Matilda Cederblad¹ (), Armand R. Tanguay, Jr^1,2, Shinsuke Shimojo¹, Noelle R. B. Stiles^1,3; ¹California Institute of Technology, Division of Biology and Biological Engineering, ²University of Southern California, Departments of Electrical Engineering, Chemical Engineering and Materials Science, Biomedical Engineering, Ophthalmology, and Physics and Astronomy; Neuroscience Graduate Program, ³University of Southern California, Department of Ophthalmology

Sensory signals of different modalities can be processed as a unified event, which in turn can affect how individual sensory signals are perceived. For example, the Double Flash Illusion shows that a single flash can be perceived as two when paired with two auditory beeps. The Audiovisual Rabbit Illusion, a variation of the Double Flash Illusion, shows that two flashes can be perceived as three when coupled with three beeps. It is however unclear whether an object defined by illusory contours could perform like that of high contrast visual objects in these multisensory illusions. In our experiments we presented visual objects constructed of illusory contours paired with beeps, following a previous setup by Stiles et al 2018. The participants reported the number of flashes they saw on a trial-by-trial basis. The illusory contour objects were defined by a white rectangular space (0.65 by 4.68 degrees, presented 8.58 degrees below the fixation cross) overlaid on numerous black intersecting lines. The black lines (without the white rectangles) were presented for 2 seconds before the onset of the illusory-contour-rectangles, which each flashed for 36 milliseconds in sequence from left-to-right. Our preliminary findings (N=8) indicate that illusory contours behave in a similar way to defined objects. We observed the suppression of veridical flashes in that the 2b3f (2-beep-3-flash sequence, Invisible Rabbit stimulus) had significantly fewer perceived flashes than 3b3f (Means=2.63 vs. 2.99 perceived flashes). We also found support for the creation of illusory flashes in that 3b2f (Audiovisual Rabbit stimulus) had significantly more perceived flashes compared to 2b2f (Means=2.44 vs. 2.19 perceived flashes). These results show that illusory contours are integrated with audition similarly to that of luminance-defined objects. Furthermore, this multisensory integration occurred over a short timescale of a few hundred milliseconds, indicating that illusory contour objects can be processed surprisingly fast.

Acknowledgements: We are grateful for support from the National Institutes of Health, National Eye Institute