A new device to quantify ocular counterroll using retinal afterimages.

BACKGROUND: The integrity of vertical semicircular canal and otolith function remains difficult to assess in the clinical setting, partly due to difficulties in recording ocular counterroll. Here, we quantify static ocular counterroll from head tilt using a new head-mounted device. METHODS: The device consists of an LED positioned 42 cm in front of one eye and a striated lens which produces a streak of light on the retina. The LED is illuminated at full intensity (80 cd) to generate a retinal afterimage. Subsequently, in darkness, the subject's head is tilted in the roll plane. Finally, the LED is illuminated dimly (0.2 cd) and the subject rotates the striated lens to superimpose the dim light streak onto the afterimage. An angular scale indicates the angle through which the lens is rotated, giving a measure of the ocular counterroll. To validate the device, we recorded binocular counterroll simultaneously with 3D computerised video-oculography of the other eye in 16 normal subjects; 2 patients with acquired bilateral loss of vestibular function were also tested. RESULTS: In the normal subjects, there was no significant difference between the two techniques (p=0.24) when recording ocular counterroll and the correlation between the two techniques was R2=0.78. The 2 avestibular patients essentially showed no ocular counterroll with both techniques. CONCLUSIONS: We have devised a non-invasive, quick and reliable test of ocular counterroll. The lack of response in the 2 avestibular patients indicates that this device is clinically applicable to assess otolith function.

Interference between postural control and mental task performance in patients with vestibular disorder and healthy controls.

OBJECTIVES: To determine whether interference between postural control and mental task performance in patients with balance system impairment and healthy subjects is due to general capacity limitations, motor control interference, competition for spatial processing resources, or a combination of these. METHOD: Postural stability was assessed in 48 patients with vestibular disorder and 24 healthy controls while they were standing with eyes closed on (a) a stable and (b) a moving platform. Mental task performance was measured by accuracy and reaction time on mental tasks, comprising high and low load, spatial and non-spatial tasks. Interference between balancing and performing mental tasks was assessed by comparing baseline (single task) levels of sway and mental task performance with levels while concurrently balancing and carrying out mental tasks. RESULTS: As the balancing task increased in difficulty, reaction times on both low load mental tasks grew progressively longer and accuracy on both high load tasks declined in patients and controls. Postural sway was essentially unaffected by mental activity in patients and controls. CONCLUSIONS: It is unlikely that dual task interference between balancing and mental activity is due to competition for spatial processing resources, as levels of interference were similar in patients with vestibular disorder and healthy controls, and were also similar for spatial and non-spatial tasks. Moreover, the finding that accuracy declined on the high load tasks when balancing cannot be attributed to motor control interference, as no motor control processing is involved in maintaining accuracy of responses. Therefore, interference between mental activity and postural control can be attributed principally to general capacity limitations, and is hence proportional to the attentional demands of both tasks.

An ambulatory dyskinesia monitor.

OBJECTIVES: New treatments are now becoming available for the management of levodopa induced dyskinesias in Parkinsons's disease. However, assessment of their efficacy is limited by the inadequacies of current methods of dyskinesia measurement. The objective was to develop and validate a portable device capable of objectively measuring dyskinesias during normal daily activities. METHODS: A portable device was developed based on a triaxial accelerometer, worn on the shoulder, and a data recorder that can record levodopa induced dyskinesias. A computer program plots raw acceleration and acceleration over 0.5 Hz frequency bands against time. The acceleration in the different bands can then be compared with the raw acceleration trace, enabling identification and exclusion of confounding activities such as tremor and walking, which have a characteristic appearance on the trace. The validity of this device was assessed on 12 patients and eight age matched controls by comparing accelerations in the 1-3 Hz frequency band with established clinical dyskinesia rating scales. While wearing the monitor, subjects were videorecorded sitting and during dyskinesia provocation tasks, including mental activation tasks, eating, drinking, writing, putting on a coat, and walking. The dyskinesias were graded with both modified abnormal involuntary movement (AIM) and Goetz scales. The clinical ratings were then compared with the mean acceleration scores. RESULTS: Acceleration in the 1-3 Hz frequency band correlated well against both scales, during all individual tasks. Acceleration produced by normal voluntary activity (with the exception of walking, which produced large accelerations, even in controls) was small compared with dyskinetic activity. With walking excluded, the mean acceleration over the rest of the recording time correlated strongly with both the modified AIM (Spearman's rank (r=0.972, p<0.001) and Goetz (r=0.951, p<0.001) scales. CONCLUSIONS: This method provides an accurate, objective means for dyskinesia assessment, and compares favourably with established methods currently used.

