Self-controlled reorienting movements in response to rotational displacements in normal subjects and patients with labyrinthine disease.
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Adult↗
Biomedical subjects
M Gresty
Publications and source records attributed to M Gresty.
Visual destabilisation of posture in normal subjects.
A new and simple method of assessing reliance on vision for postural control was evaluated in 41 normal subjects. Left-right reversal of peripheral vision induced by a head-mounted mirror device caused an initial dramatic instability in approximately half the subjects, when standing on foam to reduce the value of proprioception. Lateral reversal of central vision by means of a prism device evoked similar responses. Sensitivity to vision reversal was significantly correlated with motion sickness susceptibility. Despite some rapid habituation (partially retained over several weeks) sway remained as great as with eye closure in the anterior-posterior as well as lateral direction, indicating complete suppression of the visual input. Balancing with vision reversal caused a selective decrement in performance of a visuo-spatial memory task, suggesting that coping with misleading visual input may place continuous demands on cortical spatial processing.
Adult↗
Cervico-ocular function in patients with spasmodic torticollis.
The cervico-ocular (COR) and active and passive vestibulo-ocular reflexes (VOR) were measured in seven patients with spasmodic torticollis (ST) and six normal controls. The COR was found to be weak or absent in both groups. The VOR gain was similar in the two groups but five patients had a significant asymmetry of the response. There was no evidence of abnormal cervico-vestibular interaction during active head rotation. The study suggests that the VOR asymmetry frequently found in ST cannot be explained on the basis of an abnormal cervical input.
Adult↗
Spectral analysis of tremor: understanding the results.
Spectral analysis of a tremor record can sometimes produce a spectrum with multiple components of significant amplitude. The problem is to determine whether the presence of several peaks represents the coexistence of separate tremor mechanisms or be a consequence of fluctuations in the frequency or amplitude of a single tremor. The spectrum of a tremor whose frequency or amplitude vary and are independent has the recognisable pattern of a central carrier frequency with sidebands of equal amplitudes distributed symmetrically around the carrier. However, if tremor amplitude and frequency fluctuate and are not independent, (frequency proportional to amplitude or frequency inversely proportional to amplitude), the spectrum has a pattern of sidebands which are asymmetrical in amplitudes and may resemble the spectrum of the combined signal from different independent oscillators. The investigation of sidebands in spectra has been neglected in tremor studies and multiple irregular peaks on a tremor spectrum have sometimes been used wrongly as evidence for the coexistence of multiple tremor mechanisms or frequency components assumed to be concurrent.
Computer Simulation↗
Saccadic function in spasmodic torticollis.
Twelve patients with idiopathic spasmodic torticollis were compared with 19 normal controls on tests of saccadic eye movements thought to depend upon normal basal ganglia function. The patients were able to make random, predictive, remembered, and self-paced saccades equally as well as control subjects. This suggests that those parts of the basal ganglia which may be damaged in spasmodic torticollis, are separate from pathways responsible for the normal initiation and execution of saccades.
Adult↗
Joubert syndrome.
Joubert syndrome is an autosomal recessive condition in which there is a variable combination of central nervous system defects with a distinctive congenital retinal dystrophy, ocular motor abnormalities, and respiratory abnormalities in early infancy. The retinal dystrophy has been previously classified as a variant of Leber's congenital amaurosis. We report electrophysiologic and eye movement findings in a series of seven consecutive children with Joubert syndrome. Unlike patients with Leber's congenital amaurosis, all but one of these children had preserved flash and pattern-reversal visual evoked potentials. Six of the seven children had abnormalities of smooth pursuit, optokinetic nystagmus, and saccades. Six of the children had nystagmus: three had a pendular torsional nystagmus and three had a form of see-saw nystagmus. An alternating hyperdeviation was present in five of the patients, two of whom also had a tonic deviation of their eyes laterally. All seven patients had cerebellar vermis hypoplasia on a magnetic resonance imaging scan. Developmentally delayed children with an absent or highly attenuated electroretinogram should be investigated for Joubert syndrome.
Blindness↗
Stability of the head in pitch (neck flexion-extension): studies in normal subjects and patients with axial rigidity.
The dynamic stability of the head in pitch during normal upright posture has been studied in normal subjects and patients with neurological disease affecting neck muscle tone by examining angular head acceleration responses to unpredictable linear motion of the trunk in the direction of surge. Within the frequency range of natural head movements the transfer function between head and trunk for both normal subjects and patients approximated a second-order linear differential equation involving inertia and coefficients of viscosity and elasticity. The degree of neck rigidity was determined by the damping ratio (viscosity:elasticity), which averaged .35 for normal subjects and ranged from 0.6 to 0.96 for patients with rigid syndromes. A patient with absent labyrinthine function and a "floppy" head had a damping ratio 0.18. The technique gives a numerical measurement of neck rigidity, which could be of value in characterising severity of disorder and response to therapy.
Adult↗
Visual evoked potentials in dissociated vertical deviation: a reappraisal.
