It’s a poor sort of memory that only works backwards.
—LEWIS CARROLL
ELSEWHERE I DESCRIBE MEMORY BANKS AS A LIBRARY that we can visit in our heads. That’s the traditional way of seeing it, but it isn’t remotely accurate. Memory is an activity and not a vault. The brain stores different aspects of any one memory in different parts of the brain. What was seen, what was heard, the smell, touch, taste, the emotional input—all are contributed by their specialist areas. Visual memory’s called up from the occipital lobe, auditory memory from the temporal, working together in a synchronized way. It’s not a place, but a process, and a process not unlike music made by an orchestra. In short, it works in just the same way that consciousness does.
Why do some people have good memories and others bad? My sister has an extraordinary memory for our shared childhoods, which puts me at a disadvantage, when I’m quoted at age eight in a fight over an ice-cream scoop. Partly, the reason some people retain the “film” of the past in such vivid detail is that they use their memories more. To keep a memory you have to keep having the memory, revisiting the memory, using it, so as to keep that collection of neurons imprinted and those synaptic connections in place. If they’re not used, then they wither. To remember things you have to go through the process of remembering them again. You make a new memory each time you remember, revisiting the route from neuron to neuron. Researchers have discovered that there is an actual anatomical change in the laying down of long-term memories. The axons grow new synapses and new proteins are made in the nucleus of the neuron. There’s a change at the cellular level, something that doesn’t occur in the making of short-term memories. In his book In Search of Memory, Eric Kandel, who was awarded the 2000 Nobel Prize for medicine, elaborates on this idea that in order to convert a short-term memory into a long-term one, we need to care about it enough, whether for happy or unhappy reasons, and that our caring has physiological effects. One hit of neurotransmitter and the synapse is improved. Five hits and the cell is alerted to this (whatever it is) being something important. It sends the information, via a protein, to the nucleus that triggers the genetic switch for the growth of the new synaptic port. There are two ways in, it seems, via quantity or quality: either via repetition, thinking about something over and over, or by means of the intensity of a shock or equivalent emotional event.
The things that stick aren’t always the obvious things. Odd, oblique, incidental, tangential things stick. As the writer Elizabeth Bowen said once in an interview, “The charm, one might say the genius, of memory is that it is choosy, chancy, and temperamental: it rejects the edifying cathedral and indelibly photographs the small boy outside, chewing a hunk of melon in the dust.” Montaigne, in the sixteenth century, was more succinct but less alluring, as is his way, in writing that “[t]he memory represents to us not what we choose but what it pleases.” What you care about isn’t necessarily what you think you care about.
When you remember, it’s a memory of the memory that you’re having. You don’t go into the library of your memory and pick up the book and read your past. In a sense, you write the book all over again. And research shows that if you don’t take the trouble to rewrite the books, the books disappear. It’s rather like those wardrobe nannies who insist that anything not worn for twelve months ought to be put in a bin bag. You haven’t thought about this for years so I’m chucking it out. It sends the nanny in and chucks, and it’s only when you open the wardrobe that you discover your fake fur jacket/caravan holiday memory is missing. Or, to use another analogy, we need to keep digging out paths in the snow. If we don’t, snow eliminates them. Get out there and dig those paths. Maintain them and you can keep walking on them. Don’t maintain them and they are gone. How does the brain do this? The nanny in question’s an enzyme called PP1 that removes the phosphate from the target protein and deactivates it, in effect wiping a particular memory from the slate.
There are four levels of memory. The first, sensory memory, isn’t really memory at all. It’s stuff that the eyes see, that the brain may know (far more goes in than is retrievable), but the conscious self doesn’t notice. Take the scene in front of my eyes just to the side of the laptop, right now, for instance. The books and papers, used coffee cups, the tin of salted almonds, the box of old photographs waiting to be put into albums, the postcards, pens, mobile phone, plus the jewelry and homework the children left there—everything that’s spread on the coffee table beside me as I write this—made a brief sensory imprint in my mind, but hadn’t been processed any further until I turned my attention tableward. Perhaps a probe could find it in my head, if probes and scanners grew that sophisticated. Perhaps I might be an unwitting witness to a crime that my eyes saw but I didn’t register, while looking out of the window in the city at the cherry blossom on a busy street, where among the traffic and pedestrians, somebody was quietly and efficiently killed with a knife. I saw it but I didn’t register it. It was among the things my eyes were seeing while I was concentrating on something else. That’s the first level of memory.
The second level is the working memory. This is the material we hold in mind, temporarily, like part of a mathematical calculation we put aside while doing the second part, ready to add the two numbers together, or a phone number we need to remember that was given to us when we didn’t have a pen. It’s recited in the head and retained for as long as we need it. Then we forget it. Nancy is beginning to forget things that have just happened, things that have just been said to her, and how to finish a sentence she’s only halfway through speaking. She’s losing her working memory and is unable to hold things in mind. The man known only by the initials H.M., a neurologically much-quoted epilepsy victim—run over by a bicycle at age nine, and in his twenties at the time of being a research subject in the 1950s—with his temporal lobe function diminished and hippocampus removed, could still remember new things done or said for a few minutes. His working memory survived although his short-term memory, ordinarily the next phase in the process, no longer functioned.
