Binocular Anomalies.griffin (1)

Contents Foreword | xiii Preface I xv

Part One—Diagnosis 1 Normal Binocular Vision I 3 Valué of Normal Binocular Vision | Anatomy of the Extraocular Muscles Neurology of Eye Movements | 6 Sensory Aspects of Binocular Vision

10

Visual Skills Efficiency I 19 Patient History | 21 Maladaptive Behaviors | 21 Reading Dysfunction | 21 Saccadic Eye Movements | 22 Pursuit Eye Movements | 35 Fixation | 38 Accommodation | 40

Vergences | 49 Sensory Fusión | 57 Recommendations on the Basis of Test Results

65

Heterophoria Case Analysis I 69 Tonic Convergence and Accommodative-Convergence/ Accommodation Ratio | 70 Zone of Clear, Single Binocular Vision | 72 Morgan's Normative Analysis 76 Criteria for Lens and Prism Prescription | 76 Fixation Disparity Analysis | 78 Validity of Diagnostic Criteria | 87 Recommendations for Prism Prescription | 90 Vergence Anomalies 92 Bioengineering Model | 97

Strabismus Testing I 101 History | 101 Measurement of Strabismus | 105 Comitancy | 110 Frequency of the Deviation | 126 Direction of the Deviation 128 Magnitude of the Deviation | 130 Accommodative-Convergence/Accommodation Ratio | Eye Laterality | 132 Eye Dominancy | 133 Variabi I ity of the Deviation | 133 Cosmesis | 133 vii

132

Contents

Sensory Adaptations to Strabismus

135

Suppression | 135 Amblyopia | 143 Anomalous Correspondence ) 166

Diagnosis and Prognosis I 189 Establishing a Diagnosis j 189 Prognosis | 190 Modes of Vision Therapy | 199 Case Examples | 206

Types of Strabismus I 215 Accommodative Esotropía | 215 Infantile Esotropía | 220 Primary Comitant Esotropía | 224 Primary Comitant Exotropia | 225 AandV Patterns | 227 Microtropia | 229 Cyclovertical Deviations | 232 Sensory Strabismus | 233 Consecutive Strabismus I 234

8 Other Oculomotor Disorders I 237 Neurogenic Palsies | 237 Myogenic Palsies | 242 Mechanical Restrictions of Ocular Movement | 245 Internuclear and Supranuclear Disorders | 248 Nystagmus | 252

Part Two—Treatment 9 Philosophies and Principies of Binocular Vision Therapy I 263 Philosophies | 263 Principies | 268

10 Therapy for Amblyopia I 279 Management of Refractive Error | 280 Occlusion Procedures | 281 Monocular FixationTraining | 293 FovealTagTechniques | 300 Pleoptics | 305 Binocular Therapy for Amblyopia | 309 Case Examples | 313

11 Anomalous Correspondence Therapy I 323 Therapy Precautions | 323 Sensory and Motor Therapy Approaches ) 324 Occlusion Procedures | 325 Optical Therapy | 327 Major Amblyoscope | 328 Training in the Open Environment | 336

Contents Exotropia and Anomalous Retinal Correspondence | 340 Surgical Results ¡n Cases of Anomalous Retinal Correspondence | 341 Case Management 342 Case Examples | 343

12 Antisuppression Therapy I 347 Occlusion Antisuppression Therapy | 348 General Approach to Antisuppression Training 348 Specific Antisuppression Techniques | 354 Management Considerations | 363 Case Example 364

13 Vision Therapy for Eso Deviations I 367 Diagnostic Considerations | 368 Vision Therapy Sequence for Comitant Esotropía | 368 Vision Therapy Sequence for Esophoria 373 Specific Training Techniques 373 Case Management and Examples 391

14 Vision Therapy for Exo Deviations I 399 Diagnostic Considerations | 400 Vision Therapy Sequence for Comitant Exotropia | 400 Vision Therapy Sequence for Exophoria 405 Specific Training Techniques 406 Case Management and Examples | 419