Pattern reversal and flash evoked potentials were recorded in 13 children with dissociated vertical deviation (DVD). No electrophysiological evidence was found to support the notion that patients with DVD have an anomalous (albinoid) projection of visual fibres originating from the temporal retina of each eye. However, DVD patients had significantly smaller monocular and binocular pattern evoked responses than age matched controls. Explanations are given for this finding and for the occipital VEP asymmetries reported by other workers.
Adolescent↗
Stability of the head: studies in normal subjects and in patients with labyrinthine disease, head tremor, and dystonia.
The dynamics of postural control of the head were investigated in normal human subjects and patients with neurological disease. The technique adopted was to measure the head movements provoked by passive, unpredictable oscillations of the trunk in the frequency range 0-6-7 Hz when subjects were required (a) to try to stabilise their head "in space" and (b) to try to make their heads move "en bloc" with the trunk. Head movement responses were characterised by the gain and phase with respect to trunk movement (transfer function) and degree of linear relationship between head and trunk (coherence). The normal transfer function approximated a cascade of two second-order, underdamped, systems representing the passive inertial, viscous, and elastic properties of the muscle and joints of the head and neck. Stabilisation of the head "in space" produced about 40% reduction in transmission of body movement, was only evident at frequencies less than 1 Hz and was affected partly by voluntary movements. An alabyrinthine patient could also achieve some spatial stabilisation. The findings indicate a weak role for vestibular-collic reflexes and emphasise that the primary control of head posture during unpredictable movement is through the tonic visco-elastic properties of neck muscles that work to stabilise the head on the shoulders. In patients with dystonia of the neck and essential and cerebellar head tremor, the head showed a tendency to unstable oscillation (resonance). The instability and dystonia could be measured in terms of visco-elastic constants and damping ratios. The head movements of some tremor patients did not linearly follow the trunk movement, showing that the motion stimulus provoked abnormal phasic muscle activity at frequencies other than those of the tremor. The technique quantifies head control in movement disorders and is sensitive to abnormal function.
Adolescent↗
Frequency/amplitude characteristics of postural tremor of the hands in a population of patients with bilateral essential tremor: implications for the classification and mechanism of essential tremor.
Amplitude/frequency characteristics of postural hand tremor in 59 patients with bilateral essential tremor of various degrees of severity were assessed using accelerometric recordings and spectral analysis. Intra-subject comparisons of tremor characteristics between the more and less affected hands were used to control for variability of tremor due to age factors and intersubject differences in amplitude and frequency. Statistical analysis distinguished three different patient groups. Some patients had low amplitude (less than 0.1-0.015 cm) tremor in the less affected limb (which tended to be 7 Hz or more in frequency in the young) and a larger amplitude tremor in the more affected hand which was 1 Hz or more lower in frequency. Other subjects had either bilaterally small or bilaterally large amplitude tremors of similar frequencies. These findings imply that there is a downwards step in frequency between symptomatic tremors of small and large amplitude. The amplitude and frequency of the small amplitude tremors were unrelated but frequency declined with age. The frequency of the large amplitude tremor was generally determined by amplitude but a wide range of amplitudes were compatible with similar frequencies. The frequency of large amplitude tremor also declined with age. It was concluded that there are two types of essential tremor, the smaller amplitude tremor probably derives from an exaggeration of some or all of the mechanisms of normal physiological tremor whereas the larger amplitude tremor probably arises from a separate "pathological" central nervous mechanism. It is not known if or how one may transform into, or be replaced by, the other during progression of the disease.
Adolescent↗
Assessment of rest tremor in Parkinson's disease.
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Adult↗
Analysis of downbeat nystagmus. Otolithic vs semicircular canal influences.
The relative strengths of vertical canal and otolithic factors influencing downbeat nystagmus (DBN) were investigated in a patient whose nystagmus was of maximum intensity with the head in the upright position and abolished with the head in the supine position. The vestibuloocular reflex (VOR) was assessed by oscillating the patient about both the supine and upright positions. During oscillation about the supine position both the upward and downward VORs had equal gains in the dark (0.6) and unity gain in the light. In contrast, during oscillation about the upright, the upward VOR became hyperactive with a gain of 1.8 in the dark and 1.2 in the light, whereas the downward VOR became hypoactive with a maximum gain of 0.86 in the light. This degree of asymmetry of the VOR is greater than would be expected from a summation of spontaneous nystagmus with normal canal reflexes. We concluded that the DBN arose from an asymmetry of vertical canal function, which became manifest when the otoliths were tilted with respect to gravity. Contrasting findings are presented in a patient whose DBN was insensitive to tilt. It would seem that other cases of DBN lie on a continuum between these extreme examples.
Adult↗
Clinical and theoretical aspects of head movement dependent oscillopsia (HMDO). A review.
Head movement-dependent oscillopsia (HMDO) with peripheral vestibular, brainstem and cerebellar lesions is reviewed. The differentiation of this kind of oscillopsia is based mainly on clinical grounds. HMDO with bilateral abolition of caloric responses, and in the absence of disease of the central nervous system, is due to bilateral vestibular disease. HMDO in patients with internuclear ophthalmoplegia and other brainstem signs is probably due to a lesion of VOR pathways in or near the medial longitudinal fasciculus. The occurrence of HMDO with ataxia of gait and cerebellar eye movement disorders (rebound nystagmus, flutter-like oscillations), in the absence of brainstem lesions (medial longitudinal fasciculus), is clinical evidence for HMDO due to a cerebellar lesion. An attempt is made to associate the different kinds of oscillopsia with current knowledge of the vestibulo-ocular reflexes.
