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Biomedical subjects

A M Bronstein

Publications and source records attributed to A M Bronstein.

At least 37 records · Page 2Linked to original sources
PubMed2001

Respiratory responses of normal and vestibular defective human subjects to rotation in the yaw and pitch planes.

We evaluated the contribution of the semicircular canals to autonomic responses to rotation in 12 normal subjects and three labyrinthine defective patients. Respiration, heart rate, arterial blood pressure and blood flow in the finger were recorded during 60 degrees /s steps of constant velocity rotation. Rotation was performed in the dark about a vertical axis: (I) with head upright for yaw stimulation of horizontal canals; (II) head tilted chin down and rotated laterally to stimulate the vertical canals in both face-forwards and backwards directions. In normal subjects, rotation in the pitch plane elicited an increase in respiratory frequency due to a shortening of the time for expiration, whilst patients had inconsistent responses. We conclude that vertical semicircular canal activity exerts a regulatory drive on respiration in human subjects.

Adult↗
PubMed2001

Horizontal ocular vergence and the three-dimensional response to whole-body roll motion.

We evaluated the human binocular response to roll motion in the dark and during visual fixation with horizontal convergence. Six normal human subjects were exposed to manually driven, whole-body rotation about an earth-vertical, naso-occipital axis, under two conditions: (I) oscillation at 0.4 Hz (peak velocity 69+/-3.8 degree/s) in the dark, and whilst fixating an axial light-emitting diode at 48 cm ('near') and at 206 cm ('far'); (II) constant velocity rotation (56.5+/-3.1 degree/s) for 40 s, clockwise and counter-clockwise, in the dark, and sudden stops. Eye and head movements were monitored using scleral search coils. In head-fixed, angular velocity coordinates roll motion always evoked conjugate ocular torsion, with small conjugate horizontal and disconjugate vertical components. The resultant binocular eye responses were rotations about convergent axes. During oscillation with target fixation the convergence of the rotation axes was larger than that predicted by target geometry, producing disconjugate oscillations of vertical gaze about the target ('skewing'). Fast-phase eye movements were primarily resetting rotations about the same convergent rotation axes as the slow phases, but the small vertical velocity components had oscillatory, asymmetrical profiles. In response to velocity steps the slow-phase eye velocity decayed exponentially with time constants of 4.5+/-1.5 s for the torsional component and 5.8+/-1.9 s for the 'vertical vergence' component (right eye-left eye recordings). We conclude that in normal human subjects dynamic vertical canal stimulation with horizontal gaze convergence evokes rotation of the eyes about convergent axes and a small skewing of the eyes.

Adult↗
PubMed2001

Influence of action and expectation on visual control of posture.

Previous studies have shown that human subjects presented with a moving visual environment initiate a postural re-adjustment in the direction of motion. The present study investigated how active control or expectation of the displacement of a visual scene affects this postural response. Center of foot pressure (COP) and head displacement were recorded using a sway platform and a tracking system, respectively. The subjects faced a visual scene (1 x 1 m, at a distance of 45 cm) which moved transiently (with a velocity of 1 cm/s) in a direction parallel to the interaural axis. When the displacement of the visual scene was under the active control of the subjects, visually induced body sway was strongly inhibited, in comparison with the response to unexpected stimuli. Prior knowledge of the characteristics of the forthcoming displacement was sufficient, in most subjects, to reduce postural re-adjustment, even when subjects did not exert active control. Finally, the visually induced postural response was strongly reduced even when subjects only triggered the stimulus, without any knowledge about the direction of motion. In conclusion, it appears that although vision is of primary importance in the control of postural orientation, high level processes such as expectation can modulate its impact by providing cues as to whether forthcoming visual flow is the consequence of self-motion or object-motion.

Adult↗
PubMed2001

Force platform recordings in the diagnosis of primary orthostatic tremor.

