John Mayer has synesthesia What is unusual about his sensory organs?

Answer 1

Nothing is unusual about his sensory organs i.e. eyes, ears etc. A synesthetic perception originates IN the brain and is probably be the result of abnormal connections resulting in communication between parts of the brain that wouldn't normally be communicating. For instance, the visual cortex receives some signalling from the auditory cortex and possibly because some previously learned association a color is associated with sound/notes/or combination thereof.