In the peculiar world of synesthesia, people experience an involuntary joining of different sensations. These individuals may, for example, feel intense facial pressure when listening to music or see vivid colors in response to spoken words.
“Colored hearing,” probably the most common form of synesthesia, arises in the brain through a process similar to that responsible for hallucinations of colors, according to brain-imaging data in the April Nature Neuroscience. In these cases, a genetic mutation may foster the development early in life of an unusually direct connection between auditory and visual brain regions, theorize neuroscientist Jeffrey A. Gray of the Institute of Psychiatry in London and his colleagues.
Synesthesia runs in families and occurs more often in women than in men. This pattern suggests that a sex-linked genetic disturbance yields jumbled sensory experiences, the researchers say. However, other investigators argue that healthy brains mix senses together in the early, unconscious stages of perception. In their view, individuals with synesthesia somehow become conscious of these sensory blends.
Whatever the case, the new findings cast doubt on theories that dismiss synesthesia as the product of overactive imaginations and instead “lend such phenomena an authenticity beyond reasonable doubt,” Gray’s group says.
The scientists used functional magnetic resonance imaging to track blood-flow changes in the brains of 13 women who reported seeing colored patterns and shapes while listening to people talk and 28 women who had no such experiences.
These scans provide an indirect measure of surges and declines in brain activity.
While listening to strings of spoken words, only women with colored-hearing synesthesia showed pronounced activity in a part of the visual system known to orchestrate color identification, the researchers say. Conscious perception of colors may only require activation of this “color center,” Gray theorizes. Other visual areas, including those that initially handle incoming visual information, remained calm during colored-hearing trials.
In contrast, women without synesthesia exhibited little activity in the brain’s color center while listening to words, even after they received training to visualize colors in response to spoken words.