Seeing the body distorts tactile size perception

Abstract

Vision of the body modulates somatosensation, even when entirely non-informative about stimulation. For example, seeing the body increases tactile spatial acuity, but reduces acute pain. While previous results demonstrate that vision of the body modulates somatosensory sensitivity, it is unknown whether vision also affects metric properties of touch, and if so how. This study investigated how non-informative vision of the body modulates tactile size perception. We used the mirror box illusion to induce the illusion that participants were directly seeing their stimulated left hand, though they actually saw their reflected right hand. We manipulated whether participants: (a) had the illusion of directly seeing their stimulated left hand, (b) had the illusion of seeing a non-body object at the same location, or (c) looked directly at their non-stimulated right-hand. Participants made verbal estimates of the perceived distance between two tactile stimuli presented simultaneously to the dorsum of the left hand, either 20, 30, or 40 mm apart. Vision of the body significantly reduced the perceived size of touch, compared to vision of the object or of the contralateral hand. In contrast, no apparent changes of perceived hand size were found. These results show that seeing the body distorts tactile size perception.

Introduction

Both vision and somatosensation provide important sources of information about our body. Therefore, combining multisensory inputs is critical to perceiving the properties and current state of the body. Recent results have demonstrated widespread effects of vision of the body on somatosensation, even when entirely non-informative about stimulation, for example by enhancing tactile spatial acuity (Kennett, Taylor-Clarke, & Haggard, 2001) and reducing pain (Longo, Betti, Aglioti, & Haggard, 2009). Such results show that seeing the body can increase the sensitivity of somatosensory processing, or alter perceived intensity of somatosensory stimuli. But does vision also distort touch, altering its perceived metric properties? Here, we addressed this question, investigating how vision of the body affects the perceived size of tactile stimuli applied to the seen body part.

How might vision of the body distort tactile size perception? Intriguingly, two sets of considerations lead to opposite predictions. Weber (1834/1996) originally noted that the perceived distance between two tactile stimuli is larger on skin regions with relatively high sensitivity compared to those with lower sensitivity, an effect now known as Weber’s illusion (Taylor-Clarke, Jacobsen, & Haggard, 2004). Since seeing the body increases tactile sensitivity (Kennett et al., 2001), it is thus natural to hypothesize that this should lead to a corresponding increase in the perceived size of tactile stimuli. Indeed, this was our initial hypothesis. There is, however, another set of considerations which point in the opposite direction, suggesting that reduced tactile sensitivity can be associated with increased perceived size of the body and of touch. For example, cutting off afferent signals from a body part with local anesthesia increases the perceived size of that body part (Gandevia and Phegan, 1999, Türker et al., 2005) as well as the perceived size of objects held in the affected body part (Berryman, Yau, & Hsiao, 2006). Similarly, chronic pain often produces both reduced tactile sensitivity on the affected body part (Moseley, 2008, Pleger et al., 2006), as well as perceived swelling (Moseley, 2005, Moseley, 2008, Pelz et al., 2011). In the case of both anesthesia and pain, reduced tactile sensitivity is associated with increased perceived body part size. Vision of the body, then, could be expected to produce exactly opposite effects: since seeing the body enhances tactile sensitivity it should reduce the perceived size of the body and also shrink the perceived size of tactile stimuli.

We investigated this question using the mirror box illusion (Ramachandran, Rogers-Ramachandran, & Cobb, 1995) to induce the subjective experience of direct vision of the stimulated left hand, while simultaneously keeping vision non-informative about stimulation. In Experiment 1, participants made verbal judgments of the distance between two touches applied to the dorsum of their left hand, while looking into a mirror aligned with their body midline, with their hands symmetrically on either size of the mirror. The reflection of their right hand, thus appeared to be a direct view of their left hand, yet provided no information about the size of touch. In control conditions, participants looked at the mirror reflection of a non-hand object or at their non-stimulated right hand. To anticipate our results, we found that seeing the body reduces the perceived size of touch. In Experiment 2 we replicated this finding and additionally added a measure of perceived hand size before and after each block, finding no apparent change in any condition.