Eric M. Van (ericmvan) wrote,
Eric M. Van
ericmvan

A Model for Attention-Switching, Part III: The Contents of Active Memory

Those of you have heard my talks on the neurochemical basis of personality, or on the cognitive structure of feelings, are familiar with the big idea that contributes significantly to the former and forms the foundation of the latter. It’s probably the best single insight I’ve had about the brain.

The insight is this: the brain treats thinking as one of the things we do. For the brain, there is no fundamental difference between thinking and doing; thinking is a type of doing. And this means that any mechanism in the brain that regulates action also regulates thought, as a form of action.

All actions are motivated by rewards and punishments; the idea tells us that the thoughts we think are also motivated the same way. The size of the reward we get merely for thinking turns out to be a key personality trait. In my theory about the nature of feelings, I propose that feelings are not just about what we expect to happen to us in the future, but about what we expect to feel in the future.

Applying this insight to active memory immediately tells us what’s missing. Active memory has to be filled with everything we intend to think about, not just everything we intend to do. In other words, it contains everything that’s “on our minds.”

When we’re sitting, waiting for the bus, and we’ve got nothing to read and no music player to listen to, where do our minds go? We start thinking about things. Things that are on our minds. We might start thinking about a vexing personal problem, or a scientific hypothesis about the brain, or the ideal Red Sox batting order. All of these things would be things we were thinking about recently, and things we were expecting to think about again. That’s why we say they’re “on our minds.” Active memory is a place to store these potential thoughts in a way that makes them much quicker and easier to access than if they were stored in long-term memory, even if there were a system for tagging information in long-term memory as “active” (which I believe is the current conception of the way information is primed for preferential access).

But there’s one more aspect of active memory that we need to complete the picture, and it’s actually the first aspect I was aware of.

In the spring of 1978 I took my first ever psychology course, Robert Coles’ course in Defense Mechanisms. A little Googling makes me fairly certain that the text he used was the recently-published Adaptation to Life, by his Harvard colleague George E. Vaillant. The course met before noon, which meant (I am not proud of this) that I attended only one or two lectures. My roommate, Thomas M. Keane, Jr., was also enrolled in the course, so I was able to keep tabs on the required work through him. (The term paper assignment was to read a biography and analyze the life within in terms of defense mechanisms; I did Humphrey Carpenter’s Tolkien: A Biography and knew I was being an idiot to not make a photocopy for myself before handing it in to my section leader. In my defense, the paper was not only days late, grades were closing tomorrow; I remember running to hand it in.)

At Harvard, there is a two week break called “reading period” between the end of classes and the start of exams. It’s a godsend to people like me; by my senior year I had taken to filling it with twelve subjective days of more or less 28 hours each, with the exact sleep schedule timed to put my morning exams in my circadian afternoon or, better yet, evening. I borrowed Tom’s copy of the text and read the entire thing in two days, then aced the exam, and got an A- in the course in the days before grade inflation (there were lots of little assignments I had never completed).

When my circadian clock shattered into shards in the early 1990s, I got interested in the brain, and I did a lot of introspection about my own. I have often mentioned that what really hooked me on neuroscience was discovering that I personally falsified Robert Cloninger’s theory mapping neurotransmitters to personality traits, as presented in Listening to Prozac—I was in the 99th percentile for half the descriptors of his proposed dopamine trait, “novelty seeking,” and in the 1st percentile for the other half.

Naturally, I also did a lot of thinking about what made me smart. The Coles course was a big clue. Clearly the course and book had been designed so that each week’s lectures constituted a manageable chunk of information. I figured that other students must be absorbing that information each week and writing it to their hard drives. The next week, they’d read another chapter, and to see where it fit in the big picture, they would search their hard drives and fetch this or that relevant bit of information from it, and then they’d write it all back when they were done. And gradually, over the course of a semester, they would build up this edifice of connected knowledge. What I had done instead, it seems, is absorb all of the information at once; I could handle much more of it then my peers. And that allowed me to see all the connections effortlessly; I never had to search my hard drive to fetch missing information that was needed to make full sense of what I was reading, since that information was already in my brain. Wow, I thought: I’ve got, like twenty megabtyes of RAM (gigabytes, now).

So active memory contains not just a list of things we hope to do and a list of things we expect to think about it, but large quantities of information belonging to the latter: as much relevant information, in fact, as will fit.

As I said, there has always been an awareness that there must be a mechanism for priming some of the information in long-term memory for preferential access. What I am proposing that this mechanism in fact consists of a third, intermediate memory system, active memory, which also handles prospective memory tasks like “stop and get milk.” The chief feature that distinguishes active memory from long-term is speed of access by the executive processes of the prefrontal cortex and by working memory. It can thus be thought of as a version of working memory, but of vastly greater capacity. Again, the computer analogy seems applicable: the memory cache on the CPU chip and RAM are very similar, and quite different from the hard drive.

(As an aside, I am hopeful that the concept of active memory will be crucial in figuring out the mechanisms by which memories are consolidated and pruned during sleep.  We have much evidence that this happens, but it won’t be possible to understand what’s going on unless we correctly understand the memory systems involved.)

Next: How Active Memory Works, Part 1

Tags: neuroscience
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