Operant response rates often change within experimental sessions, sometimes increasing and then decreasing. The authors attribute these changes to sensitization and habituation to aspects of the experimental situation presented repeatedly (e.g., reinforcers) or for a prolonged time (e.g., the experimental enclosure). They describe several empirical similarities between sensitization-habituation and within-session changes in operant responding. They argue that many alternative explanations for within-session changes in operant responding can be dismissed. They also examine some implications of linking the literatures on habituation and operant responding. Because responding follows a similar pattern in several other cases (e.g., human vigilance, classical conditioning, and unconditioned responding), 2 relatively simple processes may be responsible for the temporal patterning of behavior in a wide variety of situations.

Rats and pigeons responded for food delivered according to multiple schedules. The session length varied from 10 to 120 min, and the programmed rate of reinforcement varied from 15 to 240 reinforcers per hour. Response rates usually changed systematically within experimental sessions. For both rats and pigeons, responding reached a peak after an approximately constant amount of time since the beginning of the session, regardless of session length. When rats, but not pigeons, served as subjects, the peak rates of responding occurred later in the session and the within-session changes were smaller for lower than for higher rates of reinforcement. The similarities between the results for rats and for pigeons when session length varied suggest that at least one of the factors that produces the within-session changes in responding is shared by the present species, responses, and reinforcers. The differences in results when rate of reinforcement varied are more difficult to interpret.Rate of responding changes systematically within experimental sessions under some conditions. For example, McSweeney, Hatfield, and Allen (1990) reported that response rates increased to a peak and then decreased within sessions when rats pressed levers or keys on multiple variable-interval (VI) 1-min VI 1-min schedules. These within-session changes deserve further study for several reasons. First, they are as large as, or larger than, those usually studied by operant psychologists. For example, Catania and Reynolds (1968) observed only a doubling of response rate when they changed the programmed rate of reinforcement from 8.4 to 300 reinforcers per hour. The rate of keypressing reported by McSweeney et al. changed by an average of 450% within the session. Second, the changes are orderly. Responding reached a peak at the same time in the session regardless of whether subjects' leverpressing was reinforced by Noyes pellets or whether their keypressing was reinforced by sweetened condensed milk. Finally, as has been discussed elsewhere, within-session changes may have important implications for theory and methodology in operant psychology (see, e.g., McSweeney & Roll, 1993).The present experiments examined within-session changes in responding as a function of session length and programmed rate of reinforcement. In Experiment 1, rats pressed levers for Noyes pellets and pigeons pecked keys for mixed grain. Session length varied from 10 to 120 min. In Experiment 2, rats pressed keys for sweetened condensed milk and pigeons pecked keys for mixed grain. The programmed rate of reinforcement varied from 15 to 240 reinforcers per hour. These experiments should add to our knowledge in three ways.First, the experiments help to establish the generality of within-session changes in responding. If these changes occur only under limited conditions, they are less important than they are if they occur more generally. The behavior of two species making three different responses for three different reinforcers was examined for five rates of reinforcement an...

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