Synesthesia

Synesthesia is a neurological trait in which the stimulation of one sensory or cognitive pathway involuntarily triggers an experience in a different sensory or cognitive pathway; a person who experiences it is called a synesthete. The blending of senses it produces is not limited to the traditional five senses, and can also involve the perception of pain, temperature, the passage of time, or a person's sense of their own internal body state, with the specific experience varying considerably from one synesthete to another and by the type of synesthesia involved. The most common documented forms include grapheme-color synesthesia, in which letters and numbers are automatically perceived as having a specific color, spatial-sequence synesthesia, in which sequences such as numbers, months or days of the week are experienced as occupying precise locations in space or as forming three-dimensional mental maps, and sound-to-color associations, in which musical notes or everyday sounds trigger vivid visual experiences. The neurological basis is proposed to involve unusually direct cross-activation between adjacent, specialized brain regions, such as heightened communication between the brain's grapheme-recognition area and its nearby color-processing region in grapheme-color synesthesia, or reduced inhibition along existing neural feedback pathways that normally keep sensory processing streams more separate; researchers describe the precise neurological mechanism as still incompletely understood, though structural brain differences have been documented between synesthetes and non-synesthetes.