Weird illusion makes you think fabric is moving faster than it is

Your brain can be tricked into thinking rough materials move faster than smooth ones. The illusion shows how our brains use skin vibrations to judge both speed and texture – and sometimes confuse the two.

Sliman Bensmaia at the University of Chicago and his colleagues developed an experiment to test this after noticing that our perception of a material’s texture has little to do with the way our fingers explore it.

This is unusual because signals fired from the skin to the brain are hugely dependent on the movement, and especially the speed, of our fingers over the material.

To see how the sensation of different textures affect speed perception, Bensmaia and his team blindfolded participants and strapped their right hand down with their palms facing up and index fingers extended. The researchers dragged different materials – such as corduroy, stretch denim, microsuede, metallic silk, wool and vinyl – over the participants’ fingers at speeds ranging from 20 to 120 millimetres per second. Participants were then asked to judge the speed of pairs of materials, saying which was moving faster or slower.

Bensmaia found that rougher textures tended to feel like they were moving faster than smooth ones. For instance, thin corduroy was judged to be slightly faster than thick corduroy. They also measured the vibrations on the skin, which increased with both rougher textures and higher speeds.

“The stronger the vibrations, the faster the surface was felt to move,” says Bensmaia. The effect was striking: even when moving at 20 millimetres per second, a coarse fabric huck towel felt faster to the participants than microsuede did moving at 120 millimetres per second.

When the team tested this on Rhesus macaques, they found that a specific population of nerve fibres that are sensitive to skin vibrations play a key role in this perception. “The more active these nerve fibres are, the faster the surface is felt to move,” says Bensmaia.

Some neurons are sensitive to both speed and texture, and others only to texture. Bensmaia says our perception of speed depends on both. Those sensitive to both speed and texture tell us how fast a material is moving across the skin. “The texture sensitive ones tell us how much of that signal can be attributed to the texture, but we don’t listen to the latter enough, so our perception of speed is still biased,” he says.