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PubMed1995

The visual vertigo syndrome.

Neuro-otological and posturography findings in 15 patients with visually induced vertiginous symptoms (visual vertigo, VV) are reported. Most patients were considered to have peripheral vestibular disorders but one had cerebellar degeneration and another a brainstem stroke. Five patients showed abnormally large body sway induced by full-field visual motion stimulation; including the 2 patients with CNS disease. Four of these 5 patients had additional strabismic symptoms (diplopia, squint surgery and ocular muscle weakness). VV may occur if patients with balance disorders show high visual field dependence. The presence of additional CNS or strabismic symptoms, probably by reducing the ability to resolve conflicting visual stimuli, may lead to inappropriate postural reactions.

Adult↗
PubMed1995

Unilateral vestibular deafferentation (UVD) causes permanent asymmetry in the gain of the yaw VOR to high acceleration head impulses in guinea pigs.

Using an acute scleral search coil technique for measuring eye position in alert animals we have shown that after UVD the yaw VOR in the guinea pig shows a permanent gain asymmetry. There is a reduced gain during the first 100 ms of brief, high acceleration horizontal head rotations ("yaw head impulses") towards the operated side, but only a small loss in gain for similar rotations towards the intact side. This result confirms that the horizontal E response during the first 100 ms of an abrupt high acceleration head rotation is a clear indicator of the function of the horizontal canal. These results are similar to those in human patients after unilateral acoustic neuroma operations. The asymmetry in response is large shortly after UVD and decreases over time but is permanent.

Acceleration↗
PubMed1995

Torsional eye velocity components during yaw angular acceleration identify the side of unilateral vestibular deafferentation.

Using dual torsion scleral search coils we have recorded 3-dimensional eye position during yaw angular accelerations of 20 degrees/s2 about an earth vertical axis in healthy subjects and in patients with unilateral vestibular deafferentation (UVD). These experiments produced two interesting results: i) even in healthy subjects, the axis of eye velocity did not coincide with the (earth vertical) stimulus axis during centred rotation; ii) Patients with UVD had torsional eye velocity components that were systematically different from those in normal subjects. While in normals the direction of the torsional component of the eye velocity depended on the direction of rotation and was on average approximately symmetric for CW and CCW yaw rotation, there was a clear asymmetry in patients, which was distinctly different for left and right UVD.

Acceleration↗
PubMed1995

Influence of target distance and acceleration level on eye movements evoked by lateral acceleration steps.

Lateral eye movements produced by linear acceleration along the inter-aural axis were studied in 6 normal subjects. They were seated upright, whole-body restrained, and were exposed to randomised rightward/leftward steps of 0.05 g. 0.1 g, 0.24 g of 600 ms duration. When viewing earth-fixed targets at 30, 60 or 280 cm from their eyes, mainly pure compensatory slow-phase eye movements were evoked at latencies around 50 ms measured for the closest viewing distances. At onset, slow-phase amplitude was modulated by acceleration and target distance. When the subjects were stationary and pursued moving targets at similar distances and accelerations, latencies around 140 ms were observed, and catch-up saccades were frequently made. From these experiments, we defined the dynamics of the otolith-ocular reflex for various levels of acceleration and viewing distances.

Adult↗
PubMed1995

Subjective postural vertical in peripheral and central vestibular disorders.

The perception of subjective postural vertical was assessed in normals and patients with peripheral and central vestibular disorders and spasmodic torticollis. The subjects were seated in a motorized gimbal with the head and torso restrained and their eyes closed. The gimbal executed 7-10 cycles of tilt around the vertical at 1.5 degrees/s in either pitch or roll. Subjects indicated when they began to feel upright and again when they began to feel tilted by an analogous 3-position joystick. Normal subjects felt upright within a sector of 5-6 degrees around vertical in pitch and roll. Five patients with absent vestibular function, 25 torticollis patients and 3 patients with acute unilateral peripheral vestibular lesions showed a significant increase of the sector in pitch and roll, but only the latter had a mild directional bias. Two patients with long standing complete unilateral vestibular deficit and 8 patients with up or downbeat nystagmus in the vicinity of upright had abnormally large sectors within which they felt to be upright. The results suggest that vestibular function is important for the accurate perception of the postural vertical and that a directional asymmetry in vestibulo-ocular function or a head tilt does not necessarily correlate with a directional bias of subjective verticality.

