Our brains constantly interpret visual information from our eyes, allowing us to navigate the world. Sometimes, this interpretation leads to perceiving movement where there is none, or misinterpreting actual motion. This highlights the sophisticated yet sometimes fallible nature of our visual system.
How Our Brains Detect Movement
The visual system processes motion through specialized mechanisms. When light enters the eye, it stimulates photoreceptors in the retina, converting light into electrical signals. Signals then travel along the optic nerve to the visual cortex.
Within the visual cortex, particularly in areas like V5/MT, specialized neurons respond specifically to movement in certain directions. These “motion detectors” detect visual signals moving across the retina. This feedforward processing, from eyes to dedicated brain regions, forms the foundation for our perception of motion.
Why Static Images Seem to Move
Static images can create an illusion of motion, often referred to as peripheral drift illusions. The “Rotating Snakes” illusion, for instance, features coiled patterns that appear to spin despite being still. These illusions typically occur in peripheral vision, and the perceived motion is often in a dark-to-light direction.
These illusions stem from how our visual system processes differences in luminance and contrast. Variations in neural processing speed can trick motion detectors into generating responses similar to actual movement. Eye movements, such as microsaccades and blinks, can also trigger or sustain the perception of motion in these static images.
Illusions from Changing Visuals
Visual perception can also be influenced by dynamic stimuli, leading to illusory movements. One such phenomenon is the motion aftereffect, exemplified by the “waterfall illusion.” After staring at continuous motion, a stationary scene may appear to move in the opposite direction. This occurs due to neural adaptation or fatigue in neurons sensitive to the original direction of motion.
The stroboscopic effect demonstrates how discrete static images presented rapidly can create the perception of continuous motion, which is the basis for film. The “wagon-wheel effect” is a common example, where a rotating wheel in a video or under flickering light appears to slow, stop, or even rotate backward. This occurs because the camera’s sampling rate or light flashes interact with the wheel’s rotation, causing the brain to interpret the shortest path of motion.
Another illusion is the autokinetic effect, where a stationary point of light in a dark room appears to move. Without a visual frame of reference, the brain lacks stable cues to judge the light’s position, and subtle, involuntary eye movements can be misinterpreted as the light’s motion. Induced motion occurs when the movement of a large background object makes a stationary object within it appear to move. For instance, the moon may seem to move when clouds drift past it, or your stationary car might feel like it’s moving backward when an adjacent vehicle pulls forward.
The Influence of Eye Movements
Our eye movements play a role in how we perceive stability and apparent motion. Rapid, jerky eye movements called saccades allow us to shift our gaze quickly. During these movements, the brain employs saccadic suppression, briefly blocking visual processing to prevent motion blur. This suppression helps us experience a stable visual world despite constant eye shifts.
In contrast to saccades, smooth pursuit eye movements allow us to track moving objects continuously. The brain uses signals from eye muscles, known as efference copy, to distinguish between retinal image shifts caused by our eye movements and those caused by actual object movement. This internal signal helps the brain maintain a stable perception of the world, even when actively following a moving target.