Surreal illustration of a person upside down perceiving shifting shapes under changing light.

Is Your Brain Playing Tricks? How Gravity and Light Shape What You See

Beau Callahan in Mind & Education August 2025 • 4 min read.

"Uncover the surprising ways your brain adapts to body position and light, influencing your perception of the world."

Our brains are remarkable at constructing a stable view of the world, even when our bodies are in unusual positions. To do this, the visual system relies on prior knowledge, particularly about the direction of light. The assumption that light comes from above is a key factor in how we perceive objects, determine orientation, and even reorient ourselves.

Recent research has delved into how this 'light-from-above' prior is affected by body orientation and the position of our eyes relative to gravity. Understanding these mechanisms can shed light on how our brains constantly adapt to maintain optimal object recognition.

Scientists used advanced technology to test participants' shape-from-shading perception in various body orientations. This study reveals the fascinating interplay between visual input, gravity, and our internal sense of 'up,' challenging our understanding of fixed perceptions.

The Experiment: Tilting Perspectives on Perception

Surreal illustration of a person upside down perceiving shifting shapes under changing light.

To investigate how gravity and body orientation influence our perception of shape, researchers conducted an experiment involving seven male participants. They were positioned in multiple orientations using a specialized robotic arm while viewing a monocular stimulus. The observers then made judgments about whether the shapes appeared convex or concave. By controlling the direction of simulated light and the participants' body position, the researchers could measure changes in the 'light-from-above' prior.

The study used a modified KUKA anthropomorphic robot arm to precisely control the participants' body orientations. Participants were placed in twelve different roll postures relative to gravity, including upright, supine, and various tilted positions. This allowed the researchers to monitor how the brain's interpretation of light and shape shifted as the body's orientation changed.

Precise Positioning: The KUKA robot arm allowed for accurate and repeatable positioning of participants in different orientations.
Controlled Lighting: Simulated illumination from 24 different directions was used to assess the participants' shape-from-shading judgments.
Monocular Viewing: Participants viewed the stimuli with one eye to eliminate binocular depth cues.
Convex-Concave Judgments: Observers indicated whether the central stimulus appeared convex (bulging outward) or concave (indented).

The results confirmed that the 'light-from-above' prior is not fixed but changes with body orientation relative to gravity. Intriguingly, the orientation of the retina and an additional component that was related to the vestibular system signals also influenced results. This suggests that our perception of shape is a dynamic process influenced by multiple sensory inputs.

Implications: A Constantly Adapting Brain

This research demonstrates that our brains constantly update priors, such as the 'light-from-above' assumption, by monitoring self-orientation. This adaptation allows for optimal object recognition, even when our bodies are not in an upright posture. However, extreme deviations from the upright position can lead to errors in perception. These findings highlight the brain's remarkable ability to integrate multiple sensory inputs to create a coherent and adaptable representation of the world.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1038/s41598-018-33625-2, Alternate LINK

Title: Gravity-Dependent Change In The ‘Light-From-Above’ Prior

Subject: Multidisciplinary

Journal: Scientific Reports

Publisher: Springer Science and Business Media LLC

Authors: Michael Barnett-Cowan, Marc O. Ernst, Heinrich H. Bülthoff

Published: 2018-10-11

Everything You Need To Know

How does the brain use gravity and light to perceive the world?

The brain utilizes both gravity and light to maintain a stable perception of the world. The visual system relies on prior knowledge, especially the assumption that light originates from above. Gravity helps the brain determine body orientation, while the direction of light is crucial for object recognition and understanding our sense of 'up.' The brain integrates these inputs to create a coherent and adaptable representation, even when our bodies are in unusual positions.

What is the 'light-from-above' prior, and why is it important?

The 'light-from-above' prior is the brain's assumption that light comes from above. This prior is a key factor in how we perceive objects, determine orientation, and reorient ourselves. It helps the visual system construct a stable view of the world by providing a consistent reference point for interpreting shapes and understanding their relationship to the surrounding environment. Research shows this prior is not fixed but changes with body orientation relative to gravity.

Describe the experiment conducted and the technology used to study perception.

The experiment involved seven male participants positioned in various orientations using a KUKA anthropomorphic robot arm. They viewed a monocular stimulus and made judgments about whether shapes appeared convex or concave. The experiment controlled simulated light direction and the participants' body position to measure changes in the 'light-from-above' prior. The KUKA robot arm allowed for precise and repeatable positioning in twelve different roll postures, while simulated illumination from 24 different directions assessed shape-from-shading judgments.

How does body orientation influence the 'light-from-above' prior?

The 'light-from-above' prior is not fixed but dynamically changes with body orientation relative to gravity. As the body's position shifts, the brain adjusts its interpretation of light and shape. This was observed as participants in the experiment made judgments about shapes in different orientations, demonstrating that the brain continuously updates this prior based on self-orientation relative to gravity. The orientation of the retina and signals from the vestibular system also played a role.

What are the implications of this research on how we understand perception?

This research demonstrates that the brain constantly updates priors like the 'light-from-above' assumption by monitoring self-orientation, allowing for optimal object recognition even in unusual body positions. It emphasizes the brain's remarkable ability to integrate multiple sensory inputs—visual input, gravity, and vestibular system signals—to create a coherent and adaptable representation of the world. While this adaptation is generally beneficial, extreme deviations from the upright position can lead to perceptual errors, highlighting the dynamic nature of our perception.