If you have ever observed someone’s facial expression and had that impact your perception of the emotion in their voice, then you have had a minor experience of “synesthesia.” Synesthesia is literally “joined perceptions” and a wonderful example of the complexity of the human mind.
Though infant brains have numerous neuro-anatomical pathways between sensory areas at birth, a process of neural pruning results in few people retaining the ability to experience multi-sensory perceptions.
Though synesthesia can occur between any two senses, some types are more common than others. One of the most common forms is grapheme (symbol)-color synesthesia, in which individual letters or days of the week are imbued with a color. In sound-color synesthesia, voice, music and environmental sounds trigger colors.
Number--form synesthesia involves a mental map of numbers in space. Ordinallinguistic personification is a type of synesthesia in which numbers, letters or words are associated with personalities.
A more rare synesthesia is lexical-gustatory synesthesia in which language sounds and words evoke the experience of taste sensations in the mouth.
Synesthetes (those who experience synesthesia) perform normally on standard neurological exams. However, neuro-imaging does show differences between the brains of synesthetes and non-synesthetes. Specifically, brain scans show that people with synesthesia seem to have "cross-wiring" between brain regions. A study by J. Nunn at Goldsmiths College, London, found that the visual areas of the brain were also activated in response to sound in people who are sound-color synesthetes.
Estimates are that one out of every twenty-three people do experience profound “joined sensations.” However, many synesthetes are not aware that what they experience is rare. Synesthesia in children may slow down reading or make auditory instructions difficult to absorb.
It has been known for some time that synesthesia tends to run in families and now there is evidence of a genetic basis for synesthesia. In 2009 the American Journal of Human Genetics reported that Julian Asher and colleagues in the United Kingdom collected DNA from 43 families in which there were a total of 196 members with auditory-visual synesthesia. They found that synesthesia was linked to regions on chromosomes 2, 5, 6, and 12.
Genes on chromosome 2 have also been linked to autism and epilepsy, both which co-occur with synesthesia more than expected. Both left-right confusion and difficulty with sense of direction have been shown to be more common among synesthetes as well.