Eye Movements
There are several different types of eye movements. They all aid the purpose of creating a stable, visual representation both on the retina, and in the user’s mind.
Fixation
A fixation is when your eyes remain still for a moment. By doing this, our eyes stabilize the scenery on our fovea, giving us the ability to perceive things clearly. It’s important to note that while fixations are usually correlated with attention, it is not always the case.
Fixations range in duration from 50 milliseconds during reading to several hundred milliseconds when examining a photograph or image, and are thus task-dependent.
Common fixation durations vary between 50-600ms (however longer fixations have been reported) and are based on various factors, e.g., the task at hand and the stimulus. Despite the name, fixations are not entirely fixed, but are instead composed of subtle “fixational eye movements” (microsaccades, tremor and drift). These minute eye movements help the eye align with the target and avoid perceptual fading (fixational eye movements).
Saccade
The rapid motion of the eye from one fixation to another is called a saccade. During the saccade the eyes don’t take in information the same way, making us mostly blind throughout the motion. Saccades are the fastest movement the body can produce.
Saccades can be triggered voluntarily or involuntarily, and during saccades both eyes move in the same direction.
The time to “plan” a saccade (latency) is task dependent and varies between 100-1000 ms. Once executed the average duration of a saccade is 20-40 ms.
During saccades its duration and amplitude are linearly correlated, i.e. larger jumps take longer to complete. Once initiated, the end point of a saccade cannot be changed when the eye is moving, however saccades do not always have simple linear trajectories.
Smooth Pursuit
When looking at moving targets, the eyes can continuously follow them using a technique called smooth pursuit. Smooth pursuit is dependent on there being a target to follow and can not be done without a stimulus. This is another way to shift your gaze other than using saccades, but in contrast to saccades, the eye continues to perceive detail during this movement.
Some smooth pursuits cannot be triggered voluntarily, in absence of a moving target.
Once initiated, the eye velocity is most often less than 30 deg/sec (however some individuals can smooth pursuit at velocities as high as 100 deg/sec). Once the target moves at a higher speed than 30 deg/sec we start to employ catch up saccades to keep up with the target.
Vergence
When shifting attention between near and far objects, our eyes converge (rotate more inward) or diverge (rotate to become more parallel) to keep the focused object centered in both eyes. This is referred to as vergence. This rotation allows for objects to be in the fovea of both eyes at different distances.
Vestibulo–ocular Reflex
The vestibulo–ocular reflex (VOR) is another image stabilization mechanism employed by the vision system to ensure visual information is persistently projected onto the retina. When head rotation is detected through the inner ear, a countering eye rotation is triggered. It differs from a smooth pursuit in such that a VOR-induced eye movement is essentially a compensation for head movement, while a smooth pursuit is compensation for object movement.