Within-session changes in responding during delayed matching-to-sample and discrimination procedures

Two experiments examined within-session changes in responding during discrimination procedures. In Experiment I, rate of responding changed significantly within sessions during symbolic delayed matching-to-sample tasks when the delay between the stimulus and the choice period was short (1-5 sec), but not when it was long (8-12 sec). The percentage of responses that were correct did not change within sessions. Ln Experiment 2, response rates increased and then decreased within sessions during both S+ and S- when successive discrimination procedures provided high, but not low, rates of reinforcement. Discrimination ratios sometimes increased within sessions. These results question two potential definitions of attention as explanations for within-session changes in response rates. They are more compatible with explanations based on concepts such as arousal, satiation, habituation, and interfering responses.