Enhanced Cortical Excitability in Grapheme-Color Synesthesia and Its Modulation

Devin Blair Terhune,Sarah Tai,Alan Cowey,Tudor Popescu,Roi Cohen Kadosh

doi:10.1016/j.cub.2011.10.032

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.

Highlights

Recent studies have shown that synesthetes exhibit superior modality-specific perceptual processing than nonsynesthetes [4,5,6,7]
We first tested the prediction that grapheme-color synesthetes would exhibit elevated cortical excitability in primary visual cortex
Synesthetes exhibited approximately 300% lower phosphene thresholds than controls at left [F(1, 9) = 78.18, p < 0.001, hp2 = 0.90], mid-line [F(1, 9) = 107.06, p < 0.001, hp2 = 0.92], and right [F(1, 9) = 61.30, p < 0.001, hp2 = 0.87] visual cortex, whereas the two groups did not differ in motor thresholds (F < 1.25, p > 0.30) (Figure 1)

Summary

Introduction

We used transcranial magnetic stimulation (TMS) and transcranial direct-current stimulation (TDCS) to investigate whether grapheme-color synesthetes exhibited enhanced cortical excitability and how modulation of cortical excitability affects the experience of synesthesia. We first tested the prediction that grapheme-color synesthetes would exhibit elevated cortical excitability in primary visual cortex.

Results

Conclusion