A fixed interval schedule in which the interval is initiated by a response.
Adding a simple "start" response lets you split the FI pause into two measurable pieces.
01Research in Context
What this study did
HEFFERLINE et al. (1963) built a new kind of fixed-interval schedule. The rat or monkey had to press one lever to "start the clock," then wait the full interval before a second press on the usual lever produced food.
The team recorded when the animals pressed each lever. This let them split the classic FI pause into two parts: time until the starter press, and time from starter press to the first food-producing press.
What they found
Rats and monkeys quickly learned the two-step rule. Response records showed a clear break-and-run pattern after the initiating press, giving experimenters a finer look at timing behavior.
The study proved you can pull apart the FI pause without changing the basic interval. Behavior still lined up with time, but now each piece could be measured separately.
How this fits with other research
Glynn (1970) later tested pigeons on the same response-initiated FI. Birds paused for almost the entire interval before the starter peck, showing even tighter temporal control than the early rat data.
Robertson et al. (2013) compared plain FI with the response-initiated version side by side. They found longer, more variable intervals and higher early response rates under the new schedule, yet the birds still told time just as well.
These follow-ups extend, rather than contradict, the 1963 work. Each study adds a layer: first show the split is possible, then show it sharpens or slightly shifts responding without wrecking the internal clock.
Why it matters
If you need to measure separate parts of wait time in a token system or DRL procedure, borrow the two-lever trick. Have the client tap a "start" card, then wait the interval before tapping the main button for the reinforcer. You can now graph pre-start latency and post-start pause to see which piece needs intervention.
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02At a glance
03Original abstract
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.
Journal of the experimental analysis of behavior, 1963 · doi:10.1901/jeab.1963.6-323