Stimulus strength and dominance duration in perceptual bi-stability. Part II: From binocular rivalry to ambiguous motion displays

In binocular rivalry, strengthening the stimulus to one eye does not affect the mean dominance duration of that eye, but instead decreases the dominance durations of the other eye (Levelt 1968). Here we show that this fundamental result can be generalized to another domain of perceptual bi-stability. Methods: Plaids made of rectangular-wave gratings were used. Plaids are bi-stable stimuli that can be seen as a single pattern moving coherently or two grating sliding over each other (transparency). We manipulated the strength of the transparent percept without affecting the coherent percept, relying on two facts: (i) during the transparency states, the sense of motion is dominated by the 'front' grating; (ii) there is a preference to see motion along cardinal directions (e.g. Andrews & Schluppeck VR 2000; Hupé & Rubin ARVO 2001). The plaids consisted of one cardinal and one oblique grating, with unambiguous depth ordering via occlusion cues. Shuffling the cardinal grating from back to front therefore enhanced the transparent percept, but did not affect the coherent percept (during coherency the two plaids looked indistinguishable). The mean duration of the transparency and coherency epochs were measured over 60 sec trials. Results: For all eight observers, strengthening the transparent percept led to a shortening of the mean duration of the coherency epochs, without affecting the duration of the transparency epochs, in analogy to Levelt's result. This suggests that there may be general rules governing the dynamics of bi-stable perception. Specifically, it suggests a suppressive/inhibitory coupling between rivaling stimulus interpretations as a general principle in the brain. Our results also suggest that measuring the mean durations of the coherency and transparency epochs may provide a way to asses whether a given manipulation affected motion integration or motion segmentation processes, or both. To date such independent assessment has not been possible.