Relations between behavioral arousal and plasma cortisol levels in monkeys performing repeated free-operant avoidance sessions.

The commonly believed hypothesis that increases in adrenocorticoid levels sensitively reflect behavioral arousal was tested by subjecting monkeys to repeated sessions of free-operant avoidance; these sessions produced varying degrees of behavioral arousal over time, which were quantified by a behavioral scoring technique. Cortisol was reliably elevated only in the most aroused subjects early in the first avoidance session. Although subjects were still aroused later in the session, cortisol had returned to basal levels. During subsequent avoidance sessions, cortisol returned to basal levels and did not increase significantly even when arousal was further manipulated by the superimposition of unavoidable shock during the final avoidance session. The addition of unavoidable shock was associated with a significant correlation between arousal, as reflected by increases in response rate, and magnitude of change, usually decreases, in mean cortisol levels. The frequent occurrence of dissociations between cortisol levels and behavioral arousal, as reflected by behavioral score, operant rate, and shock frequency, indicates that cortisol levels are of little use as a neuroendocrine index of arousal.