![]() The aim of this experiment is to investigate whether the Oppel-Kundt illusion arises due to the repulsion of the end ticks by the adjacent ticks. Please note that although the contrast polarity (stimulus luminance 20 cd/m 2 background luminance approaching 0 cd/m 2) was reversed in comparison with Experiment 1, no differences were observed in the pattern of results. However, any function fitted to these data would reveal a peak at no less than 10 vertical ticks, and this is in accordance with results from Spiegel (1937, as cited in Wackermann & Kastner, 2009) and Wackermann and Kastner ( 2009), which showed a maximal Oppel-Kundt illusion at 17 and 16 lines, respectively. Due to a plateau in the results between 8 to 12 ticks, we are unable to determine at which point exactly the maximum effect would occur. Additionally, taking into account results from Experiment 1, it appears that the size of the Oppel-Kundt illusion is relatively constant from 8 to 12 vertical ticks, inducing a 5% increase in the perceived size of the horizontal line. When 8, 10, or 12 ticks were present, the Oppel-Kundt illusion was approximately 5% with no significant differences between the three conditions. This difference could be explained by the large error bars for this condition in Experiment 1, which show 95% confidence intervals. This result was surprising, not only in comparison with Experiment 1, but also with our previous reports on the effects of bisection (Mikellidou & Thompson, 2013), both showing the effect of a single tick was to reduce perceived length by 7%. Results from the current experiment confirmed those of Experiment 1, albeit with a larger effect of a single tick (∼13%). The main purpose of Experiment 2 was to observe changes, if any, in the size of the Oppel-Kundt illusion when 8, 10, or 12 vertical ticks were added on a horizontal line. Figure 3 illustrates the stimuli used in this experiment. As in all subsequent experiments, no fixation point was used as we did not want to provide any means of reference to the participants, something that would allow them to use alternative strategies to carry out the task. The size of the gap between the left and right stimulus varied between 5°–7°, depending on the size of the comparator stimulus and the amount of jittering introduced in every given trial. The height of each vertical tick was 0.61° and the width was 0.12°. For the other three conditions, one, five, or nine vertical ticks crossed the standard horizontal line in a regular manner. Four conditions were interleaved in the control condition (no vertical ticks) a standard horizontal line (6.1° long, 0.12° wide) with an end tick at each end was compared with one of seven comparator stimuli composed of a similar horizontal line, varying in size from slightly smaller to slightly longer than the standard. A single-interval, spatial two-alternative forced-choice method of constant stimuli was used to construct psychometric functions from which the point of subjective equality (PSE) was determined. ![]()
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