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Biomedical subjects

A M Bronstein

Publications and source records attributed to A M Bronstein.

At least 55 records · Page 3Linked to original sources
PubMed2000

Assessment of unexplained falls and gait unsteadiness: the impact of age.

When a patient with a balance disorder reports rotational vertigo, the clinician rightly focuses his or her attention on the vestibular system. This article reviews the possible diagnoses in the many patients who primarily report falls or gait disorder. Falls can be caused by predisposing neurologic conditions impairing gait, cardiovascular conditions, or epileptic episodes. The proportion of idiopathic falls, however, remains high. In the elderly, environmental circumstances, visual defects, psychotropic medication, and poor general health are additional risk factors. Clinical assessment of gait is more revealing and less expensive than computerized posture/ gait systems. The diagnosis of orthostatic tremor, however, requires either Fourier analysis of sway platform signals or electromyography.

Accidental Falls↗
PubMed2000

Adaptation to oscillopsia: a psychophysical and questionnaire investigation.

In this study we explore the reasons why patients with bilateral vestibular failure report disparate degrees of oscillopsia. Twelve bilateral labyrinthine-defective (LD) subjects and twelve normal healthy controls were tested using a self- versus visual-motion psychophysical experiment. The LD subjects also completed a questionnaire designed to quantify the severity of handicap caused by oscillopsia. Additional standardized questionnaires were completed to identify the role of personality, personal beliefs and affective factors in adaptation to oscillopsia. During the psychophysical experiment subjects sat on a motorized Barany chair whilst viewing a large-field projected video image displayed on a screen in front of them. The chair and video image oscillated sinusoidally at 1 Hz in counter-phase at variable amplitudes which were controlled by the subject but constrained, so that the net relative motion of the chair and video image always resulted in a sinusoid with a peak velocity of 50 degrees /s. The subject's task was to find the ratio of chair versus video image motion that subjectively produced the 'most comfortable visual image'. Eye movements were recorded during the experiment in order that the net retinal image slip at the point of maximum visual comfort could be measured. The main findings in the LD subjects were that, as a group, they selected lower chair motion amplitude settings to obtain visual comfort than did the normal control subjects. Responses to the questionnaires highlighted considerable variation in reported handicap due to oscillopsia. Greater oscillopsia handicap scores were significantly correlated with a greater external locus of control (i.e. the perception of having little control over one's health). Retinal slip speed was negatively correlated with oscillopsia handicap score so that patients who suffered the greatest retinal slip were those least handicapped by oscillopsia. The results suggest that adaptation to oscillopsia is partly related to the patient's personal attitude to the recovery process and partly associated with the development of tolerance to the movement of images on the retina during self-motion. The latter is likely to be related to previously described changes in visual motion sensitivity in these patients.

Adaptation, Physiological↗
PubMed2000

Visual control of postural orientation and equilibrium in congenital nystagmus.

PURPOSE: To investigate how humans with congenital nystagmus (CN) use visual information to stabilize and orient their bodies in space. METHODS: Center of foot pressure (COP) and head displacements in the lateral plane were recorded using a sway platform and Schottky barrier photodetector, respectively. In experiment 1, a comparison was made of the oscillatory characteristics of body sway with eyes open compared with eyes closed. Experiment 2 studied the postural readjustments made in response to absolute or relative motion (motion parallax) of objects in the visual scene, generated by lateral displacement of background scenery. RESULTS: Experiment 1 revealed that subjects with CN were not able to use visual information to stabilize COP but were able to stabilize the head at frequencies lower than 1 Hz. Experiment 2 showed that in response to the displacement of a visual display, for both absolute motion and motion parallax, subjects with CN reoriented their body in space in a manner similar to control subjects. CONCLUSIONS: The results suggest that despite involuntary eye movements, subjects with CN use orientation cues to control their posture, but not dynamic cues useful to control the rapid oscillations that are particularly important at the level of COP. These findings suggest that in CN, visual control of posture is restricted by low-frequency sampling of the visual scene.

