From SciAm's Mind Matters, an interesting post on hypnotism, looking at what posthypnotic amnesia tells us about memory and the brain.
Read the whole post.Mind Matters - October 7, 2008
Hypnosis, Memory and the Brain
A new study points to specific areas of the brain affected by hypnosis. The technique could be a tool for exploring what happens in the brain when we suddenly forget.
By Amanda J. Barnier, Rochelle E. Cox and Greg Savage
Hypnosis has long been considered a valuable technique for recreating and then studying puzzling psychological phenomena. A classic example of this approach uses a technique known as posthypnotic amnesia (PHA) to model memory disorders such as functional amnesia, which involves a sudden memory loss typically due to some sort of psychological trauma (rather than to brain damage or disease). Hypnotists produce PHA by suggesting to a hypnotized person that after hypnosis he will forget particular things until he receives a “cancellation,” such as “Now you can remember everything.” PHA typically only happens when it is specifically suggested and it is much more likely to occur in those with high levels of hypnotic ability, or “high hypnotizable” people. Now a new study shows that this hypnotic state actually influences brain activity associated with memory.
High hypnotizable people with PHA typically show impaired explicit memory, or difficulty consciously recalling events or material targeted by the suggestion, and a dissociation between implicit and explicit memory, so that even though they can’t recall the forgotten information it continues to influence their behavior, thoughts and actions. The forgetting is reversible—when the suggestion is cancelled, their memories come flooding back. These last two features—the dissociation and reversibility—confirm that PHA is not the result of poor encoding of the memories or of normal forgetting, because the memories return as soon as PHA is cancelled. Rather, PHA reflects a temporary inability to retrieve information that is safely stored in memory. That makes it a useful tool for research.
Researchers have used PHA as a laboratory analogue of functional amnesia because these conditions share several similar features. Case reports of functional amnesia, for instance, describe men and women who, following a traumatic experience such as a violent sexual assault or the death of a loved one, are unable to remember part or all of their personal past. However, as in PHA, they might still show “implicit” evidence of the forgotten events. For instance, they might unconsciously dial the phone number of a family member whom they can’t consciously recall. (In contrast, explicit memories are those we consciously have access to, such as remembering a childhood birthday or what you had for dinner last night.) And, as suddenly as they lost their memories, they can just as suddenly recover them.
Forgetting in the Brain
But for the comparison between PHA and functional amnesia to be most meaningful, we need to know that they share underlying processes. One way to test this is to identify the brain activity patterns associated with PHA. In a groundbreaking study published in Neuron, neuroscientist Avi Mendelsohn and colleagues at the Weizmann Institute in Israel did just that using functional magnetic resonance imaging (fMRI). They carefully selected 25 people to participate in their experiment. Although all were susceptible to hypnosis, earlier testing had shown that half could respond to a PHA suggestion (labelled “the PHA group”) and half could not (the “non-PHA group”). In the Study session of their experiment, participants watched a 45-minute movie. One week later, in the Test session, participants returned to the laboratory and were hypnotized while they lay within the fMRI scanner. During hypnosis, people in both the PHA and non-PHA groups received a suggestion to forget the movie until they heard a specific cancellation cue.
After hypnosis, participants’ memories were tested twice while the fMRI scanner recorded their brain activity. For Test 1, they were asked 40 questions about the content of the movie (for example, the actress knocked on her neighbor’s door on the way home) and 20 questions about the context in which they saw the movie (for instance, during the movie, the door to the study room was closed). These questions required a “yes” or “no” response. For Test 2, participants were asked the same 60 recognition questions, but first they heard the cue to cancel PHA. So Test 1 measured memory performance and brain activity while the PHA suggestion was in effect and Test 2 measured memory performance and brain activity after it was cancelled.
In Test 1 Mendelsohn and colleagues found that people in the PHA group (who could experience PHA) forgot more details from the movie than people in the non-PHA group (who could not experience PHA). But in Test 2, after the suggestion was cancelled, this memory loss was reversed. People in the PHA group correctly recognized just as many details from the movie as people in the non-PHA group. Somewhat surprisingly, however, the suggestion to forget was selective in its impact. Although people in the PHA group had difficulty remembering the content of the movie following the forget suggestion, they had no difficulty remembering the context in which they saw the movie.
This finding—that PHA temporarily disrupted some people’s ability to recall the past—echoes decades of hypnosis research. What is entirely new in Mendelsohn et al.’s study is their demonstration that PHA was associated with a specific pattern of brain activation. Consistent with what normally occurs in remembering, when people in the non-PHA group performed the recognition task and successfully remembered what happened in the movie, fMRI showed high levels of activity in areas responsible for visualizing scenes (the occipital lobes) and for analyzing verbally presented scenarios (the left temporal lobe). In stark contrast, when people in the PHA group performed the recognition task and failed to remember the content of the movie, fMRI showed little or no activity in these areas. Also, fMRI showed enhanced activity in another area (the prefrontal cortex) responsible for regulating activity in other brain areas.
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