Enhanced Cortical Excitability in Grapheme-Color Synesthesia and Its Modulation

Abstract

SummarySynesthesia is an unusual condition characterized by the over-binding of two or more features and the concomitant automatic and conscious experience of atypical, ancillary images or perceptions [1–3]. Previous research suggests that synesthetes display enhanced modality-specific perceptual processing [4–7], but it remains unclear whether enhanced processing contributes to conscious awareness of color photisms. In three experiments, we investigated whether grapheme-color synesthesia is characterized by enhanced cortical excitability in primary visual cortex and the role played by this hyperexcitability in the expression of synesthesia. Using transcranial magnetic stimulation, we show that synesthetes display 3-fold lower phosphene thresholds than controls during stimulation of the primary visual cortex. We next used transcranial direct current stimulation to discriminate between two competing hypotheses of the role of hyperexcitability in the expression of synesthesia. We demonstrate that synesthesia can be selectively augmented with cathodal stimulation and attenuated with anodal stimulation of primary visual cortex. A control task revealed that the effect of the brain stimulation was specific to the experience of synesthesia. These results indicate that hyperexcitability acts as a source of noise in visual cortex that influences the availability of the neuronal signals underlying conscious awareness of synesthetic photisms.

Using transcranial electrical stimulation to enhance cognitive functions in the typical and atypical brain

“…Most of the studies so far found that anodal stimulation improved human performance, while cathodal stimulation impaired human performance [14,19]. However, some exceptions to that, in which no e ect for cathodal stimulation or the opposite pattern (i.e., enhancement rather than impairment), were reported [20][21][22]. Jacobson et al [14] have attributed these inconsistencies to several possible mechanisms.…”

Multisensory integration and cross-modal learning in synaesthesia: A unifying model

“…Enhanced excitability of the primary visual cortex in grapheme-colour synaesthetes was also demonstrated by TMS (Terhune et al 2011). These widespread differences in neuronal processing may be functionally relevant: enhanced perceptual ability is reported in synaesthetes (Banissy, Walsh and Ward, 2009) as well as more vivid imagery ͺ (Spiller, Jonas, Simner & Jansari, 2015) even for stimuli not implicated in their synaesthesia .…”

The structure of inter-individual differences in visual ability: Evidence from the general population and synaesthesia

“…Grapheme-colour synaesthetes (GCS) experience colours for letters and numbers. Not only do they have atypical visual-like experiences they also appear to have atypical (non-synaesthetic) visual functioning: they perform better at tests of colour discrimination (Banissy et al, 2013); show increased visual evoked potentials, in EEG, to high-frequency but not low-frequency Gabor gratings ; have lower phosphene thresholds to occipital lobe stimulation (Terhune, Tai, Cowey, Popescu, & Kadosh, 2011); and have been shown to have worse motion coherence (Banissy et al, 2013).…”