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Goro Maehara, Robert Hess, Mark Georgeson; Direction discrimination thresholds in binocular, monocular, and dichoptic viewing: Motion opponency and contrast gain control. Journal of Vision 2011;11(11):747. doi: 10.1167/11.11.747.
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Purpose: The present study investigated the binocular organization of motion opponency and its relation to contrast gain control. Method: We measured luminance contrast thresholds for discriminating direction of motion for drifting Gabor patterns (target) presented on counter-phase flickering Gabor patterns (pedestal; equivalent to the superposition of two Gabors drifting in opposite directions). There were four presentation conditions. (i) Binocular: all stimuli were presented to both eyes, (ii) Monocular: all stimuli were presented to the same one eye, (iii) Dichoptic: the target was presented to one eye while the pedestal was presented to the other eye, (iv) Half-binocular: the target was presented to one eye while pedestals were presented to both eyes. In addition, we tested an increment-and-decrement condition, in which the target increased contrast for one direction of movement, but decreased it by the same amount for the opposite moving component of the pedestal. The decrement was either in the same eye as the increment, or in the other eye. Results and Discussion: Threshold-versus-pedestal-contrast (TvC) functions showed a dipper shape: thresholds decreased and then increased with pedestal contrast. At low pedestal contrasts, there was binocular summation: binocular thresholds were lower than monocular. But at high pedestal contrasts there was little difference between them. The ‘dip’ was smaller for dichoptic presentation than for other presentations. Thresholds were similar for monocular and half-binocular presentations at low pedestal contrasts, but half-binocular thresholds became higher and closer to dichoptic thresholds as pedestal contrast increased. The added decremental target lowered thresholds by about a factor of 2, compared with half-binocular, when the decrement was in the same eye as the increment, or the opposite eye. Several models fit to the data strongly suggest that motion opponency and contrast gain control operate at a binocular level of processing.
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