Delay-of-reinforcement-gradient hypothesis of SHR hyperactivity. Ticks on the x-axis are responses; thick ticks are bout initiations. A reinforcer is delivered after the last response on the right of each panel. The sloped curves indicate that reinforcement is more effective with temporal proximity to the reinforcer. For SHR, only short bouts are effectively reinforced (panel A); for WKY, longer bouts are reinforced (panel B). Note that reinforcement affects more behaviors in panel B than in panel A, which entails that a smaller proportion of reinforcement strengthens bout initiation in panel B than in panel A. Therefore, relative to competing behaviors such as activities between bouts and responses within bouts, bout initiations are less effectively reinforced when reinforcement gradients are flatter.