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Patricia R. DeLucia; Effective information for TTC judgments varies during an approach event. Journal of Vision 2005;5(8):330. doi: 10.1167/5.8.330.
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Purpose. Prior studies of time-to-contact (TTC) typically measured one judgment at the end of an approach event (e.g., DeLucia, 1991; Todd, 1981) and tacitly assumed that Os used the same source of information throughout the event. However, TTC judgments are influenced by multiple sources of information (DeLucia, 2004), and the quality of some information sources varies with distance (e.g., optical expansion; Cutting & Vishton, 1995). We hypothesized that the sources of information that affect performance vary throughout an approach event (DeLucia, 2004; DeLucia & Warren, 1994). Method. Two computer-generated spheres approached the O for 8 s. The optical parameters of the spheres were controlled systematically throughout the event. During the first 5.6 s, the right sphere maintained a larger optical size and a smaller tau compared with the left sphere. After 5.6 s, the right sphere's optical size was still relatively larger, but the left sphere's tau was smaller. Relative TTC judgments were measured throughout the event. 16 Os moved a control stick to the left (or right), and kept it there as along as they thought the left (or right) sphere would reach their observation plane first. They reversed the stick's position to indicate a reversal in judgment. The stick's position was analyzed to determine whether response reversals coincided with reversals in the spheres' optical properties. Results. During the first 5.6 s, a significant number of Os selected the right sphere, p < .05. Judgments were consistent with both optical size and tau. After 5.6 s, a significant number of Os reversed their response and selected the left sphere, p < .05. This reversal was consistent with tau rather than optical size. Conclusions. The information sources that affect relative TTC judgments can change during an approach event. It is important to develop methods to measure such changes. The manner in which effective information varies throughout an event must be considered in models of TTC perception.
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