Hypnosis can induce episodes of synaesthesia, where one sense triggers perceptions in other senses, according to new research by a team of Spanish, British and Israeli researchers. The findings form an important contribution to our understanding of the causes of synaesthesia. The work, which was partly funded by a Marie Curie Intra-European Fellowship from the EU, is published in the journal Psychological Science. In people with synaesthesia, perceptions in one sense trigger experiences in other senses. In the most common form, digit-colour synaesthesia, numbers and letters have colours; a synaesthete may see the digit '8' as red, for example. In others, certain words may trigger tastes and some people even assign personalities to letters, numbers or days of the week. The causes of synaesthesia have long been debated; one of the most common theories claims that people with synaesthesia have extra neuronal connections between certain areas of the brain. Another theory attributes the phenomenon to more cross-talk between different brain regions. In this study, the scientists subjected non-synaesthetes to post-hypnotic suggestion to induce digit-colour synaesthesia. During the experiment, the volunteers reported similar experiences to those reported by real synaesthetes in their everyday lives. 'When I'm walking on the street, the car registration numbers, if those numbers are on the registration, I see them in those colours,' said one participant. Furthermore, the hypnotised test subjects failed a test which synaesthetes also tend to fail. When the participants were hypnotised to experience the number seven as red, they were unable to see a black seven on a red background. The results challenge the idea that synaesthesia is caused by extra connections in the brain, because new connections could not possibly arise and become functional in the short timescale of the experiment. Meanwhile, studies have suggested that hypnosis may increase the amount of cross-talk between different brain regions. 'Our study shows that hypnosis can induce synaesthetic experiences in people, suggesting that extra brain connections are not needed to experience cross-sensory interactions and that it is a change in inhibitory processes - more cross talk within the brain - that causes these experiences,' explains Roi Cohen Kadosh of Ben-Gurion University of the Negev in Israel. 'This takes us one step closer to understanding the causes of synaesthesia and abnormal cross-brain interactions.'