Search Coil Recordings Research Articles

Video Head Impulse Testing: From Bench to Bedside.

Over 30 years ago, the head impulse test (HIT) was measured with search coil recordings and it provided robust evidence for a new test of vestibular function that could detect impairment of a single semicircular canal, that is, the lateral canal. Over the next two decades, the diagnostic spectrum of HIT was expanded to the testing of vertical canals, differentiation of central from peripheral vestibulopathy, and incorporation of visual interaction-the suppressed head impulse. However, HIT measurement was limited to very few specialized laboratories that were able to maintain the time-consuming and expensive operation of the scleral search coil system, which is the gold standard in eye movement recording. The video HIT (vHIT) was validated for the first time over 10 years ago, against the search coils, and its introduction into dizzy clinics worldwide has revolutionized the practice of neuro-otology. Here we review the basic physiology, practical aspects, and clinical application of the vHIT.

Design, simulation and evaluation of uniform magnetic field systems for head-free eye movement recordings with scleral search coils.

The precise measurement of eye movements is important for investigating vision, oculomotor control and vestibular function. The magnetic scleral search coil technique is one of the most precise measurement techniques for recording eye movements with very high spatial (≈ 1 arcmin) and temporal (>kHz) resolution. The technique is based on measuring voltage induced in a search coil through a large magnetic field. This search coil is embedded in a contact lens worn by a human subject. The measured voltage is in direct relationship to the orientation of the eye in space. This requires a magnetic field with a high homogeneity in the center, since otherwise the field inhomogeneity would give the false impression of a rotation of the eye due to a translational movement of the head. To circumvent this problem, a bite bar typically restricts head movement to a minimum. However, the need often emerges to precisely record eye movements under natural viewing conditions. To this end, one needs a uniform magnetic field that is uniform over a large area. In this paper, we present the numerical and finite element simulations of the magnetic flux density of different coil geometries that could be used for search coil recordings. Based on the results, we built a 2.2 × 2.2 × 2.2 meter coil frame with a set of 3 × 4 coils to generate a 3D magnetic field and compared the measured flux density with our simulation results. In agreement with simulation results, the system yields a highly uniform field enabling high-resolution recordings of eye movements.

An Objective Photographic Analysis of Ocular Oblique Muscle Dysfunction

To develop a new objective method for measuring ocular oblique muscle dysfunction. Prospective, observational study. Twelve healthy volunteers and 27 patients with oblique muscle overaction or underaction in at least 1 eye were included. The corneal contour in 9-gaze photographs was transcribed into an ellipse. A newly developed simulating program depicted the corneal contour as the eyes moved, and graded oblique muscle dysfunction by analyzing the angular difference between the 2 eyes. An angular difference of 5 degrees was defined as 1 unit of overaction or underaction. To verify the accuracy of the method, a masked observer compared the photographic analysis with goniometric readings in the model eye and magnetic search coil recordings in healthy volunteers. The diagnostic accuracy of the photographic analysis was compared with the clinical gradings of patients with oblique muscle overaction or underaction. There was no statistical difference between photographic analysis and goniometric readings of the model eye. Bland-Altman plots showed that there was no overall tendency for the photographic analysis to differ from the scleral search coil values, and the difference fell mostly within ±7 degrees. There was a good correlation between the photographic analysis and the clinical grading of oblique muscle dysfunction (R(2)= 0.618; P< .001). The sensitivity and specificity for detecting oblique muscle overaction were 90.2% and 88.1%, respectively, whereas those for detecting underaction were 81.5% and 80.2%, respectively. A newly developed photographic analysis could provide an objective and accurate means of evaluating ocular oblique muscle dysfunction.

