Page 373 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Eye Movements, Stereoscopic Vision, cade), a new object can be brought into focus.
Depth Perception Damage to the cerebellum or organ of balance
(! p. 342) can result in pathological nystag-
Conjugated movement of the eyes occurs when mus.
the external eye muscles move the eyes in the The brain stem is the main center re-
same direction (e.g., from left to right), sponsible for programming of eye movements.
whereas vergence movement is characterized Rapid horizontal (conjugated) movements
by opposing (divergent or convergent) eye
such as saccades and rapid nystagmus move-
Central Nervous System and Senses of the visual axes. In addition, the pupil con- grammed in the mesencephalon. The cerebel-
movement. The axes of the eyes are parallel
ment are programmed in the pons, whereas
when gazing into the distance. Fixation of the
vertical and torsion movements are pro-
gaze on a nearby object results in convergence
lum provides the necessary fine tuning
tracts (to increase the depth of focus) and ac-
(! p. 326). Neurons in the region of the
commodation of the lens occurs (! p. 346).
Edinger–Westphal nucleus are responsible for
The three reactions are called near-vision re-
vergence movements.
sponse or convergence response.
In near vision, depth vision and three-di-
mensional vision are primarily achieved
Strabismus. A greater power of accommodation for
through the coordinated efforts of both eyes
near vision is required in hyperopia than in normal vi-
of vision (! A). If both eyes focus on point A
convergence impulse, hyperopia is often associated
(! B), an image of the fixation point is pro-
with squinting. If the visual axes wander too far
jected on both foveae (A L, A R), i.e., on the corre-
apart, vision in one eye will be suppressed to avoid
12 sion. Since accommodation is always linked with a and are therefore limited to the binocular field
double vision (diplopia). This type of visual impair-
sponding areas of the retina. The same applies
ment, called strabismic amblyopia, can be either tem- for points B and C (! B) since they both lie on a
porary or chronic.
circle that intersects fixation point A and nodal
Saccades. When scanning the visual field, the points N (! p. 347 B) of the two eyes (Vieth–
eyes make jerky movements when changing Müller horopter). If there were an imaginary
the point of fixation, e.g., when scanning a line middle eye in which the two retinal regions (in
of print. These quick movements that last the cortex) precisely overlapped, the retinal
10–80 ms are called saccades. Displacement of sites would correspond to a central point A C !
the image is centrally suppressed during the A L + A R (! C). Assuming there is a point D out-
eye due to saccadic suppression. A person look- side the horopter (! C, left), the middle eye
ing at both of his or her eyes alternately in a would see a double image (D!, D") instead of
mirror cannot perceive the movement of his or point D, where D! is from the left eye (D L). If D
her own eyes, but an independent observer and A are not too far apart, central processing
can. The small, darting saccades function to of the double image creates the perception
keep an object in focus. that D is located behind D, i.e., depth perception
Objects entering the field of vision are re- occurs. A similar effect occurs when a point E
flexively imaged in the fovea centralis (! C, right) is closer than A; in this case, the E!
(! p. 348). Slow pursuit movements of the image will arise in the right eye (E! R) and E will
eyes function to maintain the gaze on moving be perceived as being closer.
objects. Nystagmus is characterized by a com- Depth perception from a distance. When
bination of these slow and rapid (saccade-like) viewing objects from great distances or with
opposing eye movements. The direction of only one eye, contour overlap, haze, shadows,
nystagmus (right or left) is classified according size differences, etc. are cues for depth percep-
to the type of rapid phase, e.g., secondary nys- tion (! D). A nearby object moves across the
tagmus (! p. 342). Optokinetic nystagmus oc- field of vision more quickly than a distant ob-
curs when viewing an object passing across ject, e.g., in the case of the sign compared to
the field of vision, e.g., when looking at a tree the wall in plate D). In addition, the moon ap-
from inside a moving train. Once the eyes have pears to migrate with the moving car, while
360
returned to the normal position (return sac- the mountains disappear from sight.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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