Perceptual grouping impairs temporal resolution.

Performance on multisensory temporal order judgment (TOJ) tasks is enhanced when the sensory stimuli are presented at different locations rather than the same location. In our first experiment, we replicated this result for spatially separated stimuli within the visual modality. In Experiment 2, we investigated the effect of perceptual grouping on this spatial effect. Observers performed a visual TOJ task in which two stimuli were presented in a configuration that encouraged perceptual grouping or not (i.e., one- and two-object conditions respectively). Despite a constant spatial disparity between targets across the two conditions, a smaller just noticeable difference (i.e., better temporal resolution) was found when the two targets formed two objects than when they formed one. This effect of perceptual grouping persisted in Experiment 3 when we controlled for apparent motion by systematically varying the spatial distance between the targets. Thus, in contrast to the putative same-object advantage observed in spatial discrimination tasks, these findings indicate that perceptual grouping impairs visual temporal resolution.