Application of the Video Head Impulse Test to Detect Vertical Semicircular Canal Dysfunction

The video head impulse test (vHIT) is a useful clinical tool to detect semicircular canal dysfunction. However, so far, vHIT has been limited to measurement of the function of the horizontal semicircular canals. The goal of this study was to determine if vHIT can detect vertical semicircular canal dysfunction. Horizontal and vertical eye movements were recorded in response to abrupt, passive, unpredictable head turns (head impulses) in the planes of the vertical semicircular canals by high-speed video (250-Hz sampling rate) together with measures of the head movement. Head impulses were delivered diagonally in the plane of the vertical semicircular canals, whereas gaze was directed along the same plane. Patients with known vestibular loss as shown by previous scleral search coil recording were tested to identify if the vHIT testing could detect the loss. The results of patients with unilateral, bilateral, and individual semicircular canal dysfunction were compared with the results of a healthy control subject. The patient with bilateral vestibular loss had no compensatory slow eye movements in any direction. The patient with unilateral vestibular loss showed reduced response for head impulses activating the canals in their affected right ear (right anterior, right posterior, and right horizontal head impulses). The patient with isolated canal loss showed reduced response for head impulses activating the affected right posterior canal. vHIT detects peripheral deficits of both vertical and horizontal semicircular canal function and is a new tool for measuring dysfunction of individual semicircular canals in vestibular patients.

The Video Head Impulse Test (vHIT) Detects Vertical Semicircular Canal Dysfunction

BackgroundThe video head impulse test (vHIT) is a useful clinical tool to detect semicircular canal dysfunction. However vHIT has hitherto been limited to measurement of horizontal canals, while scleral search coils have been the only accepted method to measure head impulses in vertical canals. The goal of this study was to determine whether vHIT can detect vertical semicircular canal dysfunction as identified by scleral search coil recordings.MethodsSmall unpredictable head rotations were delivered by hand diagonally in the plane of the vertical semicircular canals while gaze was directed along the same plane. The planes were oriented along the left-anterior-right-posterior (LARP) canals and right-anterior-left-posterior (RALP) canals. Eye movements were recorded simultaneously in 2D with vHIT (250 Hz) and in 3D with search coils (1000 Hz). Twelve patients with unilateral, bilateral and individual semicircular canal dysfunction were tested and compared to seven normal subjects.ResultsSimultaneous video and search coil recordings were closely comparable. Mean VOR gain difference measured with vHIT and search coils was 0.05 (SD = 0.14) for the LARP plane and −0.04 (SD = 0.14) for the RALP plane. The coefficient of determination R2 was 0.98 for the LARP plane and 0.98 for the RALP plane and the results of the two methods were not significantly different. vHIT and search coil measures displayed comparable patterns of covert and overt catch-up saccades.ConclusionsvHIT detects dysfunction of individual vertical semicircular canals in vestibular patients as accurately as scleral search coils. Unlike search coils, vHIT is non-invasive, easy to use and hence practical in clinics.

Vestibular Role of KCNQ4 and KCNQ5 K+ Channels Revealed by Mouse Models

The function of sensory hair cells of the cochlea and vestibular organs depends on an influx of K(+) through apical mechanosensitive ion channels and its subsequent removal over their basolateral membrane. The KCNQ4 (Kv7.4) K(+) channel, which is mutated in DFNA2 human hearing loss, is expressed in the basal membrane of cochlear outer hair cells where it may mediate K(+) efflux. Like the related K(+) channel KCNQ5 (Kv7.5), KCNQ4 is also found at calyx terminals ensheathing type I vestibular hair cells where it may be localized pre- or postsynaptically. Making use of Kcnq4(-/-) mice lacking KCNQ4, as well as Kcnq4(dn/dn) and Kcnq5(dn/dn) mice expressing dominant negative channel mutants, we now show unambiguously that in adult mice both channels reside in postsynaptic calyx-forming neurons, but cannot be detected in the innervated hair cells. Accordingly, whole cell currents of vestibular hair cells did not differ between genotypes. Neither Kcnq4(-/-), Kcnq5(dn/dn) nor Kcnq4(-/-)/Kcnq5(dn/dn) double mutant mice displayed circling behavior found with severe vestibular impairment. However, a milder form of vestibular dysfunction was apparent from altered vestibulo-ocular reflexes in Kcnq4(-/-)/Kcnq5(dn/dn) and Kcnq4(-/-) mice. The larger impact of KCNQ4 may result from its preferential expression in central zones of maculae and cristae, which are innervated by phasic neurons that are more sensitive than the tonic neurons present predominantly in the surrounding peripheral zones where KCNQ5 is found. The impact of postsynaptic KCNQ4 on vestibular function may be related to K(+) removal and modulation of synaptic transmission.

