A tactile illusion: The rotating hourglass

Abstract

A new tactile (more properly termed haptic) illusion, the rotating hourglass, was investigated in the laboratory by rotating a rod end for end between the S's thumb and forefinger. This illusion, which is an apparent decrease in the diameter of the rod at the point of contact with the fingers, was easily observed by 19 of the 20 Ss. When the illusion was studied as a function of time, the magnitude of the illusion increased over time with a mean decrease in apparent diameter of 52.3% from the beginning to the end of the 38-sec trials. A theory of differential adaptation of the skin is postulated to explain the rotating hourglass illusion and a similar illusion. © 1974 Psychonomic Society, Inc.