Visual stimulation at adjacent borders of the retinal blind spot leads to perceptual filling in: a bar reaching through the blind spot, for example, is seen as complete. When multiple stimuli are presented reaching through the blind spot, which stimulus is to be filled in remains ambiguous. This can be demonstrated in a novel illusion, the a??jumping pen illusiona??. Hold a strip of paper horizontally and view it monocularly so that part of it reaches through the blind spot. Then hold a pen (or another object) vertically behind the paper where it also spans the blind spot region. The pen appears to jump in front of the paper. The blind spot thus provides a unique opportunity to study the resolution of perceptual ambiguity in the absence of retinal input. Here we show that the visual system resolves ambiguity by alternating between unambiguous percepts in which one filled-in stimulus occludes the other. Using a haploscopic setup, we presented an orthogonal cross formed by two isoluminant bars of different colors only to one eye, with its intersection placed in the blind spot. Observers viewed this stimulus continuously for trials of 1 minute. They indicated with continuous key presses which of the two bars appeared in front of the other; no key press meant unsure. Observers readily perceived one bar in front of the other; only brief periods were associated with a??unsurea?? responses. Which bar was perceived to be in front was stochastically alternating, akin to other cases of perceptual rivalry. Distributions of perceptual dominance durations followed gamma distributions, as found for binocular and monocular rivalry (Oa??Shea et al., 2009). These results demonstrate a novel situation: perceptual competition between two illusory percepts. Further, they suggest that filling-in at the blind spot shares common features with other mechanisms that resolve perceptual ambiguity.
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