Effect of required response force on rats' performance on a VI+ schedule of reinforcement

Three experiments were performed to examine the effect of response force on rats' performance on various schedules of reinforcement. Response force was manipulated by changing the weight of the lever in the operant chamber--a heavy lever for high response force and a light lever for low response force. Using a within-subjects design, Experiment 1 replicated previous findings that rats respond more quickly on variable ratio (VR) than on equivalent variable-interval-plus-linear-feedback (VI+) schedules. Experiment 2 replicated this finding but also showed that the use of a smaller response force abolished the response rate difference between the VR and VI+ schedules. Experiment 3 used a between-subjects design and showed a response rate difference between the VR and VI+ schedules with a high response force but no response rate difference with a low response force. This suggests that under conditions of low force, when the rats' responding can continue at prolonged high rates, these subjects show little difference in their response rates between VR and VI + schedules. These data are similar to those found for human subjects.