The peripheral nervous system and the perception of verticality

Orientation of the body with respect to gravity is based on integration of visual, vestibular and somatosensory signals. Here, we investigated the subjective postural vertical (SPV) and visual vertical (SVV) in three patients with bilateral somatosensory deafferentation and a group of age-matched normal subjects. Our hypothesis was that the patients with bilateral somatosensory deafferentation may show tilt induced bias in the construction of their SPV, with a normal SVV. Patient 1 had a severe Guillain Barré syndrome and almost complete absence of peripheral sensation, the two other patients had a thoracic spinal injury with a sensory loss from T6-7 down. On initial testing, compared with normal subjects and the patients with spinal injury, Patient 1 had a significant bias in SPV towards the side of a preceding tilt in both directions. Several months later, after significant improvement of sensation, this tilt-induced bias in SPV had resolved completely. In addition, Patient 1 had a significantly enlarged "cone of verticality", which did not change following improvement in peripheral sensation, reflecting persisting disturbance in the perception of body verticality. In the two patients with spinal injury, bias towards the side of a preceding tilt was not significant. These findings confirm the importance of somatosensory input from the trunk to the perception of SPV in the seated position.

Perception of longitudinal body axis in patients with stroke: a pilot study

BACKGROUND AND AIMS

To investigate the hypothesis that patients with a hemisphere stroke may perceive their longitudinal body axis (LBA) rotated in the frontal plane. This error in an egocentric frame of reference could be detrimental to posture, as tilted LBA would imply an unequal distribution of body mass about the true vertical.

METHOD

26 healthy subjects matched in age with 18 patients living with stroke participated in the study. The 18 patients were tested on average 80 days after a first left (n = 8) or right (n = 10) hemisphere stroke. Participants perceived their LBA by adjustments of the orientation of a luminous rod pivoting around a dorsonavel axis to the subjective direction of LBA. Participants were studied in the supine position to dissociate somaesthetic cues from graviceptive cues.

RESULTS

Patients with stroke perceived their LBA rotated to the contralesional side in comparison with controls (p = 0.004). For all controls and 10 patients with stroke, the perceived LBA was very close to true LBA (mean (SD) 0.24 degrees (1.31 degrees)). For eight patients with stroke (six right stroke, two left stroke), the perceived LBA was rotated from true body orientation in the direction opposite to the lesioned side (range 3-9.5 degrees, mean 5.2 degrees). These eight patients provided similar estimates by tactile manipulation of the rod (without vision). The rotation of perceived LBA was more pronounced for right-hemisphere strokes. The magnitudes of perceptual rotations correlated with sensory loss, signs of spatial neglect and the degree of postural and gait disability.

CONCLUSION

This is the first study showing that certain patients with a hemisphere stroke perceive their LBA rotated to the contralesional side. The consequences for perceptuomotor coordination have implications for their postural disorders.

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.

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.

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.

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

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.

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.

The Control of Head Movements during Human Balance Corrections

The assumption that the CNS regulates head stability during human balance corrections is explored in this review (an outgrowth of discussions initiated during the Head/Neck meeting held in Vail, Colorado, USA, July 1995). Two major questions were considered. First, how do the vestibulocollic (VCR) and cervicocollic (CCR) reflexes interact with intrinsic mechanical properties of the head neck system to control head position during balance corrections? Second, how is this interaction affected by factors such as vestibular loss, aging, and changes in behavioral goals or central set? The authors conclude that head velocities observed during balance corrections depend to a large extent on the movements of the head–neck mass-viscoelastic system whose properties could be altered by cocontracting the neck muscles. For experiments involving stance perturbations, much of the corrective response in neck muscles appeared to be triggered by trunk and leg proprioceptive signals, and a major role for the VCR was not established. Evidence consistent with a role for the vestibular system was found in other experimental paradigms in which the head was perturbed directly. In these paradigms the VCR modulates the amplitude of functionally stabilizing responses and damps mechanically induced instability of the head and neck.

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.

The control of head movements during human balance corrections

The assumption that the CNS regulates head stability during human balance corrections is explored in this review (an outgrowth of discussions initiated during the Head/Neck meeting held in Vail, Colorado, USA, July 1995). Two major questions were considered. First, how do the vestibulocollic (VCR) and cervicocollic (CCR) reflexes interact with intrinsic mechanical properties of the head neck system to control head position during balance corrections? Second, how is this interaction affected by factors such as vestibular loss, aging, and changes in behavioral goals or central set? The authors conclude that head velocities observed during balance corrections depend to a large extent on the movements of the head-neck mass-viscoelastic system whose properties could be altered by cocontracting the neck muscles. For experiments involving stance perturbations, much of the corrective response in neck muscles appeared to be triggered by trunk and leg proprioceptive signals, and a major role for the VCR was not established. Evidence consistent with a role for the vestibular system was found in other experimental paradigms in which the head was perturbed directly. In these paradigms the VCR modulates the amplitude of functionally stabilizing responses and damps mechanically induced instability of the head and neck.

