Automatic letter-colour associations in non-synaesthetes and their relation to grapheme-colour synaesthesia

Although grapheme-colour synaesthesia is a well-characterized phenomenon in which achromatic letters and/or digits involuntarily trigger specific colour sensations, its underlying mechanisms remain unresolved. Models diverge on a central question: whether triggered sensations reflect (i) an overdeveloped capacity in normal cross-modal processing (i.e., sharing characteristics with the general population), or rather (ii) qualitatively deviant processing (i.e., unique to a few individuals). We here address this question on several fronts: first, with adult synaesthesia-trainees and second with congenital grapheme-colour synaesthetes. In Chapter 3, we investigate whether synaesthesia-like (automatic) letter-colour associations may be learned by non- synaesthetes into adulthood. To this end, we developed a learning paradigm that aimed to implicitly train such associations while keeping participants naïve as to the end-goal of the experiments (i.e., the formation of letter-colour associations), thus mimicking the learning conditions of acquired grapheme- colour synaesthesia (Hancock, 2006; Witthoft & Winawer, 2006). In two experiments, we found evidence for significant binding of colours to letters by non-synaesthetes. These learned associations showed synaesthesia-like characteristics despite an absence of conscious, colour concurrents, correlating with individual performance on synaesthetic Stroop-tasks (experiment 1), and modulated by the colour-opponency effect (experiment 2) (Nikolic, Lichti, & Singer, 2007), suggesting formation on a perceptual (rather than conceptual) level. In Chapter 4, we probed the nature of these learned, synaesthesia-like associations by investigating the brain areas involved in their formation. Using transcranial Direct Current Stimulation to interfere with two distinct brain regions, we found an enhancement of letter-colour learning in adult trainees following dlPFC-stimulation, suggesting a role for the prefrontal cortex in the release of binding processes. In Chapter 5, we attempt to integrate our results from synaesthesia-learners with the neural mechanisms of grapheme-colour synaesthesia, as assessed in six congenital synaesthetes using novel techniques in magnetoencephalography. While our results may not support the existence of a “synaesthesia continuum,” we propose that they still relate to synaesthesia in a meaningful way