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David McCormick, Pascal Mamassian; What does the illusory-flash look like?. Journal of Vision 2006;6(6):189. doi: 10.1167/6.6.189.
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In the illusory-flash effect (Shams et al. 2000, Nature, 408, 788), one flash presented with two tones has a tendency to be perceived as two flashes. The origin of this illusory-flash is uncertain; to date, the literature surrounding this phenomenon has focused primarily on the strength of the illusion and its underlying brain mechanisms. We address the following issue - what are the perceptual attributes of the illusory-flash? A white flash of 24ms was presented prior to a flashed Pac-Man of the same radius and duration at the same location. The Pac-Man had one of twelve low contrasts above or below mean background luminance. Half of the visual stimuli were presented in silence; the other half in conjunction with two beeps. The Pac-Man could have one of two possible orientations and observers responded to their perceived orientation. We predict better orientation discrimination if the illusory-flash has the same polarity as the real flash, and poorer performance if it has the opposite polarity. We first confirmed that our stimuli produced a strong illusory-flash effect. We then compared the psychometric functions for orientation discrimination, where concomitant beeps were present or absent. Initial results show a relative shift of the psychometric functions consitent with a same-polarity illusory-flash. We discuss other situations in which our method may elucidate further properties of the illusory-flash.
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