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Biomedical subjects

R J Leigh

Publications and source records attributed to R J Leigh.

At least 19 recordsLinked to original sources
PubMed1992

The stability of human eye orientation during visual fixation.

Using the magnetic search coil technique, gaze stability in the horizontal, vertical and torsional planes was measured binocularly in human subjects during visual fixation. Horizontal and vertical eye rotations exhibited a mixture of slow drifts and resetting microsaccades yielding an average standard deviation of 0.11 and 0.10 deg, respectively. In contrast, torsional rotations showed unsystematic smooth drifts with fewer saccades yielding an average standard deviation of 0.18 deg. The lower precision of gaze control in the torsional plane may reflect (i) a discrepancy between the encoding of retinal images in two dimensions but of ocular motor control signals in three dimensions, and (ii) the visual consequences of ocular drifts in the torsional plane, which differ from those in the horizontal and vertical planes.

Adult↗
PubMed1992

High-frequency vestibuloocular reflex as a diagnostic tool.

During locomotion, the head is subject to rotational perturbations with fundamental frequencies in the range 0.5-5.0 Hz, and significant harmonics up to 20 Hz. Patients who have lost labyrinthine function complain of oscillopsia and visual impairment during locomotion. Measurements of head movements during walking and running in place in such patients indicate that head stability is similar to that in normal subjects. Therefore, head stability is mainly guaranteed by mechanical, not neurogenic, factors. On the other hand, the visual symptoms of such patients can be ascribed to instability of gaze. Thus, it seems that other mechanisms such as visual following, the cervicoocular reflex, or anticipatory eye movements cannot compensate for loss of the VOR during locomotion (though they may do so for lower-frequency or active head rotations). The indispensable role of the VOR during locomotion is probably a reflection of its short latency (16 mseconds or less in the horizontal and vertical planes), which guarantees short phase lags during high-frequency head rotations. Our results indicate that laboratory testing of patients with vestibular symptoms should employ stimuli that correspond to those occurring during locomotion.

Aged↗
PubMed1992

Extraocular muscles are spared in advanced Duchenne dystrophy.

Fast-twitch extremity muscle fibers are preferentially affected in Duchenne's and Becker's muscular dystrophy. Since saccades are thought to be mediated by fast-twitch fibers, saccadic velocities would be expected to be decreased in these patients. Using infrared oculography, we found that the peak velocities of saccades in 3 patients with advanced Duchenne's or Becker's muscular dystrophy were normal. Clinical findings in 7 other patients with these forms of dystrophy were normal. This investigation is the first study of ocular motility in Duchenne's and Becker's muscular dystrophy. It demonstrates that extraocular muscle function is preserved and suggests that fast-twitch fibers in extraocular muscles possess properties that protect against degeneration.

Adult↗
PubMed1992

Effectiveness of botulinum toxin administered to abolish acquired nystagmus.

We injected botulinum toxin into the horizontal rectus muscles of the right eyes of 2 patients who had acquired pendular nystagmus with horizontal, vertical, and torsional components. This treatment successfully abolished the horizontal component of the nystagmus in the injected eye in both patients for approximately 2 months. Both patients showed a small but measurable improvement of vision in the injected eye that may have been limited by coexistent disease of the visual pathways. The vertical and torsional components of the nystagmus persisted in both patients. In 1 patient, the horizontal component of nystagmus in the noninjected eye increased; we ascribe this finding to plastic-adaptive changes in response to paresis caused by the botulinum toxin. Such plastic-adaptive changes and direct side effects of the injections--such as diplopia and ptosis--may limit the effectiveness of botulinum toxin in the treatment of acquired nystagmus. Neither patient elected to repeat the botulinum treatment.

Adult↗
PubMed1992

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.

Adult↗
PubMed1992

Comparison of predictable smooth ocular and combined eye-head tracking behaviour in patients with lesions affecting the brainstem and cerebellum.

We compared the ability of eight normal subjects and 15 patients with brainstem or cerebellar disease to follow a moving visual stimulus smoothly with either the eyes alone or with combined eye-head tracking. The visual stimulus was either a laser spot (horizontal and vertical planes) or a large rotating disc (torsional plane), which moved at one sinusoidal frequency for each subject. The visually enhanced vestibulo-ocular reflex (VOR) was also measured in each plane. In the horizontal and vertical planes, we found that if tracking gain (gaze velocity/target velocity) for smooth pursuit was close to 1, the gain of combined eye-hand tracking was similar. If the tracking gain during smooth pursuit was less than about 0.7, combined eye-head tracking was usually superior. Most patients, irrespective of diagnosis, showed combined eye-head tracking that was superior to smooth pursuit; only two patients showed the converse. In the torsional plane, in which optokinetic responses were weak, combined eye-head tracking was much superior, and this was the case in both subjects and patients. We found that a linear model, in which an internal ocular tracking signal cancelled the VOR, could account for our findings in most normal subjects in the horizontal and vertical planes, but not in the torsional plane. The model failed to account for tracking behaviour in most patients in any plane, and suggested that the brain may use additional mechanisms to reduce the internal gain of the VOR during combined eye-head tracking. Our results confirm that certain patients who show impairment of smooth-pursuit eye movements preserve their ability to smoothly track a moving target with combined eye-head tracking.

