Eye movements during voluntary head motion with minimized cervical input

Eye movements were recorded using DC-electro-oculography in 20 healthy subjects during sinusoidal voluntary head movements and during similar voluntary head movements with minimized cervical input. The eye movements during these conditions were compared to the vestibulo-ocular reflex (VOR), to the cervico-ocular reaction (COR) and to the active COR (i.e., voluntary stabilization of the head in space while the trunk was moved sinusoidally). Each subject sat with eyes covered on a rotating chair. Sinusoidal movements having an amplitude of +/- 20 degrees and either 0.1 or 0.2 Hz were performed. Voluntary head movements and head movements with minimized cervical input showed no difference in the gain of the slow-phase velocity of the nystagmus, the saccadic activity (cumulated amplitude/time) and the maximum eye deviation. The distribution of saccades around the reversal of head movement in space represents the only significant difference between these conditions. While the saccades appear promptly after reversal during sinusoidal voluntary head movement, they appear during voluntary head movement with minimized cervical input just before the reversal of head movement in space. The direction of saccades always depends on the displacement of the head related to the trunk and is due to cervical input and/or central influences.     

Effect of intravenous diazepam and thiopental on voluntary saccades and pursuit eye movements.

The effects of diazepam and thiopental on voluntary saccades and pursuit eye movements were tested in 9 volunteers, with an interval of at least 2 weeks between tests. One, 4 and 8 h after intravenous injection of diazepam (0.3 mg/kg) or thiopental (6.0 mg/kg), voluntary saccades and pursuit eye movements were tested and blood samples taken for analysis of drug concentration. As compared to results of tests without drugs, a significant reduction both of saccadic peak velocity and gain of pursuit eye movements was found 1 h after injection of either drug, but not after 4 and 8 h. The amplitude of saccades elicited with the 60 degrees stimulus was significantly reduced 1 h after injection of diazepam. Latency of saccades increased significantly up to 4 h after injection of either drug. No significant correlation was found between peak velocity of saccades and blood concentration of either thiopental or diazepam 1 h after administration. The present results confirm that in man saccades and pursuit eye movements are reduced by benzodiazepines and barbiturates, but provide no support for the previously described efficacy of saccades in monitoring the effect of benzodiazepines. It is hypothesized that diazepam and thiopental also induce reduction of voluntary saccades and pursuit eye movements via a general sedation of the central nervous system (CNS), besides having specific effects on CNS structures important to the performance of voluntary eye movements.     

Saccades horizontal or vertical at near or at far do not deteriorate postural control

OBJECTIVE: There is a discrepancy about the effect of saccades on postural control: some studies reported a stabilization effect, other studies the opposite. Perturbation of posture by saccades could be related to loss of vision during saccades (saccades suppression) due to high velocity retinal slip. On the other hand, efferent and afferent proprioceptive signals related to saccades can be used for obtaining spatial stability over saccades and maintaining good postural control. In natural conditions saccades can be horizontal, vertical and made at different distance. The present study examines all these parameters to provide a more complete view on the role of saccade on postural control in quiet stance. METHODS: Horizontal or vertical saccades of 30 degrees were made at 1Hz and at two distances, 40 and 200cm. Eye movements were recorded with video-oculograhpy (EyeLink II). Posturography was recorded with the TechnoConcept platform. The results from "saccade" conditions are compared to "fixation control" condition (at far and near). RESULTS: The video oculography results show that subjects performed the fixation or the saccade task correctly. Execution of saccades (horizontal or vertical at near or at far distance) had no significant effect on the surface of center of pressure (CoP), neither on the standard deviation of the lateral body sway, nor on the variance of speed of the CoP. Moreover, whatever the distance, execution of saccades decreased significantly the standard deviation of the antero-posterior sway. CONCLUSION: We conclude that saccades, of either the direction and at either the distance, do not deteriorate postural control; rather they could reduce sway. Efferent and proprioceptive oculomotor signals as well as attention could contribute to maintain or improve postural stability while making saccades.     

