The speed of a caress

Put a hand on your forearm. Almost at once you know where it is, how hard it presses, whether it is skin or cloth. That news travels on thick nerve fibres wrapped in insulation, the same wiring that tells your fingertips what they are holding.

Under the same skin runs a second kind of fibre: thin, bare and slow. Researchers find it by sliding a fine needle into a nerve of an awake volunteer and listening to single fibres, one at a time, through a loudspeaker. Each impulse is a click.

They called it a C-tactile fibre. It answers to light, moving touch on hairy skin, like your forearm. On the palm, such fibres are rare.

For years nobody knew what it was for. It carries much less spatial detail: on its own, it can say only roughly where you were touched.

GL

At 31, a woman known in the research papers as GL permanently lost the large nerve fibres that carry touch. In daily life she says she cannot tell where, or whether, she is touched anywhere below her nose.

Håkan Olausson and colleagues stroked her forearm slowly with a soft brush. When she concentrated, she could tell it was there. She called it faint, and pleasant.

On the palm of her hand the same brush found nothing.

In the scanner, the brush on her arm activated her insula, a region involved in how the body feels from the inside and in emotion, and not the areas that normally map touch.

The fibres she had left could hardly tell her where she was touched. What reached her was a faint touch, and that it was pleasant.

So this slow fibre has something to say about tenderness. Here is your chance to hear it.

The slow fibre really is slow. Its impulses travel at about 0.8 metres a second, slower than a walk. In a separate study of the hand, fast touch fibres averaged about 59 metres a second, some 70 times faster.

Nobody has measured whether the pleasant part of a caress is felt later than the touch itself. What has been measured is that a late wave in the brain's electrical response to a slow brush begins about 0.7 seconds after the brush meets the skin, late enough to fit the slow fibre.

A fibre this slow is no good for catching a falling cup. Its timing suits sustained touch, not split-second judgements.

Parents of babies four to sixteen weeks old were filmed while they stroked their child, and their speed was measured from the video afterwards. Mothers averaged about 7.9 centimetres a second, fathers about 7.1.

Croy and colleagues saw something similar when people stroked a partner or their baby: slower than they stroked an artificial arm, and every one of them at speeds known to activate these fibres.

Babies respond differently to the speed. In 16 nine-month-olds, heart rate fell slightly, by about 1 per cent, while a brush moved along the arm at 3 centimetres a second, and rose slightly at 0.3 and at 30. Merle Fairhurst and colleagues found the difference from the fast stroke clear; from the slow one, less certain. They note the babies may simply have paid more attention.

In a trial with 92 babies born early, in a hospital in Milan, five minutes of stroking at about 3 centimetres a second lowered heart rate and raised blood oxygen more than still touch did. And in 13 babies only 11 to 36 days old, a slow brush on the leg already reached the insula as well as the touch areas of the brain. Only one speed was tried there, so this cannot yet be pinned on the slow fibre.

The fibre has one more preference. When the probe stroking the arm was at 32 °C, the temperature of skin, it fired about 39 times a second at 3 cm/s; with a cooler probe, at 18 °C, about 28. A warmer probe, at 42 °C, also lowered its firing at most speeds. It is tuned to the warmth of skin.

In the same study, how pleasant people found a stroke tracked the fibre only at skin temperature. The fibre is part of the story of a caress, not all of it.