Somatosensory activations during the observation of touch and a case of vision–touch synaesthesia

Abstract

In this study, we describe a new form of synaesthesia in which visual perception of touch elicits conscious tactile experiences in the perceiver. We describe a female subject (C) for whom the observation of another person being touched is experienced as tactile stimulation on the equivalent part of C's own body. Apart from this clearly abnormal synesthetic experience, C is healthy and normal in every other way. In this study, we investigate whether C's 'mirrored touch' synesthetic experience is caused by overactivity in the neural system that responds to the observation of touch. A functional MRI experiment was designed to investigate the neural system involved in the perception of touch in a group of 12 non-synesthetic control subjects and in C. We investigated neural activity to the observation of touch to a human face or neck compared with the observation of touch to equivalent regions on an object. Furthermore, to investigate the somatosensory topography of the activations during observation of touch, we compared activations when observing a human face or neck being touched with activations when the subjects themselves were touched on their own face or neck. The results demonstrated that the somatosensory cortex was activated in the non-synesthetic subjects by the mere observation of touch and that this activation was somatotopically organized such that observation of touch to the face activated the head area of primary somatosensory cortex, whereas observation of touch to the neck did not. Moreover, in non-synesthetic subjects, the brain's mirror system-comprising premotor cortex, superior temporal sulcus and parietal cortex-was activated by the observation of touch to another human more than to an object. C's activation patterns differed in three ways from those of the non-synesthetic controls. First, activations in the somatosensory cortex were significantly higher in C when she observed touch. Secondly, an area in left premotor cortex was activated in C to a greater extent than in the non-synesthetic group. Thirdly, the anterior insula cortex bilaterally was activated in C, but there was no evidence of such activation in the non-synesthetic group. The results suggest that, in C, the mirror system for touch is overactive, above the threshold for conscious tactile perception.

Cognitive control in social situations: A role for the dorsolateral prefrontal cortex

“…Our findings also provide further support for the existence of a mirror neuron system for touch (Blakemore et al, 2005). When we directly face an individual, as participants did in the present experiment, we often mirror that individual's movements and sensations in a body part (e.g., the left hand) that mirrors the body part that is being manipulated by the observed individual (e.g., the right hand) (Pelphrey et al, 2004).…”

“…Recent findings indicate that watching a person being touched on a particular part of the body results in the activation of corresponding regions within the perceiver's somatosensory cortex (Blakemore, Bristow, Bird, Frith, & Ward, 2005). Moreover, when we directly face a person, as our study participants directly faced the child, we appear to simulate their experiences in body parts that mirror those of the individual (Pelphrey et al, 2004).…”

Visual enhancement of touch and the bodily self

“…Additionally, viewing another person's hand may cause social effects that seeing a rubber hand would not. Recent studies, for example, have found that seeing someone else being touched activates primary and secondary somatosensory cortex in a manner comparable to actual tactile perception (Blakemore et al, 2005;Bufalari et al, 2007;Keysers et al, 2004), suggesting a tactile homologue of the human mirror system (cf. Rizzolatti & Craighero, 2004).…”

“…For example, I can experience touch of an object on my own body, but I cannot have a direct experience of touch on someone else's body. My brain may simulate the experience of touch (Keysers et al, 2004;Thomas, Press, Haggard, 2006), and, in the case of some individuals, the brain appears to actually produce a tactile experience when watching another person being touched (e.g., Blakemore et al, 2005).…”

Haptic contents of a movie dynamically engage the spectator's sensorimotor cortex

“…Such automated, vicarious activations contribute to our understanding of the mental and bodily states of other people and facilitate social interaction [for reviews, see, e.g., Frith and Frith, 2007; Hari and Kujala, 2009]. Although the most prominent vicarious activations have been demonstrated in pre‐motor cortical areas [for a review, see, e.g., Rizzolatti and Craighero, 2004], also somatosensory cortices are engaged during observation of actions [e.g., Avikainen et al, 2002; Möttönen et al, 2005; Oouchida et al, 2004; Rossi et al, 2002] or while seeing other people being touched [e.g., Blakemore et al, 2005; Ebisch et al, 2008; Keysers et al, 2004; Martínez‐Jauand et al, 2012; Meyer et al, 2011; Pihko et al, 2010; Schaefer et al, 2009]. …”