Yanqing chen
Lee Brown [1/23]
Yanqing Chen [9/11]
Pat Foo [2/5]
Rees, G., Howseman, A., et al. (1997) Characterizing the relationship between BOLD contrast and regional cerebral blood flow measurements by varying the stimulus presentation rate. Neuroimage, 6, 270-278.
Tim McKenna [9/24]
The genesis of new cells, including neurons, in the adult human brain has not yet been demonstrated. This study was undertaken to investigate whether neurogenesis occurs in the adult human brain, in regions previously identified as neurogenic in adult rodents and monkeys. Human brain tissue was obtained postmortem from patients who had been treated with the thymidine analog, bromodeoxyuridine (BrdU), that labels DNA during the S phase. Using immunofluorescent labeling for BrdU and for one of the neuronal markers, NeuN, calbindin or neuron specific enolase (NSE), we demonstrate that new neurons, as defined by these markers, are generated from dividing progenitor cells in the dentate gyrus of adult humans. Our results further indicate that the human hippocampus retains its ability to generate neurons throughout life. [Online copy of the article]
Wilson Truccolo [10/1]
Peaks in spike train correlograms are usually taken as indicative of spike timing synchronization between neurons. Strictly speaking, however, a peak merely indicates that the two spike trains were not independent. Two biologically plausible ways of departing from independence that are capable of generating peaks very similar to spike timing peaks are described here: covariations over trials in response latency and covariations over trials in neuronal excitability. Since peaks due to these interactions can be similar to spike timing peaks, interpreting a correlogram may be a problem with ambiguous solutions. What peak shapes do latency or excitability interactions generate? When are they similar to spike timing peaks? When can they be ruled out from having caused an observed correlogram peak? These are the questions addressed here. The previous article in this issue proposes quantitative methods to tell cases apart when latency or excitability covariations cannot be ruled out.
Gautam Vallabha [10/8]
Human subjects are known to adapt their motor behavior to a shift of the visual field brought about by wearing prism glasses over their eyes. The analog of this phenomenon was studied in the speech domain. By use of a device that can feed back transformed speech signals in real time, subjects were exposed to phonetically sensible, online perturbations of their own speech patterns. It was found that speakers learn to adjust their production of a vowel to compensate for feedback alterations that change the vowel's perceived phonetic identity; moreover, the effect generalizes across phonetic contexts and to different vowels.
Dawei Dong [10/15]
We present a new method for separating multiple task-related events and other physiological and physical events revealed by functional MRI (fMRI) signals. Each event is a three-dimensional spatial pattern of brain activation and an associated time-course. The method separates fMRI signals into different events by minimizing the spatial and temporal correlations between events. We derived a closed-form solution which does not assume any spatial or temporal structure of an event, and works well for Gaussian and non-Gaussian distribution of different event signal sources.
When applied to various fMRI data sets from subjects performing certain tasks, We found not only the activation in cortical areas corresponding to the main tasks but also other task-related as well as unrelated events. For example, we sometimes found an activation in auditory area which was consistent with the time course of the auditory instructions given to the subject. We also found other events such as slow changes probably associated with head drifts. Therefore, signals from different sources are separated by the method and we can achieve a better signal-to-noise ratio for the activations related to the main tasks.
Yanqing Chen [11/12]
Objective: To study the cortical physiology of fast repetitive finger movements. Methods: We recorded steady- state movement-related magnetic fields (ssMRMFs) associated with self-paced, repetitive, 2-Hz finger movements in a 122-channel whole-head magnetometer. The ssMRMF generators were determined by equivalent current dipole (ECD) modeling and co-registered with anatomical magnetic resonance images (MRIs). Results: Two major ssMRMF components occurred in proximity to EMG onset: a motor field (MF) peaking at 37+/-11 ms after EMG onset, and a postmovement field (post-MF), with inverse polarity, peaking at 102+/-13 ms after EMG onset. The ECD for the MF was located in the primary motor cortex (M1), and the ECD for the post-MF in the primary somatosensory cortex (S1). The MF was probably closely related to the generation of corticospinal volleys, whereas the post-MF most likely represented reafferent feedback processing. Conclusions: The present data offer further evidence that the main phasic changes of cortical activity occur in direct proximity to repetitive EMG bursts in the contralateral M1 and S1. They complement previous electroencephalography (EEG) findings on steady-state movement-related cortical potentials (ssMRCPs) by providing more precise anatomical information, and thereby enhance the potential value of ssMRCPs and ssMRMFs for studying human sensorimotor cortex activation non-invasively and with high temporal resolution.
Corey Delaplain [11/19]
Two simple experiments reveal that the ease with which an action is performed by the neuromuscular-skeletal system determines the attentional resources devoted to the movement. Participants were required to perform a primary task, consisting of rhythmic flexion and extension movements of the index finger, while being paced by an auditory metronome, in one of two modes of coordination: flex on the beat, or extend on the beat. Using a classical dual-task methodology, we demonstrated that the time taken to react to an unpredictable visual probe stimulus (the secondary task) by means of a pedal response was greater when the extension phase of the finger movement sequence was made on the beat of the metronome than when the flexion phase was coordinated with the beat. In a second experiment, the posture of the wrist was manipulated in order to alter the operating lengths of the muscles that flex and extend the index finger. The attentional demands of maintaining the extend-on-the-beat pattern were altered in a systematic fashion by changes in wrist posture, even though the effector used to respond to the visual probe stimulus was unaffected.
Dinesh Nair [2/18]
Synaptic activity modulates synaptic efficacy and is important in learning and development. Here we show that development of excitability in presynaptic motor neurons required synaptic activation of postsynaptic muscle cells. Synaptic blockade broadened action potentials and decreased repetitive firing of presynaptic neurons. Consistent with these findings, synaptic blockade also decreased potassium-current density in the presynaptic cell. Application of neurotrophin-3, but not related neurotrophins, prevented these changes. Recordings from patches of somatic membrane indicated that modifications of presynaptic potassium and sodium currents occurred in a remote, nonsynaptic compartment. Thus, activity-dependent postsynaptic signals modulated presynaptic excitability, potentially regulating transmission at all synapses of the presynaptic cell.
Susan Owens [3/3]
Performance of complex motor tasks, such as rapid sequences of finger movements, can be improved in terms of speed and accuracy over several weeks by daily practice sessions. This improvement does not generalize to a matched sequence of identical component movements, nor to the contralateral hand. Here we report a study of the neural changes underlying this learning using functional magnetic resonance imaging (MRI) of local blood oxygenation level-dependent (BOLD) signals evoked in primary motor cortex (M1). Before training, a comparable extent of M1 was activated by both sequences. However, two ordering effects were observed: repeating a sequence within a brief time window initially resulted in a smaller area of activation (habituation), but later in larger area of activation (enhancement), suggesting a switch in M1 processing mode within the first session (fast learning). By week 4 of training, concurrent with asymptotic performance, the extent of cortex activated by the practised sequence enlarged compared with the unpractised sequence, irrespective of order (slow learning). These changes persisted for several months. The results suggest a slowly evolving, long-term, experience-dependent reorganization of the adult M1, which may underlie the acquisition and retention.
Shelrie Houlton [3/10]
We present behavioral and anatomical evidence for a multi-component reading system in which different components are differentially weighted depending on culture-specific demands of orthography. Italian orthography is consistent, enabling reliable conversion of graphemes to phonemes to yield correct pronunciation of the word. English orthography is inconsistent, complicating mapping of letters to sounds. In behavioral studies, Italian students showed faster word and non-word reading than English students. In two PET studies, Italians showed greater activations in left superior temporal regions associated with phoneme processing. In contrast, English readers showed greater activations, particularly for non-words, in left posterior inferior temporal gyrus and anterior inferior frontal gyrus, areas associated with word retrieval during both reading and naming tasks.
Cheng Qian [3/24]
We studied the transition of stimuli from novel to familiar in visual search and in the guidance of attention to a particular object. Ability to identify an object improved dramatically over several days of training. The learning was specific for the object's position in the visual field, orientation and configuration. Improvement was initially localized to one or two positions near the fixation spot and then expanded radially to include the full area of the stimulus array. Characteristics of this learning process may reflect a shift in the cortical representation of complex features toward earlier stages in the visual pathway.
Chris King [3/31]
Thousands of hippocampal neurons are born in adulthood, suggesting that new cells could be important for hippocampal function. To determine whether hippocampus-dependent learning affects adult-generated neurons, we examined the fate of new cells labeled with the thymidine analog bromodeoxyuridine following specific behavioral tasks. Here we report that the number of adult-generated neurons doubles in the rat dentate gyrus in response to training on associative learning tasks that require the hippocampus. In contrast, training on associative learning tasks that do not require the hippocampus did not alter the number of new cells. These findings indicate that adult-generated hippocampal neurons are specifically affected by, and potentially involved in, associative memory formation.
Dr. Annette Schmid [4/7]
Collins Assisi [4/12]
We have investigated the role that different connectivity regimes play in the dynamics of a network of Hodgkin-Huxley neurons by computer simulations. The different connectivity topologies exhibit the following features: random topologies give rise to fast system response yet are unable to produce coherent oscillations in the average activity of the network; on the other hand, regular topologies give rise to coherent oscillations, but in a temporal scale that is not in accordance with fast signal processing. Finally, small-world topologies, which fall between random and regular ones, take advantage of the best features of both, giving rise to a fast system response with coherent oscillations.
Lina Shehadeh [4/14]
Many physical processes have distributions of times between events that have non-normalizable, power law probability density functions. The moments (mean and variance) of such distributions are not well-defined. We found that the PDF's of the times between events of atrial fibrillation have a power law form indicative of a non-normalizable distribution, and that the timing between these events cannot be meaningfully characterized by the mean frequency of such events. These results indicate that the physical processes that disrupt the normal rhythm of the heart produce a fractal pattern in the timing between these events.