Head Impulse Testing Using Video‐oculography

Head impulses are a routine clinical test of semicircular canal function. At the bedside, they are used to detect malfunctioning of the horizontal semicircular canals. So far, 3-D-search-coil recording is required to reliably test anterior and posterior canal function and to determine the gain of the vestibulo-ocular reflex (VOR). Search-coil recording cannot be done at the bedside. Here we tested whether video-oculography (VOG) is suitable to assess VOR gain for individual canals at the bedside. We recorded head impulses in healthy subjects using a mobile high-frame-rate, head-mounted VOG-device and compared the results with those obtained with standard search-coil recording. Our preliminary results indicate that high-frame-rate VOG is a promising tool to measure and quantify individual semicircular canal function not only at the bedside.

Binocular Vertical Rectus Muscle Recession For Comitant Vertical Strabismus

Binocular vertical rectus muscle recession has not been formally evaluated in the correction of comitant vertical strabismus. Eight patients with stable comitant vertical strabismus for at least 6 months were included. All underwent recession of the superior rectus muscle of the hypertropic eye combined with an equal or nearly equal recession of the inferior rectus muscle in the hypotropic eye. On the day before surgery, on one of the first three postoperative days, and at one year postoperatively, ocular alignment in vertical and horizontal gaze directions were measured with simultaneous and alternate cover test at a viewing distance of 5 meters, and with the two dimensional Hess screen test. The field of single binocular vision was determined with a Goldmann perimeter. The Lang stereopsis chart was presented at the last follow-up visit. All patients were orthotropic at the last postoperative follow-up visit. In primary gaze, the degree of vertical and horizontal phoria diminished significantly. Normal alignment was achieved in nearly all gaze directions and stereopsis was reestablished. The field of single binocular vision enlarged after the surgery. Binocular vertical rectus muscle recession is an effective surgical approach for patients with comitant vertical ocular misalignment.

Teaching Video Neuro Image : Acquired or congenital gaze-evoked nystagmus?

A 40-year-old female migraineur reported intermittent unsteadiness with headache. On examination, she had gaze-evoked horizontal-torsional nystagmus that was also evident during pursuit (video). Nystagmus recordings showed increasing-velocity slow phases typical for congenital nystagmus (CN) (figure 1). A relative subsequently confirmed that the nystagmus was long-standing. Figure 1 Three-dimensional search-coil recordings of the patient's nystagmus The patient is attempting …

Horizontal Rectus Muscle Anatomy in Naturally and Artificially Strabismic Monkeys

Structural abnormalities of extraocular muscles (EOMs) or their pulleys are associated with some forms of human strabismus. This experiment was conducted to investigate whether such abnormalities are associated with artificial or naturally occurring strabismus in monkeys. Binocular alignment and grating visual acuities were determined in 10 monkeys representing various species using search coil recording and direct observations. Four animals were orthotropic, two had naturally occurring "A"-pattern esotropia, two had concomitant and one had "V"-pattern esotropia artificially induced by alternating or unilateral occlusion in infancy, and one had "A"-pattern exotropia artificially induced by prism wear. After euthanasia, 16 orbits were examined by high-resolution magnetic resonance imaging (MRI) in the quasi-coronal plane. Paths and sizes of horizontal rectus EOMs were analyzed quantitatively in a standardized coordinate system. Whole orbits were then serially sectioned en bloc in the quasi-coronal plane, stained for connective tissue, and compared with MRI. Nerve and EOM features were analyzed quantitatively. Quantitative analysis of MRI revealed no significant differences in horizontal rectus EOM sizes or paths among orthotropic or naturally or artificially strabismic monkeys. Histologic examination demonstrated no differences in EOM size, structure, or innervation among the three groups, and no differences in connective tissues in the pulley system. The accessory lateral rectus (ALR) EOM was present in all specimens, but was small, inconsistently located, and sparsely innervated. Characteristics of the ALR did not correlate with strabismus. Major structural abnormalities of horizontal rectus EOMs and associated pulleys are unrelated to natural or artificial horizontal strabismus in the monkeys studied. The ALR is unlikely to contribute to horizontal strabismus in primates. However, these findings do not exclude a possible role of pulley abnormalities in disorders such as cyclovertical strabismus.

