Feeling someone else’s sensation of touch – the neural background, the examples, and you

Touch is the only sensation which we cannot share with another person. The immediacy of touch differentiates it from the distant impressions which sight and audition can give us. However, modern neuroscience is currently revising this picture: you can touch at a distance. One just doesn’t notice it. Can we find people who do?

A very possessive romantic partner may mind when his love interest was looked at. But it is a whole different game if touch was involved. There is something intuitively different about touch which pervades every day culture. It is, arguably, the lack of distance, the necessary intrusion of the touching object into personal space. This makes touching a very personal experience, far more so than seeing or hearing.
Modern neuroscience is currently revising this picture. The first report to challenge the immediacy of touch came out in 2004. A team led by Christian Keysers found that when people saw someone else being touched on the leg they showed activation in the same brain area as when their legs were being touched directly. Curiously, seeing touch from a first person perspective led to similar brain region activity as seeing it from a third person.
What it looks like to touch and being touched in the secondary somatosensory cortex.

What it looks like to see touch and being touched in the secondary somatosensory cortex.

This got several laboratories around the world started on the topic. For example, recently, Schaefer and colleagues showed that when hands are touched or seen to be touched, perspective does actually matter – a first person view-point increases activation more in primary areas than a third person perspective. In any event, the general picture was not a statistical fluke but instead a replicable finding – being touched is represented in a very similar way in the brain as seeing someone else being touched. But this raises two questions: why can’t I feel anything then and why does it happen?
In 2009, Ramachandran and Brang published a paper which may provide an answer to the first question. They studied four amputees who had lost one hand due to accident. When they watched an experimenter being touched on her hand, the lost hand’s phantom ‘felt the touch’ after a few seconds. One anecdote shows the power of this finding:
‘Patient 1 even added that after we had demonstrated this, he had gone home and asked his wife to massage her own hand while he watched, and watching her do so seemed to relieve his phantom pain.’
Importantly, this was not the case for intact hands – whether of control participants or the amputees. The difference, thus, appears to be whether the sensation felt by others is in competition with the own direct input from the skin. If so, the own touch wins the competition and one does not consciously feel someone else’s experience. But without a hand providing direct sensory input – as in the case of amputees – the touch felt by others becomes vivid.
Apes (of the non-human variety) collaborating.

Apes (of the non-human variety) collaborating.

This still leaves the question as to why this happens. The common explanation is that having the capacity to feel the touch of someone else – even if it is so faint as to be below the level of awareness – aids our ability to understand others. As a social species we need a high level of empathy in order to work together efficiently. Evolutionary ancestors who had a touch-empathy link may have been better at collaborating and, thus, were better able to survive and reproduce.
The current account makes an interesting prediction. Next time you have an anaesthetized hand or foot – and thus no own skin-sensation – you might want to check whether you can feel someone else’s touch. Let me know whether it worked. This experiment has not, as far as I can tell, be done, yet. You yourself could disprove the immediacy of touch.

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Keysers C, Wicker B, Gazzola V, Anton JL, Fogassi L, & Gallese V (2004). A touching sight: SII/PV activation during the observation and experience of touch. Neuron, 42 (2), 335-46 PMID: 15091347

Ramachandran VS, & Brang D (2009). Sensations evoked in patients with amputation from watching an individual whose corresponding intact limb is being touched. Archives of neurology, 66 (10), 1281-4 PMID: 19822785

Schaefer M, Heinze HJ, & Rotte M (2012). Close to you: embodied simulation for peripersonal space in primary somatosensory cortex. PloS one, 7 (8) PMID: 22912698

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Figures:

1) adapted from Keysers et al., 2004, p. 337

2) By Ikiwaner (Own work)  via Wikimedia Commons

ResearchBlogging.org

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