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PubMedDate not supplied

Role of the cervico-ocular reflex in the "flying" pigeon: interactions with the optokinetic reflex.

We studied the cervico-ocular reflex (COR) alone and in combination with the optokinetic (OKN) reflex in head-fixed pigeons. We analyzed these responses in two behavioral conditions: (1) animals were hung in a harness ("resting" condition); and (2) animals were additionally submitted to a frontal airflow that provoked a flight posture ("flying" condition). In both conditions, cervical stimulation provoked a slow phase of very low gain (around 0.05) in the opposite direction to that of the stimulation and fast phases triggered near the head-body alignment in the same direction as the stimulation. The slow phase showed a phase lag of 20 deg at 0.5 Hz. The gain of the slow phase was not modified by the velocity, amplitude, or frequency of the stimuli. This gain was not changed by the presence of a fixed visual surround. When cervical stimuli (0.05-0.5 Hz) were added to an optokinetic stimulation (30 deg/s) in the "resting" condition, the slow phase velocity (SPV) of the optokinetic reflex was modulated with a time course close to that produced by the cervico-ocular reflex alone. The SPV was alternately increased and decreased round the SPV level corresponding to the steady-state OKN. In the "flying" condition, optokinetic-cervical stimulation provoked an eye beating field and a strong SPV modulation synchronized with the position of the cervical stimulation. The number of nystagmic beats (OKN) and the amplitude and velocity of the fast phases were modulated in correlation with the SPV. Consequently, the optokinetic response was increased or decreased according to whether the cervical stimuli were in the reverse direction or in the same direction as the optokinetic stimulation, respectively. These data are interpreted as an improvement of gaze stabilization by the COR. This mechanism is context dependent, since it is strongly reinforced during the flight.

Animals↗

PubMed2001

Adaptive motor control in crayfish.

This article reviews the principles that rule the organization of motor commands that have been described over the past five decades in crayfish. The adaptation of motor behaviors requires the integration of sensory cues into the motor command. The respective roles of central neural networks and sensory feedback are presented in the order of increasing complexity. The simplest circuits described are those involved in the control of a single joint during posture (negative feedback-resistance reflex) and movement (modulation of sensory feedback and reversal of the reflex into an assistance reflex). More complex integration is required to solve problems of coordination of joint movements in a pluri-segmental appendage, and coordination of different limbs and different motor systems. In addition, beyond the question of mechanical fitting, the motor command must be appropriate to the behavioral context. Therefore, sensory information is used also to select adequate motor programs. A last aspect of adaptability concerns the possibility of neural networks to change their properties either temporarily (such on-line modulation exerted, for example, by presynaptic mechanisms) or more permanently (such as plastic changes that modify the synaptic efficacy). Finally, the question of how "automatic" local component networks are controlled by descending pathways, in order to achieve behaviors, is discussed.

Adaptation, Physiological↗

PubMed1984

Tonic cervical influences on eye nystagmus following hemilabyrinthectomy: immediate and plastic effects.

In intact guinea pigs a passive horizontal rotation of the body about the fixed head induces compensatory ocular movements (cervico-ocular reflex). When the static neck deviation is maintained, a significant ocular displacement is observed. In acutely hemilabyrinthectomized animals, static body deviation towards the lesion side tonically alters eye nystagmus. It affects slow phase eye velocity and quick phase amplitude and frequency causing the eye to reach a less eccentric orbital position. Apart from such immediate influences, a plastic effect on eye nystagmus abatement is induced. In the animals restrained with no body-on-head deviation, abatement of nystagmus is delayed with respect to the animals restrained with 35 degrees body deviation towards the lesion side. Thus the head position signal is not only a contributing factor for the correction of postural deficits but also influences the time course of the ocular balancing process following unilateral vestibular damage.

Adaptation, Physiological↗

PubMed1982

Force-sensitive interneurons in the spinal cord of the cat.

The input-output properties of interneurons mediating spinal reflexes were investigated by extracellularly recording the response of interneurons to excitation from muscle receptors in the ankle extensor muscles of decerebrated, spinal cats. A population ofinterneurons in the intermediate region ofthe spinal cord is potently excited by increases in muscle force. Unlike the discharge of Golgi tendon organs, which accurately encodes moment-to-moment variations in the force of a single muscle, the discharge of these interneurons depends in a dynamic and usually nonlinear way on the force in several muscles. Powerful input from unidentified mechanoreceptors in muscle, presumably free nerve endings, is at least partly responsible for these properties. These force-sensitive interneurons are more likely to mediate clasp knife-type inhibition than simple negative force feedback.

