The Forgetting Machine

by Rodrigo Quian Quiroga

  Optical illusions are a clear example of the way the brain uses unconscious inferences to construct meaning. In Figure 3.3 we see a classic illusion, the Kanizsa triangle, where we infer the shape of a triangle based on its angles and the apparent occlusion of another triangle in back. Even if we know that there is no such triangle, we cannot help but perceive its sides. To the right we can see two circles in relief, one appearing to recede as if pressed into the surface and the other one appearing raised. They happen to be the same circle, rotated 180 degrees. The illusion of relief stems from what seems to be the reflection of light (at bottom and top, respectively) and our assumption based on experience that light always comes from above.

  Figure 3.3

  Kanizsa triangle and illusion of relief

  Another clear manifestation of the importance of experience in giving meaning to what we see is shown by the cases of people blind from birth who become sighted as adults (for example, after cataract surgery). Says Helmholtz:

  The memory traces of previous experience play an even more extensive and influential role in our visual observations . . . The fact that people blind from birth who afterward gain their sight by an operation cannot, before they have touched them, distinguish between such simple forms as a circle and a square by the use of their eyes has been confirmed even more fully by recent studies.

  —HERMANN VON HELMHOLTZ,

  THE FACTS OF PERCEPTION, 1878

  Helmholtz’s observation coincides almost exactly with the conclusions reached some two centuries earlier by John Locke, one of the most renowned British empiricists, by sheer force of thought. Asked by his friend Molyneux how a person blind from birth would perceive, say, a sphere and a cube upon seeing for the first time, Locke states:

  [I] am of opinion that the blind man, at first sight, would not be able with certainty to say which was the globe, which the cube, whilst he only saw them; though he could unerringly name them by his touch, and certainly distinguish them by the difference of their figures felt.

  —JOHN LOCKE, AN ESSAY CONCERNING

  HUMAN UNDERSTANDING, 1690,

  BOOK II, CHAPTER 9, SECTION 8

  Similar arguments denying the possibility of knowledge divorced from experience were brought forth in 1709 by Bishop George Berkeley, another great British empiricist, in An Essay Towards a New Theory of Vision.

  There are in fact multiple reports of people blind from birth who began to use their sight (note that I don’t say “see”) after surgery as adults. In general, given their lack of experience interpreting the information collected by their eyes, these people have vision problems.6 Richard Gregory and John Wallace reported the rare case of a patient (known by his initials, S.B.) who began to use his eyes at age fifty-two, after undergoing a cornea transplant.7 When they subjected him to a battery of visual tests, Gregory and Wallace observed that, among other shortcomings, S.B. could not infer depth or perspective from two-dimensional drawings (for example, when seeing the famous Necker cube). Most interesting, however, is the description by Gregory and Wallace of S.B.’s initial visual experiences:

  S.B.’s first visual experience, when the bandages were removed, was of the surgeon’s face … He heard a voice coming from in front of him and to one side: he turned to the source of the sound, and saw a “blur.” He realized that this must be a face … He seemed to think that he would not have known that this was a face if he had not previously heard the voice and known that voices came from faces . . .

  About three days after the operation he saw the moon for the first time. At first he thought it a reflection in the window, but when he realized, or was told, it was the moon, he expressed surprise at its crescent shape, expecting a “quarter moon” to look like a quarter piece of cake! …

  It was obvious that facial expressions meant nothing to him, and that he could not recognize people by their faces, though he could immediately do so by their

  voices . . .

  Gregory and Wallace also report that S.B. could recognize capital letters, but not lowercase ones. This happened because S.B. had learned to identify capital letters by touching molds at a school for the blind but had never “felt” lowercase letters. In other words, his brain had a representation of capital letters, and when he saw them for the first time he was able to transfer this representation gleaned through another sense. On the other hand, he was not able to learn lowercase letters because he did not have a tactile representation of them.

  We are reaching the end of our chapters about vision, in which we have explored the different strategies by which the brain extracts meaning from what we see—something that goes far beyond generating a copy of the information present in our field of view. In summary, first, the brain processes most visual information in the fovea—our center of attention—disregarding the rest; second, it implements a center-surround representation to encode contrast in the retina, and third, it constructs signs arising from unconscious inferences based on previous experiences. As we will see later, this process of meaning construction continues in the cerebral cortex.

  We may appear to have digressed from our main focus, which is memory. However, I have decided to dwell on the details of vision for several reasons. Vision and memory are two very closely related processes. We cannot recognize an object if we do not have a memory of it. One of the most famous cases related by neurologist Oliver Sacks concerns a talented musician, Dr. P., who could not recognize photographs of his colleagues, his family, or even himself. Dr. P. could not recognize the faces of his students and could only tell them apart by their voices. According to Sacks, in an initial routine test this patient was unable to recognize his shoe once he had taken it off, and, absurd as it may sound, at some point mistook his wife’s head for a hat.8 Dr. P.’s is perhaps the most famous case of visual agnosia. There are essentially two types of visual agnosia, both resulting from brain damage. Patients with apperceptive agnosia have difficulty recognizing objects because they cannot see them as a whole. They instead see disparate details that they cannot integrate. Patients with associative agnosia, on the other hand, can see the objects, can even copy them flawlessly in a drawing, but cannot say what these objects are because they cannot assess their meaning; in other words, the visualization of the objects does not evoke a representation, a specific memory.

