I’ve proposed that what neuroscience recognizes as prospective memory is actually part of a third memory system, active memory, which is intermediate between short-term and long-term memory. Indeed, the need for an intermediate memory system seems obvious once you contrast the capacities (tiny versus vast) and durations (seconds versus lifetime) of short-term and long-term memory.
So, how is active memory organized? It contains the key information about everything that’s “on our mind”: everything we are thinking of doing at some point in the future, and everything we are thinking of merely thinking about. Each one of these is a potential “attendum”—something we might attend to. You can think of each attendum as having a marker or token, which corresponds to the “hot spot” of the neural representation of the information. And here’s the key feature: each marker has associated it with it a salience tag which measures the importance or relevance of the attendum at the present time. It is the salience tag which determines the likelihood of an attendum actually being attended to: the higher the salience, the likelier we are to attend.
It’s quickly obvious that these salience tags are in fact small programs (in the computer sense) which run continually and unconsciously and calculate the salience at the present time as a function of the current environment (physical and emotional / cognitive). For instance, the salience of “stop and get milk” is very low until we sight the supermarket, at which point it becomes very high. That could be done with one line of computer code, along the lines of
IF in-sight(supermarket) THEN high-value ELSE low-value.
The salience program for thinking about a troubled personal relationship would be much more complex. There are times when you would want to suppress all thoughts of the absent friend, and times when thinking through the history of the friendship (all of which would be loaded in active memory, for fast access) would be desirable.
Where do these salience tags come from? It’s one of the great recent insights of modern neuroscience that all of our decisions are mediated by conscious emotion. We are often not conscious of the information we are evaluating emotionally (hence the trusted “gut feeling” at one end of a continuum and apparent pure irrationality at the other), but without conscious emotion we are unable to choose, unable to evaluate. Everything we do (and, according to me, everything we think about) is driven by the carrot of reward and the stick of punishment, by the balance of good feelings versus bad feelings. The brain continually attempts to optimize the difference between them. Our feelings are the currency of our motivation.
It thus becomes obvious that you could not create a salience tag or write a salience program without conscious emotion. In all but the most extraordinary cases, there would be conscious thought as well. The first time we think of doing something, or have a new thought that we might want to continue later—that’s when our feelings evaluate the importance of the action or thought, and that’s when a salience program is written, and stored in active memory along with the information itself.
As you probably know, the brain chemical that mediates reward is dopamine (DA). Of all the principal neuromodulators (brain chemicals that control the state of the brain), it’s the best understood. When we perceive a potential reward, that perception is accompanied by the release of DA, and the subjective feeling of desire appears to be caused by DA release as well. So, as we evaluate the salience of a new attendum, that salience is in fact measured by the level of DA it is evoking.
(Strictly speaking, this is known to be true only of attendums that relate to potential rewards, like “stop and get milk” or “apply for that job.” Little is known about the neurochemical correlate of punishment, but I’m betting it’s DA as well. DA serves a secondary role as the mediator of movement (damage to this DA system causes Parkinson’s), and that makes sense because action is necessary to secure a potential reward. But action is also necessary to flee a potential punishment. I believe that DA is in fact responsible for turning on aspects of subjective awareness (“phenomenal consciousness” in the terminology of philosophy of mind), and subjective awareness is all about feelings, of all kinds.)
Now, here’s what’s really interesting. It’s well established that DA holds information in working memory. Without adequate DA, you can’t remember a phone number. Drugs which boost DA are the long-time preferred treatment for AD(H)D, which until relatively recently was presumed to be caused strictly by DA deficits. Yet as we have conceptualized it, the problem with ADD isn’t just in working memory, it’s in active memory. The absent-mindedness that is a core ADD trait is a problem holding information in active memory.
This ties together very neatly. The current DA level evoked by a newly thought-of attendum is a measure of its salience. We need some chemical to hold the attendum in active memory, and the more salient the attendum, the more strongly we want to hold it there—if active memory capacity is limited, we want the least salient attendums to be bumped out first. It’s simply economical to use the already-evoked DA level to hold the information in memory. Holding it there with some other chemical (virtually all signals in the brain are chemical) would just require an extra step of translation.
Again, we have given a simplified picture by talking of salience tags rather than programs. When we create a simple salience program, we are contemplating the importance of the attendum under various foreseeable circumstances. We imagine the very intense reward we expect to get from the bedtime milk and cookies, and that creates a high DA level, but we link the expectation of that reward to the sight of the supermarket, and the sight of the supermarket only. We are equally conscious of the fact that we don’t want to think about buying milk until we see the supermarket. For classic prospective memory tasks like this, there is probably a built-in program that codes for a low salience level when the evoking stimulus is absent—low enough so that it has no chance of being attended to, but not so low that it gets bumped out of active memory altogether.
What are more complex salience programs like? Let’s consider two things I’ve been thinking about a lot and that are more or less equally important to me: the Red Sox, and this series of posts. I usually wake up in the morning and think about the possibility of writing another post in the late evening, but that thought is very unlikely to cross my mind after about 5:00 PM, because the Red Sox salience program is driven by time of day and the salience starts to ramp up to very high levels a few hours before game time. I’ll usually mess around with baseball stats for an hour or two after the game is over—and then at some point, as the salience of the Red Sox drops through sheer acclimation (more about that if I eveer get around to considering boredom), the thought occurs to me that it might be time to write another post. And that thought is compelling (salient) in exact proportion to how many days it’s been since the last one. With enough introspection, it should be possible to identify a great many standard salience program modules like these. The most complex salience programs, such as those for thinking about weighty emotional issues, are probably subject to frequent revision and elaboration.
So, we have active memory filled with attendums, and each has a salience program that monitors the environment. The programs set the level of DA for each attendum at that moment, and one of those attendums is selected for attention.
How? That’s the subject of the next installment.