Animals↗

PubMed1995

Perceptual studies in patients with vestibular neurectomy.

Twelve patients undergoing unilateral vestibular neurectomy for the treatment of refractory vertigo were investigated. Vestibular motion perception was assessed using a self-rotational task and "vestibular remembered saccades". Cervical perception was also measured with remembered saccades. The tests were performed pre- and post-operatively to examine changes in vestibular and cervical perception following an acute vestibular lesion, and to monitor the progress of vestibular compensation. These perception tests were carried out in conjunction with a conventional evaluation of the vestibular ocular reflex (VOR), using electro-oculography. The patients' subjective symptoms at each stage of testing were also quantified with questionnaires. Generally, in the vestibular tests, for stimulation to the operated side, responses became strongly hypometric directly after the neurectomy, with a partial recovery during convalescence. In the cervical test, responses were bilaterally reduced immediately after operation. Results from both of the vestibular perception tests were significantly correlated with the VOR assessment of vestibular function. Scores for the patients' subjective symptoms of "vertigo" were only significantly correlated with the vestibular perception tests, and not with the conventional measures of vestibular function. Perceptual measurements afford useful complementary information in the assessment of vestibular patients.

Adult↗

PubMed2001

[Gait disorders in Parkinson disease. Neuroanatomic and physiologic organization of gait].

GAIT IS A VOLUNTARY, AUTOMATIC AND REFLEX RHYTHMIC ACTIVITY: It is generated by a central pattern generator identified from animal models. This spinal gait generator (SGG) is controlled by various parts of the central nervous system: the descending tracts and locomotor regions of the brainstem, the cerebellum, the basal ganglia, the motor and parietal cortex and the hippocampus. Kinesthetic inputs which project to the SGG and the cerebellum, play an important role in the production of postural reflex responses; vestibular and visual inputs mainly control balance. GAIT MAINLY DEPENDS ON THE RELATIONSHIP BETWEEN POSTURE BALANCE AND MOVEMENT: As concerns posture each segment is under the control of both peripheral and central nervous systems and is used as a system of reference to organize movements of adjacent segments. Balance is maintained by sensory inputs which provide corrective mechanisms: anticipatory postural responses, reflex postural responses and voluntary responses. DIFFERENT DESCRIPTIVE PARAMETERS MAY BE PROPOSED: Analysis of kinematic (displacement, speed and acceleration of segments) and kinetic parameters during the four successive stages of gait (posture, initiation, rhythmic gait and return to the initial posture) provides an understanding of neurological gait disorders. In particular the relationship between the center of pressure and the center of gravity is used to analyze infraclinical gait abnormalities. NEW AND SOPHISTICATED INVESTIGATIONS METHODS ARE AVAILABLE: The optoelectronic system provides a tridimensional analysis of movement and can be combined with forceplate and electromyographic recordings. These methods constitute an interesting contribution to the clinical analysis of gait. CLASSIFICATION: This is established according to clinical data and the positionment of the lesion among the structures of the nervous system. The physiopathological approach is then specified taking into account the lesions of the muscular, skeletal and nervous structures.

Brain↗

PubMed1984

Eye movements induced by head rotation in unresponsive patients.

Eye movements induced by head rotation were studied in 6 patients in acute coma, 4 patients in a persistent vegetative state, and 6 healthy, alert control subjects. Results from control subjects suggest that the oculocephalic response in the supine position is principally a vestibulo-ocular reflex. A position-step rotation of the head produced an initial oppositely directed eye movement, followed by a drift of the eyes back toward midline with a negative exponential time course. The time constant of this drift was greater than or equal to 10 seconds in control subjects but less than or equal to 1.5 seconds in unconscious patients and less than or equal to 0.5 seconds in vegetative patients. The rapid drift back of the eyes in unresponsive patients implies dysfunction of reticular and, possibly, cerebellar connections; the rate of this drift may indicate the severity and extent of brain injury. Sinusoidal head rotation produced slow and quick phases of nystagmus in normal subjects. Quick phases were absent in patients in acute coma; although present in vegetative patients, the quick phases did not keep the eyes close to primary position, as was the case in control subjects.

Adult↗

PubMed1998

Flexor reflex afferents reset the step cycle during fictive locomotion in the cat.

