Large Systematic Deviations in the Haptic Perception of Parallelity

Abstract

Recently, Kappers and Koenderink (1999 Perception 28 781–795) showed that what subjects haptically perceive as parallel often deviates greatly from what is actually physically parallel. In their experiment, subjects had to rotate a test bar in such a way that it felt as though it was parallel to a reference bar. Their data were obtained with the right hand on a table plane to the right side of the median plane of the subject. The present study extends that work in a number of ways: (1) the locations of the stimuli cover the total reachable table plane; (2) distances between stimuli can also be large (more than 1 m); (3) experiments are done both unimanually (with the right and left hand) and bimanually. Like in the previous study, the results show large systematic deviations that correlate significantly with horizontal (left-right) distance between the two bars but not with vertical (forward–backward) distance. Thus we have established that a description of the results in terms of a horizontal gradient in the deviations is valid over a large part of haptic space, over large distances, and in both unimanual and bimanual conditions. The subject-dependent horizontal gradients ranged from −12° m⁻¹ to −27° m⁻¹ in the present experiment. In all conditions a significant haptic oblique effect can be demonstrated.