Academic journal article International Journal of Psychology and Psychological Therapy

Instructional Ambiguity in the Discrimination of and Memory for the Duration of a Stimulus*

Academic journal article International Journal of Psychology and Psychological Therapy

Instructional Ambiguity in the Discrimination of and Memory for the Duration of a Stimulus*

Article excerpt

The ability of animals to discriminate and remember the duration of temporal intervals has received considerable attention in the past 25 years. The reasons are varied but two are notable. First, the temporal dimension is unique in its abstractness. It is not defined by the presence of certain observable attributes or by the relative relation among objects. Instead, it is marked by the abstract notion of when an event (its start) occurred in time. Thus, unlike many other events that can be identified almost as quickly as they appear, the point on the dimension of time changes continually.

The second reason that the judgment of temporal intervals by animals has been of interest to researchers is because of its importance in the lives of modern humans and the difficulty that we have in separating our natural ability to make temporal judgments from our extensive experience in making those judgments, including our general experience with our culturally invented scale of time and the external machines (clocks) we have invented for its measurement.

The study of timing by animals has been characterized largely by two procedures; one often referred to as the production procedure, the other as the discrimination procedure. In the production procedure the animal is typically presented with a stimulus and reinforcement is provided for the first response that occurs after a fixed time has passed (a fixed interval schedule of reinforcement). If the animal is given a continuous choice between responding and not responding it will generally show the typical 'scalloped' rate of responding from the onset of the stimulus to the time at which the reinforcement occurs. If on selected 'empty' trials reinforcement is omitted and the stimulus is not removed, the resulting mean response function will typically increase to the time at which reinforcement is expected and then it will typically decline such that the resulting function will generally appear bell-shaped (see S. Roberts 1981). That the response function typically is at its maximum at about the time that reinforcement would be delivered has been taken as evidence that animals have a reasonably good ability to judge the passage of time. Because the response function peaks at about the time that reinforcement is expected, this procedure has often been referred to as the peak procedure (W. Roberts, 1998).

In contrast, in the discrimination procedure, on some trials the animal is presented with a stimulus of a particular duration and is then given a choice between two test or comparison stimuli, a response to one of which is reinforced. On the remaining trials, the animal is presented with a stimulus of a different duration after which a response to the other comparison stimulus is reinforced. Most of the research involving the discrimination procedure has been done with pigeons. Because the timing models that have been developed from research using the peak and discrimination procedures have been somewhat different, I will discuss them separately.

THE PRODUCTION OR PEAK PROCEDURE

Research using the production or peak procedure has led to the development of the information processing model of timing (Church, 1978). This model, depicted in Figure 1, consists of three processes: the clock process, the memory process, and the decision process. The clock process consists of a pacemaker which puts out a series of pulses at a constant average rate, a switch controlled by the onset and offset of the to-be-timed signal, and an accumulator which collects those pulses. The memory process consists of working memory which receives input from the accumulator and reference memory which receives and stores the count in working memory when a response is rewarded. The decision process consists of a comparator which receives input from both working memory and reference memory and when the difference in counts between the two memories is sufficiently small (based on the animal's response criterion) it makes a decision to respond. …

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