Synesthesia

Synesthesia - the cross-activation of sensory modalities such that stimulation in one sense produces automatic experience in another - is among the most vivid and reliably reported perceptual phenomena during psychedelic experiences. People hear music and see corresponding shapes and colors, or feel textures that have a sound. Language and emotion may become cross-wired in ways that give words visual or spatial qualities. The ordinary separateness of the senses temporarily relaxes.

This cross-modal activation appears to reflect the increased connectivity that neuroimaging documents during psychedelic states. In ordinary experience, sensory cortices maintain a degree of functional separation; information is processed in modality-specific regions before being integrated at higher levels. Under psychedelics, this compartmentalization partially dissolves: signals from one sensory system more readily activate processing in another. The result, experientially, is that the senses seem to flow into one another.

A smaller population of people experience synesthesia as a stable, baseline feature of their perception - not induced by substances but simply present in their ordinary experience. The phenomenological accounts of synesthetic perception in this population share striking similarities with psychedelic-induced synesthesia, suggesting that the underlying mechanism (increased cross-modal connectivity) may be similar.

For many people, the synesthetic dimension of psychedelic experiences is among the most aesthetically remarkable features - music becomes something that can be watched as well as heard, color becomes something with tactile quality, the ordinary single-channel experience of each sense opens into a more integrated, multi-modal perception. This is often described as feeling like a richer or more complete engagement with the world than ordinary sensory experience allows.

The integration of synesthetic experiences is not usually a primary focus of therapeutic work, but they occasionally carry symbolic or emotional significance that is worth attending to: a particular color that seemed to accompany grief, or a sound that felt like it was communicating something specific.