The generation of locomotor-like spinal rhythms has been proposed to involve two neural centres with mutual reciprocal inhibition (Graham Brown's "half-centre" hypothesis). Much later a particular set of segmental flexor reflex pathways were described as being organized in accordance with this half-centre hypothesis. As these pathways became operative following injection of monoaminoxidase inhibitors and L-3,4-dihydroxyphenylalanine (L-dopa), i.e. under the same conditions under which a spontaneous locomotor activity may develop, it was assumed that these particular pathways and spinal rhythm generators involve the same neuronal networks. In order to give further evidence to this hypothesis, we investigated whether short trains to "flexor reflex afferents" (FRA) reset the spinal locomotor rhythm, i.e. shorten or lengthen the stimulated cycle after which the regular rhythm is resumed with step cycles of the original duration. The experiments were performed in anaemically decapitated, high-spinal curarized cats. A steady locomotor rhythm was induced by injection of nialamide and L-dopa and the influence of electrical stimulation (trains of 50-1000 ms) of FRA (joint, cutaneous, and group II and III muscle afferents) onto this rhythm was tested. Stimulation of FRA induced a clear resetting of the locomotor rhythm, which was mainly characterized by a flexion reflex pattern: during the extension phase the extensor activity was interrupted and a flexion phase was initiated; during the late flexion phase mainly a prolongation of that phase with a variable change of the following extension phase was induced. In addition to this prevailing pattern, stimulation of some nerves (in particular nerves to more distal extensors and the sural nerve) could often prolong extension, when stimulated during the late extension, or terminate the flexor burst and initiate a new extension phase, when stimulated during the late flexion phase. This pattern is probably due to the concomitant stimulation of group I afferents in the case of the muscle nerves and to separate non-FRA pathways in the case of the sural nerve. The results demonstrate that the interneurones of the FRA pathways, which are operative during L-dopa-induced locomotion in spinal animals, can be considered as neuronal elements of the rhythm-generating network for locomotion.

Animals↗

PubMed1997

Visual vestibular interaction in the dynamic visual acuity test during voluntary head rotation.

BACKGROUND: Although intact vestibular function is indispensable to maintaining spatial orientation, no good screening tests of vestibular function are implemented in the aviation community. High frequency voluntary head rotation was selected as a vestibular stimulus to isolate the vestibulo-ocular reflex (VOR) from visual influence. METHOD: A dynamic visual acuity test that incorporates voluntary head rotation was evaluated as a potential vestibular function screening tool: 27 normal subjects performed voluntary sinusoidal head rotation at frequencies from 0.7-4 Hz under 3 different visual conditions: visually-enhanced VOR, normal VOR, and visually suppressed VOR. Standardized Bailey-Lovie chart letters were presented on a computer monitor in front of the subject, who then was asked to read the letters while rotating his head horizontally. The electro-oculogram and dynamic visual acuity score were recorded and analyzed. RESULTS: There was no significant difference in gain or phase shift among 3 visual conditions in the frequency range 2.8-4 Hz. The dynamic visual acuity score shifted less than 0.3 log MAR at frequencies under 2.0 Hz. CONCLUSION: The dynamic visual acuity test at frequencies around 2 Hz can be recommended for evaluating vestibular function.

Adult↗

PubMed1986

Inhibition of vestibulo-ocular reflex by tonic stimulation of the posterior neck region in man.

The effect of posterior neck stimulation on vestibulo-ocular reflex (VOR) was studied in 10 healthy subjects. The experimental situation was designed to minimise all afferent inputs except from the neck; each subject was placed in supine position with body fixed to a tilting table inclinable in a vertical plane. The head was immovably secured at 30 degrees to an independent non-inclinable rigid frame. In this situation the body could be mechanically moved upwards in the vertical plane (ventriflexion), producing symmetrical and selective stretch of the posterior neck region. The VOR elicited by caloric monoaural stimulation was evaluated for each of the following static positions: 0 degree and 50 degrees of ventriflexion and vice versa. We observed a significant decrease in the slow-phase angular velocity of induced nystagmus as the body was tilted upwards and a significant opposite effect when the body was returned to the original position. Similar changes in VOR were observed in 4 selected patients with spontaneous 'peripheral nystagmus'. Mechanisms involved in the cervical control of VOR are discussed.

Adult↗

PubMed2001

Postural responses of forelimb extensors to somatosensory signals elicited during wrist rotation: interaction with vestibular reflexes.

