Rapid acquisition of choice and timing and the provenance of the terminal-link effect

Eight pigeons responded in a concurrent-chains procedure in which terminal-link schedules changed pseudorandomly across sessions. Pairs of terminal-link delays either summed to 15 s or to 45 s. Across sessions, the location of the shorter terminal link changed according to a pseudorandom binary sequence. On some terminal links, food was withheld to obtain start and stop times, measures of temporal control. Log initial-link response ratios stabilized within the first half of each session. Log response ratio was a monotonically-increasing but nonlinear function of programmed log terminal-link immediacy ratio. There was an effect of absolute terminal-link duration on log response ratio: For most subjects, preference for the relatively shorter terminal-link delay was stronger when absolute delays were long than when absolute delays were short. Polynomial regressions and model comparison showed that differences in degree of nonlinearity, not in sensitivity to log immediacy ratio, produced this effect. Temporal control of stop times was timescale invariant with scalar variability, but temporal control of start times was not consistent across subjects or terminal-link durations.

Response allocation in concurrent chains when terminal-link delays follow an ascending and descending series

Eight pigeons were trained in a concurrent-chains procedure in which the terminal-link immediacy ratio followed an ascending or descending series. Across sessions, one terminal-link delay changed from 2 s to 32 s to 2 s or from 32 s to 2 s to 32 s, while the other was always 8 s. For all pigeons, response allocation tracked changes in delay and was biased towards the 8-s alternative on the descending series, indicating a hysteresis effect, and was more sensitive to changes in the terminal-link delay ratio for relatively long (> 8 s) than short (< 8 s) delays. Both the hysteresis and effect of delay duration were predicted by an extended version of Grace and McLean's (2006) decision model. The extended decision model provided an overall better account of the results than a simple linear-operator model (Grace, 2002), and holds promise for an integrated account of choice in concurrent chains for both acquisition and steady-state conditions.