TEMPORAL RESOLUTION OF DICHOPTIC AND 2ND-ORDER MOTION MECHANISMS

We addressed the question of whether low-level motion analysers can in tegrate signals binocularly. We compared the temporal sensitivity in m otion discrimination tasks using monocular and dichoptic first-order m otion and monocular and dichoptic second-order motion. Three human obs ervers were required to discriminate the direction of motion of either sinusoidal gratings (1 c/deg), used as a stimulus for first-order mot ion analysers, or the envelopes of contrast-modulated stationary sinus oidal gratings (carrier frequency 5 c/deg, carrier contrast 0.1, modul ation frequency 1 c/deg), used as a stimulus for second-order motion a nalysers. Contrast sensitivity was measured as a function of temporal frequency. The moving grating or envelope was generated by summing two non-moving sinusoidally flickering gratings or envelopes in spatiotem poral quadrature. These were either combined monocularly or presented dichoptically. Sensitivity to the moving envelope was highest at a tem poral frequency between 0.5 and 2 Hz, depending on the observer, and d eclined rapidly at high temporal frequencies. None of the observers wa s able to discriminate the direction of motion of envelopes moving fas ter than 4 Hz. Dichoptic and monocular presentation produced very simi lar results. Sensitivity to a monocularly presented moving grating was fairly uniform between 1 and 8 Hz, and declined slightly at 16 Hz. In one of three observers sensitivity to the dichoptically presented gra ting was very close to that of the monocularly presented grating at al l temporal frequencies tested (from 1 to 16 Hz) All observers could di scriminate the direction of motion of the dichoptically presented grat ing at 8 Hz, but two of the three were unable to discriminate its dire ction of motion at 16 Hz. These results indicate that second-order mot ion analysers have very poor temporal resolution and that dichoptic mo tion analysers have very good resolution. We suggest that this implies that there are low-level motion analysers that are capable of integra ting information binocularly. (C) 1998 Elsevier Science Ltd. All right s reserved.