Attention, automaticity, and awareness in synesthesia.

The phenomenon of synesthesia has occupied the thoughts of philosophers and artists for decades. With the advent modern behavioral and brain imaging techniques, scientific research on synesthesia has also moved into the mainstream of thought. Here I provide a cognitive neuroscience perspective on the condition, with a particular emphasis on grapheme-color synesthesia, the most common variant, in which individuals report vivid and consistent experiences of color in association with numerals, letters, and words. Behavioral studies have revealed several fundamental properties of induced synesthetic colors. First, although they seem to arise automatically, without the need for voluntary control, they are strongly modulated by selective attention. Second, they attain salience relatively early in visual processing, and so can influence perceptual judgments and guide focal attention in cluttered, achromatic displays. Third, brain activity during synesthetic color experiences arises from within the ventral temporal lobe, including color-selective area V4. It has been speculated that grapheme-color synesthesia arises from disinhibited feedback or abnormal cross-wiring between brain regions involved in extracting visual form and color.