The responses of the forelimb extensor triceps brachii (TB) to sinusoidal wrist rotation, at 0.156 Hz, +/-5 degrees, were investigated in decerebrate cats. In most of the tested muscles [12/15) the multiunit electromyographic (EMG) activity of the TB muscle increased during dorsiflexion and decreased during plantar flexion of the ipsilateral limb extremity, showing an average (+/-SD) gain of 1.38+/-0.64 impulses/s/ degrees and a phase lead of 33+/-19 degrees with respect to the extreme dorsiflexion of the forepaw. Both parameters remained unmodified by increasing the amplitude of stimulation from 5 degrees up to 10 degrees. Rotation of the contralateral wrist had no effect on TB activity. When the rotation of the ipsilateral wrist occurred during sinusoidal roll tilt of the whole animal, a stimulus that activates vestibular receptors, the TB response to combined stimulation closely corresponded to the vectorial sum of the individual responses. Dorsiflexion of the forepaw could drive the vestibulospinal system, which excites the ipsilateral TB motoneurons. In fact, functional inactivation of the cerebellar anterior vermis, which controls vestibulospinal neurons, significantly modified the TB response to wrist rotation. It appears, therefore, that the somatosensory signals elicited by wrist rotation utilize the spinocerebellum to modulate the activity of the TB. The TB responses to wrist rotation could play a role in stabilizing posture during stance and locomotion.

Animals↗

PubMed1987

Consequences of damage to the sensorimotor cortex in neonatal and adult cats. I. Sparing and recovery of function.

Postural reflexes and locomotion were studied in order to assess the effects of unilateral sensorimotor cortical ablations in neonatal (1 day old) and adult cats. To document the infant lesion effect and to distinguish recovery from sparing of function, development of motor function was studied in neonatal operates and in normal littermates. Once neonatal operates achieved maturity, their motor performance was compared with that of chronic adult operates. The emergence of motor behavior during development in neonatal operates appeared to follow the same pattern as in normal animals although with a protracted time course and motor behavior did not attain the level of maturity of normal animals. Some deficits were not apparent immediately but evolved with time. Adult operates exhibited recovery of function of some behavior but neonatal operates exhibited greater recovery and sparing. Adult operates, like neonatal operates, were able to mask certain deficits by compensatory mechanisms. Kinematic analysis revealed that neonatal and adult operates often executed movements abnormally. It is hypothesized that somewhat different mechanisms underlie recovery in neonatal and adult operated animals.

Age Factors↗

PubMed2002

The vestibulo-ocular reflex in fourth nerve palsy: deficits and adaptation.

The effects of fourth nerve palsy on the vestibulo-ocular reflex (VOR) had not been systematically investigated. We used the magnetic scleral search coil technique to study the VOR in patients with unilateral fourth nerve palsy during sinusoidal head rotations in yaw, pitch and roll at different frequencies. In darkness, VOR gains are reduced during incyclotorsion, depression and abduction of the paretic eye, as anticipated from paresis of the superior oblique muscle. VOR gains during excyclotorsion, elevation and adduction of the paretic eye are also reduced, whereas gains in the non-paretic eye remain normal, indicating a selective adjustment of innervation to the paretic eye. In light, torsional visually enhanced VOR (VVOR) gains in the paretic eye remain reduced; however, visual input increases vertical and horizontal VVOR gains to normal in the paretic eye, without a conjugate increase in VVOR gains in the non-paretic eye, providing further evidence of selective adaptation in the paretic eye. Motions of the eyes after fourth nerve palsy exemplify monocular adaptation of the VOR, in response to peripheral neuromuscular deficits.

Adaptation, Physiological↗

PubMed1975

Reflex modulation of motoneurone activity in the cheliped of the crayfish Astacus leptodactylus.

1. The reflex activity elicited by movement of the mero-carpopodite (M-C) joint in the cheliped of the crayfish Astacus leptodactylus is investigated and the role of the different proprioceptors (chordotonal and myochordotonal organs) separately studied. 2. The reflex discharge involves mainly the tonic motoneurones of the extensor (E), the flexor (F) and the accessory flexor (AF) muscles. 3. M-C joint posture is also regulated by the cuticular stress detector (CSD2) afferents: they increase mainly the F discharge and secondarily the AF command. 4. The activity of the motor axons supplying the muscles of the meropodite can be also influenced by a variety of natural stimuli applied to other appendages. The effect usually produced is a general flexion reaction which is characterized by a reciprocity between E and F involving both central and peripheral mechanisms. 5. The AF muscle is innervated by two antagonistic motoneurones, an excitatory neurone functionally linked in its discharge with one of the four excitors supplying F and an inhibitory motoneurone, common with E. The resulting competitive effect between these two neurones has been recorded intracellularly in AF muscle fibres. 6. The role of the myochordotonal organ (MCO) in the crayfish is discussed. In particular the modulation of the AF command in relation to the discharges of the motor nerves to the main muscle E and F is studied.

