Volunteers under hypnosis showed distinct signs of brain activity when they were asked to suppress or recall memories.Israeli researchers have identified the brain circuits that play a key role in memory suppression and recall in a study of volunteers under hypnosis.
Scientists from the Weizmann Institute of Science in Rehovot combined hypnosis with functional magnetic resonance imaging (fMRI), to investigate changes in brain activity as volunteers under hypnosis were first made to forget, and then remember, memories of a recently viewed movie.
Neuroscientist, Yadin Dudai from the department of Neurobiology, and colleagues Avi Mendelsohn, Yossi Chalamish, and Alexander Solomonovich, showed volunteers a movie about a day in the life of a young woman. A week later the volunteers returned to the lab, were placed in a magnetic resonance imagine scanner, and instructed under hypnosis to forget the movie until they heard the phrase: “Now you can remember everything.”
Once the subjects were brought out of the hypnotic state, researchers tested their recall of the movie in a quiz, and found that the volunteers only answered half of the yes-no questions correctly. The test was repeated after the volunteers heard the phrase, and this time 80 percent of the answers were correct, the same as a control group that wasn’t susceptible to posthypnotic amnesia.
As expected, hypnosis-susceptible volunteers showed reduced recall of the movie, compared to those who were not susceptible.
In a report in the January issue of Neuron
, Dudai reported that the fMRI scans, collected as subjects answered questions about the movie, revealed that several regions of the brain, mostly in the occipital and temporal lobes, were unusually quiet when hypnosis-susceptible subjects suppressed memories. In contrast, activity in the rostrolateral prefrontal cortex, implicated in memory retrieval, was elevated during memory suppression.
“The paralleled recovery of brain activity and memory performance strongly suggests that suppression was exerted at early stages of the retrieval process, thus preventing the activation of regions that are crucial for productive retrieval,” wrote the researchers in Neuron
The scientists added that amnesia induced by posthypnotic suggestion affected a monitoring system which governed memory retrieval. When a question was received about the film, the retrieval process was aborted.
The findings also suggested that some forms of clinical amnesia may model hypnosis by resulting from a similar abort mechanism. The researchers added, however, that further studies will be needed to determine this.
Dudai and his research team believe their work on memory suppression, and the recall process, may yield insights into the mechanisms underlying amnesia and suppression of painful memories.