15 Management of Noncomitant Deviations, Intractable Diplopia, and Nystagmus I 429 Infantile Noncomitant Deviations | 429 Acquired Noncomitant Deviations | 430 Intractable Diplopia | 435 Congenital Nystagmus | 438 Acquired Nystagmus 443 Case Examples | 444

16 Therapy for Vision Efficiency I 451 Visual Comfort and Performance | 452 Aniseikonia | 452 Monovision | 455 Saccadic Eye Movements 455 Pursuit Eye Movements | 460 Accommodation | 462 Vergences 466 Stereopsis | 469 Case Examples | 470 Future Directions ¡n Binocular Vision Therapy | 473

Pa rt Th ree—Tec h n ¡ q u es 17 Vision Training for Eso Deviations I 489 MirrorStereoscope(T13.2,T14.4) | 490

ix

Contents

Dual Polachrome llluminated Trainer Vectograms and Tranaglyphs for DivergenceTraining at Near (T13.8) | 491 Aperture-RuleTrainer, Double Aperture (T13.13) \ 492 Orthopic Fusión (T13.15) \ 494 Remy Separator (T13.14) | 496 Pencil Push-Aways with Base-ln Prism (T14.13, Pencil Push-Ups and Push-Aways) | 497 Brock String and Beads with Base-ln Prism (T13.6) | 498 BrewsterStereoscope(T13.3,T13.4) | 499 Peripheral Fusión Rings (T13.7) | 500 Televisión Trainers and Base-ln Prisms (T12.8) j 502

18 Vision Training for Exo Deviations I 505 Voluntary Convergence (T14.1) | 506 Pencil Push-Ups and Push-Aways (T14.13, T11.13, T14.1) | 507 Brock String and Beads (T14.5, T13.6) | 509 Three-DotCard(T14.6) | 511 Aperture-Rule Trainer, Single Aperture (TI 4.12) | 512 Vectograms and Tranaglyphs: ConvergenceTraining at Near (T14.9) | 514 Vectograms and Tranaglyphs: Convergence Walk-Aways (T14.10) 517 Vectograms and Tranaglyphs: Projected Base-Out Slides (T14.11) I 518

Chiastopic Fusión—Colored Circles (T14.14) | 519 Chiastopic Fusión—Eccentric Circles (T14.14) | 521 Vergence Rock— Televisión Trainer and Prisms (T14.16) | 522 Vergence Rock—Bar Reader and Prisms (T14.17) \ 523 Vergence Rock—Framing and Prisms (TI 4.18) 525

19 Vision Training for Saccades, Pursuits, and Accommodation I 527 Electronic Fixation Instruments for Saccades (TI 6.12) | 527 Continuous Motion for Saccades (TI 6.6) | 528 Ann Arbor (Michigan) Tracking (T10.7) | 529 Sequential Fixator (T16.10) | 529 Standing Rotator for Pursuits (T16.13) | 531 Marsden Ball (T16.14) | 531 FlashlightChase(T16.17) 532 Hart Chart Near-Far Rock (T16.22) | 533 Plus-and Minus-Lens Rock (T16.23) I 537 OtherTrainingTechniques | 538

20 Sequencing of Techniques and Practice Management I 539 Suggested Sequencing of Tra'm'mgTechniques for Amblyopia | 540 Suggested Sequencing of Training Techniques for Eso Deviations | 540 Suggested Sequencing of Tra'm'mgTechniques for Exo Deviations | 541 Vision Training for Visual Skills Efficiency | 543 Practice Management in Vision Therapy | 543 Comments I 546

Contents

Appendixes | 547 A. Special Commentary: Vision, Learning, and Dyslexia—A Joint Organizational Policy Statement of the American Academy of Optometry and the American Optometric Association | 549 B. Developmental History | 551 C. Strabismus Examination Record | 554 D. Stereoacuity Calculations | 556 E. Conversión of Prism Diopters and Degrees | 557 F. Visual Acuity and Visual Efficiency | 557 G. Visual Skills Efficiency Evaluation (Testing Outline) 558 H. Visual Skills Efficiency Pass-Fail Gritería (Summary from Previous Chapters) | 559 I. Visual Symptoms Survey 561 J. Suppliers and Equipment 562

Self-Assessment Test | 567 Questions | 567 Answers | 579

Glossary | 589 Index I 593

XI

Foreword

By popular demand, and need, comes this new, updated, and expanded fourth edition of Drs. Griffin and Grisham's now classic Binocular Anomalies: Diagnosis and Vision Therapy. In the interven¡ng period since publication of the previous edition (1995), the important and challenging área of abnormal binocular visión has continued to flourish, and even expand, ¡n optometry, with spill-over into other disciplines. For example, visión therapy is beginning to be ¡ntroduced into selected grade school systems using a team approach, including consultant optometrist, teacher, therapist, and parent, with formal legislative support. Furthermore, visión therapy's unique contribution to patients with acquired brain injury ¡n which vergence, accommodative, reading, and visual-spatial dysfunctions abound has only recently been appreciated by many, including the traditional medically based rehabilitation team consisting primarily of physiatrists and occupational therapists. The updated and new topics included in this latest edition are consistent with these ¡deas. For example, the presence of ocular disease may have pronounced adverse effects on the binocular state (e.g., central scotomas ¡n macular degeneration and peripheral scotomas in retinitis pigmentosa). These will reduce disparity drive to the vergence system, resulting ¡n fusional problems and, furthermore, produce reading ¡mpairment due to scanning limitations. In addition, recent research advances ¡n basic aspects of binocular visión clearly demónstrate the subtle but important adverse effects on dynamic motor control (e.g., grasping rapidly for a small or moving object) when binocular function is compromised. This ¡s ¡n addition to the more traditional ¡y cited advantages of hav-

ing normal binocular visión, such as stereopsis and an ¡ncreased field-of-view. Furthermore, an entire section is now devoted to the objective recording and evaluation of reading eye movements using the Visagraph system. This seems most appropriate as the reading demands imposed by society continué to ¡ncrease, in both adults and children. Finally, the addition of a self-assessment test should serve to test one's newly acquired knowledge with practica! clinical examples, including full answers, for immediate feedback and reinforcement. In addition to these new components, the traditional áreas of optometric visión therapy are presented ¡n detall with abundant graphics and case examples, both with respect to diagnosis and therapeutic aspects. To the best of my knowledge, this ¡s the most complete and encyclopedic treatise on visión therapy ever produced, incorporating an appropriate blending of clinical knowledge and training protocol with substantial theoretical bases to satisfy one's ¡ntellectual cravings. Henee, once again, Drs. Griffin and Grisham share selflessiy their years of clinical experience and devotion to optometric visión therapy. The result is an outstanding book that should guide students and practitioners alike in their quest for a greater understanding of binocular anomalies and their more effective and efficacious treatment. Kenneth J. Ciuffreda, O.D., Ph.D. DistinguishedTeaching Professor and Chairman Department of Vision Sciences State College of Optometry State University of New York New York

XIII

Preface

This fourth edition of Binocular Anomalies: Diagnosis andVision Therapy follows the format and philosophy of the third edition. When the exact diagnosis of a binocular anomaly is known, exact visión therapy can be prescribed. Part One covers diagnosis, and Part Two is on treatment We have updated many of the topics, because there have been important advances in diagnostic procedures and training techniques. Deleting some of the oíd material has been necessary to allow room for discussion of new methods for diagnosis and treatment. We take the accepted view that visión therapy encompasses all modes of treatment of binocular visión problems. Besides visión training, we include the use of lenses and prisms, occlusion, pharmaceutical treatment, motivational methods, and extraocular muscle surgery when necessary. Vision therapy of binocular anomalies is for treatment of strabismus, heterophoria, amblyopia, and dysfunctions affecting educational, vocational, and avocational performance. As ¡n the previous edition, each visión training technique (active visión therapy) ¡s identified by a "T" number for easy identificaron and referencing. Diagnostic methods are referred to as procedures so that confusión can be avoided between methods of testing (procedures) and training (techniques). Although these techniques are discussed thoroughly in general terms, we thought ¡t would be helpful to include specific, detailed discussions ¡n a "how-to" format, similar to the teaching method of a preclinical laboratory for students and practitioners (especially primary-care clinicians) and other professionals and therapists wishing to review and learn new techniques. Part Three presents such detailed instructions, including illustrations, on the most frequently used vision-training techniques. These instructions are applicable to clinicians as well as to parents and patients for home training. Also ¡ncluded ¡n Part Three are recommended sequenc¡ng of techniques for specific binocular anomalies and practice management principies. A self-assessment test of 100 multiple-choice questions and explanatory answers ¡s ¡ncluded. This

addition ¡s by popular request of students and practitioners. All questions follow the exact chapter-bychapter sequence of topics presented ¡n the text. Particularly updated topics include diseases affecting binocular visión, binocular anomalies and reading dysfunction, advantages of good binocular visión, and pharmacologic treatment. Additional case examples are included to ¡Ilústrate the ¡mplementation of visión therapy and to help the clinician connect theoretical principies with specific visión therapies. A CD-ROM is included ¡n which popular training techniques are discussed; these techniques can be downloaded and modified according to the wishes of each practitioner. We have attempted to be semantically consistent with terminology so that the reader can sean through various chapters without ambiguity and always know what specified terms mean. For example, we speak of viewing at far (e.g., 20 ft or 6 m) rather than using distance, which many clinicians use in their customary discourse. (One could wonder if distance is referring to far distance, intermedíate distance, or near distance.) In addition, we have set eso and exo (and the like) apart as single words when combined with fixation disparity and deviation. We have also ¡ncluded older terms ¡n parentheses when a term ¡s ¡ntroduced. For ¡nstance, visuoscopy was once spelled as visuscopy; we provide the reader with both terms initially and retain the more accepted term throughout the book. Our ¡ntention ¡s to enhance the readability of the text. We thank the following individuáis for their help in making this new edition possible: Karen Oberheim, Judy Higgins, Judy Badstuebner, Ronda Barton, L. Ernie Carrillo, Dr. James Saladin, Dr. James Bailey, Dr. Walter Chase, Kirsten Griffin, R.N., Dr. William Ridder, Dr. Lawrence Stark, Kim Vu, David West, Donnajean Matthews, Denise Hess, Doreen Keough, Pam Bickel, Lois Keup, and Holly Hoe. J. David Grisham Berkeley

John R. Griffin Fullerton xv

chapter 1 / Normal Binocular Vision

Valué of Normal Binocular Vision 3 Anatomy of the Extraocular Muscles 5 Neurology of Eye Movements 6 Accommodation 6 Conjúgate Gaze Movements 6 Saccades 7 Vestíbulo-Ocular Eye Movements 7 Pursuits 8 Vergences 8 Sensory Aspeéis of Binocular Vision 10 Monocular Considerations 11

Binocular visión pertains to the motor coordination of the eyes and the sensory unification of their respective views of the world. This is a unitary process but, for the sake of analysis, ¡t can be broken into sensory and motor components. The sensory side starts with light emitted or reflected from physical objects ¡n the externa! environment that is brought into focus on the retina by each eye's optics. This pattern of light energy ¡s transformed by retinal photoreceptors into neuroelectrical impulses and is transmitted to the visual perceptual áreas of the cerebral cortex and certain subcortical áreas. The result of complex neural processing, which is only partially understood, is the sensation of object attributes (i.e., form, color, intensity, and position in space) that, ¡n turn, culminates ¡n an immediate, vivid perception of object identity and of the relations of objects in the external environment. The motor positioning and alignment of the eyes completely subserve the primary sensory function of image unification and allow visual perception to proceed efficiently. The task of the

Retina! Correspondence 11 Panum's Fusiona! Áreas 11 Singleness Horopter 11 Physiologic Dipiopia 12 Pathologic Dipiopia 13 Types of Sensory Fusión 13 Color Fusión 13 Form Fusión 13 Theories of Sensory Fusión 15 Binocularly Driven Cells and Ocular Dominance 16

motor system is to direct the alignment of both foveas (foveae) to the object of attention with i n the visual field and to maintain them ¡n that position as long as the individual requires. The motor system holds the eyes in alignment and sustains clear focus, thereby ensuring the maintenance of binocular visión. Frequently, however, the complete remediation of binocular visión anomalies requires attention to both sensory and motor aspects.

VALUÉ OF NORMAL BINOCULAR VISION One distinctive perceptual attribute of humans, among all primates, ¡s a high degree of stereoscopic binocular visión. Our skills in hunting, food gathering, and tool making have helped to direct our evolution. In the competition for food, shelter, and safety, stereopsis is one of several attributes that evidently provided ¡mportant advantages to those who possessed ¡t. In the mod-

4 Chapter1

Monocular Temporal Crescent

Monocular Temporal Crescent

OD

FIGURE 1-1—Extent of binocular visual field showing monocular temporal crescents. (OD = oculus dexter [right eye]; OS = oculus siníster [left eye].) '

ern age, stereoscopic visión continúes to provide individuáis with important information about their environment. Stereopsis significantly aids in making judgments of depth, whether at school, the workplace, or the sports field. It also helps to stabilize sensory and motor fusión and can be considered a "barometer" of the status of binocular visión. Besides stereopsis, there are other benefits that derive from normal binocular visión. The most obvious benefit of having two eyes is that, ín case of injury to one, there is an eye in reserve. This might be called the "spare tire" concept. Whereas the loss of sight in one eye can cause some significant problems for an individual, the loss of sight in both eyes can be devastating. The binocular individual also has the advantage of a large field of visión (Figure 1-1). The binocular field of visión usually is at least 30 degrees larger than the monocular field. Binocular visual acuity normally is better by approximately one-half line of letters on a Snellen chart, as compared with either eye alone. 1'2 The

difference is even greater when uncorrected ametropia is present in each eye. Binocular visión, in contrast to monocular visión, minimizes the effects of ocular disease. Binocular summation of ocular images significantly heightens contrast sensitivity, by approximately 40%. 3 In practical terms, this is helpful for driving at night and working under low-illumination conditions. Individuáis with certain ocular diseases (e.g., optic nerve demyelination in múltiple sclerosis) may demónstrate profound differences in contrast sensitivity between binocular and monocular sight. There are several vocational and avocational performance benefits of having good binocularity. Sheedy et al.4 described superior task performance under binocular versus monocular viewing conditions (Table 1-1). Differences favoring binocular viewing were notable ¡n such tasks as card filing, needle threading and, surprisingly speed of word decoding. No significant difference was noted, however, in letter counting on a video display terminal or in throwing beanbags accurately. Tríese inves-

Chapter1

TABLE1-1. Superíority of Task Performance under Binocular Conditíons as Compared with Monocular Conditions

Task

Percentage of Impr oyerpent of Scores under Binocular Conditíons

Puttíng sticks in holes Needle threading Card fíling Placing pegs in grooves Reading (word decoding) Letter counting on vídeo dfsplay terminal Beanbag tossing

30 20 9 4 4 2 -1

Stgníf íeañee (Studerrt*s fr-test)

Q.Ü01 0.05 NS

' NS -

NS = not signiflcant. Source: Adapted from }í Sheedy 1L Baitey, M Muri, E Bass. Binocular vs, monocular task performance. Am } Optom Physlol Qpt. 1986;63(10):839-846.

