Certain Characteristics of Formation of Conditioned Reflexes in Sheep.

Abstract

The defensive reaction evoked by the application of an electric shock to the sheep's foreleg is a convenient unconditioned reflex from which to develop various conditioned reflexes. Six lambs born in May, 1928, were, after about 18 days preliminary training in August, caused to develop auditory conditioned reflexes of the simultaneous type. That is, the auditory stimulus signalling an electric shock preceded the shock by a short, variable interval of 2 to 6 seconds. The conditioned stimulus for 2 of the lambs was the sound of a buzzer. For the others it was the sound of the metronome beating twice per second. The conditioned reflex appeared in the 6 animals after 3 to 7 combinations of sound and shock and became relatively stable (11 or more positive conditioned responses in succession) after 18 to 43 combinations. The development of a conditioned reflex of the type described and a simple maze habit exhibit a common characteristic. Both show a period of instability between the first appearance of the new response and the final, more or less stereotyped, reaction.

The most interesting feature of the formation of the conditioned reflex was the concentration of the motor response with increasing training. At first, the reaction involved a widespread activity of the skeletal musculature. The leg receiving the shock might be extended instead of flexed as the animal struggled in the harness. Later, and usually before the conditioned reflex was stable, the response was gradually concentrated in the leg stimulated by the electric current and the sound now evoked a brief and precise leg movement. The reaction of the other skeletal muscles at this stage served to give the body a posture in which the foreleg could be vigorously flexed.