Last week, I talked about the sound-induced flash illusion, in which presenting a single flash with two or more auditory beeps caused people to see two or more flashes. This week, a study showing that the same effect can be obtained by replacing the beeps with touches. Here’s the setup (unfortunately, I can’t show you this illusion)1. Participants were placed in front of a computer screen, on which they observed a small dot flash for 10 ms. At the same time, they received either no taps, one tap, or two taps (spaced 60 ms from each other) from all 8 pins of a Braille cell on their left index finger. They were then asked to report how many flashes they observed. When they received no taps or one tap as the dot flashed on the screen, only 20% reported seeing two flashes, but when they received two taps, 63% of the participants reported seeing two flashes. Thus it appears that, like sound, touch can induce illusory flashing.
What does this mean? Well, apparently we don’t know a whole hell of a lot about the integration of visual and somatosensory information in the brain, but the authors of the paper speculate that the illusion is likely induced by crossmodal perceptual interactions like those that are thought to mediate the auditory induced flash illusion, rather than by cognitive processes. Since the illlusion was first reported only last year, further research has yet to be published, but I suspect that a lower-level perceptual explanation rather than a cognitive one will be the right one. Sight and touch need to be closely for body-eye coordination, so it just makes sense that they would interact.
1Violentyev, A., Shimojo, S., & Shams, L. (2005). Touch-induced visual illusion. Neuroreport, 16(10), 1107-10.