It has long been known that definite and enduring nervous disturbances can be produced experimentally in certain mammals. Abnormal states of the nervous system were observed in the dog a number of years ago by Pavlov and his associates1 during the course of experiments on the conditioned reflex. When the conditions of the experiments were such that an animal was called on to solve a problem beyond its capacity, e. g., to distinguish between two closely similar conditioned stimuli, the dog receiving food with one and not with the other, it often happened that the behavior of the animal exhibited a profound change. The formerly friendly and quiet animal became extremely fractious and disturbed, and in this condition it was unable to solve even the simplest problem.
In 1927 Liddell and Bayne2 observed this derangement of behavior in a sheep during conditioning experiments in which the animal was
LIDDELL HS, ANDERSON OD, KOTYUKA E, HARTMAN FA. EFFECT OF EXTRACT OF ADRENAL CORTEX ON EXPERIMENTAL NEUROSIS IN SHEEP. Arch NeurPsych. 1935;34(5):973-993. doi:10.1001/archneurpsyc.1935.02250230045004