Touch and hearing are usually thought of as separate senses. A recent study of identical twins suggests that they have more in common than is generally thought and may share a genetic basis.
Hearing is so crucial to daily life that people usually notice when hearing loss is occurring. This is one reason that hearing impairments have been so extensively studied. There are 60 known mutations that impair hearing and another 60 that are suspected of doing so.
The researchers also found a connection between the subjects' hearing and touch abilities--the better an individual's hearing, the better was their sense of touch, while poor hearing often was accompanied by a poor sense of touch.
But defects in touch tend to be more subtle and may go unnoticed. There have been comparatively few studies on them and there are no known mutations that cause touch insensitivity.
Because identical twins have identical genes while fraternal twins share only about 50% of the same genes on average, any trait governed by genes should vary less among the identical twins than among the fraternal twins. That's exactly what the study found for touch sensitivity, indicating that it is inherited to some degree--under genetic control. The genetic component (heritability) appeared to be half as strong for touch (0.28) as it was for hearing (0.52).
The researchers also found a connection between the subjects' hearing and touch abilities--the better an individual's hearing, the better was their sense of touch, while poor hearing often was accompanied by a poor sense of touch. This suggests that both hearing and touch might be governed by the same genes, though it doesn't prove it.
Blind subjects, on the other hand, tended to have enhanced touch sensitivity.
Hearing was measured by the ability to detect high frequency vibrations. Touch was measured by having the subjects press their fingers onto fine gratings with ridges spaced at intervals rangers from less than a millimeter to about a centimeter and seeing how fine an interval the subjects' fingers were able to sense.
The best evidence that genes that govern hearing also govern touch came from a group of subjects with Usher's syndrome, a hereditary condition that causes both deafness and blindness. These subjects also showed a highly impaired sense of touch. This suggests that the gene USH2A, which is mutated in people with Usher's syndrome, plays a role in both the detection of sound and in the sense of touch.
The researchers plan future studies to see if other known genetic defects that cause hearing impairment also impair touch sensitivity.
An article on the study appears was published by PLoS Biology on May 1, 2012 and is freely available.