tigators concluded that stereopsis provides a performance advantage for many different Jobs, par-ticularly those requiring nearpoint eye-hand coordi-nation. Persons in several occupations (e.g., pilots, microsurgeons, cartographers) are aided by stereopsis in performing their tasks safely and efficiently. Strabismus affects only a small percentage of the population (1.3-5.4%),5 but other deficiencies of binocular visión, such as convergence insuffi-ciency and accommodative infacility, are much more prevalent and may result in bothersome symptoms and inefficient performance. Except for those individuáis who have acquired strabismus and experience persistent double visión, most constant strabismics report few extraordinary visual symptoms. On the other hand, many nonstrabismics with binocular visión dysfunctions experience a variety of anomalies that are visual in origin, such as ¡ntermittent blur at far or near, tired eyes after reading or viewing a computer monitor, "eye-strain" at day's end, the appearance of jumping or moving print, vision-related headaches, reduced depth perception, and mild photophobia. Many of these symptomatic individuáis experience "binocular efficiency dysfunction" (see Chapter 2).

ANATOMY OF THE EXTRAOCULAR MUSCLES Three pairs of extraocular muscles control the movements of each eye: a pair of horizontal rectus muscles, a pair of vertical rectus muscles, and a pair of oblique muscles. The rectus muscles, the superior oblique muscle, and the leva-tor muscle (controlling the upper eyelid) are attached to the bones at the back of the orbit by a tendinous ring (the annulus of Zinn) that sur-rounds the optic foramen and part of the superior orbital fissure. The four rectus muscles, optic nerve, ophthalmic artery, cranial nerve VI, and two branches of cranial nerve III form a muscle cone (Figure 1-2). The insertions of the rectus muscles are not equidistant from the corneal limbus but form a spiral, known as the spiral of Tillaux, with the superior rectus inserting farthest away from the limbus (7.7 mm) and the medial rectus inserting nearest to the limbus (5.5 mm) (Figure 1-3). The more advanced the ¡nsertion, the greater the mechanical advantage of the muscle (e.g., the medial rectus as compared with the superior rectus). As with the rectus muscles, the superior oblique muscle originates from the annulus of Zinn, but it courses along the superior medial wall of the orbit to the trochlea, a U-shaped fibrocartilage, that acts as a pulley. Near the trochlea, the muscle tissue becomes a tendón as it passes through the trochlea and then reflects back normally at an angle of approximately 51 degrees to the medial wall. The muscle then crosses the globe superiorly, passing under the superior rectus, to insert ¡n the posterior, superior quad-rant near the vortex veins. The trochlea, there-

5

Chapter1

Superior Rectus

Levator Superior Palpebrae^ obligue

A

LR

?

Oculomotor Foramen

SR

=

:

Inferior Rectus Pulley Inferior Oblique

IR -12mm

• - 24mm

FIGURE 1 -2—Lateral view of muscles of trie right eye.

fore, becomes the effective mechan ¡cal origin for the action of the superior oblique (Figure 1-4). The inferior oblique is the only extraocular muscle that does not origínate ¡n the orbital apex; ¡t arises from a small fossa in the anterior, inferior, orbital wall (the maxilla bone). This muscle's course parallels the reflective portion of the superior oblique muscle, again forming a 51degree angle as it courses inferiorly and laterally across the globe and over the inferior rectus to insert in the inferior, posterior quadrant. Evidence from magnetic resonance imaging studies of the orbit indícate that all rectus muscles pass through pulleys, structures composed of connective tissue and smooth muscle, that are coupled to the orbital wall and located just behind the equator of the globe.6"8 In effect, these pulleys ("sleeves")— rather than the attachments of these muscles at the annulus of Zinn in the back of the orbit—act as the origin for the action of the rectus muscles. In most people, the location of these pulleys ¡s remarkably consistent and does not shift much with rotation of the globe into the various fields of gaze. Many strabismic individuáis have been found to have normal pulleys, although some do not. Abnormal location

NEUROLOGY OF EYE MOVEMENTS The neurology of the following systems are discussed briefly: accommodation, conjúgate gaze movements, and vergence.

of rectus pulleys has been implicated as a cause of noncomitant strabismus.

