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Colour perception in synaesthesia

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We studied grapheme-colour synaesthesia in persons who inherently see characters with colour. The aim of the study was to understand the mechanism of their colour perception and identify the sensory-perceptual processing stage at which synaesthesia occurs. We varied characters – digits, Latin letters and Japanese logotypes (hiragana, katakana, kanji) – with regards to their pictorial features, associated phonetic and semantic information, while assessing change in elicited colour. Our results reveal that a similar colour was elicited under the following conditions: (i) similar shape (e.g. 0 and O); (ii) different shape but similar phonetics (e.g. ka in Japanese); (iii) different shape and different phonetics but similar meaning. Our results reveal that grapheme-colour synaesthesia is influenced by the character shape, a low-level feature. The effect is, though, overruled by the effect of the character phonetics, while the character meaning is superior to both, i.e. the phenomenon that follows the hierarchical feature extraction in the order that reflects the principle stages of human cognitive processing. Notably, symbols indicating specific pronunciation quality (e.g. diacritic umlaut in O vs. Ö) caused changes in the elicited colour that varied along the blue-yellow axis of colour space, implying opponency in colour synaesthesia. We conclude that the grapheme-colour synaesthesia occurs early in the visual pathway. It is considered to result from neural confounding, or cross-activation (Ramachandran & Hubbard, 2001) in the colour area V4 and right parietal-occipital region.

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