Mechanisms Underlying Response Suppression in Cat Visual Cortex

Ian Micah Finn

doi:https://doi.org/10.21985/N29D9V

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Mechanisms Underlying Response Suppression in Cat Visual Cortex

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A comprehensive understanding of how image processing occurs in the primary visual cortex (V1) requires learning what aspects of neuronal responses are driven by strong feed-forward input from the lateral geniculate nucleus (LGN), and what aspects arise due to the densely recurrent network operating within the cortex itself. From an anatomical perspective, the preponderance of intracortical excitatory and inhibitory connections over feed-forward excitatory connections stands as strong evidence that intracortical input is critical for determining V1 response properties. A particularly appealing hypothesis is that intracortical inhibition operates to shape response selectivity in V1, as it does in the retina for stimuli of varying size. A number of suppressive response patterns in V1 indirectly support the importance of intracortical inhibition, including: (1) contrast-invariant orientation tuning in simple cell membrane potential responses, (2) MAX-like responses in complex cells, and (3) surround suppression in both simple and complex cells. It is not clear, however, whether inhibition has a distinct role in determining these properties, or if they arise from more excitatory means. Recently, Hubel and Wiesel's hierarchical model, which invokes only excitatory feed-forward input, has been used successfully to explain the suppressive phenomena of sharp orientation tuning and cross-orientation suppression. These results suggest that their model may be able to account for other suppressive behaviors in visual cortex. In this thesis I present data from intracellular experiments designed to probe various mechanisms of suppression in V1. Based on my results, I conclude that an excitatory, feed-forward architecture can in fact explain the emergence of contrast-invariant orientation tuning in simple cells, and MAX behavior in complex cells, when modest extensions to the traditional model are considered. In addition, I conclude that surround suppression is mediated by cortical inhibition, but in a novel manner that reflects a critical and highly general contribution of inhibition to stabilizing the visual network.

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