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Michelle Jarick, Colin Hawco, Todd Ferretti, Mike Dixon; Electrophysiological Evidence Supporting the Automaticity of Synaesthetic Number-Forms. Journal of Vision 2010;10(7):875. doi: 10.1167/10.7.875.
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© ARVO (1962-2015); The Authors (2016-present)
For individuals with Number-Form Synaesthesia, numbers occupy very specific and highly consistent spatial locations. The number-form synaesthete we studied here (L) experiences the numbers 1 to 10 rising vertically from bottom to top, then extending in a horizontal left to right direction from 10-20. Using a spatial cueing paradigm, we empirically confirmed L's subjective reports of her unique number line. Digits 1, 2, 8, or 9 were centrally presented on a computer screen followed by a target square that appeared on the bottom or top of the display. L was reliably faster at detecting targets in synaesthetically cued relative to uncued locations, where controls showed no response time differences. Interestingly, L's cueing effects disappeared once the targets were misaligned with her number-forms (presented on the left and right of the display). Furthermore, L demonstrated the vertical cueing effects even at short stimulus onset asynchronies (150 ms SOAs) between cue and target onsets, suggesting her attention was automatically shifted to the cued location. Here, we used event-related brain potentials (ERPs) to provide converging evidence for L's rapid shifts of attention. Compared to non-synaesthetes, L's brain waves generated an early negative deflection occurring at about 200 ms (N2) in occipital and parietal sites following valid targets, reflecting an early enhancement in attention to validly cued locations. Importantly, this N2 component disappeared once the targets were misaligned with her number-forms (targets appeared on the left and right). Non-synaesthetes showed no ERP differences to detect the targets when presented both vertically and horizontally. These findings substantiate L's behavioural cueing effects for the vertical targets at short SOAs with converging electrophysiological evidence that revealed the emergence of early-evoked potentials to validly cued locations. These findings further strengthen our claim that for this synaesthete (L) digits automatically trigger shifts in spatial attention.
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