Vision Without Photoreceptors: Crossmodal Perception Within the Blind Spots

Multisensory illusions are a key tool to investigate crossmodal integration and plasticity, given their resilience across manipulations and unique spatial adaptability. We previously tested the classic Double Flash Illusion across retinal locations in low vision participants. These participants reported stronger double flash perception in visual impairment areas relative to neurotypical participants. To examine whether illusions could span regions of no light perception, the present study induced multisensory interactions within the blind spots of neurotypicals using a postdictive illusion. The Audiovisual Rabbit Illusion consists of a sequence of [beep-flash, beep, beep-flash]; an illusory flash is induced by the second beep, located between the first and second beep-flash pairs (all beeps are centrally located). This illusion is postdictive, as a latter sensory stimulus impacts the perception of an already-presented stimulus. We mapped each participant’s blind spots (with one eye blindfolded) and placed the beep-flash pairs 0.5° outside the borders. We tested four sequences: left-to-right, right-to-left, top-to-bottom, and bottom-to-top, and three conditions: zero-beep, two-flash (0B2F; control); 2B2F (control); and 3B2F (illusion). Participants reported strong illusory percepts within their blind spots, as well as in visible locations. Performance was comparable between these locations. For 0B2F and 2B2F conditions, participants reported perceiving ~2 flashes, confirming that the illusion only occurs when visual and auditory information are incongruent within each beep-flash sequence. There were no significant differences in the number of flashes perceived between the 0B2F and 2B2F conditions. These results support the hypothesis that filling-in within blind areas can be multisensory. In this case, audition may play a key role in inducing visual propagation across visual space, even into regions without visual input capacity. The blind spot provides an interesting test case for how the brain interprets blind regions in the retina, particularly in comparison to scotomas generated from eye diseases.