"Boundary of illusion": an experiment of sensory integration with a pseudo-haptic system

Describes a psychophysical experiment designed to study the phenomenon of illusion which occurs with pseudo-haptic feedback, and to identify the moment when this illusion occurs: the "boundary of illusion". The subjects were given the task of deciding which of two virtual springs is the stiffer, these springs being simulated with a PHANToM^TM force feedback device and displayed on a monoscopic computer screen. The first spring had a realistic behavior, since its visual and haptic displacements were identical. The second spring-the pseudo-haptic one-was stiffer on a haptic basis, but sometimes less stiff on a visual basis. The data collected allowed us to calculate the visual point of subjective equality (PSE) between the two springs, which represents the boundary of the sensory illusion. On average, a high value of PSE turned out to be -24%. This value increased monotonically when the haptic difference between the springs increased. This implies that more visual deformation is necessary to compensate for large haptic differences and qualifies the notion of visual dominance. However, this boundary varies greatly depending on the subjects and their sensory integration strategy. The subjects were sensitive to this illusion to varying degrees. They were divided into different populations-from those who were "haptically oriented" to those who were "visually oriented".