Adult↗
PubMed2000

The effect of habituation and plane of rotation on vestibular perceptual responses.

A technique was applied to assess vestibular sensation without reference to external spatial, position cues. The stimuli were stopping responses to velocity-steps of 90 deg/s in the dark. Subjects indicated their perceived angular velocity by turning a flywheel connected to a tachogenerator. Two separate experiments were conducted. In one, subjects were rotated in yaw about an earth-vertical axis before and after prolonged rotational or visual (optokinetic) stimuli. In the second experiment, subjects were rotated in roll supine, with either the head ('roll centred') or the feet ('roll eccentric') on the axis of rotation. The two aims of the paper were to (i) examine the effect of repetitive vestibular and optokinetic stimulation on the time constant of decay of vestibular sensation in yaw; (ii) to compare vestibular sensation responses to rotation in roll both with and without the addition of a Z-axis centrifugal force. The pre-habituation sensation response in yaw decayed exponentially with a median time constant of 12.8 s. The duration of the sensation responses were significantly reduced following both prolonged vestibular and optokinetic stimulation. The reduction in vestibular responses following prolonged visual and vestibular stimuli, 1) is likely to occur in velocity storage mechanisms mediating ocular and perceptual responses, 2) may represent a mechanism for reducing the disorientating consequences of visual-vestibular conflict and 3) supports the use of optokinetic stimuli as a treatment for vestibular patients. The time constant of the sensation responses in roll was shorter and not significantly influenced by head position: 5.7 s in the head-centred position compared to 4.7 s in the eccentric head position. Therefore, perceptual as well as ocular responses to rotation in roll are determined primarily by cupula dynamics and not influenced by velocity storage.

Adult↗
PubMed2000

The human linear vestibulo-ocular reflex to transient accelerations: visual modulation of suppression and enhancement.

Visual modulation of the linear vestibulo-ocular reflex (LVOR) was investigated in 4 normal subjects exposed to translational interaural transient accelerations of 0.08 g and 0.17 g. Binocular eye movements were recorded with the scleral search-coil technique. LVOR modulation with target proximity was studied using earth-fixed targets at distances of 30 and 60 cm (LVOR-E). LVOR suppression (LVOR-S) was provoked by similar targets which were head-fixed. For both conditions, linear acceleration evoked compensatory slow-phases whose velocities were, from onset, enhanced in proportion to acceleration and target proximity. At 80 ms after motion onset, i.e. before visually-guided eye movements could aid target fixation, gains (eye velocity/relative target velocity) during LVOR-E averaged 0.32 (S.D. 0.07) over all combinations of accelerations and target distances. At this time, eye velocities for LVOR-S were on average 33% lower than for LVOR-E (1.8 degrees/s vs. 2.7 degrees/s). During LVOR-S, a marked suppression of eye movements appeared at 102 ms (S.D. 10 ms). We conclude that mechanisms other than pursuit can be used to attenuate the LVOR at < 80 ms but their effect is weak. The marked suppression observed around 100 ms might be due to an early visual effect on vestibular pathways or by some independent voluntary control mechanism.

Acceleration↗
PubMed1999

Horizontal linear vestibulo-ocular reflex testing in patients with peripheral vestibular disorders.

UNLABELLED: Horizontal eye movements in response to lateral head translation [linear vestibulo-ocular reflex (LVOR)] in normal subjects and in patients with bilateral vestibular failure (n = 14), unilateral vestibular nerve section (n = 9), and benign positional vertigo (n = 14), were studied. LVORs were elicited in darkness by step acceleration (0.24 g) of the whole body along the interaural axis. RESULTS AND CONCLUSIONS: (1) In patients with bilateral vestibular failure, LVORs were either absent or abnormal with asymmetries, diminished velocities, and prolonged latencies. Measurements of dynamic visual acuity during linear self-motion showed decreased performance in patients at 1.0 and 1.5 Hz, which correlated with absent or delayed LVORs. These findings demonstrate the functional role of LVORs for dynamic visual acuity. (2) Early after vestibular nerve section, LVORs were diminished or absent with head acceleration toward the operated ear and normal in the opposite direction. After 6-10 weeks, responses were symmetrical again. Thus, a single utricle appears to be polarized with respect to the LVOR early after unilateral vestibular loss generating mostly contraversive responses. (3) Patients with benign positional vertigo showed mostly normal LVORs, which can be explained by minor utricular damage or central compensation of a chronic unilateral deficit.

