learning theory

learning theory


      any of the proposals put forth to explain changes in behaviour produced by practice, as opposed to other factors, e.g., physiological development.

      A common goal in defining any psychological (psychology) concept is a statement that corresponds to common usage. Acceptance of that aim, however, entails some peril. It implicitly assumes that common language categorizes in scientifically meaningful ways; that the word learning, for example, corresponds to a definite psychological process. However, there appears to be good reason to doubt the validity of this assumption. The phenomena of learning are so varied and diverse that their inclusion in a single category may not be warranted.

      Recognizing this danger (and the corollary that no definition of learning is likely to be totally satisfactory) a definition proposed in 1961 by G.A. Kimble may be considered representative: Learning is a relatively permanent change in a behavioral potentiality that occurs as a result of reinforced practice. Although the definition is useful, it still leaves problems.

      The definition may be helpful by indicating that the change need not be an improvement; addictions and prejudices are learned as well as high-level skills and useful knowledge.

      The phrase relatively permanent serves to exclude temporary behavioral changes that may depend on such factors as fatigue, the effects of drugs, or alterations in motives.

      The word potentiality covers effects that do not appear at once; one might learn about tourniquets by reading a first-aid manual and put the information to use later.

      To say that learning occurs as a result of practice excludes the effects of physiological development, aging, and brain damage.

      The stipulation that practice must be reinforced serves to distinguish learning from the opposed loss of unreinforced habits. Reinforcement objectively refers to any condition—often reward or punishment—that may promote learning.

      However, the definition raises difficulties. How permanent is relatively permanent? Suppose one looks up an address, writes it on an envelope, but five minutes later has to look it up again to be sure it is correct. Does this qualify as relatively permanent? While commonly accepted as learning, it seems to violate the definition.

      What exactly is the result that occurs with practice? Is it a change in the nervous system? Is it a matter of providing stimuli that can evoke responses they previously would not? Does it mean developing associations, gaining insights, or gaining new perspective?

      Such questions serve to distinguish Kimble's descriptive definition from theoretical attempts to define learning by identifying the nature of its underlying process. These may be neurophysiological, perceptual, or associationistic; they begin to delineate theoretical issues and to identify the bases for and manifestations of learning. (The processes of perceptual learning are treated in the article perception: Perceptual learning (perception).)

The range of phenomena called learning
      Even the simplest animals display such primitive forms of adaptive activity as habituation, the elimination of practiced responses. For example, a paramecium can learn to escape from a narrow glass tube to get to food. Learning in this case consists of the elimination (habituation) of unnecessary movements. Habituation also has been demonstrated for mammals in which control normally exercised by higher (brain) centres has been impaired by severing the spinal cord. For example, repeated application of electric shock to the paw of a cat so treated leads to habituation of the reflex withdrawal reaction. Whether single-celled animals or cats that function only through the spinal cord are capable of higher forms of learning is a matter of controversy. Sporadic reports that conditioned responses may be possible among such animals have been sharply debated.

      At higher evolutionary levels the range of phenomena called learning is more extensive. Many mammalian species display the following varieties of learning.

      This is the form of learning studied by Ivan Petrovich Pavlov (Pavlov, Ivan Petrovich) (1849–1936). Some neutral stimulus, such as a bell, is presented just before delivery of some effective stimulus (say, food or acid placed in the mouth of a dog). A response such as salivation, originally evoked only by the effective stimulus, eventually appears when the initially neutral stimulus is presented. The response is said to have become conditioned. Classical conditioning seems easiest to establish for involuntary reactions mediated by the autonomic nervous system.

Instrumental conditioning
      This indicates learning to obtain reward or to avoid punishment. Laboratory examples of such conditioning among small mammals or birds are common. Rats or pigeons may be taught to press levers for food; they also learn to avoid or terminate electric shock.

      In the form of learning called chaining the subject is required to make a series of responses in a definite order. For example, a sequence of correct turns in a maze is to be mastered, or a list of words is to be learned in specific sequence.

