Flow parsing and biological motion

Abstract

Flow parsing is a way to estimate the direction of scene-relative motion of independently moving objects during self-motion of the observer. So far, this has been tested for simple geometric shapes such as dots or bars. Whether further cues such as prior knowledge about typical directions of an object’s movement, e.g., typical human motion, are considered in the estimations is currently unclear. Here, we adjudicated between the theory that the direction of scene-relative motion of humans is estimated exclusively by flow parsing, just like for simple geometric objects, and the theory that prior knowledge about biological motion affects estimation of perceived direction of scene-relative motion of humans. We placed a human point-light walker in optic flow fields that simulated forward motion of the observer. We introduced conflicts between biological features of the walker (i.e., facing and articulation) and the direction of scene-relative motion. We investigated whether perceived direction of scene-relative motion was biased towards biological features and compared the results to perceived direction of scene-relative motion of scrambled walkers and dot clouds. We found that for humans the perceived direction of scene-relative motion was biased towards biological features. Additionally, we found larger flow parsing gain for humans compared to the other walker types. This indicates that flow parsing is not the only visual mechanism relevant for estimating the direction of scene-relative motion of independently moving objects during self-motion: observers also rely on prior knowledge about typical object motion, such as typical facing and articulation of humans.

Visual perception of travel distance for self-motion through crowds

“…But, during real walking, when vestibular and somatosensory cues are also available, flow passing is close to perfect ( Xie et al, 2020 ). In addition, flow parsing, can also take limb articulation or facing direction of biological motion into account to estimate the direction and speed of walkers embedded in optic flow ( Mayer, Riddell, & Lappe, 2021 ). This is similar to the situation in our present experimental conditions, in which participants have to use articulation information to extract their own speed from the optic flow by decomposing the flow into own and crowd components.…”

“…In natural environments, however, we are often confronted with dynamic scenes in which other people walk alongside us. The motion of other walking humans, known as biological motion ( Johansson, 1973 ), introduces noise to the optic flow field, thereby biasing optic flow analysis for heading, the direction of one's self-motion ( Riddell & Lappe, 2017 ; Riddell & Lappe, 2018 ; Riddell, Li, & Lappe, 2019 ; Hülemeier & Lappe, 2020 ; Koerfer & Lappe, 2020 ) and concerning flow parsing, the estimation of independent object motion within a flow field ( Mayer, Riddell, & Lappe, 2021 ). Biological motion consists of limb articulation and its associated translation through space.…”

Target motion misjudgments reflect a misperception of the background; revealed using continuous psychophysics

“…2. One thing that has been shown to affect flow-parsing gain is multisensory stimulation (e.g., Mayer et al, 2021;Xie et al, 2020). In particular, the addition of vestibular cues that are conducive with the background motion has been shown to increase β total .…”

“…Note that step 1—the measuring of β total under a range of conditions—has been performed extensively in a range of Induced Motion and Flow-Parsing studies (Dokka et al, 2013; Dokka et al, 2015; Falconbridge et al, 2022b; Mayer et al, 2021; Niehorster & Li, 2017; Xie et al, 2020). Our study is distinguished from these previous studies by the fact that we measured β total using a continuous psychophysics approach described below.…”

“…See Figure 1 for a depiction of this process. This hypothesis has received overwhelming support in recent years (e.g., Dupin & Wexler, 2013; Fajen et al, 2013; Falconbridge et al, 2022b; Foulkes et al, 2013; Mayer et al, 2021; Rogers et al, 2017; Warren & Rushton, 2008; Warren & Rushton, 2009) and provides an explanation for the misperception of target motion in the classic Induced Motion Illusion (Falconbridge et al, 2022b; Rushton & Warren, 2005; Warren & Rushton, 2004).…”