R. J. Leigh

Behavior of human horizontal vestibulo-ocular reflex in response to high-acceleration stimuli

We studied the horizontal vestibulo-ocular reflex (VOR) during transient, high-acceleration (1900-7100 degrees/s2) head rotations in 4 human subjects. Such stimuli perturbed the angle of gaze and caused illusory movement of a viewed target (oscillopsia). The disturbance of gaze could be attributed to the latency of the VOR (which ranged from 6-15 ms) and inadequate compensatory eye rotations (median VOR gain ranged from 0.61-0.83).

Measuring eye movements during locomotion: Filtering techniques for obtaining velocity signals from a video-based eye monitor

Video-based eye-tracking systems are especially suited to studying eye movements during naturally occurring activities such as locomotion, but eye velocity records suffer from broad band noise that is not amenable to conventional filtering methods. We evaluated the effectiveness of combined median and moving-average filters by comparing prefiltered and postfiltered records made synchronously with a video eye-tracker and the magnetic search coil technique, which is relatively noise free. Root-mean-square noise was reduced by half, without distorting the eye velocity signal. To illustrate the practical use of this technique, we studied normal subjects and patients with deficient labyrinthine function and compared their ability to hold gaze on a visual target that moved with their heads (cancellation of the vestibulo-ocular reflex). Patients and normal subjects performed similarly during active head rotation but, during locomotion, patients held their eyes more steadily on the visual target than did subjects.

Eye Movements During Motion After-effect

Using the magnetic search coil technique, we measured torsional eye movements in four male subjects during and after rotation of a visual display around the line of sight. During rotation of the display, subjects developed a torsional nystagmus with slow-phases in the direction of target rotation that had a typical gain of less than 0.01. Upon cessation of display motion, subjects experienced a motion after-effect (MAE) in the direction opposite prior target rotation, which persisted for greater than 15 sec. During this MAE, slow-phase eye movements of low velocity were in the same direction as the MAE, but did not persist as long as perceptual effects. In separate experiments, horizontal eye movements were recorded during horizontal stimulus motion; during MAE, no eye movements occurred due to stronger fixation mechanisms. We conclude that MAE is not caused by retinal slip of images, but MAE and the accompanying eye movements might be produced by shared or similar mechanisms.

Evaluating Large Saccades in Patients with Brain‐Stem or Cerebellar Disorders

Abstract: Clinicians conventionally test saccades at the bedside by noting the accuracy, initiation time, and speed of large movements, with the patients head stationary. Partly for methodological reasons, laboratory analysis of saccades has mainly focused on movements of 20 degrees or less. By measuring the velocity waveform of large saccades, it is possible to examine more closely the way in which brain stem and cerebellum guide the eye to the target. Large saccades made by healthy humans show a positively skewed velocity profile. Slow saccades made by patients with brain‐stem disorders show a prolonged plateau of low velocity. Some patients with cerebellar disorders may show increased acceleration and deceleration of saccades. Each of these velocity waveforms can be modeled by changing the parameters that describe medium‐lead burst neuron firing. In certain other brain‐stem and cerebellar disorders, transient decelerations or premature terminations of saccades occur; such velocity waveforms cannot be modeled solely by changing the parameters that describe burst neuron firing. Instead, it is necessary to postulate dysfunction of the mechanism that normally inhibits pontine omnipause neurons, thereby permitting burst neurons to discharge until the saccade is completed. Analysis of large, abnormal saccades calls for application of novel techniques to identify the beginning and end of the saccadic pulse command.

Effects of visual fixation and convergence on periodic alternating nystagmus due to MS

We studied a young woman with multiple sclerosis, who developed periodic alternating nystagmus (PAN) with a period of oscillation of about three minutes. Neither visual fixation of a stationary target nor large-field optokinetic stimulation substantially influenced the cycle of PAN. During convergence, induced by fixation of a near target, PAN was suppressed by over 70%. Treatment with baclofen abolished her nystagmus, but optokinetic and pursuit responses remained impaired. Convergence during viewing a near target did not increase the response (gain) of her vestibulo-ocular reflex. We postulate that visual drives were able to suppress PAN independently of any effects on vestibular responses and were prevented from exerting effects on velocity storage and vestibular gain adjustment by demyelinating lesions affecting her pontine nuclei and cerebellar circuits.

