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B. M. Sheliga, E. J. FitzGibbon, F. A. Miles; Evidence that disparities defined by luminance and contrast are sensed by independent mechanisms. Journal of Vision 2010;10(7):326. doi: 10.1167/10.7.326.
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© ARVO (1962-2015); The Authors (2016-present)
We recorded the initial vergence eye movements that were elicited by 1-D sinusoidal gratings differing in phase at the two eyes by ¼ wavelength (binocular disparity) and created by luminance modulation (LM) or contrast modulation (CM) of dynamic binary noise that was uncorrelated at the two eyes. Whether horizontal or vertical, gratings defined by either LM or CM elicited vergence responses that were always compensatory, working to reduce the ¼-wavength disparity. When LM was added to the CM, vergence responses showed a U-shaped dependence on the magnitude of the LM, reaching a minimum with in-phase LM of 3.0-5.5%, consistent with the nulling of 1st-order distortion products due to compressive nonlinearities early in the visual pathway. The minimum vergence responses here were robust, had longer latencies than the responses evoked by the LM component of the stimulus (differences ranging from 15.5 to 31.2 ms), and were attributed to cortical mechanisms that can sense disparities defined solely by contrast. In a second experiment, we found that disparities defined by LM in one eye and CM in the other eye (“LM+CM stimulus”) generated only weak vergence responses and these were always in the “wrong” direction, i.e., opposite to the imposed ¼-wavelength disparity, consistent with mediation entirely by 1st-order distortion products associated with the CM stimulus. Thus, these (reversed) vergence responses could be eliminated entirely by adding a small amount of LM to the CM stimulus (in phase), and the greater the depth of the CM, the greater the added LM required for nulling. Controls indicated that the failure of the LM+CM stimulus to elicit vergence responses (after nulling the distortion products) was not due to differences in the amplitude or timing of the inputs from the two eyes. These data suggest that disparities defined by LM and CM are sensed by independent mechanisms.
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