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Biomedical subjects

M Gresty

Publications and source records attributed to M Gresty.

At least 19 recordsLinked to original sources
PubMed1999

Attentional demands of perception of passive self-motion in darkness.

The purpose of this study was to determine whether significant attentional resources are required to accurately monitor changes in bodily orientation, using vestibular information. This question was addressed firstly using a dual-task paradigm in which orientation perception tasks and a speeded auditory tone discrimination task were carried out either singly or in combination. For the active orientation perception task, subjects were seated in darkness on a motorised chair which could be rotated about an earth-vertical axis. Following passive angular displacements, subjects were required to return the chair to their perceived starting position, using a joy-stick which controlled chair motion. For the speeded auditory task, subjects pushed a hand-held button as fast as possible when a tone was presented over headphones. When the two tasks were combined, reaction times on the auditory task increased. Reaction time also increased when subjects were simply asked to fixate during rotation. A second experiment demonstrated that if attention was occupied by performance of a demanding mental arithmetic task during the passive rotation, accuracy of subsequently repositioning the chair to the origin declined, implying that change in orientation had been less accurately registered when performing the concurrent mental task. In combination, these findings indicate that a small but significant degree of attention or cognitive effort is necessary to monitor accurately the direction and amplitude of a brief angular rotation, and to suppress vestibulo-ocular reflex eye movement.

Adult↗
PubMed1998

Postural responses to vibration of neck muscles in patients with uni- and bilateral vestibular loss.

Postural responses to vibration applied unilaterally to the dorsal neck muscles were recorded with a sway platform in 11 patients with bilateral vestibular loss (BLD), 13 patients with unilateral vestibular lesions (ULD) and 19 normal subjects. In the normals, the vibration induced a forward postural deviation. Vibration failed to induce postural sway in the BLD patients but induced a backwards head movement in 7 patients. In the ULD patients, vibration contralateral to the lesion induced normal forward sway, whereas ipsilateral vibration resulted in sway directed to the side of the lesion and backwards. The findings demonstrate the importance of concurrent vestibular signals in determining the behavioural response to neck afferent input. We propose that in normal subjects the intact vestibular signal gives no confirmation that a head movement has occurred so it is assumed that the lower body has tilted forwards which provokes a compensatory sway. In the total absence of vestibular function the neck signal may represent a real head movement so the preferential response is a head tilt to restore upright posture. The vestibular imbalance in the ULD patients is roughly equivalent to the asymmetrical signals obtained in a normal subject during head rotation to the intact side. The stretch signal induced by ipsi-lesional vibration confirms possible head rotation, thus provoking a compensatory postural sway. Copyright 1998 Elsevier Science B.V. All rights reserved

Journal Article↗
PubMed1998

Horizontal otolith-ocular responses in humans after unilateral vestibular deafferentation.

We studied horizontal eye movements evoked by lateral whole body translation in nine patients who underwent vestibular nerve section. Preoperatively, all had preserved caloric function on both sides. Testing was performed before, 1 week and 6-10 weeks after surgery. Patients were seated upright in an electrically powered car running on a linear track. The car executed acceleration steps of 0.24 g, randomly to the left and right in the dark. The normal response consisted of a bidirectionally symmetrical nystagmus with compensatory slow phases. Response asymmetry of the slow-phase velocity of the desaccaded and averaged eye position signal was less than 13% in normals (n = 21). Before surgery, patients' responses were mostly symmetrical. Postoperatively, responses were diminished or absent with head acceleration towards the operated ear in all patients, causing a marked asymmetry which averaged 56% after correction for spontaneous nystagmus. On follow-up, responses regained symmetry. Thus, early after vestibular nerve section, a single utricle produces a normal LVOR only with ipsilateral head translation. Therefore, afferents for the LVOR seem to originate from the mid-lateral area of the macula, where hair cells are stimulated in their on-direction during ipsilateral head translation. Compensation may depend on recovery of the off-directional responses from lateral hair cells of the remaining utricle.

Acceleration↗
PubMed1998

Changes in heart rate and respiration rate in patients with vestibular dysfunction following head movements which provoke dizziness.

Patients with vestibular dysfunction often complain of additional symptoms typical of panic disorder and/or hyperventilation. This study investigated whether autonomic and respiratory symptoms reported by patients with vestibular disorders were associated with objective changes in heart and respiration rate following head movements provoking dizziness. Subjective ratings of symptoms and anxiety and objective measures of heart and respiration rate were obtained from 29 patients and 16 healthy controls immediately before and after the subjects performed three standardised sets of vigorous head movements. Within-group analyses revealed greater increases in respiration rate following head movement among patients who complained of more somatic symptoms, both during the previous 2 months and following head movement.

