What is Synesthesia?

This is a rather peculiar condition in which the senses get cross-wired. For example, a person with synethesia may see colours when they hear a sound, or can actually taste words; stimulation of one sense, it seems, causes an inappropriate stimulation of another.

The most common form of synesthesia is when people see or hear words in colour. The condition affects about 1 in 25 000 people and is found more commonly in women than men. There appears to be more left handed individuals among synesthetics than in the general population although the significance of this is unclear. Often synesthetics have exceptional memories, have a tendency to have unusual 'psychic' experiences, but have problems at maths or navigation.

No one really knows what goes on in the synesthetic brain. One explanation proposed is that the region of the brain normally receiving input from the ears also gets some information from the eyes. An imaging technique called PET, which follows the flux of radiolabelled blood through the active regions of the brain, showed that areas of the cortex that normally receive information from the eyes are also activated when the person hears a sound. This would suggest that there is some cross wiring in the brain.

Other researchers argue, however, that this is too simple a notion and that it is the limbic system (the centre of emotion in the brain) that differs in synesthetics. Their theory is that the limbic system this lies directly below the cortex pulls together fragments of memories from all over the brain and pastes them together to produce a complete memory. Normally we are not conscious of this process but perhaps synesthetics are; this might explain why inappropriate sensory information pops up in their mind.

Left: This test chart points out easily whether a synesthetic person will be able to see numbers in color. If you are able to see a triangle in red twos, then you have it.



A drawing made by a synesthetic depicting how he sees the sound of a dog barking (Source: Dr. Richard E. Cytowic

One thing is for sure, synesthetics do not think their unusual ability as a handicap. Most are, indeed, rather sorry that the rest of us live in such a colourless world.

To Know More

Chudler,: Synesthesia. Neuroscience for Kids.
Cytowic, R.E.:
Synesthesia: Phenomenology And Neuropsychology. A Review of Current Knowledge. Psyche, 2(10), July 1995
Cytowic, R.E.:
The Man Who Tasted Shapes. Cambridge, MA: The MIT Press, 1998.

Published with permission. Adaptation: Renato M.E. Sabbatini, PhD
Image: Book cover by
Richard E. Cytowic.


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Brain & Mind Magazine 17 (May-August 2003)
An initiative:
Center for Biomedical Informatics
State University of Campinas, Brazil
First Published 25.May.2003