  Associative agnosia thus gives a clear example of the relation between perception and memory. Stressing this relation further, memories are usually generated from perceptions, since we tend to create memories of things we see or hear. But the most important reason to discuss vision as we probe the subject of memory is that the brain uses very similar strategies to see and to remember. Both processes are based on the construction of meaning, an interpretation of the outside world that relies on selecting a minimum of information and making abstractions—while discarding a multitude of detail.

  Chapter 4

  HOW MUCH DO WE REMEMBER?

  In which we discuss the virtues of forgetting, Ebbinghaus’s principles, the subjectivity and fickleness of memory, the reliability of eyewitnesses, the amount of information that we remember, and the difference between human and computer memory

  In the first of In Search of Lost Time’s seven volumes, Marcel Proust relates how, on a cold winter day, the flavor of a madeleine that he had let soften in a spoonful of tea unleashed a torrent of memories of his childhood in Combray. The taste of the cake on his palate led him to remember the madeleines dipped in tea that his aunt Léonie gave him on Sunday mornings, and this in turn brought back the image of his old gray house, the pavilion behind it built for his parents, the town, the town square, the streets he walked as he ran errands, the country roads, the flowers in his garden, those in Mr. Swann’s park, the water lilies, the townspeople, their houses, the church . . .

  Proust’s acclaimed narrative illustrates how a specific stimulus, here the flavor of a madeleine, can release a stream of interrelated memories—even those long lost in recesses of the brain to which we do not usua
lly have conscious access. Before he was transported there via the madeleine, Marcel had felt dejected at his inability to evoke the details of his childhood in Combray. I suspect we have all, every now and then, wished that reminiscences of our past were clearer and more detailed, and feel melancholy when we realize that even our most precious memories fade with the passage of time. In those moments, we look for triggers in photographs or elsewhere to help us summon up our pasts, we lament how precious little we remember, and wish we could remember more, much more.

  However, even as we ponder this, we may realize that remembering more is not necessarily beneficial, because unconsciously we solidify the most pleasurable of our memories and forget the less-enjoyable details. We may reminisce longingly about our childhood, but we conveniently forget the torture it was to get out of bed early for school morning after morning, the effort it took to sit for hours in class, or the tedium of homework. Forgetting gives us the pleasurable heartache of blurry photographs and unfinished stories, a tango that laments the sorrows of our scarce memory while acknowledging that some things are better glimpsed dimly.

  In “Funes the Memorious,” Jorge Luis Borges describes with remarkable insight the torment that would result from being able to remember everything. Writes Borges, “Funes remembered not only every leaf on every tree on every mountain, but also each time he had perceived or imagined it.”1 Funes ended his days lying in the darkness of his bedroom, his mind so crammed with memories and irrelevant details that he could not sleep or think. In a vision akin to Borges’s, William James, the American psychologist and philosopher who pioneered modern psychology, argued at the end of the nineteenth century that, paradoxically, it is necessary to forget in order to remember; if we were to remember everything, we would be as handicapped as if we remembered nothing.2

  The virtues of forgetting have been acknowledged since antiquity (notwithstanding the concurrent appreciation of memory we will discuss in the next chapter). In his treatise on oratory, Cicero writes that Themistocles, the Athenian general and politician, refused to learn the science of mnemonics, arguing that he preferred instead the advantages of forgetting.3 The importance of forgetting underlies the thoughts of Aristotle and especially of Aquinas. They may not express their posture as explicitly as James or Borges, but, as we saw in the previous chapter, according to Aristotle and Aquinas the interpretations we draw from the stimuli perceived by our senses are like images or ghosts that we construct in our minds and from which we abstract concepts. For example, when we see a horse, we generate the representation of an individual, a specific horse, and when we have seen many horses, we extract from such individual representations a universal, the concept of a horse. The creation of a universal concept from individual representations is based on abstracting common characteristics. This is the importance of forgetting: sweeping aside irrelevant details in order to form concepts. Borges describes this point brilliantly in “Funes the Memorious.” Says Borges of Funes:

  Let us not forget that he was almost incapable of having general, Platonic ideas. Not only was it difficult for him to understand that the generic term “dog” could embrace so many disparate individuals of diverse sizes and shapes; it bothered him that the dog seen in profile at 3:14 would be called the same as the dog at 3:15 seen from the front. His own face in the mirror, his own hands, surprised him every time … [Funes] was the lonely and perceptive spectator of a world at once multifarious, instantaneous, and almost intolerably precise.