Tim St.Cyr [4/21]
Despite growing interest in temporal aspects of auditory neural processing, little is known about large-scale timing patterns of brain activity during the perception of auditory sequences. This is partly because it has not been possible to distinguish stimulus-related activity from other, endogenous brain signals recorded by electrical or magnetic sensors. Here we use amplitude modulation of unfamiliar, ~1-minute-long tone sequences to label stimulus-related magnetoencephalographic neural activity in human subjects. We show that temporal patterns of activity recorded over particular brain regions track the pitch contour of tone sequences, with the accuracy of tracking increasing as tone sequences become more predictable in structure. In contrast, temporal synchronization between recording locations, particularly between sites over the left posterior hemisphere and the rest of the brain, is greatest when sequences have melody-like statistical properties, which may reflect the perceptual integration of local and global pitch patterns in melody-like sequences. This method is particularly well suited to studying temporal neural correlates of complex auditory sequences (such as speech or music) which engage multiple brain areas as perception unfolds in time.
Gautam Vallabha [9/15]
The emergence of language was a defining moment in the evolution of modern humans. It was an innovation that changed radically the character of human society. Here, we provide an approach to language evolution based on evolutionary game theory. We explore the ways in which protolanguages can evolve in a nonlinguistic society and how specific signals can become associated with specific objects. We assume that early in the evolution of language, errors in signaling and perception would be common. We model the probability of misunderstanding a signal and show that this limits the number of objects that can be described by a protolanguage. This "error limit" is not overcome by employing more sounds, but by combining a small set of more easily distinguished sounds into words. The process of "word formation" enables a language to encode an essentially unlimited number of objects. Next, we analyze how words can be combined into sentences and specify the conditions for the evolution of very simple grammatical rules. We argue that grammar originated as a simplified rule system that evolved by natural selection to reduce mistakes in communication. Our theory provides a systematic approach for thinking about the origin and evolution of human language.
Bill McLean [9/22]
Linear systems theory provides an inadequate characterization of sustained oscillations in nature. In this two-part exposition of oscillation in piece-wise linear dynamical systems, we guide the reader from linear concepts and simple harmonic motion to non-linear concepts and chaos. By means of three worked examples, we bridge the gap from the familiar parallel RLC network to exotic non-linear dynamical phenomena in Chua's circuit. Our goal is to stimulate the reader to think deeply about the fundamental nature of oscillation and to develop intuition into the chaos-producing mechanisms of non-linear dynamics. In order to exhibit chaos, an autonomous circuit consisting of resistors, capacitors and inductors must contain (1) at least one non-linear element, (2) at least one locally active resistor, and (3) at least three energy-storage elements. Chua's circuit is the simplest electronic circuit that satisfies these criteria. In addition, this remarkable circuit is the only physical system for which the presence of chaos has been proved mathematically. The circuit is readily constructed at low cost using standard electronic components and exhibits a rich variety of bifurcations and chaos. In part -I of this two-part paper, we plot the evolution of our understanding of oscillation from linear concepts and the parallel RLC resonant circuit to piece-wise linear circuits and Chua's circuit. We illustrate by theory, simulation and laboratory experiment, the concepts of equilibria, stability, local and global behavior, bifurcations and steady-state solutions. In part-II, we study bifurcations and chaos in a robust practical implementation of Chua's circuit.
Fred Carver [10/6]
Working memory (WM) is the ability to retain and associate information over brief time intervals. Functional imaging studies demonstrate that WM is mediated by a distributed network including frontal and posterior cortices, hippocampus, and cerebellum. In rodents, the presentation of stimuli in a WM task is followed by a reset of the phase of hippocampal theta. In this paper, we report the observation of a similar phenomenon in normal human subjects. Neuromagnetic responses were recorded during presentation of a set of digits and a subsequent probe of the retained items. All stimuli were presented with a fixed temporal pattern. We observed phase reset of ~7 Hz theta in the left hippocampus ~120 ms after probe stimuli, whereas reset of theta in right hippocampus was visible ~80 ms prior to these anticipated stimuli. The duration of stimulus-locked theta increased with memory load, with a limiting value of ~600 ms for 5-7 retained items. We suggest that, as in rats, stimulus-locked theta may index involvement of human hippocampal networks in the cognitive processing of sensory input. The anticipatory phase rest of theta indicates involvement of hippocampus in right hemisphere and cerebellar timing networks. Hippocampal structures are essential for orientation to perturbations in the sensory scene, a function that requires use of a context established by a constellation of stimuli. We suggest that the initiation and maintenance of stimulus-locked hippocampal theta observed here may facilitate processing of potentially salient and/or novel input with respect to a context established by the contents of WM.
Roger Sambrook [10/13]
The epsilon4 allele of the apolipoprotein E gene (APOE) is the chief known genetic risk factor for Alzheimer's disease. In this study, the authors performed APOE genotyping and functional magnetic resonance imaging (MRI) of the brain in 30 older persons (aged 47-82 yrs) with intact cognition to examine the relation between brain responses to tasks requiring memory and the genetic risk of Alzheimer's disease. Patterns of brain activation during functional MRI scanning were determined while Ss memorized and recalled unrelated pairs of words and while Ss rested between such periods. Memory was reassessed in 14 Ss 2 yrs later. Results show that both the magnitude and the extent of brain activation during memory-activation tasks in regions affected by Alzheimer's disease were greater among the carriers of the APOE epsilon4 allele than among carriers of the APOE epsilon3 allele. During periods of recall, the carriers of the APOE epsilon4 allele had a greater average increase in signal intensity in the hippocampal region and a greater mean number of activated regions throughout the brain than did carriers of the APOE epsilon3 allele. Longitudinal assessment after 2 yrs indicated that the degree of baseline brain activation correlated with degree of decline in memory.
Miao Tan [10/20]
Taube Lubart [11/17]
The amygdala is thought to play a crucial role in emotional and social behavior. Animal studies implicate the amygdala in both fear conditioning and face perception. In humans, lesions of the amygdala can lead to selective deficits in recognition of fearful facial expressions and impaired fear conditioning, and direct electric stimulation evokes fearful emotional responses. Here we report direct in vivo evidence of a differential neural response in the human amygdala to facial expressions of fear and happiness. Positron-emission tomography (PET) measures of neural activity were acquired while subjects viewed photographs of fearful and happy faces, varying systematically in emotional intensity. The neural response in the left amygdala was significantly greater to fearful as opposed to happy expressions. Furthermore, this response showed a significant interaction with the intensity of emotion (increasing with increasing fearfulness, decreasing with increasing happiness). The findings provide direct evidence that the human amygdala is engaged in processing the emotional salience of faces, with a specificity of response to fearful facial expressions.
| Feb 2 | Dinesh Nair
Blood, A.J., Zatorre, R.J., Bermudez, P., and Evans, A.C. (1999) Emotional responses to pleasant and unpleasant music correlate with activity in paralimbic brain regions. Nature Neuroscience , 2, 382-387. Neural correlates of the often-powerful emotional responses to music are poorly understood. Here we used positron emission tomography to examine cerebral blood flow (CBF) changes related to affective responses to music. Ten volunteers were scanned while listening to six versions of a novel musical passage varying systematically in degree of dissonance. Reciprocal CBF covariations were observed in several distinct paralimbic and neocortical regions as a function of dissonance and of perceived pleasantness/unpleasantness. The findings suggest that music may recruit neural mechanisms similar to those previously associated with pleasant/unpleasant emotional states, but different from those underlying other components of music perception, and other emotions such as fear. [Paper in PDF format] |
| Feb 12 | Shelrie
Houlton
Belin, P., Zatorre, R.J., Lafaille, P., Ahad, P. and Pike, B. (2000) Voice-selective areas in human auditory cortex. Nature , 403, 309-312. The human voice contains in its acoustic structure a wealth of information on the speaker's identity and emotional state which we perceive with remarkable ease and accuracy. Although the perception of speaker-related features of voice plays a major role in human communication, little is known about its neural basis. Here we show, using functional magnetic resonance imaging (fMRI) in human volunteers, that voice-selective regions can be found bilaterally along the upper bank of the superior temporal sulcus (STS). These regions showed greater neuronal activity when subjects listened passively to vocal sounds, whether speech or non-speech, than to non-vocal environmental sounds. Central STS regions also displayed a high degree of selectivity by responding signifcantly more to vocal sounds than to matched control stimuli, including scrambled voices and amplitude-modulated noise. Moreover, their response to stimuli degraded by frequency filtering paralleled the subjects' behavioural performance in voice-perception tasks that used these stimuli. The voice-selective areas in the STS may represent the counterpart of the face-selective areas in human visual cortex; their existence sheds new light on the functional architecture of the human auditory cortex. [Paper in PDF format] |
| Feb 16 | Gautam
Vallabha
Brooks, R.A., Breazeal, C., Irie, R. et al. (1998). Alternative Essences of Intelligence. In Proceedings of the Fifteenth National Conference on Artificial Intelligence, pp. 961-968. (Menlo Park, Calif.: AAAI Press). We present a novel methodology for building human-like
artificially intelligent systems. We take as a model the only existing
systems which are universally accepted as intelligent: humans. We
emphasize building intelligent systems which are not masters of a
single domain, but, like humans, are adept at performing a variety of
complex tasks in the real world. Using evidence from cognitive science
and neuroscience, we suggest four alternative essences of intelligence
to those held by classical AI. These are the parallel themes of
development, social interaction, embodiment, and integration. Following
a methodology based on these themes, we have built a physical humanoid
robot. In this paper we present our methodology and the insights it
affords for
facilitating learning, simplifying the computation underlying rich
behavior,
and building systems that can scale to more complex tasks in more
challenging
environments. |
| Feb 23 | Collins Assisi
Geoffrey B West et al. (1997). A general model for the origin of allometric scaling laws in Biology. Science, 276, 122-126. Allometric scaling relations, including the 3/4 power law for metabolic rates, are characteristic of all organisms and are here derived from a general model that describes how essential materials are transported through space-filling fractal networks of branching tubes. The model assumes that the energy dissipated is minimized and that the terminal tubes do not vary with body size. It provides a complete analysis of scaling relations for mammalian circulatory systems that are in agreement with the data. More generally, the model predicts structural and functional properties of vertebrate cardiovascular and respiratory systems, plant vascular systems, insect tracheal tubes, and other distribution networks.
