The ability to detect motion is a fundamental function of the visual system that enables interpreting and responding to an ever-changing environment.

Unlike static perception, motion detection requires sophisticated neural processing to discern moving objects, their direction, and speed.

Retinal Foundations of Motion Detection

The process begins in the retina, which contains photoreceptors and ganglion cells optimized for different visual tasks. Among these, magnocellular (M) pathway cells play a crucial role in motion perception due to their high temporal resolution. Located predominantly in the mid-peripheral retina, magnocellular retinal ganglion cells respond rapidly to changes in light intensity, enabling detection of moving stimuli. In contrast, parvocellular (P) cells have lower temporal resolution and focus more on high-resolution color and detail, making them less involved in motion detection.

According to Ehud Kaplan and Robert M. Shapley (1986), the primate retina contains two types of ganglion cells—one projecting to the magnocellular layers that exhibits high contrast sensitivity and rapid responses, and one projecting to the parvocellular layers with lower contrast sensitivity and slower responses.

Specifically, specialized retinal ganglion cells known as direction-selective cells can distinguish the direction of motion across the retina. These cells integrate spatiotemporal information, responding vigorously when motion occurs in their preferred direction and suppressing responses for opposite directions. This receptor-level feature extraction provides the first neural indication of motion before signals proceed to higher brain centers.

Subcortical Processing: The Lateral Geniculate Nucleus and Superior Colliculus

After the retina, motion-related signals travel primarily through the magnocellular layers (layers 1 and 2) of the lateral geniculate nucleus (LGN) in the thalamus. This relay station further processes temporal aspects of the visual input, filtering and enhancing motion information before it reaches the cortex. Some retinal signals also project to the superior colliculus, a midbrain structure involved in integrating visual motion with eye and head movement control. This pathway contributes to reflexive orienting responses, helping to track moving objects through coordinated eye movements.

Visual Cortex: Complex Motion Analysis

The visual cortex is the primary processing center for sophisticated motion perception. In the primary visual cortex (V1), neurons exhibit direction selectivity, responding preferentially to motion in specific orientations and directions. These responses form a neural map representing motion characteristics across the visual field.

From V1, motion information proceeds along the dorsal stream or "where" pathway, specialized for spatial awareness and motion processing. Within this stream, the middle temporal area (MT or V5) is crucial; approximately 90% of MT neurons are directionally selective and sensitive to speed, identifying the precise velocity and direction of moving stimuli. Damage to MT/V5 can cause motion blindness (akinetopsia), underscoring its essential role.

Beyond MT, the medial superior temporal (MST) area processes complex optic flow patterns, helping to interpret self-motion and the movement of objects relative to the observer. MST neurons integrate signals across wide visual fields, contributing to balance, navigation, and motion-based attention.

The Hassenstein-Reichardt Model: Neural Correlation for Motion Detection

One prominent theory explaining motion detection mechanisms is the Hassenstein-Reichardt model. It proposes that motion perception arises from the correlation between sequential signals from adjacent retinal receptors. The model suggests two neural subunits, each receiving light input; one signal undergoes a temporal delay, then it is multiplied with the other signal from the neighboring receptor. The difference in output between subunits encodes directional motion, with positive or negative values indicating preferred or null directions.

Hypercomplex cells in the cortex embody this function, demonstrating direction-selective responses consistent with this model. Experimental evidence from optical recordings and electrophysiology support the Hassenstein-Reichardt framework as a plausible biological mechanism for motion detection.

Integration with Eye Movements

Detecting motion involves distinguishing between actual object movement and self-induced retinal image shifts caused by eye movements. The brain utilizes corollary discharge signals—a type of internal communication about planned eye movements to differentiate stationary objects causing retinal motion from objects moving independently in the environment. Specialized real-motion neurons in cortical areas V3A and V6 mediate this distinction, allowing perception of a stable visual world despite frequent eye shifts.

According to Claudio Galletti, a neurophysiologist at the University of Bologna, "The activity of these real-motion cells is responsible for motion perception and the brain’s ability to distinguish real-world motion from motion caused by eye movements".

The detection of motion by the human visual system is an intricate, multilayered process beginning at the retina and extending through subcortical structures to the cerebral cortex. Retinal magnocellular cells and direction-selective ganglion cells initiate motion signals, which are refined through the LGN and superior colliculus before reaching cortical areas tuned for speed and direction. Advances in neuroscience have illuminated this remarkable sensory capability that supports navigation, attention, and interaction with a dynamic environment.