Abstract
After discrimination training on a multiple variable-interval extinction schedule of food reinforcement, pigeons were placed on the uncued or mixed version of the same schedule and allowed to make an optional "observing response" that converted the uncued schedule to the corresponding cued schedule by providing a 20-sec exposure to the appropriate discriminative stimulus. The schedule consisted of one hundred 40-sec components, and the probability that any one of them would be a variable-interval component was systematically varied between 0.00 and 1.00. The results showed that the amount of observing behavior was an inverted "U" function of the probability of the variable-interval component. Few observing responses occurred at probabilities of 0.00 or 1.00, and maximum responding occurred at a value less than 0.50.