synesthesia, neuropsychological trait in which the stimulation of one sense causes the automatic experience of another sense. Synesthesia is a genetically linked trait estimated to affect from 2 to 5 percent of the general population.
Grapheme-colour synesthesia is the most-studied form of synesthesia. In this form, an individual’s perception of numbers and letters is associated with colours. For this reason, in all the subject reads or hears, each letter or number is either viewed as physically written in a specific colour (in so-called projector synesthetes) or visualized as a colour in the mind (in associator synesthetes). Many synesthetes, however, have more than one type of synesthesia. A number of types have been reported, ranging from music-colour synesthesia, in which musical notes and sounds are associated with colour visualization, to tactile-emotion synesthesia, in which certain fabrics and textures conjure certain emotions in the synesthete. Examples of other types include sound-colour, spatial sequence, flavour-temperature, flavour-sound, sound-smell, time units-colours, and personality-smell.
Contemporary models agree that synesthesia involves communication between regions of the cerebral cortex in the brain that are not otherwise connected in nonsynesthetes. In essence, this suggests that in grapheme-colour synesthetes, for example, the visual/colour portion of the brain retained excess neuroconnections with the semantic/letter-processing area during development. A popular theory regarding these connections in synesthetes is that of neural pruning: excess neural connections that typically are pared away in development remain intact, and thus synesthetic neuropathways persist. Another theory suggests that neural connections between cortical regions are maintained in every person but that only some people fully experience synesthesia.
DNA analyses have suggested that several chromosome regions may be involved in synesthesia. For example, an area on the second chromosome that is linked to the TBR1 gene is thought to be involved in the synesthetic experience. Although it was once thought to be controlled by genes on the chromosomes that determine sex, the condition does not appear to be sex-linked. Some studies have suggested, however, that there may be a complex genetic linkage between synesthesia, autism, and savantism.
Though English philosopher John Locke and physician Thomas Woolhouse appear to have mentioned synesthesia or synesthesia-like conditions as early as 1689–90 and 1710, respectively, it generally is accepted that German physician Georg Tobias Ludwig Sachs provided the first medical report of synesthesia, in a thesis published in 1812. It then became a popular research subject until the early 1900s, when it was considered to be too subjective. Synesthesia research began to flourish again in the 1980s, when technical equipment was able to demonstrate that it was indeed a palpably discrete genetic condition. Synesthesia remains an area of active research, because of both its own uncomprehended nature and its neurophysiological similarity to more detrimental conditions, such as autism and schizophrenia. Synesthesia is a phenomenon that is largely a gift to those who experience it, as many synesthetes have an aptitude for the arts, a strong sense of creativity, and increased memory skills.