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Figure 1.
Schematic diagram depicting corticaland optokinetic pathways. Cortical input to temporally directed movement,which is present only in frontal-eyed animals, requires the establishmentof normal binocular cortical connections. This input is absent in humans withcongenital strabismus. Direct crossed pathways from the eye to the nucleusof the optic tract provide nasalward subcortical optokinetic responses evenwhen binocular cortical connections are absent (R and L represent monocularcortical cells corresponding to the right and left eyes, respectively). Notethat the nucleus of the optic tract (NOT) relays horizontal visuo-vestibularinformation to the vestibular nucleus (VN), where it is integrated with horizontalvestibular input from the labyrinths to establish horizontal extraocular muscletonus. LGN indicates lateral geniculate nucleus; CC, corpus callosum; V1,abducens nucleus; III, oculomotor nucleus; LR, lateral rectus muscle; MR,medial rectus muscle; AC, anterior canal; PC, posterior canal; and HC, horizontalcanal.

Schematic diagram depicting corticaland optokinetic pathways. Cortical input to temporally directed movement,which is present only in frontal-eyed animals, requires the establishmentof normal binocular cortical connections. This input is absent in humans withcongenital strabismus. Direct crossed pathways from the eye to the nucleusof the optic tract provide nasalward subcortical optokinetic responses evenwhen binocular cortical connections are absent (R and L represent monocularcortical cells corresponding to the right and left eyes, respectively). Notethat the nucleus of the optic tract (NOT) relays horizontal visuo-vestibularinformation to the vestibular nucleus (VN), where it is integrated with horizontalvestibular input from the labyrinths to establish horizontal extraocular muscletonus. LGN indicates lateral geniculate nucleus; CC, corpus callosum; V1,abducens nucleus; III, oculomotor nucleus; LR, lateral rectus muscle; MR,medial rectus muscle; AC, anterior canal; PC, posterior canal; and HC, horizontalcanal.

Figure 2.
Visual and vestibular interactionin latent nystagmus. Latent nystagmus decreases with spinning toward the fixatingeye and increases with spinning toward the occluded eye. O represents directionof ocular (visuo-vestibular) tonus; V, direction of horizontal vestibulartonus. Both O and V correspond to the slow phase of the induced nystagmus.++ Indicates stimulated horizontal semicircular canal; −−, inhibitedhorizontal semicircular canal; A, Occlusion of the left eye increases visuo-vestibulartonus to the left. B, The patient with latent nystagmus is spun to the rightto stimulate the right horizontal semicircular canal, which increases leftwardhorizontal vestibular tonus and causes a slow conjugate drift of both eyesto the left. At this point, the latent nystagmus would be enhanced by vestibularinput (if the examiner could observe it). C, When the spinning is suddenlystopped, the opposite vestibular stimulus is exerted, causing the left semicircularcanal to drive the eyes to the right. This rightward vestibular tonus imbalancenullifies the leftward visual tonus imbalance induced by monocular fixationwith the right eye, thereby reducing the intensity of the latent nystagmus.D, When the occluder is quickly switched to the right eye, the visual tonusimbalance is augmented by an ipsidirectional visual tonus imbalance, increasingthe intensity of the latent nystagmus.

Visual and vestibular interactionin latent nystagmus. Latent nystagmus decreases with spinning toward the fixatingeye and increases with spinning toward the occluded eye. O represents directionof ocular (visuo-vestibular) tonus; V, direction of horizontal vestibulartonus. Both O and V correspond to the slow phase of the induced nystagmus.++ Indicates stimulated horizontal semicircular canal; −−, inhibitedhorizontal semicircular canal; A, Occlusion of the left eye increases visuo-vestibulartonus to the left. B, The patient with latent nystagmus is spun to the rightto stimulate the right horizontal semicircular canal, which increases leftwardhorizontal vestibular tonus and causes a slow conjugate drift of both eyesto the left. At this point, the latent nystagmus would be enhanced by vestibularinput (if the examiner could observe it). C, When the spinning is suddenlystopped, the opposite vestibular stimulus is exerted, causing the left semicircularcanal to drive the eyes to the right. This rightward vestibular tonus imbalancenullifies the leftward visual tonus imbalance induced by monocular fixationwith the right eye, thereby reducing the intensity of the latent nystagmus.D, When the occluder is quickly switched to the right eye, the visual tonusimbalance is augmented by an ipsidirectional visual tonus imbalance, increasingthe intensity of the latent nystagmus.

