BINOCULAR SUMMATION IN ORIENTATION DISCRIMINATION DEPENDS ON STIMULUSCONTRAST AND DURATION

Binocular summation, an improvement in visual performance with binocul ar viewing compared to monocular viewing, has been studied extensively in detection tasks. Monocular detection thresholds for stationary sti muli are typically about 40% higher than binocular thresholds. Binocul ar summation in discrimination tasks, however, is often lower and less consistent. A possible explanation for this difference is that satura tion of responses limits the extent of binocular summation in discrimi nation tasks. To investigate this possibility, we used an orientation discrimination task and varied stimulus contrast and exposure duration . Monocular and binocular orientation discrimination thresholds were o btained using one-dimensional difference-of-Gaussian stimuli. For brie fly exposed stimuli, binocular summation is greatest at low contrasts (e.g. 66% at 8% contrast) and is reduced systematically at higher cont rasts so that monocular and binocular thresholds are approximately equ al at contrasts above 15%. Binocular summation for low-contrast stimul i is greatest at a brief exposure duration (50 msec), is reduced at lo nger durations, and is not significant at durations of 100 msec or lon ger. Thus, binocular summation in orientation discrimination is greate st for relatively low-energy stimuli. These results are consistent wit h models of binocular energy summation and the hypothesis that saturat ion of responses after binocular combination can limit binocular summa tion in discrimination tasks.