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Christopher Shooner, Luke Hallum, Jenna Kelly, J. Movshon, Michael Hawken; Prevalence of gain control effects in macaque visual cortex. Journal of Vision 2017;17(10):798. doi: https://doi.org/10.1167/17.10.798.
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© ARVO (1962-2015); The Authors (2016-present)
Visual neurons adjust their responsivity based on the total contrast energy of a stimulus. Or do they? Following classic studies of the cat visual system, it has often been assumed that contrast normalization is a ubiquitous property of neurons in primary visual cortex (V1). The prevalence of this phenomenon in the primate brain, however, has not been confirmed experimentally; this test is of particular interest given the important differences in subcortical gain control between the species. We used a simple masking experiment to characterize how cortical responsivity changes with stimulus energy. We measured the responses of 150 V1 neurons in 4 opiate-anesthetized macaque monkeys to sinusoidal gratings with optimized spatiotemporal parameters and varying contrast. We obtained contrast-response curves in the presence and absence of a superimposed, orthogonal, masking grating of 50% contrast, and compared contrast gain between masked and unmasked conditions. The effect of the mask varied widely across the population, ranging from 100% gain reduction to slight gain enhancement. This pattern differs markedly from an idealized model of contrast normalization, in which cortical gain is reduced consistently in proportion to increased contrast. Additionally, we found a relationship between masking strength and spatial receptive field properties: cells showing less gain reduction tended to have smaller receptive fields and to prefer higher spatial frequencies, suggesting they may receive significant input from the parvocellular LGN. The parvocellular pathway, unique to primates, relays a linear representation of stimulus contrast which is not subject to the gain-control mechanisms shaping magnocellular responses. Our findings suggest that this veridical contrast signal is preserved in V1, alongside more conventional gain-controlled signals.
Meeting abstract presented at VSS 2017
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