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PubMed1997

The human horizontal vestibulo-ocular reflex during combined linear and angular acceleration.

We employed binocular magnetic search coils to study the vestibulo-ocular reflex (VOR) and visually enhanced vestibulo-ocular reflex (VVOR) of 15 human subjects undergoing passive, whole-body rotations about a vertical (yaw) axis delivered as a series of pseudorandom transients and sinusoidal oscillations at frequencies from 0.8 to 2.0 Hz. Rotations were about a series of five axes ranging from 20 cm posterior to the eyes to 10 cm anterior to the eyes. Subjects were asked to regard visible or remembered targets 10 cm, 25 cm, and 600 cm distant from the right eye. During sinusoidal rotations, the gain and phase of the VOR and VVOR were found to be highly dependent on target distance and eccentricity of the rotational axis. For axes midway between or anterior to the eyes, sinusoidal gain decreased progressively with increasing target proximity, while, for axes posterior to the otolith organs, gain increased progressively with target proximity. These effects were large and highly significant. When targets were remote, rotational axis eccentricity nevertheless had a small but significant effect on sinusoidal gain. For sinusoidal rotational axes midway between or anterior to the eyes, a phase lead was present that increased with rotational frequency, while for axes posterior to the otolith organs phase lag increased with rotational frequency. Transient trials were analyzed during the first 25 ms and from 25 to 80 ms after the onset of the head rotation. During the initial 25 ms of transient head rotations, VOR and VVOR gains were not significantly influenced by rotational eccentricity or target distance. Later in the transient responses, 25-80 ms from movement onset, both target distance and eccentricity significantly influenced gain in a manner similar to the behavior during sinusoidal rotation. Vergence angle generally remained near the theoretically ideal value during illuminated test conditions (VVOR), while in darkness vergence often varied modestly from the ideal value. Regression analysis of instantaneous VOR gain as a function of vergence demonstrated only a weak correlation, indicating that instantaneous gain is not likely to be directly dependent on vergence. A model was proposed in which linear acceleration as sensed by the otoliths is scaled by target distance and summed with angular acceleration as sensed by the semicircular canals to control eye movements. The model was fit to the sinusoidal VOR data collected in darkness and was found to describe the major trends observed in the data. The results of the model suggest that a linear interaction exists between the canal and otolithic inputs to the VOR.

Adolescent↗
PubMed1991

The primate vestibulo-ocular reflex during combined linear and angular head motion.

The squirrel monkey vestibulo-ocular reflex (VOR) was studied in darkness during Earth-horizontal rotation over a frequency range, 0.01-4 Hz, with the head positioned both centrally and displaced radially relative to the axis of rotation. With the head centered, the canal-mediated angular VOR (AVOR) was recorded in isolation. However, with the head placed eccentrically, otolith-mediated linear VOR (LVOR) components interact with the AVOR to yield a combined AVOR-LVOR response. The plane of the ocular response could be manipulated by placing the head in different orientations relative to gravity (i.e. upright or nose-up). When the head was upright and centered, the horizontal AVOR was recorded. Comparisons between eye and head angular velocity showed that gain (pk eye/pk head velocity) was nearly flat, averaging 0.83, across the frequency range. Phase (difference in degrees between eye and head velocity, shifted 180 degrees by convention) was near 0 degrees, except at frequencies below 0.1 Hz where phase leads were seen. When the head was displaced eccentrically and in the nose-out position (facing away from the axis of rotation), gain rose above that of the AVOR alone. The enhancement was progressive with increasing frequency, but only for frequencies above 0.25 Hz. When the subject was turned nose-in, gain declined relative to the AVOR alone, and in a similar frequency-dependent fashion. These results are consistent with the notion that nose-out and nose-in responses to eccentric rotation represent a combined influence of the horizontal AVOR and LVOR, the latter driven by inter-aural tangential acceleration. To further evaluate this possibility, eccentric rotation was also used to assess the LVOR in isolation. With the head in the nose-up orientation, the AVOR was shifted into the head's roll plane and generated torsional ocular responses. With the head centered over the axis of rotation, no systematic horizontal responses were observed. However, when the head was displaced eccentrically and placed in the head-out and head-in positions, horizontal ocular responses were recorded which were proportional to head eccentricity and were of appropriate polarity to presume that they represented the inter-aural LVOR activated by inter-aural tangential acceleration. Response gain rose with increasing frequency, as did tangential acceleration. The LVOR in its resting state in darkness could be characterized by an average sensitivity of 40.3 degrees/s/g (g = 9.81 m/s2).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2000

The role of the veno-arteriolar reflex (VAR) in the pathogenesis of peripheral oedema in patients with chronic critical limb ischaemia (CLI).

