Why does synesthesia persist in the population?

Brang and Ramachandran do an interesting review of ideas on synesthesia, a condition present in 2%–4% of the population,in which a sensory stimulus presented to one modality elicits concurrent sensations in additional modalities. In two of the most common variants, auditory tones and achromatic (colorless) numbers produce vivid and perceptually salient colors. The authors point out that synesthesia can be associated with a wide variety of conceptual and perceptual benefits, suggesting that the gene(s) involved may have been selected for because of a hidden agenda. They speculate that increasing the range of sensory associations may extend to other systems such as creativity and metaphor (increasing the range of association between words). They point to examples of people with prodigious memories based largely on using synesthetic associations evoked by the items to be memorized.

In addition to facilitating processes in individual sensory modalities, synesthetes also show increased communication between the senses unrelated to their synesthetic experiences, suggesting that benefits from synesthesia generalize to other modalities as well, supporting their ability to process multisensory information. Furthermore, others have argued that synesthesia is the direct result of enhanced communication between the senses as a logical outgrowth of the cross-modality interactions present in all individuals. Taken collectively, these data suggest that synesthesia may be associated with enhanced primary sensory processing as well as the integration between the senses...synesthesia is a highly heritable phenomenon that is associated with numerous benefits to cognitive processing, potentially underscoring a basis for why this condition has survived evolutionary pressures.