Brain Advance Access originally published online on April 7, 2005
Brain 2005 128(7):1571-1583; doi:10.1093/brain/awh500
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Somatosensory activations during the observation of touch and a case of vision–touch synaesthesia
1 Institute of Cognitive Neuroscience, 2 Wellcome Department of Imaging Neuroscience and 3 Department of Psychology, University College London, London, UK
Correspondence to: Dr S.-J. Blakemore, Institute of Cognitive Neuroscience, Department of Psychology, University College London, 17 Queen Square, London WC1N 3AR, UK E-mail: s.blakemore{at}ucl.ac.uk
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.
Key Words: mirror neurons; somatosensory cortex; premotor cortex; tactile perception; fMRI
Abbreviations: fMRI = functional MRI; HLF = human left face; HLN = human left neck; HRF = human right face; HRN = human right neck; LF = left face; LN = left neck; MEP = motor evoked potential; OLF = object left face; OLN = object left neck; ORF = object right face; ORN = object right neck; RF = right face; RN = right neck; SI = primary somatosensory cortex; SII = secondary somatosensory cortex; STS = superior temporal sulcus; TMS = transcranial magnetic stimulation
Received July 28, 2004. Revised January 11, 2005. Accepted March 9, 2005.
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