Getting ready for Mars: How the brain perceives new simulated gravitational environments

Abstract

On Earth, we are continually exposed to gravity: sensory signals are constantly integrated to form an internal model of gravity. However, it is unclear whether this internal model is fixed to Earth’s gravity or whether it can be applied to a new gravitational environment. Under terrestrial gravity, observers show a “gravitational bias” while judging the speed of falling versus rising objects, as they comply with the physical laws of gravity. We investigated whether this gravitational bias may be present when judging the speed of objects moving upwards or downwards in both virtual reality (VR)-simulated Earth gravity (9.81 m/s2) and Mars gravity (3.71 m/s2). Our results highlighted a gravitational bias in both Earth and Mars VR-simulated gravity: the speed of downwards movement was more precisely detected than the speed of upwards movement. Although the internal model of gravity has been built up under terrestrial gravity, it can quickly expand to novel non-terrestrial gravitational environments.

Gravity prior in human behaviour: a perceptual or semantic phenomenon?

“…It is also important to note that while rockets can move against gravity, they do not move at gravitational acceleration. Here, we ensured that the acceleration of the objects was identical in both upwards and downwards conditions to closely match previous studies of the gravitational advantage (Moscatelli and Lacquaniti 2011;Torok et al 2019). Future studies might focus on whether the gravitational advantage would be modulated if the rocket was presented with a more realistic upwards acceleration profile.…”

“…To avoid discrepancies in visual saliency and to closely match previous studies (Moscatelli and Lacquaniti 2011;Torok et al 2019), we presented both the rugby ball and rocket at the same size and scale within the virtual environment. It may be possible that the rocket condition was significantly less realistic than the rugby ball condition, considering that a real rocket would be many times larger than we presented here.…”

“…Participants were instructed to press the left arrow on a keyboard if the object was moving faster and the right arrow if it was moving slower than the reference trials. The initial speed of the object during test trials was manipulated between 9.53 m/s and 0.05 m/s in nine steps resulting in nine different motion durations (0.5 s, 0.65 s, 0.7 s, 0.75 s, 0.80 s, 0.85 s, 0.90 s, 0.95 s, 1.10 s) as in previous studies (Moscatelli and Lacquaniti 2011 ; Torok et al 2019 ). Each motion duration was presented 20 times, resulting in 180 test trials per block with ISI = 2300 ms.…”

“…In addition, gravity can influence eye movements, with improved smooth pursuit of objects which move according to 1 g vs objects which move according to weightlessness (0 g), reversed gravity (− 1 g), or hypergravity (2 g) (Delle Monache et al 2015 ; Jörges and López-Moliner 2019 ). We are so exceptionally adapted to terrestrial gravity, that a gravitational advantage appears in perceptual judgements: observers are more precise in judging the speed of objects accelerating downwards compared to upwards (Bosco et al 2008 ; Moscatelli and Lacquaniti 2011 ; Torok et al 2019 ). The neural correlates of this gravitational advantage have been identified in a widespread brain network including the insular cortex, temporoparietal junction, premotor and supplementary motor areas, middle cingulate cortex, postcentral gyrus, thalamus and putamen (Indovina et al 2005 ; Maffei et al 2015 ).…”

“…However, it is not yet clear whether the gravity prior is purely made of constant exposure to online multimodal—vestibular, visual, proprioceptive, and visceral—gravitational signals, or whether it may also be built on semantic knowledge about physical gravitational constraints. Critically, in all previous studies (e.g., Moscatelli and Lacquaniti 2011 ; Torok et al 2019 ; Zago et al 2004 ), observers have been presented with objects which are most often seen to comply with the laws of gravity in the real world, such as a ball. Thus, it could be possible that the gravitational advantage was influenced by implicit semantic knowledge and expectations that a ball is normally falling down, rather than accelerating upwards.…”