ReviewTouch and the body
Section snippets
Introduction and model
Touch is often considered by neuroscientists under the general heading of somatosensation. This already reveals a strong link between tactile sensation and perception on the one hand, and the body on the other. Indeed, the receptor organ for touch, the skin, also forms the surface of the physical body. Although the interdependence between the sense of touch and the body is well recognised, this interaction can take place at a range of different levels within the nervous system, with quite
Pathway 1—the physical body structures tactile sensation
The sense of touch is the phenomenal counterpart of afferent input from mechanoceptors on the body surface. Peripheral signals from the skin are transmitted through the dorsal columns of the spinal cord to the medulla, and project via the postero-lateral thalamus, to contralateral primary (SI; area 3b and 1 in the monkey) and secondary (SII) somatosensory cortices. These are located, respectively in the postcentral gyrus and parietal operculum. Somatosensory cortical areas, and SI in
Pathway 2—tactile sensations contribute to a mental body representation
Processing of tactile information does not end at the primary somatosensory cortex. Broadly speaking, brain areas showing tactile responses beyond SI can be classified in two ways, following Katz's (1925) distinction between exteroceptive and interoceptive touch. On the one hand, areas such as SII are concerned with further processing of tactile object features. Neurons in these areas have larger receptive fields but more precise tuning properties than SI, suggesting a role in feature
Pathway 3—mental body representations reciprocally influence primary tactile processing
So far we have reviewed evidence showing that the tactile representation of the body contributes to more abstract, multimodal representations of the body in the brain. In this section we will show how MBRs in turn influence primary levels of tactile processing. In particular, we focus on how visual information related to the body affects tactile sensation (Fig. 2).
Pathway 4—MBRs mediate the formation of object representations from primary tactile sensations
The tactile interpretation of an object touching the skin is often mediated by a description of one's own body. That is, exteroceptive tactile perception depends on, and implicitly includes, information from MBRs. In this sense, tactile perceptions are always referenced to the body, even if the content of the perception is an external object.
This body-referencing can take at least four forms, which we argue are conceptually quite distinct. First, touch is inevitably body-referenced in the sense
Overall conclusion
To conclude, we have shown that the sense of touch has a close and interactive relation with higher cognitive representations of our own body. Indeed, studies of tactile perception offer one of the few ways to study mental body representations in a well-controlled and quantitative way. Modulations of tactile perception often reflect the contribution of a mental body representation. We have presented an analytic and neurally plausible model, suggesting four key pathways whereby touch and the
Acknowledgements
PH's contribution was supported by BBSRC project grant D009529, and by a research grant from Bial Foundation.
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