BACKGROUND AND AIMS: Relatively many patients with chronic critical limb ischaemia (CLI) have oedema in the afflicted limb. Previous studies in these patients indicate derangement in the Starling forces governing transcapillary fluid balance. An impaired veno-arteriolar reflex (VAR) may cause an increase in capillary pressure and hence increased filtration pressure resulting in oedema. The aim of the present study was to investigate VAR in patients with CLI to come to a better understanding of the pathogenesis of ischaemic oedema. MATERIAL AND METHODS: Sixteen patients (mean age of 78+/-9.4 years) with unilateral CLI and oedema were included. There were two control groups of similar age, one consisted of 8 patients with unilateral CLI but without oedema and the other of 9 healthy subjects. Laser Doppler fluxmeters were used to evaluate the foot skin microcirculation, with the limbs in supine as well as in the dependent position, simultaneously in 4 different areas: the pulp of the first toe (Sitel), at the level of the second metatarsal body (Site 2), at the anterolateral part of ankle (Site 3) and the pulp of the first toe of the contralateral limb (Site 4) as reference. Laser Doppler flux (LDF) values (expressed in perfusion unit, PU) were recorded with the foot dependent (PUd), in the horizontal position (PUh), and the orthostatic response (OR) was calculated at all measuring sites as PUd/PUh. RESULTS: In none of the measured sites there was a significant difference in OR between the limbs with CLI and oedema and the limbs with CLI but without oedema. The median OR of CLI limbs at site 1 [2.5 (0.61-8.96)] was greater than at sites 2 [0.99 (0.46-2.38), p < 0.01] and 3 [0.95 (0.68-10.31), p < 0.04] respectively, while the differences in OR between site 2 and 3 were not significant. The median OR in the limbs of healthy controls at sites 1, 2 and 3 were 0.58 (0.37-1.43), 0.54 (0.28-1.33) and 0.51 (0.34-0.91), respectively. There were no significant differences in OR between sites 1, 2 and 3 of the control group. The OR of CLI at site 1, 2 and 3 were significantly greater than the corresponding sites in the healthy subjects (p < 0.001, p < 0.008, p < 0.001). CONCLUSIONS: The VAR is disturbed in limbs with CLI, both in the ones with and without oedema. There were regional differences in the OR in these ischaemic feet but there were no differences in OR between those with and without oedema. Thus, disturbances in VAR may play a role in the development of ischaemic oedema, but is probably not the only causative factor.

Aged↗
PubMed2003

Gravity-specific adaptation of the angular vestibuloocular reflex: dependence on head orientation with regard to gravity.

The gain of the vertical angular vestibuloocular reflex (aVOR) was adaptively altered by visual-vestibular mismatch during rotation about an interaural axis, using steps of velocity in three head orientations: upright, left-side down, and right-side down. Gains were decreased by rotating the animal and visual surround in the same direction and increased by visual and surround rotation in opposite directions. Gains were adapted in one head position (single-state adaptation) or decreased with one side down and increased with the other side down (dual-state adaptation). Animals were tested in darkness using sinusoidal rotation at 0.5 Hz about an interaural axis that was tilted from horizontal to vertical. They were also sinusoidally oscillated from 0.5 to 4 Hz about a spatial vertical axis in static tilt positions from yaw to pitch. After both single- and dual-state adaptation, gain changes were maximal when the monkeys were in the position in which the gain had been adapted, and the gain changes progressively declined as the head was tilted away from that position. We call this gravity-specific aVOR gain adaptation. The spatial distribution of the specific aVOR gain changes could be represented by a cosine function that was superimposed on a bias level, which we called gravity-independent gain adaptation. Maximal gravity-specific gain changes were produced by 2-4 h of adaptation for both single- and dual-state adaptations, and changes in gain were similar at all test frequencies. When adapted while upright, the magnitude and distribution of the gravity-specific adaptation was comparable to that when animals were adapted in side-down positions. Single-state adaptation also produced gain changes that were independent of head position re gravity particularly in association with gain reduction. There was no bias after dual-state adaptation. With this difference, fits to data obtained by altering the gain in separate sessions predicted the modulations in gain obtained from dual-state adaptations. These data show that the vertical aVOR gain changes dependent on head position with regard to gravity are continuous functions of head tilt, whose spatial phase depends on the position in which the gain was adapted. From their different characteristics, it is likely that gravity-specific and gravity-independent adaptive changes in gain are produced by separate neural processes. These data demonstrate that head orientation to gravity plays an important role in both orienting and tuning the gain of the vertical aVOR.