Automatic control of postural sway by visual motion parallax.

The purpose of this study was to establish whether visual motion parallax participates in the control of postural sway. Body sway was measured in ten normal subjects by photoelectric recordings of head movements and by force-plate posturography. Subjects viewed a visual display ("background"), which briefly moved (2 s) along the y (horizontal) axis, under three different conditions: (1) direct fixation of the background, (2) fixation of a stationary window frame in the foreground, and (3) fixation of the background in the presence of the window in the foreground ("through the window"). In response to background fixation, subjects swayed in the same direction as stimulus motion, but during foreground (window) fixation they swayed in the opposite direction. The earlier forces observed on the force platform occurred at circa 250 ms in both conditions. The results show that motion parallax generates postural responses. The direction of these parallax-evoked postural responses-opposite to other visually evoked postural responses reported so far-is appropriate for stabilizating posture in natural circumstances. The findings show that motion parallax is an important source of self-motion information and that this information participates in the process of automatic postural control. In the "fixating through the window" condition, which does not mimic visual conditions induced by body sway, no consistent postural responses were elicited. This implies that postural reactions elicited by visual motion are not rigid responses to optokinetic stimulation but responses to visual stimuli signalling self-motion.

The effect of eye/head deviation and visual conflict on visually evoked postural responses.

Three interrelated experiments on visually evoked postural responses (VEPR) are presented to investigate the effect of lack of coplanarity between retinal and body coordinates (Experiment I) and the effect of directionally conflicting information in the visual stimulus. Experiment I showed that the direction of VEPR is modified by eye-in-orbit and head-on-trunk position signals, presumably of proprioceptive origin. Experiments II and III showed that VEPR can be critically suppressed by the presence of conflict within the visual stimulus (Experiment II: a linear, tangential component of visual motion acting in the opposite direction to the main angular component of a roll-motion display; Experiment III: a non congruent "improbable" visual motion parallax linear motion stimulus). A conceptual model of the postural system is presented, incorporating a gain control unit for the visuo-postural loop with inputs from the ocular/cervical proprioceptive system and from intra- and inter-sensory conflict detectors (comparators).

Control of the head in response to tilt of the body in normal and labyrinthine-defective human subjects.

1. Head movement responses to discrete, unpredictable tilts of the trunk from earth upright were studied in normal and labyrinthine-defective (LD) subjects. Tilts of the seated, restrained trunk, were delivered in pitch and roll about head-centred axes and approximated raised cosine displacements with peak amplitudes of 20-30 deg and durations of 1.5-2 s. Subjects performed mental arithmetic with eyes closed or read earth-fixed text. 2. At the onset of tilt the head momentarily lagged behind the trunk because of inertia. Subsequently, head control varied widely with three broad types: (i) head relatively fixed to the trunk (in normal subjects and some patients); (ii) head unstable, falling in the direction of gimbal tilt (typical of acute patients for pitch motion); (iii) compensatory head movement in the opposite direction to gimbal tilt (observed consistently in normal subjects and in well-adapted patients). 3. EMG was well developed in subjects with compensatory head movement and consisted of an initial burst of activity at minimum latencies of 25-50 ms (means 72-108 ms), followed by a prolonged peak; both occurring in the 'side up' neck muscles, appropriate for righting the head. These muscles are shortened during the initial head lag so the responses cannot be stretch reflexes. In normal subjects their origin is predominantly labyrinthine but in patients they may be an 'unloading response' of the neck. 4. Head stability in space was superior with the visual task for all subjects but vision only partially compensated for labyrinthine signals in unstable patients. 5. Modelling the responses to tilt suggests that, in LD subjects, the short-latency burst could be driven by signals from the neck of the relative acceleration between head and trunk tilt. The longer latency EMG could be driven by a signal of head tilt in space. Normally, this signal is probably otolithic. In patients it could be synthesized from summing proprioceptive signals of position of head on trunk with trunk tilt.

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.