McDevitt, M. A., Spetch, M. L., & Dunn, R. (1997).
Contiguity and conditioned reinforcement in probabilistic choice.
Journal of the Experimental Analysis of Behavior,
68, 317-327.
In a baseline condition, pigeons chose between an alternative
that always provided food following a 30-s delay (100%
reinforcement) and an alternative that provided food half of the
time and blackout half of the time following 30-s delays (50%
reinforcement). The different outcomes were signaled by
different-colored keylights. On average, each alternative was
chosen approximately equally often, replicating the finding of
suboptimal choice in probabilistic reinforcement procedures. The
efficacy of the delay stimuli (keylights) as conditioned
reinforcers was assessed in other conditions by interposing a 5-s
gap (keylights darkened) between the choice response and one or
more of the delay stimuli. The strength of conditioned
reinforcement was measured by the decrease in choice of an
alternative when the alternative contained a gap. Preference for
the 50% alternative decreased in conditions in which the gap
preceded either all delay stimuli, both delay stimuli for the 50%
alternative, or the food stimulus for the 50% alternative, but
preference was not consistently affected in conditions in which
the gap preceded only the 100% delay stimulus or the blackout
stimulus for the 50% alternative. These results support the
notion that conditioned reinforcement underlies the finding of
suboptimal preference in probabilistic reinforcement procedures,
and that the signal for food on the 50% reinforcement alternative
functions as a stronger conditioned reinforcer than the signal
for food on the 100% reinforcement alternative. In addition, the
results fail to provide evidence that the signal for blackout
functions as a conditioned punisher.
Key words: conditioned reinforcement, probabilistic
reinforcement, delay-reduction theory, hyperbolic decay model,
choice, key peck, pigeons