Researchers wanted to better understand what brain circuits suppress or assist in long-term memory retrieval.

Yadin Dudai and colleagues report in the 01/10/2008 journal of Neuron on a study performed on a group of volunteers. The group was divided into those who were susceptible to hypnotic suggestions and those not. Both groups were shown a documentary depicting a day-in-the-life of a young woman.

One week later the participants were placed into a magnetic resonance imaging scanner (MRI) and induced into a hypnotic state. While in a hypnotic state the subjects were given a posthypnotic suggestion to forget the documentary movie viewed a week earlier. They were also given a cue that would restore their memory of the film.

The subjects were brought out of the hypnotic state and asked to recall the documentary. The hypnosis-susceptible group recalled less of the film than the non-susceptible group. The participants were then given a cue to restore their memory of the film and asked again to recall the film. As expected, the hypnosis-susceptible group were now able to recall more of the film.

Researchers observed in the MRI scans a difference between the hypnosis-susceptible group and the non-susceptible group in the occipital, temporal, and prefrontal areas of the brain. Also detected were tell-tale brain activity changes in the hypnosis-susceptible group as they forgot and recalled memory of the movie. Specifically, during memory suppression they showed a reduction of activity in some brain regions and increased activity in other areas. After they were given the cue for memory recall the MRI’s showed that the hypnosis-susceptible group had a recovery of activity in what was previously a suppressed region.

There appears to be a pre-retrieval monitoring process, which decides whether to proceed or not with the memory retrieval. Further studies will be needed to determine whether their findings apply to cases of functional amnesia. Some forms of amnesia may be a consequence of the “pre-retrieval memory abort” mechanism that their findings revealed.