We studied two otherwise normal, synaesthetic subjects who `saw' a speci¢c colour every time they saw a speci¢c number or letter. We conducted four experiments in order to show that this was a genuine perceptual experience rather than merely a memory association. (i)The synaesthetically induced colours could lead to perceptual grouping, even though the inducing numerals or letters did not. (ii)Synaesthetically induced colours were not experienced if the graphemes were presented peripherally. (iii)Roman numerals were ine¡ective: the actual number grapheme was (...) required. (iv)If two graphemes were alternated the induced colours were also seen in alternation. However, colours were no longer experienced if the graphemes were alternated at more than 4 Hz. We propose that grapheme colour synaesthesia arises from `cross-wiring' between the `colour centre' (area V4 or V8)and the `number area', both of which lie in the fusiform gyrus. We also suggest a similar explanation for the representation of metaphors in the brain: hence, the higher incidence of synaesthesia among artists and poets. (shrink)

Synesthesia is the involuntary physical experience of a cross-modal association. That is, the stimulation of one sensory modality reliably causes a perception in one or more different senses. Its phenomenology clearly distinguishes it from metaphor, literary tropes, sound symbolism, and deliberate artistic contrivances that sometimes employ the term "synesthesia" to describe their multisensory joinings. An unexpected demographic and cognitive constellation co-occurs with synesthesia: females and non-right-handers predominate, the trait is familial, and memory is superior while math and spatial navigation suffer. (...) Synesthesia appears to be a left-hemisphere function that is not cortical in the conventional sense. The hippocampus is critical for its experience. Five clinical features comprise its diagnosis. Synesthesia is "abnormal" only in being statistically rare. It is, in fact, a normal brain process that is prematurely displayed to consciousness in a minority of individuals. (shrink)