Adaptation, Physiological↗
PubMed1996

Behavior of eye-movement-related cells in the vestibular nuclei during combined rotational and translational stimuli.

1. Secondary position-vestibular-pause (PVP) neurons in the vestibuloocular reflex (VOR) pathway of adult rhesus monkeys were studied during combined semicircular canal and otolith stimulation. The head was rotated at 0.5 Hz with the axis of rotation centered between the otolith organs (on-axis, ON) and with the axis of rotation 23 cm in front of the otoliths (off-axis, OFF). Both conditions were tested with two different vergence angles by the use of 14-cm (near target, NT) and 100-cm (far target, FT) targets. 2. The tangential translational stimulus to the otoliths in the OFF trials should result in a compensatory eye movement that is opposite in direction to that resulting from the angular stimulus to the canals. The otolith stimulus should be great enough to reverse the eye movement response in the NT OFF trials according to geometric calculations. This reversal in eye movement direction occurred as expected although the latency of the reversal (70 ms) was somewhat greater than expected and the magnitude of the reversal was less than predicted solely on the basis of geometric considerations. 3. The responses of the PVP neurons were corrected for eye position sensitivity to investigate the head movement response components. The amplitude of the response in 22 of 24 PVP cells was reduced in the NT OFF condition compared with the FT OFF condition. This difference was not sufficient in itself to explain the observed reversal in eye movement response. 4. The average sensitivities of the neurons to rotation during the FT and NT ON trials were 1.38 and 1.41 spikes.s-1.deg-1.s-1, respectively. This is too small an increase to account for the increase in the angular VOR gain with near targets (approximately 25%); therefore cells other than PVP neurons must be responsible. 5. The average sensitivities of the PVP neurons to translational accelerations obtained from the FT and NT OFF trials were 305 and 484 spikes.s-1.g-1, respectively, which is higher than most otolith afferent sensitivities reported for 0.5-Hz stimuli in the literature. The otolith component is modified by ocular convergence (59% increase in sensitivity), but this increase is too small to account for the change in the translational VOR gain between the two conditions. 6. Although recordings were only obtained from seven eye-head-velocity cells, the results indicate that these neurons may provide the additional signals not present in the PVP cells. These neurons exhibited large differences between ON and OFF rotations and were found to substantially increase their modulation during the NT conditions compared with that observed during the FT conditions.

Animals↗
PubMed1998

Human horizontal vestibulo-ocular reflex initiation: effects of acceleration, target distance, and unilateral deafferentation.

