Visual limitations shape audio-visual integration.

Recent studies have proposed that some cross-modal illusions might be expressed in what were previously thought of as sensory-specific brain areas. Therefore, one interesting question is whether auditory-driven visual illusory percepts respond to manipulations of low-level visual attributes (such as luminance or chromatic contrast) in the same way as their nonillusory analogs. Here, we addressed this question using the double flash illusion (DFI), whereby one brief flash can be perceived as two when combined with two beeps presented in rapid succession. Our results showed that the perception of two illusory flashes depended on luminance contrast, just as the temporal resolution for two real flashes did. Specifically we found that the higher the luminance contrast, the stronger the DFI. Such a pattern seems to contradict what would be predicted from a maximum likelihood estimation perspective, and can be explained by considering that low-level visual stimulus attributes similarly modulate the perception of sound-induced visual phenomena and "real" visual percepts. This finding provides psychophysical support for the involvement of sensory-specific brain areas in the expression of the DFI. On the other hand, the addition of chromatic contrast failed to produce a change in the strength of the DFI despite it improved visual sensitivity to real flashes. The null impact of chromaticity on the cross-modal illusion might suggest a weaker interaction of the parvocellular visual pathway with the auditory system for cross-modal illusions.