Friday, July 30, 2010

Making Sense of Sense - The roads less travelled – four paths to get from touch to the body

By Nadia Barnsley, a third year medical students from the University of New South Wales.

Nadia's research interests include bodily awareness, body ownership and immune responses. She is doing an independent learning project on Body in Mind research. Here she reviews Serino and Haggard’s paper “Touch and the Body” which was published in Neuroscience and Biobehavioural Reviews.

Serino and Haggard’s paper gives a four-part model that explains the notion that our sense of touch carries information about both the external object touching our skin and also our own body. Tactile information can influence (or be influenced by) our mental representation of the physical body. Mental body representations (MBR) are simply descriptions in our mind of the parts of the body, their position in space and their organization into a structural whole (us!)

Serino and Haggard firstly examined the idea that the physical body structures tactile sensation – that when we touch or get touched, tactile afferents will map this into a homunculus, “little man”, in the parietal lobe of the brain. This information is conveyed to what’s known as S1 (the primary somatosensory cortex) of the opposite hemisphere.

Serino and Haggard’s second pathway explains that tactile information provides an important afferent input to mental body representations. Importantly they clarified the difference between body schema and body image; where body schema is short-lived and represents the positions of body parts in space, whereas body image remains fixed over time, representing a basic appearance of the body as an object in third person perspective.

Interestingly they reviewed how visual information relates to tactile sensation; the third pathway. They found that tactile acuity improved when subjects viewed the body. Moreover, this is independent from visuo-spatial orienting to the location of the body. The paper explained those with poor tactile ability will have vast improvements in tactile sensation when visual information is added. This has clinical implications for those with reduced sensation following brain lesions.

The last pathway explained how MBRs not only contribute to our body perception, but the perception of external objects. Basically, our touch of external objects is ‘body referenced’. This implies that MBRs are not just a stored body image; they are updated to integrate current sensory information.

So the first pathway tells the brain where we are being touched, the second pathway gives our brain a representation of what we are (our body image), the third is that this representation influences how we understand our sense of touch and the last being that our body image alters how external objects are perceived.

Reference: Serino A, & Haggard P (2010). Touch and the body. Neuroscience and biobehavioral reviews, 34 (2), 224-36 PMID: 19376156 Epub 2009 Apr 17.

Touch and the body
Serino A, Haggard P. Dipartimento di Psicologia and Centro studi e ricerche in Neuroscienze Cognitive, Università degli Studi di Bologna, Italy.

The dual nature of touch has long been understood. The sense of touch seems to carry information at the same time about the external object touching our skin, and also about our body itself. However, how these two interact has remained obscure. We present an analytic model of how tactile information interacts with mental body representations in the brain. Four such interactions are described: the link between the body surface and the maps in primary somatosensory cortex, the contribution of somatosensory cortical information to mental body representations, the feedback pathway from such higher representations back to primary tactile processing in somatosensory cortex, and the modulation of tactile object perception by mental body representations.

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