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| Mar 2 | Debbie Taylor
Neuromagnetic correlates of sensorimotor synchronization Muller K.; Schmitz F.; Schnitzler A.; Freund H.-J.; Aschersleben G.; Prinz W. Neurologische Klinik, Heinrich-Heine-Universitat, MEG-Labor, Moorenstrasse 5, 40225 Dusseldorf, Germany. Sensorimotor synchronization tasks, in which subjects have to tap their finger in synchrony with an isochronous auditory click, typically reveal a synchronization error with the tap preceding the click by about 20 to 50 msec. Although extensive behavioral studies and a number of different explanatory accounts have located the cause of this so-called 'negative asynchrony' on different levels of processing, the underlying mechanisms are still not completely understood. Almost nothing is known about the central processes, in particular, which sensory or motor events are synchronized by subjects. The present study examined central-level processing in synchronization tasks with magnetoencephalography (MEG). Eight subjects synchronized taps with their right index finger to an isochronous binaural paring signal presented at an interstimulus interval of 800 msec. To gain information on central temporal coupling between 'tap'' and 'click,' evoked responses were averaged time-locked to the auditory signal and the tap onset. Tap-related responses could be explained with a three dipole model: One source, peaking at approximately 77 msec before tap onset, was localized in contralateral primary motor cortex (MI); the two other sources, peaking approximately at tap onset and 75 msec after tap onset, in contralateral primary somatosensory cortex (SI). Temporal coupling of these sources was compared in relation to different trigger points. The second SI source was equally well time locked to the tap and to the auditory click. Furthermore, analysis of the time locking of this source activity as a function of the temporal order of tap and click showed that the second event - irrespective whether tap or click - was decisive in triggering the second SI source. This suggests that subjects use mainly sensory feedback in judging and evaluating whether they are 'keeping time'.
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| Mar 9 | Wilson Truccolo --- POSTPONED ---
Rao, RPN, and Ballard, DH (1999). Predictive coding in the visual cortex: a functional interpretation of some extra-classical receptive-field effects. Nature Neuroscience, 2(1), 79-87. We describe a model of visual processing in which feedback connections from a higher- to a lower-order visual cortical area carry predictions of lower-level neural activities, whereas the feedforward connections carry the residual errors between the predictions and the actual lower-level activities. When exposed to natural images, a hierarchical network of model neurons implementing such a model developed simple-cell-like receptive fields. A subset of neurons responsible for carrying the residual errors showed endstopping and other extra-classical receptive-field effects. These results suggest that rather than being exclusively feedforward phenomena, nonclassical surround effects in the visual cortex may also result from cortico-cortical feedback asa consequence of the visual system using an efficient hierarchical strategy for encoding natural images.
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| Mar 30 | Xiaomin Yue
Platt ML, Glimcher PW (1999). Neural correlates of decision variables in parietal cortex. Nature, 400, 233-238. Decision theory proposes that humans and animals decide what to do in a given situation by assessing the relative value of each possible response. This assessment can be computed, in part, from the probability that each action will result in a gain and the magnitude of the gain expected. Here we show that the gain (or reward) a monkey can expect to realize from an eye-movement response modulates the activity of neurons in the lateral intraparietal area, an area of primate cortex that is thought to transform visual signals into eye-movement commands. We also show that the activity of these neurons is sensitive to the probability that a particular response will result in a gain. When animals can choose freely between two alternative responses, the choices subjects make and neuronal activation in this area are both correlated with the relative amount of gain that the animal can expect from each response. Our data indicate that a decision-theoretic model may provide a powerful new framework for studying the neural processes that intervene between sensation and action.
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| Apr 13 | Matt Woods
Tenenbaum JB, de Silva V, Langford JC (2000). A global geometric framework for nonlinear dimensionality reduction. Science, 290, 2319-2323. Scientists working with large volumes of high-dimensional data, such as global climate patterns, stellar spectra, or human gene distributions, regularly confront the problem of dimensionality reduction: finding meaningful low-dimensional structures hidden in their high-dimensional observations. The human brain confronts the same problem in everyday perception, extracting from its high-dimensional sensory inputs-30,000 auditory nerve fibers or 10(6) optic nerve fibers-a manageably small number of perceptually relevant features. Here we describe an approach to solving dimensionality reduction problems that uses easily measured local metric information to learn the underlying global geometry of a data set. Unlike classical techniques such as principal component analysis (PCA) and multidimensional scaling (MDS), our approach is capable of discovering the nonlinear degrees of freedom that underlie complex natural observations, such as human handwriting or images of a face under different viewing conditions. In contrast to previous algorithms for nonlinear dimensionality reduction, ours efficiently computes a globally optimal solution, and, for an important class of data manifolds, is guaranteed to converge asymptotically to the true structure.
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| Apr 20 | Felix Almonte
Feldman, J. (2000). Minimization of Boolean complexity in human concept learning. Nature, 407, 572-573. One of the unsolved problems in the field of human concept learning concerns the factors that determine the subjective difficulty of concepts: why are some concepts psychologically simple and easy to learn, while others seem difficult, complex or incoherent? This question was much studied in the 1960s but was never answered, and more recent characterizations of concepts as prototypes rather than logical rules leave it unsolved. Here I investigate this question in the domain of Boolean concepts (categories defined by logical rules). A series of experiments measured the subjective difficulty of a wide range of logical varieties of concepts (41 mathematically distinct types in six families--a far wider range than has been tested previously). The data reveal a surprisingly simple empirical 'law': the subjective difficulty of a concept is directly proportional to its Boolean complexity (the length of the shortest logically equivalent propositional formula)--that is, to its logical incompressibility. Jacob Feldman's home page (with links to an online copy of the paper and supplementary information).
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| Sep 7 | Dr. Jeffrey Bradstreet
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| Sep 21 | Olivier Oullier |
| Oct 5 | Zhiyi Zhou
Rao, S.M., Harrington, D.L, et al. (1997). Distributed neural systems underlying the timing of movements. Journal of Neuroscience , 17(4), 5528-5535. Timing is essential to the execution of skilled movements, yet our knowledge of the neural systems underlying timekeeping operations is limited. Using whole-brain functional magnetic resonance imaging, subjects were imaged while tapping with their right index finger in synchrony with tones that were separated by constant intervals [Synchronization (S)], followed by tapping without the benefit of an auditory cue [Continuation (C)]. Two control conditions followed in which subjects listened to tones and then made pitch discriminations (D). Both the S and the C conditions produced equivalent activation within the left sensorimotor cortex, the right cerebellum (dorsal dentate nucleus), and the right superior temporal gyrus (STG). Only the C condition produced activation of a medial premotor system, including the caudal supplementary motor area (SMA), the left putamen, and the left ventrolateral thalamus. The C condition also activated a region within the right inferior frontal gyrus (IFG), which is functionally interconnected with auditory cortex. Both control conditions produced bilateral activation of the STG, and the D condition also activated the rostral SMA. These results suggest that the internal generation of precisely timed movements is dependent on three interrelated neural systems, one that is involved in explicit timing (putamen, ventrolateral thalamus, SMA), one that mediates auditory sensory memory (IFG, STG), and another that is involved in sensorimotor processing (dorsal dentate nucleus, sensorimotor cortex). |
| Oct 12 | Dr. K.J.
