This review focuses on recent research on color synesthesia, the most common form of synesthesia. The authors say that over the past few years, substantial advances have been made in the understanding of synesthesia, as well as in the comprehension of perception and consciousness.
Synesthesia is a phenomenon in which individuals experience unusual visual or emotional perceptions elicited by the activation of an unrelated sensory modality (such as touch) or by a cognitive process (such as computing). More than 60 types of synethesthic phenomena have been described.
A color synesthete may see brightly colored triangles dancing in front of his eyes when he hears the sound of a trumpet. In others, letters or numbers may trigger the perception of a particular color.
Synesthesia has recently generated considerable interest among neuroscientists, but its clinical significance remains underevaluated. Imaging has demonstrated distinct patterns in cortical activation and brain connectivity in people with synesthesia and controls.
The authors write that the condition has prompted philosophical debates on the nature of perception and impacted the course of art history, with artworks of affected painters illustrating the nature of synesthetic experiences.
Like prosopagnosia, synesthesia is classified as developmental and acquired. Developmental forms of synesthesia predispose individuals to changes in primary sensory processing and cognitive functions, usually with better performances in certain aspects and worse ones in others, and to heightened creativity. Acquired forms of synesthesia commonly arise from drug ingestion or neurological disorders, including thalamic lesions and sensory deprivation (e.g., blindness).
They say that awareness of synesthesia has significantly improved, allowing synesthetes to discuss their experience instead of hiding it. However, the authors hope for understanding of synesthesia to continue to deepen and for a more comprehensive definition beyond currently used criteria, which they say can be rather restrictive for a condition quite polymorphic in nature.