The vestibulo-ocular reflex (VOR) generates compensatory eye movements in response to angular and linear acceleration sensed by semicircular canals and otoliths respectively. Gaze stabilization demands that responses to linear acceleration be adjusted for viewing distance. This study in humans determined the transient dynamics of VOR initiation during angular and linear acceleration, modification of the VOR by viewing distance, and the effect of unilateral deafferentation. Combinations of unpredictable transient angular and linear head rotation were created by whole body yaw rotation about eccentric axes: 10 cm anterior to eyes, centered between eyes, centered between otoliths, and 20 cm posterior to eyes. Subjects viewed a target 500, 30, or 15 cm away that was extinguished immediately before rotation. There were four stimulus intensities up to a maximum peak acceleration of 2,800 degrees/s2. The normal initial VOR response began 7-10 ms after onset of head rotation. Response gain (eye velocity/head velocity) for near as compared with distant targets was increased as early as 1-11 ms after onset of eye movement; this initial effect was independent of linear acceleration. An otolith mediated effect modified VOR gain depending on both linear acceleration and target distance beginning 25-90 ms after onset of head rotation. For rotational axes anterior to the otoliths, VOR gain for the nearest target was initially higher but later became less than that for the far target. There was no gain correction for the physical separation between the eyes and otoliths. With lower acceleration, there was a nonlinear reduction in the early gain increase with close targets although later otolith-mediated effects were not affected. In subjects with unilateral vestibular deafferentation, the initial VOR was quantitatively normal for rotation toward the intact side. When rotating toward the deafferented side, VOR gain remained less than half of normal for at least the initial 55 ms when head acceleration was highest and was not modulated by target distance. After this initial high acceleration period, gain increased to a degree depending on target distance and axis eccentricity. This behavior suggests that the commissural VOR pathways are not modulated by target distance. These results suggest that the VOR is initially driven by short latency ipsilateral target distance dependent and bilateral target-distance independent canal pathways. After 25 ms, otolith inputs contribute to the target distance dependent pathway. The otolith input later grows to eventually dominate the target distance mediated effect. When otolith input is unavailable the target distance mediated canal component persists. Modulation of canal mediated responses by target distance is a nonlinear effect, most evident for high head accelerations.

Acceleration↗
PubMed1998

Gaze stabilization during dynamic posturography in normal and vestibulopathic humans.

Dynamic posturography by measurement of center of pressure (COP) is a widely employed technique for evaluating the vestibular system. However, the relationship of COP motion to vestibulo-ocular reflex (VOR) function and image stability on the retina has not been determined previously. To assess these relationships, we report gaze, head, and trunk stability during dynamic posturography in 11 normal volunteers, 7 subjects with unilateral vestibular lesions, and 3 subjects with bilateral vestibular lesions. Posturographic tasks consisted of standing still and standing on a platform that was sliding (0.2 Hz), tilting (0.1 Hz), or covered with a foam cushion 6 cm thick while tilting (0.1 Hz). Each perturbation was imposed in the anterior-posterior and repeated in the medial-lateral direction, in both light and darkness. Subjects viewed (or in darkness remembered) a target located 50, 100, or 500 cm distant. COP, angular eye position, and angular and linear orbit and trunk positions were measured using magnetic search coils and flux gate magnetometer sensors. With the target visible, the velocity of image motion on the retina was on average always less than 1 degree/s, well within the range consistent with high visual acuity. In darkness, gaze velocity increased for normal and vestibulopathic subjects. During tilt, vestibulopathic subjects had a significantly greater gaze velocity than controls. Gain of the angular VOR (eye velocity/head velocity) was significantly lower in darkness than in light and in vestibulopathic as compared to control subjects. Gain of the VOR was significantly correlated with gaze instability, but variation in VOR gain accounted for only 20-40% of the variance. In darkness, the velocity of the COP was significantly greater in vestibulopathic than control subjects for every condition tested. In light, this difference was small and often not significant. Although spectral analysis of the COP indicated frequencies above 1 Hz that were not observed in motion of the trunk and orbit, root mean square (RMS) velocities of the trunk and orbit in the horizontal plane were higher in darkness and in vestibulopathic subjects, mirroring COP findings. Only in vestibulopathic subjects tested in darkness was there a correlation between COP velocity and gaze velocity; COP velocity was otherwise uncorrelated with gaze. Gaze velocity was greater with near than with distant targets. Vertical VOR gain was higher with near targets. No other significant effects of target distance were found. Head movement strategy, VOR gain, and COP were all unaffected by target proximity. These data show that gaze velocity measurements during dynamic posturography in darkness are sensitive to vestibular loss. With a visible target, both COP and gaze stability of vestibulopathic subjects are difficult to distinguish from normal. During visual feedback, it is likely that image stabilization over the range of frequencies tested is achieved through better head stability and through visual tracking, allowing vestibulopathic subjects to maintain adequate visual acuity.