Jantzen
Logothetis, N.K, Pauls, J., Augath, M. Trinath, T., & Oeltermann, A. (2001). Neurophysiological investigation of the basis of the MRI signal , Nature, 412, 150-157. Functional magnetic resonance imaging (fMRI) is widely used to study the operational organization of the human brain, but the exact relationship between the measured fMRI signal and the underlying neural activity is unclear. Here we present simultaneous intracortical recordings of neural signals and fMRI responses. We compared local field potentials (LFPs), single- and multi-unit spiking activity with highly spatio-temporally resolved blood-oxygen-level-dependent (BOLD) fMRI responses from the visual cortex of monkeys. The largest magnitude changes were observed in LFPs, which at recording sites characterized by transient responses were the only signal that significantly correlated with the haemodynamic response. Linear systems analysis on a trial- by-trial basis showed that the impulse response of the neurovascular system is both animal- and site-specific, and that LFPs yield a better estimate of BOLD responses than the multi-unit responses. These ®ndings suggest that the BOLD contrast mechanism re¯ects the input and intracortical processing of a given area rather than its spiking output. |
| Oct 26 | Lina
Shehadeh
H. Jeong, B. Tombor, A.-L. Barabasi, and Z.N. Oltvai, The global organization of cellular networks. (Computation of biochemical pathways and genetic networks workshop proceeding, Heidelberg, 2001). Abstract: Biological phenomena arise as a sum of various cellular constituents and reactions seamlessly integrated into a complex functional network. Although the large-scale topology of metabolic networks has been established, the global relationship of all cellular constituents remains unknown. Here we show that the comnplete biochemical reaction network of 43 organisms from life's three domains, and separately their information pathway components, mirror the topologic scaling properties of their metabolic networks and display striking similarity to the organization of robust and error-tolerant scale-free networks. This may indicate that the global topology of cellular networks is organized identically in all species, and its form may represent a design pattern inherent to cellular life. [ pdf ] |
| Nov 30 | Fred Carver
Sakai K, Hikosaka O, Miyauchi S, Takino R, Tamada T, Iwata NK, Nielsen M. (1999). Neural representation of a rhythm depends on its interval ratio . Journal of Neuroscience, 19(22), 10074-81. Rhythm is determined solely by the relationship between the time intervals of a series of events. Psychological studies have proposed two types of rhythm representation depending on the interval ratio of the rhythm: metrical and nonmetrical representation for rhythms formed with small integer ratios and noninteger ratios, respectively. We used functional magnetic resonance imaging to test whether there are two neural representations of rhythm depending on the interval ratio. The subjects performed a short-term memory task for a seven-tone rhythm sequence, which was formed with 1:2:4, 1:2:3, or 1:2.5:3.5 ratios. The brain activities during the memory delay period were measured and compared with those during the retention of a control tone sequence, which had constant intertone intervals. The results showed two patterns of brain activations; the left premotor and parietal areas and right cerebellar anterior lobe were active for 1:2:4 and 1:2:3 rhythms, whereas the right prefrontal, premotor, and parietal areas together with the bilateral cerebellar posterior lobe were active for 1:2.5:3.5 rhythm. Analysis on individual subjects revealed that these activation patterns depended on the ratio of the rhythms that were produced by the subjects rather than the ratio of the presented rhythms, suggesting that the observed activations reflected the internal representation of rhythm. These results suggested that there are two neural representations for rhythm depending on the interval ratio, which correspond to metrical and nonmetrical representations. |
| Jan 25 | Gautam Vallabha
Self-imitation as a window into vowel organization The imitation task provides an alternative approach to traditional problems such as the granularity of vowel representation and its relation to linguistic categories. Imitation of self-produced vowels is especially useful -- since the speaker is in principle capable of perfectly matching the target, systematic deviations can be ascribed to perceptual and production biases. I shall present data from two experiments that used the self-imitation paradigm: (1) Multiple reproduction, in which speakers imitated each target ten times, and (2) Serial reproduction, in which the imitation was played back as target for another imitation, and so on for ten steps. For both experiments, the targets were distributed across the F1/F2 space, allowing a global map of the imitation behavior. The results show that speakers are consistently inaccurate, and articulatory and perceptual modeling suggests that the inaccuracies are not due to low-level noise. I shall discuss implications for the organization of the vowel space, and whether the accuracy of self-imitation can serve as a general measure of phonetic stability. |
| Feb 22 | Ted Zanto
Rodriguez, E., et al. (1999). Perception's Shadow: Long Distance Synchronization of Human Brain Activity. Nature 397, 430-433. Transient periods of synchronization of oscillating neuronal discharges in the frequency range 30-80 Hz (gamma oscillations) have been proposed to act as an integrative mechanism that may bring a widley distributed set of neurons together into a coherent ensemble that underlies a cognitive act...Here we record electrical brain activity from subjects who are viewing ambiguous visual stimuli (perceived either as a face or as meaningless shapes). We show for the first time, to our knowledge, that only face perception induces a long-distance pattern of synchronization, corresponding to the moment of perception itself and to the ensuing motor response. A period of strong desynchronization marks the transition between the moment of perception and the motor response. We suggest that this desynchronization reflects a process of active uncoupling of the underlying neural ensembles that is necessary to proceed from one cognitive state to another. |
| Mar 1 | Ed Modestino
Just, M.A., et al. (2001). Interdependence of Nonoverlapping Cortical Systems in Dual Cognitive Tasks. Neuroimage 14, 417-426. One of the classic questions about human thinking concerns the limited ability to perform two cognitive tasks concurrently, such as a novice driver's difficulty in simultaneously driving and conversing. Limitations on the concurrent performance of two unrelated tasks challenge the tacitly assumed independence of two brain systems that seemingly have little overlap. The current study used fMRI (functional magnetic resonance imaging) to measure cortical activation during the concurrent performance of two high-level cognitive tasks that involve different sensory modalities and activate largely nonoverlapping areas of sensory and association cortex. One task was auditory sentence comprehension, and the other was the mental rotation of visually depicted 3-D objects. If the neural systems underlying the two tasks functioned independently, then in the dual task the brain activation in the main areas supporting the cognitive processing should be approximately the conjunction of the activation for each of the two tasks performed alone. We found instead that in the dual task, the activation in association areas (primarily temporal and parietal areas of cortex) was substantially less than the sum of the activation when the two tasks were performed alone, suggesting some mutual constraint among association areas. A similar result was obtained for sensory areas as well. |
| Mar 15 | Dinesh Nair
Cancelled |
| Mar 22 | Xiaomin Yue
Cancelled |
| Mar 29 | Collins Assisi
Hoppensteadt, F.C., & Izhikevich, E.M. (1996). Synaptic organizations and dynamical properties of weakly connected neural oscillators. I. Analysis of a canonical model. Biological Cybernetics 75, 117-127. We study weakly connected networks of neural oscillators near
multiple Andronov-Hopf bifurcation points. We analyze relationships
between
synaptic organizations (anatomy) of the networks and their dynamical
properties
(function). Our principal assumptions are: (1) Each neural oscillator
comprises
two populations of neurons: excitatory and inhibitory ones; (2)
activity
of each population of neuron as is described by a scalar
(one-dimensional)
variable; (3) each neural oscillator is near a nondegenerate
supercritical
Andronov-Hopf bifurcation point; (4) the synaptic connec-tions between
the
neural oscillators are weak. All neural networks satisfying these
hypotheses
are governed by the same dynamical system, which we call the canonical
model.
Studying the canonical model shows that: |
| Apr 5 | Mike Marshall
Hui, KK, Liu, J, Makris, N. et al (2000). Acupuncture modulates the limbic system and subcortical gray structures of the human brain: evidence from fMRI studies in normal subjects. Human Brain Mapping 9(1), 13-25. Acupuncture, an ancient therapeutic technique, is emerging as an important modality of complementary medicine in the United States. The use and efficacy of acupuncture treatment are not yet widely accepted in Western scientific and medical communities. Demonstration of regionally specific, quantifiable acupuncture effects on relevant structures of the human brain would facilitate acceptance and integration of this therapeutic modality into the practice of modern medicine. Research with animal models of acupuncture indicates that many of the beneficial effects may be mediated at the subcortical level in the brain. We used functional magnetic resonance imaging (fMRI) to investigate the effects of acupuncture in normal subjects and to provide a foundation for future studies on mechanisms of acupuncture action in therapeutic interventions. Acupuncture needle manipulation was performed at Large Intestine 4 (LI 4, Hegu) on the hand in 13 subjects [Stux, 1997]. Needle manipulation on either hand produced prominent decreases of fMRI signals in the nucleus accumbens, amygdala, hippocampus, parahippocampus, hypothalamus, ventral tegmental area, anterior cingulate gyrus (BA 24), caudate, putamen, temporal pole, and insula in all 11 subjects who experienced acupuncture sensation. In marked contrast, signal increases were observed primarily in the somatosensory cortex. The two subjects who experienced pain instead of acupuncture sensation exhibited signal increases instead of decreases in the anterior cingulate gyrus (BA 24), caudate, putamen, anterior thalamus, and posterior insula. Superficial tactile stimulation to the same area elicited signal increases in the somatosensory cortex as expected, but no signal decreases in the deep structures. These preliminary results suggest that acupuncture needle manipulation modulates the activity of the limbic system and subcortical structures. We hypothesize that modulation of subcortical structures may be an important mechanism by which acupuncture exerts its complex multisystem effects. |
| Apr 19 | Debbie Taylor
Sannita, WG, Bandini, F, Beelke, M. (2001). Time dynamics of stimulus- and event-related gamma band activity: contrast-VEPs and the visual P300 in man. Clinical Neurophysiology 112, 2241-2249. Objectives: To investigate the time dynamics and phase
relationship with the stimulus of the onset/offset visual evoked
potentials (VEPs),P300 and gamma band oscillatory responses to visual
(contrast) stimulation. Gamma band oscillatory activity mediates in
sensory and cognitive operations, with a role in stimulus-related
cortical synchronization, but is reportedly reduced in the time window
of the P300 response. |
| Aug 30 | Dr.
