The present study examined whether the Ebbinghaus illusion can be induced by surrounding contexts that are suppressed from conscious perception. We employed continuous flash suppression to render the contextual stimuli invisible. The surrounding contexts were presented to one eye and the masking stimulus was presented to the other eye, followed by the presentation of the two center test stimuli to both eyes. The illusory size effect was observed even when the inducers were rendered invisible, although the effect was weak- approximately one third the strength of that induced by visible contexts. The effects induced by both visible and invisible contexts decayed with equal speed as the interstimulus interval between the inducer and test stimuli increased. Moreover, interocular transfer of illusory size effect did not occur when the inducers were suppressed from awareness. These results suggest that the size-contrast effect in the Ebbinghaus illusion is mediated in V1, and the monocular pathway in V1 is involved in the unconscious effect.
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