Ulcers in Executive Monkeys
Method of the experiment
To test this Brady used a yoked control monkey. He placed an 'Executive Monkey' in the restraining chair, which could press the lever to prevent the electric shock. The yoked monkey had no control over the lever, leaving only the 'Executive' with the psychological stress of pushing the lever.
The greatest danger occurred when the sympathetic arousal stopped and the stomach was flooded with digestive hormones. This was a parasympathetic rebound associated with the Hypothalamic-pituitary-adrenal axis, which led to the development of ulcers in the Executive monkeys.
In all the variations of the experiment, no yoked control monkey ever developed an ulcer. This suggests that the ulcers were a symptom of the excessive stress induced by having control. Hans Selye's General Adaptation Syndrome proposes a similar effect in the Exhaustion phase.