Adult↗
PubMed1996

Adaptation of primate vestibuloocular reflex to altered peripheral vestibular inputs. II Spatiotemporal properties of the adapted slow-phase eye velocity.

1. The ability of the vestibuloocular reflex (VOR) to undergo adaptive modification after selective changes in the peripheral vestibular system was investigated in rhesus monkeys by recording three-dimensional eye movements before and after inactivation of selective semicircular canals. In the preceding paper we showed that the horizontal VOR gain evoked by passive yaw oscillations after lateral semicircular canal inactivation recovers gradually over time in a frequency-specific manner. Here we present the spatial tuning of the adapted slow-phase eye velocity and describe its spatiotemporal properties as a function of time after canal inactivation. 2. The spatial organization of the VOR was investigated during oscillations at different head positions in the pitch, roll, and yaw planes, as well as in the right anterior/left posterior and left anterior/right posterior canal planes. Acutely after bilateral inactivation of the lateral semicircular canals, a small horizontal response could still be elicited that peaked during rotations in pitched head positions that would maximally stimulate vertical semicircular canals. In addition, the phase of horizontal slow-phase velocity abruptly reversed through 180 degrees at positions close to upright, similarly to torsional slow-phase velocity. These spatial response properties suggest that the small, residual horizontal response components that are present acutely after plugging of both lateral canals originate from vertical semicircular canal signals. 3. As the horizontal response amplitude increased over time, consistent changes were also observed in the spatiotemporal tuning of horizontal slow-phase velocity. 1) The spatiotemporal response properties of horizontal slow-phase velocity acquired noncosine tuning characteristics, primarily in the pitch plane, in the right anterior/left posterior and left anterior/right posterior canal planes. Accordingly, horizontal response amplitude was nonzero during rotation in any head position in these planes and response phase varied significantly as a function of head orientation. 2) The peak horizontal response amplitude shifted spatially over time, such that 5-10 mo after plugging it was maximal during rotations at head positions close to upright. 4. In parallel to these unique spatiotemporal response properties characterizing the adapted horizontal VOR, torsional slow-phase velocity also exhibited small spatiotemporal changes after lateral canal inactivation that tended to precede in time the changes associated with the horizontal response components. In contrast, vertical slow-phase velocity in the plugged animals was unaltered and continued to be characterized by cosine-tuned spatial properties in three dimensions. 5. Recovery of the horizontal response gain during yaw oscillations in upright position, as well as the unique, noncosine spatiotemporal characteristics of the adapted horizontal VOR, were also observed in an animal with all but one vertical semicircular canals inactivated. There was, however, no sign of VOR gain recovery up to 2 mo after all semicircular canals were inactivated. These results suggest that the observed recovery of horizontal VOR is at least partly due to signals originating from the remaining intact vertical canal(s). Even in the presence of a single intact vertical canal, the improvement in horizontal gaze stability is at least partly restored through spatiotemporal changes in the processing of vestibuloocular signals that improve the gain and spatial tuning of horizontal VOR at the expense of temporal response properties.

Adaptation, Physiological↗
PubMed2000

[Head shaking test and low-frequency rotation-acceleration test: comparison of sensitivity and specificity].

