What does the illusory-flash look like?

In the illusory-flash effect (Shams, L., Kamitani, Y., & Shimojo, S. (2000). Illusions. What you see is what you hear. Nature, 408, 788), one flash presented with two tones has a tendency to be seen as two flashes. Previous studies of this effect have been ill-equipped to establish whether this illusory-flash is the result of a genuine percept, or that of a shift in criterion. We addressed this issue by using a stimulus comprising two locations. This enabled contrast-threshold measurement by means of a location detection task. High-contrast white or black flashes were presented simultaneously to both locations, followed by threshold contrast flashes of the same contrast polarity at the two locations in half of the trials; observers reported whether or not the low-contrast flashes had been present. Irrelevant to the task, half of the trials contained one tone, the other half contained two tones. In this way, we were able to compute the change in sensitivity and shift in criterion between illusory and non-illusory trials. We observe both a decrease in visual sensitivity and a criterion shift in the illusory-flash conditions. In a second experiment, we were interested in determining whether this change in visual sensitivity gave rise to measurable visual attributes of the illusory-flash. If it has a contrast, it should interact with a spatio-temporally concurrent real flash. Using a similar two-location stimulus presentation, we found that under certain conditions, we were able to infer the polarity of the perceived illusory-flash. We conclude that the illusory-flash is indeed a perceptual effect with psychophysically assessable characteristics.