Adult↗
PubMed1998

Asymmetry of vestibular function induced by unidirectional visual-vestibular conflict.

We describe an attempt to model unilateral vestibular dysfunction in normal man by inducing vestibular asymmetry with exposure to long-term, unidirectional, visual-vestibular conflict. Subjects were exposed to pseudo-random (0.13, 0.2, 0.25, 0.3Hz; 77 degrees/s peak) oscillation in Yaw for 30 min whilst viewing a surrounding, whole field optokinetic drum which rotated with them when they were rotating rightwards and remained earth stationary when they rotated leftwards. Adaptation to this stimulus was assessed by combined tests of "goal-directed" vestibular-ocular reflex (VOR) and vestibular memory contingent saccades (VMCS) in 5 subjects and in a further 4 subjects by combined tests of perception of reorientation (a "navigation" task) and sinusoidal VOR at 0.1 and 0.32 Hz. The exposure induced a reduction in the gain of the VMCS and an underestimation of perceived amplitude of displacement when subjects were turned rightwards. VOR gain for rightwards movement was reduced more markedly at 0.1 Hz. No change was found in the goal-directed VOR gain. Thirty minutes after adaptation, the asymmetry of the VOR gain remained at 0.1 Hz, but vestibular perception recovered to normal. Asymmetrical adaptation can be achieved with short exposures and is more marked for low frequency stimuli. Modification reflex of vestibular functions endures longer than of perception of reorientation.

Adaptation, Physiological↗
PubMed1997

The functional effectiveness of neck muscle reflexes for head-righting in response to sudden fall.

Reflex head-righting in normal and labyrinthine-defective (LD) subjects was compared to identify the relative functional effectiveness of vestibular-collic and cervico-collic myotactic reflexes. To restrict stimuli largely to the head and neck, subjects lay supine, supported up to the shoulders on a horizontal bed with their head supported in a sling over the edge. The head fell freely as the sling was released with an electromagnetic catch. Head drops were delivered with the subjects instructed to relax and accept the fall passively or to actively right the head as fast as possible. With both instructions, righting responses in normal subjects commenced with electromyographic (EMG) bursts in the sternocleidomastoid (SCM) at 24.5 ms latency, which was reflected in a deceleration of the downwards head velocity. The latency of the earliest EMG responses in LD subjects was 67.4 ms, accompanied by similar deceleration. It is assumed that the earliest response in normal subjects is vestibular, whereas in LDs the SCM stretch reflex is the earliest response. These reflexes are followed at circa 100 ms by more intense EMG activity due to voluntary movement, but braking of head fall is evident before voluntary activity takes effect. Righting was more effective in normal subjects than in LDs, and when "active" normal subjects made more vigorous righting responses than when "passive"; whereas active righting in LDs was no better than passive. The results demonstrate that reflex responses contribute significantly to head-righting. The vestibular contribution gives an advantage over stretch reflexes alone and also assists in voluntary enhancement of reflex responses.

Acceleration↗
PubMed1997

Postural responses to vibration of neck muscles in patients with idiopathic torticollis.

Vibration of the dorsal muscles of the neck, simulating lengthening, in standing man causes a visible forwards tilt of the body shown on posturography as a tonic sagittal sway deviation. According to the theory that posture is organized with respect to a 'body schema' this deviation is a result of an interpretation of the concurrent neck afferent and vestibular signals. Considering the hypothesis that neck afferent signals may be misinterpreted in patients with spasmodic torticollis (ST) causing abnormal postural responses, we recorded body sway induced by unilateral dorsal neck muscle vibration in 22 idiopathic ST patients (19 treated with botulinum toxin) during upright stance with eyes closed. Comparison groups were 19 normal subjects and 11 patients with bilateral loss of vestibular function (labyrinthine defective, LD) in whom neck afference should be intact. Both treated and untreated ST and LD patients had absent or diminished sway deviations. When sway deviation did occur, it was sagitally oriented as with normal subjects and unrelated to ST head turns. In most ST and LD patients, neck vibration induced neck extension, an effect which is observed in normal subjects only if the torso is retrained. The results suggest that neck proprioceptive input retains local postural functions in ST, however, it is relatively ignored in the context of the whole body postural control and spatial orientation. The mild disorders of vestibular function reported in torticollis patients may be due to an inability to calibrate vestibular signals by reference to corroborative signals from neck proprioception.

Achilles Tendon↗
PubMed1997

Deafness and vertigo.

