Unleashing synaesthetic-like effects by stimulating the DLPFC but not PPC

Models on the mechanisms underlying letter–colour synaesthesia diverge on a central question: whether triggered sensations reflect (a) quantitatively or (b) qualitatively deviant brain organization. We previously found evidence for (a) after observing synaesthesia-like letter–colour binding in adult non-synaesthetes following execution of a visual letter search task which employed likelihood manipulations of letter–colour pairings to implicitly train letter–colour associations (Kusnir and Thut, 2012). The newly-formed associations were synaesthesia-like, since correlating with the synaesthetic-Stroop and showing the colour-opponency effect, present in synaesthetes (Nikolić et al., 2007). This latter effect manifests as increased synaesthetic-Stroop interference when the real colour of a letter is opponent to the synaesthetic/associated colour, in line with involvement of visual/colour areas in letter–colour binding. Here, we investigated the brain areas involved in the formation of these synesthetic associations. Based on (Cohen Kadosh and Iuculano, 2011) revealing reciprocal involvement of the PPC and dlPFC in distinct aspects of learning in a numerical conception task, we hypothesized that these two areas may also be differentially implicated in the learning of synaesthesia-like letter–colour associations by non-synaesthetes. Previous results would predict that while PPC supports processing of these associations after learning (see also Esterman et al., 2006), dlPFC shows a reciprocal function, i.e., suppressing these associations under normal conditions. Using bilateral tDCS, we interfered with either PPC or dlPFC in two groups while they performed the letter–colour association task as in (Kusnir and Thut, 2012). A third group performed the same task but without tDCS (control group). All three groups used non-opponent colour pairs for letter–colour association learning, to avoid ceiling effects. In comparison to the control group, dlPFC-stimulation significantly enhanced letter–colour binding. This enhancement was substantial, leading to interference between learned and real colours in the order of the colour-opponency effect (despite the use of non-opponent colour pairs). No such effect was observed with PPC stimulation. This provides novel information regarding the network of areas implicated in the formation of automatic (letter–colour) associations. Synaesthesia-like binding of colours to letters by non-synaesthetes seems to be suppressed by dlPFC under normal conditions, and may be released by modulation of the dlPFC. We speculate that dlPFC-stimulation facilitates the formation of letter–colour binding, possibly by emphasizing relevant associations during task performance and/or by disinhibiting other brain areas (i.e., posterior parietal) involved in automatic, perceptual letter–colour binding.

Affiliations: 1: University of Glasgow, UK