Saccade abnormalities in patients with ocular flutter

In a group of 8 patients with opsoclonus or ocular flutter, the eye movements were recorded by electro-oculography (EOG). The spontaneous eye movement pattern and the amplitude and peak velocity of the refixation saccade were analysed. The EOG recording demonstrated frequent bursts of horizontally and vertically directed saccades elicited without any intersaccadic interval. Three patients also demonstrated an increased frequency of square waves. In 6 of the 8 patients the peak velocity of the voluntary saccades was increased; in 5 patients this condition was found for saccades in both directions. Over-shooting oscillations or slightly hypermetric voluntary saccades occurred in 5 patients. It is proposed that the ocular flutter and the increased saccadic velocities found in the present series of patients is caused by a disorder of the burst cells in the brain stem saccadic control system.     

Eye movements during voluntary head motion with minimized cervical input

Summary Eye movements were recorded using DC-electro-oculography in 20 healthy subjects during sinusoidal voluntary head movements and during similar voluntary head movements with minimized cervical input. The eye movements during these conditions were compared to the vestibulo-ocular reflex (VOR), to the cervico-ocular reaction (COR) and to the active COR (i.e., voluntary stabilization of the head in space while the trunk was moved sinusoidally). Each subject sat with eyes covered on a rotating chair. Sinusoidal movements having an amplitude of ±20° and either 0.1 or 0.2 Hz were performed. Voluntary head movements and head movements with minimized cervical input showed no difference in the gain of the slow-phase velocity of the nystagmus, the saccadic activity (cumulated amplitude/time) and the maximum eye deviation. The distribution of saccades around the reversal of head movement in space represents the only significant difference between these conditions. While the saccades appear promptly after reversal during sinusoidal voluntary head movement, they appear during voluntary head movement with minimized cervical input just before the reversal of head movement in space. The direction of saccades always depends on the displacement of the head related to the trunk and is due to cervical input and/or central influences.     

Effects of hypothermic anesthesia of the feet on vibration-induced body sway and adaptation

The aim of this study was to investigate the significance of information from the plantar cutaneous mechanoreceptors in postural control and whether postural control could compensate for reduced cutaneous information by adaptation. Sixteen healthy subjects were tested with eyes open or eyes closed with hypothermic and normal feet temperature during posturography where body sway was induced by vibratory proprioceptive stimulation towards both calf muscles. The hypothermic anesthesia was obtained by cooling the subject's feet in ice water for 20 minutes. Body movements were evaluated by analyzing the anteroposterior and lateral torques induced towards the supporting surface by a force platform during the posturography tests. The reduction of cutaneous sensor information from the mechanoreceptors of the feet significantly increased the vibration-induced torque variance mainly in the anteroposterior direction. However, the effects of disturbed mechanoreceptors information was rapidly compensated for through postural adaptation and torque variance was in level with that without anesthesia within 50 to 100 seconds of stimulation, both when standing with eyes open and eyes closed. Our findings suggest that somatosensory input from mechanoreceptors in the foot soles contribute significantly in maintaining postural control, but the sensory loss could be compensated for.     

Computerized analysis of voluntary eye movements. A clinical method for evaluation of smooth pursuit and saccades in oto-neurological diagnosis

