Surreal illustration of eye representing Motion Perception

Motion Perception: How Your Brain Interprets Movement

Mira Elwood in Mind & Education August 2025 • 4 min read.

"Decoding the link between shape, curvature, and motion to better understand visual processing"

Our brains are incredibly skilled at interpreting the world around us, especially when it comes to motion. Imagine watching a bird fly or a car speed by—how does your brain translate these visual cues into a seamless perception of movement? Scientists have long been fascinated by this question, diving deep into the mechanisms that allow us to understand motion in its many forms.

A recent study sheds light on this intricate process, focusing on how our brains code curvature to perceive motion. Curvature, in this context, refers to the bends and shapes of objects we see. The study reveals that this curvature coding is finely tuned for motion direction, luminance (brightness), and temporal frequency (how quickly something changes).

The researchers used shape after-effects, specifically the shape-frequency after-effect (SFAE) and the shape-amplitude after-effect (SAAE), to investigate how our brains process these visual elements. Think of after-effects as visual illusions that occur after prolonged exposure to a stimulus. The SFAE and SAAE are believed to be linked to curvature-sensitive mechanisms in our visual system.

What are SFAE and SAAE, and How do They Relate to Motion?

Surreal illustration of eye representing Motion Perception

The shape-frequency after-effect (SFAE) and shape-amplitude after-effect (SAAE) are visual illusions that help scientists understand how our brains perceive shape and curvature. In essence, these after-effects cause a shift in how we see the frequency and amplitude of shapes after we’ve been exposed to an adapting stimulus.

Imagine you stare at a sine-wave-shaped contour for a while. Afterward, when you look at a similar but slightly different contour, your perception of its shape-frequency and shape-amplitude will be altered. This distortion, this shift in perception, is what researchers call SFAE and SAAE.

Shape-Frequency After-Effect (SFAE): Adaptation to a sine-wave-shaped contour causes a shift in the apparent shape-frequency of a test contour in a direction away from that of the adapting stimulus.
Shape-Amplitude After-Effect (SAAE): Adaptation to a sine-wave-shaped contour causes a shift in the apparent shape-amplitude of a test contour in a direction away from that of the adapting stimulus.

To study the motion direction selectivity, the researchers had sinusoidal-shaped contours drift within a fixed stimulus window. In one condition, the entire contour moved (global motion), while in another, the contour was made of moving Gabor patches (local motion). The key was to see if the after-effects changed depending on whether the adaptor and test contours moved in the same or opposite directions.

Why is this Research Important?

Understanding how our brains code curvature and motion is essential for several reasons. Firstly, it deepens our fundamental knowledge of how the visual system works. Secondly, it has implications for various applications, such as improving artificial vision systems, enhancing visual rehabilitation strategies, and designing more effective visual displays. Ultimately, by unlocking the secrets of motion perception, we can gain valuable insights into the brain's remarkable ability to make sense of the dynamic world around us.

About this Article -

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Everything You Need To Know

What is motion perception?

Motion perception is the brain's ability to interpret movement from visual cues. It involves translating what we see, like a bird flying or a car moving, into a seamless understanding of motion. This process relies on intricate mechanisms within the brain that decode various elements, including shape and changes in objects, to create a coherent perception of movement.

What is curvature coding, and what factors is it tuned for?

Curvature coding, in the context of motion perception, is how the brain uses the shapes and bends of objects to understand movement. It has been found that curvature coding is finely tuned for several factors: the direction of motion, luminance (or brightness), and temporal frequency, which refers to how quickly something changes in appearance. This implies that our brain analyzes the shape of moving objects to interpret their motion, taking into account brightness and speed.

What are shape after-effects (SFAE and SAAE), and how do they relate to the study of motion perception?

Shape after-effects, like the shape-frequency after-effect (SFAE) and shape-amplitude after-effect (SAAE), are visual illusions that occur after prolonged exposure to a stimulus. SFAE causes a shift in how we perceive the shape-frequency of an object, while SAAE causes a shift in how we perceive the shape-amplitude. Both of these effects are linked to curvature-sensitive mechanisms in our visual system. The SFAE and SAAE help in studying motion perception by revealing how adapting to specific shapes affects our subsequent perception of other shapes, providing insight into how the brain processes shape and curvature in relation to movement.

Why is understanding curvature and motion coding important?

The research is important because understanding curvature and motion coding enhances the understanding of the visual system. This knowledge has implications such as improving artificial vision systems by mimicking the brain's efficient processing of motion. The research could also have implications for visual rehabilitation strategies, where understanding how the brain perceives motion could aid in developing treatments for visual impairments. Furthermore, it can inform the design of visual displays to provide more effective and natural experiences.

What is the difference between global motion and local motion?

Global motion refers to the overall movement of an entire object or shape, while local motion refers to the movement of individual components or parts within that object. The research used sinusoidal-shaped contours drifting within a fixed stimulus window to study motion direction selectivity to see if the after-effects changed depending on whether the adaptor and test contours moved in the same or opposite directions. The distinction is important because it helps to understand how the brain integrates local cues to perceive the global motion of an object, allowing the study of how these different types of motion processing interact within the visual system.