Scientists classify short-term memory differently according to length. Neurologists tend to talk about it as short-to-medium term. The things we did yesterday, last weekend, even the wedding we danced at the weekend before that, can be described as held in short-term memory. The process of converting a select few of these into long-term memory, forming strong memories that survive, can take weeks, and it’s thought most of the work’s done while we sleep.
Memory making is a single-track road. To get from the sensory memory stage through working memory into short-term and thus into long-term memory is like going along one of those winding narrow routes that stretch out into the fingers of the coast of Argyll in remote western Scotland. There’s only one road from the village of Sensory to Long Term. To get to Long Term you need to go through the other three villages first. In other words, if there is a break in the road, a flash flood, say, and then a road slip, a section of the road sagging and tipping down the hill, and the road becomes impassable, nothing can get to Long Term. That’s what happens in Alzheimer’s. The short-term memory fails, is gone for good, and so nothing new can be processed into long-term memory. The poor old village of Short Term is obliterated entirely. The brain has an alternative route out of Long Term, though not in. Eventually it, too, will be obliterated.
Once you get into long-term memory, the road branches. Down one road there’s implicit memory, and down the other, explicit. Explicit further branches, into episodic and semantic. Implicit is another way of saying procedural memory, the one that deals with the things that we do as if automatically. Riding a bike, driving a car, knowing a dance, playing the flute: these skill memories are taken care of by the cerebellum in league with the basal ganglia, four clusters of neurons at the base of the brain that help initiate and control movement. Serotonin is the neurotransmitter of choice in the making of implicit memories, and dopamine in the creating of explicit memory. Researchers think that implicit memories are laid down while we’re in REM (rapid eye movement) sleep, in which our dreams are most vivid, and that explicit memories are made during non-REM sleep.
Explicit memory is the sort we need actively to call up, “thinking” in the familiar conscious sense. Episodic is autobiographical, and locates things in time and sequence: “I ate eggs for breakfast, went to the life drawing class in the village, and after lunch Nancy and I took the dogs to the beach.” That’s episodic memory. Semantic memory is encyclopedic, intellectual, for facts.
Alzheimer’s damages the episodic (autobiographical) memory first and worst. The semantic survives longer. Sufferers might know very little about themselves, nothing whatever about what happened ten minutes ago, and yet might be able to talk at length about the history, the battles, and the princes associated with a ruined castle visited on a Sunday afternoon outing, using long-term semantic memory. Alzheimer’s sufferers of a certain generation, taught screeds of poetry by rote at school, find they can still recite their twenty verses of Longfellow with perfect accuracy, until quite late in the disease.
Because memory is a process, relying on neurons to fire up in the same sequence each time we remember, memory can be wrong. Memory, indeed, is notoriously unreliable. Why should it be, though, when we rely on it for survival? Perhaps that’s the point. Perhaps our brains are more dedicated to our psychic health than to the truth. What we see, the way that we see it, and the way we remember it are essentially subjective. The process of making memories and then remembering them is both technical and personal. The synapses may not reproduce their original pattern. It’s like the old fable of the bad carpenter’s table, in which leg number two is drawn from leg number one (and is a bit out), and then leg number three is drawn from two and is even more wrong, and number four, drawn from number three, isn’t anything like the same length or shape as number one. Something we thought, imagined, doubted, added on one occasion of remembering distorts the memory for next time it’s called up. How then can I be sure of what I have done and experienced in my life? There are some slices of time, moments, collections of moments, from the deep past that are unlikely, eccentric, unaccountably preserved, and which I treasure. But are they accurate, or are they a story I tell myself for my own reasons? There’s no way of knowing for sure. Not only do you, the reader, perhaps suspect that not all of what I write about life with Nancy is exactly as it happened, but strictly speaking, knowing the mechanism to be emotional, I ought to suspect the same. Our memories of things are never objective. We interact with them and add meaning; highlight certain aspects and throw others into shadow.
The brain is selective about memory. Not only about the details, but about the quantity. This selection and editing is important in life having a shape. The truth of this is illustrated by the problems encountered by people who have too much memory. There have been neurological cases of people who can’t forget things. Their brains can’t filter out or edit and everything is retained. They can tell you in detail exactly what was said or done on this day last year. What happens to them is that they lose the big picture, a sense of perspective, and are overwhelmed by detail. No choices can be made, no judgments. Everything is of equal importance. Because of this, they don’t always function well as humans. So it appears that in principle and in moderation, forgetting is important. As Nietzsche wrote, “There could be no happiness, cheerfulness, hope, pride, immediacy, without forgetfulness.”