Table 1. 
Peripheral Vestibular Nystagmus vs Latent Nystagmus
Peripheral Vestibular Nystagmus vs Latent Nystagmus
Table 2. 
Experimental Evidence That Latent Nystagmus Is Vestibularin Origin
Experimental Evidence That Latent Nystagmus Is Vestibularin Origin
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Clinical Sciences
February 2004

Latent NystagmusVestibular Nystagmus With a Twist

Author Affiliations

From the Departments of Ophthalmology and Pediatrics, University ofArkansas for Medical Sciences, Little Rock (Dr Brodsky), and the Departmentsof Neurology and Otolaryngology, Emory University, Atlanta, Georgia (Dr Tusa).The authors have no relevant financial interest in this article.

Arch Ophthalmol. 2004;122(2):202-209. doi:10.1001/archopht.122.2.202
Abstract

Background  Latent nystagmus is a horizontal binocular oscillation that is evokedby unequal visual input to the 2 eyes. It develops primarily in humans withcongenital esotropia.

Objective  To investigate the interrelationship between latent and peripheral vestibularnystagmus and their corollary neuroanatomical pathways.

Methods  Examination of subcortical neuroanatomical pathways producing latentnystagmus and review of the neurophysiological mechanisms by which they becomeactivated in congenital esotropia.

Results  The vestibular nucleus presides over motion input from the eyes andlabyrinths. Latent nystagmus corresponds to the optokinetic component of ocularrotation that is driven monocularly by nasal optic flow during a turning movementof the body in lateral-eyed animals. Congenital esotropia alters visual pathwaydevelopment from the visual cortex to subcortical centers that project tothe vestibular nucleus, allowing this primitive subcortical motion detectionsystem to generate latent nystagmus under conditions of monocular fixation.

Conclusions  Latent nystagmus is the ocular counterpart of peripheral vestibularnystagmus. Its clinical expression in humans proclaims the evolutionary functionof the eyes as sensory balance organs.

Vestibular disease holds little interest for the ophthalmologist. Althoughpatients with vestibular disease can develop nystagmus, diplopia, and oscillopsia,these symptoms can be treated empirically. Peripheral vestibular disease iscaused by injury to the labyrinth rather than to the eye, whereas centralvestibular disease is caused by brainstem or cerebellar disorders involvingthe central vestibular pathways along their course to the ocular motor nuclei.1,2 But the ophthalmologist encountersa unique form of vestibular nystagmus that is caused by unbalanced input fromthe two eyes rather than from the two labyrinths. This visuo-vestibular nystagmusis known as "latent nystagmus."

Congenital esotropia is associated with a clinical triad of latent nystagmus,inferior oblique muscle overaction, and dissociated vertical divergence.3 These unique eye movements conform to primitive vision-dependenttonus mechanisms that are reactivated by congenital strabismus or early abnormalvisual experience.47 Evolutionaryanalogues of primary oblique muscle overaction and dissociated vertical divergencehave been identified in lower vertebrates.5,6 Infish, these are physiologic extraocular movements that use weighted binocularvisual input to modulate extraocular muscle tonus and to maintain visual orientationduring body movements.5,6 Thestimulus for bilateral inferior oblique muscle overaction corresponds to avisuo-vestibular imbalance in the sagittal (pitch) plane, while dissociatedvertical divergence corresponds to a similar imbalance in the coronal (roll)plane.57 We proposethat latent nystagmus results from a similar visuo-vestibular tonus imbalancein the horizontal turning (yaw) plane.

WHAT IS LATENT NYSTAGMUS?

Latent nystagmus is a binocular horizontal oscillation that becomesapparent when 1 eye is covered. First described by Faucon in 1872,8 latent nystagmus develops when congenital esotropiaprecludes frontal binocular vision early in infancy.913 Inthis setting, a conjugate horizontal jerk nystagmus can be induced by covering1 eye, blurring 1 eye, or reducing image brightness in 1 eye.1,10,14 Inlatent nystagmus, the slow-phase rotation of the fixating eye is directedtoward the nose and the fast-phase rotation of the fixating eye is directedtoward the ear.1,2,14 Assuch, fixation with the right eye generates a right-beating nystagmus, whilefixation with the left eye produces a left-beating nystagmus.10 Inchildren with congenital esotropia and alternating fixation, the directionof nystagmus will spontaneously reverse when fixation is switched from oneeye to the other.1015 Evenafter the eyes have been surgically realigned, occlusion of either eye willcontinue to induce latent nystagmus. The intensity of latent nystagmus ismaximal in abduction and minimal in adduction, causing some patients to maintaina head turn to place the fixating eye in an adducted position. The intensityof latent nystagmus decreases when visual attention declines and increasesduring attempted fixation.1322 Infact, some patients can reverse the direction of their latent nystagmus bylooking at an imagined target and mentally switching fixation from one eyeto the other.16,22

