Suboptimal Choice in a Percentage‐reinforcement Procedure: Effects of Signal Condition and Terminal‐link Length

Abstract

Pigeons' choice between reliable (100%) and unreliable (50%) reinforcement was studied using a concurrent-chains procedure. Initial links were fixed-ratio 1 schedules, and terminal links were equal fixed-time schedules. The duration of the terminal links was varied across conditions. The terminal link on the reliable side always ended in food; the terminal link on the unreliable side ended with food 50% of the time and otherwise with blackout. Different stimuli present during the 50% terminal links signaled food or blackout outcomes under signaled conditions but were uncorrelated with outcomes under unsignaled conditions. In signaled conditions, most pigeons displayed a nearly exclusive preference for the 100% alternative when terminal links were short (5 or 10 s), but with terminal links of 30 s or longer, preference for the 100% alternative was sharply reduced (often to below .5). In unsignaled conditions, most pigeons showed extreme preference for the 100% alternative with either short (5 s) or longer (30 s) terminal links. Thus, pigeons' choice between reliable and unreliable reinforcement is influenced by both the signal conditions on the unreliable alternative and the duration of the terminal-link delay. With a long delay and signaled outcomes, many pigeons display a suboptimal tendency to choose the unreliable side.

Determinants of pigeons’ choice between certain and probabilistic outcomes

“…This is interesting because increasing the ITI decreases the overall rate of reinforcement available on the 50% alternative more than it decreases the overall rate of reinforcement available on the 100% side. These results are consistent with previous results obtained by Spetch et al (1990) and Mazur (1989), in suggesting that the relative value of an alternative leading to probabilistic reward is not altered by the duration of the ITI when the stimulus conditions during the ITI differ from those present during the trial. At the same time, the duration of the ITI was not completely without effect on behavior, because rate of responding to the terminal-link stimuli associated with food on both sides was higher when the ITI was long than it was when the ITI was short.…”

“…On the other hand, increases in the ITI have been found to diminish human subjects' tendency to choose uncertain large rewards over certain small ones in a discrete-trials hypothetical gambling procedure (Rachlin et al, 1986). In the discrete-trial concurrent-chains procedure with signaled outcomes, Spetch et al (1990) found no consistent effect of ITI duration on choice between 100% and 50% reinforcement. However, their study manipulated ITI only in the context of short terminal-link durations, in which pigeons displayed preference for the 100% alternative.…”

“…However, pigeons do not always respond in this fashion (e.g., Kendall, 1974Kendall, , 1985Spetch, Belke, Barnet, Dunn, & Pierce, 1990). Instead, preference depends on the contingencies associated with the 50% alternative.…”

“…Preference for the 100% side is therefore always predicted, provided that animals are sensitive to the differences in the delay. Insensitivity to such differences does not provide a plausible account of pigeons' suboptimal choice in signaled procedures because strong preferences for the 100% alternative occur under the same temporal parameters in unsignaled procedures (e.g., Spetch et al, 1990).…”

“…The contrast between the signaled and unsignaled results suggests instead a role played by the signals for food (i.e., the conditioned-reinforcing properties of the terminal-link stimuli). Along these lines, Dunn and Spetch (1990) and Spetch et al (1990) proposed a framework that borrows heavily from Fantino's (1969) delay-reduction hypothesis of conditioned reinforcement. This will be referred to as the "local delayreduction framework," both to distinguish it from the original hypothesis and to indicate that, as yet, it represents only an approach to understanding choice between certain and probabilistic outcomes, not a formalized model.…”