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The dominant feature of the functional organization of the primary visual cortex is the visuotopic organization of its cells: the visual field is systematically represented across the surface of the cortex (Figure 25–11A).
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In addition, cells in the primary visual cortex with similar functional properties are located close together in columns that extend from the cortical surface to the white matter. The columns are concerned with the functional properties that are analyzed in any given cortical area and thus reflect the functional role of that area in vision. The properties that are developed in the primary visual cortex include orientation specificity and the integration of inputs from the two eyes, which is measured as the relative strength of input from each eye, or ocular dominance.
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Ocular-dominance columns reflect the segregation of thalamocortical inputs arriving from different layers of the lateral geniculate nucleus. Alternating layers of this nucleus receive input from retinal ganglion cells located in either the ipsilateral or contralateral retina (Figure 25–12). This segregation is maintained in the inputs from the lateral geniculate nucleus to the primary visual cortex, producing the alternating left-eye and right-eye ocular dominance bands (Figure 25–11B), which receive input from the respective layers of the lateral geniculate nucleus.
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Cells with similar orientation preferences are also grouped into columns. Across the cortical surface there is a regular clockwise and counterclockwise cycling of orientation preference with the full 180° cycle repeating every 750 μm (Figure 25–11C). One full cycle of orientation columns is called a hypercolumn. Likewise, the left- and right-eye dominance columns alternate with a periodicity of 750 to 1,000 μm. The orientation and ocular dominance columns are crisscrossed over the cortical surface.
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Both types of columns were first mapped by recording the responses of neurons at closely spaced electrode penetrations in the cortex. The ocular-dominance columns were also identified by making lesions or tracer injections in individual layers of the lateral geniculate nucleus. More recently a technique known as optical imaging has enabled researchers to visualize a surface representation of the orientation and ocular dominance columns in living animals. Developed for studies of cortical organization by Amiram Grinvald, this technique visualizes changes in surface reflectance associated with the metabolic requirements of active groups of neurons, known as intrinsic-signal optical imaging, or changes in fluorescence of voltage-sensitive dyes. Intrinsic-signal imaging depends on activity-associated changes in local blood flow and alterations in the oxidative state of hemoglobin and other intrinsic chromophores.
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An experimenter can visualize the distribution of cells with left or right ocular dominance, for example, by subtracting the image obtained while stimulating one eye from that acquired while stimulating the other. When viewed in a plane tangential to the cortical surface, the ocular dominance columns appear as alternating left- and right-eye stripes, each approximately 750 μm in width (Figure 25–11B).
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The cycles of orientation columns form various structures, from parallel stripes to pinwheels. Sharp jumps in orientation preference occur at the pinwheel centers and "fractures" in the orientation map (Figure 25–11C). Superimposed on these is a third columnar system of continuously changing directional preference.
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Embedded within the orientation and ocular-dominance columns are clusters of neurons that have poor orientation selectivity but strong color preferences. These units of specialization, located within the superficial layers, were revealed by a histochemical label for the enzyme cytochrome oxidase, which is distributed in a regular patchy pattern of blobs and interblobs. In the primary visual cortex these blobs are a few hundred micrometers in diameter and 750 μm apart (Figure 25–11D). The blobs correspond to clusters of color-selective neurons. Because they are rich in cells with color selectivity and poor in cells with orientation selectivity, the blobs are specialized to provide information about surfaces rather than edges.
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In area V2 thick and thin dark stripes separated by pale stripes are evident with cytochrome oxidase labeling (Figure 25–11D). The thick stripes contain neurons selective for direction of movement and for binocular disparity as well as cells that are responsive to illusory contours and global disparity cues. The thin stripes hold cells specialized for color. The pale stripes contain orientation-selective neurons.
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For every visual attribute to be analyzed at each position in the visual field there must be adequate tiling, or coverage, of neurons with different functional properties. As one moves in any direction across the cortical surface, the progression of the visuotopic location of receptive fields is gradual, whereas the cycling of columns occurs more rapidly. Any given position in space can therefore be analyzed adequately in terms of the orientation of contours, the color and direction of movement of objects, and the stereoscopic depth. The small segment of visual cortex that deals with that particular part of the visual field represents all possible values of all the columnar systems (Figure 25–13).
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The columnar systems serve as the substrate for two fundamental types of connectivity along the visual pathway. Serial processing occurs in the successive connections between cortical areas, connections that run from the back of the brain forward. At the same time parallel processing occurs simultaneously in subsets of fibers that process different submodalities such as form, color, and movement.
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Many areas of visual cortex reflect this arrangement; for example, functionally specific cells in V1 communicate with cells of the same specificity in V2. These pathways are not absolutely segregated, however, for there is some mixing of information between different visual attributes (Figure 25–14).
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Columnar organization confers several advantages. It minimizes the distance required for neurons with similar functional properties to communicate with one another and allows them to share inputs from discrete pathways that convey information about particular sensory attributes. This efficient connectivity economizes on the use of brain volume and maximizes processing speed. The clustering of neurons into functional groups, as in the columns of the cortex, allows the brain to minimize the number of neurons required for analyzing different attributes. If all neurons were tuned for every attribute, the resultant combinatorial explosion would require a prohibitive number of neurons.