Exploring mechanisms for contextual modulation in V1




Whitmire, Matthew P.

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The brain can quickly take complex visual information in a scene and group together or distinguish relevant features, allowing us to seamlessly interpret and interact with the world around us. Evidence from recordings in visual cortex have shown that neurons not only responsive to stimuli within their receptive field, but also sensitive to stimuli in the surrounding area. This effect is referred to as contextual modulation. The primary visual cortex (V1) represents high resolution spatial information and stands as the bottleneck before all other visual processing in the cortex, making it a prime location to study complex mechanisms for sensory encoding, like contextual modulation. This dissertation investigates contextual modulation mechanisms present in V1 by: 1) testing if the large-scale population activity in V1 contributes to contour grouping while a monkey is engaged in a complex and difficult contour detection task, and 2) comparing how excitatory and inhibitory cell populations in V1 contribute to orientation tuning, size tuning, and surround modulation effects. First, I will show that we can train rhesus macaques to reliably perform a complex contour detection task with stimuli based on the statistics of contours found in natural scenes. Next, I will demonstrate that when recording voltage sensitive dye imaging signals during the contour detection task, V1 does not show evidence of participating in the contour grouping computation. This suggests V1 may only relay information from individual elements, which are then grouped together in subsequent visual areas. Continuing, I will present that inhibitory cells in macaque V1 show orientation tuning and orientation maps with broader tuning than excitatory cells. Then, I will demonstrate that inhibitory and excitatory size tuning and receptive field sizes do not differ substantially and that V1 surround modulation may be sensitive to general discontinuity, rather than only orientation difference. These results do not support key models of inhibition during size tuning effects and open the field to more work comparing contextual modulation effects across excitatory and inhibitory cells in macaque. Overall, this thesis challenges widely held models of contextual modulation in V1, both for contour grouping and how inhibition shapes surround modulation effects.



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