Comparison of Harms tangent screen and search coil recordings in patients with trochlear nerve palsy

Harms tangent screen, a subjective measurement method of three-dimensional binocular alignment, was compared with search coil recording. Twenty-three patients with unilateral trochlear nerve palsy were measured in nine gaze positions. The two methods correlated best for the horizontal gaze deviation, the vertical gaze deviation, and the vertical incomitance, but there was no correlation for the results of torsional incomitance. Using Harms tangent screen, torsional deviation underestimated the torsional incomitance measured by the search coils. Therefore, central torsional fusional mechanisms or alignment error in the Harms tangent screen are assumed.

The Active Head-Impulse Test in Unilateral Peripheral Vestibulopathy

The head-impulse test, which is sensitive and specific for detecting severe unilateral peripheral vestibulopathy, is an accepted part of the neurological examination, especially in patients with vertigo and balance disorders. To discover if the head-impulse test is just as useful diagnostically when patients are asked to rotate their own heads, the active head-impulse test, rather than when the clinician does so as in the standard passive head-impulse test. Clinical observation of compensatory saccades and search coil measurement of compensatory eye rotations, during active and passive horizontal head-impulses in 6 patients with total unilateral vestibular deafferentation. Clinical observation showed the expected compensatory saccades with rotations toward the side with the lesion with passive head-impulses but not with active head-impulses. Search coil recordings revealed 2 reasons for this. With active head-impulses not only was vestibulo-ocular reflex gain higher, but compensatory saccade latency was shorter resulting in an occult saccade that occurred during, rather than after, head rotation. Passive head-impulses are necessary to detect a severe unilateral peripheral vestibulopathy; active head-impulses will produce a false-negative result.

Vertical misalignment in unilateral sixth nerve palsy

Objective To detect and determine the magnitude of vertical deviation in patients with unilateral sixth nerve palsy. Design Prospective consecutive comparative case series. Participants Twenty patients with unilateral peripheral sixth nerve palsy, 7 patients with central palsy caused by brainstem lesions, and 10 normal subjects. Methods Subjects were tested by the prism and cover test, Maddox rod and prism test, and magnetic search coil recordings in nine diagnostic eye positions. They were also tested during static lateral head tilt by the prism and cover, and Maddox rod and prism tests. Main outcome measures The magnitudes of horizontal and vertical deviations. Results All patients had an abduction deficit and incomitant esodeviation that increased in the field of action of the paretic muscle, indicating sixth nerve palsy. Mean vertical deviations, for all positions of gaze in peripheral palsy were 0.3 ± 0.8 prism diopters (PD) by prism and cover test, 1.3 ± 1.6 PD by Maddox rod and prism test, and 2.0 ± 1.4 PD by coil recordings. Mean vertical deviations in normal subjects were 0.0 ± 0.0 PD by prism and cover test, 1.0 ± 0.9 PD by Maddox rod and prism test, and 1.9 ± 2.1 PD by coil recordings. Therefore, peripheral palsy did not cause abnormal vertical deviation. In central palsy, for all positions together mean vertical deviations were 0.9 ± 1.3 PD by prism and cover test, 1.4 ± 1.6 PD by Maddox rod and prism test, and 2.5 ± 1.6 PD by coil recordings; they were not different from normal values. During static head roll, patients with peripheral palsy had a right hyperdeviation on right head tilt and a left hyperdeviation on left head tilt, regardless of the side of the palsy. In contrast, in central palsy, head tilt caused vertical strabismus that remained on the same side on head tilt to either side. Conclusions Small vertical deviations in sixth nerve palsy are consistent with normal hyperphorias that become manifest in the presence of esotropia. In peripheral sixth nerve palsy, static head roll to either side induces hyperdeviation in the eye on the side of the head tilt. Hyperdeviation of the same eye induced by head tilt to either direction implicates a brainstem lesion as the cause of paretic abduction. Quantitative study of sixth nerve palsy demonstrates that if a vertical deviation falls within the normal range of hyperphoria, multiple cranial nerve palsy or skew deviation may not be responsible. Conversely, vertical deviation > 5 PD indicates skew deviation or peripheral nerve palsy in addition to abduction palsy.

