Research Article Summary: The timing of cortical activation in associator grapheme-colour synaesthetes using MEG
The study involved six female 'Associator' synesthetes who experienced color sensations when viewing graphemes (letters and numbers) and six matched controls who did not have synesthesia. The synesthetes and controls were matched based on age, gender, handedness, and level of education.
The researchers used magnetoencephalography (MEG), a neuroimaging technique that can measure the magnetic fields generated by neural activity in the brain. The participants were presented with black letters or numbers on a white background, and the researchers measured the timing and anatomical correlates of synesthesia-inducing activity in the brain.
The results showed that synesthesia-inducing activity occurred approximately 190 ms after grapheme presentation in extrastriate visual cortices and superior parietal lobes in the synesthetes. This activity was not present in the control group.
These findings support the Disinhibited Feedback model, which proposes that synesthesia arises from abnormal feedback processing in the brain. According to this model, neural signals from one sensory modality (such as vision) are disinhibited and spill over to other sensory areas, leading to the experience of synesthetic sensations. The results of this study suggest that this disinhibition occurs approximately 190 ms after grapheme presentation, providing further support for this model.
Overall, this study provides valuable insights into the neural mechanisms underlying grapheme-color synesthesia and may have implications for the development of future treatments for synesthetic individuals.
Michalareas, G., Kusnir, F., Thut, G., & Gross, J. (2023). The timing of cortical activation in associator grapheme-colour synaesthetes using MEG. Neuropsychologia, 181, 108491–108491. https://doi.org/10.1016/j.neuropsychologia.2023.108491
The Study Hypotheses
If Synesthesia is the result of early stage processing activity, then testing will show activity very quickly after presentation of the grapheme stimuli to be processed.
Findings and Conclusions
The results showed that synesthesia-inducing activity occurred approximately 190 ms after grapheme presentation in extrastriate visual cortices and superior parietal lobes in the synesthetes. This activity was not present in the control group. This differs form other studies that reported earlier activation. this may be the result of other studies including 'projector type' synesthetes in their studies.
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 noninducing 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.
The Short Conclusion:
In a study of six grapheme-colour synaesthetes, researchers used MEG to examine the timing and anatomical correlates of synaesthesia-inducing activity. They observed that this activity occurred approximately 190 ms after grapheme presentation in extrastriate visual cortices and superior parietal lobes, supporting the Disinhibited Feedback model. This effect was not present in matched controls. Basically, Synesthesia in this group is a result of late stages in brain activity rather than early stages.
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