This study examined whether a change in the amount of attention equallyallocated to two locations affects judgments of the simultaneity or successiveness of stimuli presented at those locations. Observers were cued to expect two brief flashes either to the left and right of fixation or above and below fixation. Stimulus onset asynchrony was randomly varied. On a small proportion of trials, the stimuli appeared at the unexpected locations. Observers were more likely to report the stimuli as simultaneous when they appeared in the unexpected locations. A model proposed to account for the data assumes that a brief stimulus event is represented by a probability distribution reflecting the uncertainty in determining the time ofthe event's occurrence, and two events are judged to be simultaneous if they are perceived to fall within some critical temporal interval, C, which is a function of the amount of attention allocated to the task It has long been recognized that the perceived order of a pair of closely timed stimuli does not always correspond to the actual order of their occurrence (see James, 1890). The idea that some stimuli could receive perceptual or attentional priority for further processing led to the notion of"prior entry," which was the notion that there were factors that could alter the time it took for a stimulus to reach awareness (see Boring, 1950). The physical intensity of a stimulus is one strong determinant of prior entry (e.g., Roufs, 1963). More recently, Sekuler, Tynan, and Levinson (1973) uncovered what seemed to be an inherent bias in the visual modality toward more accurate recognition ofthe temporal order ofleft-first-right-second stimuli over right-first-left-second stimuli. The observed bias could be due to a left-right scanning mechanism such as is assumed to occur in reading, which would make the bias an attentional effect. Using a location-cuing manipulation, Stelmach and Herdman (1991) demonstrated that the direction ofattention could bias the perception of the temporal order of two visual stimuli: stimuli appearing in the attended location were more likely to be perceived first than were stimuli appearing in the unattended location (see also Hikosaka, Miyauchi, & Shimojo, 1993).While it is clear that an attentional bias can affect judgments of temporal order and simultaneity, a related question that, to our knowledge, has not yet been addressed is whether the overall level ofattention directed toward both stimuli in a temporal order task has any effect on judgments of temporal order or simultaneity. Does attention increase the precision ofjudgments of the relative time ofThe authors wish to thank Arthur Kramer and two anonymous reviewers for their helpful comments on earlier versions of this paper. Correspondence can be addressed to either R. A. Carver or V Brown, Department of Psychology, University of Texas, Arlington, TX 76019-0528 (e-mail: rcarver@uta.edu or brown@uta.edu).occurrence of two stimulus events? Conversely, to what extent are such judgments harmed by a decrease in the...
