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Have you ever known someone who claims that
he sees colors when listening to music?

This ability is known as Synesthesia.

Stedman's Medical Dictionary defines synesthesia as,

A condition in which one type of stimulation evokes
the sensation of another, as when the hearing of a sound
produces the visualization of a color.

Have you ever wondered how such an experience
is possible?  Researcher David Eagleman  is seeking
an to answer this question.

In the video below, Dr. Eagleman defines synesthesia
and proposes a neurological basis.