The Dynamic Double Flash Illusion: Auditory Triggered Replay of Illusory Visual Expansion

In the original double flash illusion, a visual flash (e.g., a sharp-edged disk, or uniformly filled circle) presented with two short auditory tones (beeps) is often followed by an illusory flash. The illusory flash has been previously shown to be triggered by the second auditory beep. The current study extends the double flash illusion by showing that this paradigm can not only create the illusory repeat of an on-off flash, but also trigger an illusory expansion (and in some cases a subsequent contraction) that is induced by the flash of a circular brightness gradient (gradient disk) to replay as well. The perception of the dynamic double flash illusion further supports the interpretation of the illusory flash (in the double flash illusion) as similar in its spatial and temporal properties to the perception of the real visual flash, likely by replicating the neural processes underlying the illusory expansion of the real flash. We show further that if a gradient disk (generating an illusory expansion) and a sharp-edged disk are presented simultaneously side by side with two sequential beeps, often only one visual stimulus or the other will be perceived to double flash. This indicates selectivity in auditory-visual binding, suggesting the usefulness of this paradigm as a psychophysical tool for investigating crossmodal binding phenomena.