Animal learning


Laws of performance

Conditioning could have no function at all, however, if it did not involve changes in an animal’s behaviour. Nor could scientists infer that conditioning has occurred unless they could observe, at some point, a change in an animal’s behaviour attributable to certain conjunctions of events. So, although conditioning may involve the formation of associations between events or the attribution of particular events to their most probable antecedent causes, it must also include some mechanisms for translating these associations into changes in behaviour.

For an earlier generation of behaviourists, the fundamental fact about conditioning was precisely that it changed behaviour, and the theories they advanced were determined by this fact. The description of conditioning as the establishment of a new response to a stimulus that had not previously elicited that response naturally suggested that conditioning was a matter of forming new stimulus–response connections. This conceptualization led to the development of the stimulus–response theory, variations of which long provided the dominant account of conditioning. One version of the stimulus–response theory suggested that the mere occurrence of a new response to a given stimulus, as when Pavlov’s dog started salivating shortly after the metronome had started ticking, is in itself sufficient to strengthen the connection between the two. Thorndike, however, argued that the probability that a particular stimulus will repeatedly elicit a particular response depends on the perceived consequences of this response. According to this view, new stimulus–response connections are strengthened only if the response is followed by certain kinds of consequences.

There are several questions raised here, and it is important to keep them distinct. One is whether responses are sometimes (or even always) modified by their consequences. Although denied by some theorists, their denial seems distinctly paradoxical. A rat whose presses on a lever are followed by the delivery of a food pellet will press the lever again; if the only consequence of pressing the lever is the delivery of a painful shock, the rat will desist from this action. Thorndike’s law of effect—which stated that a behaviour followed by a satisfactory result was most likely to become an established response to a particular stimulus—was intended to summarize these observations, and it is surely an inescapable feature of understanding how and why humans and other animals behave. In keeping with this understanding, parents reward children for good behaviour and punish them for bad. When this fails to produce the desired behaviour, we are inclined to argue that the child is finding other sources of reward or does not find the intended punishment particularly unpleasant, or that the parents’ behaviour is hopelessly inconsistent. We are far less likely to question the assumption that, other things being equal, people (and other animals) repeat actions that have desirable consequences and avoid repeating those that have undesirable consequences.

Thorndike’s law of effect was, however, also a theory of how reward and punishment modify behaviour. This theory, which states that behaviour normally is modified by changing the strength of stimulus–response connections, finds less general acceptance today. A simple experiment suggests one reason for this. A rat is trained to press a lever in a Skinner box, being rewarded with a small quantity of sucrose solution for each press of the lever. Once the response has been established, the rat is removed from the Skinner box. The next day, while in its home cage, the animal is given sucrose solution to drink and shortly thereafter is made ill by an injection of lithium. Once this treatment has established a strong aversion to the sucrose, the rat is returned to the Skinner box, where, despite the opportunity to do so, the animal does not press the lever again. The result is hardly surprising: there is no reason to expect the rat to perform a response whose sole consequence is the delivery of the now aversive sucrose solution. But this behaviour cannot be explained by Thorndike’s theory, for according to Thorndike all that the rat learned in the first stage of the experiment was a new stimulus–response habit; stimuli from the Skinner box should, by Thorndike’s reasoning, now elicit the response of pressing the lever. Thorndike’s stimulus–response theory credits the rat with no acquired knowledge of the connection between pressing the lever and obtaining sucrose; the function of sucrose is merely to strengthen the stimulus–response connection.

That responses are modified by their consequences, therefore, need not call for Thorndike’s theoretical account of this fact. It is probably more reasonable to suppose that animals learn about the relationship between their actions and consequences (just as they can also learn about the relationship between any other classes of events), and that they then modify their actions in accordance with the current value of these consequences. The next question to consider is whether this is an entirely general principle of performance, or whether it applies only to some classes of response in some kinds of situations. Why, for example, does Pavlov’s dog start salivating to the ticking of the metronome? Is it because the response of salivating is followed by a rewarding consequence? The response is, at first, elicited by the sight of food and is shortly followed by the rewarding consequence of chewing and swallowing the food. But another simple experiment suggests that salivating to the metronome is not strengthened because it is followed by food. The experimenter can turn on the metronome for five seconds on each trial, at the end of which time the dog receives food—but only if it did not start salivating before the arrival of food. Now the response of salivating to the metronome is followed by an undesirable consequence, the cancellation of the food that would otherwise have been delivered on that trial, but the dog still cannot help salivating (at least sometimes) to the metronome. The implication is that salivating is not a response modified by its consequences, but one reflexly elicited by food and also by any stimulus associated with food. Voluntary responses can be modified by their consequences; involuntary responses (such as blushing when a person is embarrassed or the release of adrenalin when a person is angry or afraid) cannot. The reason Pavlov’s dog starts salivating to the metronome is, just as Pavlov himself supposed, that the association between metronome and food means that the metronome can substitute for food. To put it another way, the metronome now produces activity in neural centres normally responsive to the delivery of food, activity that is reflexly connected to the salivary response.

It should not be thought that only autonomic, glandular responses are involuntary in this sense. If a small light is always illuminated for five seconds before the delivery of food to a hungry pigeon, the pigeon will learn, by classical conditioning, to approach and peck at the light. Exactly the same experiment as that described above can be undertaken, with food delivered only on those trials when the pigeon does not approach and peck the light during the initial five seconds. The pigeon cannot help doing so. Pavlovian conditioning appears to be a widespread phenomenon, applying to a relatively wide range of responses.

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