Synaesthesia, from the Greek 'syn', meaning 'together', and 'aisthesis', meaning 'sense', is a sensory processing disorder in which the senses are 'crossed' - stimulation of one sensory modality leads to simultaneous stimulation of another. For synaesthetes, people who suffer from synaesthesia, this means that they may experience dual sensations that the rest of us do not; in colour-grapheme synaesthesia, perhaps the most common form of the disorder, patients will associate a particular colour with a particular grapheme (a written symbol, such as a letter, number, punctuation mark, or other character). Whenever they see a letter, they also see its corresponding colour. Many other kinds of synaesthesia also exist, including sound-colour synaesthesia, in which patients see colours when they hear sounds, and, rarely, lexical-taste synaesthesia, in which words register as particular tastes.


There are several theories for why synaesthesia occurs. Some argue that it occurs as a result of cross-activation between different sensory systems in the brain. Another possible explanation for synaesthesia is that, in synaesthetes, the neural connections between the different sensory areas of the brain that would normally have been 'pruned' over the course of development have remained.

Another theory regarding synaesthesia involves disinhibited feedback. According to this explanation, in the brains of synaesthetes, signals from later sensory processing are not inhibited as normal, allowing them to feed back into earlier stages of multi-modal processing, resulting in the mixed signals common to synaesthesia.

Strong evidence for the cross-activation theory of synaesthesia can be found in in a study of synaesthetic experiences in 3- to 4-month-old infants by Walker et al. (2010). Using the preferential looking paradigm (PLP), Walker et al. presented infants with visual displays accompanied with auditory stimuli that were either congruent (e.g., a bouncing orange ball on a black field accompanied by a sliding whistle sound that went up in pitch when the ball went up, and down in pitch when the ball went down; a morphing shape that went from rounded edges to sharp and pointy edges and back while a sound went from low to high pitch and back) or incongruent (e.g., a bouncing orange ball as in the congruent condition, but with the sound going down in pitch when the ball went up and vice versa; the same morphing shape as in the congruent condition, but with the pitch of the auditory stim going down as the shape became more pointed and vice versa). Infants preferred to look at the displays when the audio was congruent with the visual display, presenting what Walker et al. believe to be the strongest evidence to date that 'synaesthetic cross-modality correspondences are an unlearned aspect of perception.' (21)

Issues in the study of synaesthesia

One of the greatest issues facing the study of synaesthesia as a disorder involves finding subjects. It is not a particularly common disorder, and researchers also face the issue of ensuring that people who report experiencing synaesthesia actually do suffer from it. Several tests do exist to diagnose synaesthesia, including tests relating to the Stroop effect and checking to see whether a patient's reported associations remain the same over time.

Another issue, and one linked to the first, is the variability involved; not only are there different kinds of synaesthetes in terms of which modalities are involved, synaesthetes also vary in how early or how late in sensory processing the 'mixing' happens.

For a far more thorough discussion of synaesthesia, please visit the Synesthesia group's page.

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