A fixed interval schedule in which the interval is initiated by a response

The fixed interval schedule described requires the animal to initiate every time interval by making a response on a bar other than the one on which it is reinforced. This response, R_A, demarcates the postreinforcement pause (S^R-R_A interval) from the fixed interval pause (R_A-R_B interval) so that these pauses may be measured separately. Twelve rats and three monkeys, working in two-bar Skinner boxes, were trained and stabilized on this schedule. The resulting performances, presented for individual animals, are analyzed in terms of (1) the relative frequencies with which the animal waits various lengths of time between consecutive responses, (2) the relative frequencies with which various rates of responding appear, (3) the change in response rate throughout the fixed interval, (4) the average length of the postreinforcement pause, (5) the relative frequencies with which the animal waits different lengths of time between the R_A and the first R_B, and (6) the average inter-response time as a function of the rank order in the fixed interval of the inter-response time. The joint interpretation of the several measures taken leads to the following conclusions: 1. The probability of an R_B increases throughout the fixed interval. 2. The increase is discontinuous at the first R_B, at which point the probability increases sharply. 3. The frequency distributions of R_A-R_B pauses exhibit three discrete types of behavior with no intermediate cases. 4. The (main) mode of R_A-R_B interval length usually occurs just below the fixed interval requirement.