Ed Large and Shelrie
Houlton
Discussion of papers presented at the 7th International
Conference on Music Perception and Cognition, Sydney, in July 2002. |
| Sep 6 | Dinesh Nair
Discussion of the paper presented at the 7th International Conference on Music Perception and Cognition, Sydney, in July 2002 ( paper in pdf ). |
Sep 13 |
K.J. Jantzen Review of safety measures while doing functional MRI
experiments. |
| Sep 20 | Stacey Nash **********
POSTPONED **********
Haxby J, Gobbini M.I., Furey M.L., Ishai A., Schouten J.L.,
Pietrini P. Distributed and Overlapping Representations of Faces
and
Objects in Ventral Temporal Cortex. Abstract: |
Sep 27 |
Olivier Oullier Meyer-Lindenberg, A., Ziemann, U., Hajak, G., Cohen, L., & Berman, K. F. Transitions between dynamical states of differing stability in the human brain. Proc. Natl. Acad. Sci. USA. 99(17): 10948-53, Aug 20, 2002 (paper in pdf) . Abstract: What mechanisms underlie the flexible formation, adaptation, synchronization, and dissolution of large-scale neural assemblies from the 10(10) densely interconnected, continuously active neurons of the human brain? Nonlinear dynamics provides a unifying perspective on self-organization. It shows that the emergence of patterns in open, nonequilibrium systems is governed by their stability in response to small disturbances and predicts macroscopic transitions between patterns of differing stability. Here, we directly demonstrate that such transitions can be elicited in the human brain by interference at the neural level. As a probe, we used a classic motor coordination paradigm exhibiting well described movement states of differing stability. Functional neuroimaging identified premotor (PMA) and supplementary motor (SMA) cortices as having neural activity linked to the degree of behavioral instability. These regions then were transiently disturbed with graded transcranial magnetic stimulation, which caused sustained and macroscopic behavioral transitions from the less stable out-of-phase to the stable in-phase movement, whereas the stable pattern could not be affected. Moreover, the strength of the disturbance needed (a measure of neural stability) was linked to the degree of behavioral stability, demonstrating the applicability of nonlinear system theory as a powerful predictor of the dynamical repertoire of the human brain. |
Oct 4 |
Anders Ledberg Pesaran B, Pezaris JS, Sahani M, Mitra PP, Andersen RA. Temporal structure in neuronal activity during working memory in macaque parietal cortex. Nat. Neurosci. 2002 Aug; 5(8): 805-11 (paper in pdf) . Abstract: Many cortical structures have elevated firing rates during working memory, but it is not known how the activity is maintained. To investigate whether reverberating activity is important, we studied the temporal structure of local field potential (LFP) activity and spiking from area LIP in two awake macaques during a memory-saccade task. Using spectral analysis, we found spatially tuned elevated power in the gamma band (25-90 Hz) in LFP and spiking activity during the memory period. Spiking and LFP activity were also coherent in the gamma band but not at lower frequencies. Finally, we decoded LFP activity on a single-trial basis and found that LFP activity in parietal cortex discriminated between preferred and anti-preferred direction with approximately the same accuracy as the spike rate and predicted the time of a planned movement with better accuracy than the spike rate. This finding could accelerate the development of a cortical neural prosthesis. |
Oct 18 |
Mukesh Dhamala Abstract: Transient chaos is a ubiquitous phenonmenon in nonlinear dynamics of many physical, biological and engineering systems. The chaotic transients are caused by the presence of chaotic saddles. The analysis on chaotic transients is far less advanced than on chaotic signals from chaotic attractors. I will talk about some aspects of the analysis of short chaotic transients from some of my earlier work on transient chaos. I will also present an fMRI experiment in which we investigated the variation in dynamical complexity of the human brain activity for different mental loads. The experiment measured the brain activity with three different experimental conditions. The correlation dimension and dimension density were used to gauge the spatiotemporal activity complexity (the calculation involved the use of multiple time series in the Grassberger-Procaccia algorithm as in the case of transient signals). The results show that there is a direct relationship between the complexity and difficulty of the tasks. Reference papers: 1 2 3 |
| Oct 25 |
Lina
Shehadeh D'Haeseleer P, Wen X, Fuhrman S, Somogyi R. Linear modeling of mRNA expression levels during CNS development and injury. Pac Symp Biocomput 1999; 41-52 Abstract: Large-scale gene expression data sets are revolutionizing the field of functional genomics. However, few data analysis techniques fully exploit this entirely new class of data. We present a linear modeling approach that allows one to infer interactions between all the genes included in the data set. The resulting model can be used to generate interesting hypotheses to direct further experiments. |
| Nov 15 | Mike Marshall Christian M. Siedentopf, Stefan M. Golaszewski, Felix M. Mottaghy, Christian C. Ruff, Stephan Felber and Andreas Schlager. Functional magnetic resonance imaging detects activation of the visual association cortex during laser acupuncture of the foot in humans. Neuroscience Letters, 327 (2002) 53–56 ( paper in pdf ). Abstract: The aim of this study was to investigate the effect of laser acupuncture on cerebral activation. Using functional magnetic imaging (fMRI) cortical activations during laser acupuncture at the left foot (Bladder 67) and dummy acupuncture, were compared employing a block design in ten healthy male volunteers. All experiments were done on a 1.5 Tesla magnetic resonance scanner equipped with a circular polarized head coil. During laser acupuncture, we found activation in the cuneus corresponding to Brodmann Area (BA) 18 and the medial occipital gyrus (BA 19) of the ipsilateral visual cortex. Placebo stimulation did not show any activation. We could demonstrate that laser acupuncture of a specific acupoint, empirically related to ophthalmic disorders, leads to activation of visual brain areas, whereas placebo acupuncture does not. These results indicate that fMRI has the potential to elucidate effects of acupuncture on brain activity. |
Nov 22 |
Mohammad Dastjerdi Discussion of their poster at SFN 2002. M. Dastjerdi and D.W Dong. Independent Component Analysis of natural time-varying images at the center of the gaze and at the periphery. Abstract: Our purpose is understanding the relationship between the response properties of visual neurons and the statistical structures of natural time varying images, specifically the high order statistical structures. In previous studies, movies and images from camera were used. But statistics of natural time varying images are, in principle, different for different retinal locations. Because eye movements like saccades, fixations and smooth pursuits explicitly keep the focus of attention at the center of the gaze. We used natural time-varying images viewed by normal human subjects. Specifically in this study, the higher order statistics of time-varying images at the center of the gaze and at the periphery are studied by decomposing the time-varying images into spatial and temporal independent components. It is found that the statistical structures at these two locations are very different. Higher velocity components are observed more frequently at the periphery than at the center of the gaze. Moreover, the higher velocity structures have larger spatial scales than the lower velocity ones. On the other hand, lower velocity and higher spatial frequency components were observed on the center more than on the periphery. Our studies suggest that different velocity and spatial resolution components at the center of the gaze and at the periphery are consistent with physiology of vision. |
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Jan 17
|
Yonghong Chen
M. Breakspear and J. R. Terry, Topographic Organization of Nonlinear Interdependence in Multichannel Human EEG. NeuroImage 16, 822–835 (2002) ( paper in pdf ). Abstract: This paper investigates the spatial organization of nonlinear interactions between different brain regions in healthy human subjects. This is achieved by studying the topography of nonlinear interdependence in multichannel EEG data, acquired from 40 healthy human subjects at rest. An algorithm for the detection and quantification of nonlinear interdependence is applied to four pairs of bipolar electrode derivations to detect posterior and anterior interhemispheric and left and right intrahemispheric interdependences. Multivariate surrogate data sets are constructed to control for linear coherence and finite sample size. Nonlinear interdependence is shown to occur in a small but statistically robust number of epochs. The occurrence of nonlinear interdependence in any region is correlated with the concurrent presence of nonlinear interdependence in other regions at high levels of significance. The strength, direction and topography of the interdependences are also correlated. For example, posterior interhemispheric interdependence from right-to-left is strongly correlated with right intrahemispheric interdependence from back-to-front. There is a subtle change in these correlations when subjects open their eyes. These results suggest that nonlinear interdependence in the human brain has a specific topographic organization which reflects simple cognitive changes. It sometimes occurs as an isolated phenomenon between two brain regions, but often involves concurrent interdependences between multiple brain regions. |
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Jan 24
|
Felix Almonte Michael H. Herzog and Manfred Fahle, Effects of grouping in context modulation . Nature, 415(6870):433-6, 24th Jan 2002 ( paper in pdf ). Abstract Perception of a visual target and the responses of cortical neurons can be strongly influenced by a context surrounding the target. This observation relates to the fundamental issue of how cortical neurons code objects of the external world. In high-contrast regimes, embedding a target in an iso-oriented context reduces neural responses and deteriorates performance in psychophysical experiments. Performance from orthogonal surrounds is better than that from iso-oriented ones. This contextual interference is often postulated to be caused by long- or short-range interactions between neurons tuned to orientation. Here we show, using a new illusion called `shine-through' as a sensitive psychophysical probe, that the orientation difference between target and context does not determine performance. Instead, contextual modulation depends on the overall spatial structure of the context. We propose that contextual suppression vanishes if the contextual elements are grouped to an independent and coherent object. |
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Jan 31
|
Arpan Banerjee Shannon R. Campbell and DeLiang Wang, Relaxation oscillators with time delay coupling. Physica D, 111(1998) 151-178 ( paper in pdf ). Abstract We study networks of relaxation oscillators coupled with time delay synapses. A pair of oscilltors is analysed and shown to attain loosely synchronous solutions for a wide range of initial conditions and time delays. Simulations of one- and two-dimensional oscillator networks indicate that locally coupled oscillators are loosely synchronous. Desynchronous solutions are possible when system parameters are varied. To characterize loosely synchronous networks, we introduce a measure of synchrony, the maximum time difference between any two oscillators. In locally excitatory globally inhibitory oscillator networks with time delays, we find that desynchronous solutions for different groups of oscillators are maintained, and the maximum number of groups that can be segregated is related to the maximum time difference within each group. To examine the maximum time difference, we display its histograms for oscillator networks in one and two dimensions. Also, a range of initial conditions is given so that the maximum time difference is contained as the system evolves. |
