Previous studies have shown that the effect of concurrent nontemporal processing on time estimation may vary depending on the level of difficulty ofthe nontemporal task. This is commonly interpreted within the context of so-called distraction/interruption models of temporal processing, which propose that as concurrent task difficulty or complexity is increased, temporal processing receives less attention. We hypothesize that the effect of nontemporal processing does not depend on the level of difficulty as such, but rather on the extent to which the concurrent nontemporal task specifically involves processing in short-term memory. Four experiments were run in which the short-term memory requirements of concurrent tasks were systematically varied, although all of the tasks were of comparable levels of difficulty. In the first experiment, the effect of memory search on simultaneous temporal productions was proportional to the number of items to search. As with reaction time, produced intervals were shown to increase linearly with the number of items in the memorized set. In Experiment 2, a visual search involving some load on short-term memory interfered in the same way with time production, although to a lesser extent. The last two experiments showed that performing attention-demanding visual search tasks that did not involve short-term memory did not lengthen simultaneously produced time intervals. This suggests that interference of nontemporal processing on time processing may not be a matter of nonspecific general purpose attentional resources, but rather of concurrent shortterm-memory processing demands.Prospective timing occurs when one is required to estimate the duration of a temporal interval that is to be presented. In such a situation, it is often reported that estimated duration decreases as a result of increasing concurrent nontemporal processing demands. This is commonly interpreted within the context of so-called distraction/interruption models of temporal processing, which propose that as concurrent task difficulty or complexity is increased, temporal processing receives less attention (Block, 1990;Brown, 1985;Fortin & Rousseau, 1987;Fraisse, 1984;Hicks, Miller, & Kinsbourne, 1976;McClain, 1983; Rousseau, Fortin, & Kirouac, in press;Thomas & Cantor, 1978;Vroon, 1970). The reduction of attention to temporal processing is assumed to result in less temporal information and, consequently, to shorter perceived duration. In spite of the accumulation of evidence, the distraction models still remain quite informal, which led Block to state that "Terms like attention to time and temporal information processing are unacceptedly vague" (p. 22).We believe that two basic questions should be addressed for some progress to be achieved. First, temporal-

Short-term memory or working memory has been proposed as a cognitive structure contributing to time estimation. Thus, in a previous experiment, retrieving a stored item during a temporalinterval production lengthened the interval in proportion to the number of items in the memory set. In the present study, this issue was analyzed further by testing whether the proportional lengthening is induced by the load itself (i.e., the number of items) or by comparing the probe with memorized items. In a first experiment, a memory set was maintained during a temporal production, and the comparison of the probe with memorized items was postponed until the end of time production. Varyingthe number of items in the memory set had no effect on temporal intervals produced during its retention, suggesting that mental comparison was the source of the lengthening of time intervals. In succeeding experiments, tasks requiring processing in working memory but involving no memory load were combined with temporal production. In Experiment 2, increasing the number of syllables in a rhyme-judgment task proportionally lengthened temporal intervals that were produced simultaneously. In Experiment 3, increasing the amount of mental rotation in a task involving visuospatial processing also lengthened simultaneous temporal production. This interference between processing in working memory and time estimation suggests that working memory, defined as a work space for active processing of current information, contributes to time estimation.Time estimation is often represented as relying on the activity of two main components of a timing system: a pacemaker or a clock generating pulses, and a counter accumulating them and signaling when the number of pulses reaches a criterion value corresponding to a given duration (e

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The interference from nontemporal processing on concurrent time estimation is usually attributed to disruption in timing caused by attentional requirements of nontemporal processing. Here, we examined interruption in timing without concurrent nontemporal processing. Empty breaks of various durations, during time-interval production, lengthened produced intervals. Moreover, an effect of break location was observed: Intervals lengthened proportionally to prebreak duration. When cued and uncued uninterrupted trials were introduced, the lengthening was proportional to the duration for which a break was expected. It was concluded that attentional time-sharing between time estimation and expectation of its interruption contributed to the interference effect in time-estimation research, independently of any concurrent processing requirements during time estimation.

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The influence of cognitive processing on time estimation w~investigated. A temporal-interval production and memory-search dual task was devised so that some operations needed by the search took place during the time interval. Subjects were required to produce time intervals concurrently with a memory-search task similar to Sternberg's (1966). On the average, duration increased in proportion to the number ofelements in the positive set. In general, temporal-production duration displayed the features previously observed, with speeded responses in memory-search tasks. The additive effect of memory scanning on time estimation made possible an interpretation involving inhibition of timing during concurrent processing in short-term memory. This concurrent processing situation appears to be a fruitful procedure for the study of the interaction between time estimation and cognitive processes. However, since a methodological feature ofthe search task could favor a successive processing strategy, the possibility that subjects performed the time production and the item-recognition tasks successively makes a conclusive interpretation difficult.Cognitive processes active during a given time interval are known to contribute to the judgment made about its duration (e.g., Michon, 1970). Systematic investigation of the relationship between time estimation and cognitive processes has proven to be a difficult task. A major step in clarifying the issue comes from Hicks, Miller, and Kinsboume (1976). They reported that under the prospective time-estimation paradigm, in which a subject has to estimate the duration of a completed time period, judged time decreases with increases in amount of other information processing required. This is interpreted to indicate that prospective judgments require attention to time. Concurrent cognitive processing distracts the subject from time processing, thus reducing the amount of temporal information accumulated over an interval; this results in an underestimation of a given physical duration.In support of this conclusion, Hicks, Miller, Gaes, and Bierman (1977) had subjects perform card sorting or verbal rehearsal for an experimenter-controlled length of time ranging from 8 to 22 sec. They showed that verbal estimation of judged time decreased monotonically with the processing demands of the concurrent task, expressed in terms of stimulus uncertainty.

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A temporal reproduction task is composed of two temporal estimation phases: encoding of the interval to be reproduced, followed by its reproduction. The effect of short-term memory processing on each of these phases was tested in two experiments. In Exp. 1, a memory set was presented, followed by two successive tones bounding the target interval to be reproduced. During the reproduction of the target interval, a probe was presented, and the subject ended the reproduction by pressing one of two keys, depending on the presence or absence of the probe in the memory set. In Exp. 2, probe recognition was required during the encoding of the interval to be reproduced. Whereas in Exp. 1 reproductions lengthened as a function of memory-set size, in Exp. 2 temporal reproductions decreased with set size. These results support attentional models of time estimation and suggest that short-term memory processing interrupts concurrent accumulation of temporal information.