Microsaccades are minimal movements of the eyes that play a crucial role in visual perception. Without one microsaccade per second, the brain does not achieve visual perception because only microsaccades provide a shift of light on the retina. This shift is important for the retinal receptors to relay visual information to the brain.
What are microsaccades?
A cross-section of the human eye showing its anatomical components. Click image to enlarge. There are several types of eye movement. One is fixation, which corresponds to a resting position of the eye on a particular fixation point. However, even when the eye is in apparent motionless fixation, micromovements still occur every second. Such micromovements are called microsaccades. The eye makes between one and three microsaccades per second. During these jerky flash movements with amplitudes between three and 50 angular minutes, the incident light is shifted on the retina. It is only through these microsaccades that visual perceptions are ultimately possible. The receptors on the retina of the eyes react predominantly to changes in light. Thus, the shifting of light from one receptive retinal area to the next causes the receptors to respond and ultimately enables vision. The term ‘local adaptation’ refers to a visual phenomenon that allows people to perceive fixed stimuli in the visual image, but not perceive them as fixed. This effect occurs under certain environmental conditions. The fact that humans do not have visual perception problems in everyday life due to local adaptation of the eyes is in turn related to microsaccades.
Function and task
The amplitude of microsaccades ranges from five to 50 angular minutes. For microsaccades, the maximum velocity of the motion depends linearly on the distance. Thus, it is about eight deg/s each to amplitudes starting from five angular minutes. Similarly, it is about 80 deg/s at amplitudes around 50 angular minutes. Microsaccades either correspond to low velocity drifts or are a part of the so-called micromovements among eye movements. In the context of micromovements, saccades are also called the microtremor part of the movement. Each microsaccade reorients the lines of gaze to the fixated point. Physiologically, the eyes permanently deviate from fixed points by drifting movements to avoid the phenomenon of local adaptation. The microsaccades are thus one of the most essential components of visual perception ability. They ensure that the eye permanently transmits visual stimuli from the environment to the brain and does not filter them out of perception in the context of local adaptation. Local adaptation is necessary because without it the human being would permanently perceive the fine veins of his own eye above the stimuli from the environment. Humans are among the eye-controlled creatures that find their way in their environment primarily by means of visual perception. That they are able to do so is sometimes due to phenomena such as local adaptation and microsaccades. Typically, microsaccades occur one to three times every second. The respective rate depends on the person and is also related to influencing factors such as fatigue. Scientists now assume that neuronal processes similar to those involved in the generation of saccades play a role in the generation of microsaccades. Common neuronal structures appear to underlie the movements. The drift movements of the eye from a fixation point are as automatic and involuntary as the corrective microsaccades that reorient the eye to the fixation point. These processes are rarely consciously perceived and occur in time spans of less than a second.
Diseases and disorders
Microsaccades have clinical relevance primarily in the context of paralysis of the eye muscles. In most cases, paralyses of this type are associated with neurologic diseases and thus correspond to neuronal lesions in the area supplied by the affected muscles. When the eye muscles are paralyzed, sometimes microsaccades can no longer take place. This can have fatal consequences for visual perception. Since the receptors on the retina react almost exclusively to changing light conditions, microsaccades cause a shift of light on the retina.When microsaccades can no longer take place, only constant light stimuli reach the eyes when the head is fixed. This phenomenon is accompanied by an absolute loss of total vision. This is also referred to as vision loss due to receptor fatigue. In this way, a patient with paralyzed eye muscles would temporarily go blind if his head were fixed from the outside. Head movement can shift the light stimuli on the retina in a similar way to microsaccades. Thus, once the patient can move his head again, he may well see something again, despite eye muscle paralysis. A doctor can follow the paralysis of eye muscles by fixation of the head, because microsaccades prevented by paralysis in this position would lead to temporary blindness. The light constantly falling on the retina is not shifted to different receptors without microsaccades, which has effects mainly on peripheral vision in the corner of the eye. The receptive retinal fields are too large at the retinal cells in the periphery to allow a stimulus shift of the incident light stimuli in the context of microsaccades elsewhere. In the central field of view, shifting of light stimuli may occur through other micromovements because central retinal cells are of smaller size than peripheral retinal cells. This makes the receptive fields in the center smaller, so that shifts of the light stimuli are more easily possible.