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Feb 7
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Andrea Brovelli Michael S. A. Graziano, Charlotte S. R. Taylor and Tirin Moore, Complex movements evoked by microstimulation of Precentral Cortex . Neuron, Vol. 34, 841–851, May 30, 2002 ( paper in pdf ) Abstract Electrical microstimulation was used to study primary motor and premotor cortex in monkeys. Each stimulation train was 500ms in duration, approximating the time scale of normal reaching and grasping movements and the timescale of neuronal activity that normally accompanies movement. This stimulation on a behaviorally relevant time scale evoked coordinated, complex postures that involved many joints. For example, stimulation of one site caused the mouth to open and also caused the hand to shape into a grip posture and move to the mouth. Stimulation of this site always drove the joints towards this final posture, regardless of the direction of movement required to reach the posture. Stimulation of other cortical sites evoked different postures. Postures that involved the arm were arranged across cortex to form a map of hand positions around the body. This stimulation-evoked map encompassed both primary motor and the adjacent premotor cortex. We suggest that these regions fit together into a single map of the workspace around the body. |
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Feb 21
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Ted Zanto Olivier Bertrand & Catherine Tallon-Baudry, Oscillatory gamma activity in humans: a possible role for object representation . Int J Psychophysiol; 2000 Dec, 38(3): 211-223 ( paper in pdf ). Abstract: The coherent representation of an object has been suggested to be established by the synchronization in the gamma range (20-100 Hz) of a distributed neural network. So-called '40-Hz' activity in humans could reflect such a mechanism. We have presented here experimental evidence supporting this hypothesis, both in the visual and auditory modalities. However, different types of gamma activity should be distinguished, mainly the evoked 40-Hz response and the induced gamma activities. Only induced gamma activities seem to be related to coherent object representations. In addition, their topography depends on sensory modality and task, which is in line with the idea that they reflect the oscillatory synchronization of task-dependent networks. They can also be functionally and topographically distinguished from the classical evoked potentials and from the alpha rhythm. It was also proposed that the functional role of gamma oscillations is not restricted to object representation established through bottom-up mechanisms of feature binding, but also extends to the cases of internally driven representations and to the maintenance of information in memory. |
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Feb 28
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Carey Witkov R. Quian Quiroga and H. Garcia, Single-trial event-related potentials with wavelet denoising. Clinical Neurophysiology (February 2003),114: 376-390 ( paper in pdf ) Abstract: The application of a recently proposed denoising implementation for obtaining event-related potentials at the single-trial level is shown. We study its performance in simulated data as well as in visual and auditory event-related potentials. For the simulated data, the method gives a significantly better reconstruction of the single-trial event-related responses in comparison with the original data and also in comparison with a reconstruction based on conventional Wiener filtering. Moreover, with wavelet denoising we obtain a significantly better estimation of the amplitudes and latencies of the simulated ERPs. For the real data, the method clearly improves the visualization of both visual and auditory single-trial event-related potentials. This allows the calculation of better averages as well as the study of systematic or unsystematic variations between trials. Since the method is fast and parameter free, it could complement the conventional analysis of event-related potentials. |
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Mar 14
|
Collins Assisi Silvia De Monte, Francesco d'Ovidio and Erik Mosekilde, Coherent Regimes of Globally Coupled Dynamical Systems. Physical Review Letters, 7 Feb 2003, Vol 90, No 5 ( article in pdf ). Abstract This Letter presents a method by which the mean field dynamics of a population of dynamical systems with parameter diversity and global coupling can be described in terms of a few macroscopic degrees of freedom. The method applies to populations of any size and functional form in the region of coherence. It requires linear variation or a narrow distribution for the dispersed parameter. Although an approximation, the method allows us to quantitatively study the transitions among the collective regimes as bifurcations of the effective macroscopic degrees of freedom. To illustrate, the phenomenon of oscillator death and the route to full locking are examined for chaotic oscillators with time scale mismatch. |
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Mar 21
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Julien Lagarde Antoine Lutz, Jean-Philippe Lachaux, Jacques Martinerie, and Francisco J. Varela, Guiding the study of brain dynamics by using first-person data: Synchrony patterns correlate with ongoing conscious states during a simple visual task. PNAS, Feb 5 2002, Vol 99, no 3, 1586-1591 ( article in pdf ). Abstract Even during well-calibrated cognitive tasks, successive brain responses to repeated identical stimulations are highly variable. The source of this variability is believed to reside mainly in fluctuations of the subject's cognitive "context" defined by his/ her attentive state, spontaneous thought process, strategy to carry out the task, and so on ... As these factors are hard to manipulate precisely, they are usually not controlled, and the variability is discarded by averaging techniques. We combined first-person data and the analysis of neural processes to reduce such noise. We presented the subjects with a three-dimensional illusion and recorded their electrical brain activity and their own report about their cognitive context. Trials were clustered according to these first-person data, and separate dynamical analyses were conducted for each cluster. We found that (i) characteristic patterns of endogenous synchrony appeared in frontal electrodes before stimulation. These patterns depended on the degree of preparation and the immediacy of perception as verbally reported. (ii) These patterns were stable for several recordings. (iii) Preparatory states modulate both the behavioral performance and the evoked and induced synchronous patterns that follow. (iv) These results indicated that first-person data can be used to detect and interpret neural processes. |
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Mar 28
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Balazs Szemes Fischl B, Sereno MI, Tootell RBH, Dale AM, High-resolution intersubject averaging and a coordinate system for the cortical surface. Human Brain Mapping (1999) 8 (4): 272-284 ( article in pdf ) Abstract The neurons of the human cerebral cortex are arranged in a highly folded sheet, with the majority of the cortical surface area buried in folds. Cortical maps are typically arranged with a topography oriented parallel to the cortical surface. Despite this unambiguous sheetlike geometry, the most commonly used coordinate systems for localizing cortical features are based on 3-D stereotaxic coordinates rather than on position relative to the 2-D cortical sheet. In order to address the need for a more natural surface-based coordinate system for the cortex, we have developed a means for generating an average folding pattern across a large number of individual subjects as a function on the unit sphere and of nonrigidly aligning each individual with the average. This establishes a spherical surface-based coordinate system that is adapted to the folding pattern of each individual subject, allowing for much higher localization accuracy of structural and functional features of the human brain. |
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Apr 4
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Anil Bollimunta Tiesinga PH, Fellous JM, Sejnowski TJ, Attractor reliability reveals deterministic structure in neuronal spike trains. Neural Comput 2002 Jul;14(7):1629-50 ( article in pdf ) Abstract When periodic current is injected into an integrate-and-fire model neuron, the voltage as a function of time converges from different initial conditions to an attractor that produces reproducible sequences of spikes. The attractor reliability is a measure of the stability of spike trains against intrinsic noise and is quantified here as the inverse of the number of distinct spike trains obtained in response to repeated presentations of the same stimulus. High reliability characterizes neurons that can support a spike-time code, unlike neurons with discharges forming a renewal process (such as a Poisson process). These two classes of responses cannot be distinguished using measures based on the spike-time histogram, but they can be identified by the attractor dynamics of spike trains, as shown here using a new method for calculating the attractor reliability. We applied these methods to spike trains obtained from current injection into cortical neurons recorded in vitro. These spike trains did not form a renewal process and had a higher reliability compared to renewal-like processes with the same spike-time histogram. |
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Apr 11
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Walter Hoover Paul Gabbott, Anthony Headlam and Sarah Busby, Morphological evidence that CA1 hippocampal afferents monosynaptically innervate PV-containing neurons and NADPH-diaphorase reactive cells in the medial prefrontal cortex (Areas 25/32) of the rat, Brain Research, Vol 946, Issue 2, 16 August 2002, 314-322 ( article in pdf ). Abstract Immunocytochemical and ultrastructural evidence is presented indicating that direct inputs from the hippocampal CA1 field to prelimbic (area 32) and infralimbic (area 25) cortices in the rat, innervate not only `spiny' (presumed pyramidal) neurons but also monosynaptically contact NADPH-diaphorase reactive cells and parvalbumin-containing local circuit neurons - the latter cell type is shown to be GABA immunoreactive. Similar evidence of direct CA1 input to local circuit neurons containing either calbindin or calretinin was not found. |
| Sept 12 |
Dr. David Lewkowicz
