The variability in the behavioral equilibrium established by six basic schedules was characterized. The measures were the pause preceding the first response in each interreinforcement interval; the mean rate of responding in each interreinforcement interval; and the relative frequency of each interresponse time. The temporal windows ranged across the 780-session exposure, across a session, and across the interreinforcement interval. A display of individual interresponse times as a function of time in the interreinforcement interval indicated clear recurrent responding at somewhat less than 3 Hz in every bird, even after extended exposure to a schedule and regardless of the contingency. No strong sequential dependencies in the interresponse-time distributions were identified. A simulator, based on a simple recurrent pulser, was presented that produced output similar to the obtained data. An archival data base of the behavior chronically maintained by the simple schedules was also generated.

The interresponse-time structures of pigeon key pecking were examined under variable-ratio, variableinterval, and variable-interval plus linear feedback schedules. Whereas the variable-ratio and variableinterval plus linear feedback schedules generally resulted in a distinct group of short interresponse times and a broad distribution of longer interresponse times, the variable-interval schedules generally showed a much more continuous distribution of interresponse times. The results were taken to indicate that a log survivor analysis or double exponential fit of interresponse times may not be universally applicable to the task of demonstrating that operant behavior can be dichotomized into bouts of engagement and periods of disengagement.Key words: interresponse time, variable-ratio, variable-interval, yoked schedules, Shull machine, key peck, pigeonsIn the past, the conceptualization of behavior as two-state (Gilbert, 1958) has not enjoyed widespread research interest because of the difficulty in validly dichotomizing the behavior stream into visits and nonvisits by simply specifying some specific interresponse time (IRT) duration as a criterion. The split between within-visit IRTs and visit-initiation IRTs is unlikely to be at the same value across individuals, or even within an individual across procedures.Shull, Gaynor, and Grimes (2001) described one possible solution for resolving the categorization problem. They displayed the total distribution of obtained IRTs as a survivor plot with a logarithmically scaled y axis. In this semilogarithmic plot, the slope between any two points on the x axis is an indicator of the relative decrease in the frequency of the IRTs per opportunity between those points. If a single exponential decay governed the occurrence of all responding, then the IRT distribution would appear as a single straight line. If, on the other hand, behavior occurred in short bouts of responding separated by longer delays, then the short IRTs governed by one exponential decay process would result in an initial steep slope, while the longer delays between response bouts characterized by a different exponential would be spread across a broad range of values and would generate a second shallower slope.Shull and his colleagues (Shull et al., 2001) found that the log survivor plots of the output of their explicitly two-state emulator had two relatively straight lines intersecting at an angle less than 180u, or had a ''broken-stick'' appearance, such that the function and therefore the two classes of behavior were easily dichotomized by simple inspection. Figure 1 provides example log survivor plots of the output of Shull's model. The ratio of the within-visit response rate to the between-visit response rate and the probability of disengaging are given above each frame. The columns of frames illustrate how increasing the relative value of the probability of entering the engaged state, p(V), with respect to the probability of a response during a visit, p(R), (i.e., larger ratios of within-visit to the be...

An inexpensive microprocessor-based experiment controller available as a commercially produced bare printed circuit board is described. It provides for up to 8 inputs and 40 outputs. An 8K Experiment Control BASIC (ECBASIC) and a compiled Experiment Control Language (ECL) are also available. Both languages have internal structures that enable time and responses to be dealt with in a very convenient and error-free manner. Time and response count are automatically incrementing variables that need only to be tested by the user program. ECBASIC provides a 10•msec accuracy, whereas ECL provides a l•msec accuracy. ECL provides extensive support for event-driven programs. The board and software are designed to provide an orderly migration from a stand-alone controller running ECBASIC as a relay-rack replacement, to a network of ECBASIC machines on a small computer, to a true error-checking network of ECL machines on a time-sharing operating system of a large computer. Network communication is over the three wires of a single unmodified RS232 asynchronous serial communication line.

Food was presented to pigeons, irrespective of their behavior. The fixed 60-s interfood interval was segmented into ten 6-s periods, each signaled by a distinctive stimulus color, ordered by wavelength. This "interfood clock" reliably generated and maintained successively higher rates of key pecking at stimuli successively closer to food. Under extinction, key pecking ceased. When the standard stimulus sequence was changed to a different sequence for each bird, accelerated responding again emerged and was sustained under each of the new color sequences. However, responding was neither maintained nor acquired when each successive interfood interval provided a different random sequence of the ten stimuli. Thus, the interfood clock generated and maintained sign-tracking under stimulus control, and the resulting behavior was attributable neither to stimulus generalization nor to a simple temporal gradient.

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