The effect of multiple S periods on responding on a fixed-interval schedule: IV. Effect of continuous S with only short S probes.

Pigeons were studied under FI 500 sec in which an S(Delta) was present throughout the interval except during the terminal 50-sec segment and one earlier 50-sec segment. Very little responding occurred during the presence of S(Delta). The rate of responding in the earlier 50-sec S(D) segments was lower than in the terminal S(D) segment. There was a clear trend for the rate of responding in the earlier S(D) segment to be progressively higher the later it occurred in the course of the FI 500 sec. This trend was shown roughly to parallel the increasing rate of responding in a conventional FI 500 sec with no interruption by S(Delta). Since the changing tendency to respond through the FI survives massive disruption by S(Delta), it is concluded that the control of responding through the FI does not require continuous mediating behavior. It is suggested that it is the decaying retroactive influence of the reinforcer on responses that occurred longer and longer before the reinforcer occurred which produces the familiar scalloped pattern of responding under FI schedules.