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Luke E. Hallum, J. Anthony Movshon; Suppressive signals from within and beyond the classical receptive field of macaque V1 neurons. Journal of Vision 2014;14(15):72. doi: 10.1167/14.15.72.
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© ARVO (1962-2015); The Authors (2016-present)
Single neurons in V1 respond selectively to luminance-modulated stimuli. These responses are often influenced by context, e.g., when stimuli extend outside the classical receptive field (CRF), or when a mask is summed with the stimulus. These contextual phenomena reflect a fundamental cortical computation, and may support perception by signaling 2nd-order visual features defined by spatial relationships of contrast, orientation and spatial frequency. In the anesthetized macaque, we measured single-unit responses to the sum of a circular patch of drifting preferred sinusoidal grating and an annular mask grating drifting in a different direction. Eighteen of 44 neurons were reliably surround suppressed by the preferred grating. The mask had a range of effects, from suppressive (15 of 44 neurons) through excitatory. We found the mask's suppressive effects were contained to the CRF, and the mask did not alleviate surround suppression when it was summed with the target beyond the CRF perimeter. We modeled these receptive field subregions by extending the difference-of-two-Gaussians model of center and surround to include a third concentric Gaussian, sensitive to the mask. We extracted the model's fitted parameters to analyze excitation and suppression dynamics. The center and surround Gaussians showed susceptibility to adaptation. However, the third Gaussian, which may be of subcortical origin, showed none. Using a simple simulation, we illustrate how mask suppression increases 2nd-order sensitivity, and propose that it enables cortical neurons to do double duty, providing signals about both 1st- and 2nd-order form.
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