Performance and feedback function on variable differential reinforcement of low rates: Tempus urgit vel abundat?

The variable-differential-reinforcement-of-low-rates (VDRL) schedule introduces temporal uncertainty by requiring organisms to withhold responding for variable intervals, yet its effects on behavior remain largely unexplored. The present study examined rat performance under a 10-s VDRL schedule, focusing on the temporal organization of responding. Ten rats were exposed to VDRL after baseline training, and performance was analyzed using survival analysis of interresponse times. Results showed a consistent three-stage structure across subjects, comprising within-bout responding, an intermediate waiting phase, and bout initiation. This intermediate phase was characterized by sustained but imperfect response inhibition, suggesting adaptive waiting under temporal uncertainty. Disruption via presession feeding extended the observed range of response rates and revealed that longer pauses could increase reinforcement rates despite reduced overall responding. In parallel, we evaluated the reinforcement feedback function (RFF) associated with the VDRL. A Hill-type function provided a candidate RFF. Moreover, simulated and observed data showed close agreement, supporting the use of computational models in RFF estimation. Overall, the findings indicate that VDRL schedules generate a distinctive pattern of temporally structured behavior shaped by a trade-off between responding and waiting.