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L.E. Hallum, J.A. Movshon; Mechanisms of selectivity for orientation-defined form in macaque visual cortex. Journal of Vision 2012;12(9):1120. doi: 10.1167/12.9.1120.
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Humans perceive texture-defined form in a way that suggests that second-order stimuli are encoded by mechanisms computing a localized difference of orientation signals like those arising from V1 neurons responding to conventional luminance gratings. Those responses can be modulated by optimally and orthogonally oriented contrast falling in or near the receptive field (RF) center. One model for second-order processing relies on the orientation tuning and the spatial organization of RF center-surround modulation to create selectivity for form. We tested this model by measuring the responses of 25 single units in V1 and V2 of opiate-anaesthetized macaques. We stimulated neurons with summed contrast-modulated (CM) gratings covering both the RF center and surround. CM grating 1 was a sinusoid of optimal spatial frequency, orientation, and drift rate. CM grating 2 was similar, but of orthogonal orientation. Each grating was independently contrast-modulated across 6 drift directions and 5 spatial frequencies. The responses of most neurons were modulated by both optimally and orthogonally oriented contrast. Suppression arose from optimally and orthogonally oriented contrast that was similar in its overall magnitude, but different in its spatial organization, thus conferring second-order selectivity to responses. For both optimally and orthogonally oriented contrast, we used the difference of an excitatory and a suppressive Gaussian to model this organization. In many modulated cells, optimally oriented contrast suppressed responses when it appeared in a surround that was organized anisotropically relative to the RF center. Orthogonally oriented contrast typically suppressed responses when it was coextensive with the RF center. Selectivity for second-order form may not depend on dedicated "second-order neurons" that collect orientation-selective signals from V1 neurons. Rather, selectivity may arise by spatially organized, orientation-selective interactions that modulate the response of the RF center to conventional luminance gratings.
Meeting abstract presented at VSS 2012
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