Discrimination between Plaids: Do Oriented Filters Combine Dichoptically?

Two oblique gratings combine perceptually to form a chequerboard appearance under monoptic presentation but not under dichoptic presentation (Georgeson and Meese, 1996 Perception 25 Supplement, 121), suggesting an early combination process. Combining processes that operate over orthogonal orientations are also observed in complex pattern discrimination tasks (Olzak and Thomas, 1991 Vision Research 31 1885 – 1898). We ask here whether the interactions observed in discrimination tasks occur before or beyond the site of binocular interaction.

Observers discriminated between two patterns that differed slightly in their spatial-frequency content. On each trial, one of the two patterns was presented. Observers judged which stimulus had been presented on a 1 – 6 confidence scale. In control conditions, the stimuli were single sinusoidal gratings near 3 cycles deg⁻¹, and were always presented monoptically. In test conditions, a second component was added to the first in each stimulus. The second component was either presented to the same eye as the first component (monoptic presentation), or to the other eye (dichoptic presentation). The second component was either a mask of exactly 3 cycles deg⁻¹, or varied to give a second cue to discrimination. Second cues either varied with the first (both components lower frequency or both higher) or varied in opposite sign (high with low). Rivalry was eliminated by using low contrast (10 × threshold) and small (1.2 deg) patterns. Our results suggest that the combination processes we observe occur beyond the site of binocular combination and differ from those mediating the percept.