The look of music.

The look of music

Spearmint is a cool column, so smooth it must be glass. Lemon pricks face, arms and hands with sharply pointed spears. Taste has shape, sound has color.

Synesthetes are people with a brain condition that leads to a hallucinatory welding of senses. And the poetry of their perceptions, neurologist Richard Cytowic suggests, may be a matter of relating to the world in an evolutionarily older manner.

Synesthesia has been described for centuries, says Cytowic, who has a private practice in Washington, D.C., but because there was neither technology nor vocabulary in neurophysiology for an understanding of the phenomenon, it was chalked up as a psychological quirk. However, it is "brain-based, not mind-based," Cytowic says.

Cytowic measured the blood flow, simultaneously, to various regions of the brain. Among synesthetes, he found that, compared with controls, flow throughout the brain was reduced during sensory stimulation; the greatest reduction was in the blood going to the cortex, where sensory sensations are normally processed. According to Cytowic, the tests show that during synesthesia, "the cortex is going on the back burner ... everything is turned down."

Cytowic also studied the effect of context on stimulus perception -- whether, for instance, changing the musical context of a note in relation to other notes would change its perceived color. He found that context was relatively unimportant, indicating that the senses are linked "neither at the lowest level [of stimulus processing and response], like a knee jerk, nor at the highest level," where imaginative or metaphoric connections might be made. The limbic region, an evolutionarily older part of the brain where neurons from all senses converge, is the most likely site of the sensory mixing, Cytowic says.

The limbic system in humans is very similar to that in more primitive mammals, which have a less elaborate cortex. Synesthetes are "living cognitive fossils," Cytowic says. "They have a more fundamentally mammalian way of perceiving."