In children with latent nystagmus, the development of amblyopia or therecurrence of ocular misalignment can disrupt binocular vision and make alatent nystagmus become manifest.23 The magnitudeof the resulting manifest latent nystagmus is proportional to the degree ofthe interocular visual disparity.1 Most patientswith clinical latent nystagmus actually have a small spontaneous jerk nystagmusthat can be measured with both eyes open using eye movement recording.14 However, successful treatment of amblyopia or strabismuscan convert a manifest latent nystagmus to a clinical latent nystagmus.23 Manifest latent nystagmus has also been reportedin children with unilaterally reduced vision and sensory esotropia resultingfrom congenital disorders such as cataract or optic nerve hypoplasia.911 In this setting, achild will often maintain a head turn to position the fixating eye in adduction.911

Various theories have been advanced to explain latent nystagmus.24 These include a primitive tonus imbalance,1 an egocentric disorder,14 adisorder of the subcortical optokinetic system,21 asubcortical maldevelopment of retinal slip control,22 abnormalcortical motion processing,25,26 adisorder of proprioception,27 and an evolutionarypreponderance of the nasal half of the retina.3,28 Thesedisparate theories can be reconciled by considering the critical evolutionaryfunction of the eyes as sensory balance organs.

NASOTEMPORAL ASYMMETRY AND LATENT NYSTAGMUS

Latent nystagmus is associated with nasotemporal asymmetry of the horizontaloptokinetic response during monocular viewing.25,26 However,not all patients with nasotemporal asymmetry have latent nystagmus.29 In patients with nasotemporal asymmetry, the monocularoptokinetic responses to nasally moving targets are brisk, while those totemporally moving targets are poor in each eye. This "nasalward" movementbias under monocular viewing conditions corresponds both in direction andin waveform to the nasalward slow-phase drift of the fixating eye in latentnystagmus.21,29 To our knowledge,Roelofs30 first observed horizontal optokineticasymmetry in patients with latent nystagmus in 1928. Fifty years later, experimentsby van Hof–van Duin31 and Wood et al32 suggested that reduced binocularity in strabismuscan lead to nasotemporal asymmetry. In 1977, Kommerell33 suggestedthat latent nystagmus could be regarded as the consequence of horizontal optokineticasymmetry. In 1982, Hoffman34 developed a modelto explain nasotemporal asymmetry based on combined cortical and subcorticalinput to the nucleus of the optic tract in the cat. In 1983, Schor21 proposed that latent nystagmus and nasotemporal optokineticasymmetry are mediated by the nucleus of the optic tract.

Human nasotemporal asymmetry has received considerable attention becauseit persists throughout life in humans with congenital strabismus.15,21,25,3436 Evenafter surgical realignment, nasotemporal asymmetry remains as a "footprintin the snow" of abnormal visual development.36 Nasotemporalasymmetry is seen in rabbits, kittens, monkey infants, and human infants withinthe first 6 months of life.36 The evolutionaryretention of this primitive nasotemporal asymmetry in human infancy showshow ontogeny recapitulates phylogeny during human visual development.37,38

In ordinary life, large parts of the visual field move together duringself-motion.39 Optic flow occurs during translation(which is signaled by the otoliths and linear optic flow) and rotation (whichis signaled by the semicircular canals and rotational optic flow).39 The low sensitivity to nasal to temporal optic flowin afoveate, lateral-eyed animals is commonly assigned the function of preventingthe locomoting animal from responding to the image motion of stationary contoursduring forward motion, while permitting full compensation for rotational inputduring turning movements.3941 Theabsence of nasotemporal optokinetic responses in lateral-eyed animals assuresthat during forward movements, ineffective temporalward eye movements do notdestabilize images of objects that are directly ahead of the animal.3941 The optokinetic responsesof both eyes are controlled by whichever eye is stimulated by temporal-to-nasalmovement of the visual world.40 Latent nystagmusrecapitulates this monocularly driven horizontal optokinetic movement.