To permit rapid and exact quantification of the oculomotor function in clinical practice, a computerized program has been designed for the recording and analysis of pursuit eye movements and voluntary saccades. In a pursuit sequence the subject tracks a moving target, projected onto a screen at a constant speed of 20 degrees/sec over a horizontal visual angle of 60 degrees. The pursuit sequence is followed by a refixation saccade when the subject rapidly shifts his gaze back to the starting point of the target. A complete test procedure consists of ten consecutive pursuit sequences and refixation saccades in each direction. The EOG signal is fed to a PDP11/23 computer for storage and analysis. The pursuit eye movements are quantified and arranged in five velocity intervals: less than 8, 8-16, 16-24, 24-32 and greater than 32 degrees/sec. The relative distribution of the velocity content is calculated for these intervals and presented in histogram form. Saccades superimposing on the smooth pursuit are identified and grouped according to amplitude and direction. The refixation saccades are quantified as mean peak velocities and also the highest and lowest velocities of the refixation saccades are determined. In a material of 70 healthy subjects, normative data and limits for pathological function were established. In the smooth pursuit, 69% of the velocity values were located within the 16-24 degrees/sec interval. Pathological limits were set for each velocity interval and impaired pursuit tracking ability was considered to be present when those limits were reached in at least three of the five intervals. Normal mean peak eye velocity of the refixation saccade was found to be 460 degrees/sec with a range of 354-575 degrees/sec. Application of the test procedure and method of analysis is described in two patients with impairment of the oculomotor function due to a disturbance in the cerebellar brain stem area.     

The relationship of optokinetic nystagmus to pursuit eye movements, vestibular nystagmus and to saccades in humans. A clinical study

The relationship of mean velocity of optokinetic nystagmus (OKN) to pursuit eye movements (PEM), to vestibular nystagmus and to voluntary saccades was analysed in 10 patients with peripheral vestibular lesions and in 30 patients with central vestibular lesions. PEM and vestibular nystagmus were significantly correlated to OKN, suggesting that a common neural pathway is used in the generation of these eye movements. Weak or no correlation was found between saccadic peak velocity and slow phase velocity of OKN. Using multiple linear regression analysis, it was found that 78.5% of the variation in the slow-phase velocity of OKN could be explained by a synthesis of PEM and vestibular test data. PEM test data were more powerful than those of vestibular nystagmus in deduction of OKN. The possible appearance of slow build-up of OKN could not be deduced from the reduction of PEM. Hence, the relationship between PEM and OKN in man is not a simple linear one, but is more complex.     

Cervical and vestibular afferent control of oculomotor response in man.

Oculomotor response in the absence of vision has been compared in a group of 12 normal humans in two experimental conditions testing (a) the vestibulocular reflex by whole-body oscillation on a turntable, and (b) the cervico-ocular reflex by oscillation of the body with the head held stationary. The stimulus was a sinusoidal oscillation (peak angular velocity +/- 50 degrees/sec) at frequencies between 0.2 and 1.3 Hz. The slow-phase eye movements of the vestibulo-ocular response were compensatory for head movement and showed a mean gain of 0.54--0.90, increasing with frequency. The cervicoocular response was found to be very variable. The slow-phase eye movements were of low velocity (mean gain 0.05) and did not generally compensate for body movement. During neck torsion, some subjects exhibited large overall eye deviations composed of both slow and fast phase eye movements.     

Eye movements during active head turning with different vestibular and cervical input

Eye movements were measured in 15 volunteers during vestibulo-ocular reflex (VOR), cervico-ocular reflex with the head fixed from the ceiling (passive COR), during voluntary stabilization of the head in space while the trunk was moved sinusoidally (active COR) and active head movements with and without additional vestibular or cervical stimuli. The subjects were sitting with eyes covered on a rotating chair swinging sinusoidally at 40 degrees peak to peak amplitude at 0.05, 0.1 and 0.2 Hz. The saccadic activity during passive COR is below the VOR and increases slightly during active COR. During voluntary head movements it shows a marked increase and is further activated if cervical or vestibular stimuli are added. The amplitudes of eye shifts of passive and active COR are not different. During active head movements and more with additional cervical or vestibular input, they increase significantly. The phase of the maximum eye shifts to head position is anticompensatory during passive COR and compensatory during VOR. The phase lead of about 45 degrees during active head movements is less during active COR but is larger with additional cervical and vestibular stimuli reaching 90 degrees.     

