When Up Is Down in 0g: How Gravity Sensing Affects the Timing of Interceptive Actions

Abstract

Humans are known to regulate the timing of interceptive actions by modeling, in a simplified way, Newtonian mechanics. Specifically, when intercepting an approaching ball, humans trigger their movements a bit earlier when the target arrives from above than from below. This bias occurs regardless of the ball's true kinetics, and thus appears to reflect an a priori expectation that a downward moving object will accelerate. We postulate that gravito-inertial information is used to tune visuomotor responses to match the target's most likely acceleration. Here we used the peculiar conditions of parabolic flight-where gravity's effects change every 20 s-to test this hypothesis. We found a striking reversal in the timing of interceptive responses performed in weightlessness compared with trials performed on ground, indicating a role of gravity sensing in the tuning of this response. Parallels between these observations and the properties of otolith receptors suggest that vestibular signals themselves might plausibly provide the critical input. Thus, in addition to its acknowledged importance for postural control, gaze stabilization, and spatial navigation, we propose that detecting the direction of gravity's pull plays a role in coordinating quick reactions intended to intercept a fast-moving visual target.

Sound-evoked vestibular stimulation affects the anticipation of gravity effects during visual self-motion

“…Accordingly, we argued that vestibular signals play a role in the anticipation of gravity effects during visually simulated self-motion, much in the same way in which the vestibular detection of the direction of gravity's pull plays a role in coordinating the manual interception of a fast-moving visual target (Senot et al 2012) or self-motion evaluation ( de Saedeleer et al 2013).…”

Visuomotor adaptation to a visual rotation is gravity dependent

“…This impact is well illustrated by the widespread role of visual vertical/gravity on spatial and temporal aspects of human activity. For instance, a large body of evidence demonstrates that the gravitational component plays a fundamental role in human action production (Crevecoeur et al 2009;Gaveau and Papaxanthis 2011;Papaxanthis et al 1998a) and perception of living (Troje and Westhoff 2006) and nonliving (McIntyre et al 2001;Senot et al 2012;Zago et al 2009;Zago and Lacquaniti 2005) object motion. Therefore, the probability to deal with gravity is strong even when its involvement is artificially created and also the processes of motor adaptations are shaped by an internal model of gravity.…”

Neural Representation of Orientation Relative to Gravity in the Macaque Cerebellum

“…Multiple studies have indeed documented the importance of sensing gravity for both motor planning and sensory perception (Gaveau et al 2011; MacNeilage et al 2007; Senot et al 2012; Zago and Lacquaniti 2005). Sensing gravity is complicated, however, because gravitational and translational accelerations are physically indistinguishable (‘equivalence principle’; Einstein, 1907).…”