Effect of aging on the human initial interaural linear vestibulo-ocular reflex

To determine age-related changes, the initial linear vestibulo-ocular reflex (LVOR) of eight older subjects of mean age 65+/-7 years (mean +/- SD, range 56-75 years) was compared with that of nine younger subjects of mean age 24+/-5 years (range 18-31 years) in response to random transients of whole-body heave (interaural) translation at peak acceleration of 0.5 g delivered by a pneumatic actuator. Binocular eye rotations were measured with magnetic search coils, while linear head position and acceleration were measured with a potentiometer and piezoelectric accelerometer. Subjects viewed targets 200 cm, 50 cm, or 15 cm distant immediately before the unpredictable onset of randomly directed translation in darkness (LVOR) and in light (LVVOR). All subjects maintained ideal vergence of 1.5-2 degrees for the 200-cm target, 6-8 degrees for the 50-cm target, and 21-26 degrees for the 15-cm target, with actual vergences depending on individual interpupillary distances. Search coil recording of angular position of the upper teeth showed head rotation to be negligible (less than 0.5 degrees ) for the first 250 ms after onset of head translation, excluding a role for the angular VOR in the responses studied. The LVOR response to heave translation was an oppositely directed eye rotation occurring after a mean latency of 62+/-3 ms for older and 42+/-3 ms (mean +/- SD) for younger subjects ( P<0.0001). The peak of the latency distribution was 60-100 ms for older and 20-60 ms for younger subjects. During the early interval, 70-80 ms from head motion onset prior to a pursuit contribution or saccades, all subjects had significantly enhanced LVOR with decreasing target distance. In this interval, the LVOR position amplitude of younger subjects was 0.17+/-0.01 degrees, 0.40+/-0.01 degrees, 0.57+/-0.01 degrees (mean +/- SE), respectively, in descending order of target distance. Early sensitivities were significantly reduced for older subjects to 0.07+/-0.01 degrees, 0.23+/-0.01 degrees, 0.40+/-0.01 degrees ( P<0.0001). There was no significant effect of target visibility in either group during the first 110 ms ( P>0.05). Visual-otolith interaction was mainly reflected not by the vestibular slow phase, but by vestibular catch-up saccades (VCUS) in the compensatory direction. The effect of aging on the initial human LVOR is thus to: prolong latency, reduce early sensitivity, and reduce occurrence of vestibular catch-up saccades.

Dynamic visual acuity during transient and sinusoidal yaw rotation in normal and unilaterally vestibulopathic humans.

The vestibulo-ocular reflex (VOR) stabilizes gaze to permit clear vision during head movements. It has been supposed that VOR function might be inferred from dynamic visual acuity (DVA), the acuity during imposed head motion. We sought to determine effectiveness of DVA for detection and lateralization of unilateral vestibulopathy, using rigorous psychophysical methods. Seventeen normal and 11 unilaterally vestibulopathic subjects underwent measurement of optically best corrected DVA during head motion. A variable size letter "E" 6 m distant was displayed in oblique random orientations to determine binocular DVA by a computer controlled, forced choice method. Three types of whole-body yaw rotation were delivered by a servo-controlled chair synchronized with optotype presentation. Two types of motion were predictable: (1) steady-state 2.0-Hz rotation at 10-130 degrees/s peak velocity with repetitive optotype presentation only during head velocity exceeding 80% of peak; and (2) directionally predictable transients at peak accelerations of 1000, 1600 and 2800 degrees/s2 with optotype presentation for 300 ms. For neither of these predictable motions did DVA in vestibulopathic subjects significantly differ from normal, with suggestions from search coil recordings that this was due to predictive slow and saccadic eye movements. Unilaterally vestibulopathic subjects experienced a significant decrease in DVA from the static condition during ipsilesional rotation for all three peak head accelerations. Only during directionally unpredictable transients with 75 ms or 300 ms optotype presentation was the sensitivity of DVA in unilaterally vestibulopathic subjects significantly abnormal during ipsilesional rotation. The ipsilesional decrease in DVA with head motion was greater for 75 ms than 300 ms optotype presentation. Search coil recordings confirmed hypometric compensatory eye movements during DVA testing with unpredictable, ipsilesional rotation. Receiver-operator characteristic analysis indicated ideal detection and lateralization of unilateral vestibulopathy by DVA tested with a 75-ms optotype exposure for unpredictable transient rotations to a peak acceleration of 2800 degrees/s. DVA can reliably detect unilateral deafferentation only if precautions are taken to prevent compensation by predictive slow eye movements and saccades.

