Laboratory studies of habituation and conditioning usually employ very simple stimuli, such as lights, buzzers, and ticking metronomes in Pavlov’s experiments. Some of the other examples of learning considered earlier have already suggested that animals can actually respond to additional, more complex stimuli. Even the solution of simple spatial discriminations in the laboratory requires the animal to learn about spatial relationships between different landmarks; migration or navigation over hundreds of miles demands abilities at least as complex as this. Song learning requires the young bird to discriminate between different sequences of subtly varying notes and calls, and the individual recognition involved in imprinting requires response to elaborate configurations of features.

Thus, one way in which a problem may become more difficult is if its solution depends on response to more subtle changes in stimuli. Numerous laboratory studies have examined the abilities of a variety of animals to perform such discriminations. The phenomenon of transposition, first studied in chicks by the Gestalt psychologist Wolfgang Köhler, suggests that animals may solve even simple discriminations in ways more complex than the experimenter had imagined. Köhler trained his chicks to perform simple discriminations—say, to choose a large white circle (five centimetres in diameter) in preference to a small white circle (three centimetres in diameter). He then sought to discover whether the animal was responding to the relationship between the two stimuli or to the absolute characteristics of the stimuli. In other words, had the chick learned to select the larger of the two circles, or had it learned to pick the five-centimetre circle? If the former were the case, Köhler reasoned that given the choice between the five-centimetre circle and an even larger one (eight centimetres in diameter), the animal should transpose the relationship and choose the larger circle. This was indeed the result, demonstrating that the animal was responding in terms of the relationship between stimuli rather than, or at least in addition to, their absolute properties.

Transposition experiments show that animals can respond to relationships between stimuli varying along a particular continuum of physical characteristics: size, brightness, hue, etc. Another question is whether animals can respond to an abstract property of a stimulus array, independent of the actual physical stimuli making up that array. In experiments on counting, the animal must choose between an array containing, say, five stimuli and one containing three. The actual stimuli in the array vary from trial to trial, in order to rule out the possibility that the animal is responding in terms of other features, such as differences in total area or brightness, between the arrays. Counting experiments have been tried on birds more frequently than on any other class of animal, and several species, notably ravens, rooks, and jackdaws, have solved this type of problem. This success may not be entirely by chance, for there is reason to believe that the stimulus that controls when a female bird stops laying eggs is something to do with the number of eggs already laid and in the nest. Chimpanzees, however, have been trained to label pictures of various objects (e.g., spoons, shoes, padlocks, and balls) with the numeral specifying the number of objects in the picture. Moreover, rats and other standard laboratory animals have solved similarly abstract discriminations, for example, of temporal duration. A rat can learn to perform one response after a stimulus has been turned on for two seconds and a different response after the stimulus has been turned on for five seconds. The nature of the actual stimuli employed can vary without disrupting the rat’s discrimination, suggesting that it is the duration of the stimuli to which the rat responds.

Concept learning makes up another class of discriminations that may be solved by the abstraction of a particular property or set of properties from a very wide array of individual stimuli. In a typical experiment, a pigeon is shown a large number of colour photographs of natural scenes: half of these contain, somewhere within the scene, all or part of a tree or group of trees; the other half contain no tree (although there might be flowers, a climbing rose, or other plants). Responding to the pictures of trees is rewarded, but responding to the remaining pictures is not. Pigeons rapidly learn the discrimination. In one sense, perhaps this is not surprising: birds that roost in trees, one is inclined to argue, must be able to recognize them. But pigeons can learn other discriminations with almost equal facility; for example, they can be trained to distinguish between underwater scenes containing a fish and similar views with no fish present. In such cases, the class of stimuli in question is one for which their evolutionary history can hardly have prepared pigeons. The question, of course, is how the pigeons solve such problems. Are they, in some sense, abstracting a conceptual rule for categorizing the world into classes of stimuli? Or are they responding to what is no doubt a very large number of particular features that differentiate trees or fish from other objects in the world?

Pigeons, in common with most birds, rely more heavily on vision, and certainly have better developed colour vision, than most mammals—with the exception of primates. There is evidence that monkeys can solve the concept discriminations that have been set to pigeons, but there is no evidence that other mammals can. For extensive comparative analysis, therefore, it is necessary to turn to different kinds of tasks. One that has been studied almost to excess is discrimination reversal. In reversal tasks, an animal is first trained on a simple discriminative problem: for example, to choose the left-hand arm of a T-maze, where it is rewarded, rather than the right arm, where it is not. Once the animal has solved the problem, the experimenter reverses the reward assignments, so that the food is now in the right arm rather than the left. Training continues until the animal has learned this reversal, whereupon the assignment of reward is switched back to the left arm. And so on. Rats trained on this series of reversals eventually become extremely adept at the task. Although the initial reversal causes considerable problems, with animals making many more errors than on the original discrimination, after a few more reversals these difficulties vanish. Eventually, rats solve each new reversal in fewer trials than they took to solve the original discrimination, often with no more than a single error.

