Superior Colliculus: a recap

From the discussion in class we learned that the Superior Colliculus (SC) coordinates visual, somatic and auditory information, adjusting movements of the head and eyes towards the stimulus. The various sensory maps are ALIGNED SPATIALLY with one another – for e.g. in the superficial visual map, neurons that receive information from the contralateral temporal visual field are located above neurons in the deeper auditory map that receives information from the same contralateral region of the animal’s auditory space. In this way, different sensory information about the location of a stimulus with respect to a particular part of the body is conveyed to a common region in the SC.

I. Afferents:

We have seen that the superficial layers of the SC receive multiple sensory inputs and that the SC is one of the main areas responsible for multimodal integration of these inputs. Although not complete, below is a list of brain areas from which the SC receives its sensory input.

Acoustic information à is relayed from the Inferior Colliculus to the deep layers of the SC.

Visual input à from posterior parietal, prestriate, middle temporal & frontal eye field

Somatosensory information à from the spinal cord (spinotectal), cerebellum, pretectal region, Substantia Nigra and periaqueductal gray.

II. Efferents:

What projections does the SC have to cortical and sub-cortical structures?

• The sensory maps in turn connect to the motor map located in the intermediate deep layers of the SC. In fact neurons in the superficial layer do not project directly to the intermediate layers. Instead they send fibers to the pulvinar of the thalamus and signals are relayed from there to the cortical regions and then to the intermediate layers of the SC. Combining sensory information from the superficial layers, with information from the cortical inputs (especially the frontal eye fields), the SC controls SACCADIC eye movements that orient the eye towards the stimulus.

• Motor output drives burst cells of the paramedian pontine reticular formation that specify how far the eyes should move. These cells have “movement fields” analogous to the receptive fields of the sensory neurons.

• Brain stem – i) TECTOSPINAL – axons cross the midline and descend to the upper spinal cord; involved in reflex control of head and neck movements ii) TECTOPONTINE – through the pons to the cerebellum – coordination of eye movements.

• In addition to the tracts mentioned above, SC also has connections with the Inferior Olive, Cuneiform nucleus, Substantia Nigra, Raphe nucleus and also have commissural connections.

III. Some notes of interest:

1) Attention and SC: When a subject attends visually to an object, cells in the posterior parietal cortex that respond to the object discharge, but cells in the SC discharge only when the subject moves his eyes towards the object.

2) Electric stimulation of the SC (and frontal eye field) – evokes saccades into the movement field of the stimulated neurons; conjugate eye movements in cats and monkeys. But stimulation of bilateral frontal eye fields resulted in vertical saccades.

3) Lesions: With reference to the following figure, let us try to see what certain lesions (indicated by red lines in the figure) do:

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1. Lesions of small part of SC transiently affect latency, accuracy and velocity of saccades. Deficits in orientation to visual and acoustic cues. Disturbances in conjugate eye movements can occur with lesions of the SC, although, this deficit is not unique to lesions of SC.

2. Lesions of the entire colliculus transiently render the monkey unable to make any contralateral saccades, but this slowly recovers. (frontal - brainstem projection is intact).

3. Lesions of frontal eye field – transient contralateral neglect and contralateral gaze paresis (weakness). No predictive or memory-guided saccades, but can make visually-guided saccades.

4. Bilateral frontal eye fields + SC à No saccades.

5. Parietal lesion à nothing significant as the frontal signals are functional.

From my medical books:

1) Pinealomas – tumors of the pineal gland compress the SC and result in paralysis of upward gaze. This is also generally called Parinaud’s syndrome.

2) Progressive Supranuclear Palsy – Neurodegenerative disorder affecting the basal ganglia and the brainstem. Symptoms are similar to Parkinson’s disease, but there is also degeneration unique to this disease involving the SC and the adjacent peri-tectal region. à paralysis of voluntary eye movements, loss of spontaneous visual grasp reflex.

3)

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Sprague effect: Left Right

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Removing Occipital and Parietal lobes on the right side à result in symptoms similar to left hemispatial neglect. Subsequently if the left SC is also removed à improved vision in the left field – because of the removal of inhibition from the left SC. This is called Sprague effect. (So if the left SC is not removed, the right SC receives a double hit due to cortical damage and increased inhibition from the opposite SC).

Dinesh Nair

nair@walt.ccs.fau.edu

Last modified : 22 Sept 2001.