Animals↗

PubMed2002

The clinical identification of peripheral neuropathy among older persons.

OBJECTIVE: To identify simple clinical rules for the detection of a diffuse peripheral neuropathy among older outpatients. DESIGN: Observational, blinded, controlled study. SETTING: A tertiary-care electrodiagnostic laboratory and biomechanics laboratory. PARTICIPANTS: One hundred research subjects, 68 with electrodiagnostic evidence of peripheral neuropathy, between the ages of 50 and 80 years. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASUREMENTS: One examiner, unaware of the results of electrodiagnostic testing, evaluated Achilles' and patellar reflexes, Romberg testing, semiquantified vibration, and position sense at the toe and ankle in all subjects, and unipedal stance time and the Michigan Diabetes Neuropathy Score in a subset of subjects. RESULTS: Significant group differences were present in all clinical measures tested. Three signs, Achilles' reflex (absent despite facilitation), vibration (128Hz tuning fork perceived for <10s), and position sense (<8/10 1-cm trials) at the toe, were the best predictors of peripheral neuropathy on both univariate and logistic regression (pseudo R(2)=.744) analyses. The presence of 2 or 3 signs versus 0 or 1 sign identified peripheral neuropathy with sensitivity, specificity, and positive and negative predictive values of 94.1%, 84.4%, 92.8%, and 87.1%, respectively. Values were similar among subgroups of subjects with and without diabetes mellitus. When other clinicians applied the technique to 12 more subjects, excellent interrater reliability regarding the presence of peripheral neuropathy (kappa=.833) and good to excellent interrater reliability for each sign (kappa range,.667-1.00) were shown. CONCLUSION: Among older persons, the presence of 2 or 3 of the 3 clinical signs strongly suggested electrodiagnostic evidence of a peripheral neuropathy, regardless of etiology. Age-related decline in peripheral nerve function need not be a barrier to the clinical recognition of a diffuse peripheral neuropathy among older persons.

Achilles Tendon↗

PubMed2003

Adaptations and deficits in the vestibulo-ocular reflex after peripheral ocular motor palsies.

Palsy of a nerve might be expected to lower vestibulo-ocular reflex (VOR) responses in its fields of motion, but effects of peripheral neuromuscular disease were unknown. We recorded the VOR during sinusoidal head rotations in yaw, pitch, and roll at 0.5-2 Hz and static torsional gain in 43 patients with unilateral nerve palsies. Sixth nerve palsy (n = 21) reduced both abduction and adduction VOR gains in darkness. In light, horizontal visually enhanced VOR (VVOR) gains were normal in moderate and mild palsy. In severe palsy, horizontal VVOR gains remained low in the paretic eye when it was fixating, whereas gains in the nonparetic eye became higher than normal. Third nerve palsy (n = 10) decreased VOR and VVOR gains during abduction, adduction, elevation, depression, extorsion, and intorsion. Fourth nerve palsy (n = 13) reduced VOR gains of the paretic eye during intorsion, extorsion, elevation, depression, abduction, and adduction, but in light vertical and horizontal VVOR gains were normal. In the nonparetic eye, all gains were normal. Reduced VOR gains in the direction of paretic muscles and also in the direction of their antagonists, together with normal gains in the nonparetic eye, indicate a selective adjustment to the antagonists of paretic muscles. Increase of VVOR gains to normal in the paretic eye, when used for fixation, without conjugate increase in gains in the occluded nonparetic eye, provides further evidence of selective adaptation for the paretic eye. Motions of the eyes after nerve palsies indicate monocular VOR adaptation in three dimensions.

Abducens Nerve Diseases↗

PubMed1993

Adaptive control for backward quadrupedal walking. III. Stumbling corrective reactions and cutaneous reflex sensitivity.

