A single moving dot creates a sense of depth

What is it about?

How little information does the brain need to create a vivid sense of depth? Surprisingly little. In this study, we show that even a single dot moving across a screen can evoke a compelling impression of motion in three dimensional space. A dot following an elliptical path on a flat display could be seen as nothing more than a 2D trajectory. Yet many observers instead experience it as motion in depth, despite the complete absence of classical depth cues. We asked participants to view single dots moving along elliptical trajectories. These speed variations were crafted to match what a circular trajectory slanted in depth would look like when projected onto the screen. Participants then reported whether they perceived the motion as a flat 2D ellipse or as a 3D circular path tilted in depth, and if they perceived depth they also reported the strength of the depth impression. As the dot traced its elliptical path, observers reliably experienced it as looping in and out of the screen. Although the dot’s speed changed along the 2D path, the visual system interpreted these changes as the projection of motion in 3D with constant speed. When the dot instead moved with the sinusoidal speed pattern expected from a perfect side view of a circular path, essentially a dot sliding back and forth along a straight line, the impression of depth vanished and a purely 2D motion was seen. This reflects a general principle: the brain prefers interpretations that preserve constant speed in 3D space, as long as the inferred viewpoint is not an accidental, perfectly aligned side view. In other words, the visual system chooses the explanation that makes the motion physically plausible, and in line with previous occurrences of moving objects. We also tested whether perspective projection, normally a strong depth cue in images, would enhance the impression of depth compared to parallel projection. It did not. For this stimulus, perspective, where the dot moves faster when its closer to the observer, added almost nothing beyond what the elliptic motion already provided. A robust “view from above” bias also emerged: when the motion was ambiguous, observers tended to interpret it as if looking down on the scene, so the circular motion appeared to lie on a ground surface rather than on a ceiling. This bias reflects ecological priors, built in expectations shaped by everyday experience with objects moving on the ground. Together, these findings show that the brain is not a passive camera but an active interpreter, constantly constructing the most plausible 3D percept from sparse and ambiguous information. They reveal how strongly our visual system relies on motion based assumptions to build a coherent 3D world.

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Photo by Joshua Hoehne on Unsplash

Photo by Joshua Hoehne on Unsplash

Why is it important?

Drawing on principles from Gestalt psychology and Bayesian inference, the study highlights how variations in the speed of a single moving element can generate striking impressions of depth. Beyond advancing our understanding of human motion perception, these insights may inform the design of visual displays, animation techniques, and more effective virtual reality environments.