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Most of us take for granted that our senses – hearing, touch, taste, smell, vision – are discrete. For people with a condition called synesthesia, however, the experience of these sensations is mixed. Some ‘synesthetes’ see colors when hearing musical notes, for example, or feel distinct shapes on the tongue when tasting certain foods – lemon is pointy, for example, or chocolate is round. Now, new research by Professor Tony Ro of The City College of New York and Graduate Center of the City University of New York and his colleagues might explain the origins of this phenomenon. They find that synesthesia may result from a particular type of cross-wiring in the brain, and they show that a form of this kind of crossing of sensory information occurs even in the normal brain.
An example of this can be experienced when hearing the whine of a mosquito hovering around the face. The high frequency alone can induce the feeling of tickling in the skin and heighten the skin’s sensitivity, making one more aware of the light touch of the mosquito’s feet when it lands.
In a recent study, Ro and his colleagues show how this occurs. They had study participants sit in a sound attenuating testing chamber where all sound and touch information delivered could be precisely controlled. Through headphones the subjects wore, they played pure frequency, single note tones or complex broad spectrum sounds. For touch, they applied sensations with small devices called ‘vibrotactile stimulators’ attached to the hands or feet that vibrate much as a muted cellphone does. They found that a region of the normal brain known to process feelings of touch also responds to sounds.
In additional studies, brain structure and function was measured using magnetic resonance imaging, electroencephalography, and optical imaging. For certain tests they disrupted brain activity using transcranial magnetic stimulation to see how different brain areas were involved with interactions between the senses. One study visualized connections between brain areas, finding the existence of physical connections that carry sensory information between brain regions for sound and touch. The researchers then showed that both the overlap in function and the shared brain connections were more pronounced in a brain-damaged patient with acquired sound-touch synesthesia.
These results suggest not only that the two senses use similar processing mechanisms in the body and in the brain, but also that hearing actually evolved out of the sense of touch. Such findings could help develop therapies for the visually impaired to substitute touch sensations for lost vision and could aid rehabilitation after brain damage.