Elsevier

Consciousness and Cognition

Volume 17, Issue 4, December 2008, Pages 1181-1191
Consciousness and Cognition

Visual enhancement of touch and the bodily self

https://doi.org/10.1016/j.concog.2008.01.001Get rights and content

Abstract

We experience our own body through both touch and vision. We further see that others’ bodies are similar to our own body, but we have no direct experience of touch on others’ bodies. Therefore, relations between vision and touch are important for the sense of self and for mental representation of one’s own body. For example, seeing the hand improves tactile acuity on the hand, compared to seeing a non-hand object. While several studies have demonstrated this visual enhancement of touch (VET) effect, its relation to the ‘bodily self’, or mental representation of one’s own body remains unclear. We examined whether VET is an effect of seeing a hand, or of seeing my hand, using the rubber hand illusion. In this illusion, a prosthetic hand which is brushed synchronously—but not asynchronously—with one’s own hand is felt to actually be one’s hand. Thus, we manipulated whether or not participants felt like they were looking directly at their hand, while holding the actual stimulus they viewed constant. Tactile acuity was measured by having participants judge the orientation of square-wave gratings. Two characteristic effects of VET were observed: (1) cross-modal enhancement from seeing the hand was inversely related to overall tactile acuity, and (2) participants near sensory threshold showed significant improvement following synchronous stroking, compared to asynchronous stroking or no stroking at all. These results demonstrate a clear functional relation between the bodily self and basic tactile perception.

Section snippets

Participants

Twenty-two volunteers (13 female) between 18 and 54 years of age participated. Participants were naïve to the experimental hypotheses, and the study was approved by the local ethical committee.

Apparatus and stimuli

Participants sat with their right hand resting palm up on a table. Their right index finger rested on a pedestal which kept it stationary so that stimuli could be applied consistently to the fingertip. Participants wore a black smock which covered their arms so they could not see them directly. They

Visual enhancement effects

The inverse efficiency principle suggests that multisensory integration effects are strongest when individual sensory channels are weakest. Thus, we would expect that VET effects should be largest for participants performing close to chance. VET effects were computed by taking the difference in performance following synchronous stroking and both asynchronous stroking and no brushing; overall acuity was computed by taking the overall average performance, across conditions. There was a

Discussion

Two principal effects were observed in the present study. First, the characteristic inverse efficiency pattern for VET was observed even when stimuli were held constant at test; performance overall was negatively related to the difference in performance following synchronous stroking and both asynchronous stroking and no brushing. This confirms the similarity between VET and other forms of multisensory interaction (Stein & Meredith, 1993). It also points to a possible functional role of VET, in

Acknowledgments

This research was supported by Biotechnology and Biological Sciences Research Council (BBSRC) grant BB/D009529/1 and a grant from the Bial Foundation to PH. Portions of the data were previously presented at the annual conference of the European Society for Cognitive Psychology, Marseille, France, August, 2007.

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