The use of visual cues in gravity judgements on parabolic motion

Abstract

Evidence suggests that humans rely on an earth gravity prior for sensory-motor tasks like catching or reaching. Even under earth-discrepant conditions, this prior biases perception and action towards assuming a gravitational downwards acceleration of 9.81 m/s. This can be particularly detrimental in interactions with virtual environments employing earth-discrepant gravity conditions for their visual presentation. The present study thus investigates how well humans discriminate visually presented gravities and which cues they use to extract gravity from the visual scene. To this end, we employed a Two-Interval Forced-Choice Design. In Experiment 1, participants had to judge which of two presented parabolas had the higher underlying gravity. We used two initial vertical velocities, two horizontal velocities and a constant target size. Experiment 2 added a manipulation of the reliability of the target size. Experiment 1 shows that participants have generally high discrimination thresholds for visually presented gravities, with weber fractions of 13 to beyond 30%. We identified the rate of change of the elevation angle (ẏ) and the visual angle (θ) as major cues. Experiment 2 suggests furthermore that size variability has a small influence on discrimination thresholds, while at the same time larger size variability increases reliance on ẏ and decreases reliance on θ. All in all, even though we use all available information, humans display low precision when extracting the governing gravity from a visual scene, which might further impact our capabilities of adapting to earth-discrepant gravity conditions with visual information alone.

Determining Mean and Standard Deviation of the Strong Gravity Prior through Simulations

“…We restrict this comparison to the 0.7g/0.85g/1g/1.15/1.3g condition, as we expect 314 the gravity model not to be activated for inverted gravitational motion. For a discussion of factors 315 impacting the performance of the model for short occlusions, see (Jörges et al, 2018). We first simulate 316 a range of sensible standard deviations (from 0, corresponding to an impossibly precise representation, 317 to 0.28, corresponding to a quite imprecise representation with limited impact on the final percept, in 318 steps of 0.03) to determine the lower and upper bounds of the optimization interval (see Error!…”

Visuomotor Interactions and Perceptual Judgments in Virtual Reality Simulating Different Levels of Gravity

“…Smith et al (2018) showed that the extrapolation of ballistic pendulum motion is idiosyncratic and erroneous when people draw the trajectories, but consistent with accurate physical inferences under uncertainty when people must process pendulum trajectories to catch a ball or they release a pendulum to hit a target. Also, when observers were asked to judge which of two visually presented parabolic motions of a virtual target presented against a blank background had the higher simulated gravity, they generally showed high discrimination thresholds, suggesting that a prior of Earth gravity does not give rise to a discriminability of different gravity accelerations better than that for other arbitrary accelerations (Jörges et al, 2018).…”

“…For instance, humans should be able to detect minimal departures from veridical gravitational acceleration in visual scenes. However, the evidence that this is the case remains controversial Ceccarelli et al, 2018;Jörges et al, 2018;Vicovaro et al, 2019). The issue is especially relevant in rehabilitation applications requiring visuomotor interactions of the patients with the virtual reality setup.…”

“…Interceptions can still be accurate even when the target is virtual and it moves vertically or on a parabolic path under simulated Earth gravity in a visual scene with sufficient context cues about the environmental reference and metric scale, whereas the timing errors (TEs) increase considerably when the background scene lacks context cues (Miller et al, 2008) or the target moves under simulated levels of gravity departing from Earth gravity (Miller et al, 2008;Zago et al, 2011;Bosco et al, 2012;Russo et al, 2017;Jörges et al, 2018). Likewise, ocular tracking of a virtual target that moves on a parabolic path accelerated by Earth gravity is more accurate than tracking a target that moves at constant speed, hypo-or hyper-gravity (Delle Monache et al, 2019;Jörges et al, 2018).…”

Earth-Gravity Congruent Motion Benefits Visual Gain For Parabolic Trajectories

“…There is a growing corpus of research indicating that humans use their knowledge of earth gravity in a broad range of tasks, such as grasping (Verheij, Brenner, & Smeets, 2013), catching and interception (Bosco, Delle Monache, & Lacquaniti, 2012;Jörges, Hagenfeld, & López-Moliner, 2018;Lacquaniti et al, 2013;McIntyre, Zago, & Berthoz, 2001;McIntyre, Zago, Berthoz, & Lacquaniti, 2003;Senot, Zago, Lacquaniti, & McIntyre, 2005;Senot et al, 2012;Zago et al, 2004;Zago, McIntyre, Senot, & Lacquaniti, 2009;, duration estimation (Moscatelli & Lacquaniti, 2011) or the perception of biological motion (Jokisch & Troje, 2003;Maffei et al, 2015;Simion, Regolin, & Bulf, 2008), while arbitrary accelerations are generally neglected (Benguigui & Bennett, 2010;Benguigui, Ripoll, & Broderick, 2003) or used insufficiently. Here we examined ocular pursuit and spatial estimation in a linear prediction motion task that emphasized extrapolation of occluded accelerative object motion.…”