This review follows closely on the publication of significant handbooks and symposia concerned with neuro-otology, pharmacology of emesis, imaging, cochlear prostheses and aspects of vertigo which reflect the considerable advances that have been made in clinical and basic neuroscience in these areas in recent years. The value of the cochlear prosthesis has been demonstrated convincingly and may well be a model for the future of brain implantations in diverse disorders of the central nervous system. Imaging of the inner ear has made spectacular advances to provide invaluable aids to diagnosis. Neuro-otologists are becoming aware of the diversity of diseases, particularly those related to hormonal regulation, which may cause or exacerbate symptoms in patients with vertigo. Fortunately for the sufferer, a clearly focussed view is emerging of the pharmacology of overlapping and interrelated problems of emesis, vertigo and migraine which promises an early solution to their integrated management.

Cochlear Implants↗
PubMed1997

Motion detection deficits in infantile esotropia without nystagmus.

PURPOSE: To investigate whether adults with infantile strabismus but without latent nystagmus have abnormalities of horizontal motion detection. METHODS: Eleven adult subjects with infantile esotropia but without latent nystagmus and 15 control subjects were required to detect the onset of motion and drift direction of a sinusoidal, spatial frequency grating that moved with linearly increasing velocity. The grating was presented monocularly in paracentral vision at an eccentricity of 16.5 degrees with a field size of 18 degrees. The contrast of the grating was just above contrast threshold for visibility. RESULTS: The mean velocity threshold for detection of motion was raised significantly in the patient group compared with the control group. Nine of the 11 subjects with infantile esotropia demonstrated directional asymmetry for the detection of motion. Thresholds were elevated more often when the grating was moving nasally in the squinting eye and temporally in the nonsquinting eye, and raised thresholds were more prevalent in the squinting eye. CONCLUSIONS: The findings indicate that in infantile esotropia, the presence of motion perception deficits are not always associated with the development of latent nystagmus. The predominance of nasally directed motion deficits in the squinting eye and temporally directed motion deficits in the nonsquinting eye was unexpected and may have been caused by abnormal development of cortical motion processing.

Adult↗
PubMed1996

Postural responses to vibration of neck muscles in patients with unilateral vestibular lesions.

Postural responses to vibration applied unilaterally to dorsal neck muscles were recorded with a sway platform in nine patients with unilateral vestibular lesions and 19 normal subjects. In normals, the vibration induced a forward postural deviation. In patients, vibration of the neck contralateral to the lesion induced normal forward sway, whereas ipsilateral vibration resulted in sway of lower amplitude than normal and predominantly in the direction of the lesion or backwards. It is suggested that the proprioceptive error signal introduced by the neck vibration combined with an asymmetrical vestibular input due to a unilateral vestibular lesion provoked an erroneous representation of head position in patients resulting in a redirection of their body sway.

Humans↗
PubMed1996

Thresholds for detection of motion direction during passive lateral whole-body acceleration in normal subjects and patients with bilateral loss of labyrinthine function.

To investigate the effect of velocity, acceleration, and gradient of acceleration on self-motion perception, thresholds for detection of direction of whole-body interaural acceleration were determined for various stimulus profiles. For acceleration steps, acceleration thresholds at 67% correct detection of motion direction were similar for eight normals (mean 4.84 cm/s2 (range 2.9-6.3), peak gradient = 22 cm/s2) and five labyrinthine-defective subjects (mean 5.65 cm/s2 (4.85-6.6), peak gradient = 25 cm/s2). Velocity thresholds were 7.93 cm/s for a proportion of correct responses of 73% for normals and 9.67 cm/s for 69% of correct detection for avestibular subjects. For linear and parabolic accelerations, high intersubject variability was observed both among nine normals and three labyrinthine-defective subjects. Mean normal and avestibular subjects' acceleration thresholds for 74% of correct responses were respectively 12.1 cm/s2 (7.3-20.4) and 16.4 cm/s2 (13.2-20) for a ramp with gradient of acceleration = 2.8 cm/s3, 19.2 cm/ s2 (10.4-35.3) and 28.2 cm/s2 (21.4-32.8) for a ramp with gradient = 7.9 cm/s3 and 16.7 cm/s2 (10.5-25) and 20.6 cm/s2 (18.4-24.2) for a parabola with second derivative = 1.52 cm/s4. The corresponding velocity thresholds for normals were 21.2 cm/s (5.2-50.3), 22.0 cm/s (7-56.6), and 22.2 cm/s (9.5-43.7). The lowest thresholds were obtained for acceleration steps indicating that a high acceleration gradient facilitates motion perception. For linear and parabolic accelerations, motion perception seemed to follow an integration of acceleration, but a high intersubject variability was observed. For all stimuli, the range of thresholds for normals and avestibular subjects overlapped showing that detection of motion was not a sole prerogative of the otoliths but could also be performed using somatosensory cues.

