Active head rotations and eye-head coordination.
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Acceleration↗
Biomedical subjects
L Stark
Publications and source records attributed to L Stark.
Dynamic vergence eye movements in strabismus and amblyopia: asymmetric vergence.
This report investigates line-of-sight asymmetric disparity vergence in patients having either intermittent strabismus, constant strabismus with amblyopia, or amblyopia without strabismus. We find an absence of disparity vergence in all patients with strabismus and in some with amblyopia only. Accommodative vergence and saccades place the dominant eye on the targets moving in depth. These accommodative vergence responses have normal dynamic characteristics, thus indicating a properly functioning vergence motor system. We purpose there is a higher-level central defect in which incoming information of one eye is suppressed, so that the disparity vergence system is rendered inoperable.
Adolescent↗
Voluntary nystagmus, saccadic suppression, and stabilization of the visual world.
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Afterimage↗
Disordered inhibition in internuclear ophthalmoplegia: analysis of eye movement recordings with computer simulations.
High resolution infra-red oculographic recordings were obtained in 19 patients with clinically evident internuclear ophthalmoplegia. The major findings were attenuated phasic and tonic components of adducting saccades, fractionated phasic components of abducting saccades, equally long durations for phasic components of adducting and abducting saccades with refixation, and nasal drift of the abducting eye which initiated abducting nystagmus. Ipsilateral gaze paresis and abduction lag were occasionally associated with primarily unilateral cases of internuclear ophthalmoplegia. These findings were interpreted using available anatomical, electromyographic and oculographic data as well as computer simulations of internuclear ophthalmoplegia. We concluded that deficient excitation of the ipsilateral medial rectus was due to interruption of burst-tonic neurons within the medial longitudinal fasciculus which mediate horizontal eye movements. This resulted in a decreased pulse height and step of the agonist neural controller signal. We were also able to determine that variably slowed fractionated abducting saccades were caused by deficient intrasaccadic inhibition of the antagonist medial rectus. When medial rectus excitation was more deficient than medial rectus inhibition of the opposite eye, then a typical internuclear ophthalmoplegia resulted; when the amount of medial rectus excitation was equal to the amount of medial rectus inhibition of the opposite eye, then a gaze paresis occurred; and when medial rectus excitation was less deficiennt than medial rectus inhibition of the opposite eye, abduction lag resulted in the oculographic appearance of internuclear ophthalmoplegia of abduction. Abducting nystagmus appeared to be initiated by a tendency for the abducted eye to drift nasally from the increased level of tonic inhibition of the antagonist medial rectus. Some oculographic patterns were attributed to higher level adaptive changes in innervation. These changes were a consequence of disordered excitatory and inhibitory controller signals at the lower, internuclear level. Possible anatomical pathways which might carry these inhibitory controller signals were discussed. High resolution eye movement recordings of patients with internuclear ophthalmoplegia were interpreted directly and by computer simulations as being most consistent with disordered inhibitory and excitatory control of the medial rectus motor pool during rapid eye movements and eccentric gaze.
Adolescent↗
An unexpected role for normal accommodative vergence in strabismus and amblyopia.
Accommodative vergence was measured using the Müller experimental paradigm in which target movement along the line of sight of the viewing eye results in large movement of the fellow (covered) eye. Our paper demonstrates an unexpected predominance of accommodative vergence over disparity vergence in patients with constant strabismus amblyopia, intermittent strabismus, and amblyopia without strabismus. Comparison of accommodative vergence responses in these patients with responses from normal subjects using symmetrical and asymmetrical targets under binocular and monocular conditions established the normal character of the patients' accommodative vergence and the absence of disparity vergence in most of them. The intermittent absence of disparity vergence in certain of our patients--some with surgically corrected strabismus, some with moderate amblyopia, and some with amblyopia treated with orthoptics--raises questions of clinical and neurophysiological interest. Preliminary results on the effects of amblyopia on accommodative responses are also presented.
Accommodation, Ocular↗
Unequal saccades during vergence.
An examination of saccades during vergence eye movements (accommodative and disparity) reveals that binocular saccades have unequal magnitudes in each eye, the smaller saccade of each pair opposing the vergence movement. Furthermore, this inequality cannot be explained by linear summation of the saccadic amplitudes onto the ongoing vergence. Saccadic inequalities are usually greater at the start of the vergence; the amplitude of the smaller saccade may be reduced 70% from that of the larger. Using a sixth-order nonlinear model of the eye movement plant and linear summation of saccadic (pulse-step) and vergence (step) control signals to drive the plant, we have produced a reduction in saccadic amplitudes between the two eyes during vergence that is similar to those found in our subjects. These results suggest that the reduction of saccadic amplitudes is due to an interaction in the muscular system moving the eye, not to a nonlinear neuronal interaction more centrally located.