Primary orthostatic tremor (OT) consists of rhythmical muscle contractions at a frequency of around 16 Hz, causing discomfort and/or unsteadiness while standing. Diagnosis has hitherto relied on recording Electromyography (EMG) from affected muscles. The main aim of this study was to see if the characteristic postural tremor in OT can be identified with force platforms. We also quantified postural sway in OT patients to assess their degree of objective unsteadiness. Finally, we investigated the time relations between bursts of activity in the various affected muscle groups. Subjects stood on a force platform with concurrent multichannel surface EMG recordings from the lower limbs. Seven patients with clinical and EMG diagnosis of OT were examined and the force platform data compared with those of 21 other neurological patients with postural tremor and eight normal controls. All OT patients had high frequency peaks in power spectra of posturography and EMG recordings (12--16 Hz). No such high frequency activity was evident in patients with Parkinson's disease, cerebellar degenerations, essential tremor or in healthy controls. Additionally, OT patients showed increased sway at low frequencies relative to normal controls, suggesting that the unsteadiness reported by OT patients is at least partly due to increased postural sway. Examination of EMG timing showed fixed patterns of muscle activation when maintaining a quiet stance within but not across OT patients. These data show a high correlation between EMG and posturography and confirm that OT may be diagnosed using short epochs of force platform recordings.

Blood Pressure↗
PubMed2001

Abnormal interaction between vestibular and voluntary head control in patients with spasmodic torticollis.

The functional status of vestibulo-collic reflexes in the sternocleidomastoid (SCM) muscles was investigated in 24 patients with spasmodic torticollis using small, abrupt 'drops' of the head. None had been treated with botulinum toxin injections during at least 4 months preceding the study. Eight of the patients, four of whom had been studied before surgery, were also studied after selective peripheral denervation of neck muscles. The reflex was of normal latency and duration in the 'passive drop' condition, in which subjects were instructed not to oppose the fall of the head. To study voluntary interaction with the reflex response, subjects were then asked to flex the neck as quickly as possible after onset of the head drop ('active drop'). In this condition, voluntary responses in patients were delayed, smaller and less effective in counteracting the head fall than in normal subjects. The same abnormalities were also found in patients after surgery when the head posture was improved. Somatosensory/auditory voluntary reaction times in SCM were normal, as was the latency of the startle reflex. We conclude that voluntary interaction with the vestibulo-collic reflex is disrupted in patients with spasmodic torticollis, a finding which corroborates the patients' aggravation of their symptoms by head or body perturbations. Lack of effective interaction between two major systems controlling head position may contribute to torticollis.

Adult↗
PubMed2001

Visual vertigo: symptom assessment, spatial orientation and postural control.

Certain patients with balance disorders report a 'visual vertigo' in which their symptoms are provoked or aggravated by specific visual contexts (e.g. supermarkets, driving or movement of objects). In order to determine the causes of visual vertigo (VV), we assessed symptoms, anxiety and the influence of disorienting visual stimuli in 21 such patients. In 17 out of 21 patients, a peripheral vestibular disorder was diagnosed. Sixteen bilateral labyrinthine-defective subjects (LDS) and 25 normal subjects served as controls. Questionnaire assessment showed that the levels of trait anxiety and childhood motion sickness in the three subject groups were not significantly different. Reporting of autonomic symptoms and somatic anxiety was higher than normal in both patient groups but not significantly different between LDS and VV patients. Handicap levels were not different in the two patient groups, but the reporting of vestibular symptoms was higher in the VV than in the LDS group. The experimental stimuli required subjects to set the subjective visual vertical in three visual conditions: total darkness, in front of a tilted luminous frame (rod and frame test) and in front of a large disc rotating in the frontal plane (rod and disc test). Body sway was also measured in four visual conditions: eyes closed, eyes open, facing the tilted frame and during disc rotation. In psychophysical and postural tests, both LDS and VV patients showed: (i) a significant increase in the tilt of the visual vertical both with the static tilted frame and with the rotating disc; and (ii) an increased postural deviation whilst facing the tilted frame and the rotating disc. The ratio between sway path with eyes closed and eyes open (i.e. the stabilizing effect of vision) was increased in the LDS, but not in VV patients, compared with normal subjects. In contrast, the ratio between sway path during disc rotation and sway path during eyes open (i.e. the destabilizing effect of a moving visual stimulus) was increased in the VV patients but not in LDS. Taken together, these data show that VV patients have abnormally large perceptual and postural responses to disorienting visual environments. VV is not related to trait anxiety or a past history of motion sickness. The results indicate that VV emerges in vestibular patients if they have increased visual dependence and difficulty in resolving conflict between visual and vestibulo-proprioceptive inputs. It is argued that treating these patients with visual motion desensitization, e.g. repeated optokinetic stimulation, should be beneficial.

