The resistence of memory in hypnotic amnesia

Research just published in neuroscience journal Neuron has discovered some of the brain networks behind post-hypnotic amnesia. Importantly, the study might give us an insight into how memories are repressed from consciousness.

Psychogenic amnesia is a type of memory disorder where there is no brain damage to explain the memory loss. Unlike amnesia after brain injury, which usually causes an inability to form new memories, psychogenic amnesia typically results in the person being unable to remember past events.

Most memory research involves comparing how well people recognise or recall information that they’ve been shown earlier.

One of the difficulties in studying psychogenic amnesia is that you’re never sure whether the memories you’re asking about were taken in, but are inaccessible, or whether the person simply didn’t register the information in the first place.

Amnesia caused by hypnosis is remarkably similar to psychogenic amnesia in many ways, but has the advantage of being temporary and reversible.

This is important because it allows researchers to show people information, then induce hypnotic amnesia and check memory, and then reverse the effects and check memory again.

The final memory check shows that the person genuinely took the information in to start with, so you know that the amnesia was for memories that were definitely there already.

Post-hypnotic amnesia is where a suggestion is given during hypnosis that the person won’t remember a specific event after the hypnosis is over. Because hypnotisability varies between individuals, it doesn’t work for everybody, but for those who experience this type of temporary memory loss, the effect can be quite dramatic.

In an initial session, researchers showed high and low hypnotisable participants a 45 minute film which they were told to remember.

A week later, they were put in an fMRI scanner, hypnotised and told to forget the film when the hypnosis was over. Crucially, they were told that their memories would return when given a specific command.

They were then scanned while being asked “yes/no” questions about both the film itself and other details about the initial session (such as whether the door to the testing room was open).

Unlike facts about the film, the participants were never told to forget these other details, allowing the researchers to test how specific the amnesia was.

The ‘hypnosis resistant’ low hypnotisable participants were equally good at recalling facts about the film and the testing session.

For high hypnotisable participants, although they were good at remembering session details, they were no better than chance at answering the questions about the film. In other words, they would have got the same number of questions right if they flipped a coin – suggesting their memory was quite impaired.

When given the command to remove the amnesia, the high hypnotisable participants could then recall the film as well as the others.

(Partly owing to the scepticism about hypnosis, the researchers also tested another group of people who were told just to pretend to be hypnotised. They performed quite differently – vastly exaggerating their memory difficulties – indicating that the high hypnotisable participants weren’t faking or ‘conforming’).

When trying to recall information when post-hypnotic amnesia was in effect, activity in the temporal lobes and occipital lobes was reduced, while activity in part of the frontal lobes increased.

The areas of the temporal and occipital lobes are known to be involved in dealing with factual and visual information, while the frontal lobes are known to be involved in coordinating other brain areas.

In this case, they seem to be inhibiting the function of other areas, perhaps preventing recall and explaining the amnesia.

Interestingly, when the amnesia was reversed, brain circuits involved in long-term memories became more active as the participants were able to answer questions.

This study might explain how psychogenic amnesia works. Perhaps this syndrome results from the same brain mechanism being ‘locked’ in place, persistently ‘repressing’ memories.

In fact, there’s a whole range of apparently neurological problems but where the person has no recognisable brain damage. These usually get diagnosed as conversion disorder and can involve everything from blindness to paralysis.

Two studies have just come out which point in the same direction as this hypnosis study.

In one, several patients with structurally normal brains were found to have under-activation in certain areas corresponding to their conversion disorder paralysis.

In another, when a patient with conversion disorder was asked to recall the traumatic event which triggered her paralysis, brain activation suddenly dropped in the brain areas that controlled movement in her immobile limbs.

What these studies are suggesting is that problems can arise in the operation of seemingly intact brains that can lead to what appear to be neurological problems.

An analogy might be that while the roads are intact, traffic jams can still bring a city to a standstill. The trick, of course, is to get the traffic flowing again.

The more we understand about how the flow gets disturbed, the more likely we are to help patients get things running smoothly again.

Link to abstract of post-hypnotic amnesia study.
Link to write-up of study from Science News.

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