Superior oblique myokymia: quantitative description of the eye movement.

We studied superior oblique myokymia (intermittent uniocular microtremor) using magnetic scleral search coils. The monocular disorder consisted of both phasic and tonic components consistent with the primary, secondary, and tertiary actions of the superior oblique muscle. These observations support the hypothesis of a disorder restricted to the superior oblique motor unit.

Epileptic gaze deviation and nystagmus.

We studied a patient with stereotyped focal seizures characterized by leftward conjugate eye- and head-turning followed by nystagmus. Eye deviation was associated with the appearance of seizure activity, recorded over the right temporo-occipital scalp, that did not spread frontally. The initial eye deviation consisted of a staircase of small saccades. The subsequent nystagmus showed rightward decreasing-velocity exponential slow phases and normal leftward quick phases. Saccadic eye movements due to seizures may occur via projections from posterior cortical areas as well as from the frontal eye fields.

Hyperactive vestibulo-ocular reflex in cerebellar degeneration: pathogenesis and treatment.

We studied a patient with a cerebellar degeneration and hyperactive vestibulo-ocular reflex (VOR). He complained of oscillopsia and blurred vision with head movement. A twofold increase in VOR gain (peak eye velocity/peak head velocity) at high frequencies was associated with a VOR time constant of 6 seconds (low normal). Visual cancellation ("suppression") of the VOR and smooth pursuit were also abnormal. We hypothesized that his high VOR gain was due to dysfunction of olivocerebellar projections. Physostigmine reduced his VOR gain, consistent with the hypothesis that these projections are cholinergic.

Effects of retinal image stabilization in acquired nystagmus due to neurologic disease.

We studied the effects of variable amounts of artificial retinal image stabilization (RIS) upon oscillopsia and visual acuity in eight patients with acquired nystagmus due to neurologic disease. We measured horizontal and vertical eye movements with the magnetic search coil technique and used these electronic signals to control the position of a visual stimulus on a screen in front of the patient. We also used an optical device to stabilize images of the real world upon the retina. During electronic stabilization, RIS was progressively increased until oscillopsia was abolished; this was achieved in all eight patients and corresponded to retinal image drift of 5 degrees/sec or less. In five patients with downbeat nystagmus, further increases in RIS caused the oscillopsia to reappear, but in the opposite direction. Electronic stabilization also improved visual acuity in four of five patients; the limitation of improvement could be related to coexistent visual system defects. Using electronic feedback, we could measure the range of RIS that any individual required to abolish oscillopsia; from this measurement, the components of the optical device that were best suited to provide a stable field of vision could be calculated.

Slow saccades and hypometria in anticonvulsant toxicity.

We report a patient with abnormal saccades in association with anticonvulsant toxicity (phenytoin 27.5 micrograms/ml, phenobarbital 18.8 micrograms/ml). The patient looked toward visual targets either with multiple, small, hypometric saccades or with single slow saccades. These abnormalities resolved when anticonvulsant levels returned to therapeutic range. Thus, slow saccades may be clinical evidence of anticonvulsant toxicity.

Oscillopsia, retinal image stabilization and congenital nystagmus.

Most individuals with congenital nystagmus (CN) do not complain of oscillopsia (visual inconstancy) even though the amount of retinal image slip varies considerably according to gaze angle and CN waveform. We induced oscillopsia in four subjects with CN by artificially stabilizing images upon the retina under several conditions. Every subject reported oscillopsia during retinal image stabilization, but the condition of stabilization varied from one individual to another. Our results indicate that a variety of mechanisms operate to maintain spatial constancy in congenital nystagmus; some individuals appear to use one mechanism more than another. Possible mechanisms include use of extra-retinal signals, elevated threshold for motion detection and "suppression" of visual input except during foveation periods.

Effect of monocular visual loss upon stability of gaze.

Using the eye-coil/magnetic field method, we measured horizontal and vertical movements of both eyes in four patients with monocular loss of vision while they attempted steady, binocular fixation of a visual target. We also measured gaze stability in two normal subjects while they fixed upon a target monocularly, and in one patient with congenital, bilateral blindness. In the patients with monocular visual loss, gaze instability was greater in the blind eye, both vertically and horizontally, compared either with their seeing eye or with nonviewing eyes of control subjects. Gaze instability due to monocular blindness resulted from: (1) low-frequency, low-amplitude, bidirectional drifts that were more prominent vertically; and (2) unidirectional drifts, with nystagmus, that were more prominent in the horizontal plane. Gaze-evoked nystagmus, however, was not a feature of monocular blindness. Thus, the gaze instability of monocular blindness may reflect disruption of: (1) a monocular visual stabilization system; (2) fusional vergence mechanisms; or (3) both. In contrast, bilateral congenital blindness led to nystagmus with horizontal and vertical components and a wandering null point, indicative of an abnormal neural integrator.