Small Vertical Saccades Have Normal Speeds in Progressive Supranuclear Palsy (PSP)

Slow vertical saccades are a cardinal feature of PSP that allows differentiation from other parkinsonian disorders.1 The premotor signals for vertical saccades are generated by burst neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF).2–4 Thus, characterization of the dynamic properties of vertical saccades in PSP presents an opportunity to understand better the pathogenesis of the saccadic palsy. The goal of this study was to follow up on the unpublished observation of Lea Averbuch-Heller, M.D. (1958–2000), that small saccades in PSP appeared to be of normal velocity. We studied six patients with PSP; their ages ranged from 68–76 years, and mean duration of disease was 1.8 years. All patients gave informed consent. We compared saccadic properties with seven healthy controls (age range 62–75 years) previously studied.1 We measured eye movements using the search coil technique (bandwidth 0–150 Hz) and defined saccade onset interactively using an eye velocity criterion of 15 deg/sec. We analyzed a total of 1054 vertical saccades from the PSP patients and fitted plots of saccadic amplitude and peak velocity with an equation of the form:

The torsional vestibuloocular reflex can be canceled but not enhanced by visual stimuli.

The torsional vestibuloocular reflex (VOR) in humans shows different properties from the VOR in the horizontal or vertical plane; the gain is lower, and “velocity storage” appears to be absent.’ Despite the low gain of the torsional optokinetic system, it is possible to reduce the gain of the torsional VOR by viewing a head-fixed visual display during head rotations in roll. In the horizontal plane, the ocular motor system can nullify more than 90% of the VOR during combined eye-head tracking, and a significant portion of this reduction is believed to be due to cancellation by smooth pursuit signals.* It has also been shown that these visual inputs can enhance the horizontal VOR if a subject attempts to track a target moving opposite to the direction of head rotation.2 In the torsional plane it has been shown that the VOR can be reduced by about 30% while viewing a head-fixed visual d i~play .~ However, attempts to enhance or reduce the gain of the torsional VOR with visual stimuli moving at a variety of speeds with respect to the head have not been reported. Torsional eye and head movements were measured in four subjects using the magnetic search coil technique, and digitized for later analysis. Analog head position signals were differentiated in real time to yield velocity information, which was then fed to the command input of a torsional optokinetic stimulator. This stimulator employed a servo motor to rotate a circular (70 cm diameter) randomly spotted disk subtending > 70 deg. Using this servo system we could rotate the disk at positive or negative multiples of head velocity. Subjects were instructed to fixate a small red spot in the center of the visual display, and to roll their head ear-to-shoulder continuously in both directions at approximately 0.5 Hz. During this head rotation, visual feedback ranging between -1 and +3 in gain was provided to the subject. Following data collection the crossand autopower spectral densities of the input and output were then estimated using Welch periodogram techniques. This allowed us to estimate gain and coherence as a function of frequency. Coherence was always > 0.86. Results from one subject are shown in FIGURE 1, and cumulative results are shown in TABLE 1. In all subjects, the visually enhanced VOR (i.e., X O visual feedback) was of greater gain than the VOR in the dark. Subjects were not able to further increase VOR gain with visual stimuli. In fact, for two subjects, VOR gain actually decreased by more than 25% when the display rotated in a direction

Vestibulo‐Ocular Responses during Mirror‐Viewing

When we view a target located at optical infinity, eye rotations will compensate for head rotations if they are equal and opposite in direction. During viewing of a near target, the situation is more complicated because the eyes do not lie at the center of head rotation and are separated by several centimeters. In general, the gain of compensatory eye movements is increased during near viewing and the gain is greater for the eye that is closer to the object of interest.1 We have recently shown that, during close viewing of one’s own image in a mirror, the visual demands during head rotation differ from when viewing a real, near target.2 This is mainly because translations of the subject’s head in a plane parallel to that of a mirror are matched by translations of the image. Our calculations indicated that the main factor determining gain differences between mirror-viewing and near-viewing condition is the eccentricity of the target, which is zero during nearviewing, but varies as a function of head rotation in the mirror-viewing condition. Vergence angle was not the main determinant of VOR gain because it was similar during nearand mirror-viewing conditions, but gain values differed substantially under the two test conditions.2 Overall, median vergence angles differed by only 0.8 degrees (4%) under the two viewing conditions, but median gain was 65% greater during near-viewing than mirror-viewing.2 The mirror paradigm might be regarded as a combined vestibulo-ocular reflex (VOR)–smooth pursuit task in which the pursuit stimulus (the virtual image of bridge of the subject’s nose) moves as a linear function of head rotation. To investigate this possibility, we used the experimental strategy of switching to darkness and measuring VOR gain while the eyes were still converged but visual feedback was not possible.