1. Four cats were trained to walk backward (BWD) and forward (FWD) on a motorized treadmill. Mechanical (taps) or electrical (pulses) stimuli were applied to the dorsal or ventral aspect of the hind paw during swing or stance. Hindlimb kinematic data, obtained by digitizing 16-mm high-speed film, were synchronized with computer-analyzed electromyograms (EMG) recorded from anterior biceps femoris (ABF), vastus lateralis (VL), lateral gastrocnemius (LG), tibialis anterior (TA), and semitendinosus (ST). Responses to taps and pulses, as well as the modulation in cutaneous reflex sensitivity to pulses, were described for both walking directions and stimulus locations. 2. After dorsal taps that obstructed FWD swing, the hindlimb initially drew back away from the obstacle with knee flexion and ST activation, ankle extension with TA suppression and LG activation, and hip extension with ABF facilitation. Next, the limb was raised over the obstacle with resumed TA activity and enhanced knee and ankle flexion, and then compensatory knee and ankle extension positioned the limb for the ensuing stance phase. 3. For ventral taps that obstructed BWD swing, the initial response also tended to draw the limb away from the obstacle with hip and ankle flexion and TA facilitation and reduced knee flexion with weak VL facilitation and suppression of ST activity. Next, ST activity resumed as knee and ankle flexion raised the limb over the obstacle, and then compensatory extension completed the swing phase for BWD walking. Thus the initial kinematic and EMG responses to obstacles were opposite for BWD versus FWD swing, and these responses were consistent with active avoidance of the obstacles. Responses during BWD walking were subtle, however, compared with those for FWD. 4. After nonobstructing taps (ventral FWD, dorsal BWD), ST and TA activation and knee and ankle flexion were coincident, demonstrating that the aforementioned differences in responses to obstructing obstacles were not simply location dependent. Regardless of the direction of walking or the location of stimulation, taps applied during stance had little immediate kinematic effect, but the subsequent swing phase was usually exaggerated, as if the response was programmed to avoid any lingering obstacle. 5. Electrical pulses did not elicit the full-blown responses typically evoked by taps. The sequencing in activation of ST and TA characteristic after laps was absent after pulses, and there were rarely dramatic kinematic responses to pulses like those easily elicited by taps. There were, in fact, few differences in responses to electrical stimulation for BWD versus FWD walking.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed2002

Adaptations and deficits in the vestibulo-ocular reflex after sixth nerve palsy.

PURPOSE: The effects of paralytic strabismus on the vestibulo-ocular reflex (VOR) have not been systematically investigated in humans. The purpose of this study was to analyze the VOR in patients with unilateral peripheral sixth nerve palsy. METHODS: Twenty-one patients with unilateral peripheral sixth nerve palsy (6 severe, 7 moderate, 8 mild) and 15 normal subjects were studied. Subjects made sinusoidal +/-10 degrees head-on-body rotations in yaw and pitch at approximately 0.5 and 2 Hz, and in roll at approximately 0.5, 1, and 2 Hz. Eye movement recordings were obtained using magnetic scleral search coils in each eye in darkness and during monocular viewing in light. Static torsional VOR gains, defined as change in torsional eye position divided by change in head position during sustained head roll, were also measured. RESULTS: In all patients, horizontal VOR gains in darkness were decreased in the paretic eye in both abduction and adduction, but remained normal in the nonparetic eye in both directions. In light, horizontal visually enhanced VOR (VVOR) gains were normal in both eyes in moderate and mild palsy. In severe palsy, horizontal VVOR gains remained low in the paretic eye during viewing with either eye, whereas those in the nonparetic eye were higher than normal when the paretic eye viewed. Vertical VOR and VVOR were normal, but dynamic and static torsional VOR and VVOR gains were reduced in both eyes in all patients. CONCLUSIONS: In darkness, horizontal VOR gains were reduced during abduction of the paretic eye in all patients, as anticipated in sixth nerve palsy. Gains were also reduced during adduction of the paretic eye, suggesting that innervation to the medial rectus has changed. After severe palsy, vision did not increase abducting or adducting horizontal VVOR gains to normal in the paretic eye, but caused secondary increase in VVOR gains to values above unity in the nonparetic eye, when the paretic eye fixated. In mild and moderate palsy, vision enhanced the VOR in the paretic eye but caused no change in the nonparetic eye, suggesting a monocular readjustment of innervation selectively to the paretic eye. Vertical VOR and VVOR gains were normal, indicating that the lateral rectus did not have significant vertical actions through the excursions that we tested (+/-10 degrees ). Reduced torsional VOR gains in the paretic eye can be explained by the esotropia in sixth nerve palsy. Torsional VOR gain normally varies with vergence. We attribute the reduced torsional gains in the paretic eye to the mechanism that normally lowers it during convergence. The low torsional gains in the nonparetic eye may be an adaptation to reduce torsional disparity between the two eyes.

Abducens Nerve Diseases↗