Sub-second temporal processing : effects of modality and spatial change on brief visual and auditory time judgments
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The present thesis set out to investigate how sensory modality and spatial presentation influence visual and auditory duration judgments in the millisecond range. The effects of modality and spatial location were explored by considering right and left side presentations of mixed or blocked visual and auditory stimuli. Several studies have shown that perceived duration of a stimulus can be affected by various extra-temporal factors such as modality and spatial position. Auditory stimuli lead to more precise duration judgments than visual stimuli and often last subjectively longer than visual stimuli of equal duration. The circumstances under which these modality differences occur are not clear yet. Recent studies indicated an interaction between temporal and spatial processing. Overestimation of durations was associated with right side presentation of visual stimuli, underestimation with left side presentation. However, the effect of spatial presentation has not been explored in the auditory temporal judgments. Furthermore, there is a debate concerning the mechanisms underlying processing of visual and auditory intervals with some researchers supporting the view that there is a central, amodal temporal mechanism and others arguing in favour of distinct, modality specific temporal mechanisms. The above issues were examined in a series of experiments using the duration discrimination paradigm. Processing demands where varied between experiments by varying the number of stimuli positions and the way that different modality trials were presented (mixed or blocked). Across all experiments we found no effect of location either in visual or auditory domain. However, in experiments in which different modality trials were intermixed, participants in the visual versions of the task tended to overestimate durations of comparison stimuli that were presented at different locations to the standard stimuli. In such conditions, visual stimuli were also judged to be longer than the auditory. However, when the location of the comparison stimulus was at the same side as the standard a reverse effect was observed. These findings call into question an influence of the position per se on temporal judgments as the visual duration judgments were affected rather by the change of the location. Auditory judgments were not affected by location manipulations, suggesting that different mechanisms might underlie visual and auditory temporal processing. Based on these results, we propose the existence of an error-correction mechanism, according to which a specific duration is added in order to compensate for the loss of time caused by spatial attention shifts. This mechanism is revealed under some circumstances (such as mixed modality) where it is over-activated, resulting into a systematic bias. This work has important implications for the contemporary research in time perception as it is shedding new light on the possible ways that a unified experience of timing arises from modally and spatially specific temporal mechanisms.