  —JORGE LUIS BORGES, “FUNES

  THE MEMORIOUS” FROM FICCIONES, 1994

  Here I stop discussing the affinity of these ideas with Borges’s thoughts, because that is precisely the topic of another book.4 Clearly we do not want to remember everything, but neither do we want to remember nothing. There must be a balance between remembering and forgetting. But where is that balance? How much do we remember? And, in particular, how can we estimate our memory capacity?

  In the late nineteenth century, Gustav Spiller, a Hungarian-born English psychologist, set upon himself the momentous task of quantifying how much he remembered.5 To this end, he wrote down all the experiences he recalled from the different stages of his life and enumerated all the specific memories that constituted each and every one of those experiences. This remarkable thought experiment led Spiller to estimate having about 100 memories during the first nine years of his life,6 some 3,600 up to age twenty, an additional 2,000 between twenty and twenty-five, and around 4,000 more in the subsequent nine years, concluding that the average thirty-five-year-old has about 10,000 memories. Moreover, Spiller calculated that the sum of this person’s memories, in time lived (or, rather, relived), would amount to about half a day. Of course, these numbers are estimates, but it is worth noting that Sir Francis Galton7 and other more recent researchers have arrived at similar figures.8 It is possible that at thirty-five we may have not ten, but fifteen, twenty, or even thirty thousand memories, and that evoking them all may take us not half a day but two, or even a whole week. Spiller’s quantitative estimates, as he himself acknowledges, are not completely reliable, but leaving aside the exact values of these numbers, it is astounding to grasp that such a massive amount of information is lost to oblivion.

  The experimental and systematic study of memory was pioneered by Hermann Ebbinghaus, a German psychologist who in 1885 published the results of a set of precise—though unbearably tedious—experiments on the capacity of human memory.9 Ebbinghaus constructed 2,300 nonsensical words, each composed of three letters (two consonants surrounding a vowel), then selected random groups of these words and measured 1) how the number of words he could remember varied at different time intervals, and 2) how repeatedly revisiting the list of words made it easier to remember them later. From these experiments, Ebbinghaus derived two fundamental principles. First, given the fast decline in the number of words he could remember as time went by, he concluded that while some memories persist for hours, months, or years, others last for only a few minutes or even seconds. Nowadays, this principle is reflected in our division between long-term and short-term memory. Short-term memory allows us to remember information for brief time intervals and be conscious of the unfolding of present events. This is the memory that I use, for example, to remember what I want to say in this sentence as I fumble for the right words. Long-term memory, on the other hand, is made up of the particulars that we select from the present—those that will become part of the past as we relive them in the future. Long-term memory is what I remember from my last birthday, the flavor of a fine wine, or the mathematical trick I need to compute an integral. Only a minute fraction of our short-term recollections end up being cemented in our brains. But how do short-term memories become long-term memories? Ebbinghaus’s second principle deals with precisely this issue: repetition and practice make memories last; the more he repeated the nonsensical words, the longer he could remember them.

  Figure 4.1: Ebbinghaus’s forgetting curves

  The number of remembered words diminishes with time, but this decline in memory is reduced as the words are repeated.

  From Ebbinghaus’s results we see that repetition helps to strengthen memories, what is technically known as memory consolidation. Persistent, long-term memories are composed of those salient events that capture our attention—those memories that we repeatedly bring back to our consciousness. In Theaetetus, Plato describes memory as etchings on a block of wax. The more we recall a memory, the firmer the etching. Plato’s image corresponds to the intuitive idea we have of memory, but, as we shall soon see, while repetition does indeed reinforce memory, the notion of static memories etched on our brains is very far removed from reality.

  Breaking with the line of experimental studies pioneered by Ebbinghaus, Frederic Bartlett, a British philosopher and psychologist of the early twentieth century, showed just how malleable and subjective memory actually is. According to Bartlett, the use of nonsensical words leads to a controlled situation that is too far removed from real life and
neglects a most important factor: the creation of meaning. In other words, Bartlett argued that the study of the number of nonsensical words that can be remembered at different time intervals cannot fully explain the workings of day-to-day memory. Take, for example, what I remember about my breakfast this morning. The series of events that constitute my memory of this morning’s breakfast are interrelated; they have a context; they are not isolated facts. Suppose that I had toast and jam. This simple fact can have different contexts: perhaps I had toast instead of my usual bacon and eggs because I ate too much for dinner the night before, or maybe it was because I wanted to try a homemade jam that I bought at a craft fair a few days ago. The extraction of meaning is driven by the context. In the first case, I may not even remember the kind of jam I ate because it is irrelevant; in that context, what mattered is that I ate toast because I wanted a light breakfast. In the second case, I will remember the kind of jam I ate because I chose my breakfast specifically to try it. In other words, even though the event itself is the same, the subjective experience, the memory that I will potentially store in my brain, is completely different. The context also helps recall the memory. If in a few days I try to remember what I had for breakfast today, remembering that I wanted to eat something light or try a homemade jam will lead me to remember, by different streams of associations, that I had toast and jam.