Academic journal article The Psychological Record

Does the Temporal Placement of Food-Pellet Reinforcement Alter Induction When Rats Respond on a Three-Component Multiple Schedule?

Academic journal article The Psychological Record

Does the Temporal Placement of Food-Pellet Reinforcement Alter Induction When Rats Respond on a Three-Component Multiple Schedule?

Article excerpt

A series of recent studies have reported that when rats press a lever for 1% liquid-sucrose reinforcement delivered by a random-interval (RI) schedule, they respond at a higher rate if food-pellet reinforcement will be available later in the session than if sucrose reinforcement will continue to be available in the session (e.g., Weatherly, Plumm, Smith, & Roberts, 2002b; Weatherly, Stout, McMurry, Rue, & Melville, 1999). This increased rate of responding, termed induction, is of interest because it is opposite of what one might expect to observe. That is, given that food pellets represent an increase in the conditions of reinforcement relative to 1% sucrose (see Weatherly et al., 2002b, for a more detailed explanation), one might expect to observe a decrease in response rates for sucrose (i.e., a contrast effect; Reynolds, 1961) rather than an increase.

Several studies have investigated why this induction effect occurs. Weatherly et al. (2002b), for instance, investigated whether Pavlovian processes play a role. To do so, they had rats respond in two types of session. In one, the rats lever pressed for liquid-sucrose reinforcers in both the first and second half of the session. In the other, they lever pressed for sucrose reinforcers in the first half of the session and for food-pellet reinforcers in the second. Which type of session was in effect was signaled by which lever (right or left) the subjects pressed to obtain reinforcement. After training, subjects displayed induction in the first half of the sessions in which food-pellet reinforcement was upcoming. To test whether lever location was serving as a conditioned exciter (because it was paired with upcoming food-pellet reinforcement), they then had rats respond in a similar procedure with the location of the lever used in the first half of the session being perfectly predictive of the type of reinforcer (continued sucrose or food-pellet reinforcement) that would be available in the second half. However, at the midpoint of each type of session, the lever that had been used in the first half was retracted from the chamber and, at an equal probability, either the right or left lever was inserted into the chamber and was used to collect reinforcement in the second half of the session. Thus, although lever location in the first half of the session was predictive of upcoming reinforcer type, both the right and left levers were used equally often to collect both sucrose and food-pellet reinforcement.

Weatherly et al. (2002b) reasoned that, if Pavlovian processes contributed to induction, then rates of responding in the first half of sessions in which food-pellet reinforcement was upcoming should be higher than those observed in the first half of sessions in which continued sucrose reinforcement was upcoming. However, this result did not occur. Response rates did not differ in the first half of the different types of session. Furthermore, when subjects responded in concurrent-choice sessions in which both levers were inserted into the chamber, little evidence existed to suggest they had a response preference for either lever. These results led Weatherly et al. to conclude that induction was likely under operant control. More specifically, in the presence of stimuli that indicate that food-pellet reinforcement will be available during the session, the rate of operant responding increases. Because, in the above procedure, both levers were used equally often to collect food-pellet reinforcement, no stimulus differentially signaled food-pellet reinforcement and thus induction was not observed.

Weatherly et al. (2002b) went on to suggest that, if this account for induction was indeed correct, then induction in responding for sucrose reinforcement should occur in either half of the session when food-pellet reinforcement was available in the other half. In other words, the appearance of induction should not require that food-pellet reinforcement be upcoming. …

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