The Russian physiologist Ivan Pavlov (1849-1936) was the first to publicize event timing in animals. He recognized the trait when his salivating dogs began to `wait' to salivate after lengthy durations of the conditioned stimuli (story retold by Roberts, 1998). Since that time, psychophysics has developed a number of techniques for assaying animal event timing. Before exposing the established contributions of psychophysics to animal event timing, I would like to present the three most prevalent assays, as I will refer to them frequently.
One of the first and most easily used assays of animal event timing is the fixed-interval procedure (FI). FI schedules present a subject with a lever and reinforce lever presses after a fixed interval of time. There is no deterrent for lever presses prior to the reinforcement, lending subjects to press the lever at will until food is finally procured. Animals that are able to temporally regulate behavior based on experience typically show scalloped response curves in cumulative response records over time (Figure 1). The scallop is created by the increase in response rate as the reward time approaches. The alternative to this behavior is the `break-and-run' response, which is characterized by a short pause in procuring the reward followed by a steady response rate until the next reward (Figure 2). The inference from the break-and-run response is that the animal is indifferent to the temporal interval.
A more informative variation of FI is the peak procedure. In the peak procedure, animals learn that a lever will deliver food after a certain interval has passed since the initiation of a light cue. The difference between peak procedure and FI is that in the peak procedure, approximately 20 percent of the time, there is no food reward. Instead, the light cue stays on for a few minutes regardless of how the animal responds. It is during these no reward trials that responses are recorded. Inevitably the response rate increases up until the time of the reward and then decreases until the light is turned off (Figure 2). The peak rate matches the peak time.
The bisection procedure requires the subject to discriminate between two signals, long and short in duration (e.g., by pressing the left or right levers, respectively). The point of subjective equality (PSE) is the signal duration at which the subject responds with equal probability long and short. This data is usually presented as in Figure 3. This procedure has direct relevance to models of foraging theory, because it verifies the animal's ability to discriminate temporal intervals either in transit time between patches, in time between captures at a patch, or in handling time.
While the principles that I report here are not established for all animals, they do seem to be well established for vertebrates (i.e., birds, mammals, and fish). I will point out exceptional species in `Species Comparisons' (Section 2.6).