From multisensory integration to synaesthesia: a boundary-preserving continuum and research framework.

Synaesthesia is commonly defined as an involuntary and consistent phenomenon in which stimulation in one sensory or cognitive domain elicits an additional experience in another domain. Yet definitions derived from perceptual psychology, neuroscience, aesthetics, cultural anthropology, and artistic practice often speak past one another. This article proposes a boundary-preserving conceptual review that uses the evolutionary history of sensing as a meta-theoretical platform for dialogue among these definitions. It makes four contributions. First, it clarifies the boundaries among congenital or developmental synaesthesia, acquired or trained synaesthesia, synaesthesia-like associations, cross-modal correspondences, multisensory integration, and cultural or metaphorical synaesthesia. Second, it argues that multisensory integration and cross-modal correspondence provide shared cross-modal resources, whereas synaesthesia requires stricter evidence of individual consistency, automaticity, phenomenological vividness, durability, and behavioral interference. Third, it reconstructs the continuum as a heuristic framework rather than a developmental ladder, thereby preserving diagnostic boundaries while enabling communication across competing definitions. Fourth, it explores a dialogue space between synaesthetic artists and psychologists of perception, with implications for scientific research, education, arts-based learning, and cultural psychology.