Second-order schedules : a comparison of the effects of paired and unpaired brief stimuli on component fixed interval performance

Pigeons were exposed to second-order schedules in which completion of a component fixed-interval schedule was reinforced according to a variable-ratio schedule. The completion of each component FI resulted in presentation of either a brief stimulus previously paired with food (paired brief stimulus), a brief stimulus not previously paired with food (unpaired brief stimulus), or no brief stimulus presentation (tandem). Three fixed-interval durations were employed in the study, with all birds exposed to the different durations in an ascending order (PI 15-sec, PI 30-sec, FI 1-min). The results demonstrated both similarities and differences in effects produced by the paired and unpaired brief stimuli. The pause following a brief stimulus, whether paired or unpaired, increased as the component PI duration increased. However, for a particular FI duration, the longest pauses always followed the paired brief stimulus. The results are consistent with the suggestion that the brief stimulus functions as a discriminative stimulus, with the pairing operation serving to enhance the discriminative properties of the brief stimulus