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Avesh Raghunandan, Frank E Visco, Scott B Stevenson; Contrast interactions in two-frame motion discrimination imply a binocular site of contrast gain control for motion. Journal of Vision 2003;3(9):535. doi: https://doi.org/10.1167/3.9.535.
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© ARVO (1962-2015); The Authors (2016-present)
Background: Previous studies have shown that two-frame motion detection thresholds are elevated if one frame's contrast is raised, despite the increase in average contrast. This “contrast paradox” phenomenon implies the existence of a gain control mechanism that operates in conjunction with the detection of motion. In this study, we used dichoptic presentation to investigate whether the gain control mechanism in motion detection is essentially monocular or binocular.
Methods: Stimuli were vertical 1 cpd Gabor patches presented as a two-frame movie. Thresholds for left-right motion discrimination were measured with a single interval, forced choice, method of constant stimuli. Each Gabor had a contrast of either 0.1 or 0.4, producing two matched contrast motion sequences (low-to-low and high-to-high) and two mixed contrast sequences (low-to-high and high-to-low). The mixed stimuli were presented in two ways. In the “binocular” condition, both eyes saw the same sequence (low-to-high or high-to-low). In the “dichoptic” condition, each eye saw the opposite order. Thus, if the left eye saw low-to-high, the right eye saw high-to-low, and vice versa.
Results: Matched contrast sequences showed an overall threshold improvement from 0.1 to 0.4 contrast. In agreement with previous reports, mixed contrast sequences were dramatically elevated in the “binocular” condition, where both eyes saw the same mixed sequence. In the “dichoptic” condition, however, thresholds were consistent with the average contrast of the two frames. The “contrast paradox” did not occur.
Conclusions: The lack of a contrast paradox in the “dichoptic” condition indicates that the gain control accompanying motion energy extraction includes signals from both eyes. That is, it is at or beyond the site of binocular combination. Preliminary results from interocular motion experiments suggest that an independent, binocular gain control mechanism operates as well.
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