Drug discrimination in rats under concurrent variable-interval variable-interval schedules.
Unequal concurrent VI schedules keep drug-appropriate responding sharp and dose curves clear.
01Research in Context
What this study did
Scientists taught rats to tell pentobarbital from saline.
The rats pressed one lever when they felt the drug and another when they felt plain salt water.
The levers paid off under two VI schedules that ran at the same time.
Sometimes the schedules paid equally fast. Sometimes one paid four times faster.
What they found
When both sides paid at the same speed, the rats mixed up their choices.
When one side paid much faster, the rats picked the drug lever almost every time.
The unequal setup made the dose-response curve clean and easy to read.
How this fits with other research
Carr et al. (2002) later swapped VI for VR schedules and used pigeons.
They still saw smooth curves, showing the finding holds across species and schedule types.
Bacotti (1979) saw pentobarbital flip behavior in the other direction, but that study mixed FR and interval parts and looked at rate, not stimulus control.
The old result seems to clash until you notice the different question: rate change versus clear discrimination.
Why it matters
If you run concurrent schedules in a drug-discrimination task, make the richer side four times denser.
This simple tweak sharpens stimulus control and gives you a cleaner dose curve.
The same rule can guide any two-choice procedure where you need one response to dominate.
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02At a glance
03Original abstract
Eight rats were trained to discriminate pentobarbital from saline under a concurrent variable-interval (VI) VI schedule, on which responses on the pentobarbital-biased lever after pentobarbital were reinforced under VI 20 s and responses on the saline-biased lever were reinforced under VI 80 s. After saline, the reinforcement contingencies programmed on the two levers were reversed. The rats made 62.3% of their responses on the pentobarbital-biased lever after pentobarbital and 72.2% on the saline-biased lever after saline, both of which are lower than predicted by the matching law. When the schedule was changed to concurrent VI 50 s VI 50 s for test sessions with saline and the training dose of pentobarbital, responding on the pentobarbital-biased lever after the training dose of pentobarbital and on the saline-biased lever after saline became nearly equal, even during the first 2 min of the session, suggesting that the presence or absence of the training drug was exerting minimal control over responding and making the determination of dose-effect relations of drugs difficult to interpret. When the pentobarbital dose-response curve was determined under the concurrent VI 50-s VI 50-s schedule, responding was fairly evenly distributed on both levers for most rats. Therefore, 6 additional rats were trained to respond under a concurrent VI 60-s VI 240-s schedule. Under this schedule, the rats made 62.6% of their responses on the pentobarbital-biased lever after pentobarbital and 73.5% of their responses on the saline-biased lever after saline, which also is lower than the percentages predicted by perfect matching. When the schedule was changed to a concurrent VI 150-s VI 150-s schedule for 5-min test sessions with additional drugs, the presence or absence of pentobarbital continued to control responding in most rats, and it was possible to generate graded dose-response curves for pentobarbital and other drugs using the data from these 5-min sessions. The dose-response curves generated under these conditions were similar to the dose-response curves generated using other reinforcement schedules and other species.
Journal of the experimental analysis of behavior, 2000 · doi:10.1901/jeab.2000.73-103