Elsevier

Neuroscience Research

Volume 65, Issue 3, November 2009, Pages 307-311
Neuroscience Research

Double tactile sensations evoked by a single visual stimulus on a rubber hand

https://doi.org/10.1016/j.neures.2009.08.005Get rights and content

Abstract

The classical rubber hand illusion involves individuals misattributing tactile sensations ‘felt’ by their real hand hidden from view to a rubber prosthetic hand that they ‘see’ being tactilely stimulated in synchrony. However, from previous studies, it is not clear whether individuals actually feel the tactile sensation in the rubber hand, real hand, or both because the same part of the rubber and real hands were stimulated simultaneously. Here, we attempted to isolate subjects’ sensations attributed to the rubber hand from those sensed in the real hand by placing the rubber and real hands in opposing orientations (e.g., palm up versus dorsum up). Interestingly, half of the subjects reported two tactile sensations for one visual stimulus, that is, one in the rubber finger stimulated visually with a light source and one in the real finger overlapping the rubber finger. This finding suggests that the tactile sensation induced by the visual stimulus is referred to the rubber hand and real hand simultaneously. Thus, both visuo-spatial and somatic codes are used in the localization of tactile sensation in the rubber hand illusion.

Introduction

In the rubber hand illusion (Botvinick and Cohen, 1998, Rorden et al., 1999, Farne et al., 2000), a realistic life-sized rubber prosthetic hand is placed on a table in front of the subject while their real hand is hidden from view on or under the table, and the subjects continuously watches the rubber hand. When the rubber hand and real hand are touched simultaneously, the subject ‘feels’ a tactile sensation in the rubber hand as though the rubber hand was their own hand. The brain interprets the rubber hand as the real hand because the visual sense can override tactile sense when tactile input on the real hand is consistent with the visual input.

Previous research on the rubber hand illusion has measured perceptual location errors in the classical form of the illusion and has shown that subjects can discriminate between coordinated and mismatched touch (e.g., Pavani et al., 2000, Ehrsson et al., 2004, Holmes and Spence, 2005, Tsakiris and Haggard, 2005). Recently, Durgin et al. (2007) reported that subjects ‘felt’ tactile sensations from a light source, without any tactile stimulation. Such evidence suggests that visual input strongly influences tactile sensation. However, the illusion was weakened or disappeared in cases of physically impossible mismatch situations between rubber and real hands, such as misorientation between the front and heel of the hand (Pavani et al., 2000, Durgin et al., 2007). Thus, it is postulated that body state (somatic) inputs also influence tactile sensation.

To examine how visual and tactile inputs contribute to the rubber hand illusion, we attempted to isolate subjects’ sensations attributed to the rubber hand from those felt in the real hand by placing the rubber and real hands in opposing orientations. The present study therefore set up congruent and incongruent conditions. The congruent condition consisted of placing the real and rubber hands in identical orientations, a condition which was selected in order to verify the results of Durgin et al. (2007) who reported that the rubber hand illusion is induced by visual stimulus alone. We examined whether subjects could accurately ascertain the location where they felt tactile sensation (palm or dorsum of the hand and which of the five digits) under the condition of visual stimulus alone. The incongruent condition differed only in the orientation of the rubber and real hands, that is, palm up versus dorsum up (Fig. 1B).

We made three predictions as follows: (1) when only visuo-spatial coding is used to localize the tactile sensation, tactile sensation should occur on the finger of the rubber hand where a light spot is presented; (2) when only somatic coding is used to localize the tactile sensation, tactile sensation should occur on the finger of the actual hand (hidden from view) when a light spot is presented on the rubber hand; (3) when both codings are presented simultaneously, two tactile sensations should be perceived, one on the rubber hand and the other on the actual hand.

Previous investigations have measured skin conductance response (SCR) to verify sensation in the rubber hand illusion (e.g., Armel and Ramachandran, 2003), where a subject who does not succumb to the rubber hand illusion will not display any change in SCR when the rubber hand is stimulated. Conversely, a subject who does experience the illusion will show a change in SCR. In this research, we measured SCR when the experimenter pricked the rubber hand with a needle.

Section snippets

Subjects

Thirty-five volunteers (age, 18–35 years; mean age, 21.9 years) naïve to the purpose of the experiment and with normal or corrected-to-normal visual acuity were recruited to the study. All subjects were right handed. Written informed consent was obtained from all subjects prior to participation. The study protocol was approved by Rikkyo University Research Ethics Committee.

Apparatus and stimuli

The finger of a prosthetic rubber hand was exposed to a red laser light (630 nm) that moved a distance of 10 cm, at a right

Results

In the congruent condition, 27 of the 35 subjects experienced a tactile sensation. All subjects who experienced a sensation reported that the location of tactile sensation corresponded with movement of the laser light. The tactile sensation was experienced in their own hand, on the same finger as the rubber finger that was illuminated by the laser light (Table 1, left). For example, some subjects experienced a tactile sensation in the little finger of their real hand as the laser light was

Discussion

In the congruent condition, subjects experienced a tactile sensation in their own hand, on the same finger as the rubber finger being illuminated by the laser light. This concurs with the finding that the rubber hand illusion can be induced by a visual cue alone (Durgin et al., 2007), and indicates that subjects can accurately ascertain the kinds of finger on which they experience a sensation.

Results from the incongruent condition demonstrated that both visuo-spatial and somatotopic codes are

Acknowledgments

We wish to thank M. Okumura and Y. Suzuki for assistance with the experiments. This work was supported by the Rikkyo University Special Fund for Research and the Open Research Centre Project for Private Universities: Matching Fund Subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology), 2005–2009. The work was also supported by a Grant-in-Aid for Young Scientists (B) awarded to S.K. (19730467, 21730586).

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