Interocular interactions reveal the opponent structure of motion mechanisms

Interactions between motion sensors tuned to the same and to opposite direc tions were probed by means of measuring summation indexes for sensitivities (d') to contrast increments and/or decrements applied to drifting gratings presented in binocular and in dichoptic vision. The data confirm a phenome non described by Stromeyer, Kronauer, Madsen & Klein (1984, J. Opt. Sec. Am . A 1, 876-884), whereby opposite polarity contrast changes applied to bino cular gratings drifting in opposite directions yield sensitivities signific antly higher than same sign changes for which performance complies with pro bability summation (PS). The effect disappears in dichoptic vision where op posite sign contrast changes yield a performance close to, or below PS. whe ther they are applied to same or to opposite direction stimuli. Same sign c hanges in dichoptic drifting stimuli yield a performance higher than PS ind ependently of their relative directions and close to the performances obtai ned when these same sign changes are applied to dichoptic, static +/- 45 de grees gratings. Opposite sign changes applied to such static gratings yield PS. The data support the view according to which: (i) motion direction is extracted at the monocular site; (ii) motion sensors exhibit mutual inhibit ion within each eye when tuned to opposite directions: and (iii) binocular summation when tuned to the same direction. The data also suggest that (iv) independently of their directional tuning, all motion sensors converge on a binocular, motion non-specific ('flicker') unit; and that (v) signals car ried by ON and OFF pathways are slightly inhibitory to each other. (C) 2001 Elsevier Science Ltd. All rights reserved.