Failure to replicate visceral learning in the acute curarized rat preparation.

An attempt was made to replicate a series of experiments reported to demonstrate robust visceral learning (autonomic instrumental learning) in rats during acute (2-4 hr) pharmacological paralysis. The results of exploratory procedures involving more than 2,000 animals are described, and six complete experiments, representing systematic observations on an additional 500 animals, are presented. In the first three experiments, which closely followed the original procedures, the characteristics of the preparation were reproduced with the exception of initial heart rhythm and visceral learning. In the second three, the respiration procedure was modified to satisfactorily reproduce the heart rhythm, and the PaO2, PaCO2, and pH were verified to be within the range of freely moving, normally behaving animals; nevertheless, visceral learning was not observed in these experiments either. After more than 2,500 rats were studied, it is concluded that the original visceral learning experiments are not replicable and that the existence of visceral learning remains unproven; however, neither the original experiments nor the replication attempt included the necessary controls to support a general negative conclusion about visceral learning. Because continuing and extensive citation of the original experiments indicates their widely perceived significance, specific requirements for critically testing the visceral learning hypothesis are given, and the limited theoretical implications and practical value of neuromuscular blockade are discussed.