IS LATENT NYSTAGMUS A VESTIBULAR NYSTAGMUS?

Vestibular eye movements are reflex contraversive rotations of the eyesthat occur during involuntary head movements, acting to stabilize the positionof the eyes in space and thereby maintain visual orientation.1,2,42,43 Accordingto Walls, " . . . vestibularly-controlled reflex eye movements are historicallythe oldest of all, with all other kinds of eye-muscle controls and operationsaccreted to them above the primitive fish level of evolution."44(p71) During head movements, input to the semicircular canals withinthe 2 labyrinths provides the afferent stimulus for the vestibulo-ocular reflex.1,2,43,45 Thesemicircular canals respond to angular acceleration and produce dynamic vestibulo-oculareye movements. Damage to a horizontal semicircular canal pathway producesa nystagmus in the plane of the injured canal.43,45 Inlateral- and frontal-eyed animals, the geometry of the semicircular canalsconforms closely with the orientation of the extraocular muscles.46 When the head is rotated in a particular plane, asemicircular canal within the labyrinth detects acceleration and sends excitatoryinnervation to the corresponding extraocular muscles. Within the brainstemand cerebellum, peripheral vestibular input is summated to produce innervationto the appropriate extraocular muscle subnuclei and to maintain the positionof the eyes in space. Each horizontal semicircular canal provides excitatoryinput to the ipsilateral medial rectus muscle and the contralateral lateralrectus muscle.1,2,43,45

Visual stabilization mechanisms act in concert with labyrinthine reflexes.43 In normal life, optokinetic responses are elicitedmainly by head movements, which also stimulate the vestibular system.39 Because vestibular neurons receive such prominentvisual and vestibular inputs, disrupting either input reduces the tonic activationof these neurons, with the effect of disturbing the responses to the othersensory modality.39 Thus, labyrinthectomy eliminatesoptokinetic nystagmus in rabbits,47,48 whereasblocking optic nerve activity with tetrodotoxin reduces the gain of the vestibulo-ocularreflex.39,49 According to Miles,each sensory modality "has played such a major role in the evolution of theother that it is impossible to understand the operation of either one in isolation."41(p393) A confluence of neuroanatomical,clinical, evolutionary, and experimental evidence has led us to conclude thatlatent nystagmus is a vestibular nystagmus that is brought about by unequalvisual input from the 2 eyes rather than from the 2 ears (ie, a visuo-vestibularnystagmus). The evidence that latent nystagmus arises when the 2 eyes revertto their primitive function as balance organs can be summarized as follows.

Neuroanatomy of Latent Nystagmus

Studies in subhuman primates have shown that latent nystagmus arisesas a result of incomplete development of visual input from occipitotemporalcortex to subcortical vestibular pathways.50,51 Inmonkeys with latent nystagmus, there is a loss of binocularity in the nucleusof the optic tract (NOT), the subcortical structure that feeds into the vestibularsystem, with most cells driven by the contralateral eye.50,51 Theareas that normally provide binocular input to the NOT are the middle temporal(MT) visual area and the medial superior temporal (MST) visual area in occipitotemporalcortex. When strabismus is surgically induced in infant monkeys during thefirst 2 weeks of life, these monkeys also develop latent nystagmus and visualarea MT/MST loses binocularity. If either eye is covered during infancy, visualarea MT/MST and NOT develop normal binocularity, but the striate cortex stillshows loss of binocularity and these monkeys do not develop latent nystagmus.52 This finding suggests that the initial cause of latentnystagmus is loss of binocularity in visual area MT/MST from the misalignedeye during the first few weeks of life.52

Neuroanatomical experiments have confirmed the Schor hypothesis21 that the NOT is the generator of latent nystagmus.21,34,5052 Alatent nystagmus occurs in monkeys following artificial induction of esotropiawithin the first 2 weeks of life.53 Unilateralelectrical stimulation of the NOT in binocularly deprived monkeys inducesa conjugate nystagmus with the slow phases directed toward the side of stimulation.54,55 Latent nystagmus can be abolishedby direct injection of muscimol, a potent γ-aminobutyric acid A agonistinto the NOT in monkeys.50 Simultaneous bilateralblockage of the NOT virtually abolishes latent nystagmus for the durationof the blockade.50

