Sliman Bensmaia, PhD, assistant professor of organismal biology and anatomy at the University of Chicago, studies the neural basis of tactile perception, or how our hands convey this information to the brain. In a new study published in the Journal of Neuroscience, he and his colleagues found that the timing and frequency of vibrations produced in the skin when you run your hands along a surface, like searching a wall for a light switch, play an important role in how we use our sense of touch to gather information about the objects and surfaces around us.
The sense of touch has traditionally been thought of in spatial terms, i.e. receptors in the skin are spread out across a grid of sorts, and when you touch something this grid of receptors transmits information about the surface to your brain. In their new study, Bensmaia, two former undergraduates, and a postdoctoral scholar in his lab—Matthew Best, Emily Mackevicius and Hannes Saal—found that the skin is also highly sensitive to vibrations, and that these vibrations produce corresponding oscillations in the afferents, or nerves, that carry information from the receptors to the brain. The precise timing and frequency of these neural responses convey specific messages about texture to the brain, much like the frequency of vibrations on the eardrum conveys information about sound.
Neurons communicate through electrical bits, similar to the digital ones and zeros used by computers. But, Bensmaia said, “One of the big questions in neuroscience is whether it’s just the number of bits that matters, or if the specific sequence of bits in time also plays a role. What we show in this paper is that the sequence of bits in time does matter, and in fact for some of the skin receptors, the timing matters with millisecond precision.”
Researchers have known for years that these afferents respond to skin vibrations, but they studied their responses using so-called sinusoidal waves, which are smooth, repetitive patterns. These perfectly uniform vibrations can be produced in a lab, but the kinds of vibrations produced in the skin by touching surfaces in the real world are messy and erratic.