Now Tony Ro, a neuroscientist at the City College of New York and the Graduate Center of the City University of New York, might have figured out the cause of this synesthesia. Sophisticated imaging of the womanâ€™s brain revealed that new links had grown between its auditory part, which processes sound, and the somatosensory region, which handles touch.
â€œThe auditory area of her brain started taking over the somatosensory area,â€ says Ro, who used diffusion tensor imaging, which focuses on the brainâ€™s white matter connections, to spot the change.
This connection between sound and touch may run deep in the rest of us as well, Ro and colleagues said during presentations May 25 at a meeting of the Acoustical Society of America. Both hearing and touch, the scientists pointed out, rely on nerves set atwitter by vibration. A cellphone set to vibrate can be sensed by the skin of the hand, and the phoneâ€™s ringtone generates sound waves â€” vibrations of air â€” that move the eardrum.
Elizabeth Courtenay Wilson, a neuroscientist who did not attend the Seattle meeting, has also seen strong connections between areas of the brain that process hearing and touch. â€œWeâ€™re suggesting that the ear evolved out of the skin in order to do more finely tuned frequency analysis,â€ adds Wilson, of Beth Israel Deaconess Medical Center in Boston.
Wilson earned her Ph.D. in an MIT laboratory focused on studying whether vibrations could boost hearing aid performance. She published a series of papers showing that people with normal hearing were much better at detecting the combination of an extremely weak sound and an extremely weak vibration applied to the skin than either stimulus on its own.
Other researchers have shown that hearing a sound can boost touch sensitivity. Ro calls this the mosquito effect: The bugâ€™s buzz makes our skin prickle. The frequency of the sound and the frequency of the vibrations our hands feel must match for this to work, according to a 2009 paper he published in Experimental Brain Research.
Frequency may be a two-way street in the brain that unites these two senses, says Jeffrey Yau, a neuroscientist at the Johns Hopkins University School of Medicine in Baltimore. A vibration that has a higher or lower frequency than a sound, he found, tends to skew pitch perception up or down. Sounds can also bias whether a vibration is perceived.