Your eyes never stop moving. When you look at something, your eyes constantly move from feature to feature, taking in parts and pieces to create a whole image in your mind. Even when you try to focus on something, your eyes move off your subject quickly. Saccadic eye movement constantly forces your eyes back onto the subject. It happens so quickly that you usually are not even aware of it.
Saccades are small, abrupt movements. Saccadic eye movement is when both eyes simultaneously jerk back and forth and up and down, fixating on and then leaving one point and then another. The velocity of saccadic eye movements can be as much as 1,000 degrees per second. In addition to the saccadic motion, the eyes vibrate at a rate of about 30 to 70 hertz. These vibrations cause the eye to refresh the image to the brain similar to when you refresh a web browser. The vibrations are microsaccades.
The direction of the saccade eye movements depends on which eye muscles the brain is activating. The muscles are controlled by a group of neurons--specialized nerve cells--in two gaze centers in the reticular formation of the brain. The reticular formation is a group of nerve pathways that control waking, sleeping and other levels on consciousness. The gaze centers control horizontal movement and vertical movement. Each gaze center activates independently.
The brain triggers saccades in total darkness during the dream or rapid eye movement phase of sleep. During waking hours, saccades are stimulated when a movement, light or change in color attracts the observer’s attention. The observer directs the foveae--small depressions in the retina where the cones are concentrated--toward the stimuli. Cones are the light and color sensitive cells in the retina of the eye.
The brain could not retain an image if your eyes fixated on it without moving. The function of the eye-brain connection is complicated and requires the constant movements to continually make impressions on the brain. Mapping of saccadic eye movements when the observer gazes at a portrait show that most movement traces patterns from each eye to the mouth and back again, periodically moving to hair, ears and other features. This suggests that your brain identifies individuals most significantly by their eyes and mouth.
Injury and several diseases contribute to a loss of saccadic eye movement and that loss can help lead to an early diagnosis if the warning is heeded. Parkinson’s disease and dementia with Lewy bodies both impair saccadic eye movements. Brain injury, neural diseases and eye injuries also impair saccadic eye movements and affect the way the brain impresses images.
Measurements of saccadic eye movement are being used as a method of determining if a person has attention deficit hyperactivity disorder. The tests offer stimuli to induce saccadic eye movement while the individual attempts to fix a gaze on an object and then analyzes saccadic eye movements. The theory suggests that sufferers of ADHD are unable to voluntarily cause eye movement to correct their attention back to the fixed object.