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      Multivariate pattern analysis of fMRI data for imaginary and real colours in grapheme-colour synaesthesia.

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          Abstract

          Grapheme-colour synaesthesia is a subjective phenomenon related to perception and imagination, in which some people involuntarily but systematically associate specific, idiosyncratic colours to achromatic letters or digits. Its investigation is relevant to unravel the neural correlates of colour perception in isolation from low-level neural processing of spectral components, as well as the neural correlates of imagination by being able to reliably trigger imaginary colour experiences. However, functional MRI studies using univariate analyses failed to provide univocal evidence of the activation of the "colour network" by synaesthesia. Applying multivariate (multivoxel) pattern analysis (MVPA) on 20 synaesthetes and 20 control participants, we tested whether the neural processing of real colours (concentric rings) and synaesthetic colours (black graphemes) shared patterns of activations. Region of interest analyses in retinotopically and anatomically defined visual areas revealed neither evidence of shared circuits for real and synaesthetic colour processing, nor processing difference between synaesthetes and controls. We also found no correlation with individual experiences, characterised by measuring the strength of synaesthetic associations. The whole brain searchlight analysis led to similar results. We conclude that revealing the neural coding of the synaesthetic experience of colours is a hard task which requires the improvement of our current methodology: for example involving more individuals and achieving higher MR signal to noise ratio and spatial resolution. So far, we have not found any evidence of the involvement of the cortical colour network in the subjective experience of synaesthetic colours.

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          Author and article information

          Journal
          Eur J Neurosci
          The European journal of neuroscience
          Wiley
          1460-9568
          0953-816X
          September 2020
          : 52
          : 5
          Affiliations
          [1 ] Centre de Recherche Cerveau et Cognition, Université de Toulouse Paul Sabatier & CNRS, Toulouse, France.
          [2 ] Grenoble Institut des Neurosciences, Université Grenoble Alpes, INSERM & CHU Grenoble Alpes, Grenoble, France.
          Article
          10.1111/ejn.14774
          32384170

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