Differential effects of attention and contrast on transition appearance during binocular rivalry.

Binocular rivalry occurs when two eyes are presented with two conflicting stimuli. Although the physical stimulation stays the same, the conscious percept changes over time. This property makes it a unique paradigm in both vision science and consciousness research. Two key parameters, contrast and attention, were repeatedly shown to affect binocular rivalry dynamics in a similar manner. This was taken as evidence that attention acts by enhancing effective stimulus contrast. Brief transition periods between the two clear percepts have so far been much less investigated. In a previous study we demonstrated that transition periods can appear in different forms depending on the stimulus type and the observer. In the current study, we investigated how attention and contrast affect transition appearance. Observers viewed binocular rivalry and reported their perception of the four most common transition types by a button press while either the stimulus contrast or the locus of exogenous attention was manipulated. We show that contrast and attention similarly affect the overall binocular rivalry dynamics, but their effects on the appearance of transitions differ. These results suggest that the effect of attention is different from a simple enhancement of stimulus strength, which becomes evident only when different transition types are considered.