The Timing of Cortical Activation in Associator Grapheme-Colour Synaesthetes using MEG

Grapheme-colour synaesthetes experience an anomalous form of perception in which graphemes systematically induce specific colour concurrents in their mind’s eye (“associator” type). Although grapheme-colour synaesthesia has been well characterized behaviourally, its neural mechanisms remain largely unresolved. There are currently several competing models, which can primarily be distinguished according to the anatomical and temporal predictions of synaesthesia-inducing neural activity. The first main model (Cross-Activation/ Cascaded Cross-Tuning) posits early recruitment of occipital colour areas in the initial feed-forward sweep of brain activity. The second (Disinhibited Feedback) posits: (i) later involvement of a multisensory convergence zone (for example, in parietal cortices) after graphemes have been processed in their entirety; and (ii) subsequent feedback to early visual areas (i.e., occipital colour areas). In this study, we examine both the timing and anatomical correlates of associator grapheme-colour synaesthetes (n=6) using MEG. Using unbiased analysis methods with little a priori assumptions, we applied Independent Component Analysis (ICA) on a single-subject level to identify the dominant patterns of the induced, synaesthetic percept. We observed evoked activity that significantly dissociates between synaesthesia-inducing and non-inducing conditions at approximately 190 ms following grapheme presentation. This effect is present in grapheme-colour synaesthetes, but not in matched controls, and exhibits an occipito-parietal topology localised consistently across individuals to extrastriate visual cortices and to superior parietal lobes. Due to the observed timing of this evoked activity and its localisation, our results support a model predicting relatively late synaesthesia-inducing activity, more akin to the Disinhibited Feedback model. In light of previous findings, our study suggest that differential timing and neural mechanisms may account for associator grapheme-colour synaesthetes, as compared to projectors. Highlights - first MEG study to-date examining associator grapheme-colour synaesthetes - we use single-subj. ICA to dissociate synaesthesia-inducing vs. non-inducing activity - we observe evoked activity ∼190 ms following grapheme presentation effects localize consistently to extrastriate visual and superior parietal cortices - the observed timing is consistent with a model of synaesthesia supporting relatively late activity of synaesthesia-inducing stimuli - our findings suggest that differential mechanisms may underlie different sub-types of grapheme-colour synaesthesia