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Björn Jörges, Lena Slupinski, Joan López-Moliner; Moderate Influence of Target Size Variability on Visual Gravity Judgements. Journal of Vision 2018;18(10):300. doi: 10.1167/18.10.300.
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A strong earth gravity prior lends an important contribution to many functions regarding perceptuo-motor skills like reaching, catching or throwing (Jörges & López-Moliner, 2017). When earth-discrepant gravities are presented (f. e. visually in Virtual Reality), humans can not rely on this prior and need to extract the gravity value from the visual scene. A previous experiment suggests that, for parabolic motion, both Visual Angle and Elevation Angle information play a role in this computation. In addition to these online cues, it would be beneficial for observers to establish and rely on a representation of target size. To determine whether this is the case, the present experiment investigates how variability of object size influences visual gravity judgments for parabolic motion. To this end, participants (n = 12, 2 excluded) were asked to judge in a two-alternative forced-choice paradigm which of two parabolic motions had the higher underlying gravity. The test parabola could be governed by one of seven gravities (0.7 g – 1.3 g), while the reference parabola was always governed by 1 g. Their order was randomized. Both could have one out of two initial vertical velocities (3.7 or 5.2 m/s) and one out of two horizontal velocities (6 or 8.3 m/s). Furthermore, we manipulated variability of ball size (drawn from a Gaussian distribution with a mean of 0.033 m and a SD of 0.02 m or 0.005 m, respectively presented in blocks). Participants displayed a slight decrease in sensitivity in the high variability condition and a strong bias to judge smaller targets as being affected by higher gravities. Furthermore, sensitivity of some participants increased when the displayed ball size was within 10 % of the mean ball size. We conclude that an accurate representation of object size has some beneficial effects for the extraction of gravity from parabolic motion.
Meeting abstract presented at VSS 2018
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