Have you ever wondered how the blind can navigate with canes in their hands? The secret lies in an interesting part of the brain responsible for the sensation of touch, that sometimes works in ways we do not even expect!
Neurobiologist, Luke Miller, was once playing with a curtain rod in his apartment when he realized he could sense where objects were touching that rod, without even looking at it! He then went on to study the tool in more detail in a lab, because while scientists had long studied how humans used such tools, the sensory aspect was neglected.
In a 2018 Nature study, Miller and his colleagues at Claude Bernard Lyon 1 University in France reported that humans are actually quite good at pinpointing where an object comes into contact with a handheld tool using touch alone, as if the object were touching their own skin. Results in a follow-up study revealed that the brain regions involved with sensing touch on the body similarly processes it on the tool. “The tool is being treated like a sensory extension of your body,” Miller said.
The initial study involved 16 participants sensing different touches on a one-meter rod, a total of 400 times. When the rod was touched on different locations, the participants did not respond, but when it was touched on the same spot, they would press a pedal with their feet. On average, in 96% of the instances, the participants detected these touches accurately.
What’s interesting is that scientists checked the participants’ brain activities during the experiment, using scalp electrodes, and they found that the cortex rapidly processed where the tool was touched. When the rod was touched in the same location twice in a row, there was a marked suppression of neural responses in brain areas previously shown to identify touch on the body, including the primary somatosensory (touch) cortex and the posterior parietal cortex.
Some of the same subjects were then touched on their arm instead of the rod, and the brain was recorded to have similar repetition suppression in the same regions of the brain, on similar time scales, showing that neural mechanisms for detecting touch location on tools were very similar to touches on our own body. And while the rod vibrated for around 100 milliseconds after each contact, Miller mentioned that our brain detects the location dozens of milliseconds even before that.
Scott Frey, a cognitive neuroscientist researching neuroprosthetics at the University of Missouri, who was not involved with the studies, believed that the results could help inform the design of better prostheses because it suggests that “insensate objects can become, potentially, ways of detecting information from the world and relaying it toward the somatosensory systems,” he says. “And that’s not something that I think people in the world of prosthetics design really thought about. But maybe this suggests that they should. And that’s kind of a neat, novel idea that could come out of it.”