Multiple approaches to investigate the existence of an internal clock using attentional resources
Introduction
In considering how the brain processes duration, it is important to distinguish between the tasks in which subjects do not know in advance that a temporal judgment will be required at the end of the experiment (retrospective conditions) and those tasks in which subjects are required to judge time at the start of the experiment (prospective conditions). Let us mention here that this distinction between retrospective and prospective conditions is different from that one used in animal research area (see Killeen and Fetterman, 1988). The retrospective condition, as defined previously, can be applied to experiments with humans only.
In the area of time perception by humans, it is now theorized that different types of processing are involved under retrospective and prospective conditions. Several studies have suggested that judging a duration under retrospective conditions consists of using the information present in the environment to reconstruct elapsed time (Ornstein, 1969, Block, 1990). Ornstein (1969) proposed that the more information being processed during the interval to be judged, the longer the duration will be perceived. Subjective duration would then depend on the size of the information stored in memory: the greater the storage size, the longer the judged duration. Therefore, when information is more numerous or more complex in the environment, subjective duration is longer. This way of judging time does not necessitate the existence of specific mechanisms of temporal processing. However, the involvement of mechanisms specifically devoted to the processing of temporal information is hypothesized when subjects judge time under prospective conditions. It is now theorized that, under prospective conditions, attentional resources are essential. The present paper is centered on these conditions in which subjects are explicitly required to process the duration of a given interval. It presents evidence in favor of the existence of a temporal processor using attentional resources to be efficient in these conditions. The first part investigates the effects of attention in temporal processing and the second part explores the electrophysiological activity of the cortex recorded when human subjects focus their attention on the temporal parameters of a task.
Section snippets
Attentional models of time estimation
For the past 20 years, a number of studies have suggested the existence of neural mechanisms specifically devoted to the processing of information concerning the duration of a stimulus or of a motor activity. One paradigm that was commonly used is the dual-task in which subjects had to simultaneously perform a temporal and a non-temporal task (Hicks et al., 1976, McClain, 1983). For example, subjects sort cards (Hicks et al., 1976, Hicks et al., 1977) or categorize words (Macar, 1996) and at
Electrophysiological correlates of time estimation
Another approach to investigate the relationship between temporal processing and attentional resources is to examine brain activity during temporal tasks. The question is to check whether some cortical areas are prominently activated when attention focuses on temporal parameters. In particular, in the dual-task described hereafter, subjects simultaneously performed a linguistic and a temporal task, and we expected to identify specific areas related to timing, to linguistic processes and/or to
Conclusion
This set of experiments supports the idea that a temporal processor does exist. The manipulation of attention by instructions or by the presentation of an interfering stimulus produces strong effects on temporal judgments. This result confirms that the attentional resources are necessary to ensure the functioning of the temporal processor. Moreover, the fact that a default of attentional resources produces systematic underestimation suggests that subjective duration depends on the amount of
Acknowledgements
The authors are grateful to Naomi Shimizu for her help with the English language.
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