The isolation of motivational, motoric, and schedule effects on operant performance: a modeling approach

Abstract

Dissociating motoric and motivational effects of pharmacological manipulations on operant behavior is a substantial challenge. To address this problem, we applied a response-bout analysis to data from rats trained to lever press for sucrose on variable-interval (VI) schedules of reinforcement. Motoric, motivational, and schedule factors (effort requirement, deprivation level, and schedule requirements, respectively) were manipulated. Bout analysis found that interresponse times (IRTs) were described by a mixture of two exponential distributions, one characterizing IRTs within response bouts, another characterizing intervals between bouts. Increasing effort requirement lengthened the shortest IRT (the refractory period between responses). Adding a ratio requirement increased the length and density of response bouts. Both manipulations also decreased the bout-initiation rate. In contrast, food deprivation only increased the bout-initiation rate. Changes in the distribution of IRTs over time showed that responses during extinction were also emitted in bouts, and that the decrease in response rate was primarily due to progressively longer intervals between bouts. Taken together, these results suggest that changes in the refractory period indicate motoric effects, whereas selective alterations in bout initiation rate indicate incentive-motivational effects. These findings support the use of response-bout analyses to identify the influence of pharmacological manipulations on processes underlying operant performance.

Keywords: bouts; effort; extinction; lever press; motivation; rats; tandem ratio.

Fig 1

A diagram of the refractory bi-exponential model of operant performance. The lever press requires time δ to complete. Following a response, the rat either remains in the engaged state with probability 1−q and responds on the lever at rate w, or exits the engaged state with probability q and returns at rate b.

Fig 2

Mean (± SEM) reinforcement rate (Panels A and B) and overall response rate (Panels C and D) for experimental conditions in Phase 1. Left panels show daily means for low and high workload levers. Right panels show means averaged over the last four sessions for VI and Tandem condition and the mean for the single Food Dep session. Asterisks indicate significant (p < .05) effects of condition (schedule/deprivation manipulations).

Fig 3

Log survival plots of IRTs produced by a representative rat in all experimental conditions of Phase 1 (continuous curves). Left and right panels show data from low and high workload levers, respectively. The maximum likelihood fits of the refractory single exponential (Ex) and bi-exponential (Bi-ex) models are also shown. The best fitting parameters for the two models are displayed in each graph. q: proportion of IRTs separating bouts; w (responses/sec): within-bout response rate; b (responses/sec): bout-initiation rate; δ (sec): refractory period. The rat was selected by ranking the overall response rate on each lever in the last session of each condition in Phase 1 for each rat, then averaging the rank across levers and conditions, and selecting the rat with the third highest average rank.

Fig 4

Mean (± SEM) bi-exponential model parameters: probability of quitting a bout, q (Panels A and B); the refractory period, δ (Panels C and D); within-bout response rate, w (Panels E and F); and bout initiation rate, b (Panels G and H). Left panels show daily means for low and high workload levers. Right panels show means averaged over the last four sessions for VI and Tandem condition and the mean for the single Food Dep session. Asterisks indicate significant (p < .05) effects of condition (schedule/deprivation). Pound signs indicate significant effects of workload.

Fig 5

Cumulative lever presses emitted during extinction on the low and high workload levers for individual rats. Vertical drop lines indicate the time at which each subject emitted half of its total lever presses in the extinction session. Rat numbers are indicated at the end point of each record.

Fig 6

Interresponse times (IRTs) as a function of time t in extinction. The broken, solid, and dotted lines are, respectively, traces of average bout initiation IRT (1/b_t), average within-bout IRT (1/w_t), and constant minimum IRT (δ), drawn from the dynamic refractory bi-exponential model selected in Table 4 and fitted using the maximum likelihood method. Traces of 1/b_t have been joined by a smooth straight line for illustrative purposes (see main text).