10.7: Synesthesia

Synesthesia is a condition in which a sensory stimulus presented in one area evokes a sensation in a different area. In the 19th century Francis Galton observed that a certain proportion of the general population who were otherwise normal had a hereditary condition he dubbed "synesthesia"; a sensory stimulus presented through one modality spontaneously evoked a sensation experienced in an unrelated modality. For example, an individual may experience a specific color for every given note (“C sharp is red”) or printed number or letter- is tinged with a specific hue (e.g. 5 is indigo and 7 is green). The specificity of the colors remains stable over time within any given individual but the same note or letter doesn’t necessarily evoke the same color in different people. Although long regarded as a curiosity there has been a tremendous resurgence of interest in synesthesia in the last decade. Synesthesia used to be regarded as a rare condition but recent estimates suggest that it affects 4% of the population. The most common of which appears to be letter sounds associated with color. Most individuals report having had the experience as far back in childhood as they can remember. As Galton himself noted, the condition tends to run in families and recent work suggests a genetic basis.

Synesthesia was previously believed 6 times more common in women than in men according to responses from newspaper ads. However, Simner and colleagues showed no difference between the sexes testing a large population for synesthesia. Sometimes, sensory deficiency can lead to one sensory input evoking sensations in a different modality. For example, after early visual deprivation due to a disease that attacked eye retinas, touch stimuli can produce “visual light” or after a thalamic lesion leading to a loss of tactile sensation, sounds can elicit touch sensations. This probably occurs because the tactile or auditory sensory input now begins to cross-activate the deprived cortical areas. This could be regarded as a form of acquired synesthesia.