Adult↗
PubMed1995

Neck muscle responses to abrupt free fall of the head: comparison of normal with labyrinthine-defective human subjects.

1. EMG responses from sternocleidomastoid (SCM) and orbicularis oculi were recorded in subjects who lay supine with their heads cradled in a sling. When the sling released abruptly, their heads fell freely. Normal and bilateral labyrinthine-defective subjects (LDs) were studied. 2. The normal response in SCM was a small burst of excitation at 22-25 ms latency, of 18 ms duration. This merged into a larger, later burst. The drop also produced eye blinks at 22-38 ms. 3. The onset of the SCM response in LDs was delayed (56-73 ms) even though the latency of their eye blinks was normal. 4. We conclude that the early response at approximately 22 ms in normal subjects is mediated by a vestibulocollic reflex. The delayed activity in LDs may be a stretch reflex. This is the first demonstration of the latency of the vestibulocollic pathway to natural stimulation in man.

Acceleration↗
PubMed1995

Click-evoked vestibulocollic reflexes in torticollis.

A total of 26 patients with torticollis were studied using a recently developed technique for recording vestibulocollic reflexes from the sternocleidomastoid muscles in addition to conventional caloric tests of vestibular function. Previous reports of abnormalities of vestibulo-ocular reflexes in these patients were confirmed with just fewer than half having significant canal pareses or directional preponderances (nine of 20 tested). In addition, there was a high incidence of abnormal click-evoked vestibulocollic reflexes (17 of 26 tested), which were not simply the result of prior treatment with botulinum toxin, nor due to unequal levels of muscle activation. In patients never previously treated with botulinum toxin (14 patients), the effect almost always consisted of suppressed responses in the sternocleidomastoid muscle ipsilateral to the direction of head turning. Because responses were not abnormal in all patients tested, and more commonly so in those with a history of torticollis of > or = 5 years (eight of nine patients) than in de novo patients, we suggest that the changes are more likely to be compensatory than causal.

Acoustic Stimulation↗
PubMed1993

Assessing the impact of essential tremor on upper limb function.

We compared the impact of essential tremor on the performance of three manual tasks: drawing spirals, holding a cup full of water and a joystick-controlled tracking test. Tremor amplitude and frequency were measured by accelerometry during the tracking test, when holding the cup and whilst a standard posture was maintained. The inter-relationships between tremor amplitude, frequency and task impairment were then examined. The results showed that the amplitude and frequency of essential tremor (measured from the principal spectral peak) changed with different activities, with the mildest postural tremors changing most in frequency (by up to 4-5 Hz). The amplitude of tremor decreased in almost every case during the tasks, relative to posture, and this decrement was greatest for the most severe tremors. We also demonstrate that for practical purposes, such as routine clinical situations and therapeutic trails, the effect of essential tremor upon upper limb function can be usefully assessed by two simple complementary techniques: rating spirals and measuring the volume of water split from a cup. The impairments in carrying out these tasks and the tracking test were highly correlated with one another and also with the amplitude and frequency of postural tremor. The concept of tremor "suppressability" is introduced: the relative percentage decrease in the amplitude of a particular tremor during the performance of a specific task compared to that recorded whilst holding a standard posture.

Acceleration↗
PubMed1993

Management of symptomatic latent nystagmus.

Most patients with latent nystagmus are asymptomatic and do not require treatment. We discuss the management by botulinum toxin injection and surgery of five cases of latent nystagmus in which the patients suffered loss of visual acuity on certain manoeuvres as a consequence of an exacerbation of the nystagmus amplitude. The importance of eye movement recordings for accurate diagnosis is stressed and the investigative role of botulinum toxin injection is discussed. Extraocular muscle surgery is helpful in some cases of symptomatic latent nystagmus.

Adult↗
PubMed1993

A case of "voluntary nystagmus" and head tremor.

A 24-year-old woman presented with nystagmus and head tremor. Both were transient and could be executed simultaneously at different frequencies of oscillation. It is unusual for these two movements, which can be voluntary, to occur together. Suspicions of an acquired disease were not confirmed on further investigation using DC-coupled electro-oculography and angular accelerometry, and on an admission by the patient that the nystagmus could be voluntary. Voluntary nystagmus can become semi-automatic and a patient may be able to simulate more than one "involuntary movement" simultaneously yet at different frequencies.

Adult↗