Acceleration↗
PubMed1999

The interaction of otolith and proprioceptive information in the perception of verticality. The effects of labyrinthine and CNS disease.

A review of recent experiments in patients with labyrinthine and neurological disorders assessing the subjective postural vertical (SPV) and the subjective visual vertical (SVV) is presented. The SPV was measured with subjects (Ss) seated in a motorized flight simulator tilting at 1.5 deg/s in roll and pitch; the Ss' task was to indicate when they entered and left self-verticality. The SVV was measured by Ss adjusting a straight line to what they perceived as gravitational upright. Clear dissociations between the SVV and SPV were found, for example, patients with acute unilateral vestibular disorders had marked tilts of the SVV toward the side of the lesion but a "lean" (bias, tilt) of the SPV was never found. Dissociations of the SPV and SVV could also be induced in normal subjects by roll-plane visual motion stimuli: the SVV was tilted in the direction of motion, but the SPV was not. Prolonged lateral body tilt did, however, bias the SVV (the "A" effect) and the SPV, but these effects are likely to be mediated by somatosensory rather than otolithic input. Evidence for the latter came from (i) findings in patients with absent vestibular function, who showed an enhanced "A" effect, and (ii) from a patient with a thalamic infarction, who showed absence of the "A" effect when leaning on the hemihypesthetic side. In separate experiments where normal Ss indicated space-vertical and space-horizontal with saccadic eye movements, we found differences between these percepts, that is, subjective external space lost orthogonality. The findings in these various experiments can be interpreted if we abandon the idea of a single, "internal representation" of verticality. Different sensory modalities convey different and sometimes conflicting messages about verticality. Otolithic and somatosensory signals can have opposite sign effects during verticality estimates while tilted. In man, somatosensory cues have a prominent role in verticality perception.

Body Image↗
PubMed1999

Cardiovascular responses elicited by linear acceleration in humans.

Although activation of otolith receptors is known to elicit cardiovascular responses in animals, it is unclear whether vestibular stimulation can evoke changes in blood pressure and heart rate (which are independent of motion sickness) in humans. In the present study, ten normal subjects and three patients with profound bilateral reduction in vestibular function, who were seated upright with the torso aligned with the gravitation vector, were subjected to fore, aft, or lateral linear acceleration (approximately 0.2 g, attaining approximately 2 m/s in 900 ms, and decelerating for 3 s at 0.07 g). The head was fixed in the upright position, pitched maximally downward (chin on chest) or maximally backward (approximately 40-50 degrees) during the accelerations. In normal subjects, all directions of linear acceleration produced an average increase in systolic blood pressure of approximately 7-9 mm Hg and a rapid decrease in the interval between R-waves of the electrocardiogram of 14-27 ms; these responses persisted for only a few seconds. In contrast, the cardiovascular responses in patients with vestibular dysfunction were much smaller (e.g., the maximal pressor response to forward linear acceleration was <4 mm Hg). Head position during accelerations had little effect on the cardiovascular responses that were elicited in the population of normal subjects. However, although the population response was similar across directions of acceleration and head positions, many individuals exhibited larger cardiovascular changes during some stimulus conditions than during others. These data suggest that vestibular stimulation during linear accelerations can produce cardiovascular responses in humans and support the hypothesis that the vestibular system contributes to maintaining stable blood pressure during movement and changes in posture.

