Intro | Optic Chiasm | Superior Colliculus | Visual Cortex | Lateral Geniculate Nucleus | Optic Nerve | Pulvinar Nucleus | Retina
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Fibers of the secondary visual pathway leave the optic chiasm on their way to the Superior Colliculus of the midbrain. (See Figure 6 for additional information on the superior colliculus). After processing in the superior colliculus, information in the second visual pathway is conveyed to the Pulvinar Nucleus of the thalamus. (See Figure 7 for additional information on the pulvinar.) From the pulvinar, visual information is conveyed to secondary regions of visual cortex (outside of primary visual cortex yet still within the occipital lobe) for additional processing. (See Figure 15) for more detailed information on the primary and secondary regions of visual cortex.)
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Whereas the lateral geniculate nucleus of the primary visual pathway receives information from the cone-rich areas of the retina, the superior colliculus receives information from the rod-rich areas found in the periphery of the retina. The collection of cones in the foveal region of the retina provides the greatest visual acuity, that needed for the feature detection to occur further up the pathway in primary visual cortex. The peripheral rods, although not high in acuity, are particularly sensitive to movement. This ability to detect movement is central to the function of the superior colliculus. In addition to information from the retina, the superior colliculus receives auditory information, vestibular information about head position, and visual information that has already been processed by primary visual cortex. This mixture of information is integrated by the superior colliculus in such a way as to control movements of the head and body made in orientation to visual targets of novelty or interest. In addition, the superior colliculus is involved in the control of two types of eye movements, rapid eye movements (called saccades) that are made toward moving visual targets and slow movements (called pursuit eye movements) that are used for tracking moving visual targets. The superior colliculus is phylogenetically older than the lateral geniculate nucleus, and the dominant destination for retinal projections in lower mammals. The lateral geniculate nucleus, however, is the dominant target for retinal information in higher mammals, particularly primates.
The function of the pulvinar in the secondary visual pathway has eluded researchers until recently. Some evidence suggests that this large region of the thalamus helps to maintain stability of our visual world. It does so by compensating for the effects that body movement has on the placement of images on the retina. The pulvinar receives information from both the lateral geniculate nucleus and the superior colliculus (which is reciprocally connected with all regions of visual cortex). Via these connections, the pulvinar is kept informed about both eye movements (superior colliculus) and movements of the visual image (lateral geniculate nucleus). The pulvinar may help secondary visual cortex subtract out eye movements from movements of the retinal image.
The secondary visual cortical area, known as V5 or MT, contains neurons that are particularly responsive to movement. This region of visual cortex is a primary target for both the superior colliculus and pulvinar. Inputs from both the superior colliculus and primary visual cortex contribute to the movement sensitivity of V5 neurons. The distinct roles that each of these inputs to secondary visual cortex play in the perception of movement are not yet understood.