There is, however, also a well-established division by temporal direction: retrospective memory, memory for the past, versus prospective memory, memory for things we plan to do in the (usually near) future. As mentioned at the end of Part 1, the classic example of prospective memory is telling yourself to “stop and get milk.” As you finish the last drop of milk with your Cheerios and contemplate the delicious chocolate chip cookies you expect to have for dessert that evening, you tell yourself, do not forget to stop and get milk on the way home. The thought may well not cross your mind again all day at work. But if your prospective memory functions well, when you approach the supermarket on your homeward commute, its sight will trigger it, the thought popping miraculously into consciousness —even if you are deep in thought about, say, evening plans even more alluring than cookies and milk.
Prospective memory has been fairly well-studied. For instance, it has been established that holding a task in prospective memory takes a very small but measurable toll on performance of other tasks.
A question that, as far as I know, remains unasked by memory researchers: is prospective memory a type of short-term memory, or long-term? I believe the answer to both is “pretty obviously not.” Short-term memory lasts for seconds, not hours. Long-term memory lasts for lifetimes, not hours. These answers thus lead to an even more provocative unasked question: is there a third fundamental memory system intermediate between short- and long-term memory, which includes but is almost certainly not limited to prospective memory?
(The reason why such a hypothesized memory system is likely to store more than just prospective memory is, I think, obvious: prospective memory just isn’t important enough. Even if we realize that it stores not just “stop and get milk” but “look for a new job some time before the end of this month” and a host of other such important intentions, it still falls short. Prospective memory researchers believe it to be the repository of elaborate career plans (even if these are hard to study in the lab), yet no one has suggested that it’s a major component of memory on a par with short-term and long-term.)
Is there any other reason to believe in a third, intermediate, memory system? While it’s often perilous or even unwise to draw parallels between the brain and computers, I think this is one of the times when it yields insight. And computers, of course, have three levels of memory: the memory cache on the CPU chip, RAM, and the hard drive. The memory cache quite obviously corresponds to working memory—it’s the information being processed right now, directly, by the CPU or by the executive engine in the prefrontal cortex. The hard drive obviously corresponds to long-term memory; they both have very large capacities and contain information which is potentially permanent. So we ought to be asking: is there something in the brain that corresponds to RAM? The question becomes even more intriguing when we realize that the chief distinguishing feature of the three memory levels in a computer is not the duration of storage, but the speed of access by the CPU. If it’s true that permanence of storage and speed of access represent a fundamental trade-off in information storage, then the need for an intermediate third memory system in the brain becomes even more apparent.
We could call this hypothesized intermediate memory system “medium-term memory,” but that would tell us nothing we already didn’t know. So let’s emphasize the possible importance of speed of access and call it active memory. Prospective memory refers to a type of information that can be stored in active memory.
And as just argued, there must be other types of information stored there as well. These other types must bear some relationship to prospective memory as currently defined, and yet no one has expanded the concept of prospective memory to embrace them, and hence discovered the existence of active memory. So what insight is everyone missing?
Next: What’s the Big Idea?