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Charlotte Boeykens, Johan Wagemans, Pieter Moors; Biases in the perception of the ambiguous motion quartet across spatial scale. Journal of Vision 2019;19(10):152a. doi: https://doi.org/10.1167/19.10.152a.
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© ARVO (1962-2015); The Authors (2016-present)
Perception of the ambiguous motion quartet is predominantly characterized by apparent motion in one of two directions. A vertical percept emerges when two dots presented in one frame are perceptually grouped over frames in a vertical direction (i.e. up-down) and vice versa for a horizontal percept (i.e. left-right). Previous studies have documented a vertical bias, indicating vertical apparent motion even when distances between dots across frames are equally large in both directions (Gengerelli, 1948). Wexler (2018) studied percepts of the motion quartet across different orientations and observed large inter-individual variability in biases varying within individuals over time. In this study, we asked whether vertical and horizontal percepts differed between and within individuals for presentations of stimuli across different spatial distances. More specifically, observers reported their percepts of motion quartets with an ISI of 320 ms displaying two dots with radii of 0.36 deg positioned in combinations of 25 horizontal and vertical distances ranging from 0.6 to 5.4 deg. A different group of observers did this task for combinations of 17 horizontal and vertical distances within the same range and repeated this task again after 30 minutes. In between, we measured individual points of subjective equality (PSE) in vertical distances for fixed horizontal distances (0.9, 1.5, 3.0 and 4.5 deg) by means of an adaptive procedure. Our results replicate the vertical bias in most observers, with considerable inter-individual variability where a minority showed veridical perception or even a horizontal bias. Biases did not vary substantially after 30 minutes. Interestingly, transitions between vertical and horizontal percepts seemed to scale with larger spatial distances, indicating that biases in perception are not invariant across spatial scale (i.e., the adaptive procedure showed that aspect ratios for PSEs became bigger and slopes of the psychometric functions became less steep for increasing spatial distances).
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