The one certainty in the world of synaesthesia is this: there is no clear answer to why a fraction of the population has it, and others don’t. There are as many theories about the origins and significance of synaesthesia as there are scientists studying it.
Simon Baron-Cohen, a professor of developmental psychopathology at the University of Cambridge, postulates synaesthesia as an organic aberration. He espouses a theory of neonatal synesthesia; until we are about 4 months old, all babies experience the senses in a conflated way. For example, sounds would result in auditory, visual, and tactile sensations. During normal development, the senses differentiate. Sensory processing becomes modal: we see with our eyes, hear with our ears, etc. Synesthesia occurs when this differentiation is left incomplete, allowing for cross-modal associations between two or more senses.
Many contemporary neuroscientists believe there is a genetic basis for synaesthesia. The research of VS Ramachandran at The Center for Brain and Cognition at The University of California, San Diego supports a hyper-connectivity theory. Synaesthesia might be caused by a genetic mutation that creates defective pruning of connections in different parts of the brain. This mutation may further be expressed selectively in various structures in the brain, including the fusiform or angular gyrus. This may explain why there are different forms of synaesthesia. Additionally, there may be extensive cross-wiring between brain regions that represent abstract concepts, which would explain the link between creativity, metaphor, and synaesthesia.
The Serotonergic Hyperactivity model purports synesthesia is a defect in the serotonergic system leading to a blockage of regular gating mechanisms for the neurotransmitter serotonin. Professor Berit Brogaard, director of the Brogaard Lab for Multisensory Research at the University of Miami, studies acquired synaesthesia. A synaesthetic state can be induced in some subjects with drugs such as LSD or psilocybin. Additionally, traumatic brain injury (TBI) occasionally creates synaesthsia in people who did not have conflated senses prior to their trauma.
Theories of synaesthesia continue to develop, as does insight into its meaning and value. For now, “why” remains a point of contention and an unanswered question.
Carolyn “CC” Hart