Response-bout analysis of interresponse times in variable-ratio and variable-interval schedules

The purpose of this study was to analyze the response pattern difference between variable-ratio (VR) and variable-interval (VI) schedules of reinforcement by modeling interresponse time distributions of rats' lever presses. All eight rats showed higher response rates under VR 30 than under inter-reinforcement intervals yoked VI. The 30 models consisting of single Exponential (with and without the lower limit on interresponse times), Weibull, Normal, Log-Normal or Gamma distributions, all possible two component combinations of those, and 3 and 4 component models consisting of Weibull, Normal, Log-Normal, or Gamma distribution combinations were compared. The 4 component Log-Normal model was the best in terms of the Akaike information criterion and visual inspection of fitting outcome. Parameter estimates for the L4 model showed that the VR-VI response rate difference is due to a difference in short interresponse times or within bout responses. This results suggests that the VR-VI response rate difference is not an indication of a difference in the overall tendency to respond but it is rather a difference in terms of what types of response patterns are engendered between the two schedules.