Analysis of fixed-ratio behavior maintained by drug reinforcers.
Drug reinforcers bend fixed-ratio performance in their own ways—cocaine and ketamine slow the run, alfentanil stretches the pause, and every drug follows an upside-down U dose curve.
01Research in Context
What this study did
Gerhardt et al. (1991) gave rats three different drugs as rewards for pressing a lever.
The rats had to finish a fixed-ratio schedule—every 20 presses earned one drug shot.
The team watched how fast the rats ran through the ratio and how long they paused after each reward.
What they found
Cocaine and ketamine slowed the lever-pressing run when the dose got high.
Alfentanil did the opposite—it made the rats stop longer after the reward instead.
All drugs made an upside-down U: low doses helped, high doses hurt, middle doses worked best.
How this fits with other research
Lejuez et al. (2001) later saw the same rate-dependent twist with cocaine, but on a fixed-interval schedule and with wheel-running as the prize.
STEBBINS et al. (1959) had already shown that sweeter sugar water makes rats press faster on fixed-interval; P et al. now show the drug type can pull the same lever on fixed-ratio.
Smith et al. (1975) found that amphetamine scrambles extra behaviors like drinking while leaving lever pressing orderly; P et al. keep the lens on the main ratio run and still catch unique drug signatures.
Why it matters
If you ever consult on cases where medication is added, remember: the same drug can speed, slow, or stop the very behavior you are counting.
Watch for the inverted-U pattern—more milligrams does not always mean more responding.
Track run time and pause time separately; one can lag while the other stays flat, telling you which part of the chain the drug hits.
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02At a glance
03Original abstract
Behavior maintained by intravenously delivered alfentanil, cocaine, or ketamine was assessed using a fixed-ratio schedule of reinforcement. As the dose of each drug was increased, rate of responding also increased up to a maximum. Further increases in dose resulted in decreased response rates (inverted U-shaped curve). An analysis of postreinforcement-pause-time and run-time measures for the ascending limb of the inverted U-shaped functions revealed that behavior was characterized by systematic decreases in both pause time and run time as dose and rate increased. An examination of the descending limb of the dose-response functions revealed that lowered response rates for cocaine and ketamine were correlated with increases in run time and small and inconsistent effects on postreinforcement pause time. Behavior maintained by rate-reducing doses of alfentanil was characterized by lengthened postreinforcement pauses with small increases in run time. These data suggest that at larger doses, drug reinforcers may have unconditioned or direct effects on the behavior that the drug is maintaining, and more important, that the nature of these unconditioned effects depends on the drug that is maintaining behavior.
Journal of the experimental analysis of behavior, 1991 · doi:10.1901/jeab.1991.56-331