Animals↗
The relationship between head and eye movement in congenital nystagmus with head shaking: objective recordings of a single case.
Head shaking and congenital nystagmus were recorded in a patient presented with visual tasks. When she was at rest the nystagmus took a 6 cycles per second saw-tooth wave-form. When she was attentive the nystagmus beat at a 2 to 2.6 cycles per second with a saddle-shaped deformation which permitted foveation. The head shaking occurred occasionally when the patient was attentive and was phase-locked to the nystagmus with resemblances in wave form and direction. Deceleration of the head shaking to zero velocity and peak displacement (to the left) coincided with the onset of the saddle of the nystagmus and hence assisted foveation; all other parts of the head-shaking cycle were detrimental to vision. It is proposed that the head shaking has a common pathological origin with the nystagmus and that, just as an isolated congenital nystagmus wave form becomes altered with attention to permit periods of foveal fixation, the pattern of combined head and eye nodding in this patient provided similar peroids of fixation.
Adult↗
Gaze failure, drifting eye movements, and centripetal nystagmus in cerebellar disease.
Three abnormalities of eye movement in man are described which are indicative of cerebellar system disorder, namely, centripetally beating nystagmus, failure to maintain lateral gaze either in darkness or with eye closure, and slow drifting movements of the eyes in the absence of fixation. Similar eye movement signs follow cerebellectomy in the primate and the cat. These abnormalities of eye movement, together with other signs of cerebellar disease, such as rebound alternating, and gaze paretic nystagmus, are explained by the hypothesis that the cerebellum helps to maintain lateral gaze and that brain stem mechanisms which monitor gaze position generate compensatory biases in the absence of normal cerebellar function.
Adult↗
Eccentric head positions reveal disorders of conjugate eye movement.
The effect of head position on conjugate horizontal gaze was studied in healthy adults, in patients with multiple sclerosis without eye movement signs, and in patients with downbeat nystagmus indicative of low brain stem lesions. Displacements of gaze from primary position to 30 degrees left and right were recorded using the electro-oculogram, with the head in the primary position, and turned voluntarily to the left and right (in yaw). The quality of eye movements was noted and peak velocities of saccades were measured. The head turning test trebled the incidence of abnormal eye movements found in the multiple sclerosis patients and increased it by tenfold in the patients with downbeat nystagmus. Disorders of eye movement were also found in approximately 20--30% of healthy subjects tested. Weakness of abduction was the most common eye movement defect and appeared to be posterior internuclear ophthalmoplegia. A hypothesis is made which unifies the theoretical explanations of anterior and posterior internuclear ophthalmoplegia. The most likely cause of the disorders of eye movement observed is vertebrobasilar ischaemia induced by stretching and compression of the vertebral arteries during eccentric head posture.
Adult↗
Coordination of the head and eyes in pursuit of predictable and random target motion.
Subjects were required to use their head and eyes in pursuit of visual targets which moved randomly or sinusoidally in the horizontal plane. All subjects disliked moving their heads to pursue the random motion, apparently because the motion broke fixation which resulted in a predominance of the vestibulo-ocular compensatory reflex over the smooth pursuit reflex. As a consequence gaze (head plus eye movement) was at times in the opposite direction to the motion of the target. In steady state pursuit of sinusoidal targets, eye movement consisted of a combination of pursuit and vestibulo-ocular reflex eye movements. At frequencies below 0.8 HZ, the vestibular reflex was used at times of minimum target velocity to stabilize fixation whereas during maximum target velocity the head movement was slowed and the smooth pursuit reflex predominated. At 1 HZ and over, there was a failure to suppress the compensatory vestibulo-ocular reflex; however, the saccades of vestibular nystagmus were used to "catch up" the target. There was a preference not to use the head in predictable pursuit.
Adult↗
The assessment of position of stationary targets perceived during saccadic eye movement.
The experiment was performed to establish the accuracy with which visual targets perceived during saccadic eye movement are localised. Subjects were presented with the task of executing saccades of 30 degrees plus amplitude, passing through primary gaze, about the time of peak velocity a 5 ms red flash was presented at some random position (up to 30 degrees left or right of centre) on a horizontal visual display. Subjects were required to indicate the direction in which they thought the flash was localised by fixating in that direction. Observations were made under conditions of prolonged total darkness and in the presence of a contrasting background. Measurement was made of saccade velocity and eye displacement as an index of target positions. Eye displacement was linearly scaled with respect to true target direction. Targets were localised with an average error of 5 degrees-6 degrees although the variance was high. No systematic differences were found between conditions or subjects. Error was unrelated to saccade velocity. It is concluded that during saccadic eye movements the appreciation of target position is maintained with an acceptable degree of accuracy.
Eye Movements↗