People with developmental synesthesia typically report having the condition for as long as they can remember, and generally cannot provide an explanation or an account of how they may have learned how to associate the experiences. Individuals with acquired synesthesia report first experiencing synesthesia later in life after taking hallucinogenic drugs, or after a neurological condition such as epileptic seizures.
One theory suggests children learn to associate numbers or letter with colors, possibly to aid memory. This account of synesthesia cannot by itself explain why siblings with synesthesia reared in similar environments report different colors for the same inducer, or experience different variants of synesthesia.
Family studies show various forms can exist within one family, with phenotypes differing even among close relatives. This suggests there is some common genetic basis for most, if not all, synesthetic experiences. In fact, there is at least one study of synesthesia that suggests the condition may be inherited.
The major neural hypotheses propose synesthesia occurs due to neural hyper connectivity among sensory areas in the cortex, perhaps due to faulty axonal pruning, or differences in axon guidance. Evidence supporting hyper-connectivity hypotheses comes from a diffusion tensor imaging study that showed grapheme-color synesthetes — in which a particular letter or number evokes a certain color — had increased structural connectivity.
Asher, J., Lamb, J. A., Brocklebank, D., Cazier, J. B., Maestrini, E., Addis, L., et al. (2009). A whole-genome scan and fine-mapping linkage study of auditory-visual synesthesia reveals evidence of linkage to chromosomes 2q24, 5q33, 6p12, and 12p12. American Journal of Human Genetics, 84, 279-285.
Baron-Cohen, S, & Harrison, J. (Eds.). (1997). Synaesthesia: Classic and contemporary readings: Blackwell.
Baron-Cohen, S., Harrison, J., Goldstein, L.H., & Wyke, M. (1993). Coloured speech- Perception: is synaesthesia what happens when modularity breaks down? Perception, 22(4), 419-426.
Baron-Cohen, S., Wyke, M., & Binnie, C. (1987). Hearing words and seeing colours: An experimental investigation of a case of synaesthesia. Perception, 16, 761-767.
Nunn, J., Gregory, L., Morris, R., Brammer, M., Bullmore, E., Harrison, J., et al.(2002). Functional magnetic resonance imaging of synaesthesia: Activation of colour vision area V8 by spoken words. Nature Neuroscience 5, 371-375.
Paulesu, E., Harrison, J., Baron-Cohen, S., Watson, J., Goldstein, L., Heather, J., Frackowiak, R.S.J., Frith, C. (1995). The physiology of coloured hearing. A positron emission tomography activation study of coloured-word synaesthesia. Brain, 118, 661-676.