The indeterminate character of the broad theoretical formulations offered to account for transfer of training and the often unsuccessful ways in which they were applied to the practical problems of classroom teaching led some psychologists to retreat to the laboratory in the hope of identifying more clear-cut, fundamental processes in transfer of training. As a result, a number of different transfer-of-training phenomena were discovered, several of which may be reviewed as follows.
Stimulus and response similarity
The method of paired-associate learning, in which a person is asked to learn to associate one syllable or word with another (e.g., complete–hot, safe–green, wild–soft), encouraged the investigation of the influence of stimulus and response similarity on transfer of learning. Typically these pairs of verbal items are presented to the laboratory subject so that the first, or stimulus, member (e.g., complete) is exposed alone, followed after a short interval by the second, or response, member (e.g., hot). The subject’s task is to respond to the stimulus term before the response term appears, as when an English-speaking student in learning French is supposed to respond to le livre with the book.
When two successive lists of paired associates are learned in which the stimulus elements are the same but the response terms are changed (e.g., complete–hot in the first list and complete–new in the second), negative transfer typically results. Apparently, in learning the second list the subject tends to respond to the stimulus term (e.g., complete) with the previously learned correct response term (e.g., hot), the result being interference with new learning to produce negative transfer. If he were learning the second list without having learned the first, the subject would not be so handicapped.
Another question concerns the sort of transfer that results when response terms are different and stimulus elements are similar but not identical; for example, entire is similar to complete. After one has learned complete–hot, the experimental evidence is that his ability to learn entire–new becomes definitely more difficult. Both entire and complete seem to have a tendency to evoke the response hot and to be incompatible with subsequently learning the association of entire with new. The principle that appears to operate in such situations is that the greater the similarity in stimulus elements, the greater the degree of negative transfer.
The influence of response (rather than stimulus) similarity on transfer of training is more complex; in paired-associate learning, the subject needs to learn the response term of each pair (response learning) and then to remember that it is linked with its appropriate stimulus partner (associative learning). When response terms are relatively difficult to learn (as in the case of unfamiliar or foreign words), the subject tends to profit considerably from learning the first list. But when response terms already have been learned (or are easy to learn), little if any positive transfer is likely to occur. The degree of transfer between lists that contain similar response terms depends both on how similar they are and on their level of difficulty; increasing the similarity between response terms is most likely to increase positive transfer when the response terms are relatively difficult to learn.
Although attempts have been made to formulate an all-embracing theory that would account for the effects of similarity among paired associates on transfer of training, a major obstacle that has prevented fully satisfying results is that the degree of positive or negative transfer is typically a product of many interacting influences beyond those of stimulus and response similarity. For example, the amount of training that the subject receives also has significant effects on transfer. When initial training is given on a simulated task (e.g., learning to operate a set of dummy controls in preparation for a second task of acquiring a complicated skill, such as flying an airplane), negative transfer effects frequently appear during the initial stages of learning the second task and then give way with further training to generally positive transfer effects.
Another stumbling block in developing theoretical explanations has to do with the meaning of the central concept of similarity. In such experiments as those in which the salivary reflex is conditioned to different auditory stimuli, similarity is measured in terms of physical stimulus properties (e.g., pitch or loudness); in other studies, as in paired-associate learning, similarity typically is expressed in terms of verbal meaning. In neither case has a universally adopted method yet been devised to measure similarity in a reliable and precise way; perhaps none can be, simply since there are so many different aspects of physical and linguistic or semantic similarity. Despite these difficulties, efforts to analyze transfer experimentally in terms of the properties of stimulus and response events have been productive in identifying conditions that can be varied to alter the direction and the degree of transfer of training.
Retroactive and proactive inhibition
Closely related to stimulus and response similarity are phenomena called retroactive inhibition and proactive inhibition; these demonstrate how forgetting seems to result from interfering activities.
In a study of retroactive inhibition, both the experimental and control groups of people learn task A (for example, a list of adjectives) and are tested for their ability to recall A after a specified time interval. The groups differ in what they are asked to do during the interval; the experimental group learns a similar task B (say, another list of words), while the control group is assigned some unrelated activity (for example, naming a series of coloured chips) designed to prevent them from rehearsing task A. The results of numerous studies of retroactive inhibition show that the experimental subjects typically are poorest in recalling information from task A. The interpolated activity, particularly a comparable one such as memorizing a second list of adjectives, apparently interferes with one’s ability to recall words from the first list. Habit competition, or what is sometimes called interference, between the items of the original and the interpolated word lists at the time of recall is considered to be one of the major sources of the negative transfer exhibited in retroactive inhibition.
Experimental designs for demonstrating proactive inhibition differ from those used for showing retroactive inhibition in that the experimental group learns task B before, instead of after, task A. Whereas B was a task that was interpolated between the learning and the recall of task A in the retroactive inhibition study, B is a task that precedes the learning of task A in the proactive inhibition study. To evaluate the effects on the experimental subjects of their having learned B prior to A, the control people are instructed to relax during the time the experimental group is learning B. Typically an experimental subject’s ability to recall from task A is inferior to that of a control person, the degree of inferiority depending in part on how similar the two tasks are; the greater the similarity, the poorer the recall tends to be. Although proactive inhibition, so called to indicate that it acts forward from the first-learned task to the second, produces appreciably less forgetting than does retroactive inhibition, they both support the theory that interference can produce forgetting (see memory: Theories of forgetting).
Stimulus predifferentiation
Educational films can be considered as everyday examples of stimulus predifferentiation, in which the individual gets preliminary information to be used in subsequent learning. The student who sees a film describing the various parts of a microscope is likely to be better prepared to learn the requisite skills when confronted with the instrument itself. In laboratory studies of stimulus predifferentiation, the subject is given experience with a particular stimulus situation ahead of time; later he is asked to learn new responses in the same situation. In one illustrative study, subjects first practiced labelling four different lights and then later were asked to learn to press selectively one of four switches, each connected to one light. The rate at which they learned the appropriate pressing reactions was related to how well they had learned to label the lights.
