Memory (psychology)

I

Introduction

Memory (psychology), faculty by which things from the past are brought to mind or retained there, or that which is so recalled. Memory is perhaps a person’s most distinctive characteristic. Our unique memories define who we are, a record of our personal past that also acts as a guide to our present and future. Memories endure: in our ninth decade we can recall episodes from our remote childhood. Every molecule and most of the cells in our body will have been replaced many millions of times, and yet our memories persist, and with them our sense of identity. This is why the loss of memory, by diseases such as Alzheimer’s or as a result of accidental brain damage, are so devastating.

Memory is so important to our lives that it is not surprising that its nature and mechanisms are a major theme of research in both psychology and neuroscience. Sixteen centuries ago, St Augustine devoted an entire chapter of his famous Confessions to puzzling over how it was that we could conjure up in our memories entire scenes and conversations, full of colour, scent, and sound, and yet our memory itself was spaceless, colourless, soundless. In the 17th century René Descartes proposed that memories were stored in the pineal gland in the brain by the bending of minute hairs with which the surface of the gland was studded, and the idea that memories are indeed in some way preserved in the brain in the form of some type of lasting change in structure or connections is still held by most researchers.

The modern scientific study of human memory began in the 19th century, when the losses of memory resulting from brain damage or alcoholism began to be catalogued. Normal human memory was studied experimentally by Hermann Ebbinghaus, who asked people to remember lists of words or nonsense syllables and noted that a large proportion were forgotten within the first hour or so, but that after that those items that remained persisted in memory—giving rise to the short-term, long-term distinction. The psychological study of memory has been built around the two poles of normal and abnormal ever since.

II

Types of Memory

Over the last decades, psychologists have generated a taxonomy of normal human memory, distinguishing first between so-called procedural and declarative memory, that is, memory for skills versus memory for facts. The former is memory for how to do something—to ride a bicycle, for instance; the latter is memory for what things are—that a bicycle is called a bicycle, for instance. Declarative memory may further be subdivided into semantic (for example, knowing the names of days of the week) and episodic, or autobiographical (for example, remembering that I went on a bicycle ride last Tuesday). Procedural memory persists even in disease states such as Alzheimer’s where episodic and semantic memories are progressively lost, implying that the ways in which procedural and declarative memories are made and stored may differ. But this is not the only memory classification possible. For instance, events with a high emotional content are remembered much better than purely cognitive ones. Some memories are held for only a very short time—minutes or even seconds (working, or short-term memory). Others may persist for a lifetime (long-term memory).

III

Memory and Brain Structures

Until recently the brain processes on which memory depends could only be inferred from the study of patients with memory loss. Thus an operation in the 1950s to treat a Canadian patient known only as HM resulted in the destruction of his hippocampus and parts of his temporal lobes. The result was catastrophic; HM retains declarative memory only for events prior to the operation; the succeeding half century of his life he glimpses only fleetingly, remembering events for only a few seconds. The conclusion that the hippocampus is in some way crucial for the transfer of short- to long–term memory is generally accepted. Similarly, a brain region closely adjacent to the hippocampus, the amygdala, is necessary for the registration of the emotional content of memory. The advent of brain imaging (positron emission tomography, functional magnetic resonance imaging, magnetoencephalography) has confirmed the importance of these brain structures in undamaged brains too. In addition many cortical regions are engaged in working memory tasks, and the lower and medial regions of the frontal cortex in choice and decision-making based on memory.

A fundamental limitation to such studies is that even imaging does not allow one to discover the intimate physiological and biochemical mechanisms that might be required for memories to be made. At the beginning of the 20th century Ivan Pavlov, working in St Petersburg, brought learning and memory into the laboratory by showing that it was possible to teach dogs to associate the sound of a bell or a flash of light with the imminent arrival of food (so-called classical conditioning). By the 1930s the American psychologist B. F. Skinner invoked another form of learning when he showed that rats could be taught to press a lever or run a complex maze to obtain food (instrumental conditioning). And in 1948, Donald Hebb, in Montreal, proposed that such conditioning could result in alterations in the physical structure of synapses—the junction points between the nerve cells (neurons) in the brain—which would as a result generate new patterns of connections between the neurons. Granted that the hundred billion neurons in the human brain are connected by up to a hundred trillion synapses, such a model would allow ample possibility to encode the memories of a lifetime. Hebb’s hypothesis for this type of associative memory has opened the way for neural modellers to create computer-generated neural nets capable of showing “learning”. (See Cognitive Science.)

