Tau Effect

The tau effect is a perceptual illusion in which the timing between successive stimuli distorts how far apart they seem to be in space. When two intervals are spaced at fixed physical distances but separated by unequal time gaps, people tend to perceive the stimulus that appeared after a shorter time interval as also being spatially closer, so that perceived distance follows perceived time rather than the true physical distance. The effect was first described in the sense of touch by Gelb in 1914, later named by Helson in 1930, and detailed further by Helson and King in 1931; it has since been demonstrated across visual, auditory and tactile stimuli. The tau effect has a temporal counterpart, the kappa effect, in which a fixed time interval is perceived as longer when the spatial distance between the two stimuli marking it is greater; both phenomena are taken as evidence that the brain interprets a sequence of stimuli partly through built-in expectations about how something should be moving through space and time.