What Is Synaesthesia? When One Sense Automatically Triggers Another

Researchers use a set of criteria to distinguish the genuine condition from vivid imagination or learned association:

• •Involuntary and automatic, arriving with the trigger rather than being summoned
• •Consistent over time, which is the standard diagnostic test, with subjects retested after months or years and scoring far higher on consistency than controls who were told to memorise associations
• •Specific rather than vague, so that a letter has a particular shade rather than a general impression
• •Perceptual in quality, experienced as located either in the world or in the mind's eye, which divides synaesthetes into projectors and associators
• •Present from early childhood rather than acquired, in the developmental form
• •Memorable and emotionally laden, with synaesthetes frequently reporting that a wrongly coloured printed letter is actively unpleasant

More than sixty types have been described and a few account for most cases. Grapheme-colour synaesthesia, in which letters and numbers have colours, is the most studied. Chromesthesia links sounds to colours and is associated with musicians. Spatial sequence synaesthesia places ordered sequences such as months, days or numbers at specific locations in space around the body, which is far more common than the colour forms and frequently goes unrecognised because people assume everyone does it. Lexical-gustatory synaesthesia gives words tastes. Mirror-touch synaesthesia produces a felt sensation when observing another person being touched, and is associated with elevated empathy measures. Ordinal linguistic personification assigns personalities and genders to letters and numbers. Prevalence estimates vary widely with the method, and the frequently cited figure from a large randomised sample is around four percent for at least one type, with grapheme-colour at roughly one percent.

Two competing explanations have been proposed and the evidence supports a combination. Cross-activation holds that adjacent brain regions have unusual direct connections, which is attractive for the grapheme-colour form because the area processing letter shapes sits next to the colour area in the fusiform gyrus, and imaging has found colour-selective regions activating when synaesthetes view letters. Disinhibited feedback holds that the connections exist in everyone and are normally suppressed, with synaesthesia resulting from reduced inhibition, which explains why comparable experiences can be induced temporarily by certain drugs and occasionally follow sensory loss or brain injury. A related developmental account proposes that infants have unpruned connections producing something like synaesthesia universally, and that most people lose it, which would make synaesthetes people in whom the pruning was incomplete. The condition runs in families, appears to be polygenic, and can differ in type between related synaesthetes, which suggests that a general predisposition to unusual connectivity is inherited rather than a specific mapping.

The most revealing finding is that the mappings show patterns across unrelated individuals. Letter-colour associations are non-arbitrary at the group level, with A disproportionately red across many synaesthetes who have never met, and the frequency of a letter in the language correlating with the brightness of its colour. Sound-shape mapping is close to universal even in non-synaesthetes, demonstrated by the bouba and kiki experiment in which almost everyone across cultures assigns the rounded nonsense word to the rounded shape and the sharp one to the jagged shape. Higher pitch is consistently mapped to higher spatial position, to smaller size and to brighter colour by nearly everyone. This suggests that synaesthesia is an extreme form of cross-modal correspondence that exists in everyone in a weaker form, which would explain why ordinary language is full of cross-sensory metaphor, describing a sharp taste, a loud shirt, a warm voice and a heavy silence without anyone finding it strange.

Synaesthetes generally regard it as a positive or neutral feature rather than a condition, and the measurable effects are mixed and modest. Memory advantages are real and specific, since an extra coded dimension helps recall of exactly the material that carries it, so a grapheme-colour synaesthete remembers phone numbers and names better and shows no general memory advantage. Several have exceptional abilities that appear related, including Daniel Tammet, who reports experiencing numbers as shapes and colours and recited pi to over twenty-two thousand places. Associations with creativity and with artistic professions are reported and difficult to interpret, since creative people may simply be more likely to notice and report their experiences. The disadvantages are minor and real, including irritation at incorrectly coloured text and occasional interference in learning where a mapping conflicts with a taught convention. A substantial number of synaesthetes discover in adulthood that not everyone perceives this way, having assumed their experience was universal.

Synaesthesia is an involuntary, automatic and highly consistent experience in which one kind of stimulus triggers a perception in another modality, verified by retesting associations years apart. More than sixty types exist, with letters having colours the most studied and ordered sequences occupying spatial positions the most common. Explanations involve either extra connections between adjacent brain regions or reduced inhibition of connections everyone has, and the mappings are non-random across individuals, which links the condition to cross-sensory correspondences present in everyone.