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PHGY 1030 (28)


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PHGY 1030
Luming Wang

PHYSIOLOGY OF THE STRIATE CORTEX Receptive Fields The receptive fields of neurons in layer IVC are similar to the magnocellular and parvocellular LGN neurons providing their input. Outside layer IVC, new receptive field characteristics, not observed in the retina or LGN, are found. Binocularity Neurons in layers IVCα and IVCβ receive afferents from a layer of the LGN representing either eye. Monocular neurons from either eye are also clumped together in V1 rather than randomly intermixed and this accounts for ocular dominance columns. Axons leaving layer IVC diverge and innervate more superficial cortical layers. As a result, there is a mixing of inputs from the two eyes (see Fig. 10.17). Most neurons superficial to IVC are binocular and have binocular receptive fields. Orientation Selectivity Most receptive fields in the retina, LGN and layer IVC are circular and give greatest response to a spot of light matched in size to the receptive field center. Outside layer IVC, cells do not follow this pattern. Hubel and Wiesel found that many neurons in V1 respond best to an elongated bar of light moving across their receptive fields, but the orientation of the bar is critical.  A particular orientation triggers the greatest response whereas perpendicular bars generally elicit much weaker responses (see Fig. 10.20). This is orientation selectivity. Orientation selectivity of nearby neurons is related and the preferred orientation remains the same for all the selective neurons encountered from layer II down through layer VI. Such a radial column of cells are called orientation columns. As an electrode passes tangentially (i.e. parallel to the surface) through the cortex in a single layer, the preferred orientation progressively shifts. There is a mosaic-like pattern of optimal orientations in striate cortex. If an electrode is passed at certain angles through this mosaic, the preferred orientation rotates like the sweep of the minute hand of a clock (see Fig. 10.21).  A complete 180° shift in preferred orientation required a traverse of about 1mm within layer III.  Orientation selective neurons are thought to be specialized for the analysis of object shape. Direction Selectivity Many V1 receptive fields exhibit direction selectivity and respond when a bar of light at the optimal orientation moves perpendicular to the orientation in one direction but not in the opposite direction (see Fig. 10.22). Sensitivity to the direction of the stimulus motion is a hallmark of neurons receiving input from the magnocellular layers of the LGN.  Direction-selective neurons are thought to be specialized for the analysis of object motion. Simple and Complex Receptive Field Many orientation-selective neurons have a receptive field elongated along a particular axis, with an ON-center or OFF-center region flanked on one or both sides by an antagonistic surround.  Cortical neurons receive a converging input from three or more LGN cells with receptive fields that are aligned along one axis (see Fig. 10.23b). These are simple cells and the segregation of ON and OFF regions is their defining property. Simple cells are also orientation specific.  Other orientation selective neurons in V1 do not have distinct ON and OFF regions are called complex cells. These cells give ON and OFF responses to stimuli throughout the receptive field (see Fig. 10.24). Simple and complex cells are typically binocular and sensitive to stimulus orientation. Blob Receptive Fields There is a presence of the distinct cytochrome oxidase blobs outside layer IV of striate cortex.  The neurons in the interblob areas have some or all of the following properties: binocularity, orientation selectivity and direction selectivity. They are both simple and complex cells and generally are not wavelength sensitive.  Blobs cells, however, are wavelength-sensitive and monocular and lack orientation/direction selectivity. They receive input from the koniocellular layers and magnocellular/parvocellular inputs via layer IVC. The visual responses of blob cells resemble koniocellular and parvocellular input. The receptive fields of most blob neurons are circular. Some have color-opponent center- surround organization and others have color opponency in the center of their r
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