Is there a relationship between hearing and touch?

There are good reasons to suspect that hearing and touch might have a common genetic basis. Sound-sensing cells in the ear detect vibrations and transform them into electrical impulses. Likewise, nerves that lie just below the surface of the skin detect movement and changes in pressure, and generate impulses. The similarity suggests that the two systems might have a common evolutionary origin—they may depend on an overlapping set of molecules that transform motion into signals that can be transmitted along nerves to the brain.

People with a certain form of inherited hearing loss have increased sensitivity to low frequency vibration. The research findings, which were published in Nature Neuroscience,reveal previously unknown relationships between hearing loss and touch sensitivity. Those suffering from hereditary DFNA2 hearing impairment is caused by a mutation which disrupts the function of many hair cells in the inner ear. This mutation, the researchers suspected, might also affect the sense of touch. Tiny, delicate hairs in our inner ear vibrate to the pressure of the sound waves. The vibrations cause an influx of positively charged potassium ions into the hair cells. This electric current produces a nerve signal that is transmitted to the brain which results in hearing. The potassium ions flow through a channel in the cell membrane and again out of the hair cells. This potassium channel, a protein molecule called KCNQ4, is destroyed by the mutation in such hearing-impaired people. The sensory cells gradually die off due to overload. They found that KCNQ4 is present not only in the ear, but also in some sensory cells of the skin. Clearly there are parallels to hearing, As a first step, the researchers in the Jentsch lab created a mouse model for deafness by generating a mouse line that carries the same mutation in the potassium channel as a patient with this form of genetic hearing loss. The touch receptors in the skin where the KCNQ4 potassium channel is found did not die off due to the defective channel like they did in the ear, but instead showed an altered electric response to the mechanical stimuli in the mutated mouse. They reacted much more sensitively to vibration stimuli in the low frequency range. The sensation of touch varies greatly from person to person -- some people are much more sensitive to touch than others. DFNA2 patients are extremely sensitive to vibrations,

In recent years about 70 genes have been identified in humans, mutations in which trigger hearing loss or deafness. Surprisingly, no genes have been found that negatively influence the sense of touch,

In another study, to see whether the sense of touch also has a hereditary component, the researchers first studied 100 pairs of twins -- 66 pairs of monozygotic twins and 34 dizygotic pairs of twins. Monozygotic twins are genetically completely identical; dizygotic twins are genetically identical to 50 percent. The tests showed that the touch sensitivity of the subjects was determined to more than 50 percent by genes. Furthermore, hearing and touch tests showed that there is a correlation between the sense of hearing and touch.

The researchers decided it would take too much time to analyze which of the approximately 70 genes that adversely affect the sense of hearing may also negatively affect the sense of touch. Therefore, the researchers focused specifically on patients with Usher syndrome, a hereditary form of hearing impairment, in which the patients progressively become blind. Usher syndrome patients have varying degrees of hearing impairment, and the disease is genetically very well studied. There are nine known Usher genes carrying mutations which cause the disease. The studies revealed that not all patients with Usher-syndrome have poor tactile acuity and touch sensitivity. The researchers showed that only patients with Usher syndrome who have a mutation in the gene USH2A have poor touch sensitivity. This mutation is also responsible for the impaired hearing of 19 patients. The 29 Usher-syndrome patients in whom the mutation could not be detected had a normal sense of touch. The researchers thus demonstrated that there is a common genetic basis for the sense of hearing and touch.

Citations:

1. Max Delbrück Center for Molecular Medicine (MDC) Berlin-Buch. "People with DFNA2 hearing loss show increased touch sensitivity, study shows." ScienceDaily, 12 Dec. 2011. Web. 3 Mar. 2013.
2. Matthias Heidenreich, Stefan G Lechner, Vitya Vardanyan, Christiane Wetzel, Cor W Cremers, Els M De Leenheer, Gracia Aránguez, Miguel Ángel Moreno-Pelayo, Thomas J Jentsch, Gary R Lewin. KCNQ4 K+ channels tune mechanoreceptors for normal touch sensation in mouse and man. Nature Neuroscience, 2011; DOI:10.1038/nn.2985
3. Helmholtz Association of German Research Centres. "Hearing and touch have common genetic basis: Gene mutation leads to impairment of two senses." ScienceDaily, 1 May 2012. Web. 3 Mar. 2013.
4. Henning Frenzel, Jörg Bohlender, Katrin Pinsker, Bärbel Wohlleben, Jens Tank, Stefan G. Lechner, Daniela Schiska, Teresa Jaijo, Franz Rüschendorf, Kathrin Saar, Jens Jordan, José M. Millán, Manfred Gross, Gary R. Lewin. A Genetic Basis for Mechanosensory Traits in Humans. PLoS Biology, 2012; 10 (5): e1001318 DOI:10.1371/journal.pbio.1001318