Similarly efficient performance has been observed in a relatively wide range of mammals. More interesting was the early suggestion that the few species of fish (goldfish, African mouthbreeders, and Paradise fish) trained on similar problems showed no evidence of the increase in efficiency displayed by mammals. The fish would learn the first reversal slowly and laboriously, and the 20th reversal equally slowly. Subsequent experiments have established that this was an unfairly pessimistic assessment, for improvements in experimental techniques have been accompanied by a significant improvement in the fish’s performance, a finding that highlights the extreme difficulty of assessing the relative efficiency of widely differing animals on supposedly the same task. Nevertheless, it remains doubtful that goldfish are as adept at reversal tasks as rats are.

The theoretical question, however, is how rats attain such efficiency. What processes allow them eventually to learn the reversal of a discrimination faster than they originally learned the discrimination itself, and often with only a single error? The most plausible suggestion is that they develop a “win–stay, lose–shift” strategy. They learn, in other words, to characterize the alternatives between which they must choose not in terms of their physical features but in terms of whether or not they chose it on the previous trial. They then learn that, if the alternative they chose on the last trial was rewarded, choice of that alternative will be rewarded again on the current trial; while, if it was not, choice of the other alternative will now be rewarded. A variety of other experiments have shown that rats can rapidly learn to use the outcome of one trial to predict the outcome of the next, and hence keep track of regular sequential dependencies in the availability of food or other rewards.

Generalized rule learning

Second only to the reversal task in popularity as a tool for the comparative analysis of learning has been the learning set task. The latter is designed to measure the animal’s ability “to learn to learn”—in other words, to discover whether after having learned a new behaviour the animal can then more readily learn other related behaviours. For example, an animal is trained on a simple discrimination between two objects, A and B. Once the problem has been solved, the experimenter substitutes a new pair of objects, C and D, for the original pair; when the animal has solved this new problem, yet another new pair, E and F, is substituted, and so on. Rhesus monkeys trained on such a series of problems become progressively more efficient at solving each new problem. Like rats trained on reversal tasks, the monkeys eventually solve each new problem after a single trial, choosing at random on the first trial with each new pair of stimuli but thereafter selecting with essentially perfect accuracy.

Performance on learning sets, as on reversals, was once thought to discriminate between more intelligent and less intelligent animals. Apes and rhesus monkeys were extremely efficient at such tasks, more so even than New World monkeys, who were, in turn, more efficient than any nonprimate mammals. Again, however, there are grave difficulties in the way of making valid comparisons. Primates have better developed visual systems than most other mammals, so it is not surprising that they should be better at solving a series of visual discrimination problems. Even the difference in performance between rhesus and cebus monkeys (Old World versus New World monkeys) turns out to be attributable to differences in colour vision more than anything else. Rats appear to solve learning set tasks very efficiently if olfactory stimuli are used.

Nevertheless, there may be important intellectual differences also underlying the differences in performance. One reason for thinking so arises from consideration of the processes probably involved in mastering learning sets. The win–stay, lose–shift strategy that explains the progressive improvement in reversal learning can also explain the same improvement in the learning set task—but only if the animal can generalize the strategy to novel stimuli. Successful performance requires that the animal learn that the alternative chosen on the last trial, and the outcome of that choice, predict which alternative will be rewarded on this trial, whatever the nature of the alternatives. Some evidence suggests that primates can generalize rules of this sort more readily than many other animals can. Monkeys trained on a series of reversals of a single discrimination will learn the reversal of any new discrimination with equal facility. By contrast, cats trained on comparable problems show little evidence of such transfer.

A discriminative problem widely used in the study of transfer is the “matching-to-sample” discrimination. A pigeon, for example, is required to choose between two disks, one illuminated with red light and the other with green light. The correct alternative on any one trial depends on the value of a sample stimulus, which is also part of each trial. If this third light is red, then the red disk is correct; if green, then green is correct. The correct alternative is the one that matches the sample. Although naturally more difficult than the simple red–green discrimination, matching-to-sample discriminations are learned readily enough by a wide variety of animals; however, there appear to be differences among animals in their capabilities to transfer this learning to a new set of stimuli. Primates and dolphins have shown good evidence of such transfer, but pigeons have shown at best only limited transfer. If pigeons are trained with two or three colours to the point where they are responding with essentially no errors, a substitution of a new colour for one of the trained colours may result in a complete breakdown in the discrimination; there is even some question as to whether they can learn a new matching-to-sample discrimination with new stimuli any faster than pigeons with no prior experience of matching problems.

The abilities to respond in terms of certain relationships between stimuli, to abstract those relationships and invariant features from a complex and changing array of stimuli, and, above all perhaps, to transfer such learning to a completely novel set of physical stimuli seem to be some of the more important processes underlying the solution of complex discriminative problems. The fact that certain evidence suggests that animals may differ in some of these abilities has implications for studies of other forms of problem solving.