Acquisition of skill
      Within limits, laboratory animals can be taught to regulate the force with which they press a lever or to control the speed at which they run down an alley. Such skills are learned when a reward is made contingent on quantitatively constrained performance. Among human learners complex, precise skills (e.g., tying shoelaces) are routine.

discrimination learning
      In discrimination learning the subject is reinforced to respond only to selected sensory characteristics of stimuli. Discriminations that can be established in this way may be quite subtle. Pigeons, for example, can learn to discriminate differences in colours that are indistinguishable to human beings without the use of special devices.

      An organism is said to have learned a concept when it responds uniquely to all objects or events in a given logical class as distinct from other classes. Even geese can master such concepts as roundness and triangularity; after training, they can respond appropriately to round or triangular figures they have never seen before.

Principle learning
      A subject may be shown sets of three figures (say, two round and one triangular; next, two square and one round, and so on). With proper rewards, the subject may learn to distinguish any “odd” member of any set from those that are similar. Animals as low in the evolutionary scale as the pigeon can master the principle of this so-called oddity problem.

Problem solving
      Examples of human problem solving are familiar: finding the roots of a quadratic equation, solving a mechanical puzzle, and navigating by the stars. Among other animals, chimpanzees have been observed to solve problems requiring toolmaking.

      This list only samples from the remarkable array of animal activities categorized as learning. Beginning with habituation, they range from the simple adjustments of single-celled animals up to the highest intellectual accomplishments of mankind. It would be wonderful indeed if a single theory of learning were enough to account for all this diversity. So far, however, no theory of learning adequately covers more than a small fraction of these phenomena.

The state of learning theories
      Yet, at the start of the 20th century, vast psychological systems, such as behaviourism and Gestalt psychology, indeed were offered as explanations of learning (and of much wider ranges of behaviour as well). And as late as the 1940s, comprehensive theories of learning were still believed to be reasonably near at hand. But during the next three decades it grew clear that such theories are tenable only for very limited sets of data. By the late 20th century learning theory seemed to consist of a set of hypotheses of limited applicability.

Important earlier theorists
      Beginning in the 1930s a number of general theories were advanced in attempts to organize most or all of the psychology of learning. The most influential of the contributing theorists are noted below.

      E.R. Guthrie (Guthrie, Edwin Ray) (1886–1959) wrote that learning requires only that a response be made in a changing situation. Any response was held to be linked specifically to the situation in which it was learned. Guthrie argued that learning is complete in one trial, that the most recent response in a situation is the one that is learned, and that responses (rather than perceptions or psychological states) provide the raw materials for the learning process.

      For E.C. Tolman (Tolman, Edward C.) (1886–1959) the essence of learning was the acquisition by the organism of a set of what he called Sign-Gestalt-Expectations. These referred to propositions said to be made by the learner that his own specific response to given signs (or stimuli) would result in such and such circumstances later on. Tolman seemed to be saying that what the learner acquires is a specific knowledge of “what leads to what.” In brief, his theory was that the learner develops expectations based on experience and that learning depends entirely on successions of events. Although less vocal on the point than others, Tolman implied that learning was a gradual process.

      The theory offered by Clark L. Hull (Hull, Clark L.) (1884–1952), over the period between 1929 and his death, was the most detailed and complex of the great theories of learning. The basic concept for Hull was “habit strength,” which was said to develop as a function of practice. Habits were depicted as stimulus-response connections based on reward. According to Hull, responses (rather than perceptions or expectancies) participate in habit formation, the process is gradual, and reward is an essential condition.

      Comparison of these theories yields major questions for empirical investigation. Is learning continuous or discontinuous; is it a gradual or sudden (one-trial) process? Is learning a matter of establishing stimulus–response (S–R) connections or does it depend on the learner's understanding of perceptual relationships? Is reward necessary for learning?

