Developing synaesthesia: a primer

Synaesthesia is a variation of human experience that involves the automatic activation of unusual concurrent experiences in response to ordinary inducing stimuli. The causes for the development of synaesthesia are not well understood yet. Synaesthesia may have a genetic basis resulting in enhanced cortical connectivity during development. However, in some cases synaesthesia has a sudden onset, for example, caused by posthypnotic suggestions, drug exposure, or brain injury. Moreover, associative learning during a critical developmental period also seems to play an important role. Synaesthesia may even be acquired by training in adulthood. In this research topic, we bring together topical hypotheses, theories and empirical studies about the development of synaesthesia. According to the immune hypothesis introduced by Carmichael and Simner (2013), the genes that are responsible for normal cortical development are also involved in the development of synaesthesia. As many of these genes have a function for both the immune system and for altering connectivity via axonal guidance, synaptic connectivity, and pruning, the interaction between the central nervous system and the immune system during early life may play a pivotal role in the development of synaesthesia. A common genetic basis of synaesthesia and autism spectrum disorders may also explain the higher prevalence of synaesthetes among patients diagnosed with autism spectrum disorder reported by Neufeld et al. (2013). Likewise based on a neurochemical basis, Brogaard (2013) put forward the serotonergic hyperac-tivity hypothesis for the development of synaesthesia. Accordingly, excessive serotonin in the brain may be a common cause for synaesthesia induced by psychedelic drugs, brain injury, and in individuals with autism. Motivated by similar assumptions, Terhune et al. (2014) used a psychophysical approach to test this hypothesis. They found no support, neither for the serotinergic hyperactivity hypothesis nor for a related, reduced GABA levels hypothesis. Similarly, null findings were reported by Sinke et al. (2014) in an investigation on the relationship between multisensory integration and event related potentials. Nevertheless, consistent with previous research, they found evidence for alterations in early visual processing in synaesthesia. According to the neonatal hypothesis synaesthetic associations between basic shapes and colors may be present already early in childhood. Brang et al. (2013) hypothesized that even when these associations can be refined by experience they can interfere with learning novel shape-color associations later. In an empirical study they found support for this hypothesis. Other determinants of synaesthetic associations include ordinality (e.g., the position in a grapheme sequence; e.g., …