Adult↗
PubMed1995

Effect of vergence on the gain of the linear vestibulo-ocular reflex.

We measured the linear vestibulo-ocular reflex (LVOR) and vergence, using binocular search coils, in 3 humans. The subjects were accelerated sinusoidally at 0.5 Hz and 0.2 g peak acceleration, in complete darkness, while performing three different tasks: i) mental arithmetic; ii) tracking a remembered target at either 0.34 m or 0.14 m distance; and iii) maintaining vergence at either of these distances by means of audio biofeedback based on vergence. Subjects could control vergence using the audio feedback; there was greater convergence with the near audio target. However, there was no significant difference in vergence between the near and far remembered target conditions. With audio feedback, the amplitude of smooth tracking was not consistently different for the near and the far conditions. However, the amplitude of tracking (saccades and smooth component) in the remembered target conditions was greater for near than for far targets. These results suggest that linear VOR amplitude is not determined by vergence alone.

Attention↗
PubMed1995

Influence of gravity on the orientation of vestibular induced quick phases.

In rabbits and cats the orientation of the quick phases (QPs) of the vestibulo-ocular reflex (VOR) was studied varying the head position in space. At different head tilt positions, QPs induced by step vestibular stimulation disaligned with respect to the stimulus toward the orientation of the earth's horizontal axis. The rabbits' QPs were horizontal during yaw stimulation and remained horizontal in a range of head pitch of +/- 90 degrees (reorientation gain = 1). Therefore, the slow compensatory responses (CSPs) progressively disaligned compared with the QPs. QPs induced by roll stimulation also showed horizontal orientation, although these were rare in the upright position and occurred more frequently when the head was pitched. In cats only the yaw-induced QPs were coplanar with the stimulus, while QPs induced by pitching were mostly oblique. It followed that in either yawing or pitching, the QPs had their end point scattered within a horizontally elongated area of the visual field. When tilting cats in the frontal plane, the orientation of QP trajectories changed with respect to the stimulus so that the end point distribution tended to remain aligned toward the horizontal instead of being fixed in the orbit. The reorientation gain decreased from 1 to 0.5 by increasing the head tilt. On the basis of difference regarding eye implantation and motility it was suggested that the effect of gravity on the orientation of QPs could be aimed at maintaining the interocular axis aligned with the horizon in the rabbit and at orientating the visual scanning system in the horizontal plane in the cat.

Animals↗
PubMed1995

Vestibular evoked responses: a new frontier in equilibriometry.

Modern neurootology has proved that many vertigo patients suffer from central dysequilibrium states. As vertigo patients are very frequent amongst our patient population we have to increase our diagnostic as well as therapeutical efforts. Since 1965, eliciting vestibular evoked cortical potentials has been possible. A method of evoked responses based upon computation of several stimulus responses has been used since then. Besides we are also using the frequency analysis of representative epochs of ongoing vestibular-ocular or retino-ocular reactions has been used since then. For diagnostic purposes we are combining various techniques like systematic history taking, ENG, ECG, CCG and BEAM. Brain electrical activity mapping (BEAM) is a newly developed tool, which we can also use for differentiating central from peripheral vestibular diseases. We are applying this technique for topographic correlates of vertigo related functional changes as well as for spatiotemporal analysis of cerebral evoked potentials due to vestibular stimulation. By means of a specific vestibular evoked potential technique, 6 typical cortical wave patterns can be elicited, which are displayed in the EEG curves as well as in colour maps of the whole scalp. The interactions between the various cortical structures are turning into an image similar to a weather map. BEAM, the fourth plan for projecting vertigo disorders, is now also bound into our concept for planning and monitoring a drug based therapy.