Accommodation Accommodation ¡s one member of the oculomotor triad that also includes pupillary constriction and accommodative convergence, all mediated by the third nerve nucleus ¡n the midbrain. Accommodation is a reflex initiated by retinal blur; ¡t can, however, be consciously controlled. The afferent pathway extends from the retina to the visual cortex and projects from área 19 to the pretectum and superior colliculus before entering the Edinger-Westphal nucleus of the third nerve complex. Projections from the frontal eye fields (traditionally referred to as Brodmann's área 8) also enter the third nerve complex that, in part, mediates conscious control of accommodation. The efferent component of the reflex are from the third nerve complex synapses ¡n the ciliary ganglion and again ¡n the ciliary muscle which, in turn, effectuates the change of lens power (Figure 1-5).

Conjúgate Gaze Movements Conjúgate eye movements are tándem movements of the two eyes, known as versions. These are saccades, vestíbulo-ocular movements, or pursuits. These three eye movement systems share a common final pathway to the extraocular muscles, but

Chapter1

51 <

Medial Wall

FIGURE 1-4—Relation between the superior oblique muscle and the superior rectus muscle. (Note: Both the inferior and superior oblique muscles form a 51-degree angle with the medial wall, and both the inferior and superior rectus form a 23-degree angle with the medial wall. The action fields for clinical purposes are approximately 50 and 25 degrees for the oblique and vertical recti, respectively.)

they are neurologically distinct, with different central pathways and dynamic properties.

Saccades Saccadic eye movements refer to ballistic-type eye movements that carry the eye quickly from one target in space to another (i.e., a change in fixation). There are several types of saccades: (1)the fast phases of either vestibular or optokinetic nystagmus; (2) spontaneous saccades occurring approximately 20 times per minute and used to sean the environment; (3) reflexive (nonvolitional) saccades that occur in response to any new environmental stimulus; and (4) intentional saccades

that carry the eyes from one target to another predetermined target.9 The anatomy subserving voluntary saccades has been partly established by monkey studies and clinical observation in humans. For example, if there ¡s an ¡ntention for dextroversion (eye movement to the right), stimulation occurs ¡n Brodmann's área 8 (frontal eye field) in the frontal lobe of the left hemisphere. Impulses then travel to the right pontine gaze center and are forwarded to the ipsilateral nucleus of cranial nerve VI. Subsequently, the lateral rectus muscle of the right eye contracts. Simultaneously, impulses travel from the ipsilateral pontine gaze center up through the medial longitudinal fasciculus that decussates to the left third nerve nucleus. That results in contraction of the medial rectus of the left eye (Figure 1-6). Because yoked muscles have equal ¡nnervation (Hering's law), 10 the two eyes move ¡n tándem. Versions are not restricted because of the simultaneous relaxation of the antagonistic yoked muscles (Sherrington's law of reciproca! ¡nnervation)10 (Figure 1-7). Vestíbulo-Ocular Eye Movements The vestíbulo-ocular system stabilizes the eyes on a target during head movements and can be tested with the "doll's-head" maneuver. The dynamics of vestibular eye movements are relatively fast, having a latency of only 16 milliseconds as compared with the 75-millisecond latency of the pursuit system.11 As the head turns, vestíbulo-ocular reflexes are initíated by the movement of fluid wíthin the semicircular cañáis of the inner ear. For example, stimulation of the left vestibular nucleus causes impulses to travel to the right pontine gaze center. From there, the pathway to the extraocular muscles is the same as that described for saccadic eye

A F F E R E N T LGN

Retinal image blur

Lens of eye -2.5GD*

2.25 D 1.7S-2.00D

1.50D 18 14-18 10-13 6-9

Binocular* >10 8-10 6-7 4-5

18

>26

>18

14-18 12-13 11