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
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 Move-
ments). 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).
In the sense of vision, one consciously perceives the various objects being
looked upon. In addition to this, there are a number of protective reactions to visual
input--the visual reflexes.
a. When an unexpected visual stimulus occurs within the visual field, the
individual's response will often include movement and other types of reaction. This is a
part of the startle reflex.
b. When there is a change in the amount of light entering the eyeball, the size
of the pupil will change. This is the pupillary reflex. The muscles of the iris
automatically constrict or dilate to control the amount of light entering the eyeball.
c. In the blink reflex, the eyelids automatically move over the exterior surface
of the eyeball. This reflex results in the automatic washing of the exterior surface of the
eyeball with the lacrimal fluids. It also helps to keep the surface moist.