Brain Mapping↗
PubMed1995

Vestibular evoked responses in man: methodological aspects.

We have developed a stimulation method where the subject is sitting and the head is rotated with shock bursts elicited by a shaker with a repetition rate of 2 Hz. The head movement is monitored with an accelerometer mounted on the cheek by a head band. The maximum amplitude of the head movement is 3 degrees. The electrodes were places on vertex with a negative electrode on the mastoid. During stimulation, 90 dB white noise was applied to the ears to mask the noise generated by the stimulus. We recorded following responses: i) VER from ipsilateral ear, ii) VER from contralateral ear, iii) eye movement with EOG, and iv) movement of the head. Amplification and averaging of the signal were made with an evoked response recorder (Nihon Kohden, Neuropac four). From 200 to 2,000 averaged responses were collected and stored for further analysis on a floppy disc. During second stage filtering the data were fed into a microcomputer where appropriate programs were used to eliminate the EMG and movement artifacts.

Acceleration↗
PubMed1995

Short latency vestibular evoked responses to linear acceleration stimuli in small mammals: masking effects and experimental applications.

Different potential were recorded from a site close to the 8th nerve in chinchillas in response to linear acceleration pulses. Acoustic masking allowed us to distinguish between an early response (within 1 ms after initiation of the acceleration) of probable vestibular origin and later responses of probable cochlear origin. The latter were abolished by intense acoustic masking and by surgical ablation of the cochlea. The early potential was slightly reduced by simultaneous acoustic masking with white noise above 65 dB SPL and was most sensitive to 1 kHz narrow band masking. Vestibular neurons seem to be stimulated by high frequency movements of their hair cell cilia, and vestibular compound action potentials can be recorded as soon as a sufficient number of neurons are brought to a synchronized response. These vestibular evoked potentials may provide a tool for experimental studies on vestibular function in laboratory animals.

Acceleration↗
PubMed1995

Atypical "reversed" paroxysmal positioning nystagmus in benign paroxysmal positional vertigo.

An atypical paroxysmal positioning nystagmus (PPNy), "reversed" in its directional components, is a rare finding in patients suffering from benign paroxysmal positioning vertigo (BPPV). It poses problems of pathophysiogenetic interpretation, differential diagnosis with a CNS lesion and therapeutic strategy. Such a finding was observed in 7 patients (out of a total of 450 BPPV) and took on the typical PPNy pattern after repeated diagnostic manoeuvres. These aspects are incompatible with the "heavy cupula" mechanism (cupulolithiasis) and are better explained by the presence of "free endolymph aggregates" (canalolithiasis) within the posterior semicircular canal (p.s.c). Moreover, canalolithiasis would also permit a more convincing interpretation of all the clinical findings observed in typical p.s.c.-BPPV.

Adult↗
PubMed1995

Characteristics of human gait related variables in association with vestibular system disorders.

Gait analysis was performed in patients with vestibular system disorders, using foot switches and electromyography. They were divided into three groups: 11 cases with vestibular neuronitis (VN), 10 cases with large acoustic neuroma (LAN) who had central lesion evidenced by neurotological examinations, and 10 cases with olivopontocerebellar atrophy (OPCA). A total of 14 healthy adults served as controls. Results showed that time from heel strike to forefoot strike was the most sensitive variable to indicate gait abnormality but had no specificity. As for the variables of stance, swing, and time to reach the initial peak of the tibialis anterior's activity from forefoot off, occurrence rate of abnormality was high in the OPCA group, followed by the LAN and VN groups. Concerning the double support period which is related with body weight transfer from one leg to the other, the LAN and OPCA groups showed higher occurrence rate of abnormality than the VN group. These differences among the three groups could reflect different effects to the gait control systems caused by lesion.

Adult↗
PubMed1995

The effect of unilateral posterior semicircular canal inactivation on the human vestibulo-ocular reflex.

