A New Tactile Illusion: Temporal Limits on the Processing of Spatiotemporal Patterns

A novel illusion was observed when 12 tactile point-stimulators arranged in a circle were sequentially activated so that each quadrant was first traced in a clockwise manner, then counterclockwise, and again in a clockwise manner, after which the next quadrant was similarly traced. Under certain temporal conditions this stimulus pattern was experienced as a point moving through an overall circular path but looping inwardly about once per quadrant. The effects of variations in rate of presentation of such stimuli were investigated by having subjects make drawings of their perceptual experiences. Three skin surfaces were used (palm, fingers, and forearm), and all produced similar results except for lower confidence and reliability of drawings made from forearm stimulation. Pattern presentation rate, however, had a consistent and powerful effect, with the looping illusion most frequently observed at rates around 25 pattern points per s and with different perceptual organizations dominating as rates departed widely from this optimum. The illusory perceptual organizations were interpreted as compromises between past and present stimulation necessitated by the relative slowness of tactile processing of spatio-temporal patterns.