Subcortical optokinetic responses are also mediated by the pretectalNOT.15,21,3436 Themonocular pathways subserving nasotemporal asymmetry and its neutralizationby binocularly driven pathways from the visual cortex were first elucidatedby Hoffman in the cat.34,35 Thecat NOT is a diffuse cell aggregation in the pretectum that is optimally locatedto integrate direct retinal and diffuse cortical projections.34 Thesenuclei have high levels of spontaneous activity and operate in a push-pullfashion such that the sum of their opponent innervation determines the optokineticresponse.21,34

The NOT contains neurons that are sensitive to visual motion.54 Many units in the primate NOT have large receptivefields that are appropriate for encoding full-field visual motion to supportoptokinetic eye movements.54 Stimulation ofthe right and left NOT results in optokinetic nystagmus with slow phases tothe right and left, respectively.21 Outputfrom the NOT is maximal for horizontal movements but 0 for vertical movements.34

This phylogenetically ancient subcortical system is depicted in Figure 1. Crossed connections from each eyeto the contralateral NOT transmit horizontal visual motion information tothe vestibular nucleus before impinging on the ocular motor nuclei.40,56 Pretectal neurons in the left NOTreceive only crossed input from the right eye and respond only to leftwardmotion, while those in the right NOT receive only crossed input from the lefteye and respond only to rightward motion.15,21,36 Inthe first 6 months of infancy, this subcortical system predominates in humans,so that temporally directed monocular optokinetic responses are poor in earlyinfancy compared with nasally directed optokinetic responses.36 By6 months of age, cortical binocular pathways, which are responsive to temporallydirected motion, provide a route whereby the NOT, with its specialized directionalresponses, can be accessed from either eye.37,38 Inanimals with well-developed foveae and frontal, stereoscopic vision, the visualinputs feeding directly to the pretectum are supplemented by inputs routedthrough the visual cortex that selectively respond to moving images with nopositional disparity in the 2 eyes.57 Thiscoupling between optokinetic nystagmus and stereopsis allows frontal-eyedanimals to selectively stabilize the moving images of those parts of the scenewithin a selected depth plane, while disregarding induced image motion ofthe visual world at other distances.40,57 Inhumans with congenital strabismus, binocularly driven cortico-pretectal pathwaysnever become established, allowing the primitive monocular nasotemporal asymmetryto predominate.

Clinical Signs of Vestibular Origin

Bilateral positioning of the eyes and ears promotes survival by enablingthe organism to crosslink input from different sense organs to impart balance.Each eye and its ipsilateral semicircular canals share the same directionalbias to movement. For example, the right horizontal semicircular canal isactivated by head rotation to the right (which induces a rotation of the visualworld to the left) and inhibited by head rotation to the left (which inducesa rotation of the visual world to the right).2,43,45,58,59 Themonaural and monocular directional biases summate, so that activation of theright horizontal semicircular canal during rightward head rotation is reinforcedby the physiologic activation of the right eye by the induced nasal rotationof the visual world. The close geometrical relationship between the semicircularcanals and the extraocular muscles presumably facilitates the integrationof head motion and visual movement and their orderly summation to producetransformation to an appropriate ocular motor response.46,60

Latent nystagmus usually conforms to Alexander's law, which states thatthe intensity of a peripheral vestibular nystagmus increases when the eyesare moved in the direction of the fast phase and decreases when the eyes aremoved in the direction of the slow phase.2,21,6063 Latentnystagmus damps when the fixating eye is turned toward the nose (which isalso the direction of the slow phase) and increases in intensity when thefixating eye is turned toward the ipsilateral ear (which is in the directionof the fast phase).14,15,62,63 Asimilar damping of horizontal nystagmus is seen in peripheral horizontal vestibularnystagmus after disease or injury to 1 horizontal semicircular canal. By contrast,Alexander's law does not apply to congenital nystagmus, which reverses directionin different positions of gaze. The contraversive head turn in latent nystagmus(ie, a head turn opposite in direction to the deviation of the fixating eye)also characterizes vestibular eye movements.2

