Hypnosis can induce synaesthetic experiences – where one sense triggers the involuntary use of another – according to a new study by UCL (University College London) researchers. The findings suggests that people with synaesthesia, contrary to popular belief, do not necessarily have extra connections in their brain; rather, their brains may simply do more 'cross talking' and this can be induced by changing inhibitory processes in the average brain.

To explore the alternative theory of more cross talk (disinhibition) between brain areas in synaesthetes, Dr Roi Cohen Kadosh and colleagues used posthypnotic suggestion to show that people who are not synaesthetes can be induced to have synaesthetic experiences.

After inducing digit-colour synaesthesia, the volunteers reported similar experiences to those undergone by real synaesthetes in their everyday life. For example, one participant described seeing the numbers on car number plates in specific colours, while walking around under posthypnotic suggestion. Moreover, hypnotized participants failed trick tests which were also failed by real synaesthetes: in one test, when subjects were hypnotized to experience seven as red, they could not detect the number when a black seven was presented on a red background.

Dr Roi Cohen Kadosh, UCL Institute of cognitive Neuroscience, says: "Our study shows that posthypnotic suggestion can induce synaesthetic experiences in people, suggesting that extra brain connections are not needed to experience cross-sensory interactions and that it is more cross talk within the brain that causes these experiences. This takes us one step closer to understanding the causes of synaesthesia and abnormal cross-brain interactions."

Other Types of Synaesthesia

• Sound → color synesthesia - Experiencing colours in response to sounds.
• Number form synesthesia - Numbers evoke a mental map where they exist in a 3-dimensional space, individual numbers always occupy the same location.
• Ordinal-Linguistic-Personification - Ordered sequences, such as numbers or days of the week have particular human characteristics. E.g., 7 is an aggressive female.
• Lexical → gustatory synesthesia - Experiencing words as tastes

Source: Adapted from materials provided by EurekAlert (Press Release)

Reference

Kadosh, R.C., Henik, A., Catena, A., Walsh, V. & Fuentes, L.J. (2008), Induced cross-modal synesthetic experience without abnormal neuronal connections. Psychological Science.