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-

Three experiments address the cause of the different performance levels found in time discrimination of empty intervals with durations near 250 msec. Performance differed according to the kind of sensory modality that marked the intervals. With a procedure in which the type of marker was randomized from trial to trial, it was shown that variability of discrimination judgments could not be attributed entirely to the variability of the criterion on which a judgment was based. Such a randomization slightly affects discrimination but provokes a reorganization related to marker conditions of the probabilities of judging an interval to be short or long. Moreover, it was shown that within intramodal conditions, physical characteristics of markers influence the discrimination performances. To account for the results generated with different marker-type intervals at 250 msec, we propose that two types of processor may be involved in duration discrimination: one is specifically related to a given sensory modality, whereas the other is aspecific and responsible for discrimination of intermodal intervals.

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.

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.

In 1973, Rousseau and Kristofferson reported that short empty intermodal time intervals marked by a light flash and a brief tone were poorly discriminated by subjects, and that 4T 75 was constant overa large range of durations. It led them to suggest that short intramodal empty intervals, marked by stimuli from the same sensory modality, might be handled by a "more efficient mechanism" to whichintermodal intervals would not have access. Unfortunately, their study lacked the basic evidence needed to make a strong statement: no direct comparison between inter-and intramodal duration discrimination and no within-subject discrimination function were available. To clarify these two issues, three experiments were performed. The data indicate that intermodal time intervals are discriminated more poorly than intramodal ones, and that intermodal duration discrimination functions follow Weber's law. Analysis of data from different experiments lead to the conclusion that inter-and intramodal intervals are timed by a commontimekeeper and that intermodal intervals induce a large noise component in the timekeeping operation.541

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Cats (Felis catus) find an object when it is visibly moved behind a succession of screens. However, when the object is moved behind a container and is invisibly transferred from the container to the back of a screen, cats try to find the object at or near the container rather than at the true hiding place. Four experiments were conducted to study search behavior and working memory in visible and invisible displacement tests of object permanence. Experiment 1 compared performance in single and in double visible displacement trials. Experiment 2 analyzed search behavior in invisible displacement tests and in analogs using a transparent container. Experiments 3 and 4 tested predictions made from Experiments 1 and 2 in a new situation of object permanence. Results showed that only the position changes that cats have directly perceived are encoded and activated in working memory, because they are unable to represent or infer invisible movements.

In a stop-reaction-time (stop-RT) task, a subject is presented with a regular, isochronous sequence of brief signals separated by a constant time interval, or stimulus onset asynchrony (SOA). His/her task is to press a response key as fast as possible when the sequence stops. As the sequence Unfolds, an internal representation of the SOAduration builds up. Stop-RT is assumed to be triggered when an internal clock, operating as an "alarm clock," reaches a time criterion. Criterion setting is contingent upon variability in the SONs internal representation. In Experiment lA, stop-RT was measured for isochronous sequences ofbrieftones, light flashes, and also sequences of tones and flashes presented in regular alternation (tone-light-tone ...). Stop-RT was a linear function of SOA duration (ranging from 250 to 1,000msec), regardless of modality, supporting a "central-clock" hypothesis. On the other hand, taken together, the results of Experiments lA, 1B, 2, and 3 suggest that no internal representation of the bimodal (tone-light) SOA of alternating sequences builds up. Indeed, an alternating sequence is physically equivalent to two interlaced isochronous subsequences, one auditory and one visual. So, two internal representations, one for the auditory (tone-tone) and one for the visual (light-light) SOA, could build up, and two time criteria running "in parallel" could thus support stop-RT. To provide a critical test of parallel timing, stop-RT was measured for bimodal 5:3 polyrhythms formed by the superposition of auditory and visual isochronous sequences that had different SOAdurations (Experiment 4). Parallel timing accounted for a large proportion of variance in polyrhythmic stop-RT data. Overall findings can be accounted for by assuming a functional architecture of an internal clock in which pulses emitted by a central pacemaker are available in parallel with two modalityspecific switch-accumulator "timing modules."