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Simone Gori, Lothar Spillmann; Grouping thresholds are several times larger than detection thresholds - a new approach toward the psychophysics of Gestalten. Journal of Vision 2009;9(8):922. doi: 10.1167/9.8.922.
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© ARVO (1962-2015); The Authors (2016-present)
Two experiments were performed to compare detection thresholds for spacing or size with thresholds for grouping. Stimuli were horizontal chains of 12 black dots or circles that were varied either in inter-dot distance or size. Three naïve observers served as subjects. In the first experiment, the distance after the 3rd, 6th, and 9th dot was increased in random steps to determine the threshold at which the observer detected the difference in spacing. Thereafter, spacing was increased further to find the threshold at which the observer perceived 4 groups of 3 dots each. The method of constant stimuli was used with “regularly vs irregularly spaced” as response criteria. Each condition was repeated 5 times. Dots had a diameter of 27.50 arcmin, a spacing of 34.38 arcmin and were presented for 1 s. Fixation was in the center slightly below each chain. Results: The detection threshold for the cumulated data was 38.16 arcmin, while the grouping threshold was 48.26 arcmin. This difference translates into a factor of 3.7. In the second experiment, a chain of empty circles was used and the size of the 4th – 6th and 10th – 12th circle randomly increased to measure the threshold for either detection or grouping. The method was the same as before. Circles had a diameter of 27.50 arcmin and were spaced at 82.50 arcmin. Results: Here the detection threshold was 28.32 arcmin, while the grouping threshold was 33.55 arcmin. This difference translates into a factor of 7.4. Conclusion: The results of both experiments demonstrate that the threshold for perceiving stimuli as irregularly spaced or dissimilar in size is not the same as the threshold for grouping. In order to perceive stimuli as grouped, stimulus differences had to be approximately 4 times (for dot spacing) and 7 times (for size) larger than for detection.
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