Hebb’s hypothesis also offered a route by which neuroscientists could begin to explore whether training animals on learning tasks actually did produce measurable synaptic changes. One early approach was to compare rats reared in “deprived” versus “enriched” environments; the latter, it could be assumed, would learn and remember more than the former. The “enriched” rats had a thicker cerebral cortex and increased numbers of synapses, in line with Hebb’s ideas, but to be sure that such changes were specifically related to learning and memory required a more precise experimental model and the development of analytical methods sensitive enough to measure the small changes in biochemical, morphological, or physiological properties that might be expected to result from such learning experiences. The study of memory has become the study of the brain events occurring when an animal learns and subsequently is called upon to show that it remembers a particular task—running a maze, finding food, escaping unpleasant stimuli, and so on.

IV

Animal Models for the Study of Learning and Memory

Four animal models have proved particularly useful. The first is perhaps the most surprising: the fruit fly Drosophila, long the subject of study by geneticists for the speed with which it breeds and the ease with which mutants could be generated. Fruit flies will spontaneously fly towards particular odours, but if they receive an electric shock as they approach they can learn to avoid that particular odour. A series of mutants have been generated that either could not learn to respond by avoidance, or forgot after varying periods of time. Each class of mutants had a specific biochemical abnormality—the loss of activity of a particular enzyme or of one of the factors required for the synthesis of specific proteins. Hence it is argued that the missing enzyme or factor is also necessary for learning and memory to occur.

Eric Kandel, working at Columbia University in New York, has exploited a different animal, the large sea slug, Aplysia californica. This creature can undergo a form of classical conditioning, learning to respond by a particular muscular movement when a jet of water is squirted at it. The Aplysia nervous system contains a small number of rather large and readily identifiable neurons, and Kandel’s group were able to identify the circuitry required for this gill withdrawal reflex, dissect it out, and study it in isolation. They were able to mimic the learning behaviour of the intact animal in this “reduced preparation” and show that the physiological behaviour of the cells changed as a result of experience, or by the application of the neurotransmitter serotonin. After enough trials, there was an increase in the synthesis of proteins and structural changes in the neurons themselves.

The author’s group at the Open University in the UK works with young chicks, which tend to peck at small bright objects (such as a bead) in their field of view. If the bead is made to taste bitter the chick pecks once and avoids beads of a similar colour and size subsequently. Learning this avoidance triggers a rapid release of the neurotransmitter glutamate across the synaptic junctions in a specific region of the chick forebrain which in turn results in a cascade of biochemical processes in both pre- and post-synaptic neurons. Within a few hours these culminate in the synthesis and insertion into the synapses of a family of proteins called cell adhesion molecules (CAMs), which alter the strength of connections of the synapses. Under the microscope it can be shown that there are actual increases in the number of branching processes (dendrites) that each neuron possesses and in the numbers and dimensions of the synapses connecting them. It may be relevant that one particular CAM is the amyloid precursor protein, APP, whose metabolism is abnormal in Alzheimer’s disease, resulting in the characteristic plaques that gradually accumulate in the brain as the disease progresses. The normal functioning of APP is essential for the transition between short- and long-term memory.

The final model, developed by Tim Bliss at the National Institute for Medical Research in London, is not really of learning and memory at all, but a physiological process which can be studied in thin slices of tissue taken from the hippocampus, usually of rats. The neurons in specific hippocampal regions can be stimulated by firing a train of electrical pulses into them from the nerve tract which connects with them, and under the right conditions the output property of the stimulated neurons changes and they become more excitable. This phenomenon, called Long Term Potentiation (LTP), is regarded by physiologists as being an analogue of memory—an adaptive change in behaviour as a result of experience—and shows all the properties required if the Hebb hypothesis were correct. The physiology and pharmacology of LTP have now been studied in exquisite detail.