The results of a large number of experiments covering a variety of stimulus predifferentiation techniques suggest that when a learner has an opportunity to become generally acquainted with an environment, he retains some information about its different components that prepares him for learning to make new responses to them. Various explanations have been offered to account for this facilitation; some investigators suggest that the process of labelling enhances the distinctiveness of environmental stimuli for the labeller; others hold that perceptual acquaintance can more sharply differentiate an environment into its component parts for the perceiver or that it may encourage appropriate responses of observing or attending. Nevertheless, no single process has been identified as fundamental in stimulus predifferentiation. Perhaps a number of these processes operate in different combinations from one stimulus-predifferentiation transfer experiment to another, each process representing a different method by which a learner can become familiar with the details of his environment.
Transposition
Another phenomenon that has received considerable attention in theories of transfer of training is called transposition. An initial report of transposition came from a study in which chickens were trained by rewards to respond to the darker of two gray squares. After this discrimination task was learned, the chickens were shown the originally rewarded gray square along with one that was still darker. They seemed to prefer the darkest gray to the square that had been previously rewarded. This finding was interpreted to support the hypothesis that the birds had initially learned to respond to a relationship (what a human being would call the concept “darker”) and that this response to a relationship had been transposed or transferred to the new discrimination. This relational interpretation later was challenged by theorists who offered a formulation to show, on the basis of principles of stimulus generalization, how a response to a relational stimulus could be explained by assuming that organisms do indeed respond to the absolute properties of the stimuli. Both explanations were found to be too simple for the variety of findings obtained with transposition studies. As a result, the interest of many investigators shifted away from demonstrating the relative merits of absolute versus relational interpretations to identifying conditions that seem to influence transposition behaviour. Within this context, newer, more sophisticated formulations have been proposed that consider both the absolute and relational characteristics of the stimuli in transposition studies.
Learning to learn
When people are asked to learn successive lists of words, their performance tends to improve from one task to another so that much less time is commonly required to learn, say, the tenth list than was needed for mastering the first list. This improvement suggests that information beyond the specific content of lists of words is also learned. It would seem as if the subjects are learning how to learn; that is, they seem to be acquiring learning sets, or expectancies, that transfer from list to list to produce continually improving performance.
Some of the most intensive work on learning sets has been carried out with monkeys that were learning how to solve several hundred discrimination problems in succession. In each problem, the monkey learned which one of two objects (for example, a bottle cap and a cookie cutter) consistently contained a piece of food. Although the solution of each successive problem required the animals to discriminate between two previously unfamiliar objects, performance tended to improve on successive tasks; the monkeys made increasing numbers of correct choices on the second trial of each problem as the process continued. Manifestly there was no cue to indicate the correct choice on the first trial of any specific problem. If the animal responded correctly on the first trial, then on the second trial it would only have to choose the same object to be correct thereafter; if the monkey made an error on the first trial, then the other object would inexorably be the one that should be chosen next. During their efforts to solve the first few problems the monkeys were correct approximately half the time on the second attempt to solve each problem. This success increased to an average of 80 percent correct after each animal had solved 100 problems, to 88 percent after 200 correct solutions, and eventually to 95 percent after 300. Thus, after a long series of separate tasks, all of the same type, the monkey’s first response to the next problem usually provided sufficient information for the animal to make the correct choice.
Since each of the successive discrimination problems was different, what actually was being transferred from problem to problem? In these discrimination problems, the monkeys seemed to have several items of information to learn in addition to which one of the two objects contained the rewarding bit of food. The animals apparently had to learn to pay attention to that part of their environment where the objects were placed. To make the correct choice, it would seem that a monkey would have to learn to abandon any preference it might exhibit for objects on either the left or the right; indeed, the animals usually did show such preferences. (The correct object was shifted from side to side in a random sequence to control for these preferences.) Ostensibly, the monkeys also had to learn that one object consistently contained food while the other was always empty. Although these learning sets by themselves would not serve to identify the correct object in each new discrimination problem, it seems likely that they could help the animal locate the reward very rapidly by eliminating initially unprofitable responses.
Reversal learning
In reversal learning, the individual first learns to make a discrimination, such as choosing a black object in a black–white discrimination problem, and then is supposed to learn to reverse his choice—i.e., to choose the white object. Such reversals tend to be difficult for most learners since there are negative transfer effects; e.g., the individual tends to persist in responding to the black object that was originally correct. Eventually, however, one’s tendency to make the originally learned selection typically becomes weaker, and he makes the competing response (e.g., to white) more frequently until a point is reached where it is almost consistently evoked. Reversal learning can be accomplished very rapidly when a laboratory animal, such as a monkey, is presented with a series of reversal-learning problems in which the same sequence of shifts is repeated (as when black is initially correct, then white, then black, then white, and so on). After extended reversal training, some animals are able to make the next reversal in the sequence in one trial. They behave as if they have mastered the abstract concept of alternation or of regular sequence.
The speed with which representatives of a given species of animal, including human beings, can be taught to make a reversal of this kind seems to be related to the place biologists assign them in a hierarchy of evolutionary development. On first being exposed to a reversal-learning problem, normally competent adult humans who can use language are likely to achieve a solution with great rapidity. Monkeys can learn to perform equally well after a relatively longer series of reversal-learning tasks; but isopods such as pill bugs or sow bugs, small relatives of crabs and shrimp, have such primitive brains that they seem to be unable to improve their performance at all during a series of reversal-learning tasks.