Are theories of learning necessary?
 Such major investigators of learning as B.F. Skinner (Skinner, B.F.) and J.A. McGeoch maintained in the 1930s and 1940s that preoccupation with theory was misguided. For them the approach simply was to discover the conditions that produce and control learned behaviour. Beyond this, their interests diverged. Skinner studied instrumental conditioning (operant conditioning, as he called it) among rats; McGeoch specialized in human rote memory. Although study of rote verbal learning had become heavily theoretical by the 1970s, Skinner and his associates stuck to their empirical guns, guiding a variety of programs for the practical control of behaviour. Teaching machines and computer-aided instruction, behaviour modification (e.g., the use of tokens to reward desired behaviour among psychiatric patients), and planned utopian societies (Walden II) all found scientific origins in Skinner's rejection of theory in favour of direct efforts to produce results.

Intervening variables and hypothetical constructs
      Learning is a concept and not a thing, and the activity called learning is inferred only through behavioral symptoms. The distinction implicit here between behaviour and inferred process is one of Tolman's major contributions and serves to reconcile influential views that might seem completely at odds. Classical behaviourism, as developed by John B. Watson (Watson, John B.) (1878–1958), rejected every mentalistic account and sought to limit analysis to such physiological mechanisms as reflexes. Watson argued that these are objective in a way that so-called thoughts, hopes, expectancies, and images cannot be. The opposing view holds that experiential (introspective) activity (exactly what Watson sought to dismiss) does require discussion.

      Tolman called himself a behaviourist and ostensibly was bound by Watson's insistence on objectivity. But he also was interested in thinking, expectancy, and consciousness. Tolman found his solution to this problem of incompatible theories after his association with the Vienna Circle of Logical (Logical Positivism) Positivists, whose deterministic teachings he brought to the attention of U.S. psychologists about 1920. He maintained that learning is inexorably produced (determined) by such independent (directly manipulable) variables as the organism's previous training and physiological condition and by the response the environment requires. According to Tolman, the development of learning is revealed through the changing probability that given behaviour (the dependent variable) will result. He held that learning itself is not directly observable; it is an intervening variable, one that is inferred as a connecting process between antecedent (independent) variables and consequent (dependent) behaviour.

      An attractive possibility is that intervening variables may have discoverable physiological bases. Psychologists Paul E. Meehl and Kenneth MacCorquodale proposed a distinction between the abstractions advocated by some and the physiological mechanisms sought by others. Meehl and MacCorquodale recommended using the term intervening variable for the abstraction and hypothetical construct for the physiological foundation. To illustrate: Hull treated habit strength as an intervening variable, defining it as an abstract mathematical function of the number of times a given response is rewarded. By contrast, Edward L. Thorndike (Thorndike, Edward L.) (1874–1949) handled learning as a hypothetical construct, positing a physiological mechanism: improved conduction of nerve impulses.

      Intervening variables and hypothetical constructs need not be incompatible; Thorndike's hypothetical neural process could empirically be found to be the mechanism through which Hull's abstraction operates.

Miniature theories
      With growing realization of the complexity of learning, the grand theories of Guthrie, Hull, and Tolman generally have been abandoned except as historic landmarks. Hope for any impending, comprehensive theory was almost dead in the 1970s. More modest miniature theories remain, many likely to be of temporary value. An account of their major themes and issues, however, should have more enduring interest.

Major themes and issues

      A dominant ancient theme in theories of learning has been that of association. Although the concept was accepted by Aristotle, it was brought into the developing psychology of learning by British empiricist philosophers (Locke, Berkeley, Hume, the Mills, and Hartley) during the 17th, 18th, and 19th centuries. Popular acceptability of the notion of association was related to progress in the physical sciences. The physical universe had been shown to consist of a limited number of chemical elements that can combine in innumerable ways. By analogy, a science of “mental chemistry” seemed appealing. The theorized elements in this new “science” were called ideas, said to be based on what were named sensations. The synthesizing principles by which these posited ideas combined in conscious experience were expressed as so-called laws of association. It was suggested that such conditions as temporal and spatial contiguity, repetition, similarity, and vividness favoured the formation of associations, and each was called a law of association. Thus, there were “laws” of repetition, of similarity, and so on.

