The timing of cortical activation in associator graphene-colour synaesthetes using MEG

Michalareas, G., Kusnir, F., Thut, G. and Gross, J. (2023) The timing of cortical activation in associator graphene-colour synaesthetes using MEG. Neuropsychologia, 181, 108491. (doi: 10.1016/j.neuropsychologia.2023.108491) (PMID:36707026)

Abstract

Graphene-colour synaesthetes experience an anomalous form of perception in which graphemes systematically induce specific colour concurrents in their mind's eye (“associator” type). Although graphene-colour synaesthesia has been well characterised 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 and its variants) 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 graphene-colour synaesthetes (n = 6) using MEG. Using innovative and 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 activity corresponding to the induced, synaesthetic percept. We observed evoked activity that significantly dissociates between synaesthesia-inducing and non-inducing graphemes at approximately 190 ms following graphene presentation. This effect is present in graphene-colour synaesthetes, but not in matched controls, and exhibits an occipito-parietal topology localised consistently within individuals to extrastriate visual cortices and superior parietal lobes. Due to the observed timing of this evoked activity and its localization, our results support a model predicting relatively late synaesthesia-inducing activity, more akin to the Disinhibited Feedback model.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Thut, Professor Gregor
• Authors: Michalareas, G., Kusnir, F., Thut, G., and Gross, J.
• College/School: College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
• Journal Name: Neuropsychologia
• Publisher: Elsevier
• ISSN: 0028-3932
• ISSN (Online): 1873-3514
• Published Online: 25 January 2023
• Copyright Holders: Copyright © 2023 Elsevier
• First Published: First published in Neuropsychologia 181:108491
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher