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26 Theory and Practice of Squint and Orthoptics
6. Role of muscle pulleys. Muscle pulleys,
formed by stiff connective tissue have been
described to exist for rectus muscles as part of
their sleeves at or just posterior to the equator
of globe. The muscle pulleys redirect the
extraocular muscles and act as their functional
origins; and thus affect the ocular movements
in different positions of gaze.
CLASSICAL CONCEPT OF ACTIONS
OF THE EXTRAOCULAR MUSCLES
The basic and classical concept implies that the
actions of the extraocular muscles depend upon
the position of the globe at the time of muscle
contraction. The primary action of a muscle is its
major effect, when the eye is in the primary
position. The additional effects (other than the
primary action) in the primary position are
called subsidiary (secondary and tertiary)
actions. Duane
7–9
proposed that opposed vertical
muscles act as synergists in certain gaze
positions and that their functions vary with
change in direction of gaze.
Based on the old classical concept by Duane,
7–9
the muscle actions may be summarized as below.
Horizontal rectus muscles
The horizontal recti have a common muscle
plane which is horizontal in primary position
and their axis of rotation coincides with the 'Z'
axis of the globe.
When the eye is in primary position, the
horizontal recti are purely horizontal movers
around the vertical 'Z' axis and have only a
primary action. Thus lateral rectus causes
abduction and medial rectus causes adduction.
Vertical rectus muscles
The superior and inferior rectus muscles have a
common muscle plane which is in the same line
as the orbital axis and thus form an angle of 23°
with the optical axis (Fig. 2.6).
Actions of superior rectus
• In the primary position, the primary action of the
superior rectus is elevation (Fig. 2.8A). This
movement occurs about the horizontal 'X' axis.
The secondary action is intorsion (around 'Y' axis)
and tertiary action is adduction (around 'Z' axis).
• When the globe is abducted 23°, the axis of rotation
of superior rectus muscle and optical axis coincide
so that the muscle has no subsidiary actions and
can only act as elevator (Fig. 2.8B). This is, therefore,
the best position of the globe for testing the
function of the superior rectus muscle.
• If the globe could be adducted 67°, the superior
rectus would produce pure incycloduction
(Fig. 2.8C). Since the globe cannot adduct,
therefore, there is some elevating component
to the action of superior rectus even in
adduction.
Actions of inferior rectus
Actions of inferior rectus are analogous to the
superior rectus, i.e.:
• In primary position, its primary action is
depression, secondary action is extorsion and
tertiary action is adduction.
Fig. 2.8 Actions of superior rectus muscle. (A) in primary position; (B) when the globe is abducted 23o; and (C) when the
globe could be adducted 67o.
27Physiology of Ocular Motility
• When the globe is abducted 23°, its only action
is depression.
• If the globe could be adducted 67°, inferior
rectus will produce only extorsion.
Oblique muscles
The oblique muscles are inserted behind the
equator and form an angle of 51° with the optical
axis (Fig. 2.7) and thus have following actions.
Actions of superior oblique
• In primary position, the primary action of
superior oblique is intorsion (Fig. 2.9A), which
occurs about anteroposterior (Y) axis; the
secondary action is depression and the tertiary
action is abduction.
• When the globe is adducted 51°, the axis of muscle
rotation coincides with the optical axis so that
it can only act as a depressor (Fig. 2.9B). This is
the best position of the globe for clinically
testing the action of superior oblique muscle.
• When the eyeball is abducted by 39°, the optical
axis and line of pull of the superior oblique
make on angle of 90° with each other. In this
position, the superior oblique can only cause
intorsion (Fig. 2.9C).
Actions of inferior oblique
These are analogous to the superior oblique as
follows:
• In primary position, main action is extorsion,
secondary action is elevation and tertiary
action is abduction.
• When the globe is adducted 51°, its only action is
elevation.
• When the globe is abducted 39°, its only action is
extorsion.
Contribution of different extraocular muscles in
vertical eye movements
The vertical recti are primary elevators and
depressors, while the obliques are the primary
torsional muscles.
Elevation in primary position and in
abduction is contributed by superior rectus—
60%, inferior oblique—30% and the rest 10% by
the medial and lateral recti. While in adduction
between 10° and 30°, the contribution of
superior rectus, inferior oblique and horizontal
recti is about 40%, 30% and 30%, respectively.
Depression in primary position, in abduction
and up to 10° adduction is contributed by
inferior rectus—80%, superior oblique—10%
and horizontal recti—10%. The corresponding
figures in a position of adduction between 10°
and 30° are inferior rectus—50%, superior
oblique—20% and horizontal recti—30%.
KINEMATICS OF EXTRAOCULAR MUSCLE PULLEYS
The kinematics of extraocular muscle pulleys
play a crucial role in understanding eye
movements and their coordination. The term
"pulleys" refers to the regions around which
the paths of the extraocular muscles are
deviated. These pulleys are thought to be
connective tissue structures that help guide
Fig. 2.9 Actions of superior oblique muscle. (A) in primary position; (B) when the globe is adducted 51°; and (C) when
the globe is abducted 39°.
28 Theory and Practice of Squint and Orthoptics
and stabilize the paths of the muscles as they
move the eye. Understanding the pulleys is
important because they influence the mechanics
of eye movements and impact the control of
gaze.
Rectus muscle pulleys are fundamental to
ocular kinematics. Mechanics of action of extraocular muscles is presently being considered
on the basis of kinematics of muscle pulleys.
Anatomical aspects of system of muscle pulleys
and their connections are described on pages
10–11.
Key points related to the kinematics of
extraocular muscle pulleys are as below:
1. Anatomic variation: Pulleys are not fixed
structures; rather, they can exhibit variations in
position and morphology between individuals.
This anatomical variability is thought to affect
how the extraocular muscles function and
interact during eye movements.
2. Pulley function: The primary function of
pulleys is to provide a stable point around
which the paths of the extraocular muscles can
curve. This helps in maintaining efficient eye
movement mechanics by reducing the angles
of pull and minimizing unnecessary forces that
could result from direct muscle insertions on
the globe.
3. Globe movement and pulleys: During eye
movement, the extraocular muscles act in
concert to move the eyeball. The pulleys act as
points of reference for the muscles, ensuring
that their paths are well-coordinated,
minimizing friction, and optimizing the
distribution of forces.
4. Clinical implications: The understanding of
pulley kinematics is crucial in diagnosing and
treating various eye movement disorders,
including strabismus (misalignment of the eyes)
and other conditions affecting eye movements.
Surgical procedures for such disorders may
involve adjustments to the positions of the
pulleys and extraocular muscle insertions to
restore proper eye movement dynamics.
5. Research and modeling: Researchers use
imaging techniques such as magnetic resonance
imaging (MRI) to study the positions and
movements of pulleys. Computational models
are also developed to simulate the behavior of
extraocular muscles and pulleys during
different types of eye movements.
6. Adaptive changes: In some cases, extraocular
muscle pulleys can exhibit adaptive changes
due to factors such as aging or long-term
changes in eye movement patterns. These
adaptive changes can impact the mechanics of
9a
eye movements and contribute to certain vision
problems.
7. Neural control: The nervous system plays a
role in controlling the movements of the eye
muscles and coordinating their actions. The
brain takes into account the positions of the
pulleys to orchestrate precise and coordinated
eye movements.
Physiological aspects, of kinematics of pulleys
need deliberations on the following:
• Relationship of the pulley to rotational axis of
EOMs.
• Half-angle-rule
• Pulley suspension forces
• Kinematics of individual EOM pulley
Relationship of pulley to
rotational axis of EOMs
• Pulleys of rectus EOM located just behind the
equator, serve as functional origin of the
muscles (Fig. 2.10).
• Rotational axis of all rectus EOMs is
perpendicular to the segment between the
pulley and scleral insertion and thus vertical
in central gaze (Fig. 2.10A).
• Muscle pulley is pulled posteriorly during EOM
contraction by the orbital layer (OL) of the
muscle inserted over it. These muscle pulleys
move in coordination with the insertion and
the sclera.
Half-angle-rule
• Rotation axis of the eyeball shifts equivalent to half
of the angle by which eyeball moves during
duction movement from the primary position.
This has been depicted on dynamic MRI
studies. For example, if in supraduction the
eyeball moves up by angle , the horizontal
axis, around which supraduction occurs,
moves posteriorly by half the angle (i.e. /2)
(Fig. 2.10B).
29Physiology of Ocular Motility
• Half-angle-rule is maintained due to appropriate
location of the rectus muscle pulleys in such a way
that the distance from pulley ring to centre of
globe rotation (L1) is equal to the distance from
center of rotation to the muscle insertion on
the sclera (L2) (Fig. 2.10B).
• Half-angle-rule allows the prerequist for the
Listing’s law, which states that the eyeball can
reach all positions of gaze by rotations around
the axis that lie on the Listing’s plane (vertical
plane passing through the centre of rotation
of eyeball).
Pulley suspension forces
As mentioned above, the muscle pulleys move
along the length of the EOMs by the action of
orbital layer of the muscles. However, the
pulleys are located quite stably and stereotypically in the transverse direction. Stability is
provided by the suspensory forces due to
interconnections of the pulleys to muscle sleeves,
intermuscular septa, pulleys of other muscles
and the periorbita (see Fig. 1.6)
Pulley kinematics of individual EOMs
Active pulley hypothesis (APH) states that the
pulley shifts during the eye movements are
generated by the contractile activity of the orbital
layer of EOMs.
• Figure 2.10C is the diagrammatic depiction of
the muscle pulleys, horizontal recti and the
suspension forces in the central gaze (axial
view)
Fig. 2.10 Schematic depiction of relationship of pulleys to the rotational axis of horizontal rectus muscles: (A) Rotational
axis of medial rectus (MR) muscle is perpendicular to the segment between the pulley and scleral insertion, and is thus
vertical in central gaze; (B) In supraduction by angle , the distance L1 from the pulley (ring) to globe center is equal to
distance L2 from globe center to the insertion, this causes the rotational axis of MR to tilt posteriorly by approximately angle
/2, the half-angle rule to implement Listing's law; (C) Axial view showing pulleys (depicted as spindles) of the horizontal
rectus muscles in central gaze; (D) In adduction, the orbital layer of contracting MR shifts its pulley posteriorly, while the
relaxing lateral rectus (LR) orbital layer allows its pulleys to move anteriorly (modified from Demer II, Invest Ophthalmol
Visc Sci 2004).
30 Theory and Practice of Squint and Orthoptics
• During adduction, the contracting orbital
layer of the MR muscle pulls its pulley
posteriorly, while the relaxing lateral rectus
orbital layer allows its pulley to move
anteriorly (Fig. 2.10D).
Rectus EOMs activity during eye movements is
as below:
• Global layer (GL) contraction leads to
movement of eyeball. The mechanical load of
GL is predominantely the viscosity of relaxing
antagonist EOMs and a load proportional to
the speed of rotation.
• Orbital layer (OL) contraction moves the
pulley posteriorly by the same distance as the
scleral insertion. The mechanical load of OL
is due to the elasticity of the pulley
suspension. This load is independent of
speed of eyeball movement but proportional
to the gaze angle.
• Neural command needed by OL and GL of EOMs
during contraction is different. The ratio of
motor nerve fibre to muscle fibre in GL is low
(about 1:1) for rectus EOMs, reflecting high
precision for ocular rotation.
However, for OL this ratio is higher (1:5 in
horizontal recti and 1:2.5 in vertical recti);
reflecting less precision for pulley control.
Inferior oblique (IO) muscle kinematics are as
below:
• Half-angle-kinematics is also observed by IO
muscle.
• Orbital layer (OL) of IO muscle is inserted on
the pulley of IR and LR, and so the pulley of
IO moves by half of the vertical ocular duction.
• During oblique gaze shift from supraducted
adduction to infraducted abduction, the IO
pulley moves anteroposteriorly by half the
movement of IR pulley.
Superior oblique pulley and its kinematics is as
below:
• Superior oblique pulley is fixed and so does not
move during ocular movements.
• Half-angle-kinematics is also exhibited by SO,
since the distance from trochlea to centre of
globe is approximately equal to the distance
from its insertion to centre of globe; so the
rotational axis of SO muscle shifts by half the
horizontal duction.
FIELD OF ACTION
This term is used in two ways to describe
entirely separate and distinct concepts. Field
of action may be used: (1) to indicate the
direction of rotation of the eye, when a muscle
contracts, and (2) to refer to the gaze position
in which the effect of the muscle is most
readily observed. The field of action for lateral
rectus muscle and medial rectus muscle is same
by both the concepts, i.e. abduction and
adduction, respectively. However, they are not
the same for other muscles, for example, the
inferior oblique muscle creates some vertical,
torsional, and horizontal movements whenever
it contracts. Furthermore, the amount of vertical,
horizontal, and torsional changes depends on
the position of the eye. Thus, a field of action is
not a single unvarying movement for the
inferior oblique muscle. Also, only attempted
elevation of the eye increases inferior oblique
muscle activity; it does not increase with
attempted abduction. (The vertical rectus
muscles also tend to remain at similar levels of
innervation across the horizontal plane.) "Field
of activation" would perhaps be a better term for
this innervation sense of what a muscle does.
The inferior oblique muscle is usually tested by
its contribution to vertical eye movement in the
adducted position; however, this is only because
this is its field of greatest vertical action. Thus,
one must keep in mind three separate things:
(1) the plane of the muscle action, (2) the gaze
direction, which increases or decreases the
innervation to the muscle, and (3) the vector
distribution of the muscle's force (vertical,
horizontal, torsional) in various gaze positions.
The importance of fields of action is that a
deviation (strabismus) that increases with gaze
in some directions is possibly due to weakness
of the muscle normally pulling the eye in that
direction. For example, esotropia increasing
with gaze to the right may be due to right lateral
rectus weakness.
AGONIST, SYNERGISTS, ANTAGONISTS AND YOKE MUSCLES
Agonist
It refers to any particular extraocular muscle
producing a specific ocular movement; e.g. for
31Physiology of Ocular Motility
producing abduction in right eye, the right
lateral rectus muscle is agonist.
Synergists
Two muscles moving an eye in the same
direction are called synergists. For example,
superior rectus and inferior oblique muscles of
the same eye act as synergists for elevation
movement.
Antagonists
These are the muscles having opposite action in
the same eye, e.g. medial and lateral recti.
Further, as we know the superior rectus and
inferior oblique from one eye act as synergistic
elevators; however, in respect to torsion
movement, they act as antagonists, as the
superior rectus produces intorsion while the
inferior oblique produces extorsion.
Each extraocular muscle has two synergists
and two antagonists with exception of medial
and lateral recti which have two synergists and
three antagonists (Table 2.2).
Yoke muscles (contralateral synergists)
It refers to a pair of muscles (one from each eye)
which contract simultaneously during version
Table 2.2 Agonist, synergist and antagonist
extraocular muscles
Agonist Synergists Antagonists
Medial rectus Superior rectus Lateral rectus
Inferior rectus Superior
oblique
Inferior oblique
Lateral rectus Superior oblique Medial rectus
Inferior oblique Superior rectus
Inferior rectus
Superior rectus Inferior oblique Inferior rectus
Medial rectus Superior
oblique
Inferior rectus Superior oblique Superior rectus
Medial rectus Inferior oblique
Superior oblique Inferior rectus Inferior oblique
Lateral rectus Superior rectus
Inferior oblique Superior rectus Superior
oblique
Lateral rectus Inferior rectus
Table 2.3 Yoke muscle pairs
Agonist Synergists Antagonists
Cardinal direction of gaze Yoke muscle pair
Dextroversion Right lateral rectus
Left medial rectus
Levoversion Left lateral rectus
Right medial rectus
Dextroelevation Right superior rectus
Left inferior oblique
Levoelevation Left superior rectus
Right inferior oblique
Dextrodepression Right inferior rectus
Left superior oblique
Levodepression Left inferior rectus
Right superior oblique
movements, e.g. right lateral rectus and left
medial rectus muscles act as yoke muscles for
dextroversion movement. The yoke muscle pairs
for six cardinal positions of gaze are listed in
Table 2.3.
According to recent theories, a pair of muscles
in one eye can be yoked with a pair in the other
eye.10 For example, the elevators of one eye
(superior rectus and inferior oblique muscles),
are yoked as a unit to the elevators of the fellow
eye. Similarly, a pair of depressors of one eye
are yoked with a pair of depressor from the
fellow eye. Further, yoking may change
according to the different types of eye movement;
e.g. left medial rectus is yoked with right lateral
rectus for dextroversion and with the right
medial rectus for convergence.
Contralateral antagonists
(antagonist of the yoke muscles)
This refers to a pair of muscles (one from each
eye) having opposite action; e.g. right lateral
rectus and left lateral rectus muscles. The term
contralateral antagonist is commonly used in
inhibitional palsy. For example, in paralysis of
right lateral rectus muscle, there occurs
inhibitional palsy of the left lateral rectus
muscle. However, this term is contradictory and
is thus not much used in general.
32 Theory and Practice of Squint and Orthoptics
FUNDAMENTAL LAWS GOVERNING OCULAR MOTILITY
Donders’ law
Donders’ law (1848) states that for each tertiary
position, there is one and only one orientation
of the vertical and horizontal meridians of
retina.11 This orientation depends solely upon
the amount of elevation and horizontal
movement and is independent of the path by
which this position was arrived. There is no
rotation around the anteroposterior (Y) axis, i.e.
no torsion or twist occurs. The eye reaches all
tertiary positions without movement around the
AP axis. However, the eye always returns to the
same orientation from which it started.
From the Donders' law, it can be inferred that
only one orientation of the retinal meridians is
permissible with each position of the eyes. This
one and only one orientation does not allow for
the infinite number of other orientations around
the line of sight that could exist, if the freedom
of cyclorotations was unrestricted.
Listing’s law
Listing12 did not add anything essentially new
to the Donders' law; but he elaborated in
considerable details on the geometry and
mathematics of the ocular rotations. In the
analysis of eye movements, the question arises
whether the eye performs a torsional movement
around its anteroposterior axis, when it turns
from the primary position into a tertiary
position. Listing's law implies that this is not the
case. It states that every eye movement from the
primary into tertiary position can be described
as a rotation around one axis. For a given
movement, this axis would be perpendicular to
the plane that contains the line of sight in the
primary position and the line of sight in the
tertiary position into which the eye has moved.
Such an axis would always be in Listing's plane.
Compared to the objective vertical of space, the
vertical meridian of the cornea is tilted, when
the eye is in tertiary position, i.e. instead of true
torsion movement, there occurs pseudotorsion
(defined below). According to Listing's law, this
can be expected, if the eye turns around an
oblique axis in Listing's plane and is not the
consequence of an active torsional movement
around the anteroposterior (Y) axis.
True torsion is a true movement around the
anteroposterior (Y) axis. As said above, true
torsion does not enter into the usual ocular
movements. Pseudotorsion is the deviation of a
plumb line or true vertical and the vertical
meridian of the eye. When we have two
coordinate systems, one fixed (orbital) and one
movable (globe), there is an angle produced,
when the moved system is compared to the fixed
system. Such motion can occur without any true
torsional movement having actually been made.
Hering’s law of equal innervation
This law, also known as Hering’s law of motor
correspondence, states that an equal and
simultaneous innervation flows from the brain
to a pair of muscles of both eyes (yoke muscles)
which contract simultaneously in different
binocular movements. For example, an equal
and simultaneous innervation flows to:
• Left lateral rectus and right medial rectus
muscles during levoversion (Fig. 2.11);
• Both medial recti during convergence; and
• Right superior rectus and left inferior oblique
muscles during dextroelevation.
This law is the major physiologic principle
involved in the understanding of binocular
motor co-operation of the eyes. When this law
was formulated by Hering in 1868, reference was
made only to the voluntary eye movements.
Many authors still write that Hering's law is
valid for voluntary movements. Actually, it
applies to all normal binocular eye movements
Fig. 2.11 Equal (+++) and simultaneous innervations flow
to left lateral rectus and right medial rectus (yoke muscles)
during levoversion.
13
33Physiology of Ocular Motility
including vergences and other involuntary
movements. However, it is not must that both
eyes should make an observable equal movement.
Because, under certain circumstances in spite of
equal innervation to the yoke muscles, the two
eyes may make unequal movements.
Clinical applications of Hering's law
1. Secondary deviation (deviation of normal eye
under cover, when patient fixates with the
squinting eye) is more than the primary
deviation (deviation of squinting eye, when
patient fixates with the normal eye) in patients
with paralytic squint. This is based on the
Hering's law; since when the patient fixates with
the squinting eye, an excess innervation is
required to the paralysed muscle to fixate and
the concomitant excess supply to the yoke
muscle from the normal eye causes excess
contraction leading to more, the so-called,
secondary deviation.
2. Inhibitional palsy of contralateral antagonist
muscle developing in patient with paralytic
squint is also based on Hering's law. For
example, when right lateral rectus muscle is
paralysed, left lateral rectus muscle develops
inhibitional palsy.
Hering's law is also necessary to explain the
condition in superior oblique muscle paresis in
which there is said to be an inhibitional paresis
of the contralateral antagonist, when the paretic
eye is fixating. The term contralateral antagonist,
when used in conjunction with the concept of
inhibitional paresis, is a contradiction in terms.
A more accurate description would be an
inhibitional paresis of the antagonist of the yoke
muscle of the paretic muscle.
For example, if a patient has a right superior
oblique muscle paresis and fixates with the right
eye an object that is located up and to the
patient's left, less innervation of the right inferior
oblique muscle is required to move the eye into
this gaze position, because it does not have to
overcome the normal antagonistic effect of the
right superior oblique muscle. Therefore,
according to Hering's law, less innervation is
received by the right inferior oblique muscle's
yoke muscle, namely the left superior rectus
muscle. This could lead to the incorrect
impression that the left superior rectus muscle
is paretic. This is what is implied by the phrase
"inhibitional paresis of the antagonist (left
superior rectus muscle) of the yoke muscle (left
inferior rectus muscle) of the paretic muscle
(right superior oblique muscle)".
Sherrington's law of reciprocal innervation
This law states that during ocular motility, an
increased flow of innervation to the contracting
agonist muscle is accompanied by a decreased
flow of innervation to the relaxing antagonist
muscle.14 For example, during abduction, an
increased innervational flow to the lateral rectus
is accompanied by a decreased flow to the
medial rectus of the same eye (Fig. 2.12).
With the help of electromyographic studies,
the validity of Sherrington's law of reciprocal
innervation has been established in intact
human eyes (Fig. 2.13). Co-contraction of
antagonistic muscles (instead of relaxation of the
antagonist muscle) occurs in certain pathologic
conditions (e.g. Duane's retraction syndrome
and retraction nystagmus). These conditions
would seem to represent as exceptions to the
Sherrington's law.
Clinical applications of Sherrington's law
• Occurrence of strabismus following paralysis
of an extraocular muscle is explained by
Sherrington’s law of reciprocal innervation.
• Reciprocal innervation must be kept in mind
while performing surgery of extraocular
muscles.
Fig. 2.12 During abduction, an increased innervational
flow (+++) to the lateral rectus muscle is accompanied by
proportionate decreased flow to the medial rectus muscle
(– – –).
34 Theory and Practice of Squint and Orthoptics
Fig. 2.13 Electromyographic (EMG) tracing during adduction movement depicts increased activity from the medial rectus
(MR) and decreased activity from the lateral rectus (LR).
OCULAR MOVEMENTS
MONOCULAR EYE MOVEMENTS (Ductions)
These are called 'ductions' and include the
following:
1. Adduction. An inward movement (medial
rotation) along the vertical axis.
2. Abduction. An outward movement (lateral
rotation) along the vertical axis.
3. Supraduction (sursumduction). An upward
movement (elevation) along the horizontal axis.
4. Infraduction (deosursumduction). A downward
movement (depression) along the horizontal
axis.
5. Incycloduction (intorsion). A rotatory movement
along the anteroposterior axis in which superior
pole of the cornea (12 o'clock point) moves
medially.
6. Excycloduction (extorsion). A rotatory movement
along the anteroposterior axis in which superior
pole of the cornea (12 o'clock point) moves
laterally.
BINOCULAR MOVEMENTS
These are of two types—versions and vergences.
VERSIONS
Versions, also known as conjugate movements,
are synchronous (simultaneous) symmetric
movements of both eyes in the same direction.
Versions may be voluntary or involuntary
movements. Involuntary versions are semireflex
movements occurring in response to optical,
acoustic, or other stimuli.
Classification of versions
According to the direction of binocular movement
1. Dextroversion. In it, both eyes rotate to the right
(Fig. 2.1D). It results due to simultaneous
contraction of right lateral rectus and left medial
rectus muscles.
2. Levoversion. In it, both eyes rotate to the left
(Fig. 2.1F). It is produced by simultaneous
contraction of left lateral rectus and right medial
rectus muscles.
