A part of the brain called the insular cortex appears to be behind why a tender stroke can feel so nice.
Parts of the skin that have hairs on them, such as the backs of hands but not the palms, have nerve fibres, which respond to gentle touch. Normally when mammals are touched, these fibres send a signal through the spinal cord to the part of the brain called the primary somatosensory cortex, which responds to changes on the surface of the body.
But for pleasurable touch, this signal takes a detour to the insular cortex first, which is involved in self-awareness. To understand how these pleasure signals are processed, Louise Kirsch at Sorbonne University in France and her colleagues compared touch responses of 59 people who had strokes with 20 who hadn’t.
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The team gently touched the participants with a soft-bristled brush on their right and left forearms at two speeds – the slower of the two is known to stimulate the pleasure-sensing nerve fibres. They asked everyone to rate how intensely they felt the touch and how pleasant it was on a scale of 0 to 10, where 0 was the feeling of sandpaper and 10 was the feeling of velvet.
All the participants found the slower brush stroke speed more pleasurable, but some of those who had experienced a stroke rated it less so than the control group.
When the team analysed the brain scans of the participants, they found that over 80 per cent of people who’d had a stroke and reported less sensitivity to pleasant touch on their arm had a lesion on the their insular cortex, suggesting that the disruption in enjoying pleasurable touch is tied to this part of the brain.
The nerve fibres involved in pleasure were only discovered in humans in the 1990s, says Francis McGlone at Liverpool John Moores University in the UK. We’ve previously found that they related to the insular cortex, but this study gives more concrete evidence, he says.
Biorxiv
