How someone with synesthesia might perceive (not "see") certain letters and numbers. Synesthetes see characters just as others do (in whichever color actually displayed), yet simultaneously perceive colors as associated to each one. Synesthesia (also spelled synæsthesia or synaesthesia; from the Ancient Greek σύν syn, "together", and αἴσθησις aisthēsis, "sensation") is a neurological phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes. Difficulties have been recognized in adequately defining synesthesia: many different phenomena have been included in the term synesthesia ("union of the senses"), and in many cases the terminology seems to be inaccurate. A more accurate term may be ideasthesia. Characteristics There are two overall forms of synesthesia: projecting synesthesia and associative synesthesia.
Related: The human brain and the mind