How do Different Types of Synaesthesia Cluster Together? Implications for Causal Mechanisms

It is unclear whether synaesthesia is one condition or many and, this has implications for whether theories should postulate a single cause or multiple independent causes. Study 1 analyses data from a large sample of self-referred synaesthetes (N = 2925), who answered a questionnaire about N = 164 potential types of synaesthesia. Clustering and factor analysis methods identified around seven coherent groupings of synaesthesia, as well as showing that some common types of synaesthesia do not fall into any grouping at all (mirror-touch, hearing-motion, tickertape). There was a residual positive correlation between clusters (they tend to associate rather than compete). Moreover, we observed a “snowball effect” whereby the chances of having a given cluster of synaesthesia goes up in proportion to the number of other clusters a person has (again suggesting non-independence). Clusters tended to be distinguished by shared concurrent experiences rather than shared triggering stimuli (inducers). We speculate that modulatory feedback pathways from the concurrent to inducers may play a key role in the emergence of synaesthesia. Study 2 assessed the external validity of these clusters by showing that they predict performance on other measures known to be linked to synaesthesia.