Choice Between Constant and Variable Alternatives by Rats: Effects of Different Reinforcer Amounts and Energy Budgets

Abstract

Two experiments, using rats as subjects, investigated the effect of different reinforcer amounts and energy budgets on choice between constant and variable alternatives under a closed economy. Rats were housed in the chamber and were exposed to a modified concurrent-chains schedule in which the choice phase was separated from a rest phase during which the rats could engage in other activities. In the choice phase, a single variable-interval schedule arranged entry into one of two equal terminal links (fixed-interval schedules). The constant terminal link ended with the delivery of a fixed number of food pellets (two or three, depending on the condition), whereas the variable terminal link ended with a variable number of food pellets (means of two or three, depending on the condition). Energy budget was defined as positive when body weights were over 90% of free-feeding weights, and as negative when they were under 80% of free-feeding weights. The different body weights were produced by varying the duration of the equal terminal-link schedules within daily 3-hr sessions. In Experiment 1, rats chose between a constant and a variable three pellets under both energy budgets. Rats preferred the constant three pellets more under the positive energy budget, whereas they were indifferent under the negative energy budget. In Experiment 2, rats chose between a constant three pellets and a variable two pellets, and chose between a constant two pellets and a variable three pellets under both energy budgets. The rats strongly preferred the constant three pellets over the variable two pellets under both energy budgets. In contrast, rats preferred the variable three pellets over the constant two pellets only under the negative energy budget, whereas they were indifferent under the positive energy budget. These results indicate that rats choices are sensitive to the difference in reinforcer amounts and to the energy budgets defined by the level of body weight. The present results are consistent with those obtained with small granivorous birds as well as with the predictions of a recent risk-sensitive foraging theory.

Risky Choice in Pigeons: Preference for Amount Variability Using a Token‐reinforcement System

“…The VA distributions used in the current experiment were positive and negative exponential and rectangular, providing a minimum of zero and a maximum of 12 tokens. It has been noted that exponential distribution types (negative binomial in particular) represent food sources that many species experience in their natural foraging environments (Ito, Takatsuru, & Saeki, 2000; Iwasa, Higashi, & Yamamura, 1981). Because a range of distribution types have been employed in risky choice studies, such as bimodal (two‐valued; e.g., Caraco, Martindale, & Whittam, 1980; Hamm & Shettleworth, 1987; Leventhal, Morrell, Morgan, & Perkins, 1959), it is not entirely clear how differently programmed variability may affect risk‐sensitive preferences.…”

The effect of rate of reinforcement and time in session on preference for variability

“…These particular schedules were chosen in order to provide a wide range of rates of reinforcement and to exceed the range that had been used in other studies to take animals from a positive to a negative energy budget (e.g., Ito et al, 2000). They were FI and VI 15-sec, FI and VI 105sec, FI and VI 45-sec, FI and VI 5-sec, and FI and VI 225-sec schedules in different conditions, conducted in that order.…”

“…Positive energy budgets provide enough energy for the animal to meet its fitness needs. As a result, larger animals, such as pigeons and rats, have been used to test RST (e.g., Ito, Takatsuru, & Saeki, 2000). Early versions of RST defined fitness solely in terms of survival.…”

Basolateral amygdala lesions and sensitivity to reinforcer magnitude in concurrent chains schedules

“…For example, when presented with two sources of reinforcement, animals allocate their behavior to match the ratio of reinforcement schedules [10,11], reinforcer amounts [12,13], reinforcer immediacies [14,15], and the relative rate of conditioned reinforcers [16]. Although the factors determining steady-state preference are well described [17], the factors controlling how preference adapts when reinforcer properties change are poorly understood.…”