On some portion of the trials, typically 50%, the timing signal stays on for three times the normal interval length and ends without any reinforcement being presented. These are referred to as empty, peak, or probe trials. Because the timing signal stays on and there is no food presented, the animal's clock presumably continues timing and we see how they respond after the time interval has elapsed.

The central finding of the PI procedure is the peak function, a simulation of which is shown in Figure 2.14. As in the FI procedure, responding starts slowly and increases to its maximum rate at approximately the reinforced interval length. However, in the probe trials of the PI procedure we can see that responding then decreases in an almost symmetrical fashion, perhaps just a trifle slower than the rate of increase. The resulting curve is approximately Gaussian with some degree of positive skew.

This clean curve is typically found only after many, many weeks of training and represents averaged data, not individual trials. Typically, on the very first probe trial, the subject's response rate stays high for a long period of time (although it is not clear how general this tendency is). It is only after repeated experiences with

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