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

Georgios Michalareas; Flor Kusnir; Gregor Thut; Joachim Gross

doi:10.1016/j.neuropsychologia.2023.108491

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

Neuropsychologia. 2023 Mar 12:181:108491. doi: 10.1016/j.neuropsychologia.2023.108491. Epub 2023 Jan 25.

Authors

Georgios Michalareas¹, Flor Kusnir², Gregor Thut³, Joachim Gross⁴

Affiliations

¹ Max Planck Institute for Empirical Aesthetics, Grueneburgweg 14, 60322, Frankfurt, Germany. Electronic address: g.m@ae.mpg.de.
² Department of Psychology, The Hebrew University of Jerusalem, Israel.
³ Centre for Cognitive Neuroimaging, Institute of Neuroscience and Psychology, University of Glasgow, Glasgow, United Kingdom.
⁴ Centre for Cognitive Neuroimaging, Institute of Neuroscience and Psychology, University of Glasgow, Glasgow, United Kingdom; Institute for Biomagnetism and Biosignal Analysis, University of Münster, Malmedyweg, Muenster, Germany; Otto Creutzfeldt Center for Cognitive and Behavioral Neuroscience, University of Münster, Muenster, Germany.

PMID: 36707026
DOI: 10.1016/j.neuropsychologia.2023.108491

Abstract

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 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 grapheme-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 grapheme presentation. This effect is present in grapheme-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.

Keywords: ICA; MEG; Multisensory; Occipito-parietal cortex; Synaesthesia.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Color
Color Perception* / physiology
Humans
Synesthesia