Persistent Memories

What happens when a memory disappears? Once upon a time, I could actually recall the details of organic chemistry. But then I took the class final and promptly forgot every piece of information related to the chemical properties of the carbon atom. This raises the obvious question: What does it mean to forget? Do we actually delete our memories, like an unwanted computer file expunged from the hard drive? Or does the memory always remain, a persistent neuronal ensemble that we just forget how to find?

A new paper in Neuron by scientists at UC-Irvine and Princeton suggests the latter alternative. The forgotten memory doesn't disappear - we just can't remember where we put it. The experiment went like this: a few dozen undergrads were put in an fMRI machine and shown a series of different words. They were then asked to perform various cognitive tasks, such as thinking about how the word is used, or pronouncing the word backwards.

A short while later, the students were shown the same words a second time and were asked to recall any details associated with the words. Once again, the fMRI machine captured their brain activity by measuring shifts in cortical blood flow. Here's where things got interesting: When the students demonstrated a strong recollection of the task details, their brain produced a pattern of activity that was nearly identical to that produced during the task itself. In other words, the memory was a facsimile of experience. However, when the students failed to recall the task details the brain scanner still revealed a recognizable pattern of activity. (The pattern was weaker, but persistent.) Although the students had no memory of the task, their brain clearly did.

One of the interesting implications of this experiment has to do with why memory declines with age. The conventional assumption is that memory loss occurs because our memories vanish, because cells die and the hippocampus gets tired. But what if memory loss is actually triggered by the steady degradation of the frontal cortex, a brain area associated with memory retrieval? (The frontal cortex starts to lose cell density at about the same time we start to lose our memory - in our mid-thirties.) This suggests that our memories are still there, waiting to be found, like a misfiled piece of paper. The struggle of aging, then, isn't simply a matter of holding on to the past - the brain has a seemingly infinite hard drive. Instead, the challenge is remembering where all of our memories are.

For more on the subject, check out this article on tip-of-the-tongue moments.

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I don't want to discount degradation of the frontal cortex and its role in cognitive decline. However, hippocampal atrophy is very real, and it happens to almost everyone to some extent. I worked in a memory and aging lab for two years, and even healthy older adults who perform well on various tests of memory show a pretty significant amount of atrophy in the medial temporal lobes. I'll grant that there is probably a relationship between decline in the cell density of frontal cortex and likelihood of recall for any given memory, but it's certainly not the only contributing neural factor.

By Will Moore (not verified) on 09 Sep 2009 #permalink

I'll grant that there is probably a relationship between decline in the cell density of frontal cortex and likelihood of recall for any given memory, but it's certainly not the only contributing neural factor.

I want to add that, related to Jonah's class final anecdote - many (most?) college kids cram before exams, which can prevent good sleep and thus prevent the quality consolidation of nascent information into long-term memory. I wonder how that poor consolidation relates to this paper - whether the memory fails to take root or just never gets indexed properly.

My personal experience is that much of memory remains, even if not being used. I had a cognitive deficit decade and now that I am recovering, I occasionally find myself being able to retrieve all sorts of information, like images of children in my first grade class, and where they sat. Obviously, I had not thought about that in not just 10 years, but probably 50 years, when I last sat there.

Additionally, when testing my recall, I found myself able to remember not just the presidents since Eisenhower, but also their vice-presidents, wives, children and pets. Remembering their opponents was a real surprise. I am left to believe that much of memory does remain intact.

I had the recent experience of remembering a movie, The Lady from Shanghai, in terms of the story, but as I watched it, so many scenes seemed totally unfamiliar, and I would have been wrong on many details, and certainly did not remember key events. Yet when thinking (visually remembering) about the film I had seen that very evening before going to sleep, I was watching it playback in my own mind, only to have the original film (viewed at least five-ten years earlier) appear as some sort of transparency off to the side, in the background of my mind. It was really remarkable watching both images in my mind as if being played on different projectors.

Needless to say, I am glad to be getting my visual memory back.

By Elizabeth (not verified) on 09 Sep 2009 #permalink

This doesn't surprise me, my daughter has FAS (Fetal Alcohol Syndrome,) and one of the quirks of FAS is something I call on again off again. She might learn a series of facts - like multiplication facts and she'll be able to recite them for a few days. Then they'll disappear and it will be like she has never learned them. The facts will be "gone" for some time and then magically one day they "reappear" and she knows them again. The theory goes that she just can't reliably access the information it isn't "forgetting" its the frontal cortex struggling to retrieve the facts.