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.

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.

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.

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.

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.

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.

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.

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.

Assessing tremor severity

A clinical rating scale which measured the severity of tremor in 20 patients (12 with essential tremor and 8 with "dystonic" tremor) was assessed at specific anatomical sites for both inter and intra-rater reliability using four raters. The scores obtained with the scale were compared with the results of upper limb accelerometry, an activity of daily living self-questionnaire and estimates of the tremor induced impairment in writing and drawing specimens. The results show that, for the purposes of routine assessment and therapeutic trials, a clinical rating scale can produce reliable results which are a more valid index of tremor induced disability than standard postural accelerometry.

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.

The use of commercially available disposable Ag-AgCl electrodes for DC-coupled electro-oculography

Disposable bioelectrically compatible electrodes for direct-coupled electro-oculography have advantages over reusable electrodes in terms of hygiene and convenience. The electrical characteristics of examples of one type of such electrodes were determined. They were found to have suitable noise levels and contact impedance, with acceptable signal drift. They were also found to suppress artefacts on vertical eye movement.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The differential diagnosis of congenital nystagmus

The decision whether a nystagmus is congenital or acquired may be difficult and is of importance in patients presenting with neurological complaints. In this article, established diagnostic criteria are critically reviewed with particular emphasis on types of nystagmus waveform and their relationship to pursuit and optokinetic responses. Attention is drawn to certain acquired nystagmus which may have similar features which have hitherto been accepted as pathognomonic of congenital nystagmus. Symptoms due to congenital nystagmus are discussed and related to the oculomotor abnormalities. The importance of the characteristics of congenital nystagmus are evaluated for use in differential diagnosis.

Primary position upbeating nystagmus. A variety of central positional nystagmus

We report a collaborative study of 11 patients with upbeat nystagmus in the primary position of gaze. In most cases the nystagmus behaved in accordance with Alexander's Law; in 3 patients convergence enhanced the nystagmus. Lateral gaze was without effect in 7 instances. Static tilt to prone and supine positions altered the characteristics of the nystagmus in 7 patients. The effects were variable and, in one case, there was reversal of the direction of the nystagmus to downbeating. There was pathological or radiological confirmation of lesions in the pontomedullary junction (2 cases) and the pontomesencephalic junction (2 cases). The findings support previous reports that primary position upbeat nystagmus occurs predominately with intra-axial brainstem lesions. There is one report of its occurrence with an intrinsic cerebellar lesion. Modification of the amplitude of upbeat nystagmus by tilt of the head with respect to gravity in the majority of patients implies an otolith-related component in the genesis of the nystagmus.

The effects of the "vestibular sedative" drug, Flunarizine upon the vestibular and oculomotor systems

The effects of the "vestibular sedative" drug Flunarizine upon the oculomotor functions of pursuit and voluntary saccades and upon the vestibular response (to rotational stimuli) were assessed in twenty volunteer subjects. The study was then extended to three patients with chronic imbalance of central origin who had reported a beneficial symptomatic response to the drug. Three of the volunteer subjects were found to have a directional preponderance (presumed to arise from peripheral dysfunction). In the remaining seventeen normal subjects Flunarizine was found to reduce the amplitudes of fast phases of vestibular nystagmus. The directional preponderance in the other three subjects was redressed through production of fast phases which were of lower and more uniform amplitude. In the patients, in addition to a reduction in fast phase amplitude, there was a reduction or abolition of after nystagmus. In no case was any reduction in slow phase velocity observed. Pursuit and voluntary saccades were unaffected by the drug. It was concluded, on the basis that the fast phases of nystagmus are centrally generated, that Flunarizine has a central action rather than a depressant effect upon the vestibular end organ. In view of known oculomotor physiology and pharmacology it is proposed that vestibular sedatives act by depression of Type II vestibular neurons, and modification of the functional relationships between the vestibular nuclei, the perihypoglossal nuclei and the flocculus of the cerebellum. A trial of vestibular active drug is indicated particularly in patients in whom asymmetry of the vestibular response and/or abnormal after nystagmus is demonstrated.

Following eye movements on the absence of central vision

Following responses to constant velocity and sinusoidally oscillating movements of the whole visual environment were examined in normal subjects wearing blinkers to obscure central vision or subjected to a photo flash to induce a central scotoma, in patients with central scotomas of pathological origin and in one patient with a central scotoma in an immobile eye which provided open loop testing. Good following and brisk nystagmus were produced in patients with central scotomas and subjects with flash scotomas; it was subjectively evident that the scotoma itself could be used as a target to generate open loop pursuit and augment peripherally induced following responses. Following responses in subjects with blinkers were weak, possibly reflecting that, in everyday life, eye movements induced by movements of the visual background have to be suppressed. Open loop responses were strong, suggesting that the periphery has the latent potential to mediate good following. The findings provide a unified explanation for the various patterns of optokinetic nystagmus.

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.

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.

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.

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.