Learning and stimulus control
Although the ability of the discriminative stimulus to evoke a (conditioned) motivational state is undoubtedly important, this still does not fully explain how it controls instrumental responding. It is difficult to believe that a rat that receives food for leverpressing in the presence of a tone is insensitive to the conditional nature of the task – in other words, that it fails to learn that the response yields food only if the tone is on. But the version of two-process theory just described proposes only that the rat will form two simple associations – stimulus–food and response–food. There is no room in this account for the learning of a conditional relationship of the form ‘only lever-pressing in the presence of the tone results in the presentation of food’. This issue has been addressed experimentally in recent years, and several researchers have demonstrated that animals are capable of conditional learning. The stimulus control of performance revealed by these experiments cannot be explained in terms of standard two-process theory, in which discriminative stimuli have their effects solely by virtue of orthodox associations with reinforcers. Instead, it shows that animals are capable of learning the conditional relationship between a stimulus and a particular response–reinforcer relationship. So, discriminative stimuli exert their effects because they are able to trigger not just the representation of the reinforcer but also the more complex, response–outcome representation produced by instrumental training. This represents the learning of a conditional relationship.
Although the ability of the discriminative stimulus to evoke a (conditioned) motivational state is undoubtedly important, this still does not fully explain how it controls instrumental responding. It is difficult to believe that a rat that receives food for leverpressing in the presence of a tone is insensitive to the conditional nature of the task – in other words, that it fails to learn that the response yields food only if the tone is on. But the version of two-process theory just described proposes only that the rat will form two simple associations – stimulus–food and response–food. There is no room in this account for the learning of a conditional relationship of the form ‘only lever-pressing in the presence of the tone results in the presentation of food’. This issue has been addressed experimentally in recent years, and several researchers have demonstrated that animals are capable of conditional learning. The stimulus control of performance revealed by these experiments cannot be explained in terms of standard two-process theory, in which discriminative stimuli have their effects solely by virtue of orthodox associations with reinforcers. Instead, it shows that animals are capable of learning the conditional relationship between a stimulus and a particular response–reinforcer relationship. So, discriminative stimuli exert their effects because they are able to trigger not just the representation of the reinforcer but also the more complex, response–outcome representation produced by instrumental training. This represents the learning of a conditional relationship.
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