Developmental Origins of Synesthesia

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Synesthesia is when a stimulus not only induces the typical percept, but it also induces another automatic percept that is often in a different sensory modality (Spector & Maurer, 2009). For example, a particular sound may induce color. The pitch or tone of this sound may induce a more specific color. A stimulus that triggers the synesthesia is called the inducer. The modality in which the synesthesia is encountered is called the concurrent (Ward & Simner, 2003). There are at least 54 different types of synesthesia (Spector & Maurer, 2009), mixing different aspects of our senses. Synesthesia can manifest its behavioral markers in children as young as six years old. There are over 170,000 grapheme-color synesthetes ages 0-17 in the UK and over 930,000 grapheme-color synesthetes ages 0-17 in the USA (Simner et. al., 2009). There are two main developmental theories of synesthesia and this paper will explore both perspectives; the neonatal synesthesia theory and the disinhibited feedback theory.
Developmental Theories of Synesthesia
There are two predominant theories in regards to the developmental origins of synesthesia. One theory is called the neonatal synesthesia theory. It posits that synesthesia arises when the pruning of synapses is not completed between some contiguous brain areas (Spector & Maurer, 2009). Essentially all neonates are synesthetes until the appropriate pruning and apoptosis occurs. The other theory is called the disinhibited feedback theory. This theory posits “synesthesia arises when the reentrant feedback that develops postnatally from higher cortical areas onto lower sensory cortical areas is not strong enough to inhibit effects from connections between primary sensory cortical areas” (Grossenbacher & Love...

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...eory, the connections between these areas may remain but are typically inhibited. Synesthesia will also change and evolve over time through childhood. Their associations may begin rather chaotically and become more consistent. Synesthesia has many individual differences and many studies focus on the performance of a single synesthete or small sample of synesthetes. Thus, it is hard to generalize such studies to all synesthetes. Future research should include more fMRI’s and DTI research to fully map the neurological basis. It would also be beneficial to perform these on infants. This is difficult because infants are fussy, won’t stay still, and permission to expose infants to this type of analysis is difficult to obtain. It is also not possible to have access to another person’s subjective experiences so testing the genuineness of synesthesia can be more difficult.

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