Memory is a Liar

The NY Times Magazine had an interesting article on deja vu and memory. It's about a group of cognitive psychologists who are using patients afflicted with a continual sense of deja vu (sounds a little hellish to me) in order to understand the neural mechanisms of remembering.

This is a relatively new field. While psychologists and neuroscientists have long wondered how we create new memories, they have shied away from a far more complicated question: how we remember our old memories.

But now that's beginning to change. The Times' article doesn't discuss this research, but I think some of the most interesting memory work is being done in the field of reconsolidation, or how we continually recreate our past experiences. It all began with a set of extraordinary experiments done in 2000 by Karim Nader, Glenn Shafe and Joseph LeDoux at NYU. To make a long story short, they demonstrated that the act of remembering changes your memories. Nader, et.al. proved this by conditioning rats to associate a loud noise with a mild, electrical shock. (When it comes to pain, the mind is a quick learner.) As predicted, injecting a chemical that stops new proteins from being created also prevented the rats from creating a fearful memory. Since their brains were unable to connect their context to the electrical shock, the shock was always shocking.

But Nader, Ledoux and Shafe took this simple experiment one step further. First, they made sure that the rats had a strong memory associating the shock with the noise. They wanted rodents that would cower in fear whenever the sound was played. After letting this memory solidify for up to 45 days, they re-exposed the rats to the scary noise and injected a protein inhibitor into their brain. But what made their experiment different was its timing. Instead of interrupting the process of making a memory, they interrupted the process of remembering a memory, injecting the noxious chemical at the exact moment the rats were recalling what the noise meant. According to the dogma of remembrance, nothing much should have happened. The long-term memory should exist independently of its recall, filed away in one of the brain's protected file-cabinets. After the poison is flushed out of their cells, the rats should remember their fear. The noise should still remind them of the shock.

But this isn't what happened. When Nader, et.al. blocked the rats from remembering their fearful memory, the original memory trace also disappeared. After only a single interruption of the recollection process, their fear was erased. The rats became amnesiacs.

At first glance, this experimental observation seems incongruous. After all, we like to think of our memories as being immutable impressions, somehow separate from the act of remembering them. But they aren't. A memory is only as real as the last time you remembered it. The more you remember something, the less accurate the memory becomes.

The Nader experiment, simple as it seems, requires science to completely re-imagine its theories of remembering. It reveals memory as a ceaseless process, not a repository of inert information. It shows us that every time we remember anything, the neuronal structure of the memory is delicately transformed, a process called reconsolidation. (Freud called this process Nachtraglichkeit, or "retroactivity".) The memory is altered in the absence of the original stimulus, becoming less about what you remember and more about you. So the purely objective memory is the one memory you will never know. The moment you remember something is the same moment you forget what it was really like.

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Very interesting, but technically they didn't interrupt the process of remembering a memory, but the reconsolidation following remembering. They injected anisomycin after the tone (and therefore the process of remembering) and if the injection was timed soon enough (less than 6 hr) after tone, the memory was lost.

Sounds like a useful mechanism for erasing PTSD (which Nader's early work focused on), if only anisomycin wasn't such a nasty drug.

By Crusty Dem (not verified) on 04 Jul 2006 #permalink

"A memory is only as real as the last time you remembered it. The more you remember something, the less accurate the memory becomes."

This seems like quite a stretch to me. This experiment was about learning to experience a situation differently, not just memory.

I'd mention the last time I was impressed by just how vivid a memory I had triggered of something I hadn't thought of in years, but I've forgotten. Nevertheless I remember the amazement, even if not the details, and I'm sure some day soon, another distant memory will come back, triggered by some sensation or situation. Where was it hiding? Where did it go that I can't think of an example now?

I don't know how to study that in rats. Hopefully there will be some meaningful ways to trigger long term memories
in subjects being measured with functional neuroimaging. I'm not up to date to know how much like that has already been done. With so many interesting phenomena in long term memory, like repression, false memories, and tip-of-the-tongue phenomenon, I would think there could be some very interesting findings in human subjects.

Why were they testing on rats to decide for humans? Yes, brain function may be similar, but it will never be the same, so there is no reason to rest on an animal if it isn't directly relation to a human, so why not just test on a human?