We compared observers' contrast sensitivities for monocularly presented drifting gratings with their sensitivities to various pair-combinations of these gratings. If the two gratings are presented one to each eye, are of low spatial frequency, and move in same direction, contrast sensitivity is nearly twice the monocular value; if the gratings move in opposite directions, there is little or no sensitivity difference between the monocular and dichoplic conditions. As spatial frequency increases, the difference between the same- and opposite-direction conditions becomes less marked, and both pair-combinations are about 1.4 times as detectable as their monocular components. A monocular combination of gratings drifting in opposite directions (a counterphase modulated grating) gives results much like the dichoptic opposite-direction combination at all spatial frequencies. If spatial frequency is fixed at a moderate value, and drift rate is varied, then the difference between same and opposite conditions increases with drift rate; the natural conclusion that stimulus velocity determines this difference is not. however, completely borne out by experiments in which spatial and temporal frequency are varied to hold velocity constant. Thus under conditions where velocity and direction information are known to be available at detection threshold, our results show that binocular combination depends on the direction of movement: directional information must thus be extracted at or prior to the confluence of binocular signals.
ASJC Scopus subject areas
- Sensory Systems