Motion perception enables us to track objects, navigate through complex environments, avoid collisions, and understand the actions of other people and animals. It relies on specialized neural mechanisms that detect changes in the retinal image over time and distinguish true object motion from retinal motion caused by eye, head, and body movements.
Detecting Retinal Motion
At the earliest stages, motion-sensitive neurons in V1 respond to local motion energy — changes in luminance at specific locations over time. These neurons have directionally selective receptive fields: they respond strongly to motion in one direction and weakly or not at all to motion in the opposite direction. The Reichardt detector model proposes that direction selectivity arises from comparing signals from adjacent photoreceptors with a slight temporal delay.
Directionally selective filters are constructed from oriented space-time receptive fields.
Area MT/V5 and Global Motion
While V1 neurons detect local motion within small receptive fields, the middle temporal area (MT or V5) integrates these local signals into representations of global motion patterns. MT neurons have larger receptive fields, respond to coherent motion of dot patterns, and are crucial for perceiving the overall direction of movement in noisy displays. Lesions to MT produce akinetopsia — a rare condition in which the world appears as a series of static frames rather than continuous motion.
The famous patient L.M., studied by Josef Zihl and colleagues (1983), could not perceive fluid motion after bilateral damage to MT. She reported that pouring coffee was difficult because the liquid appeared frozen in mid-air, then suddenly appeared at a higher level in the cup.
Biological Motion
Humans are remarkably sensitive to biological motion — the movement patterns of living creatures. Gunnar Johansson (1973) demonstrated that just 10-12 point-lights attached to major joints of a walking person in darkness are instantly perceived as a human figure in motion. From these sparse displays, observers can extract information about the person's gender, emotional state, identity, and even whether they are carrying a heavy load.
A fundamental challenge in motion processing is the aperture problem: a local motion detector viewing a moving edge through a small aperture cannot determine the true direction of motion — only the component of motion perpendicular to the edge's orientation. The visual system must integrate information across multiple local detectors to recover the true velocity. This integration, formalized by the intersection of constraints model, is thought to occur in area MT.
Optic Flow
When we move through the environment, the entire visual field streams in a characteristic pattern called optic flow. James Gibson emphasized the rich information in optic flow for specifying the observer's heading direction, speed, and the three-dimensional layout of the environment. Neurons in the medial superior temporal area (MST) respond selectively to the expansion, rotation, and spiral components of optic flow patterns.
Apparent Motion and Illusions
The visual system infers motion even from static stimuli presented in rapid succession — apparent motion. This is the basis of cinema and animation. Max Wertheimer's 1912 study of the phi phenomenon (perceived motion between alternating lights) is often considered the founding experiment of Gestalt psychology. Motion aftereffects — such as the waterfall illusion, where staring at downward motion causes a subsequently viewed stationary pattern to appear to move upward — reveal the adaptation of direction-selective neurons.