Cortical Ocular Dominance Columns
Cells within the six ocular dominance layers of the lateral geniculate project to distinct ocular dominance stripes or columns within Brodmann's area 17 (also called area VI) of the primary visual cortex (see Fig. 11). Pairs of adjacent ocular dominance columns—one with ipsilat-eral projections from the lateral geniculate nucleus (LGN), and the other with contralateral projections— receive information about the same point in visual space. Adjacent pairs are collectively designated as a hyper-column. Hypercolumns form the basic structural modules of the visual cortex, and each displays at least three left retina right retina left optic nerve optic chiasma
M pathway hypercolumn in the V1 cerebral cortex left retina right retina
left optic nerve
M pathway hypercolumn in the V1 cerebral cortex
e cortical layers
FIGURE 11 Projections of the M and P pathways from retina to the lateral geniculate nucleus and visual cortex. Projections from the left eye are shown in light shades; projections from the right eye are dark shades. Two channels of information leave the eye: the magnocellular (M) channel (gray), starting with large ganglion cells that have large cell bodies and large dendritic fields, and the parvocellular (P) channel (blue), starting with the more numerous and smaller ganglion cells. Both
e cortical layers
FIGURE 11 Projections of the M and P pathways from retina to the lateral geniculate nucleus and visual cortex. Projections from the left eye are shown in light shades; projections from the right eye are dark shades. Two channels of information leave the eye: the magnocellular (M) channel (gray), starting with large ganglion cells that have large cell bodies and large dendritic fields, and the parvocellular (P) channel (blue), starting with the more numerous and smaller ganglion cells. Both different functional subdivisions in addition to ocular dominance columns. These include input/output layers, orientation-specific columns, and color-specific blobs. Layer 4 functions as the major input layer within a hyper-column. It contains stellate cells that receive information from axons of lateral geniculate cells. In turn, stellate cells synapse on pyramidal cells in other cortical layers. Pyramidal cells in layers 2, 3, and 4 project to higher centers in the visual association cortex: Brodmann's area 18 (also called areas V2 and V3) and Brodmann's area 19 (also called areas V4 and V5). Pyramidal cells in layer 5 project to visual reflex areas in the upper brain stem; cells in layer 6 project back to the lateral geniculate.
Orientation-Specific Columns and Color-Specific Blobs
Like most retinal and LGN neurons, the stellate cells of the visual cortex have circular center-surround receptive fields (Fig. 12). Several stellate cells with overlapping receptive fields synapse on each simple pyramidal cell, and the summation of this input establishes a bar-shaped, center-surround receptive field for the pyramidal cell. For example, an OFF center simple pyramidal cell would be maximally stimulated by a dark bar on a light background positioned on the appropriate region of the retina. In addition, different pyramidal cells have different preferred orientations so that the degree of stimulation is dependent on the orientation of the bar. Cortical cells with the same orientation selectivity are grouped together in orientation columns. Present in each ocular dominance column is a complete set of orientation columns. Each column of cells is preferentially responsive to a given axis of orientation ±5°. LGN cells in the M pathway project to stellate cells and pyramidal cells in orientation columns. LGN cells in the P pathway have a split projection (Fig. 13). Some synapse in orientation columns; others, designated as the Kchannel, carry color-specific information to color-specific blob regions in the cortex that are not orientation selective.
types of ganglion cells project to the lateral geniculate nucleus (LGN) by way of the optic nerve, chiasm, and tract. The LGN has six layers. The M channel projects to layers 1 and 2 (gray) and the P channel to layers 3, 4, 5, and 6 (blue). Contralateral neurons go to layers 1, 4, and 6 (light shade), and ipsilateral neurons go to layers 2, 3, and 5. Neurons leave these layers as the optic radiations and enter the primary visual area (V1) of the cerebral cortex. A column of cortex, about 1 mm x 1 mm, extending from the surface (layer 1) down about 2 mm to the deepest layer (layer 6), is designated as a hypercolumn. The major inputs of most LGN neurons are to layer 4C. The M channel synapses in 4Ca and the P channel synapses in 4Cb. The input from the two eyes remains segregated, with the contralateral (light color) and ipsilateral (dark) information forming ocular dominance columns.
Cells of the M pathway continue their projection from area VI to association areas designated as V2, V3, V4, V5 (Brodmann's areas 17, 18, and 19) and finally to other regions in the parietal lobe. This parietal stream carries spatial and movement information important for localizing objects in space (Fig. 14). Cells from both components of the P pathway project to association areas V2 and V4 and finally to inferior regions of the temporal lobe. At each stage of processing, receptive field properties change from spots (stellate cells) to bars (simple pyramidal cells), angles, and so on, in higher order neurons. This pathway is concerned with object recognition. About 10% of the cells in the inferotemporal cortex have very large and complex receptive fields and are selective for very specific stimuli, such as the hand or a familiar face. A condition known as prosopagnosia has been reported in patients with bilateral lesions along the inferior surface of the occipital and temporal lobes. In this condition, patients have impaired recognition of familiar faces. Although there is considerable crossover and mixing of the information at several points along the M and P pathways, it does appear that specific visual processing tasks (determining where an object is versus what an object is) are generally assigned to different regions of visual association cortex (see Fig. 14). Other higher order visual tasks may likewise be localized to specific cortical regions.
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