Our eyes constantly gather information. While we often perceive our vision as smooth and continuous, it is actually a series of rapid, jerky movements. These movements, known as saccades, are fundamental to how we perceive our surroundings, shifting our gaze quickly and efficiently between points of interest.
What Are Saccades?
Saccades are quick, simultaneous movements of both eyes that shift our gaze from one point of focus to another. These movements are ballistic, meaning that once initiated, their trajectory cannot be consciously altered or stopped mid-flight. Saccades are extremely fast, with peak speeds reaching up to 700 degrees per second for larger movements. They are the most common type of eye movement, occurring multiple times per second.
These rapid eye movements allow us to reorient our vision. They are distinct from smooth pursuit movements, which are slower and used to track moving objects continuously. Saccades can be executed horizontally, vertically, or obliquely. This ability to quickly jump between fixation points is central to how we interact with our visual environment.
Why We Make Saccades
Saccades play a central role in visual perception by allowing us to efficiently gather detailed information. Our sharpest vision is concentrated in a small central area of the retina called the fovea. Saccades rapidly direct this high-resolution area to different parts of a scene, ensuring that important visual details are processed clearly. Without these quick shifts, our vision would be largely blurry and limited.
Consider everyday tasks like reading; our eyes do not glide smoothly across the text. Instead, they make a series of small, rapid saccades, jumping from word to word and line to line. This discontinuous movement allows us to process distinct chunks of information. When scanning a room, searching for a specific object, or looking at faces, saccades enable us to quickly survey and focus on various features. They help us locate and process relevant stimuli.
How Saccades Work
The generation of saccades involves a complex interplay of specific brain regions. Key areas include the brainstem, cerebellum, and frontal eye fields, which coordinate to initiate and execute these rapid movements. The frontal eye fields, located in the frontal cortex, are considered a control center where saccadic motor commands are initiated. These commands are then relayed to the superior colliculus in the midbrain, which plays a role in selecting targets and initiating the movements.
Six extraocular muscles surround each eye, and these muscles receive signals from the brain to perform the swift, precise movements of a saccade. The brain predicts where the eyes need to land and sends a burst of neural activity to these muscles. This burst causes the eyes to move rapidly, and once initiated, the movement is ballistic. During the brief period of a saccade, visual processing is temporarily suppressed, a phenomenon called saccadic suppression. This suppression prevents the perception of motion blur that would otherwise occur due to the rapid eye movement, contributing to our perception of a stable visual world.
When Saccades Go Wrong
When saccades do not function properly, it can significantly impact visual processing and daily activities. Impairments can manifest as issues with saccadic accuracy, speed, or the ability to initiate movements. For instance, saccades might be too slow, too fast, or inaccurate, causing the eyes to overshoot or undershoot their target. Such abnormalities can indicate underlying neurological conditions, particularly those affecting areas like the brainstem or cerebellum, which are involved in saccade control.
Problems with saccades can lead to difficulties in tasks requiring precise eye movements. Reading can become challenging, with individuals losing their place or skipping lines, making comprehension difficult. Visual tracking of moving objects may also be impaired, affecting activities like sports or driving. Overall visual exploration of an environment can become less efficient. Conditions such as progressive supranuclear palsy, certain spinocerebellar ataxias, and traumatic brain injuries can affect saccade function, indicating a need for further investigation.