Visual System

The primary visual cortex (Brodmann area 17) is located in and on either side of the calcarine sulcus, on the medial surface of the occipital lobe. It functions primarily in discerning the intensity, shape, size, and location of objects in the visual field. As with other cortical areas, the primary visual cortex is composed of six layers. Visual input from the lateral geniculate nucleus of the thalamus terminates primarily in layer 4. This input is then relayed through layers 2 and 3 to secondary and association visual areas. In addition, an intracortical loop is established, with information from layers 2 and 3 being transferred to layer 5 and then to layer 6. Visual information is also relayed to the brain stem from the layer 5 of the cortex and back to the lateral geniculate nucleus of the thalamus from layer 6.

As in the primary somatosensory cortex, cells in the primary visual area are continuously active and respond maximally to specific stimuli within their receptive fields. The primary visual cortex also possesses columnar organization with each column of cells preferentially responding to a specific visual stimulus, such as a line of a specific orientation. In addition, it is retinotopically organized with the upper half of the visual field represented in the cortex inferior to the calcarine sulcus and the lower half of the visual field represented in the cortex superior to the calcarine sulcus. Moreover, there is a disproportionately large amount of cortex reserved for the area of the retina with the highest density of receptors (the macula).

The primary visual cortex, or striate cortex, sends information to the secondary visual, or prestriate cortex (Fig. 6B; Brodmann areas 18 and 19), and to the inferior temporal cortex (Brodmann areas 20 and 21). These secondary visual areas are important for color, motion, and depth perception. Information from the occipital cortex is further transmitted to two separate association areas of the cerebral cortex: superiorly to the parietal lobe (Brodmann areas 5 and 7) for processing of spatial features and movement and inferiorly to the temporal lobe for object recognition.

Figure 6 Schematic diagram of the localization of sensory and motor functions in the cerebral cortex. (A) Connections of the somatosensory system. (B) Connections of the visual, auditory, vestibular, gustatory, and olfactory sensory areas. (C) Connections of the somatomotor system. Numbers indicate Brodmann's areas. MI, primary motor area; SI, primary somatosensory area; SII, secondary somatosensory area.

Figure 6 Schematic diagram of the localization of sensory and motor functions in the cerebral cortex. (A) Connections of the somatosensory system. (B) Connections of the visual, auditory, vestibular, gustatory, and olfactory sensory areas. (C) Connections of the somatomotor system. Numbers indicate Brodmann's areas. MI, primary motor area; SI, primary somatosensory area; SII, secondary somatosensory area.

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