Lessons From The Psychophysics Of Time

The Russian physiologist Ivan Pavlov (1849-1936) recognized the learned perception of time in animals when his salivating dogs began to "wait" to salivate after lengthy durations of the conditioned stimuli (story retold by Roberts, 1998). What was at first an unconditioned response to food had become a finely timed prediction about the arrival of food. Since that time, psychophysics has developed a number of techniques for assaying animal event timing. Before covering some of the more established contributions of psychophysics to animal event timing, I present the three most prevalent assays for measuring time perception in animals, as I will refer to them frequently.

One of the first and most easily used assays of animal event timing is the fixed-interval (FI) schedule. FI schedules present a subject with an operandum (e.g., lever) and reward 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 do not temporally regulate behavior based on experience typically show 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, as illustrated in Figure 4.1a. An alternative to this behavior is the production of scalloped curves in cumulative response records over time, as shown in Figure 4.1b. The scallop is created by the increase in response rate as the reward time approaches, much like the timed increased in salivation observed by Pavlov (1927).

A more informative variation of the FI schedule is the peak-interval (PI) procedure (e.g., Church et al., 1994; Roberts, 1981). In the PI procedure, animals learn that a response will deliver food after a certain interval has passed following the initiation of a signal. The major difference between the FI schedule and the PI procedure is that in the latter, approximately 50% of the time there is no food reward. Instead, the signal stays on for a set period regardless of how the animal responds. It is during these no-reward trials that responses are recorded. Inevitably, the animal increases its response rate until the approximate time of the reward and then decreases its response until the signal is turned off, as shown in Figure 4.2. Movement of the peak function rate left or right is interpreted as a change in the rate of perceived time by the animal.

The temporal 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) (Church and Deluty, 1977; Maricq and Church, 1983). This is another psychophysical method for measuring the lengthening or foreshortening of perceived time. The signal duration at which the subject responds long and short with equal probability is called the point of subjective equality (PSE), with typical results illustrated in Figure 4.3.

While the principles that I report here are not established for all animals, they are well established for vertebrates (i.e., birds, mammals, and fish) (see Paule et al., 1999). I will point out exceptional species in "Species Comparisons" (Section 4.2.6).

Time (sec)

FIGURE 4.1 Typical results from the FI procedure. (a) Break-and-run response. Response rates are constant following a slight delay after the reward (downward slash). (b) Scalloped response. The scalloped curves are the result of increasing response rates near the point of reward. Response rates are not as smooth in real traces, because lever presses lead to discrete jumps in cumulative response number. (Adapted from Ferster, C.B. and Skinner, B.F., Schedules of Reinforcement, Appleton-Century-Crofts, New York, 1957.)

Time (sec)

FIGURE 4.1 Typical results from the FI procedure. (a) Break-and-run response. Response rates are constant following a slight delay after the reward (downward slash). (b) Scalloped response. The scalloped curves are the result of increasing response rates near the point of reward. Response rates are not as smooth in real traces, because lever presses lead to discrete jumps in cumulative response number. (Adapted from Ferster, C.B. and Skinner, B.F., Schedules of Reinforcement, Appleton-Century-Crofts, New York, 1957.)

The following list of phenomenological characteristics represents a basis for understanding what a mechanistic definition of event timers must ultimately explain. This list will also help us make predictions about how we may expect animals to behave in the wild in response to specific constraints on their perception of temporal events.

4.2.1 Temporal Memory Is Scalar

The ability to discriminate two temporal cues is reduced in a predictable way as the duration of those cues is increased. This property, known as Weber's law, describes

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