Illusory percepts of curvilinear self-motion when moving through crowds

Abstract

Self-motion generates optic flow, a pattern of expanding visual motion. Heading estimation from optic flow analysis is accurate in rigid environments, but it becomes challenging when other human walkers introduce independent motion to the scene. Previous studies showed that heading perception is surprisingly accurate when moving through a crowd of walkers but revealed strong heading biases when either articulation or translation of biological motion were presented in isolation. We hypothesized that these biases resulted from misperceiving the self-motion as curvilinear. Such errors might manifest as opposite biases depending on whether the observer perceived the crowd motion as indication of his/her self-translation or self-rotation. Our study investigated the link between heading biases and illusory path perception. Participants assessed heading and path perception while observing optic flow stimuli with varying walker movements. Self-motion perception was accurate during natural locomotion (articulation and translation), but significant heading biases occurred when walkers only articulated or translated. In this case, participants often reported a curved path of travel. Heading error and curvature pointed in opposite directions. On average, participants perceived the walker motion as evidence for viewpoint rotation leading to curvilinear path percepts.

Limb articulation of biological motion can induce illusory motion perception during self-motion

“…While the combination of limb articulation and body translation in natural locomotion allows surprisingly accurate heading perception despite massive perturbations in the flow field ( Hülemeier & Lappe, 2020 ; Riddell & Lappe, 2018 ), significant heading errors occur when only limb articulation without translation is presented ( Hülemeier & Lappe, 2020 ; Riddell & Lappe, 2018 ). These heading biases might be explained by erroneous self-motion perception that results from attributing an illusory motion to the crowd based on its articulation ( Hülemeier & Lappe, 2020 , 2023 ). The present study aimed to test whether such an illusory crowd motion is perceived.…”

“…Another reason lies in the information conveyed by biological motion about the translation of the crowd. While this information is helpful when limb articulation and crowd speed match, its contribution is seen most dramatically in the heading biases that are reported when articulation and crowd speed do not match ( Hülemeier & Lappe, 2020 , 2023 ). These biases can reach several tens of degrees of visual angle.…”

“…In the virtual scene, half of the group was placed 7 m and 9 m from the observer, and the other ones 14 to 18 m. Objects that are placed at different distances from the observer give rise to motion-parallax ( Hanes et al, 2008 ; Koenderink, 1986 ). Motion-parallax contributes to the accurate perception of self-motion ( Hülemeier & Lappe, 2023 ). The simulation lasted for 2500 ms.…”

“…Thought further, participants should therefore be able to accurately determine their self-motion, thus accurately discerning the absence of crowd speed. Considering the observed self-motion biases in the presence of only-articulating walkers ( Hülemeier & Lappe 2020 , 2023 ; Koerfer & Lappe, 2020 ; Riddell & Lappe, 2017 , 2018 ), these heading errors may be related to backscroll illusion ( Fujimoto, 2003 ; Fujimoto & Sato, 2006 ; Fujimoto & Yagi, 2008 ): In the backscroll illusion, when a walker articulates as if on a treadmill in front of a background, this background is perceived as moving in opposite direction than the walker faces. During self-motion, this illusory motion might indicate an additional component of self-translation or self-rotation of the observer.…”

“…During self-motion, this illusory motion might indicate an additional component of self-translation or self-rotation of the observer. When the walkers are the only visible elements in the scene, such an illusory self-motion percept might affect the heading perception from the optic flow ( Hülemeier & Lappe, 2020 , 2023 ). This consideration, in turn, implies that the participants may alternatively deduce crowd speed from the limb articulation pattern due to their knowledge of the natural connection between limb articulation and body translation ( Masselink & Lappe, 2015 ).…”