Our perception of the world feels continuous and stable, but this is an active process managed by the brain. Every time our eyes move, the brain must prevent the disorienting blur that would otherwise result from this rapid motion. This phenomenon, known as saccadic suppression, is the brain’s method of blocking visual processing during eye movements. Without it, our visual experience would be a blur, much like a rapidly panning camera, instead of the steady world we perceive.
The Mechanics of Rapid Eye Movements
The eye movements that threaten visual stability are called saccades: quick, simultaneous shifts of both eyes between two fixation points. Humans make thousands of saccades per day. The peak angular speed of the eye during a saccade can reach up to 700 degrees per second, a velocity that creates significant motion blur on the retina as the visual field sweeps by.
The duration of a saccade is brief, typically lasting between 20 to 200 milliseconds, depending on the distance the eye travels. These movements are too fast to be consciously controlled once initiated. This speed is a functional necessity, allowing us to rapidly shift our fovea—the small central part of the retina responsible for high-resolution vision—to points of interest.
How the Brain Creates Visual Stability
Saccadic suppression is an intricate neural process that actively inhibits visual information just before, during, and immediately after a saccade. This suppression is not uniform, as research indicates the process selectively targets specific neural channels, most notably the magnocellular pathway. This pathway is particularly sensitive to changes in brightness and motion, the information that would create the most disruptive blur.
By reducing this pathway’s sensitivity, the brain filters out the motion signals generated by the eye’s rapid movement. This suppression begins even before the eye starts to move, initiated by a central command from the brain related to the intention to move the eyes. This proactive management allows for a seamless transition between fixation points. Evidence suggests that while the magnocellular pathway is strongly suppressed, other pathways, like the parvocellular pathway which processes color and fine detail, may be less affected.
The Stopped Clock Illusion
A tangible example of saccadic suppression is the “stopped-clock illusion,” also known as chronostasis. This illusion occurs when you glance at an analog clock and perceive the second hand to be frozen for a moment longer than an actual second. This experience is a direct consequence of the brain’s handling of the visual data gap that occurs during the saccadic eye movement toward the clock.
During the fraction of a second your eyes are in motion, visual input is suppressed. To maintain a sense of continuous time, the brain engages in “backdating.” It takes the visual information it receives at the end of the saccade—the image of the second hand at its new position—and retroactively fills the temporal gap with this image. You perceive the second hand’s position for the entire duration of the eye movement, which creates the illusion of a prolonged first second.
Saccadic Suppression in a Neurological Context
The study of saccadic suppression has implications in neuroscience and clinical settings. Researchers investigate this mechanism to gain insight into how the brain coordinates sensory and motor signals. Irregularities in this process have been observed in individuals with certain neurological and psychiatric conditions, offering a window into their underlying neural function.
For example, studies have explored saccadic suppression in conditions like schizophrenia. While research has shown that the fundamental suppression mechanism often remains intact, other related eye movement behaviors, such as the amplitude of saccades under cognitive load, can be altered. These subtle differences are areas of active research, as they may help to explain some of the visual disturbances reported by individuals with the condition.