Roses are red, and sixes are blue: The basics of grapheme-colour synaesthesia

In synaesthesia, ordinary stimuli can elicit extraordinary experiences. For example when people with grapheme-colour synaesthesia are shown black digits or letters, they perceive not only the black graphemes, but also perceive highly specific colours called photisms (e.g., 5 is green, 2 is red, C is blue). Not all grapheme-colour synaesthetes experience photisms in similar fashion. For “projector” synaesthetes, photisms are perceived in external space as coloured overlays that sit atop the digits or letters. For “associator” synaesthetes, photisms are not experienced in external space but rather “in my mind’s eye” or “in my head”. First, I will present data from “projector” synaesthetes tested on perceptual grouping, backward masking, and object substitution masking tasks. These data that show that projected coloured overlays can influence synaesthetes’ ability to identify ordinary black graphemes. Second, I will present data from various Stroop-type tasks that suggest that photisms are an automatic consequence of viewing black graphemes. Third, I will show that “projector” and “associator” synaesthetes can be distinguished using not only by their self reports, but also by their patterns of results on these Stroop-type tasks. Fourth I will present data from an experiment using “ambiguous” graphemes (e.g., a scoreboard 5 that can be interpreted as the digit 5 or the letter S). The results indicate that both the form of the grapheme, and the meaning of the grapheme (whether it is interpreted as a digit or letter) ultimately determines the colour of photisms. Finally I will present data showing that photisms can have a profound influence on memory.

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