In studying the vestibular-oculomotor reflex (VOR) in patients with equilibrium disorders, both active and passive methods are used and at different stimulation frequencies, particularly to evaluate vestibular compensation. The present study compares the sensitivity and specificity of the low-frequency pendular test and the HST in normal subjects and in subjects suffering from various vestibular pathologies but showing no signs of spontaneous nystagmus, even under optimal evaluation conditions (infrared videonystagmoscopy). These spontaneous nystagmus-free subjects underwent a routine study including: case history, infrared videonystagmoscopy (HST with 2 Hz stimulation) and ENG recording (saccadic movement, smooth pursuit, OKN, VOR, VOR-fix during pendular stimulation at 0.05 Hz and VOR after Fitzgerald-Hallpike heat stimulation). On the basis of the results the subjects were classified as follows: normal (N): case history free of any equilibrium disorders and all tests negative (122 cases; 8.1%); peripheral vestibular pathology (P): case history of equilibrium disorders and labyrinthine predominance (LP) in excess of 20% upon caloric testing (716 cases; 47.6%); central pathology (C): case history of equilibrium disorders and at least 3 pathological results from among the following tests: saccadic movement, smooth pursuit, OKN, VOR-fix (226 cases; 15.0%); mixed pathology (M): with both signs of P and C (440 cases; 29.3%). The pendular test showed signs of directional dominance (DP) higher than 10% (normal limit) in 7 cases of N (5.7%), 308 P (43.0%), 33 C (14.6%) and 162 M (36.8%). DP was higher than 10% in 55.2% of the P and M cases with onset less than 1 month before, in 42.8% of those with onset within the year and in 37.2% of those with onset more than a year before. A pathological response to the HST was observed (characterized by a series of at least 3 nystagmus shakes after a maximum latency of 15 seconds) in 0 N (0.0%), 378 P (52.8%), 4 C (1.8%) and 247 M (56.1%). The nystagmus seen was nearly always monophasic (92.5%), biphasic nystagmus was only seen in only a few cases (7.5%); moreover it was predominantly horizontal in nature (94.9%) while it was vertical in only a few of the C cases (5.1%). The HST proved pathological in 46.5% of those pathologies with onset less than 1 month before, in 55.8% of those with onset within the year and in 54.4% of those with onset more than a year before. In 213 of the cases presenting pathological response to the HST there was agreement between VOR DP at the pendular test and the direction of the nystagmus evoked by the HST: 138 P (74.6%), 0 C (0.0%), 75 M (68.2%): moreover there was no agreement in 83 cases. When the DP was lower than 10% at the pendular test, the HST proved pathological in 213 P (52.2%), 3 C (1.6%), 150 M (53.9%). Both tests gave negative results in 112 N (91.8%), 163 P (22.8%), 186 C (82.3%), 123 M (27.9%). In cases of peripheral vestibular deficit (P and M) the sensitivity of the rotoacceleration test was 40.7%, specificity 88.5%. The sensitivity of the HST was 54.1%, specificity 98.8%. The sensitivity of the association using both tests was 75.2%, specificity 85.7%.

Biophysical Phenomena↗
PubMed1986

The physiology of orthostatic tremor.

This article describes the physiological findings in a patient with orthostatic tremor. The tremor primarily affected the legs, was alternating between antagonist muscle groups, and had a remarkably rapid frequency of 16 Hz. It was present only during certain postures and appeared to be of central origin. A tremor of the same frequency was also recorded in the arms during particular movements, but was cocontracting. We suggest that orthostatic tremor may be generated by spontaneous oscillation in those central structures responsible for organizing the motor programs for standing.

Electromyography↗
PubMed2006

Cortical and spinal abnormalities in psychogenic dystonia.

OBJECTIVE: The pathophysiology of psychogenic dystonia has not been examined, but a growing body of literature suggests that abnormal sensory input from repetitive movements can lead to plastic cortical changes. Reduced cortical and spinal inhibition is well documented in organic dystonia. We tested the hypothesis that aberrant sensory input associated with abnormal posture may cause similar abnormalities by testing patients with psychogenic dystonia. METHODS: We assessed cortical and spinal inhibitory circuits and cortical activity associated with voluntary movement in 10 patients with clinically definite psychogenic dystonia, 8 patients with organic dystonia, and 12 age-matched healthy control subjects. RESULTS: Three measures of cortical inhibition, resting short- and long-interval intracortical inhibition and cortical silent period, were reduced in both psychogenic dystonia and organic dystonia. Cutaneous silent period mediated by spinal circuitries was increased in psychogenic and organic dystonia. Forearm spinal reciprocal inhibition was reduced in psychogenic dystonia. INTERPRETATION: Psychogenic and organic dystonia share similar physiological abnormalities. Previous findings of abnormal cortical and spinal excitability in organic dystonia may, in part, be a consequence rather than a cause of dystonia. Alternatively, these findings may represent endophenotypic abnormalities that predispose to both types of dystonia.

Adult↗