IntroductionTime perception is the experience or awareness of the passage of time.
The human experience of change is complex. One primary element clearly is that of a succession of events, but distinguishable events are separated by more or less lengthy intervals that are called durations. Thus, sequence and duration are fundamental aspects of what is perceived in change.
Manifestly, duration is relative to the events people isolate in the sequences through which they live: the duration of a kiss, of a meal, of a trip. A given interval always can be subdivided into a sequential chain delimiting briefer durations, as with the regular units that provide empirical measures of time: the second, the day, the year. Indeed, human experience is not simply that of one single series of events, but of a plurality of overlapping changes. The duration of a radio program, for example, can combine with that of a breakfast, both being inserted within the longer period of an ocean voyage.
Humans seem to be unable to live without some concept of time. Ancient philosophies sought to relate the concept of time to some objective reality to which it would correspond. René Descartes (1596–1650) inaugurated a critical era of philosophy by stressing the ancient problem of the origin of ideas, including the idea of time. Immanuel Kant (1724–1804), providing a radical answer to the epistemological problem of time, wrote that we do not appreciate time objectively as a physical thing; that it is simply a pure form of sensible intuition. Other philosophers of the 18th and 19th centuries sought to explain the notion of time as arising from association and memory of successive perceptions.
A move to empirical psychology emerged with the growth of research on the introspective data of experience. From about mid-19th century, under the influence of the psychophysical notions of Gustav Theodor Fechner, psychologists conducted experiments to study the relationship between time as perceived and time as measured in physics. Their work with adults gradually spread to the study of children and of animals. The psychologists then broadened their investigations of time to cover all forms of adaptation to sequence and duration.
Adaptation to successive eventsOne may respond to stimulation in an immediate way (as in unconditioned reflex action) without taking the element of time into account. Stimulation, however, can also signal an event to follow; then it has meaning only as part of the sequence of which it is the first term: bell announcing dinner, a road sign, or an approaching danger. People react to such stimuli with anticipatory behavior that is adapted to a stimulus or action that has not yet occurred. The principles that govern such time-binding adaptation are none other than those of conditioning. One event becomes conditioned as the signal for another stimulus that is to be sought or avoided.
The bottle-fed infant who initially reacts to the nipple on his lips with a simple sucking reflex is gradually conditioned to stop crying when he sees the bottle (the signal for feeding). Later he may learn to react to even more secondary signals that announce the arrival of the bottle; e.g., being lifted from the crib or hearing the sounds of his mother warming the milk in the kitchen. His behavior has come to incorporate the temporal dimension of the events.
According to the principles of instrumental conditioning, one stimulus becomes the signal for an ensuing event only if the second stimulus elicits an adaptive reaction (consummatory or aversive) and only if the order of the sequence is repeated. Conditioning tends to be established most rapidly when the interval between the signal (conditioned stimulus) and the unconditioned stimulus is quite brief. Ivan P. Pavlov estimated that the optimum interval for such a sequence was 0.5 second, which corresponds approximately to the intervals characteristic of sequences that are most accurately discriminable perceptually (see below Perception of sequence and duration).
Aside from adapting the individual to the order of a sequence, conditioning also adapts to the duration between signal and immediately effective stimulus. Response to signal tends to occur after about the same interval that separated the two stimuli during conditioning. Thus, an animal may be trained to delay a response for some time after the signal (delayed conditioning).
This form of adaptation is most pervasive in human behavior, permitting people to anticipate sequences of events in their environment so that they can prepare to cope appropriately with what is yet to happen.
Adaptation to periodic changeIn 1912 one of Pavlov's students (I.P. Feokritova) demonstrated that a dog accustomed to being fed every 30 minutes would begin to drool toward the end of each half-hour period. It was clear evidence of conditioning to time; the between-feedings interval itself served as a conditioned stimulus.
That discovery underscores the ever-present periodicity of daily living, especially on the biological level: rhythms of activity and sleep, rhythms of eating and lovemaking. As conditioning intervenes, anticipatory experiences of hunger, fatigue, or arousal serve our adaptation to ecological demands.
Allowance should also be made for the daily, or circadian, rhythms in metabolic activity (e.g., daily cycles of temperature change). There is evidence that these fundamental biological functions can synchronize with the rhythmic phases of environmental (exogenous) change. Thus within a few days after a factory worker has been assigned to the night shift, highs and lows of his daily fluctuations of temperature will be inversed. The rhythmic changes in body temperature persists, nevertheless, suggesting an innate (endogenous) basis for circadian phenomena. Such a hypothesis would mean that the gradual establishment of human circadian rhythms of sleep or temperature results from maturation of the nervous system rather than from conditioning in the strict sense. Experiments begun in 1962, in which men lived in caves or other enclosures for months deprived of temporal cues from the environment, also demonstrated the enduring nature of rhythms in body temperature and in sleep–wakefulness. The rhythmic periods, however, sometimes expanded, the subject beginning to live on an approximately two-day cycle without being aware of it.
Through conditioning to time and by way of circadian rhythms, human physiology provides a kind of biological clock that offers points of reference for temporal orientation.