Bias of phencyclidine discrimination by the schedule of reinforcement.
Schedule payoffs alone can make pigeons look like they’re under stimulus control when they’re just following the money.
01Research in Context
What this study did
Bryant et al. (1984) worked with pigeons trained to peck one key when given phencyclidine and another key when given saline. The birds worked under a second-order schedule where brief color flashes signaled upcoming food.
The team varied how many pecks were needed for food and how often the color signals appeared. They used signal-detection math to split true drug detection from simple response bias.
What they found
Changing the schedule rules alone could push pigeons to peck the “drug” key even when they got only saline. The birds weren’t confused about the drug; the payoff structure nudged their choices.
Signal-detection numbers showed the effect was pure bias, not weaker drug cues. Schedule tweaks shifted the “decision criterion” without changing how well the birds sensed the drug.
How this fits with other research
Tanguay et al. (1982) set up the same pigeon PCP task but focused on boosting response rates with color tracking. The 1984 paper keeps the prep and asks a new question: can schedules fool the bird? It does, showing the earlier color gains came with hidden bias costs.
McMillan et al. (1999) later repeated the bias claim using concurrent FR schedules and pentobarbital. Pigeons again shifted keys when payoff odds changed, proving the 1984 finding holds across drugs and schedule types.
Newman et al. (1991) extended the idea by adding stimulus disparity. They showed that dim drug-versus-saline cues make reinforcer ratio effects even bigger, warning that weak sensory contrast plus lean payoffs can double the bias trouble.
Why it matters
Before you say “the client can’t tell the difference,” check the reinforcement ledger. If one choice pays more often or faster, you may be seeing schedule bias, not stimulus failure. Run a quick probe with equal payoffs or use signal-detection splits to separate true discrimination from payoff pull. This guards against needless program changes and keeps your data clean.
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02At a glance
03Original abstract
Pigeons, trained to discriminate phencyclidine from saline under a procedure requiring the bird to track the location of a color, received cumulative doses of phencyclidine, pentobarbital, or d-amphetamine with a variety of schedules of reinforcement in effect (across phases). When the same second-order schedules were used to reinforce responding after either saline or phencyclidine administration, stimulus control by phencyclidine did not depend on the schedule parameter. When different second-order schedules were used that biased responding toward the phencyclidine-correlated key color, pigeons responded on the phencyclidine-correlated key at lower doses of phencyclidine and pentobarbital than when the second-order schedule biased responding toward the saline key color. A similar but less marked effect was obtained with d-amphetamine. Attempts to produce bias by changing reinforcement magnitude (duration of food availability) were less successful. A signal-detection analysis of dose-effect curves for phencyclidine under two of the second-order schedules employed suggested that at low doses of phencyclidine, response bias is a major determinant of responding. As doses were increased, position preferences occurred and response bias decreased; at higher doses both response bias and position preference decreased and discriminability increased. With low doses of pentobarbital, responding again was biased but increasing doses produced position preference with only small increases in discriminability. At low doses of d-amphetamine responding also was biased, but bias did not decrease consistently with dose nor did discriminability increase. These experiments suggest that the schedule of reinforcement can be used to bias responding toward or away from making the drug-correlated response in drug discrimination experiments, and that signal-detection analysis and analysis of responding at a position can be used to separate the discriminability of the drug state from other effects of the drug on responding.
Journal of the experimental analysis of behavior, 1984 · doi:10.1901/jeab.1984.42-51