Acceleration↗
PubMed1999

Torsional eye movements are facilitated during perception of self-motion.

Visual motion in the roll plane elicits torsional optokinetic nystagmus (tOKN) with intermittent periods of illusory, contradirectional self-motion (circularvection, CV). The CV may also have a component of whole-body tilt if the axis of stimulus rotation is not aligned with the direction of gravity. We report how the characteristics of tOKN are affected by the presence of CV. Subjects had their eye movements recorded by VOG whilst viewing a full-field stimulus rotating at 30-60 degrees/s about their naso-occipital axis. They were tested in upright and supine posture and signalled the presence-absence of CV with a pushbutton. In both postures, during CV, tOKN slow-phase gain was found to be enhanced and average torsional eye position shifted in the direction opposite to stimulus rotation. When supine, slow-phase gain was greater than when upright both during the perception of object-motion and during CV. The effects may be explained in terms of a relegation of restraining vestibular input to the torsional oculomotor system during CV and illusory tilt.

Adult↗
PubMed1999

Visual and oculomotor responses induced by neck vibration in normal subjects and labyrinthine-defective patients.

Three-dimensional scleral search coil eye movement recordings were obtained in five normal subjects and four patients with absent vestibular function, during unilateral vibration of the neck in the supine position. The purpose of the experiments was to investigate any role played by eye movements in the illusion that a small fixation target, viewed in an otherwise dark room, moves when vibration is applied to the neck (propriogyral illusion). Vibration was applied to the right dorsal neck muscles in three visual conditions: total darkness, fixating a light-emitting diode (LED) in an otherwise totally dark room and LED fixation in the normally lit room. Normal subjects reported that during vibration, with LED fixation in an otherwise dark room, the target appeared to move predominantly leftwards and patients reported a predominantly downward movement. Eye movements were consistently elicited in all subjects. In normal subjects there was a slow-phase eye movement predominantly to the right, interrupted by nystagmic quick phases in the opposite direction, whereas in the patients slow phases were predominantly upward with quick phases downward. Eye movements were larger in the dark but the velocity of the initial slow-phase component (<200 ms) did not change with visual conditions. Mean latencies of the eye movements were typically 80 ms but in individual trials could be as short as 40- 60 ms. The eye movements were considerably larger in the patients (e.g. mean cumulative slow-phase displacement in the dark 12 degrees vs 2 degrees; maximum velocity ca. 5 degrees /s vs 1 degrees /s). These results indicate that the propriogyral illusion is secondary to vibration-induced eye movements, presumably mediated by the cervico-ocular reflex (COR). The difference in direction of the illusion and eye movements in the patients may be related to a predominant enhancement of the vertical COR, secondary to the prominent exposure to vertical retinal slippage experienced by these patients during daily activities such as locomotion.

Adult↗
PubMed1999

Vestibular perception of angular velocity in normal subjects and in patients with congenital nystagmus.

A technique is described for the assessment of vestibular sensation. The two main goals of the study were (i) to compare the perception of angular velocity with the eye velocity output of the vestibulo-ocular reflex and (ii) to study vestibular function in patients with congenital nystagmus; this was needed since most previous studies, based on eye movement recordings, have been inconclusive. Subjects indicated their perceived angular velocity by turning by hand a wheel connected to a tachometer. The vestibular stimuli used consisted of sudden deceleration from rotation at a constant horizontal velocity of 90 degrees /s ('stopping' responses). Eye movements were recorded simultaneously with electro-oculography. In normal subjects the perceived angular velocity decayed from the moment of deceleration in an exponential fashion. The mean time constant of sensation decay was approximately 16 s. Eye movement velocity decayed with a similar exponential trajectory (time constant 16 s). Congenital nystagmus patients showed markedly shortened vestibular sensation (mean time constant 7 s). The following conclusions can be drawn: (i) the similarity of the eye velocity and perceptual responses suggests that these two systems receive a vestibular signal which has been similarly processed; (ii) the time constant of the responses indicates that this vestibular signal probably originates in the same brainstem 'velocity storage' integrator; (iii) the technique described is useful for clinical assessment of vestibular function, particularly in patients with ocular motility disorders; (iv) patients with congenital nystagmus have short vestibular time constants, which is probably due to changes induced in velocity storage processing by the persistent retinal image motion present in these patients.

