Neural thresholds and state-dependent learning

In this experiment we tested the hypothesis that state-dependent learning is mediated by changes in brain excitability levels as determined by changes in the intensity of a conditioned stimulus (CS) delivered to the central nervous system, sufficient to maintain a conditioned response (CR). Four cats were trained in the normal state; CR thresholds determined for dosages of sodium pentobarbital ranging from 0 to 15 mg/kg; and then retrained to give CRs while in a drug state for 15 mg/kg pentobarbital. Another group of four cats was trained in the reverse order. They were trained to give CR in the drug state; thresholds taken as pentobarbital decreased from 15 mg/kg; and retrained in the normal state. Results showed that shifts in brain excitability levels cannot explain the state-dependent phenomenon; that the effect of pentobarbital upon the conditioning thresholds was a function of the animal's prior experience with the drug and not directly a function of its pharmacological action; and that the drug and nondrug states were distinct before training, but not after training in both states.