Primary visual cortex excitability is not atypical in acquired synaesthesia

A wealth of data suggests that psychedelic drugs elicit spontaneous perceptual states that resemble synaesthesia although it is unclear whether these different forms of synaesthesia share overlapping neural mechanisms. Multiple studies have shown that developmental and trained synaesthesia is characterized by selective hyperexcitability in primary visual cortex and it has been proposed that cortical hyperexcitability may contribute to induced and acquired synaesthesia. This study tested the prediction that a case of acquired synaesthesia (LW) would display selectively elevated primary visual cortex excitability, as reflected in lower transcranial magnetic stimulation (TMS) phosphene thresholds, but no difference in motor thresholds, relative to controls. In contrast to this prediction, LW’s phosphene threshold was well within the threshold range of controls. These results suggest that acquired synaesthesia is not characterized by atypical visual cortex excitability.