      At the end of the 19th century the notion of association was widely accepted among psychologists. German psychologist Wilhelm Wundt (Wundt, Wilhelm) (1832–1920) took a position nearly identical with that of the British empiricist philosophers. Also in Germany, Hermann Ebbinghaus (Ebbinghaus, Hermann) (1850–1909) began to study rote learning of lists of nonsense verbal items (e.g., XOQ, ZUN, ZIB). He maintained that the association of each word with every succeeding word was the primary mechanism in learning these lists. Pavlov in Russia offered temporary associative connections in the nervous system as a hypothetical basis for conditioned reflexes.

      These European influences coalesced in North America. Wundt's notions were introduced there when a student of his from England, Edward Bradford Titchener (Titchener, Edward Bradford) (1867–1927), came to teach at Cornell University in Ithaca, New York. Ebbinghaus' method and theory became standard in Canadian and U.S. studies of verbal learning; Watson and other behaviourists applied Pavlov's conceptions to their learning experiments. Experimental psychology in the Western Hemisphere came to be dominated by what seemed to be a search for laws of association.

What is associated?
      Investigators asked whether associations are formed between observable stimuli and responses (S–R) or between subjective sensory impressions (S–S). One group that included Hull (Hull, Clark L.), Guthrie, and Thorndike took the relatively objective S–R position, while Tolman and others favoured the more introspective, perceptual S–S approach. For a time S–R theorists held popularity; behavioral responses are readily observable evidence of learning, and many included them in the associative process itself.

      But the reduction of learning to mere external stimuli and overt responses raised discordant theoretical objections that the inner activities of the organism were being ignored. S–R theories failed to account for a host of learned phenomena. For example, people could be trained to say they heard sounds even when such auditory stimuli were absent. They said they dreamed about what they had learned, too; yet there need be no immediate external stimulus, nor does the dreamer always make the responses he dreams about.

      Physiological psychologists and biologists found ways of delivering electrical stimulation directly to the brain; this eliminated the sensory stimuli and vocal or motor responses on which S–R theories hinge. Direct neural stimulation was found to be an adequate signal and the electrical response of the brain itself proved susceptible to conditioning. At this level of the nervous system, distinctions between stimulus and response mean less than at the periphery, and the S–S versus S–R controversy is no longer such a burning issue.

Direction of association
      Classical conditioning dependably has been shown to proceed only forward in time. Bell must precede food if a conditioned reaction is to be established. If it had any effect, the reverse procedure (food before bell) would be called backward conditioning; but at most it only inhibits other reactions. There seems to be a relatively brief optimal interval in classical conditioning at which associations are most easily made. For quick reflexes such as the eyeblink, this interval is about one-half second; longer or shorter intervals are less effective. For slower reactions such as salivation the interval is longer, perhaps two seconds or so.

      In learning verbal associations the situation appears to be quite different. When one learns the Russian–English forward association da–“yes,” he also learns the English–Russian backward association “yes”–da. Moreover, timing is much less critical than in classical conditioning. Verbal pairs are learned with almost equal ease whether presented simultaneously or separated by several seconds.

      In what is called context association, the general environment may begin to elicit a response that is being conditioned to a specific stimulus. Thus, a dog may salivate simply on being brought into the experimental room—before any bell rings. Verbal associations also can be weakened by changes in the general situation.

      A major theoretical issue concerns whether associations grow in strength with exercise or whether they are fully established all at once. Evidence is that learning usually proceeds gradually; even when a problem is solved insightfully, practice with similar tasks tends to improve performance. Some (perhaps most) learning theorists have concluded that repetition gradually enhances some underlying process in learning.

      The view that associations develop at full strength in a single trial leads to a typical question. How can the gradual nature of most learning be explained if all-or-nothing is the rule? One possible answer suggested by Guthrie has led to so-called stimulus-sampling theory. The theory assumes that associations indeed are made in just one trial. However, learning seems slow, it is said, because the environment (context) in which it occurs is complex and constantly changing. Given a changing environment, the sample of stimuli will differ from trial to trial. Thus, it is reasoned, it should take many trials before a response is associated with a relatively complete set of all possible stimuli.

      In this light, the strength (or probability) of a response should increase with practice even if the elementary associative process occurs in a single trial.

      These stimulus-sampling notions translate easily into mathematical form; they are an example of statistical learning theory, a more general development in the quantitative treatment of learning.

      Repetition alone does not ensure learning; eventually it produces fatigue and suppresses responses. An additional process called reinforcement has been invoked to account for learning, and heated disputes have centred on its theoretical mechanism.

      Objectively reinforcement refers to the use of stimuli that have been found to facilitate learning. Under appropriate conditions, these include praise, food, water, opportunity to explore, sexual stimuli, money, electric shock, and direct brain stimulation.

      More theoretically, the term reinforcement expresses various theoretical hunches about some specialized subjective quality all such stimuli might share. Food for a hungry animal is a well-established reinforcer, conceivably through its distinctive appearance and odour. It tends to elicit a set of responses: approaching, chewing, tasting, swallowing; these may produce additional perceptual activities that reduce the drive or desire for food (e.g., by halting stomach contractions that are experienced as hunger pangs). But no single subjective quality imagined by theorists seems invariably effective in reinforcement studies. Perhaps some combination of introspective influences is critical, or it may be that perceptual processes apply differently from one learning situation to another.

Anti-associationistic positions
      Not all psychologists have accepted the general validity of association theories; many have suggested that considerations other than association are crucial to learning.

      Major critics of association theory included such Gestalt (Gestalt psychology) psychologists as Wolfgang Köhler (Köhler, Wolfgang) (1887–1967), who held that learning often entails a perceptual restructuring of environmental relationships. Köhler cited his own studies of insightful learning by a chimpanzee. The animal learned to join two sticks (akin to a jointed fishing pole) as a tool to pull in a banana that was out of arm's reach and of either short stick alone. The ape was described as sitting quietly (as if in thought), and then suddenly fitting the sticks together to rake in the fruit. It was argued that the ability to perceive new ways of relating the sticks to the banana was essential in solving the problem.

      Similar organizational processes in perceiving can be demonstrated in serial verbal learning. Memorizing the list thick, wall, it, tea, of, myrrh, seize, knots, trained should demand some rehearsal. Yet, notice the phonetic resemblance to Shakespeare's famous line from The Merchant of Venice: “The quality of mercy is not strained. . . .” With that kind of perceptual organization, learning can become quick and easy.

      A powerful argument also was made by psycholinguists who criticized what they took to be the associationistic account of language learning. Even assuming one-trial acquisition, it was held that such individually learned associations could not account for all combinations of words people use; there are simply too many. They suggested that learning a language requires some general organizing structure on which words are hung. Some proponents of this position hold that this structure does not depend on learning, being transmitted genetically from parent to child.

      Gestalt interpretations often reject the associationistic hypothesis wholesale. Other theorists endorse the notion of association, but hold it to be less important than is a process of inhibition through which errors in learning are eliminated. Such theorists find support in evidence for the development of learning sets (what is called learning to learn).

      For example, a monkey may learn a long series of discriminations; e.g., red versus green, black versus white, round versus square, large versus small, triangle versus ellipse. After solving several hundred such problems, some monkeys learn to master each new one in a single trial, as if insightfully. The animal is said to have learned to learn such discriminations.

      Evidence clearly shows that the monkey gradually abandons erroneous tendencies as learning proceeds. At first it might be prone to choose stimuli that are red, black, round, large, or triangular. Correct choices do not always correspond to the animal's initial biases, and their suppression (inhibition, extinction) eventually permits single-trial learning. Theoretically, organisms learn to learn by inhibiting erroneous behaviour; thus, Harry F. Harlow, a proponent of this view, called it an error-factor theory.

motivation in learning
      Motivation popularly is thought to be essential to learning. Yet many theorists suggest that motives make little or no direct contribution—that they simply tend to promote practice.

Motivation and performance
      Learning was defined above as a change in a behavioral potentiality. Realization of such potential seems to be related to the learner's level of motivation. A pupil who has learned the names of all members of the British Commonwealth of Nations would be expected to recite them with particular energy under some sort of incentive (reward or punishment). The incentive is said to raise his level of motivation.