Adult↗
PubMed1999

Visual-vestibular control of posture and gait: physiological mechanisms and disorders.

The scientific analysis of clinical disorders of posture and gait is an emerging field. Precise definition of the forces and postural movements involved has been pivotal to understanding many aspects of the visual and vestibular contributions to balance. However, a great deal of argument still surrounds the question of how much gait and posture laboratories actually contribute to improve the clinical management of individual patients. One of the reasons why gait analysis techniques have not penetrated rehabilitation clinics may be that the research questions asked have been aimed at understanding mechanisms rather than at quantifying disability. The condition known as primary orthostatic tremor, which is not too well known to many neuro-otologists and posturographists, is briefly reviewed here. We propose that posturography could be the easiest way to diagnose this treatable condition.

Electromyography↗
PubMed1999

EMG responses to free fall in elderly subjects and akinetic rigid patients.

OBJECTIVES: The EMG startle response to free fall was studied in young and old normal subjects, patients with absent vestibular function, and patients with akinetic-rigid syndromes. The aim was to detect any derangement in this early phase of the "landing response" in patient groups with a tendency to fall. In normal subjects the characteristics of a voluntary muscle contraction (tibialis anterior) was also compared when evoked by a non-startling sound and by the free fall startle. METHODS: Subjects lay supine on a couch which was unexpectedly released into free fall. Latencies of multiple surface EMG recordings to the onset of free fall, detected by a head mounted linear accelerometer, were measured. RESULTS AND CONCLUSIONS: (1) EMG responses in younger normal subjects occurred at: sternomastoid 54 ms, abdominals 69 ms, quadriceps 78 ms, deltoid 80 ms, and tibialis anterior 85 ms. This pattern of muscle activation, which is not a simple rostrocaudal progression, may be temporally/spatially organised in the startle brainstem centres. (2) Voluntary tibialis EMG activation was earlier and stronger in response to a startling stimulus (fall) than in response to a non-startling stimulus (sound). This suggests that the startle response can be regarded as a reticular mechanism enhancing motor responsiveness. (3) Elderly subjects showed similar activation sequences but delayed by about 20 ms. This delay is more than can be accounted for by slowing of central and peripheral motor conduction, therefore suggesting age dependent delay in central processing. (4) Avestibular patients had normal latencies indicating that the free fall startle can be elicited by non-vestibular inputs. (5) Latencies in patients with idiopathic Parkinson's disease were normal whereas responses were earlier in patients with multiple system atrophy (MSA) and delayed or absent in patients with Steele-Richardson-Olszewski (SRO) syndrome. The findings in this patient group suggest: (1) lack of dopaminergic influence on the timing of the startle response, (2) concurrent cerebellar involvement in MSA may cause startle disinhibition, and (3) extensive reticular damage in SRO severely interferes with the response to free fall.

Accidental Falls↗
PubMed1999

A case of thalamic syndrome: somatosensory influences on visual orientation.

The ability to set a straight line to the perceived gravitational vertical (subjective visual vertical, SVV) was investigated in a 21 year old woman with long standing left hemihypaesthesia due to a posterior thalamic infarct. The putative structures involved were the somatosensory and vestibular thalamus (VPL, VPM) and associative (pulvinar) thalamus. The SVV was normal when seated upright. When lying on her right side, line settings deviated about 17 degrees to the right, which is the normal A-effect. When lying on the hypaesthetic side the mean SVV remained close to true vertical-that is, the A-effect was absent, and there was a large increase in variability of the SVV settings. The findings support the view that the body tilt-induced bias of the SVV (A-effect) is largely mediated by somatosensory afferents. The finding that the A-effect was absent only when lying on the hypaesthetic side suggests that, during body tilt, the somatosensory system participates in visuogravitational orientation.