Surgery for DVD: New Approaches

The oblique extraocular muscles appear to play a greater role than previously thought in dissociated vertical deviation (DVD). Scleral search coil recordings show that the vertical vergence component of DVD, because of the accompanying torsional changes, receives significant contribution from the oblique extraocular muscles.1,2 Based on these findings, new surgical approaches for DVD have been attempted, with the purpose of decreasing the vertical action of the oblique muscles. Recession and anterior transposition of all the oblique muscles has given an excellent result in some patients but a poor result in others, with correction averaging about half of the amplitude of the DVD. Large weakening procedures of all the oblique muscles, without anterior transposition, have not been helpful, not correcting more than 40% of the DVD in any case. Bilateral weakening of all four elevators has decreased the amplitude of DVD by 50 to 70%, but in two of three cases yielded objectionable retraction of both upper lids. The best surgical approach to DVD is yet to be determined and may depend upon specific assessment of the torsional and vertical components of the particular patient's DVD response.

See-saw nystagmus and congenital nystagmus identified in the non-decussating retinal-fugal fiber syndrome

Purpose The purpose of the present study was to accurately document the oculomotor misalignments and instabilities associated with the non-decussating retinal-fugal fiber syndrome, a rare inborn, isolated, achiasmatic condition. 1–3 To date, the achiasmatic syndrome described has thus far been identified in three unrelated females. 1–4 A comparable achiasmatic condition has also been identified in a canine breed. 5,6 Materials and Methods The unique, inborn visual pathway malformation in the form of an isolated absence of the optic chiasm was confirmed by metabolic assay, the visual evoked potential (VEP) misrouting paradigm and neuroradiological evaluation, including magnetic resonance imaging (MRI). Horizontal and vertical, left and right eye, oculomotor scleral search coil recordings were measured, in one of the achiasmats, at 15 years of age. Comparable oculomotor recordings were implemented in three controls, including a normal age-matched control, an adult control and an adult with idiopathic congenital nystagmus (CN). In addition to central and eccentric target fixation with binocular and monocular viewing, binocular and monocular pursuit and optokinetic nystagmus (OKN) also were recorded. Results Following a binocular and monocular fixation paradigm, classic congenital nystagmus (CN) profiles were recorded in the achiasmat in the horizontal planes together with see-saw nystagmus (SSN) in the vertical planes; an alternating esotropia and vertical tropias also were documented. The CN showed interocular conjugacy and waveforms typically exhibited well-defined foveation periods. In general, both achiasmatic horizontal and vertical eye movements as well as corresponding interocular misalignments varied with stimu-lus and recording conditions. Moreover, horizontal eye movements showed sustained interocular conjugacy while vertical eye movements typically showed disconjugacy. Previous oculomotor studies in the achiasmatic canine breed revealed comparable oculomotor instabilities, albeit the latter were reported to present primarily with unyoked eye movements and uniocular saccades. In the present study, in addition to target fixation with binocular and monocular viewing, binocular and monocular pursuit and OKN responses also were recorded. For the achiasmat, high-gain pursuit results, as with eccentric fixation, emphasized a preference for fixation with the adducting eye. Also at eccentric gaze with monocular viewing, the eye under cover showed, relative to the fixating eye, a hypodeviation concomitant with adduction and a hyperdeviation concomitant with abduction. OKN profiles in the achiasmat with stimuli presented under both normal viewing and open-loop, retinal-field locked conditions proved rather complex; however, when an OKN response was identified, reversed OKN tracking was not apparent. Conclusions In general, the oculomotor results reported herein demonstrate that the non-decussating retinal-fugal fiber syndrome is an afferent visual pathway disorder associated with vertical and horizontal misalignments concurrent with congenital nystagmus and also with the rarer see-saw nystagmus.

