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Melchi Michel, Yuzhi Chen, Wilson Geisler, Eyal Seidemann; A novel shape illusion predicted by the effect of local orientation on retinotopic-scale V1 population responses. Journal of Vision 2012;12(9):79. doi: https://doi.org/10.1167/12.9.79.
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© ARVO (1962-2015); The Authors (2016-present)
Population responses to visual stimuli in the primate primary visual cortex are topographically organized at multiple spatial scales, each of which may be useful for a subset of visual judgments. At a fine scale, these responses are organized into orientation columns; signals at this scale might be useful for discriminating between different textures. At a larger (retinotopic) scale, the population responses seem to encode the contrast envelope of visual stimuli; signals at this scale might be useful for computing global shape. Are responses at this larger scale independent of the local orientation structure of the stimulus? To answer this question, we used voltage-sensitive dye imaging in fixating monkeys to measure V1 population responses to small isolated Gabor patches with a fixed contrast envelope and varying carrier orientations. We found that V1 response at the retinotopic scale is significantly elongated along the direction corresponding to the orientation of the carrier. Moreover, increasing the carrier frequency reduces this effect. Both of these results can be explained by an elongation of the V1 population receptive field along the preferred orientation, similar to findings in single V1 neurons. If we rely on these retinotopic-scale V1 population responses when making judgments about global shape, the results above suggest that local orientation might bias these judgments. We tested this prediction in a psychophysical task by having human subjects determine which of two small patches was more circular. Consistent with the physiological results, we found that human subjects perceive Gabor patches to be elongated along the orientation of the sinusoidal carrier, and that increasing the carrier frequency reduces this effect. Taken together, these results suggest that we make use of information from the retinotopic scale of V1 population responses when determining shape, and that for small stimuli, local orientation information can bias our perception of global shape.
Meeting abstract presented at VSS 2012
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