Abstract
During binocular rivalry, incompatible images compete for perceptual dominance. The alternation between dominant percepts generally occurs gradually: the suppressed image gradually ‘washes in’ and becomes the dominant percept (also dubbed a traveling or dominance wave). Here we investigate what determines at what location the suppressed image starts washing in. We used a concentric and a radial grating to initiate binocular rivalry. In experiment 1, we varied the contrast within the radial grating (between 20 and 80%), keeping that of the concentric grating constant at 50%. In experiment 2, we varied the spatial frequency within the radial grating (between .75 and 3 cpd, or between 3 and 12 cpd), keeping that of the concentric grating constant at the mean frequency of the radial grating. Observers indicated at what part of the stimulus the suppressed grating started becoming perceptually dominant.
In experiment 1, a perceptual alternation was most likely to start at the location where the contrast of the suppressed grating was higher than that of the dominant grating. In experiment 2, a perceptual alternation was most likely to start at the location where the spatial frequency of the suppressed grating was lower than that of the dominant grating. The results indicate that the spatial origin of a perceptual alternation is related to stimulus strength, but not necessarily to sensitivity: if the local strength of the suppressed grating is higher than that of the dominant one - because of a higher contrast - the probability of a perceptual alternation is increased at that location. For stimuli defined by spatial frequency, the probability of a perceptual alternation is highest at the location where the frequency of the suppressed grating is lowest, not where sensitivity to spatial frequency is highest (which, for our observers, was around 3 cpd).