Sometimes when we touch, honesty is absent

Close your eyes and pull a length of textile – a ribbon or necktie, say – through your fingers.

What you feel, researchers have now established, cannot be trusted. The perception of the speed at which any substance moves across skin is a function of the nerve fibres within the dermis and the texture of the material involved.

In short, the feeling of speed is not wholly dependent on actual velocity. That’s the conclusion reached by scientists led by Sliman Bensmaia of the University of Chicago, US, and published in the journal PLOS Biology.

To make their findings Bensmaia and colleagues used two approaches. 

In the first, blindfolded human volunteers had a range of fibres run across their fingertips at varying speeds. Sometimes, fingers on both hands were involved, with the subjects asked to estimate which of two fibres was moving faster. 

In the second part of the research, different fibres at different speeds were run across the fingertips of monkeys – some anaesthetised, and others conscious – and their brain activity monitored.

The results, among the humans, showed a consistent pattern.

“We found that the perception of speed is heavily influenced by texture: some textures are systematically perceived as moving faster than are others, and some textures provide a more informative signal about speed than do others,” the researchers conclude.

Silk, it turned out, was particularly liable to fool humans, with the volunteers consistently assessing it to be moving faster than it was. Corduroy was usefully indicative, with a thinly ribbed version being judged to be moving faster than a thick version, despite identical velocities.

Other textures tested included vinyl, denim and crocodile skin.

The monkey studies revealed that a particular type of nerve cell tissues – called Pacinian corpuscle–associated (PC) fibres – were triggered by movement across the skin. Other types of nerve cells – in families known as “rapidly adapting” and “slowly adapting” – also increased activity in response to speed, but in less pronounced and less consistent manners.

Crunching the numbers across both human and non-human experiments Bensmaia and colleagues found that the activity of the PC nerves was by itself sufficient to explain the differences between perceived speed, actual speed and textile type.

“In summary,” they write, “the firing rates of tactile nerve fibres systematically increase with increasing speed, but the perceptual sensitivity to speed and the dependence of perceived speed on surface texture seem to reflect PC firing rates.”