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Yih-Shiuan Lin, Chien-Chung Chen, Mark W. Greenlee; Lateral modulation of orientation discrimination of center-surround sinusoidal stimuli in peripheral vision. Journal of Vision 2019;19(10):78d. doi: https://doi.org/10.1167/19.10.78d.
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Mounting evidence shows that the center-surround modulation is subject to a normalization process influenced by the surround contrast and orientation. We investigated such orientation specific surround modulation by measuring the adaptation effect of different surrounds on target orientation discrimination. The sinusoidal adaptor consisted of two parts: a central patch and a surround annulus, with orientations assigned independently from one of the five levels between 90° (vertical) and 180° (horizontal), yielding 25 different orientation combinations. The target was a Gabor the same size as the central patch, with orientation ranging from 82° (clockwise tilt, CW) to 98° (counter-clockwise tilt from vertical, CCW). In a two-alternative forced-choice task, observers were asked to judge the orientation of the target (i.e. tilted CW or CCW from vertical) after adaptation to either the annulus or the central patch. We estimated the target thresholds for CW and CCW directions independently using two adaptive staircases. We presented stimuli in the upper right visual field 10° from the central fixation point. Results showed that when the center adapting orientation was at 101.25° CCW threshold was the highest while CW threshold was the lowest. CCW threshold first decreased then increased with adapting surround orientation. The opposite was true for CW threshold: discrimination threshold first increased than decreased with surround orientation. The maxima and minima of these threshold curves were close to when the center and surround were of the same orientation. Our results suggest an orientation-selective lateral modulation is involved, in which the surround inhibited the adaptation effect (release from suppression) for CCW stimuli, while enhancing the adaptation effect for CW stimuli. The results can be explained by a version of divisive inhibition model, in which the response of the target mechanisms are their linear excitations divided by a normalization signal plus a constant and modulated by the adaptor.
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