      Incentives do seem to invigorate performance up to a point; however, when motivation seems particularly intense, some studies show performance to deteriorate. From such data some theorists conclude that the effect of drive intensity on performance follows a U-shaped course, first helping and later hindering.

      Greatly increased motivation also may change performance qualitatively by introducing new inefficient modes of behaviour. A student may be so tautly driven to do well on an examination that his tension, fear of failure, and his visceral and muscular discomfort interfere with performance.

Motivation and learning
      To show that motivation affects performance of what has been learned is not the same as demonstrating its effect on the process of learning itself. This would require that individuals learn under various levels of motivation and be tested under the same incentive levels. (This is to control for the effects of motivation on performance alone.) And, indeed, the best-controlled experiments of this design indicate learning effects to be the same under different levels of motivation.

Varieties of learning
      It is debated whether all forms of learning represent the same process. This question applies even to relatively primitive phenomena such as classical and instrumental conditioning.

      In instrumental conditioning reinforcement is contingent on the learner's response; a rat receives food only if it presses the lever. In classical conditioning there is no such contingency; a dog is fed whether or not it salivates. But this is a distinction in experimental procedure. Whether the underlying process of learning is the same for both is quite another question.

      Classical conditioning usually has been reported for glandular, autonomically mediated, involuntary responses (e.g., salivation, heart rate). By contrast, voluntary movements of skeletal muscles more typically have been found to be conditionable instrumentally. However, to theorize that classical conditioning is exclusively effective for one class of responses while instrumental conditioning is uniquely applicable to others seems to be a mistake.

      Evidence that seems to demolish such theorizing comes from a series of experiments directed by Neal E. Miller (Miller, Neal E.) at the Rockefeller University in New York City. Rats were immobilized with curare; this drug blocks the junction between muscle and nerve to paralyze the skeletal muscles. However, a curarized individual still can show autonomic, involuntary signs of emotional activity such as a rapidly beating heart.

      Electrical stimulation of selected parts of the brain seems to be rewarding; animals behave as if they seek such stimulation and will learn to press a switch for it (voluntary muscle function). Using curarized animals, Miller and others made the rewarding stimulation contingent on such typically involuntary responses as changes in heart rate, blood pressure, contractions of the bowel, and salivation. Their research has shown such instrumental conditioning to be effective for all these responses. The evidence appears to destroy the once-popular hypothesis that involuntary autonomic reactions are subject only to classical conditioning. In this sense the two primitive forms of learning seem to be the same.

Stages of learning
      Should the basic process prove to be the same for all varieties of learning, there would still be reason to believe that it operates differently from one stage of practice to another. For example, in coping with painful stimuli (e.g., electric shocks) laboratory animals seem to learn in two successive, distinguishable phases. Apparently they first learn to fear the situation, then to avoid it.

      For example, when an animal learns to avoid painful shock (by turning a paddle wheel or by running away), a warning signal can be given; e.g., with a flash of light or a buzzer. The two stages of learning then can be studied separately. The animal first is subjected to pairings of signal and unavoidable shock to establish (by classical conditioning) signs of fear in response to the signal. In the second stage it is allowed to stop the frightening signal by making an appropriate response. Preconditioned members of the many animal species have learned to avoid the signal itself, even though shock never was presented again.

      Theoretically, the classically conditioned signs of fright in response to the initially neutral signal have a motivating function. Termination of that stimulus is seen as instrumental—that is, as rewarding the animal by reducing learned experiences of fear.

      A two-stage process has been suggested even for classical conditioning. One theory is that in the first stage the subject learns that a neutral stimulus (a ringing bell) is to be presented along with another stimulus (food) whether or not it exhibits a reaction (salivation). Conditioning of any reaction is held to constitute the second stage of learning. The skimpy supporting evidence points to the first stage as a prerequisite, suggesting that responses can only be conditioned (conditioning) after the sensory conditions are recognized.

Verbal learning
      Theories that interpret verbal learning as a process that develops in stages also have been worked out. In one variety of rote learning the subject is to respond with a specific word whenever another word with which it has been paired is presented. In learning lists that include such paired-associates as house–girl, table–happy, and parcel–chair, the correct responses would be girl (for house), happy (for table), and chair (for parcel). By convention the first word in each pair is called the stimulus term and the second the response term. Paired-associate learning is theorized to require subprocesses: one to discriminate among stimulus terms, another to select the second terms as the set of responses, and a third to associate or link each response term with its stimulus term. Although these posited phases seem to overlap, there is evidence indicating that the first two (stimulus discrimination and response selection) precede the associative stage.

Remembering and forgetting
      Learning, remembering, and forgetting often have been considered separate processes. Yet these distinctions seem to blur in the face of contemporary research and theory.

Transient and enduring memory
      Evidence for stages of learning comes from observations of learners over relatively extended series of trials (or comparatively long periods). The empirical data suggest that several alterations in memory function occur even during a single trial. The process that commits information to memory also seems to have several stages.

      Most theorists attribute at least three stages to memory function: immediate, short-term, and long-term. Immediate memory seems to last little more than a second or so. For example, subjects may be asked to remember where specific objects are located within a complex array they have just seen. Their performance shows that considerable information is retained only briefly, rapidly fading unless it is given special attention.

      Short-term memory lasts about 15–30 seconds, as after looking up a telephone number. One makes the call, discovers he has forgotten the number (perhaps in the midst of dialing), and has to look it up again. Nevertheless, such short-term retention does make information available long enough to be rehearsed; if the learner repeats it to himself, the number can be transferred to some sort of longer term storage.

      Thus, rehearsal seems to facilitate transfer of data from short-term to long-term memory. Once committed to long-term memory, the results of learning tend to endure but can be abruptly abolished when specific parts of the brain are injured or removed; they also are vulnerable to interference from other learning. Nevertheless, conditioned responses may undergo little or no forgetting over periods of months or years. And electrical stimulation of the surgically exposed brain while a person is awake can make him remember experiences long thought forgotten. recall is reported to be similarly enhanced during hypnosis.

      The amount of information one readily can retrieve from what is stored in memory is prodigious. In locating an item in memory, he apparently activates a system that stores a set of related data; then he searches for the item within that system. For example, a person is shown a long, randomly mixed list of words that belong to different categories (e.g., names of animals, plants, professions, tools). When asked to remember as many words as he can, he spontaneously will tend to group them by category; this is called clustering of recall. Thus, names of animals (spread throughout the original list) are likely to be remembered one after the other.

      Studies of the familiar tip-of-the-tongue experience yield analogous results. College students who heard definitions (of this sort: a small, open Chinese boat) were asked to supply the right word (in this case it would be sampan). Those who said they might have it somewhere on the tip of the tongue were significantly accurate in guessing the first letter and the number of syllables. Their tendency also to recall words that sounded the same or that had similar meanings is reminiscent of clustering.

      Considerable evidence of this kind supports the theory that the process of retrieval first locates stored data in some sort of associative network and then selects an item with specific characteristics.

      Whether immediate and short-term data simply decay or are lost through interference is a matter of controversy. However, evidence is clearer that interference affects retention of information in long-term storage. Retention of the word happy (learned as a paired associate of table) seems to be subject to the interference of a strong tendency to associate table with chair. Thus, the paired associate table–happy becomes more readily forgotten when followed by parcel–chair as the very next item in a list; this seems to help chair reassert its old tendency to be associated with table. In general, it is found that associations tend to interfere with or to inhibit one another. Interference deriving from earlier (and later) associations is called proactive inhibition (and retroactive inhibition). These two forms of inhibition commonly are accepted as major processes in forgetting, proactive inhibition being assigned greater importance.

