a. Convergent and Conjugate Eye Movements. In a conjugate eye movement, both eyeballs move through an equal angle in one direction, such as right or left. In a convergent eye movement, both eyeballs turn toward the midline to focus upon a nearby object. In both cases, the movement of the left and right eyeballs is highly coordinated so that an object may be viewed by both eyes. Therefore, the object can be perceived within both cerebral hemispheres in a binocular fashion.
b. "Searching" and "Following" Eye Movements. "Searching" and "following" movements of the eyeball are also called, respectively, voluntary fixation movements and involuntary fixation movements. For the first type of movement, the eyeballs move in a searching pattern, without focusing on a particular object until it is located. Once an object is located, the eyeballs will continually fix on that object in a following-type motion.
c. Eye Movements During Reading. During reading of printed or written material, the eyeball demonstrates several physical characteristics. The amount of material that can be recognized at a given glance occupies a given width of a written line. Each glance is referred to as a fixation. During a fixation, the eyeball is essentially not moving, and each eyeball is oriented so that the image falls upon the macula lutea (the maximum receptive area). Reading is a series of motions in which the eyeballs fixate on a portion of the written line and then move very rapidly to the next portion.
d. Compensation for Head Movements (Vestibular Control of Eye Movements). Since the human body cannot be held absolutely still, the eyeballs must move in order to remain fixed upon an object. For this purpose, the eyeballs must be moved in the opposite direction and at the opposite speed of the movement of the head. This is accomplished by a delicate and complicated mechanism. This mechanism includes the motor neurons of the muscles of the eyeball and the vestibular nuclei of the hindbrain (responsible for balance and spatial orientation).
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