Adult↗
PubMed2001

The endocrine system, vertigo and balance.

Steroid, amine and peptide hormones affect the peripheral vestibular system. Vasopressin hypersensitivity of the endolymphatic sac may be implicated in the pathogenesis of Meniere's disease. Specific vasopressin antagonists will help define the role of vasopressin in Meniere's disease. The modulation of central vestibular pathways by neuroactive steroids may involve effects on gamma-aminobutyric acid-ergic and glutaminergic pathways. The vestibular nuclei also express enzymes that are important in the synthesis of steroids and the modulation of their activity. Steroids mediate both facilitatory and deleterious effects of stress on vestibular compensation. The quality and quantity of stressor that determines the pattern of hormonal output, may be important. Clinical observation suggests that episodic ataxia type 2, a P/Q calcium channelopathy, may be phenotypically modulated by endocrine fluctuations. Steroid hormones may affect the episodic ataxia type 2 phenotype by modulation of voltage-gated calcium channel activity via second messenger systems and ion channel subunit expression. Despite evidence to support the link, the role of the endocrine system in vestibular function and disease is as yet virtually unexplored.

Animals↗
PubMed2001

Elevated visual motion detection thresholds in adults with acquired ophthalmoplegia.

AIMS: To test the hypothesis that in patients with acquired chronic bilateral ophthalmoplegia, abnormal retinal image slippage during head movements would result in abnormal thresholds for visual perception of motion. METHODS: Five patients (two males and three females) with ophthalmoplegia were included in the study. The average age was 44 years (range 30-69 years). The aetiology of ophthalmoplegia was myasthenia gravis (MG; n=2), chronic progressive external ophthalmoplegia (CPEO; n=2), and chronic idiopathic orbital inflammation. Visual motion detection thresholds were assessed using horizontal and vertical gratings (spatial frequency) set at thresholds for visibility. The grating was then accelerated at 0.09 deg/s(2). The subject's task was to detect the drift direction of the stimulus. RESULTS: Visual motion detection thresholds were raised to a mean of 0.434 deg/s (SD 0.09) (mean normal value 0.287 deg/s (SD 0.08)) for horizontal motion; and to a mean of 0.425 deg/s (SD 0.1) (mean normal value 0.252 deg/s (SD 0.08)) for vertical motion. The difference in values for both horizontal and vertical motion detection were statistically significant when compared with age matched controls; p <0.023 for horizontal motion and p<0.07 for vertical motion (two tailed t test). CONCLUSION: Abnormally raised visual motion thresholds were found in patients with ophthalmoplegia. This may represent a centrally mediated adaptive mechanism to ignore excessive retinal slip and thus avoid oscillopsia during head movements.

Adult↗
PubMed2001

Paroxysmal alternating skew deviation and nystagmus after partial destruction of the uvula.

A patient with suspected brain stem glioma involving the area of the left vestibular nuclei and cerebellar peduncle, developed paroxysmal alternating skew deviation and direction changing nystagmus after biopsy of the inferior cerebellar vermis resulting in destruction of the uvula. Between attacks she had right over left skew deviation with asymptomatic right beating horizontal nystagmus. Slow phases of the resting nystagmus showed increasing velocity, similar to congenital nystagmus. At intervals of 40-50 seconds, paroxysmal reversal of her skew deviation occurred, accompanied by violent left beating horizontal torsional nystagmus lasting 10-12 seconds and causing severe oscillopsia. It is proposed that this complex paroxysmal eye movement disorder results from (1) a lesion in the left vestibular nuclei causing right over left skew and right beating resting nystagmus and (2) a disruption of cerebellar inhibition of vestibular nuclei, causing alternating activity in the vestibular system with intermittent reversal of the skew deviation and paroxysmal nystagmus towards the side of the lesion.

