any memory; no referring to Rachel K. as Rachel C. once she's changed her name. It's no exaggeration to say that postal-code memory is a key component of virtually every modern computer.
But not, alas, in humans. Having postal-code memory would
*Google for 'change blindness' if you've never seen a demonstration; if you haven't seen Derren Brown's 'person swap' video on YouTube (www.youtube.com/watch?v= CFaY3YcMgiT), you're missing something special.
have been terrifically useful for us, but evolution never discovered the right part of the mountain range. We humans rarely — if ever — know precisely where a piece of information is stored (beyond the extremely vague 'somewhere inside the brain'), and our memory evolved according to an entirely different logic.
In lieu of postal-code memory we wound up with what I'll call 'contextual memory': we pull things out of our memory by using context, or clues, that hints at what we are looking for. It's as if we say to ourselves, every time we need a particular fact, 'Urn, hello, brain, sorry to bother you, but I need a memory that's about the War of 1812. Got anything for me?' Often, our brain obliges, quickly and accurately yielding precisely the information we want. For instance, if I ask you to name the director who made the movies E.T. and Schindler's List, you might well come up with the answer within milliseconds — even though you may not have the foggiest idea where in your brain that information was stored.* In general, we pull what we need from memory by using various clues, and when things go well, the detail we need just 'pops' into our mind. In this respect, accessing a memory is a bit like breathing — most of it comes naturally.
And what comes to mind most naturally often depends on context. We're more likely to remember what we know about gardening when we are in the garden, more likely to remember what we know about cooking when we are in the kitchen. Context, sometimes for better and sometimes for worse, is one of the most powerful cues affecting our memory.
Contextual memory has a very long evolutionary history; it's found not just in humans, but also in apes and monkeys, rats and mice, and even spiders and snails. Scientists picked up the first hints of the power of contextual cues almost a hundred years ago, in 1917, when Harvey Carr, a student of the famous behaviorist psychologist John Watson, was running a fairly routine study that involved training rats to run in a maze. Along the way, Carr discovered that the rats
*f m speaking, of course, of Steven Spielberg.
were quite sensitive to factors that had nothing to do with the maze itself. A rat that was trained in a room with electric light, for example, would run the maze better when tested in a room with electric light rather than natural light. The context in which the rat was tested — that is, the environment to which it had grown accustomed — affected its memory of how to run in the maze, even though lighting was not germane to the task. It has since become clear that just about every biological creature uses context, relevant or not, as a major guiding force in accessing memory.
Contextual memory may have evolved as a hack, a crude way of compensating for the fact that nature couldn't work out a proper postal-code system for accessing stored information, but there are still some obvious virtues in the system we do have. For one thing, instead of treating all memories equally, as a computer might do, context- dependent memory prioritizes, bringing most quickly to mind things that are common, things that we've needed recently, and things that have previously been relevant in situations that are similar to our current circumstances — exactly the sort of information that we tend to need the most. For another thing, context-dependent memories can be searched rapidly in parallel, and as such may represent a good way of compensating for the fact that neurons are millions of times slower than the memory chips used by digital computers. What's more, we (unlike computers) don't have to keep track of the details of our own internal hardware; most of the time, finding what we need in our memory becomes a matter of asking ourself the right question, not identifying a particular set of brain cells.*
Nobody knows for sure how this works, but my best guess is that each of our brain's memories acts autonomously, on its own, in re
*The trick — the same goes for using search engines — is to employ as many distinctive cues as possible, thereby eliciting fewer and fewer extraneous memories. The more specific your cues are ('where I left my car last night when the street was full' rather than 'where I left my car'), the greater the chance that you'll zero in on the fact you need.
sponse to whatever queries it might match, thereby eliminating the need for a central agent to keep a map of memory storage locations. Of course, when you rely on matches rather than specific locations that are known in advance, there's no guarantee that the right memory will respond; the fewer the cues you provide, the more 'hits' your memory will serve up, and as a consequence the memory that you actually want may get buried among those that you don't want.
Contextual memory has its price, and that price is reliability. Because human memory is so thoroughly driven by cues, rather than location in the brain, we can easily get confused. The reason I can't remember what I had for breakfast yesterday is that yesterday's breakfast is too easily confused with that of the day before, and the day before that. Was it yogurt Tuesday, waffles Wednesday, or the other way around? There are too many Tuesdays, too many Wednesdays, and too many near-identical waffles for a cue-driven system to keep straight. (Ditto for any pilot foolish enough to rely on memory instead of a checklist — one takeoff would blur together with the next. Sooner or later the landing gear would be forgotten.)
Whenever context changes, there's a chance of a problem. I, for example, recently found myself at a party where I was awestruck by the sudden appearance of the luminescent and brilliantly talented actress who played the role of Claire Fisher in the television show Six Feet Under. I thought it would be fun to introduce myself. Ordinarily, I probably would have had little trouble remembering her name — I'd seen it in the credits dozens of times, but at that moment I drew a total blank. By the time I got a friend to remind me of her name, the actress was already leaving; I had missed my chance. In hindsight, it's perfectly clear why I couldn't remember her name: the context was all wrong. I was used to seeing the person in question on TV, in character, in a fictional show set in Los Angeles, not in real life, in New York, in the company of the mutual acquaintances who had brought me to the party. In the memory of a human being, context is all, and sometimes, as in this instance, context works against us.
Context exerts its powerful effect — sometimes helping us, sometimes not — in part by 'priming' the pump of our memory; when I hear the word doctor, it becomes easier to recognize the word nurse. Had someone said 'Lauren' (the first name of the actor in question), I probably could have instantly come up with her last name (Ambrose), but without the right cue, I could only draw a blank.
The thing about context is that it is always with us — even when it's not really relevant to what we are trying to remember. Carr's experiment with rats, for instance, has a parallel with humans in a remarkable experiment with scuba divers. The divers were asked to memorize a list of words while underwater. Like the rats that needed
