Abstract
Are the kinds of abnormal cross-modal interactions seen in synaesthesia or following brain damage due to hyperconnectivity between or within brain areas, or are they a result of lack of inhibition? This question is highly contested. Here we show that posthypnotic suggestion induces abnormal cross-modal experience similar to that observed in congenital grapheme-color synaesthesia. Given the short time frame of the experiment, it is unlikely that new cortical connections were established, so we conclude that synaesthesia can result from disinhibition between brain areas.