The human eye is the unit of perceptual motion, perception is the input part, and motion is the output part. 

The important functions of eye movement system:

1. Expand the observation range by turning the field of vision to the area of fixation;

2. The target of gaze is projected on the macula and can be maintained;

3. Binocular coordination enables binocular monocular vision.

In a broad sense, eye movement includes convergence and dispersion, adjustment, fixation, eye rotation, saccade movement and follow-up movement. Eye convergence and dispersion and adjustment have been introduced in the previous two chapters. This chapter focuses on three eye movement phenomena: fixation, eye rotation, saccade movement and follow-up movement. These four movement phenomena are also called eye functional movement.

Fixation is to aim the fovea of the eyeball at a certain point, and its duration is about 150 ~ 400 milliseconds. During fixation, the eyeball is not absolutely motionless. In fact, the eyeball is constantly experiencing rapid micro fibrillation, with a frequency of 20 ~ 150 Hz and a micro fibrillation amplitude of 1 ~ 3 radians. Eyeball fixation inching is also called eyeball physiological fibrillation. When staring at a point, the eyeball is often in inching state, even when reading normally. The micro fibrillation movement ensures that the retina constantly changes the response of sensory cells to the fixation target, so as to overcome the decrease of sensitivity caused by the adaptation mechanism of each photoreceptor cell.

(1) Concept Of Eye Movement

When the eyeball moves in all directions, there is a point in the eyeball whose position is always the same, which is called the rotation center, or gyration point. The turning point is located about 13.5mm behind the anterior apex of cornea and about 10.5mm in front of the posterior pole of eyeball in emmetropia. Rotation centered on the turning point is called eye movement, which refers to the process of changing the angle between the visual axis and the frontal plane.

(2) Form Of Eye Movement

Under normal circumstances, the direction of eye movement includes outward rotation, inward rotation, upward rotation, downward rotation, rotation and oblique rotation.

(3) Limitation Of Eye Movement

Exophthalmos, also known as abduction, is limited by the corneal limbus reaching 0 ~ 2mm in front of the lateral canthus; Intraocular rotation, also known as adduction, is limited by the vertical connection between the upper and lower lacrimal dots at the inner edge of the pupil; The eyeball turns upward, with the lower edge of the cornea reaching the horizontal line between the inner and outer canthus as the limit; The eyeball turns downward, with the upper edge of the cornea reaching the horizontal line between the inner and outer canthus as the limit (as shown in figure 1-3-1).

(4) Extraocular Muscle Excitation Of Eye Movement

Taking exophthalmos as an example, exophthalmos, the eyeball rotates to the horizontal temporal side, and the active external rectus muscle contracts to resist the relaxation of internal rectus muscle. However, the exorbitant movement of the eyeball is far from so simple. The upper and lower rectus muscles are excited at the same time according to the proportion of demand to ensure the uniform exorbitant movement of the eyeball; The upper and lower oblique muscles are excited in proportion to the demand to maintain smooth exotropia of the eyeball. Therefore, the dynamic excitation results of all extraocular muscles during eye movement. The same is true of eye movements in all other directions.

Eye rotation is divided into monocular movement and binocular movement

1. Monocular Movement

2. Binocular Movement

There are two kinds of eye movements: the same direction movement and the different direction movement

Movement in the same direction: the rotation of both eyes to the second eye position or the third eye position is called movement in the same direction.

Opposite movement: the movement of both eyes in the opposite direction is called opposite movement, also known as convergence and dispersion movement. It can be divided into two kinds: collective motion and scattered motion.