Long-term deficits of goal-directed vestibulo-ocular function following total unilateral loss of peripheral vestibular function

Brief whole-body movements (+/- 5 to 170 degrees/s peak velocity; approximately equal to 0.5 s duration), applied during 1.44-s intervals of total darkness while subjects "looked" at a just-viewed target, were used to examine vestibulo-ocular function in 3 patients who had compensated to total unilateral loss of peripheral vestibular function. We found that the combined effects of slow-phase and saccadic eye movements both tended to keep the eyes stabilized on the unseen target. Compensatory slow-phases elicited during rapid head movements ipsilateral to (i.e., towards) the lesion were only about 60% as effective as those elicited during rapid contralateral movements. Compensatory gaze-correcting saccades tended to supplement deficient slow-phase movements, especially during rapid ipsilateral head movement. However, the gaze-correcting effect of saccades was only about half of the required for perfect stabilization. Thus, two functional vestibular deficits were observed during rapid ipsilateral head movements: (1) reduced slow-phase stabilization, and (2) reduced saccadic ability to adequately supplement the deficient slow phases. However, overall vestibular functional capability, as assessed by observation of the net effect of both slow phases and saccades, was much better than would be indicated by conventional observation of slow-phase movements alone.     

Computerized study of the velocity of the rapid eye movements

The average velocity-amplitude characteristics of rapid eye movements (voluntary saccades, and fast phases of caloric and optokinetic [OKN] nystagmus) in normal humans were analyzed with an analogue-digital computer and compared among them. For the same amplitude of movement the velocity of the saccades is considerably greater than that of the fast phases of the OKN, and these in turn are greater than those of their vestibular counterparts. The complexity in oculomotor neurophysiology makes this observation difficult to explain.     

Disturbances in the control of saccadic eye movement and eye-head coordination in schizophrenics.

Some schizophrenic patients have been known to have frontal cortical dysfunction. In view of the evidence that voluntary purposive eye movements and rapid head movements involve areas of the frontal cortex, investigations of saccade performance have been carried out on schizophrenics in various laboratories. We have compared performance of schizophrenic patients in tasks involving inhibition of reflexive saccades (no-saccade) and initiation of saccades without target (memory-saccade) with performance in the antisaccade task. These measures were also compared with results of eye-head coordination tasks. Schizophrenics showed more errors and significantly longer latencies, with lower peak velocities at large amplitudes, in both the antisaccade task and the memory-saccade task. Performance with coordinated eye-head movement was basically similar, except for significantly longer latencies of head movement. These results suggest that schizophrenics may have a disturbance in initiating and executing purposive saccades without targets, and that dysfunction of the frontal cortex may contribute to this disturbance.     

Quantitative analysis of horizontal,vertical, and oblique saccades using a search coil

Saccades occur horizontally, vertically, and obliquely, and in movements including adduction and abduction, up and down, centric and eccentric. Few studies have been on vertical and oblique saccades, and no general consensus has developed on the effect of movement on saccades. We recorded horizontal, vertical, and oblique saccades using a search coil in 9 normal subjects, and quantitatively studied saccades in (1) adduction and abduction, (2) up and down, (3) centric and eccentric, and (4) horizontal, vertical, and oblique. Abducting saccades were faster than adducting at an amplitude of 10 deg, while centric saccades were faster than eccentric at amplitudes of 20 and 30 deg. Both vertical up and centric saccades were faster than others at all amplitudes. The peak velocity of the horizontal component of oblique saccades decreased with the increase in the angle of the stimulus. The peak velocity of vertical component of oblique saccades tended to be faster with the decrease in the angle of the stimulus. We thus found significant differences in velocities of saccades in different directions. Velocity is affected by the direction of eye movement, eye position, and target amplitude. In clinical studies on saccadic eye movement, we should pay more attention to the effect of these factors on saccades. In oblique saccades, note that horizontal and vertical components are completely independent, but interacted.     

Eye movements in patients with speech dyspraxia

Saccades, smooth pursuit and angular acceleration induced nystagmus were analysed quantitatively in 10 patients with speech dyspraxia. The saccades were less accurate, had a prolonged reaction time and showed a tendency to reduced peak velocity, though only contralateral to the lesion. Smooth pursuit was imparied, with a reduction in maximum velocity gain. The vestibular responses tended to be hyperactive, indicating facilitated brain-stem reflexes. The findings show that a lesion in the frontal eye field can produce various oculomotor disturbances, in which the triggering of eye movements and their control, and pacing of the various movement sequences are disturbed. In addition, anticipation of a movement pattern requiring volition may be greatly impaired.     

Effect of microgravity on spatial orientation and posture regulation during Coriolis stimulation

OBJECTIVE: To elucidate spatial orientation and posture regulation under conditions of microgravity. MATERIAL AND METHODS: Coriolis stimulation was done with five normal subjects on the ground (1 g) and onboard an aircraft (under conditions of microgravity during parabolic flight). Subjects were asked to tilt their heads forward during rotation at speeds of 0, 50, 100 and 150 degrees/s on the ground and 100 degrees/s during flight. Body sway was recorded using a 3D linear accelerometer and eye movements using an infrared charge-coupled device video camera. Flight experiments were performed on 5 consecutive days, and 11-16 parabolic maneuvers were done during each flight. Two subjects boarded each flight and were examined alternately at least five times. RESULTS: Coriolis stimulation at 1 g caused body sway, nystagmus and a movement sensation in accordance with inertial inputs at 1 g. Neither body sway, excepting a minute sway due to the Coriolis force, nor a movement sensation occurred in microgravity, but nystagmus was recorded. CONCLUSIONS: Posture, eye movement and sensation at 1 g are controlled with reference to spatial coordinates that represent the external world in the brain. Normal spatial coordinates are not relevant in microgravity because there is no Z-axis, and the posture regulation and sensation that depend on them collapse. The discrepancy in responses between posture and eye movement under conditions of microgravity may be caused by a different constitution of the effectors which adjust posture and gaze.     

Direction of galvanically-induced vestibulo-postural responses during active and passive neck torsion

The direction of a postural response induced by galvanic vestibular stimulation depends on the head and trunk position. The relative importance of afferent information (proprioception) and efferent motor command/corollary discharge is unknown. We studied the direction of body sway evoked by galvanic vestibular stimulation in 9 healthy subjects during active and passive head positioning at 0 degrees frontal position, 35 degrees to the left, and 75 degrees to the right, using a custom-built collar. At 0 degrees and 75 degrees there were no significant differences in sway direction between active and passive head positioning. The galvanic stimulation invoked sway toward the anode, mainly in the inter-aural direction. The sway direction differed significantly between active and passive positioning at 35 degrees to the side (p < 0.05). When the head was actively kept in this position, the body sway was mainly in an inter-aural direction. The sway shifted to a naso-occipital direction when the head was passively positioned at 35 degrees. Our results indicate that the afferent proprioceptive information has the largest influence on the direction of the galvanically-induced postural response, although some dependence on efferent motor commands and non-linear cervical proprioception cannot be ruled out entirely.     

Long-term postural abnormalities in benign paroxysmal positional vertigo

Benign paroxysmal positional vertigo (BPPV) is a disorder in which patients suffer from acute rotatory vertigo due to the presence of free otoconial debris migrating into one or more semicircular canals during head movements and resulting in abnormal stimulation of the ampullary crest. A prolonged loss of equilibrium of unclear origin is also present. Static posturography is a useful tool for the study of postural control systems and their role in these abnormalities. The aim of the present study was to evaluate the frequency of body sway and long-term instability of BPPV patients by posturography frequency analysis. Twenty patients with canalithiasis of the posterior semicircular canal and 20 normal controls were subjected to static posturography. Informed consent was obtained from all subjects. Patients were tested 1 h after diagnosis, and 3 days and 12 weeks after the characteristic Epley repositioning maneuver. Patients with BPPV showed significantly increased body sway both on lateral (X) and anteroposterior (Y) planes compared to normal subjects. Corporal oscillation with a broad-frequency spectrum was observed in both closed and open eye tests. The repositioning maneuver decreased the X plane body sway, while the anteroposterior sway was unchanged. Twelve weeks after treatment, a normalization of the anteroposterior sway was observed. Results of this study suggest that the long-term postural disturbance associated with BPPV differs from the acute disequilibrium that subsides after canalith repositioning: the former is a sagittal plane/broad spectrum body sway, while the latter is primarily a frontal plane/low frequency sway. The Epley maneuver was shown to reduce frontal sway, a postural abnormality that might therefore be linked to posterior semicircular canal function. Conversely, the observed sagittal body sway was only partially relieved by the restoration of canal function, and therefore, may be more related to the chronic dizziness observed in these patients.     

Low-frequency loud acoustic stimulation and goal-directed arm movements

OBJECTIVE: To analyse the effects of low-frequency loud acoustic stimulation on goal-directed movements involving the arm. Low-frequency sound stimulation impairs eye stability, evokes a subjective tilt of the visual surround in subjects presenting Tullio's phenomenon and induces, in normal subjects, short-latency evoked potentials in the neck and limb muscles. MATERIAL AND METHODS: Healthy subjects performed goal-directed movements in the horizontal plane with the right (dominant) arm to a fixed 3 degree-wide target positioned at an angle of 30 degrees, with the instruction to perform fast and accurate movements to the target and to hold the final position. This fast-pointing task was performed in association with sound-induced vestibular-otolithic stimulation (110 dB SPL, 500 Hz) in the absence of visual guidance (i.e. pointing at a memorized target in the absence of target or pointer cues). Pointing errors were analysed by computing the constant errors made by the subjects (mean error). Pointing errors were also correlated with movement kinematics (movement duration, peak velocity, time to peak velocity) and with the reaction time of movement. RESULTS: The low-frequency loud acoustic stimulation modified the final position of the arm-pointing task at the memorized target in the absence of vision. CONCLUSION: Goal-directed movements are achieved by means of sensory interactions between visual, somatosensory and vestibular information and the vestibular-otolithic signals contribute to the accuracy of voluntary arm movements.     

Oculomotor findings mimicking a cerebellar disorder and postural control in severe Meniere's disease

Objective

We explored whether vestibular derangement in patients with severe Meniere's disease is related to gaze and fixation difficulties and could explain complains of visual blurring and visual neglect.

Methods

Twenty-six patients with severe Meniere's disease treated with intratympanically applied gentamicin of the affected ear were examined. Saccades (with pseudo-random timing, size and side of the step) and pursuit eye movements (PEMs; pseudo-random target trajectory made of a combination of two sinusoidal waves of different frequencies) and posturography were tested in patients. For normative data of voluntary eye movements, 45 control subjects were tested, and for posturography 29 control subjects. Severity of symptoms in Meniere patients was assessed according to the total handicap score of AAO-HNSF (1985).

Results

Saccadic eye movements and postural stability were severely deteriorated in Meniere patients, and these disturbances correlated significantly with each other. Most of PEM parameters were significantly worse in Meniere patients than in control subjects. Prolongation of saccadic latency and reduced gains due to saccadization in PEMs were characteristic features for Meniere's patients. Severity of symptoms in Meniere patients correlated with prolongation of latency in saccades and with poorer gains in PEMs. Postural instability measured with posturography correlated with saccadic latency and PEMs.

Conclusion

In severe Meniere's disease, programming of oculomotor and postural responses show the same pattern of disturbance, an increased delay of processing and inaccuracy in the motor output. A fluctuant vestibular lesion with advanced severity can cause disorders of voluntary eye movements that are reflected with clinical complaints of difficulties in reading and visual scanning of surrounding.