Adult↗
PubMed2001

Cervical vertigo.

Explore the source record for details and available documents.

Diagnosis, Differential↗
PubMed2001

Effect of visual surrounding motion on body sway in a three-dimensional environment.

Unidirectional motion of a uniplanar background induces a codirectional postural sway. It has been shown recently that fixation of a stationary foreground object induces a sway response in the opposite direction (Bronstein & Buckwell, 1997) when the background moves transiently. The present study investigated factors determining this contradirectional postural response. In the experiments presented, center of foot pressure and head displacements were recorded from normal subjects. The subjects faced a visual background of 2 x 3 m, at a distance of 1.5 m, which could be moved parallel to the interaural axis. Results showed that when the visual scene consisted solely of a moving background, the conventional codirectional postural response was elicited. When subjects were asked to fixate an earth-fixed foreground (window frame) placed between them and the moving background, a consistent postural response in the opposite direction to background motion was observed. In addition, we showed that this contradirectional postural response was not transient but was sustained for the 11 sec of background motion. We investigated whether this contradirectional postural response was the consequence of the induced movement of the foreground by background motion. Although induced movement was verbally reported by subjects when viewing an earth-fixed target projected onto the moving background, the contradirectional sway did not occur. These results indicate that foreground-background separation in depth was necessary for the contradirectional postural response to occur rather than induced movement. Another experiment showed that, when the fixated foreground was attached to the head of the observer, the contradirectional sway was not observed and was therefore unrelated to vergence. Finally, results showed that the contradirectional postural response was, in the main, monocularly mediated. We conclude that the direction of the postural sway produced by a moving background in a three-dimensional environment is determined primarily by motion parallax.

Adolescent↗
PubMed2000

Effects of caloric stimulation on respiratory frequency and heart rate and blood pressure variability.

Heart rate variability (HRV), blood pressure variability (BPV) and respiratory frequency were measured by power spectrum techniques in six normal humans (25-34 years old) and one labyrinthine-defective patient (33 years old) during cold (30 degrees ) vestibular caloric stimulation. Caloric stimuli were delivered intermittently for 2 min, under two different breathing conditions: (1) spontaneous breathing and (2) breathing paced with a metronome (0.25 Hz). During the spontaneous breathing condition, in the normal subjects, the caloric stimuli induced a significant increase in the absolute magnitude of the power spectrum density of the high frequency component (0.15-0. 40 Hz) of HRV and the total bandwidth (0.04-0.4 Hz) of mean BPV. These responses were related to a shift in the weighted average of the respiration frequency on the respiration spectrum, from a median value of 0.27 Hz (range, 0.17-0.29 Hz) during baseline to 0.31 Hz (0. 26-0.31 Hz) following caloric stimulation. This change was not observed in the labyrinthine-defective patient, who had weighted averages of 0.37 Hz and 0.34 Hz, respectively. No significant changes in the normalised units of the low frequency component (0. 04-0.15 Hz) or the high frequency component (0.04-0.4 Hz) of HRV and BPV were observed. During the paced breathing condition, no consistent effect on HRV or BPV was evident. For both breathing conditions, the proportions of HRV and BPV power linearly independent from respiration did not show any caloric-induced change. This study shows that caloric vestibular stimulation produces changes in HRV and BPV by modifying the respiratory pattern.

Adult↗
PubMed2000

Evidence for a vestibulo-cardiac reflex in man.