The cervico-ocular reflex of normal human subjects in response to transient and sinusoidal trunk rotations.

We used the magnetic search coil technique to measure the horizontal cervico-ocular reflex (COR) of 8 subjects in response to transient or sinusoidal (0.1-1.0 Hz) trunk rotations while their heads were firmly immobilized. Although we were able to resolve eye rotations of < 0.05 degrees, the COR was hardly measurable (gain was always < 0.07). This finding, made with the most precise measurement technique used to date, suggests that the COR makes a negligible contribution to the stability of gaze in normal subjects during natural activities.

The association of spinal and gastroesophageal anomalies. Case report and review.

Severe cervical radiculopathy occurred in a child with gastroesophageal duplication who had a previously unrecognized vertebral malformation. We review the literature on the association of vertebral and gastroesophageal anomalies and discuss the recent studies in human embryology which detail the relations of notochord to endoderm and neural ectoderm at about 3 weeks of gestation. This case illustrates the opportunity of preventing nerve root or spinal cord compromise due to such vertebral anomalies when the possibility of their association with gastroesophageal duplication is considered.

Two distinct deficits of visual tracking caused by unilateral lesions of cerebral cortex in humans.

We studied horizontal visual tracking in 20 patients with unilateral cerebral lesions and in 10 age-matched control subjects. Five patients, all with posterior lesions, showed impaired smooth pursuit of predictable targets moving toward the side of the cerebral lesion. Using nonpredictable step-ramp stimuli, we identified two distinct deficits of visual tracking. The first was a unidirectional deficit of smooth pursuit, for targets moving toward the side of the lesion, in response to stimuli presented into either visual hemifield. The second deficit, identified in a sixth patient who did not show pursuit asymmetry to predictable targets, was a bidirectional inability to estimate the speed of a moving target in the visual hemifield contralateral to the side of the lesion; this caused inaccurate saccades to moving (but not stationary) targets and impaired smooth pursuit initiation. These visual tracking deficits were independent of homonymous hemianopia or hemispatial neglect. These two tracking deficits are similar to those described in rhesus monkeys with lesions of the medial superior temporal and middle temporal visual areas.

Frequency and velocity of rotational head perturbations during locomotion.

We used the magnetic search coil technique to record horizontal (yaw) and vertical (pitch) head rotations of 20 normal subjects during (1) walking in place, (2) running in place, (3) vigorous, voluntary, horizontal head rotation and (4) vigorous, voluntary, vertical head rotation. During walking or running, the predominant frequency of pitch rotations was at least twice that of yaw rotations. During running, the median, predominant pitch frequency from all subjects was 3.2 Hz, but significant harmonics were present up to 15-20 Hz in several subjects. Group median maximal head velocity during walking or running did not exceed 90 degrees/second. During vigorous, voluntary head rotations median frequency for yaw and pitch was similar and did not exceed 2.6 Hz. However, group median maximal head velocity during vigorous voluntary yaw rotation was 780 degrees/second. Thus, (1) during locomotion, the head is stabilized in space incompletely but adequately so that the vestibulo-ocular reflex (VOR) is not saturated, (2) during vigorous, voluntary head rotations, the maximum head velocity exceeds the range where the VOR can stabilize gaze, (3) the frequencies of head rotations that occur during locomotion greatly exceed frequencies conventionally used in the laboratory for testing the VOR.

Comparison of smooth pursuit and combined eye-head tracking in human subjects with deficient labyrinthine function.

The effects of deficient labyrinthine function on smooth visual tracking with the eyes and head were investigated in ten patients with bilateral peripheral vestibular disease. Ten normal subjects served as controls. In the patients active, combined eye-head tracking (EHT) was significantly better than smooth pursuit (SP) with the eyes alone with a target frequency of 1.0 Hz. Normal subjects pursued equally well with SP and with active EHT. The gain of compensatory eye movements during active head rotation in darkness was also measured. Compensatory eye movements in labyrinthine-deficient patients (attributable to residual vestibulo-ocular reflex (VOR), cervico-ocular reflex (COR) and pre-programmed eye movements) were always less than in normal subjects. These data were used to examine current hypotheses that postulate central cancellation of the VOR (or compensatory eye movements) during EHT. A model that proposes summation of an internal smooth pursuit command and VOR/compensatory eye movements accounted for the findings in normal subjects and labyrinthine-deficient patients. In seven labyrinthine-deficient patients and nine normal subjects, passive EHT was measured during en bloc rotation while they viewed a head-fixed target. With a target frequency of 1.0 Hz, both subjects and patients showed significantly better tracking during passive EHT than during SP. Normal subjects also showed superior tracking during passive EHT compared with active EHT. These findings support the notion that during passive EHT, parametric gain changes contribute to modulation of the VOR.