When you figure out how to fix it, let me know.

By amybuilds (not verified) on 09 Sep 2009 #permalink

The old adage "use it or lose" should read "Use it or be less likely to access it". Apparently this retrieval problem as opposed to actual loss of information also applies to old habitual behaviors. A therapist who works with substance abusers recently remarked that an addict could go for years without shooting up but even after not using all of that time, still could find himself shooting up, without having made a conscious decision to do so, just because the conditions were right. The substance was available, the time of day was right, the price was right, the right music was playing, he was in the company of fellow users, etc., his state of mind could benefit from a fix, etc. I found that hard to beleive until a day or so later I found myself with my hand in the cookie jar without consciously having decided to have a cookie.

The short story "Coleridge's Dream" by Jorge Luis Borges, relates Coleridge's loss of memory about the poem "Kubla Khan." In Coleridge's case he remained unable to recall the lost fragment of the poem.

By OftenWrongTed (not verified) on 09 Sep 2009 #permalink

Jonah, your hard-drive analogy actually matches the memory loss-of-access theory, as opposed to the deletion one. On most file systems, "deletion" of a file simply means removing the entry from the file access table. The actual bits in the sectors occupied by the file aren't touched by the deletion, unless you use an NSA-grade file shredding utility.

to quote Nietzsche "The existence of forgetting has never been proved: we only know that some things do not come to our mind when we want them to."

The last commenter says that "it would be nice" if there were a link to the article you cite as the basis for this post. Here's what would be nicer: Your knowing anything regarding the subject about which you write with an authority that could only grow from ignorance. Ebbinghaus was already considering the role of retrieval in forgetting in the 1880s. The role of retrieval in forgetting was much discussed when I was in grad school a quarter of a century ago. Indeed, a major impetus for the so-called "cognitive revolution" in psychology was an interest in the process of forgetting and the role of retrieval failure therein.

Also, you appear to be confused about what is forgotten. First of all, the blood flows that show up in an MRI are neither the memory nor the storage system for the memory. They simply indicate activation of a group of cells. We have very little if any idea what is going on in and between those cells that results in the experience of recall. So the strength of the flow says next to nothing about the status of the memory. Without having the advantage of reading the article in question, I'm going to guess that the authors know this, although you do not seem to understand at all what is at issue.

Your use of the phrase "facsimile of experience" shows how little actual thinking you have done about this subject. Whatever a memory is, it is not a copy of a person's experience, as Elizabeth Loftus and others have shown again and again, and as we can also know through logical analysis. We have known for a long time that experience does not come into our eyes and ears (and other sense organs) for reliable storage somewhere in the brain. The process of retrieval itself distorts memory, which is one of the reasons it has been difficult to disentangle the roles of decay and retrieval failure in forgetting.

The list of your errors does not stop here, but I am going to. Read cognitive psychology textbooks and at least one history of psychology. I would be glad to direct your reading. While you do the actual work necessary to acquaint yourself with the basics of the field, I suggest that you suspend your blog on cognitive neuroscience. You know nothing.

Perhaps Mr Lehrer is writing out of a mere, layman interest on this subject, Mr Gerard? At no point in this article does he appear to claim any expertise in the topic under discussion, and quite clearly in the Profile/About section is described as a contributing editor to a website (while his other listed achievements appear to be predominantly journalistic in nature).
Whilst it would be good practice in journalism to cite, specifically, his source[s]; I for one would not mistake this article for anything other than journalism on a scientific topic. As for suspending his blog? Were everyone to suspend their writings owing to ever having been heckled for amateurism, I would be surprised if any "blogger" updated ever again!

(I myself, for example, take a fairly keen interest in Quantum Mechanics - I don't for a moment claim to be an expert on the subject, nor could I write about it in anything remotely approaching a "professional" capacity; but were I to blog about it, I wouldn't make any such claims [neither - in this article at least - has Mr Lehrer] and my neophytism would undoubtedly show through in my writing...)

In short: the world would be a terribly dull place were people not allowed to voice and discuss their interests and should those interests concern topics of science and an individual writes an article about them that happens to be more journalistic than professionally scientific, so what? If people want expert information on a subject, they will no doubt seek out expert research; if they're simply after some basic information that is understandable in "layman's terms" then I daresay this article would suffice.

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