A recent study published in the Journal of Vision reveals that humans are better at estimating the motion of objects moving downwards compared to upwards, regardless of the direction relative to gravity. This new research suggests that our perception of downward-moving objects is enhanced by the congruence of their motion with the direction toward our legs rather than the influence of gravity detected through our inner ear.
“Previous studies have shown that the people can catch a descending object more accurately than an ascending object. When people heard this asymmetric performance for catching ascending and descending objects, they generally thought that this asymmetric aligned with the gravity axis because we are used to seeing the moving object toward the gravity in daily life,” said study author Takashi Hirata, a member of the Geometrical Understanding of Spatial Orientation research project at the Japan Science and Technology Agency.
“Our team considered another possibility: the asymmetric performance for catching might depend on the observer’s body axis, as we spend most of our time in an upright posture where the observer’s body axis are aligned with the direction of gravity axis. To clarify whether the asymmetry in catching performance for ascending and descending objects depends on the gravity axis or longitudinal body axis, we conducted this study.”
The researchers conducted two experiments involving 59 college students, in which the participants donned virtual reality headsets that displayed moving targets in a controlled setting.
In the first experiment, the VR simulation showed objects moving either upwards or downwards along the participant’s body axis. The movements occurred in three different gravitational scenarios: normal Earth gravity (1 G), weightlessness (0 G), and high gravity (−1 G). Each scenario was designed to test the participants’ ability to predict when these objects would reach a specific point in their trajectory.
The objects were displayed moving along a vertical axis aligned with either the participants’ upright (standing or sitting) or supine (lying down) body positions. This setup allowed the researchers to test if the body’s orientation — regardless of the direction of gravitational pull — influences how well one can predict the motion of ascending versus descending objects. The participants were tasked with pressing a button at the moment they believed an object would arrive at the goal, and the timing of these presses was recorded to assess accuracy.
The second experiment investigated another aspect of motion perception: the ability to track moving objects using smooth pursuit eye movements (SPEM). SPEM are the eye movements that allow us to smoothly follow a moving object. This experiment aimed to determine whether the direction of an object’s motion relative to the body’s axis affects the accuracy of these eye movements under different gravitational scenarios, similar to those in the first experiment.
Across both experiments, a consistent pattern emerged: participants were better at estimating and tracking the motion of descending objects compared to ascending ones. This was true regardless of the gravitational conditions (1 G, 0 G, and −1 G), suggesting that the congruence between the motion of the object and the natural orientation of the body (downward towards the legs) enhances perceptual accuracy. This effect was observed in both upright and supine postures, indicating that the gravitational pull—whether aligned with the body or not—does not significantly impact the estimation abilities.
The results also showed that SPEM performance was better for tracking downward moving objects than upward ones. This was true even when the gravitational cues were misaligned with the body’s orientation, such as in the supine posture.
The findings highlight that visual and bodily cues play a more crucial role than vestibular cues (which detect gravity) in the context of this task. This suggests that our daily experiences, which predominantly involve observing objects falling downwards due to gravity, may tune our perceptual systems to be more sensitive and accurate to downward motions. Essentially, it appears that the brain prioritizes consistent visual and physical experiences over the specific directional pull of gravity when it comes to processing moving objects.
“Generally, we understand that a falling object is moving in the direction of gravity,” Hirata told PsyPost. “However, as our study shows, we may recognize the object moving toward the leg direction as a falling object and moving toward the head direction as a rising object. In other words, actual gravity information sensed by the otolith organ in our inner ear does not significantly contribute to our recognition when interacting with falling and rising objects.”
The study, “Human estimates of descending objects’ motion are more accurate than those of ascending objects regardless of gravity information,” was authored by Takashi Hirata, Yutaka Hirata, and Nobuyuki Kawai.
