Timing and choice in concurrent chains.

To investigate the role of timing processes in choice, we used a new procedure that provided simultaneous measures of ongoing choice and timing behavior. Pigeons responded in a peak procedure in which the delays to reinforcement signaled by red and green center-key stimuli were 10 and 20, or 20 and 40 s. After 25 sessions of training, the peak procedure was embedded within concurrent chains: The inter-trial interval was replaced by a choice phase in which the two side keys were illuminated white; responses to the left and right keys occasionally changed the center-key to red or green, respectively; and the terminal links signaled by the center-key stimuli were identical to the trials of the peak procedure. The temporal control of responding on no-food trials was the same regardless of whether the no-food trials occurred in the peak procedure or as the terminal links of concurrent chains. After an intervening condition with the peak procedure in which the delay for the 10 s stimulus was changed to 40 s (or vice versa), the pigeons were returned to concurrent chains. Choice responding did not reflect the changed delay, despite the fact that the pigeons timed the delays in both terminal links accurately as indexed by responding on no-food trials. This result challenges current accounts of choice based on timing processes, such as scalar expectancy theory, which assume that choice responding is mediated by a representation of terminal link delays to reinforcement. Apparently, pigeons' choice and timing behavior in a single session can be controlled by temporal information from different temporal epochs.