https://www.youtube.com/watch?v=0e8zAWt_TnIPsychologists have used hypnosis to give people the ability to see numbers as colors.

That form of synesthesia is naturally possessed by roughly one in 1,000 people, among them such historical luminaries as physicist Richard Feynman and writer Vladimir Nabokov, who saw "q as browner than k, while s is not the light blue of c, but a curious mixture of azure and mother-of-pearl."

Observations like these, long dismissed as extravagant fantasy, are now considered a window into the mysteries of perception. But despite a surge of scientific interest, synesthesia's mechanisms remain unknown.

Researchers have settled on two possible explanations. Each begins with a human infant born with connections between different sense-related brain regions. According to one hypothesis, these are lost during development, with synesthetics somehow hanging on to them or growing new connections. According to the other, the synesthetic connections merely atrophy, but can be accessed under the right conditions.

The latest findings, published in Psychological Science, support the atrophy explanation — and though caveats remain as to whether hypnosis-induced synesthesia is equivalent to the natural kind, they raise the possibility that the potential for synesthesia is actually quite common.

"The fact that they induced it so quickly means that the brain's not sprouting new neurons or making new connections," said Lawrence Marks, a Yale University psychologist and synesthesia researcher who was not involved in the study. "Maybe the connectivity always exists."

The researchers, led by Roi Kadosh of University College, London and

Luis Fuentes of Spain's University of Murcia, put three women and one man under hypnosis, then instructed them to perceive digits in color:

one as red, two as yellow, three as green, and so on.

Upon waking, the subjects found it difficult to find numbers printed in black ink against correspondingly colored backgrounds. The numbers seemed to blend in — a telltale sign of synesthesia. When the hypnosis was removed, the ability vanished.

How the synesthesia formed so suddenly isn't clear, but the researchers said that new neural connections are probably not responsible. "Such new anatomical connections could not arise, become functional, and suddenly degenerate in the short time scale provided by the current experiment," they wrote.

Instead they suggest that hypnosis broke down neurological barriers between sensory regions. Marks agreed, but cautioned against extrapolating the findings too broadly: Many different varieties of synesthesia exist, from seeing emotions to tasting sounds, and may have different neurological and psychological origins.

Further attempts at inducing synesthesia are required, said Marks, who suggested that people may someday be able to synesthetize themselves through hypnosis. There is, however, a catch.

"In theory, anybody could — but only if they were hypnotizable," he said.

Induced Cross-Modal Synesthetic Experience without Abnormal Neuronal Connections [Psychological Science] (not yet online)

Image: Lady Orlando

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