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.