The responses to rapid, passive, unpredictable, low amplitude (10-20 degrees), high acceleration (3,000-4,000 degrees/s2) head rotations were used to study the human vestibulo-ocular reflex (VOR) in pitch and yaw plane after unilateral posterior semicircular canal occlusion (uPCO) in 10 subjects. The results from these 10 uPCO subjects were compared with those from 18 normal subjects. The VOR gains at a head velocity of 200 degrees/s in the uPCO subjects were: pitch upward = 0.62 +/- 0.06, pitch downward = 0.87 +/- 0.11, yew ipsilesion = 0.78 +/- 0.06, yaw contralesion = 0.79 +/- 0.10 and in normal subjects were: pitch upward = 0.92 +/- 0.06, pitch downward = 0.96 +/- 0.04, yaw right = 0.88 +/- 0.05, yaw left = 0.91 +/- 0.12 (group means +/- twotailed 95% confidence intervals). The results showed that the pitch-vVOR gain was significantly (p < 0.05) decreased in response to upward head impulses whereas in response to downward, ipsilesion and contralesion head impulses were not significantly different (p > 0.05) from the normals. This study shows that there is 30% permanent residual deficit of the upward pitch-vVOR with an up-down asymmetry in pitch-vVOR gain following inactivation of a single posterior semicircular canal and that compensation of pitch-vVOR function is incomplete.

Adult↗
PubMed1995

Postural control and vestibular function in patients selected for cochlear implantation.

Postural control and vestibular and eye motor function were evaluated in 7 postlingually deaf patients before cochlear implantation as vibration toward the calf muscles or galvanic electrical stimulation of the vestibular nerves perturbed stance. The patients were compared with 21 control subjects. Vibration-induced bodysway was increased in the patients compared to the normals. Galvanic stimulation induced a bodysway that was not significantly different from that of the control group suggesting that the patients selected for cochlear implantation, and with otherwise reduced postural control, are sensitive to electrical stimulation of the vestibular nerve. This finding may contribute with a complementary hypothesis to the causes of dizziness among cochlear implant patients.

Adult↗
PubMed1995

Influence of centrifugal force on angular velocity estimation.

In darkness, subjects were positioned face forward (or backward, resp.) on a rotating disk at a radial distance r of 0-1.6 m. They were then accelerated within 0.8 s to a constant rotation (omega = 0.35-0.87 rad/s), and successively indicated whenever they felt turned through 180 degrees. Fairly veridical at first, these indications lagged progressively as though subjective velocity declined exponentially to zero. Plots of ordinal number of indications over time of indication revealed idiosyncratic time constants (20-90 s) that were independent of disk velocity at r = 0, but increased with radial distance, namely, with r omega 2, hence depended on centrifugal force. When, after constant rotation of at least 2 min the subjects were stopped (within 1.2 s), and asked to indicate 180 degrees turns as above, the time constants of all subjects were independent of radius and disk velocity, as expected, if the added orthogonal force caused the prolonged time constants in the former paradigm.

Acceleration↗
PubMed1995

Modeling human vestibular responses during eccentric rotation and off vertical axis rotation.

A mathematical model has been developed to help explain human multi-sensory interactions. The most important constituent of the model is the hypothesis that the nervous system incorporates knowledge of sensory dynamics into an "internal model" of these dynamics. This internal model allows the nervous system to integrate the sensory information from many different sensors into a coherent estimate of self-motion. The essence of the model is unchanged from a previously published model of monkey eye movement responses; only a few variables have been adjusted to yield the prediction of human responses. During eccentric rotation, the model predicts that the axis of eye rotation shifts slightly toward alignment with gravito-inertial force. The model also predicts that the time course of the perception of tilt following the acceleration phase of eccentric rotation is much slower than that during deceleration. During off vertical axis rotation (OVAR) the model predicts a small horizontal bias along with small horizontal, vertical, and torsional oscillations. Following OVAR stimulation, when stopped right- or left-side down, a small vertical component is predicted that decays with the horizontal post-rotatory response. All of the predictions are consistent with measurements of human responses.

Acceleration↗