Detecting a nonevent: Delayed presence-versus-absence discrimination in pigeons.
Pigeons treat 'nothing' as a weak signal, not a missing memory, so you can tune their choice by changing the payoff, not the prompt.
01Research in Context
What this study did
Pigeons pecked keys in a delayed matching task. Some trials showed a color sample. Other trials showed nothing at all.
After a delay, birds chose between two keys. The task was to tell 'something' from 'nothing' after waiting up to several seconds.
What they found
Accuracy stayed high on 'nothing' trials even after long waits. Accuracy on 'something' trials dropped as the delay grew.
When the researchers made 'nothing' trials pay off more often, pigeons quickly pecked the 'nothing' key every time. The birds acted as if they read the signal strength, not as if they flipped a yes/no switch.
How this fits with other research
Green et al. (1987) ran an earlier pigeon delay task. They found that adding position cues sped learning. Dougherty et al. (1996) now show that even without extra cues, pigeons still track signal strength across delays.
Rincover et al. (1975) reinforced wrong pecks on purpose. Sensitivity fell, a result that signal-detection theory said should not happen. Dougherty et al. (1996) turn the same theory in a new direction: they use it to explain why pigeons stay accurate on 'nothing' trials.
Davison et al. (1989) varied payoff rates for color detection. Pigeons shifted their bias but kept good sensitivity. Dougherty et al. (1996) repeat this pattern with presence-versus-absence, strengthening the idea that choice is guided by continuous evidence, not an all-or-none memory state.
Why it matters
When a learner stalls after a delay, do not assume the memory vanished. Check whether the 'absence' cue is simply paying off more. Shift reinforcer rates or add salient sample cues, then watch if accuracy rises. Signal-detection thinking gives you a lever: tweak the payoff, not just the prompt.
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Join Free →Plot correct and error trials separately for 'sample' and 'no-sample' conditions; if 'no-sample' accuracy is higher, rebalance reinforcer rates across choices.
02At a glance
03Original abstract
Eight pigeons were trained on a delayed presence-versus-absence discrimination paradigm in which a sample stimulus was presented on some trials but not on others. If a sample was presented, then a response to one choice key produced food. If no sample was presented, a response to the other choice key produced food. The basic finding was that performance remained constant and well above 50% correct on no-sample trials as the retention interval increased, whereas performance dropped precipitously (to below 50% correct) on sample trials. In the second phase of the experiment, all of the trials were no-sample trials, and reinforcers were delivered probabilistically for one group of pigeons and according to time-based schedules for the other group. The exact reinforcement probabilities used in Phase 2 were those calculated to be in effect on no-sample trials in Phase 1 (according to a discrete-state model of performance). Subjects did not show exclusive preference for the richer alternative on no-sample trials in the first phase, but those in the probabilistic group developed near-exclusive preference for the richer alternative during the second phase. These data are inconsistent with the predictions of the discrete-state model, but are easily accommodated by an account based on signal detection theory, which also can be applied effectively to discrimination of event duration and the "subjective shortening" effect.
Journal of the experimental analysis of behavior, 1996 · doi:10.1901/jeab.1996.65-81