Auditory Conditioned Reflexes in the Thyroidectomized Sheep and Goat.

Abstract

The lethargy of the cretin sheep or goat, and its apparent indifference to its environment, suggest a fundamental defect in cerebral function, but when the animal is subjected to simple learning tests the deviation from the normal, under experimental conditions, is less striking. For example, the animal, unless it is suffering one of its recurrent periods of prostration, learns the maze with a single cul de sac as well as the control. It is only when the blind alley is reversed in position, at every trial, that striking deviations from normal behavior appear. The extreme complexity of the animal's response in the labyrinth makes the analysis of the data in physiological terms very difficult. For this reason the maze studies have been abandoned in favor of the conditioned motor reflex method. A systematic investigation of cerebral activity involves, according to Pavlov, two problems, a study of the analyzers (receptors with their central terminations) and of the mechanism for the formation of temporary nervous connections. In a preliminary study it was demonstrated that the formation of a simple tactile conditioned reflex was effected as easily in the cretin as in the normal sheep. Secondary conditioned reflexes of proprioceptive origin added to the primary tactile reflex were observed in both operated and control animals. Since the alternating maze problem exhibited marked differences between cretin and normal, which were shown neither in the simple maze nor in the formation of the simple conditioned reflex, it was decided to attempt to form more difficult conditioned reflexes, viz., discriminative and delayed reflexes to the sound of the metronome.