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Tanja Nijboer, Titia Gebuis, Sarah Plukaard, Edward de Haan, Maarten van der Smagt; Neural mechanisms underlying grapheme-colour synesthesia. Journal of Vision 2008;8(6):1038. doi: 10.1167/8.6.1038.
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© ARVO (1962-2015); The Authors (2016-present)
In grapheme-colour synesthesia, perception of a grapheme induces the percept of a specific colour. In the present study, the neural processes underlying grapheme-colour synesthesia were investigated using an oddball paradigm and EEG recordings. For each of the 8 synesthetes, we presented a set of 6 different graphemes that elicited a synesthetic colour experience within one colour category (e.g. yellow (standards)). These standards were presented in black on a grey background. There were two types of oddballs: first, graphemes that induced a synesthetic colour from a different colour category (e.g. red) than the standards; and second, physically coloured graphemes. Each of the 8 control participants received the stimuli that were similar to the stimuli of one of the synesthetes. The task was to respond as quickly as possible when an oddball stimulus appeared. The reaction times of synesthetes to physically coloured oddballs were comparable to those of the controls. Reaction times to ‘synesthetic’ oddballs, however, were significantly longer (on average 53ms). The P300 ERP component measured at Pz showed a significantly larger amplitude for the physically coloured oddball compared to the standards in control participants and synesthetes. Moreover, the synesthetes also revealed a larger amplitude for synesthetic oddballs compared to the standards. Interestingly, the P300 related to the synesthetic oddball was significantly delayed (56.5 ms on average) compared to that of the physical oddballs. This delay shows a striking resemblance to the delay in reaction times for synesthetic oddballs. In conclusion, the synesthetic percept takes longer to develop than a normal colour percept and this is reflected in the ERPs as well as the reaction times.
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