Biphasic M-Cell Responses to Stimuli Producing Sound-Induced Flash Illusions

The illusion of a single pulse of light seen as two flashes has been known for nearly a century (Dunlop, 1915). Bowen (1989) extended this earlier work for multiple bright pulses of light—two flashes seen as three, etc. He attempted to explain these illusions in terms of putative multiphasic responses in the visual system. The sound induced flash illusion (SIFI) is another example of extra flashes being reported beyond what is presented. Following Bowen, we modelled the biphasic temporal responses of actual fast ON- and OFF- midget parasol M cells to visual stimuli we had previously used in our SIFI research. Could the auditory stimuli (beeps) be directing attention to short duration peaks in stimulation, thus biasing the response of the observer in reporting the number of transient events? We show that for square wave temporal profile such response bias could easily be the case, especially given that three or four clicks rarely yield an illusion of more than two flashes. On the other hand, we experimentally produced more numerous and reliable fission SIFIs in sawtooth profiles that did not generate such peaks of excitation. This strongly suggests that a decisional-level explanation of the SIFI is not adequate.