Changes in posture demand rapid cardiovascular adjustments to maintain blood pressure and volume distribution. We demonstrated a vestibulo-cardiac reflex in supine individuals by measuring electrocardiogram and arterial blood pressure after small backwards drops of the head triggered at varying intervals after the R-spike. In normal volunteers heart rate was accelerated by drops occurring within 500-600 ms of a beat, but no rapid effect was noted in patients with a vestibular defect. The speed at which the vestibular signal of head drop accelerated heart rate implies a direct reflex. Impairment of the vestibulo-cardiac reflex would help to explain the vaso-vagal consequences of labyrinthitis.

Adult↗
PubMed2000

Changes in horizontal oculomotor behaviour coincide with a shift in visual motion perception.

During full-field rotation of the visual field, subjects commonly experience an initial perception of object-motion which 'switches' to a perception of self-motion. We studied the characteristics of the horizontal optokinetic nystagmus responses evoked by a moving visual stimulus in these two perceptual states over a range of stimulus velocities. During self-motion perception mean eye position was found to shift more in the direction of the newly appearing stimulus elements with a slight reduction in slow phase gain in comparison to the nystagmus evoked during object-motion perception. The results may reflect a modified strategy of spatial attention with increased emphasis on anticipatory eye movements during visually induced self-motion perception.

Adult↗
PubMed2000

Reorientation of a visually evoked postural response during passive whole body rotation.

Visually evoked postural responses (VEPR) to a roll-motion rotating disk were recorded from normal subjects standing on a yaw axis motorised rotating platform. The disk was fluorescent so that subjects could be tested in an otherwise dark room. Movements of the head and centre of foot pressure were measured while subjects looked at the disk with their eyes and head in the primary position and while the rotating platform moved the subjects randomly to 0, +/-45 degrees and +/-90 degrees angles from the visual stimulus. Subjects were instructed to maintain fixation on the centre of the rotating disk but the amount of horizontal eye and head movement used was not specified. Platform rotational velocity was set near threshold values for perception of self-rotation (approximately 2 degrees/s) so that subjects would find it difficult to reconstruct the angle travelled. The data showed that the VEPR occurred in the plane of disk rotation, regardless of body position with respect to the disk, and despite the subjective spatial disorientation induced by the experiment. Averages of the response revealed a good match (gain=0.95) between disk orientation and sway direction. The horizontal gaze deviation required to fixate the centre of the disk was largely achieved by head motion (head 95%, eye 5%). The results confirm previous results that VEPRs are reoriented according to horizontal gaze angle. In addition, we show that the postural reorientation is independent of cognitively or visually mediated knowledge of the geometry of the experimental conditions. In the current experiments, the main source of gaze position input required for VEPR reorientation was likely to be provided by neck afferents. The results support the notion that vision controls posture effectively at any gaze angle and that this is achieved by combining visual input with proprioceptively mediated gaze-angle signals.

Adult↗
PubMed2000

Influence of motion parallax in the control of spontaneous body sway.

Visual control of postural sway during quiet standing was investigated in normal subjects to see if motion parallax cues were able to improve postural stability. In experiment 1, six normal subjects fixated a fluorescent foreground target, either alone or in the presence of full room illumination. The results showed that subjects reduced body sway when the background was visible. This effect, however, could be mediated not only by parallax cues but also by an increase in the total area of visual field involved. In experiment 2, other parameters such as image angular size and target distance were controlled for. Twelve subjects fixated a two light-emitting diode (LED) target placed at 45 cm from their eyes in a dark room. A second similar two-LED target was placed either at 170 cm (maximum parallax) or at 85 cm (medium parallax) from the fixated target, or in the same plane of the fixated target (0 cm, no parallax). It was found that the amplitude of sway was reduced significantly, by approximately 20%, when the two targets were presented in depth (parallax present) as compared to when they were in the same plane (no parallax). The effect was only present in the lateral direction and for low frequency components of sway (up to 0.5 Hz). We confirmed in experiment 3 on eight subjects with a design similar to that used in experiment 2 that the effect of motion parallax on body sway was of monocular origin since observed with monocular and binocular vision. Geometrical considerations based on these results support the existence of two modes of visual detection of body sway, afferent (retinal slippage) and efferent (extra-retinal or eye-movement based).

Adult↗