Adult↗
PubMed1992

Refinement of an optical device that stabilizes vision in patients with nystagmus.

An optical device that produces partial stabilization of retinal images has been described. It consists of a high-plus spectacle lens used in combination with a large, high-minus, polymethyl methacrylate (PMMA) contact lens. This device is designed to provide more clear and stable vision in patients with acquired nystagmus due to neurological disease. We report success in using a smaller, rigid, gas permeable (RGP) contact lens that achieves the required degree of stabilization, but is more comfortable for patients than the PMMA contact lens originally described. We have confirmed that the degree of retinal image stabilization is similar to that achieved with the PMMA lens. We present a case report to illustrate its use.

Adult↗
PubMed1992

Experimental tests of a superposition hypothesis to explain the relationship between the vestibuloocular reflex and smooth pursuit during horizontal combined eye-head tracking in humans.

1. We used a modeling approach to test the hypothesis that, in humans, the smooth pursuit (SP) system provides the primary signal for cancelling the vestibuloocular reflex (VOR) during combined eye-head tracking (CEHT) of a target moving smoothly in the horizontal plane. Separate models for SP and the VOR were developed. The optimal values of parameters of the two models were calculated using measured responses of four subjects to trials of SP and the visually enhanced VOR. After optimal parameter values were specified, each model generated waveforms that accurately reflected the subjects' responses to SP and vestibular stimuli. The models were then combined into a CEHT model wherein the final eye movement command signal was generated as the linear summation of the signals from the SP and VOR pathways. 2. The SP-VOR superposition hypothesis was tested using two types of CEHT stimuli, both of which involved passive rotation of subjects in a vestibular chair. The first stimulus consisted of a "chair brake" or sudden stop of the subject's head during CEHT; the visual target continued to move. The second stimulus consisted of a sudden change from the visually enhanced VOR to CEHT ("delayed target onset" paradigm); as the vestibular chair rotated past the angular position of the stationary visual stimulus, the latter started to move in synchrony with the chair. Data collected during experiments that employed these stimuli were compared quantitatively with predictions made by the CEHT model. 3. During CEHT, when the chair was suddenly and unexpectedly stopped, the eye promptly began to move in the orbit to track the moving target. Initially, gaze velocity did not completely match target velocity, however; this finally occurred approximately 100 ms after the brake onset. The model did predict the prompt onset of eye-in-orbit motion after the brake, but it did not predict that gaze velocity would initially be only approximately 70% of target velocity. One possible explanation for this discrepancy is that VOR gain can be dynamically modulated and, during sustained CEHT, it may assume a lower value. Consequently, during CEHT, a smaller-amplitude SP signal would be needed to cancel the lower-gain VOR. This reduction of the SP signal could account for the attenuated tracking response observed immediately after the brake. We found evidence for the dynamic modulation of VOR gain by noting differences in responses to the onset and offset of head rotation in trials of the visually enhanced VOR.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1992

An adjustment to eye movement measurements that compensates for the eccentric position of the eye relative to the center of the head.

We describe a procedure for modifying horizontal eye movement measurements in a way that effectively removes signal contributions due to the eccentric location of the eyes relative to the center of rotation of the head. This procedure helps standardize measured signals so that data collected under different experimental conditions and from different subjects may be compared directly. We also present techniques for accurately determining specific parameters of head geometry that are required by the procedure. This method may be used for modifying eye movement responses to visual target motion, head motion, or both, and does not assume continual fixation of the visual target. Also, this method may be employed when either curved (iso-distance) or flat display screens are used for presenting the visual stimuli.

Eye↗
PubMed1992

Clinical features and pathogenesis of acquired forms of nystagmus.

Nystagmus is a common finding in patients with disease affecting the brain stem and cerebellum. Basic research into mechanisms that normally control eye movements has led to a better understanding of the pathogenesis of different types of acquired nystagmus. Nystagmus is caused by disorders of the mechanisms that normally function to hold gaze steady: the vestibular system, the gaze-holding mechanism, the visual stabilization system and the smooth pursuit system. Thus, evaluation of a patient's nystagmus requires a systematic examination of each functional class of eye movements. In addition, it is important to determine the direction of the nystagmus for different angles of gaze and head positions, and the effects of monocular and binocular visual fixation upon the nystagmus. Measurement of the nystagmus waveform, using reliable methodology, is often helpful in securing a diagnosis. Such measurements help differentiate acquired nystagmus from congenital forms of nystagmus and from saccadic disorders that lead to instability of gaze.

