When you have your cell phone set to vibrate, you can usually tell what kind of message you’re getting without looking at it. On an iPhone at least, a series of long, sustained vibrations is a phone call. Two shorts bursts is a text message, and a single pulse is usually something from another app, like a reply on Twitter or a baseball score.
The reason you can tell the difference is because the receptors embedded in your hand (or in the side of your leg if you carry your phone in your pocket) respond to these vibrations and carry a message that’s meaningful to the brain. But it doesn’t stop at the skin. New research by UChicago neurobiologist Sliman Bensmaia shows that the nervous system reproduces the same frequency and intensity of these vibrations in the nerves, and all the way to the brain.
In December, Science Life spoke to Bensmaia about how vibrations in the skin play a big role in our sense of touch. The skin has receptors that transmit messages to the brain. As you run your hand along a surface, it produces vibrations that convey information about its texture, much like airborne vibrations convey information about sound to the auditory system. These same vibrations are recapitulated, or reproduced, in the nerves that carry this information to the brain.
For this study, published in PLOS Biology, Bensmaia and his colleagues, Michael Harvey, Hannes Saal and Frank Dammann, took their experiment one step further and looked at the response of neurons in the primary somatosensory cortex, the part of the brain that interprets touch. Using a probe attached to a special motor that could produce vibrations at any frequency and strength, they gently touched the hands of rhesus macaque monkeys and monitored the elicited neural response.
Bensmaia said what they found was surprising. The neural response could follow the frequency of the skin vibrations up to 800 Hz (the natural world doesn’t produce tactile vibrations much beyond this frequency). That means that the neurons in the brain fire at the same rate and, more importantly, in the same pattern as vibrations produced by the surface texture of any object we’re likely to encounter in the real world.
“800 Hz is really on the high end of what you actually experience,” Bensmaia said. “That, I think, is very surprising and as far as I know the first time that’s ever been shown in the brain.”
In a separate experiment, the researchers also asked human subjects to distinguish between various vibrations on the skin. Then, they tried to predict these perceptual judgments based on the measured neuronal responses. They found that taking the timing of the neuronal response into account predicted how vibrations are perceived better than going by the strength of the response alone.
“It’s always the timing and the strength, because of course the vibrations don’t just vary in frequency. They also vary in intensity. The question is does the timing matter too?” he said. “We show that you can predict how a vibration is perceived better when you take the timing of the response into consideration.” In other words, the precise timing of spikes in the brain shapes how stimuli are perceived.
A single population of neurons in the somatosensory cortex is attuned to both the amplitude of this signal and its frequency. Bensmaia called this “multiplexing” of information and may be a general processing principle in the brain.
“If you’re a neuron upstream trying to decode what the amplitude of the stimulus is, just count the spikes of the vibration. If you want to know what the frequency is, ignore the counts but look at the timing,” he said. “You have two types of information encoded by the same neurons in two different ways.”
But what does the brain do with these signals? For different sensations such as motion or perceiving the shape of objects, the somatosensory cortex immediately begins to decode the neural response. But for the sense of touch, the neurons simply recapitulate the signal essentially verbatim.
Bensmaia speculates that the job of interpreting the information carried by this neural response is being “outsourced” to some other part of the brain better-suited to handling frequency information, such as the auditory system. This is far from settled though, and Bensmaia and the students working in his lab are continuing their work to understand how the brain processes touch.
If it takes a while to re-learn what each vibration means after you play with the settings on your phone, it’s not just because it’s difficult to remember which one is a call and which one is a text message. It’s also because your whole nervous system is vibrating with it.
Harvey MA, Saal HP, Dammann JF 3rd, & Bensmaia SJ (2013). Multiplexing Stimulus Information through Rate and Temporal Codes in Primate Somatosensory Cortex. PLoS biology, 11 (5) PMID: 23667327