An Adaptation-Induced Repulsion Illusion in Tactile Spatial Perception

Abstract

Following focal sensory adaptation, the perceived separation between visual stimuli that straddle the adapted region is often exaggerated. For instance, in the tilt aftereffect illusion, adaptation to tilted lines causes subsequently viewed lines with nearby orientations to be perceptually repelled from the adapted orientation. Repulsion illusions in the nonvisual senses have been less studied. Here, we investigated whether adaptation induces a repulsion illusion in tactile spatial perception. In a two-interval forced-choice task, participants compared the perceived separation between two point-stimuli applied on the forearms successively. Separation distance was constant on one arm (the reference) and varied on the other arm (the comparison). In Experiment 1, we took three consecutive baseline measurements, verifying that in the absence of manipulation, participants’ distance perception was unbiased across arms and stable across experimental blocks. In Experiment 2, we vibrated a region of skin on the reference arm, verifying that this focally reduced tactile sensitivity, as indicated by elevated monofilament detection thresholds. In Experiment 3, we applied vibration between the two reference points in our distance perception protocol and discovered that this caused an illusory increase in the separation between the points. We conclude that focal adaptation induces a repulsion aftereffect illusion in tactile spatial perception. The illusion provides clues as to how the tactile system represents spatial information. The analogous repulsion aftereffects caused by adaptation in different stimulus domains and sensory systems may point to fundamentally similar strategies for dynamic sensory coding.

Commentary: An Adaptation-Induced Repulsion Illusion in Tactile Spatial Perception

“…Exposure to this stimulation resulted in an inwards position shift (i.e., repulsion from the stimulus) of a stimulus at the edge of the gap (Brooks et al, 2015 ). The size of the shift was approximately six percent of the gap size, comparable to the effect observed by Li et al ( 2017 ). Together these observations are most consistent with changes to receptive field geometry, but whether these manifest sub-cortically or cortically is unknown.…”

“…For instance, creating the visual illusion that the arm is larger improves tactile discrimination (Kennett et al, 2001 ) and increases perceived distances on the arm (Taylor-Clarke et al, 2004 ). Thus, the perceived elongation of a two-point stimulus in the Li et al ( 2017 ) study affords higher resolution perception of objects touching the adapted region.…”

“…In the study of Li et al ( 2017 ) these questions were addressed over three experiments using a force-controlled two-point stimulus. The first experiment used a two-alternative forced choice task in which participants were given the two-point stimulus on the reference arm (always given 30 mm) and subsequently on the comparison arm (counterbalanced order).…”

“…The main experiment found that adaptation results in perceived spatial expansion. Li et al ( 2017 ) propose that the spatial distortion arises from changes in receptive field size, given two assumptions about the neural processing of tactile information. First, the brain remains largely unaware of peripheral adaptation (Seriès et al, 2009 ).…”

“…Whether any such processes operate for shorter windows of sensory input in other brain regions remains an open question. One way in which the new results of Li et al ( 2017 ) can be functionalized is under the hypothesis that the body is used as a tool to perceive external objects (Linkenauger et al, 2010 ; Van der Hoort and Ehrsson, 2016 ). Under this hypothesis, the rapid malleability of higher order body representations (Taylor-Clarke et al, 2004 ) can afford enhanced object perception.…”