The nature of hypnotically suggested changes in perception has been controversial throughout the history of hypnosis. 

The major current hypotheses of hypnosis hold that we always actively use our own imagination to bring about the effects of a suggestion - for example, the occurrence of visual hallucinations always requires active use of goal directed imagery and can be experienced both with and without hypnosis. In other words, it isn't really hypnosis, but people susceptible to suggestion become more so when drowsy.

A multidisciplinary group  from the University of Turku, University of Helsinki and University of Skövde say in two papers that they have evidence that hypnotic suggestion can modify processing of a targeted stimulus before it reaches consciousness. 

They say experiments show that it is possible to hypnotically modulate even highly automatic features of perception, such as color experience.  In this case of one paper published in the open access journal PLoS One, it involved only two very highly hypnotizable participants who can be hypnotized and dehypnotized by just using a one-word cue. 

The researchers measured brains oscillatory activity from the EEG in response to briefly displayed series of red or blue shapes (squares, triangles or circles). The participants were hypnotized and given a suggestion that certain shapes always have a certain color (e.g. all squares are always red). Participant TS-H reported constantly experiencing a change in color immediately when a suggested shape appeared on the screen (e.g. seeing a red square when the real color was blue).

The researchers found that this experience was accompanied with enhanced high-frequency brain activity already 1/10 second after the stimulus appeared and it was only seen in response to the shapes mentioned in the suggestion.

The second participant did not experience the color change or the enhanced activity. However, she reported a peculiar feeling when a suggestion-relevant shape was presented: "sometimes I saw a shape that was red but my brain told me it had a different color".

Evoked potential responses. Time-frequency representations of the evoked responses to suggestion-relevant and -irrelevant shapes and their difference scalogram in the posthypnotic and simulation conditions at 15–35 Hz over the left occipital cortex for (A) TS-H and (B) RM. The maps on the right side show the scalp distribution of the shape related difference at the central frequency of 22 Hz in the 70–120 ms post stimulus time window. Credit and link: doi:10.1371/journal.pone.0070900.g002

This enhanced oscillatory brain activity is proposed to reflect automatic comparison of input to memory representations. In this case the hypnotic suggestion "all squares are red" led to a memory trace that was automatically activated when a square was presented. Furthermore, for the participant TS-H the effect was strong enough to override the real color of the square. The matching must have occurred preconsciously because of the early timing of the effect and the immediacy of the color change. Also, both participants reported having performed under posthypnotic amnesia without conscious memory of the suggestions.

In the article published in International Journal of Clinical and Experimental Hypnosis TS-H was tested in a similar type of setting, however, only behavioral data, including accuracy and response times in color recognition, were collected. These results further support that a hypnotic suggestion affects her color perception of targeted objects before she becomes conscious of them. Furthermore, TS-H was not capable of changing her experience of visually presented stable images without the use of hypnotic suggestions i.e. by using mere mental imagery.

Importantly, both of these experiments were done by using a posthypnotic suggestion. The effect was suggested during hypnosis but the experience was suggested to occur after hypnosis. Thus all the experiments were carried out while participants were in their normal state of consciousness.

This result indicates that all hypnotic responding can no longer be regarded merely as goal directed mental imagery. They say it shows that in hypnosis it is possible to create a memory trace that influences early and preconscious stages of visual processing already about 1/10 second after the appearance of a visual target. 

Citation: Koivisto M, Kirjanen S, Revonsuo A, Kallio S (2013) A Preconscious Neural Mechanism of Hypnotically Altered Colors: A Double Case Study. PLoS ONE 8(8): e70900. doi:10.1371/journal.pone.0070900

Citation:  International Journal of Clinical and Experimental Hypnosis, 61(4): 1-17, 2013 (forthcoming)