Additional evidence for the duality of optic and vestibular innervationcan be elicited by occluding 1 eye in the patient with latent nystagmus, spinningthe patient, suddenly stopping the spin, then immediately observing the effectof the postrotational nystagmus on the latent nystagmus when either eye isoccluded. A horizontal nystagmus induced by body spinning nullifies or accentuateslatent nystagmus depending on the direction of spin relative to the fixatingeye (Figure 2). For example, spinningthe patient to the right excites the right horizontal canal and inhibits theleft horizontal semicircular canal to induce a nystagmus with a slow-phaserotation to the left and a fast-phase rotation to the right. If the spin issuddenly stopped (after approximately 10 rotations), a shift in endolymphdeflects the cupula in the opposite direction, causing transient excitationof the left horizontal semicircular canal and transient inhibition of theright horizontal semicircular canal and inducing a left-beating nystagmus(termed "postrotational nystagmus"). If the left eye is occluded to inducelatent nystagmus prior to this maneuver, the latent nystagmus will diminishor disappear immediately following cessation of the spin. If the occluderis quickly moved to cover the right eye, the intensity of the latent nystagmuswith the left eye viewing will be correspondingly increased relative to thatobserved with the left eye fixating before the spin. In this way, the cliniciancan observe how visual input is summated with vestibular input to establishcentral vestibular tone in the horizontal plane.

The more visual input is dominated by 1 eye in latent nystagmus, thehigher the velocity of the slow-phase rotations in the direction toward theopposite eye.22 Simonsz and Kommerell63 performed eye movement recordings before and afterocclusion therapy for amblyopia in patients with latent nystagmus. After prolongedocclusion, the slow-phase velocity of the nystagmus in the amblyopic eye decreasedto the same extent that the slow-phase velocity of the nystagmus in the preferredeye increased. The sum of the 2 slow-phase velocities remained the same instraight-ahead gaze, demonstrating that visual input to the 2 eyes (just likerotational input to the 2 horizontal canals) maintains a push-pull relationship.21,63 This observation lends further supportto a vestibular underpinning for latent nystagmus. The clinical similaritiesbetween latent nystagmus and peripheral vestibular nystagmus are summarizedin Table 1.

Evolutionary Underpinnings of Latent Nystagmus

The notion of latent nystagmus as a horizontal visuo-vestibular tonusimbalance provides conceptual unification with its associated inferior obliqueoveraction and dissociated vertical divergence in patients with congenitalesotropia. The evolutionary progenitors of all of these visuo-vestibular movementsuse binocular input to establish physical orientation in space. These primitivereflexes rely on a dissociated form of binocular vision between the 2 laterallyplaced eyes, which has been superseded by normal cortical binocular visionin humans.5 In congenital esotropia, however,these primitive subcortical reflexes are not erased by binocular corticalinput. Eye movement recordings have demonstrated that dissociated verticaldivergence incorporates a vertical latent nystagmus, suggesting a shared commonorigin for these movements.64

Given that visual and labyrinthine input are pooled together withinthe central vestibular system of lower animals,6567 avisual counterpart to peripheral vestibular nystagmus would seem necessaryon evolutionary grounds. Many authors have attributed latent nystagmus asa tonus imbalance of the horizontal extraocular muscles.1,4,7,14,19,68 Latentnystagmus corresponds to a tropotactic vision-induced tonus imbalance (ie,one that functions to reestablish binocular equilibrium rather than to directionallyorient an eye toward incoming light).69 Ohmrecognized the physiologic coaptation of visual and vestibular innervationand its role in the generation of latent nystagmus long before others did(as was also the case with dissociated vertical divergence and primary obliquemuscle overaction).7073 Ina monograph written near the end of his life, he stated, "The impulses thatoriginate from both eyes keep both vestibular nuclei in equilibrium. The equilibriumbecomes unbalanced when one eye is being occluded. Then, a nystagmus beatingtowards the side of the open eye appears."72 Kestenbaum10 emphasized that latent nystagmus could not be attributedto luminance per se, since shining a bright light in the right eye workedlike occlusion of the right eye and caused a left-beating nystagmus. He notedthat the presence of a sharper visual image on the retina of one eye thanthe other appeared to be the decisive stimulus for inducing latent nystagmus.

