These 3 types of fixational eye movements play a vital role in our visual perception, cognitive function, and are instrumental for our ability to see the world around us.
We spend over 80% of our visual day fixating our gaze on stationary objects, but when was the last time you thought about or evaluated your patient’s fixational stability? Fixation is not a static passive state, but rather an active neural process.
These microscopic flickers of motion may seem insignificant, as they are unnoticed by most people and undetectable on standard clinical exam; however, these minute eye movements play a vital role in our visual perception, cognitive function, and are instrumental for our ability to see the world around us with 20/15 clarity and precision.
As foveate animals, humans necessitate fixational eye movements because of the uneven distribution of photoreceptors in the retina. Our eyes need to be precisely pointed at objects of visual interest so that the light image from these objects is focused on the fovea. If the light is focused parafoveally or peripherally, our visual acuity begins to decline. Thus, fixational eye movements help maintain eye alignment.
An additional function of fixational eye movements is to prevent retinal neural adaptation, a phenomenon where the sensitivity of our retinal receptors decreases over time when exposed to a constant stimulus. Thus, the eyes need to constantly microscopically move or our visual system will desensitize to the visual stimulus. Quite the conundrum, because if the eyes move too much during fixation, then the patient will lose clarity and complain of blur and/or oscillopsia, but if they don’t move enough the patient will complain of visual fading.1,2
There are 3 types of fixational eye movements: microsaccades, tremor, and drift. Microsaccades are fast, jerk-like eye movements that occur about 1-4 times a second. They are the largest of fixational eye movements, moving the image across several dozen to hundreds of photoreceptors. Drift, on the other hand, refers to the slow, smooth motion that occurs in between microsaccades, moving the image across about a dozen photoreceptors. Lastly, tremor, the smallest fixational eye movement, refers to the high frequency vibration of the eyes that occurs simultaneously with drift, and moves the image over about the diameter of a foveal cone.3-5
Technological advancement in the last few decades has led to a revival of interest in fixational eye movements and it is one of the fastest moving fields in visual neuroscience today. Anatomically, fixational eye movements require a vast portion of brain circuitry including every lobe of the brain, brainstem, cerebellum, thalamus, basal ganglia, cranial nerves, and afferent visual pathway.6 Thus, fixational eye movements are susceptible to ophthalmologic and neurologic insults.
Researchers have found abnormalities in fixational eye movements in mild traumatic brain injury,7 multiple sclerosis,8 mild cognitive impairment, Alzheimer’s disease,9 Parkinson’s disease,10 amblyopia,11 maculopathy,12 and schizophrenia.13 These findings suggest that analyzing fixational eye movements could potentially serve as a non-invasive tool for early detection and monitoring of certain neurological disorders.