Those of us without prehensile ears tend to think of our senses of hearing and touch as separate. But our sensory abilities overlap with each other more often than our kindergarten teachers let on. Our sense of smell gets help from our vision centers. Tasting food is mostly done with our noses. And a new study says hearing is just another sense of touch. The same genes can make you good--or deficient--at both.
Hearing a bird chirp and picking up a pencil from your desk, though they seem like wildly different tasks, are really the same trick performed by your body twice. Cells that receive physical input from the outside world (the edge of the pencil against your fingertips, or the vibration of sound waves rattling your inner ear) have to turn that information into an electrical signal and send it back to your brain.
So researchers in Germany guessed that the same genes affect both senses, in people as well as in other vertebrates. They used human subjects to ask the question in several different ways.
First, the team studied a group of more than 500 hearing subjects. In a battery of tests, people listened to tones and clicks, responded to tiny vibrations on their fingertips (measuring their touch sensitivity), and felt surfaces with narrow ridges (touch acuity).
There was plenty of variability in subjects' hearing and touch abilities. Both senses clearly grew worse with age. And in many of the tests, women outperformed men. These data let researchers chart how a healthy person's senses should act throughout their lives.
Then 100 pairs of twins came in for testing. Geneticists love twins: Identical sets have all the same genes, so any differences between them must come from somewhere outside their DNA. Fraternal sets share about half their genes, but are the same age and grew up (usually) in the same environment, so they can be easily compared to identical twins.
Subjecting the twin sets to the same tests, the researchers saw that touch sensitivity and touch acuity both have major genetic components, just like hearing does. And they saw that the two senses correspond to each other: People with good hearing are more likely to have good sense of touch too.
What about people with bad hearing? The researchers recruited another set of 39 teenagers and young adults from a school for the hearing impaired. About a fifth of the deaf subjects had "very poor touch performance."
There are plenty of genetic and non-genetic reasons people are born deaf, and the researchers don't know which of these factors were present in their subjects. But it seems that for some hearing-impaired people, whatever damaged their hearing did the same to their sense of touch. The simple explanation is that the same genetic mutation affects both senses.
Finally, the researchers examined a group of subjects with an illness called Usher syndrome that causes deafness and blindness. Scientists know of several genes that are involved--a mutation in any one of then can cause Usher syndrome. The German team found that patients with a certain Usher gene mutation also had significantly worse touch acuity than normal.
The twins and the deaf young adults showed that hearing and touch go along with each other, seeming to rely on shared genes. The Usher syndrome patients let researchers go a step further and identify one specific gene that affects both senses. "Both senses require cells that convert tiny changes in mechanical force into an electrical signal," said senior author Gary Lewin. "Genes may code for proteins that play a similar role in this process in the two types of cells." In other words, if our cells use the same genes for hearing and touch, they may also share a set of molecular tools.
I asked Lewin whether science is on the path to trimming down our number of senses. If we begin to understand hearing and touch as one mechanism, and if taste barely exists without smell, do we really have three senses? "No, I think we will stick with five and even more senses," Lewin said. "The cellular mechanisms and the way these different sensory cells are connected are highly unique."
Even so, our sensory skills continue to surprise us. Our kindergarten teachers may not have been wrong, but our understanding of human senses is growing up.
Frenzel, H., Bohlender, J., Pinsker, K., Wohlleben, B., Tank, J., Lechner, S., Schiska, D., Jaijo, T., Rüschendorf, F., Saar, K., Jordan, J., Millán, J., Gross, M., & Lewin, G. (2012). A Genetic Basis for Mechanosensory Traits in Humans PLoS Biology, 10 (5) DOI: 10.1371/journal.pbio.1001318