3. Supraversion (sursumversion). In it, both eyes
rotate straight upward (Fig. 2.1B). It results due
to simultaneous contraction of bilateral superior
recti and inferior oblique muscles.
4. Infraversion (deosursumversion). It is straight
downward rotation of both eyes (Fig. 2.1H). It
results due to simultaneous contraction of
bilateral inferior recti and superior obliques
muscles.
5. Dextroelevation. In it, both eyes rotate up and
to the right (Fig. 2.1A). It results from
simultaneous contraction of right superior
rectus and left inferior oblique muscles.
6. Dextrodepression. In it, both eyes rotate down
and to the right (Fig. 2.1G). It is brought about
by simultaneous contraction of right inferior
rectus and left superior oblique muscles.
35Physiology of Ocular Motility
7. Levoelevation. In it, both eyes rotate up and to
the left (Fig. 2.1E). It results from simultaneous
contraction of left superior rectus and right
inferior oblique muscles.
8. Levodepression. In it, both eyes rotate down and
to the left (Fig. 2.1I). It results from simultaneous
contraction of left inferior rectus and right
superior oblique muscles.
9. Dextrocycloversion. It is rotational movement
around the anteroposterior axis, in which
superior pole of both the cornea tilts towards
right. It results from simultaneous contraction
of inferior rectus and inferior oblique muscles
of the right eye and superior rectus and superior
oblique muscles of the left eye.
10. Levocycloversion. It is just the reverse of the
dextrocycloversion.
Depending upon the specific features of the eye
movements (irrespective of the direction of
movement)
The versions include the following movements:
1. Saccadic movements. These are rapid
conjugate eye movements performed to bring
the image of an object quickly on the fovea.
These movements may be voluntary or
involuntary which may occur following a
number of stimuli which may be optical,
acoustic or other. Examples of different types
of saccadic (jumping) or rapid eye movements
are as follows:
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• Command random movements.
• Voluntary refixation saccades.
• Sensory evoked saccades (visual, auditory).
• Nystagmus fast phase: (a) pathological,
(b) induced: optokinetic nystagmus (OKN),
vestibular.
• Rapid eye movement (REM) of sleep.
• Rapid pursuits (above 45°/sec.).
• Microsaccades.
Characteristic features of saccades are as follows:
• The purpose of saccades is to place the image
on the fovea and to keep it there as long as it
attracts attention.
• For production of saccades, alertness is
required.
• During saccadic movements, although the
visual world is rapidly sweeping across the
retina, there is no sense of blurring. This
phenomenon is called 'saccadic omission'.
• Saccades are ballistic movements and proceed
according to the preprogrammed velocity.
Once initiated, they cannot be stopped or
modified during the course of movement.
• There is a long delay of about 200 msec from
stimulus to execution.
• Velocity of saccadic movement ranges from
100°/sec. to 700°/sec.
2. Smooth pursuits or following movements. These
are made when tracking a moving object. Thus
the function of the saccadic eye movements is to
correct the position error between the target and
fovea; and the function of the pursuit system is to
match eye velocity to target velocity. For example,
the pursuits are performed while watching a bird,
which is moving right and left, up and down.
Thus, the pursuits help to keep the image of a
moving object on the fovea constantly.
Characteristic features of pursuits are as follows:
• Images moving away from the fovea constitute
the strongest stimuli for pursuit movements.
• Pursuit movements are elicited after a latency
of 125 msec.
• Smooth following movements can keep up
with targets moving up to 30° to 40°/sec.
Beyond that point, the eyes tend to fall behind
and saccades have to be made to catch up.
• The system has a very limited ability to follow
targets moving back and forth; beyond 2 Hz,
it breaks down.
• Only one image can be tracked normally.
• Usually, pursuits are performed to track an
image of some real object in space. But it can
also be an after image placed on the retina of
some real object in space or a bright light. Also,
a few people are able to track hallucinated
targets, suppressing the saccadic system.
• The effectiveness of pursuit system is
dependent on the degree of alertness.
3. Position maintenance movements. These help
to maintain a specific gaze position by means of
rapid micromovements called 'flicks' and slow
micromovements called 'drifts'.
4. Stabilization movements. These include
dynamic and tonic movements: