Performing a familiar task and learning a new rule are different demands. A 1979 experiment made that distinction unusually clear: the same four pigeons alternated between a stable discrimination task and one whose rules changed between sessions, while investigators examined acute and repeated d-amphetamine exposure.
The study’s enduring value is its experimental question. If a drug’s disruptive effect becomes smaller with repetition, has tolerance developed equally for learning, familiar performance, accuracy and speed? The answer was not uniform across those measurements or across animals.
How did the tasks differ?
Moerschbaecher and colleagues studied four drug-naive Silver King pigeons with prior experience learning conditional discriminations. Drug-naive did not mean task-naive. Animals were maintained at approximately eighty percent of free-feeding body weight, and correct responding earned access to grain.
The birds responded to combinations of colors and shapes. In the performance component, the correct rules remained the same across sessions. In the acquisition component, the combinations requiring particular responses changed from session to session, producing repeated opportunities to learn new discriminations.
The components alternated after ten reinforcements or fifteen minutes, whichever came first. Each session ended after sixty reinforcements. An incorrect response produced a ten-second timeout, after which the same stimulus returned.
This arrangement compared different demands within the same animal and session. It did not make the conditions identical: baseline errors and the frequency of food reinforcement also differed between components.
Which drug exposure and outcomes were measured?
The investigators administered d-amphetamine sulfate or cocaine hydrochloride intramuscularly into the pectoral muscles five minutes before a session. Doses were expressed as the salts. Acute administrations were separated by five days, with baseline and saline sessions between them.
The two central outcomes were percentage errors and running time. Percentage errors used errors divided by correct responses plus errors. Running time excluded time spent in timeout. A longer running time therefore was not simply an arithmetic consequence of including the timeout penalty.
These were controlled animal task measurements. No blood amphetamine concentration, driving outcome or human clinical diagnosis was established by this design.
Was new learning more sensitive to amphetamine?
In three of the four birds, disruption generally appeared at lower doses in the acquisition component than in familiar performance. For one bird, P116, 1.8 mg/kg d-amphetamine increased acquisition errors, while several lower doses increased acquisition running time without affecting familiar performance.
The fourth bird did not follow the predominant pattern in the same way and already showed relatively poor within-session acquisition under control conditions. That exception matters in a study with four subjects: a majority pattern is not a universal response.
Within-session records added information beyond aggregate errors. For P116 at 1.3 mg/kg, total errors did not increase, yet the pattern indicated slower acquisition across the session. An unchanged total error count therefore did not necessarily mean that the learning process was unchanged.
Did repeated exposure normalize every outcome?
With repeated d-amphetamine administration, the initial increases in errors and running time generally became smaller. Where both task components initially showed increased errors, tolerance developed first for familiar performance. In two birds, P117 and P118, acquisition errors and running time did not return to their predrug control ranges.
Another example separated accuracy from speed. For P116 receiving 2.4 mg/kg, acquisition errors returned to the control range relatively rapidly while tolerance to the running-time effect developed more slowly. The dose was subsequently increased to 3.2 mg/kg, producing renewed disruption that again diminished during repeated sessions.
These observations make the endpoint part of the definition. A bird could show less disruption on a familiar discrimination, continued difficulty learning changing rules, or different recovery in errors and running time. Calling the animal simply “tolerant” loses that structure.
What did the cocaine comparisons establish?
The experiment also examined cocaine, but its findings should retain the correct drug labels. After chronic cocaine exposure, d-amphetamine dose-effect curves shifted toward higher doses in two birds, consistent with cross-tolerance for the tested behaviors.
The reverse sequence was less conclusive. After chronic d-amphetamine, the investigators retested only a single high cocaine dose in the other two birds. Familiar-performance errors were reduced, but acquisition errors and running-time effects were unchanged or greater. The authors therefore could not substantiate broad cross-tolerance in that direction from the limited retesting.
Cross-tolerance in selected task outcomes is not a general statement about every action of the two drugs, their concentrations, or their health effects.
Why might reinforcement complicate the explanation?
The acquisition component normally delivered reinforcement less frequently than familiar performance. Drug-induced errors and pauses could change that frequency further. Consequently, differences attributed to learning demands could also involve differences in the consequences of responding.
The authors identified reinforcement frequency as an unresolved influence. The study demonstrated task-dependent effects, but did not isolate one complete mechanism explaining why those effects differed.
What can be carried into an evidence review?
This small, historical animal experiment supports asking precisely which task and outcome showed tolerance. It does not establish an amphetamine dose at which a person can safely drive, learn unfamiliar procedures or perform a skilled occupation.
The practical appraisal question is narrower and useful: did the cited experiment measure familiar performance, acquisition of new behavior, speed, accuracy or all of them—and did every measured outcome actually return to its control range? An opinion about preserved function should identify those distinctions before generalizing from a tolerance finding.
References
Moerschbaecher JM, Boren JJ, Schrot J, Fontes JC. Effects of cocaine and d-amphetamine on the repeated acquisition and performance of conditional discriminations. Journal of the Experimental Analysis of Behavior. 1979;31(1):127–140. doi:10.1901/jeab.1979.31-127. Historical primary research in four pigeons.
Leave a Reply