Predominance of a primitive visuo-vestibular imbalance provides an evolutionarybasis for the shift in egocenter that has been invoked to explain latent nystagmus.14 According to this hypothesis, the egocenter is localizedto the median body plane under normal binocular conditions, but shifts tothe side of the fixating eye under monocular conditions. Dell'Osso et al14 hypothesized that humans with latent nystagmus retainan abnormal egocenter in the median plane even under monocular conditions,causing the fixating eye to drift toward midline. In the lateral-eyed animal,fixation with the right eye would instantaneously shift the egocenter to theleft of the object of regard, necessitating a body turn to frontalize theobject and a contraversive eye rotation to maintain fixation.14 Asneatly summarized by Dichgans and Brandt:

. . . the results of visual and vestibular stimulation on egocentriclocalization indicate the close similarity in the perceptual consequencesof stimulation of the two organs. The assumption of a unitary central representationof egocentric space, based on visual and vestibular (as well as acoustic andsomatosensory) afferents is perceptually obvious.42(pp763-764)

It remains to be determined whether a higher order egocentric shiftcould cause the visuo-vestibular imbalance which generates the linear slowphase of latent nystagmus.

Experimental Evidence That Latent Nystagmus Is Vestibular in Origin

Optokinetic responses are fundamentally intertwined with vestibularresponses, and a major site of this commingling is the vestibular complex.39 Waespe and Henn58 andHenn et al59 performed single-cell recordingsfrom the medial vestibular nucleus in monkeys and found that single neuronscan be activated either by body rotation or optokinetic stimulation. Unitsthat were excited by head acceleration to the left were also exited by motionof optokinetic stripes to the right. Most cells responded to both the whole-fieldvisual motion, as well as to the vestibular indications of head rotation,and the responses of vestibular neurons followed approximately the same timecourse as the delayed component of optokinetic nystagmus.58,59 Assummarized by Dichgans and Brandt:

All of the recent studies performed in awake animals show a tonicmodulation of resting discharge of vestibular units in response to exclusiveconstant velocity motion of the visual surround. The modulation, althoughto a variable degree, seems to occur in the great majority of horizontal semicircularcanal-dependent units of all vertebrate species tested. A unit which is excitedby a head acceleration, say, to the left is also excited by motion of thesurround to the right, which represents the optokinetic stimulus that in manwould cause the sensation of turning to the left.42(pp781,783)

This underlying vestibular response to both visual motion and body rotationalstimuli may explain the overlapping nystagmus response that characterizeslatent, optokinetic, and peripheral vestibular nystagmus.2,19,21,62,71 Thisoverlap may reflect the fact that all 3 movements subserve a similar physiologicfunction (ie, detection of rotation of the body and visual environment).

Latent nystagmus, optokinetic nystagmus, and the vestibuloocular reflexalso show velocity storage, a phenomenon in which constant vestibular inputor visual flow in the same direction is stored for up to 20 seconds in thebrainstem, even when the stimulus is terminated (Table 2).50,51 The presenceof velocity storage serves to enhance the slow-tracking eye movements to vestibularstimulation and optic flow response at low frequencies of rotation.2,74 Although latent nystagmus has variouslybeen attributed to anomalous cortical motion processing,25,26 ora cortical pursuit asymmetry,53,75 theabsence of velocity storage mechanism within the pursuit system implicatesthe vestibular system as the generator of latent nystagmus.

CONCLUSIONS

Latent nystagmus is a unique form of vestibular nystagmus that is evokedby unbalanced visual input from the 2 eyes rather than unequal rotationalinput from the 2 labyrinths. The neurophysiological substrate for latent nystagmusis operative in lateral-eyed animals and in human infants with undevelopedbinocular corticopretectal pathways. When congenital esotropia disrupts theestablishment of these binocular visual connections, visual input from thefixating eye to the contralateral NOT evokes a visuo-vestibular counterrotationof the eyes that corresponds to a turning or twisting movement of the bodytoward the object of regard ("vestibular nystagmus with a twist"). In thissetting, unbalanced binocular visual input can induce a motion bias in thevestibular nucleus to generate the visual counterpart of horizontal labyrinthinenystagmus, namely, latent nystagmus. As the eyes rotate frontally during evolution,this visuo-vestibular function is sacrificed, but the central nervous systemretains these latent subcortical visual pathways. Latent nystagmus is nature'sproclamation that our 2 eyes, when dissociated from birth, can revert to theirancestral function as sensory balance organs.

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Article Information

Corresponding author and reprints: Michael C. Brodsky, MD, Departmentsof Ophthalmology and Pediatrics, Arkansas Children's Hospital, 800 MarshallSt, Little Rock, AR 72202.

Submitted for publication October 8, 2002; final revision received September15, 2003; accepted October 14, 2003.

This study was supported in part by a grant from Research to PreventBlindness Inc, New York, NY.

We thank Guntram Kommerell, MD, for his valuable suggestionsduring the preparation of the manuscript.

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