Humans↗
PubMed1991

Lateral deviation of the eyes on forced lid closure in patients with cerebral lesions.

We examined 35 patients with unilateral cerebral lesions to determine the incidence of lateral deviation of the eyes under forcefully closed lids and the reliability of this sign in predicting the side of the lesion. Only patients with radiologically confirmed unilateral lesions were studied. Over 70% of patients had contralateral ocular deviation (Cogan's "spasticity of conjugate gaze"), 20% had ipsilateral deviation, and less than 9% of the patients had no deviation. Lateral ocular deviation was as sensitive, but not as specific, as a unilateral Babinski plantar response in determining the side with the lesion. Contralateral deviation was more common with parietotemporal localization, suggesting that the phenomenon reflects an underlying disturbance of attentional mechanisms.

Aged↗
PubMed1991

Superior oblique myokymia. Quantitative characteristics of the eye movements in three patients.

Using the magnetic search coil technique, we measured horizontal, vertical, and torsional rotations of both eyes of two patients with idiopathic superior oblique myokymia, and of the affected eye in a third patient. Superior oblique myokymia was strictly monocular and consisted of an initial intorsion and depression of the affected eye and subsequent oscillations with torsional and vertical components. The peak-to-peak torsional and vertical amplitudes of the oscillations were less than 1 degree, but peak velocities frequently exceeded 4 degrees/sec in both planes. Fourier analysis indicated two features: (1) a broad range of frequencies up to about 50 Hz, indicating irregular oscillations; and (2) a superimposed larger-amplitude oscillation in the range from 1.5 to 6 Hz. Taken with electromyographic data from other studies, these results indicate that superior oblique myokymia reflects spontaneous discharge of trochlear motor neurons that have undergone regenerative changes.

Adult↗
PubMed1991

Effect of anticholinergic agents upon acquired nystagmus: a double-blind study of trihexyphenidyl and tridihexethyl chloride.

We conducted a randomized, double-blind, crossover trial of two anticholinergic agents--trihexyphenidyl and tridihexethyl chloride (a quaternary anticholinergic that does not cross the blood-brain barrier)--in patients with acquired nystagmus and measured visual acuity and nystagmus before and at the end of 1 month on each medication. Of the 10 patients admitted to the study, only five completed trials of both drugs due to intolerance of medication or intercurrent illness. Of six patients who completed the trial of trihexyphenidyl, only one showed improvement. Of six patients who completed a trial of tridihexethyl chloride, four showed improvement. We conclude that (1) trihexyphenidyl is not a reliable treatment for acquired nystagmus, although occasional patients may benefit; (2) anticholinergic agents may suppress nystagmus by peripheral rather than central mechanisms; and (3) the side effects of anticholinergic agents limit their effectiveness in the treatment of nystagmus.

Adult↗
PubMed1991

Acquired pendular nystagmus in toluene addiction.

We studied the ocular motor abnormalities in 4 patients chronically addicted to sniffing glue containing toluene. They showed acquired pendular nystagmus with horizontal and vertical components. One patient also showed saccadic oscillations. The pendular nystagmus may be a manifestation of a disturbance in brainstem-cerebellar connections secondary to the toxic effect of toluene on white matter.

Adult↗
PubMed1990

Instability of gaze during locomotion in patients with deficient vestibular function.

We measured the stability of gaze in the horizontal and vertical planes, in 2 patients with bilaterally deficient vestibular function while they sat, stood still, walked in place, and made active horizontal and vertical head rotations. During sitting and standing, gaze was equally as stable as that in normal subjects. During walking in place, however, gaze velocity was double that of normal subjects. Thus, our patients' complaints of impaired vision and oscillopsia during walking could be ascribed to excessive motion of images on their retinas. Eye movements compensated for head rotations more effectively (higher gain) during active head rotations than during locomotion; this difference may be due to the predictable nature of active head movements. We conclude that testing of patients with vestibular symptoms should include stimuli corresponding to the rotational head perturbations that occur during locomotion; such head rotations have nonpredictable characteristics and a frequency range of 0.5 to 5.0 Hz.

Aged↗
PubMed1990

Nonlinear parameter estimation applied to a model of smooth pursuit eye movements.

We present a procedure that optimally adjusts specified parameters of a mathematical model to describe a set of measured data. The technique integrates a dynamic systems-simulation language with a robust algorithm for nonlinear parameter estimation, and it can be implemented on a microcomputer. Sensitivity functions are generated that indicate how the operation of the model is affected by each updated parameter. This procedure offers a greater resolution of optimal parameter values than other, less rigorous methods. To illustrate this technique we have applied it to the model of human smooth pursuit eye movements proposed by D.A. Robinson and colleagues (1986).

Eye Movements↗