Background: In the double flash illusion, a visual flash can be doubled by the presentation of two beeps, one simultaneously with the flash and one following (Shams, et al., 2000). The current study found that a visual flash of a static spatial gradient (black at the center to white at the edges on a white background), can generate the perception of visual expansion then contraction. Furthermore, when multiple beeps are played during and after the visual gradient flash, the visual expansion then contraction is perceived twice. Methods: A single flash of either a black circle with sharp edges (SE) or a circular gradient (G) is presented for 20 ms. One, two, or three beeps of 6 ms each are paired with this flash, randomly across trials. Participants (N = 7) were asked to report the number of flashes perceived and the type of perception (e.g., for 1 flash: a circle expanding then shrinking, a circle shrinking then expanding, or a flash of constant size). Results: Participants reported significantly more flashes for the two and three beep conditions compared to the single beep condition for both the SE and G flashes (p 0.005). Participants indicated significantly more dynamics (expansion or shrinking) for the G flash as compared to the SE flash (p 0.005). Participants also verbally indicated that the illusory flash for the G stimulus (i.e., the second flash when two flashes were reported) had the same dynamic visual expansion as the real first flash. Discussion: The double flash illusion occurs even with a gradient stimulus. We hypothesize that the gradient flash generates a perception of expansion due to higher contrast regions being processed faster (Seiffert and Cavanagh, 1999). As the center region has the highest instantaneous contrast, it is processed faster than the rest of the gradient.
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