- There are two relevant papers for this presentation. Lewkowicz, D. J. (In Press). Perception of Serial Order in Infants. Developmental Science. Abstract: Serial order is fundamental to perception, cognition, and behavioral action. Three experiments investigated infants’ perception, learning, and discrimination of serial order. Four- and 8-month-old infants were habituated to three sequentially moving objects making visible and audible impacts and then were tested on separate test trials for their ability to detect auditory, visual, or auditory-visual changes in their ordering. The 4-month-old infants did not respond to any order changes and instead appeared to attend to the “local” audio-visual synchrony part of the event. When this local part of the event was blocked from view, the 4- month-olds did perceive the serial order feature of the event but only when it was specified multimodally. In contrast, the 8-month-old infants perceived all three kinds of order changes regardless of whether the synchrony part of the event was visible or not. The findings show that perception of spatiotemporal serial order emerges early in infancy and that its perception is initially facilitated by multimodal specification. (paper in pdf) Lewkowicz, D. J. (In Press). Learning and Discrimination of Audiovisual Events in Human Infants: The Hierarchical Relation Between Intersensory Temporal Synchrony and Rhythmic Pattern Cues. Developmental Psychology. Abstract: This study examined 4- to 10-month-old infants’ perception of audio-visual (A-V) temporal synchrony cues in the presence or absence of rhythmic pattern cues. Experiment 1 established that infants of all ages could successfully discriminate between two different audiovisual rhythmic events. Experiment 2 showed that only 10-month-old infants detected a desynchronization of the auditory and visual components of a rhythmical event. Experiment 3 showed that 4- to 8-month-old infants also could detect A-V desynchronization but only when the audiovisual event was nonrhythmic. These results show that initially in development infants attend to the overall temporal structure of rhythmic audiovisual events but that later in development they become capable of perceiving the embedded intersensory temporal synchrony relations as well (paper in pdf). |
| Sept 19 |
Dr. Phil Fink Shaffer, D. M., & McBeath, M. K. (2002). Baseball Outfielders Maintain a Linear Optical Trajectory When Tracking Uncatchable Fly Balls. Journal of Experimental Psychology: Human Perception and Performance, 28(2), 335–348. Abstract: The authors investigated whether behavior of fielders pursuing uncatchable fly balls supported either (a) maintenance of a linear optical trajectory (LOT) with monotonic increases in optical ball height or (b) maintenance of optical acceleration cancellation (OAC) with simultaneous lateral alignment with the ball. Past work supports usage of both LOT and OAC strategies in the pursuit of catchable balls headed to the side. When balls are uncatchable, fielders must choose either optical linearity or alignment at the expense of the other. Fielders maintained the LOT strategy more often and for a longer period of time than they did the OAC alignment strategy. Findings support the LOT strategy as primary when pursuing balls headed to the side, whether catchable or not. (paper in pdf) |
| Sept 26 |
Balazs
Szemes Arthurs, O. J., & Boniface, S. J. (2003). What aspect of the fMRI BOLD signal best reflects the underlying electrophysiology in human somatosensory cortex? Clinical Neurophysiology, 114, 1203–1209. Abstract: The interpretation of task-induced functional imaging of the brain is critically dependent on understanding the relationship between observed haemodynamic responses and the underlying neural changes. However, the precise nature of this neurovascular coupling relationship remains unknown. In particular, it is unclear which easure of functional magnetic resonance imaging blood oxygen level dependent (fMRI BOLD) activity is the best correlate of neural activity. We measured the somatosensory evoked potential (SEP) amplitude at the scalp, and fMRI BOLD signal to increases in intensity of contralateral median nerve electrical stimulation in healthy non-anaesthetised subjects. We compared correlation analyses between SEP amplitude and both peak voxel fMRI BOLD percentage signal change and mean voxel fMRI BOLD percentage signal change across a somatosensory cluster, and we also performed a voxel-by-voxel correlation between fMRI BOLD activity and SEP amplitude. We found that fMRI BOLD changes in primary somatosensory cortex correlate significantly with SEP amplitudes, suggesting a linear neurovascular coupling relationship under the conditions investigated. We also found that mean changes across a cluster correlate less well with SEP amplitude than peak voxel levels. This suggests that the area of haemodynamic activity correlating with SEP amplitude is smaller than the entire cluster observed. (paper in pdf) Background paper: Arthurs, O.J., Williams, E.J., Carpenter, T.A., Pickard, J.D., and Boniface, S.J. (2000). Linear coupling between functional magnetic resonance imaging and evoked potential amplitude in human somatosensory cortex. Neuroscience, 101, 803–806. Abstract: The interpretation of task-induced functional imaging of the brain is critically dependent on understanding the relationship between observed blood flow responses and the underlying neuronal changes. However, the exact nature of this neurovascular coupling relationship remains unknown. In particular, it is unclear whether blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) responses principally reflect neuronal synaptic activity. In order to address this issue directly in humans, we measured the increase in somatosensory evoked potential amplitude and fMRI BOLD changes to increases in intensity of median nerve electrical stimulation in five healthy non-anaesthetized subjects. We found that mean N20–P22 amplitudes increased significantly with stimulus intensity in all subjects, as did fMRI BOLD percentage signal intensity change. Moreover, the intensity of the BOLD signal was found to correlate linearly with evoked potential amplitude in four of the five subjects studied. This suggests that the BOLD response correlates with synchronized synaptic activity, which is the major energy consuming process of the cortex. (paper in pdf) |
| Oct 3 |
Blake Simpson Thompson, J. K., Peterson, M. R., and Freeman, R. D. (2003). Single-Neuron Activity and Tissue Oxygenation in the Cerebral Cortex. Science, 299, 1070-1072. Abstract: Blood oxygen level–dependent functional magnetic resonance imaging uses alterations in brain hemodynamics to infer changes in neural activity. Are these hemodynamic changes regulated at a spatial scale capable of resolving functional columns within the cerebral cortex? To address this question, we made simultaneous measurements of tissue oxygenation and single-cell neural activity within the visual cortex. Results showed that increases in neuronal spike rate were accompanied by immediate decreases in tissue oxygenation. We used this decrease in tissue oxygenation to predict the orientation selectivity and ocular dominance of neighboring neurons. Our results establish a coupling between neural activity and oxidative metabolism and suggest that highresolution functional magnetic resonance imaging may be used to localize neural activity at a columnar level. (paper in pdf) (supporting documents) |
| Oct 10 |
Heng Wu Wu, H. (unpublished research). Mode of Taurine’s Neuroprotective Effect. Abstract: Overexposure to glutamate, major excitatory neurotransmitter in mammalian brain, causes neuronal damage, which is called neuronal excitotoxicity. The neuronal excitotoxicity is believed to be mediated through intracellular calcium overload. Previously, we have shown that taurine can protect neurons against glutamate-induced neurotoxicity (Tang et al., 1996) by lowering the intracellular calcium level, [Ca2+]i. Here we reported that taurine decreases glutamate-induced elevation of [Ca2+]i by inhibiting calcium influx through L-, P/Q-, N-type voltage-gated calcium channels (VGCC). Furthermore, taurine was found to prevent glutamate-induced membrane depolarization. Hence, we propose that taurine exerts its neuroprotective functions partially through its effect in preventing glutamate-induced membrane depolarization resulting in reduction in calcium influx through VGCCs and lowering in [Ca2+]i. Alternatively, taurine may inhibit VGCC through metabotropic taurine receptors and coupled inhibitory G-proteins, a mechanism analogous to the GABAB receptor system. |
| Oct 17 |
Dr. Emmanuelle Tognoli Medvedev, A. V. (2002). Epileptiform spikes desynchronize and diminish fast (gamma) activity of the brain: An "anti-binding" mechanism? Brain Research Bulletin, 58 (1), Abstract: Fast (20–100 Hz) rhythms of electrical activity of the brain have been suggested to be important for perception and cognition providing a mechanism for temporal binding of neural activities underlying mental representations. Also, fast rhythms often precede epileptiform discharges inpatients and some experimental models. Generalized slow (2–3 Hz) spike activity after systemic kainic acid (KA) in the rat has been shown to be preceded by intense gamma activity. A relationship between the intensified gamma rhythms and the subsequent spike activity was studied during kainate-induced acute epileptogenesis. Power, multiple coherence and phase were analyzed at frequencies 1–100 Hz in the EEG recorded from the hippocampal–neocortical structures of the rat. Gamma rhythms, extremely intense and highly coherent at the onset of discharges, were followed by a slow rhythm of epileptiform spikes/sharp waves. During this spike activity and immediately afterwards, the gamma power and coherence were significantly decreased. These data show an antagonism between gamma rhythms and spike activity and ability of the latter to desynchronize and suppress the former. They are supportive to the hypothesis that epileptiform spike activity may result from the extreme activation of the “anti-binding” mechanism controlling temporal binding at high frequencies. It is suggested that when fast activity is abnormally intensified, “over-binding” with global synchrony of gamma rhythms can occur in the neural networks. It may lead to inadequate synaptic modifications. To prevent this process, epileptiform discharge develops as a protective mechanism suppressing fast activity. This proposal has implications for our understanding of temporal binding in the brain and how its excessive activationmay precipitate the development of pathological states. (paper in pdf) |
| Oct 24 |
Ed Modestino Muller, M. M., Malinowski, P., Gruber, T., Hillyard, S.A. (2003). Sustained division of the attentional spotlight. Nature, 424(6946), 309-12. Abstract: By voluntarily directing attention to a specific region of a visual scene, we can improve our perception of stimuli at that location. This ability to focus attention upon specific zones of the visual field has been described metaphorically as a moveable spotlight or zoom lens that facilitates the processing of stimuli within its 'beam'. A long-standing controversy has centred on the question of whether the spotlight of spatial attention has a unitary beam or whether it can be divided flexibly to disparate locations. Evidence supporting the unitary spotlight view has come from numerous behavioural and electrophysiological studies. Recent experiments, however, indicate that the spotlight of spatial attention may be divided between non-contiguous zones of the visual field for very brief stimulus exposures (<100 ms). Here we use an electrophysiological measure of attentional allocation (the steady-state visual evoked potential) to show that the spotlight may be divided between spatially separated locations (excluding interposed locations) over more extended time periods. This spotlight division appears to be accomplished at an early stage of visual-cortical processing. (paper in pdf) |
| Oct 31 |
Poster session 1: Practice for Dynamical
Neuroscience 2003 & SFN 2003 - cancelled |
| Nov 7 |
Dynamical Neuroscience 2003 &
SFN 2003 |
| Nov 14 |
Poster session 2: Dynamical Neuroscience
2003 & SFN 2003 - postponed due to FAU Presidential Inauguration
ceremony. The reshceduled date is to be announced. |
| Nov 21 |
