Saccadic eye movements are the rapid, simultaneous movements of both eyes in the same direction to shift the point of fixation. These shifts are fundamental to how we visually engage with our surroundings, occurring constantly and often without conscious awareness. They represent the primary way we explore our environment, moving our gaze from one point of interest to another.
The Function of Saccades
The primary reason for saccadic movement is rooted in the anatomy of the human eye. Vision is sharpest in a very small area of the retina called the fovea. To perceive a scene in high detail, the brain directs the eyes to move this small region to various points of interest. Without saccades, our perception of the world would be limited to a small, clear central point surrounded by a blurry periphery.
This mechanism is fundamental to many everyday activities. When reading, our eyes do not move smoothly across the page. Instead, they perform a series of saccades, jumping from one word or group of words to the next. Between these jumps, the eyes remain still in what are called fixations, allowing the brain to process the text. A similar process occurs when we look at a person’s face or scan a room, as our eyes dart between key features—like the eyes, nose, and mouth—to gather relevant information.
The saccadic system processes information about a target’s distance and direction from our current point of gaze to generate these high-velocity eye movements. This allows the image of the target to be brought onto or near the fovea, ensuring clear vision.
How Saccades Work
Saccadic movements are the fastest movements the human body can produce, reaching speeds of up to 900 degrees per second. This velocity is achieved through specialized neurons in the brainstem that directly activate the eye muscles. These neurons operate in a “bang-bang” control mode, meaning they are either fully inhibited or firing at their maximum rate to generate the movement.
Once a saccade is initiated, its path cannot be altered. This is referred to as a ballistic movement; if the target moves after the saccade has begun, the eye will miss it, and a new, corrective saccade will be required. The command is calculated by brain regions like the frontal eye fields and superior colliculus, which determine the required direction and distance. This command is then sent to the brainstem for execution.
Saccadic suppression, or masking, occurs during the brief period of rapid movement. The brain significantly reduces its processing of visual information, which prevents us from perceiving a blurry smear as our eyes jump from one point to another. The suppression primarily affects the magnocellular pathway, which is responsible for detecting high-velocity motion. We remain largely unaware of the thousands of saccades we make every day.
Saccades and Brain Health
The characteristics of saccadic eye movements, such as their speed and accuracy, are controlled by extensive neural circuits. Abnormalities in these movements can serve as early indicators of brain health and are often associated with various neurological, developmental, and psychiatric conditions, as well as brain injuries.
For instance, individuals with certain neurodegenerative disorders may exhibit altered saccadic patterns. In Parkinson’s disease, saccades can become slower and less accurate. Other conditions can lead to involuntary saccadic movements, such as saccadic intrusions, which prevent the gaze from remaining steady on a single point. These issues can arise from lesions in the cerebellum, brainstem, or basal ganglia.
Traumatic brain injuries, including concussions, can also disrupt the brain’s control over eye movements, affecting saccadic function. Distinct saccadic patterns have been observed in individuals with developmental conditions like ADHD and psychiatric conditions such as schizophrenia. In some cases, continuous and multidirectional saccadic oscillations, a condition known as opsoclonus or “dancing eyes,” can occur.
Studying Saccadic Movements
Scientists and clinicians study saccadic movements using a technology called eye-tracking. Modern eye-trackers employ high-speed cameras and infrared light to precisely record eye position and movement. This technology allows researchers to measure the various parameters of saccades, including their velocity, amplitude, and latency.
In neuroscience and psychology, eye-tracking provides insights into cognitive processes like attention, memory, and decision-making. Clinically, it is used to help diagnose and monitor the progression of neurological conditions. The technology has also found use in commercial sectors, such as in marketing and user-experience design, to analyze how people visually interact with advertisements or websites.