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Temporal integration and attention : contributions of visible persistence and masking

University of British Columbia

The perception of moving objects and scenes illuminated by lightning are influenced by the inherent sluggishness of our visual system. This phenomenon, called visible persistence, has been considered a hard-wired aspect of human vision. As such, it has been assumed that its duration was independent of higher level goals. This assumption was tested in this thesis by experiments in which attention was manipulated and the duration of visible persistence was measured. Visible persistence was measured using a temporal integration task. Observers searched for the letters 'F or 'h' in displays consisting of two successive frames of complementary line segments. The interval between frames was varied to index the extent to which integration was possible. A review of temporal integration research made it clear that accuracy in such tasks is determined by both visible persistence and masking. To separate these influences, the task was studied under scotopic conditions, where masking does not interfere with persistence. These results were compared with those collected under photopic conditions, where persistence and masking play opposing roles. The observer's focus of attention was manipulated by varying the number of potential search items (set size). The similarity between target and distractor items was varied to distinguish between attentional and non-attentional accounts of the results. Experiment 1 established baseline performance for search in brief displays under photopic and scotopic conditions. The effect of set size was largest when target and distractor items were most similar. The effect of similarity was least pronounced for scotopic viewing. This reduced effect was attributed to an increase in persistence for scotopic viewing which provided an effectively longer exposure duration and therefore an easier search. Experiment 2 added the manipulation of frame interval in order to measure temporal integration. The effect of set size was constant across frame interval in the scotopic condition, whereas, it increased with frame interval in the scotopic condition. This indicated that attention had no effect on visible persistence, but could reduce the extent of metacontrast masking. Experiment 3 was designed to rule out non-attentional causes for the set size effects in the photopic condition. Set size was held constant while relevant target locations were indicated by instruction. The effect of frame interval was less pronounced with fewer items to attend. These results support two main conclusions: attention does not influence visible persistence, and attention reduces the effects of metacontrast masking. In addition, the methodologies used to study temporal integration and visual search were extended in several important ways.

Thesis/Dissertation

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2009-04-03

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http://hdl.handle.net/2429/6801

Psychology

University of British Columbia

UBCV

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