We’ve talked about the superior colliculus a lot in lecture. One thing that we h

Question

We’ve talked about the superior colliculus a lot in lecture. One thing that we haven’t talked about is called “visual grasp”. In this phenomenon, visual stimulation results in a saccade that rotates the two eyes just the right amount to direct each of the foveas to the location of the visual field. This would suggest that there is a one-to-one correspondence between the visual map of the superficial layer of the superior colliculus and the motor maps in the deeper motor layers. What evidence is there to suggest this is or is not an accurate description of superior colliculus function?

Explanation / Answer

The superior colliculus (Latin, upper hill) is a paired structure of the mammalian midbrain. In other vertebrates the homologous structure is known as the optic tectum or simply tectum. The adjective form tectal is commonly used for mammals. It is a layered structure, with a number of layers that varies by species. The layers can be grouped into the superficial layers (stratum opticum and above) and the deeper layers (the remaining layers). Neurons in the superficial layers receive direct input from the retina and respond almost exclusively to visual stimuli. Many neurons in the deeper layers also respond to other modalities, and some respond to stimuli in multiple modalities. The deeper layers also contain a population of motor-related neurons, capable of activating eye movements as well as other responses.

The superficial layers receive input mainly from the retina, vision-related areas of the cerebral cortex, and two tectal-related structures called the pretectum and parabigeminal nucleus. The retinal input encompasses the entire superficial zone, and is bilateral, although the contralateral portion is more extensive. The cortical input comes most heavily from the primary visual cortex (area 17), the secondary visual cortex (areas 18 and 19), and the frontal eye fields. The parabigeminal nucleus plays a very important role in tectal function that is described below.

In contrast to the vision-dominated inputs to the superficial layers, the intermediate and deep layers receive inputs from a very diverse set of sensory and motor structures. Most areas of the cerebral cortex project to these layers, although the input from "association" areas tends to be heavier than the input from primary sensory or motor areas.[citation needed] However, the cortical areas involved, and the strength of their relative projections differs across species.[7] Another important input comes from the substantia nigra, pars reticulata, a component of the basal ganglia. This projection uses the inhibitory neurotransmitter GABA, and is thought to exert a "gating" effect on the superior colliculus. The intermediate and deep layers also receive input from the spinal trigeminal nucleus, which conveys somatosensory information from the face, as well as the hypothalamus, zona incerta, thalamus, and inferior colliculus.

In addition to their distinctive inputs, the superficial and deep zones of the superior colliculus also have distinctive outputs. One of the most important outputs goes to the pulvinar and lateral intermediate areas of the thalamus, which in turn project to areas of the cerebral cortex that are involved in controlling eye movements. There are also projections from the superficial zone to the pretectal nuclei, lateral geniculate nucleus of the thalamus, and the parabigeminal nucleus. The projections from the deeper layers are more extensive. There are two large descending pathways, traveling to the brainstem and spinal cord, and numerous ascending projections to a variety of sensory and motor centers, including several involved in eye movements.

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