Adult↗
PubMed1998

Fast eye movement initiation of ocular torsion in mesodiencephalic lesions.

Three patients with episodic ocular torsion and skew deviation due to mesodiencephalic lesions were studied by using binocular three-dimensional scleral search coils. The conjugate ocular torsion (upper pole of each eye rotating toward the side of the brainstem lesion) was initiated by a torsional fast eye movement. During prolonged episodes, torsional nystagmus was also present. Cessation of the ocular torsion and skew deviation occurred by slow eye movements with exponentially decreasing velocities in 2 patients, and by multiple fast torsional movements in 1 patient. In 1 patient, the abnormal eye movements were temporally linked to dystonic movements in the limbs on the side opposite the brainstem lesion. The occurrence of skew deviation with conjugate ocular torsion in brainstem lesions has been attributed to functional asymmetry in vestibular pathways responsible for the slow-phase compensatory eye movement response to roll. In comparison, the findings in our patients show that in mesodiencephalic lesions conjugate ocular torsion with skew deviation may be generated by torsional fast eye movements, indicating activation of the burst cells of the rostral interstitial nucleus of the medial longitudinal fasciculus.

Adult↗
PubMed1998

Assessment of the perception of verticality and horizontality with self-paced saccades.

We investigated the ability of human subjects (Ss) to make self-paced saccades in the earth-vertical and horizontal directions (space-referenced task) and in the direction of the head-vertical and horizontal axis (self-referenced task) during whole body tilts of 0 degrees, 22.5 degrees, 45 degrees and 90 degrees in the frontal (roll) plane. Saccades were recorded in the dark with computerised video-oculography. During space-referenced tasks, the saccade vectors did not fully counter-rotate to compensate for larger angles of body tilt. This finding is in agreement with the 'A' effect reported for the visual vertical. The error was significantly larger for saccades intended to be space-horizontal than space-vertical. This vertico-horizontal dissociation implies greater difficulty in defining horizontality than verticality with the non-visual motor task employed. In contrast, normal Ss (and an alabyrinthine subject tested) were accurate in orienting saccades to their own (cranio-centric) vertical and horizontal axes regardless of tilt indicating that cranio-centric perception is robust and apparently not affected by gravitational influences.

Adult↗
PubMed1998

Otolith and semicircular canal contributions to the human binocular response to roll oscillation.

Three normal human subjects were oscillated about their naso-occipital axis in a supine position at 0.4 Hz and 0.1 Hz, both in darkness and in the light with a structured fixation target. The same subjects were oscillated in roll about an upright position, at the same frequencies, in darkness; and also about axes directed 20 degrees and 40 degrees to the left and to the right of the midsagittal plane, at 0.4 Hz, in darkness. Three-dimensional binocular eye movements were recorded using video-oculography. All stimuli induced a predominantly torsional nystagmus with small disconjugate head-vertical (skew) and conjugate head-horizontal components. For roll oscillation, the torsional slow phase velocity gain was higher in the light and generally increased with the stimulation frequency. In darkness, only one subject had significantly higher torsional gains in the upright compared to the supine position (12% difference), suggesting that the otolith contribution to the roll response is minimal at the frequencies tested. The slow phase velocity gain of the skew increased with fixation in two subjects. but decreased in the third subject; these changes were related to changes in eye vergence. In the upright position, with oblique axes of rotation, the head-vertical eye movements were asymmetrical, with the outermost eye executing a larger amplitude movement. The disconjugate head-vertical eye movements observed can be explained by the pattern of vertical semicircular canal stimulation and their connections to the extraocular muscles. In humans, skewing of the eyes may compensate for the eccentricity of the foveae which lie in the temporal retina and undergo vertical translations in opposite directions when the eyes tort.

Adult↗