Debra Taylor Stenneken, P., Aschersleben, G., Cole, J., and Prinz, W. (2002). Self-induced versus reactive triggering of synchronous movements in a deafferented patient and control subjects. Psychological Research, 66, 40-49. Abstract: The present study investigates the contribution of tactile-kinesthetic information to the timing of movements. The relative timing of simultaneous tapping movements of finger and foot (hand-foot asynchrony) was examined in a simple reaction time task and in discrete self-initiated taps (Experiment 1), and in externally triggered synchronization tapping (Experiment 2). We compared the performance of a deafferented participant (IW) to the performance of two control groups of different ages. The pattern of results in control groups replicates previous findings: Whereas positive hand-foot asynchronies (hand precedes foot) are observed in a simultaneous reaction to an auditory stimulus, hand-foot asynchronies are negative with discrete self-initiated as well as auditorily paced sequences of synchronized finger and foot taps. In the first case, results are explained by a simultaneous triggering of motor commands. In contrast, self-initiated and auditorily paced movements are assumed to be controlled in terms of their afferent consequences, as provided by tactile-kinesthetic information. The performance of the deafferented participant differed from that of healthy participants in some aspects. As expected on the basis of unaffected motor functions, the participant was able to generate finger and foot movements in reaction to an external signal. In spite of the lack of movement-contingent sensory feedback, the deafferented participant showed comparable timing errors in self-initiated and regularly paced tapping as observed in control participants. However, in discrete self-initiated taps IW's hand-foot asynchronies were considerably larger than in control participants, while performance did not differ from that of controls in continuous movement generation. These findings are discussed in terms of an internal generation of the movement's sensory consequences (forward-modeling). (paper in pdf) |
| Nov 28 |
Thanksgiving |
| Jan 23 |
No Journal Club |
| Jan 30 |
Mike Marshall Marshall, M. L., Fuchs, A., Zanto, T. P., and Modestino, E. J. (2003, November). Short- and long-duration needle stimulation of acupoint GB37 - an EEG study. Poster session presented at the Society for Neuroscience 33rd Annual Meeting, New Orleans, LA. Abstract: An 84-channel electrode array was used to investigate the spatiotemporal dynamics of electroencephalographic (EEG) activity related to manual bilateral needle stimulation of acupoint GB37, a vision-related acupuncture point. In contrast to earlier studies, that used only short stimulation of up to 2 min, we recorded EEG in two sessions where during one of them the needles where left in place for 30 min, and in the other the needles were inserted and twirled for about 2 sec to create de qui and then removed. For both conditions, 10 min of EEG were recorded before and after the 30 min phase. A total of ten healthy subjects, half of them with previous acupuncture experience, were blindfolded and placed in a supine position inside a sound-attenuating booth. The EEG data was manually inspected for movement artifacts and bandpass filtered in the range 1-50 Hz. Three types of comparisons were made within and between subjects for the three phases of a single session and between the two conditions: Spatial distribution of total power, Spatial patterns obtained from a principle component analysis, and time course of the power in the top five PCA patterns using a moving window of two minutes (or 30720 data points). All three measures were highly reproducible within the single subjects for all phases and sessions. The spatial shape of PCA patterns were the same up to a mode (the ninth) that contributed only about 1% to the variance in the signal. However, no differences were found between the phases of either the short or the 30 min stimulations compared to baseline or between the two sessions in a given subject. Likewise, no differences were found between participants with or without acupuncture experience. |
| Feb 6 |
Summer Rankin Pfordresher, P. Q. (2003). Auditory Feedback in Music Performance: Evidence for a Dissociation of Sequencing and Timing. Journal of Experimental Psychology: Human Perception and Performance, 29(5), 949–964. Abstract: Four experiments examined temporal relationships between actions and auditory feedback in music performance. Experiment 1 incorporated phase shifts of feedback, which disrupted produced timing but not overall accuracy. Experiment 2 incorporated period shifts of pitch contents for synchronized feedback that primarily disrupted accuracy more than timing. Experiment 3 incorporated combined phase and period shifts, which caused moderate disruption to timing and accuracy and revealed interactive effects of period and phase shifts on production. A 4th experiment included all feedback conditions in the same session to confirm differences across Experiments 1–3. These results are consistent with the view that actions and their perceptual consequences are coordinated in a way that distinguishes timing (phase shifts) from sequencing (period shifts). (paper in pdf) |
| Feb 13 |
Heather Chapin Levitin, D. J., Menon, V., Schmitt, J. E., Eliez, S., White, C. D., Glover, G. H., Kadis, J., Korenberg, J. R., Bellugi, U. and Reiss, A. L. (2003). Neural Correlates of Auditory Perception in Williams Syndrome: An fMRI Study. NeuroImage, 18, 74–82. Abstract: Williams syndrome (WS), a neurogenetic developmental disorder, is characterized by a rare fractionation of higher cortical functioning: selective preservation of certain complex faculties (language, music, face processing, and sociability) in contrast to marked and severe deficits in nearly every other cognitive domain (reasoning, spatial ability, motor coordination, arithmetic, problem solving). WS people are also known to suffer from hyperacusis and to experience heightened emotional reactions to music and certain classes of noise. We used functional magnetic resonance imaging to examine the neural basis of auditory processing of music and noise in WS patients and agematched controls and found strikingly different patterns of neural organization between the groups. Those regions supporting music and noise processing in normal subjects were found not to be consistently activated in the WS participants (e.g., superior temporal and middle temporal gyri). Instead, the WS participants showed significantly reduced activation in the temporal lobes coupled with significantly greater activation in the right amygdala. In addition, WS participants (but not controls) showed a widely distributed network of activation in cortical and subcortical structures, including the brain stem, during music processing. Taken together with previous ERP and cytoarchitectonic studies, this first published report of WS using fMRI provides additional evidence of a different neurofunctional organization in WS people than normal people, which may help to explain their atypical reactions to sound. These results constitute an important first step in drawing out the links between genes, brain, cognition, and behavior in Williams syndrome. (paper in pdf) |
| Feb 20 |
Di Sha Beg, A. A., and Jorgensen, E. M. (2003). EXP-1 is an excitatory GABA-gated cation channel. Nature Neuroscience, 6(11), 1145-1152. Abstract: y-aminobutyric acid (GABA) mediates fast inhibitory transmission by activating anion-selective ligand-gated ion channels. Although electrophysiological studies indicate that GABA may activate cation-selective ligand-gated ion channels in some cell types, such a channel has never been characterized at the molecular level. Here we show that GABA mediates enteric muscle contraction in the nematode Caenorhabditis elegans via the EXP-1 receptor, a cation selective ligand-gated ion channel. The EXP-1 protein resembles ionotropic GABA receptor subunits in almost all domains. In the pore-forming domain of EXP-1, however, the residues that confer anion selectivity are exchanged for those that specify cation selectivity. When expressed in Xenopus laevis oocytes, XP-1 forms a GABA receptor that is permeable to cations and not anions. We conclude that some of the excitatory functions assigned to GABA are mediated by cation channels rather than by anion channels. (paper in pdf) |
| Feb 27 |
Jeanna Winchester Bartzokis, G. (2004). Age-related myelin breakdown: a developmental model of cognitive decline and Alzheimer’s disease. Neurobiology of Aging, 25, 5–18. Noble, M. (2004). The possible role of myelin destruction as a precipitating event in Alzheimer’s disease. Neurobiology of Aging, 25, 25–31. Abtract for the first paper: A hypothetical model of Alzheimer’s disease (AD) as a uniquely human brain disorder rooted in its exceptional process of myelination is presented. Cortical regions with the most protracted development are most vulnerable to AD pathology, and this protracted development is driven by oligodendrocytes, which continue to differentiate into myelin producing cells late into the fifth decade of life. The unique metabolic demands of producing and maintaining their vast myelin sheaths and synthesizing the brain’s cholesterol supply make oligodendrocytes especially susceptible to a variety of insults. Their vulnerability increases with increasing age at differentiation as later-differentiating cells myelinate increasing numbers of axonal segments. These vulnerable late-differentiating cells drive the protracted process of intracortical myelination and by increasing local cholesterol and iron levels, progressively increase the toxicity of the intracortical environment forming the basis for the age risk factor for AD. At older ages, the roughly bilaterally symmetrical continuum of oligodendrocyte vulnerability manifests as a progressive pattern of myelin breakdown that recapitulates the developmental process of myelination in reverse. The ensuing homeostatic responses to myelin breakdown further increase intracortical toxicity and results in the relentless progression and non-random anatomical distribution of AD lesions that eventually cause neuronal dysfunction and degeneration. This process causes a slowly progressive disruption of neural impulse transmission that degrades the temporal synchrony of widely distributed neural networks underlying normal brain function. The resulting network “disconnections” first impact functions that are most dependent on large-scale synchronization including higher cognitive functions and formation of new memories. Multiple genetic and environmental risk factors (e.g. amyloid _-peptide and free radical toxicity, head trauma, anoxia, cholesterol levels, etc.) can contribute to the cognitive deficits observed in aging and AD through their impact on the life-long trajectory of myelin development and breakdown. This development-to-degeneration model is testable through imaging and post mortem methods and highlights the vital role of myelin in impulse transmission and synchronous brain function. The model offers a framework that explains the anatomical distribution and progressive course of AD pathology, some of the failures of promising therapeutic interventions, and suggests further testable hypotheses as well as novel approaches for intervention efforts. (paper in pdf), (paper 2 in pdf) |
| Mar 5 |
Mohammad Dastjerdi Sharpee, T., Rust, N. C., and Bialek, W. (in press). Analyzing neural responses to natural signals: Maximally informative dimensions. Neural Computation. Abstract: We propose a method that allows for a rigorous statistical analysis of neural responses to natural stimuli which are non-Gaussian and exhibit strong correlations. We have in mind a model in which neurons are selective for a small number of stimulus dimensions out of a high dimensional stimulus space, but within this subspace the responses can be arbitrarily nonlinear. Existing analysis methods are based on correlation functions between stimuli and responses, but these methods are guaranteed to work only in the case of Gaussian stimulus ensembles. As an alternative to correlation functions, we maximize the mutual information between the neural responses and projections of the stimulus onto low dimensional subspaces. The procedure can be done iteratively by increasing the dimensionality of this subspace. Those dimensions that allow the recovery of all of the information between spikes and the full unprojected stimuli describe the relevant subspace. If the dimensionality of the relevant subspace indeed is small, it becomes feasible to map the neuron's input-output function even under fully natural stimulus conditions. These ideas are illustrated in simulations on model visual and auditory neurons responding to natural scenes and sounds, resp |