Comparisons between variable-interval and fixed-interval schedules of electric shock delivery.

Responding maintained in rats under a variable-interval 35-sec food schedule was suppressed more by 60-sec and 240-sec fixed-interval schedules of shock delivery than by 60-sec and 240-sec variable-interval schedules of shock delivery. When the delivery of shock was preceded by a 5-sec visual stimulus, little overall response suppression was found with either fixed-interval or variable-interval schedules. In a third experiment, the percentage of occasions on which a cue preceded each shock delivery was varied from 0% to 100%. For the fixed-interval shock condition, the most suppression occurred with the 0% treatment, the least with 100%, and an intermediate amount with the 50% treatment. For the variable-interval groups, the most suppression occurred in the 50% condition, the least in the 100% group, and an intermediate amount with 0%.