Contemporary trends in learning theory
      In the early 1930s the distinction between learned and inherited behaviour seemed clearer than it does now. The view that any bit of behaviour either was learned or simply developed without learning seemed straightforward. Studies based on these expectations led investigators to conclude that rat-killing behaviour among cats is learned rather than instinctive, that human fears are all acquired, or that intelligence is completely the result of experience. Learning theorists were saying then that most behaviour is learned and that biological factors are of little or no importance.

      Forty years later this position seemed grossly untenable. The once-implied sharp distinction between learned and inherited behaviour had become badly blurred. For example, it has been found that the young of many animal species automatically will learn to follow the first large, moving, noisy object presented (as if it were their mother). This special form of learning is called imprinting and seems to occur only during a critical early stage of life. Among mallard ducklings imprinting is most feasible about 15 hours after hatching. During this period a duckling will imprint as easily on an old man or on a rubber ball as it will on a mother duck. Is this instinctive or learned behaviour? Manifestly it is both. The instinctive tendency to be imprinted is part of the duckling's biological heritage; while the object on which it is imprinted is a matter of experience. What is significant for learning theory is that the contribution of biology cannot be ignored.

      Learning theorists once ruled a number of concepts out of court on the ground that they seemed objectively unclean. Image, cognition, awareness, and volition, all are concepts that were denied acceptance on this basis. They sounded mentalistic, subjective, introspective, and unverifiable. Yet, in the late 20th century these were being given more serious scientific consideration.

      For example, the concept of image in learning has begun to show real viability. It has long been reported that the more meaningful a list of words is, the easier it will be to learn. Degree of meaningfulness for a word may be defined by the objectively observed probability that people quickly can give another word in response. Using such empirical scales of meaningfulness, a reliable and substantial relationship has been found between meaningfulness and ease of learning. However, meaningful words also may evoke vivid images that subjects can describe when asked. When they do evoke such imagery, they seem to be learned and remembered even more easily. Thus, learning theory seems to be enriched when introspective data are used.

      A final fault in much learning theory stems from earlier tendencies to use the laws of physics as a model. Theorists once sought general laws of wide applicability that tended to obscure differences among individuals. For example, so complete was Hull's faith in universal “laws” of animal behaviour, that he based his hypothesis about the optimal interval for classical conditioning in humans, other mammals, and birds on the pattern of nerve conduction in the optic nerve of the horseshoe crab. There was little concern even for species differences. Within the same species, individual differences were viewed as a mere nuisance; it was believed that, by studying many subjects and by computing averages, basic laws of learning could be found. However, so-called laws were developed in this way that failed to represent even one individual whose behaviour contributed to the average. More than any other consideration, this has led learning theorists to take a belated look at the importance of individual differences and species differences in learning.

Gregory A. Kimble

Additional Reading
John F. Hall, The Psychology of Learning (1966), provides a comprehensive account of the empirical data that theories of learning are designed to integrate. Ernest R. Hilgard and Gordon H. Bower, Theories of Learning, 5th ed. (1981), is the standard reference on this subject. Ernest R. Hilgard and Donald G. Marquis, Hilgard and Marquis' Conditioning and Learning, 2nd ed. rev. by Gregory A. Kimble (1961), describes theoretical issues as they apply to simple learning. The separate articles reproduced in Gregory A. Kimble (ed.), Foundations of Conditioning and Learning (1967), develop some of these arguments in more detail and also present synopses of the theoretical positions of Hull, Guthrie, and Tolman. Gregory A. Kimble, Norman Garmezy, and Edward Zigler, Principles of Psychology, 6th ed. (1984), is a general textbook that covers several theoretical issues and should be particularly useful on the topics of memory and retrieval. William Bechtel and Adele Abrahamsen, Connectionism and the Mind: An Introduction to Parallel Processing in Networks (1991), gives an overview of connectionist theory and higher-level cognitive processing. David E. Rumelhart et al., Parallel Distributed Processing: Explorations in the Microstructure of Cognition, 2 vol. (1986), is a landmark collection of articles on the connectionist theory of learning. Philip Johnson-Laird, The Computer and the Mind, 2nd rev. ed. (1993), provides an easy introduction to cognitive science.Gregory A. Kimble Ed.

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