Stimulus intensity modulates perceived tactile distance.

Several features of tactile stimuli modulate the perceived distance between touches. In particular, distances are perceived as farther apart when the time interval between them is longer, than when it is shorter. Such effects have been interpreted as a form of 'psychological relativity', analogous to Einstein's conception of a four-dimensional space-time. We investigated whether similar effects occur for stimulus features other than time, specifically stimulus intensity. We hypothesised that perceived distance would be increased when the two stimuli differed in intensity, since they would then be farther apart in a multi-dimensional feature space. Participants made verbal estimates of the perceived distance between two touches on their left hand. Intensity was manipulated such that both stimuli could be intense, both could be light, or one could be intense and the other light. We found no evidence for change in perceived tactile distance when stimuli intensity mis-matched. In contrast, there were clear effects of average stimulus intensity on perceived distance. Intense stimuli were judged as farther apart than light stimuli, and mixed stimuli were intermediate. These results are consistent with theories of general magnitude representation, which argue that multiple dimensions of magnitude are dependent on a shared underlying representation of domain-general magnitude.