A comparison of video and magnetic search coil recordings of mouse eye movements

Interest in connecting molecular biology and behavior is motivating research into the eye movements of mice. Unfortunately, recording eye movements in this diminutive animal is technically difficult. We present the first method for obtaining calibrated video oculography, and contrast the results with simultaneously obtained scleral search coil recordings in C57BL/6 pigmented mice. We determined the distance of the pupil from the center of corneal curvature, based upon relative motions of the pupil and corneal reflections during camera movements, and used the distance to convert subsequent video measurements of pupil motion to eye rotation. We recorded responses during sinusoidal rotation (0.1–1.6 Hz) in the light, by video prior to search coil implantation, and by video and search coil simultaneously following implantation. Pre-implantation, video-derived gains ranged from 0.86±0.03 (mean±SD) at 0.1 Hz to 0.95±0.03 at 0.8 Hz. Phase progressed monotonically from −3.1±2.6° (eye leads head) at 0.1 Hz to +5.9±1.1° at 1.6 Hz. Coil implantation reduced the range of video-derived gains to 0.64–0.79. This reduction reflects disruption of normal behavior by the coil. Coil data confirmed the video results. Video and search coil techniques each have advantages. Specific precautions are required when designing and interpreting experiments using the coil technique.

Vestibular influence on the binocular control of vertical-torsional nystagmus after lesions in the interstitial nucleus of Cajal.

The interstitial nucleus of Cajal (iC) is a center of the velocity-to-position integration for vertical and torsional eye movements. In addition, iC has projections to and from the vestibular nuclei. Therefore the vestibular influence on the binocular alignment of vertical-torsional nystagmus after unilateral reversible iC inactivations was investigated in the alert monkey using 3D binocular search-coil recordings. The nystagmus was compared with the eye muscle rotation axes, which were corrected for the tonic ocular torsion elicited by the iC inactivation. Rotation axes of nystagmus were different for both eyes and revealed a co-activation of eye muscles similar to the effects of electrical stimulation of the anterior canal nerve. This suggests that, in addition to the deficient neural integrator, a vestibular imbalance contributes to the vertical-torsional nystagmus after iC inactivations.

Directional abnormalities of vestibular and optokinetic responses in cerebellar disease.

Directional abnormalities of vestibular and optokinetic responses in patients with cerebellar degeneration are reported. Three-axis magnetic search-coil recordings of the eye and head were performed in eight cerebellar patients. Among these patients, examples of directional cross-coupling were found during (1) high-frequency, high-acceleration head thrusts; (2) constant-velocity chair rotations with the head fixed; (3) constant-velocity optokinetic stimulation; and (4) following repetitive head shaking. Cross-coupling during horizontal head thrusts consisted of an inappropriate upward eye-velocity component. In some patients, sustained constant-velocity yaw-axis chair rotations produced a mixed horizontal-torsional nystagmus and/or an increase in the baseline vertical slow-phase velocity. Following horizontal head shaking, some patients showed an increase in the slow-phase velocity of their downbeat nystagmus. These various forms of cross-coupling did not necessarily occur to the same degree in a given patient; this suggests that different mechanisms may be responsible. It is suggested that cross-coupling during head thrusts may reflect a loss of calibration of brainstem connections involved in the direct vestibular pathways, perhaps due to dysfunction of the flocculus. Cross-coupling during constant-velocity rotations and following head shaking may result from a misorientation of the angular eye-velocity vector in the velocity-storage system. Finally, responses to horizontal optokinetic stimulation included an inappropriate torsional component in some patients. This suggests that the underlying organization of horizontal optokinetic tracking is in labyrinthine coordinates. The findings are also consistent with prior animal-lesion studies that have shown a role for the vestibulocerebellum in the control of the direction of the VOR.