Accommodation, Ocular↗
Increased drift in amblyopic eyes.
Reports are conflicting on the presence of increased drift in amblyopic eyes. Furthermore, the individual effects of either amblyopia or strabismus alone on ocular drift have not been systematically investigated. We therefore used a photoelectric method to record horizontal eye position during monocular and binocular fixation in patients having amblyopia without strabismus, intermittent strabismus, or constant strabismus amblyopia. Our principal finding was increased drift amplitude (up to 3.5 degree) and velocity (up to 3.0 degrees per second) in amblyopic eyes during monocular fixation. While increased drift was found 75% of the time in amblyopia without strabismus and 50% of the time in constant strabismus amblyopia, it was found only 20% of the time in intermittent strabismus. Amblyopic drift could be either error-producing or error-correcting in nature. Increased drift was not present during monocular fixation with the dominant eye or during binocular fixation in any of our 16 patients. We therefore conclude that amblyopia and not strabismus is a necessary condition for the presence of markedly increased fixational drift. Increased drift amplitude but not velocity may adversely affect visual acuity in the amblyopic eye.
Adolescent↗
Dynamic vergence eye movements in strabismus and amblyopia: symmetric vergence.
Dynamic vergence eye movements in response to step target displacements along the midline were measured by an infrared reflection technique in 11 patients having either intermittent strabismus, constants-strabismus amblyopia, or amblyopia without strabismus. We found the absence of normal disparity (fusional) vergence in all patients having strabismus and in some patients having amblyopia without strabismus. A characteristic response consisting of a binocular accommodative vergence component and an early binocular saccadic component was used to foveate the target of interest with the dominant eye. Vergence responses in our control subjects and patients with the nondominant eye occluded were similar to those recorded in our patients during binocular viewing. These results suggest that disparity information is not utilized by patients, probably as a result of long-term, ongoing suppression in the deviated or amblyopic eye. Accommodative vergence with the aid of an early foveating saccade was the primary mechanism for tracking targets in three-dimensional space.
Accommodation, Ocular↗
Head rotation trajectories compared with eye saccades by main sequence relationships.
A helmet apparatus permitted duration, peak velocity, and peak acceleration measurements as functions of magnitude of horizontal head rotation; these "main sequence" data give evidence for multipulse-step neurological signals appropriate for time optimal control of head rotation similar to those of saccadic eye movements.
Acceleration↗
Disparity vergence: a proposed name for a dominant component of binocular vergence eye movements.
A proposal to name the dominant aspect of vergence "disparity vergence" is put forward. Support for this proposal is gathered by briefly reviewing the components of vergence: (1) disparity vergence, the visual feedback control system for removing binocular disparity with an appropriate binocular convergence or divergence, (2) accommodative vergence, a synkinesis with accommodative motor control, (3) proximal vergence, and (4) tonic vergence. Sensory fusion is a higher level perceptual process aided by disparity vergence, and historically the term fusional vergence called attention to this relationship. However, it now appears to obscure the fact that disparity is the only direct stimulus to vergence.
Accommodation, Ocular↗
Cyclovergence: the motor response to cyclodisparity.
Static photographic evidence of the occurrence of cyclovergence is presented that supports and extends the result of Crone and Everhard-Halm (1975). Wide-angle complex targets were a necessary condition; simple horizontal line targets were insufficient. Our asymmetrical disparity targets supported in part the conformance of cyclovergence to Hering's Law but raised questions relating to the computational process that also acts to remove cyclodisparity and permits cyclofusion. Saturation and hysteresis nonlinearities were observed.
Eye Movements↗
Visual-motor adaptation. Quantitative demonstration in patients with posterior fossa involvement.
Short-term visual-motor adaptation to magnifying spectacle lenses was studied in normal subjects and in patients with nonacute posterior fossa lesions. When normal subjects, looking through magnifying lenses, pointed open loop to targets without viewing their hands, they initially underestimated the distance (magnification effect). After a 20-minute close-loop training or adaptation exposure period during which they viewed the performance of their hands, a modified visual-motor scheme evolved, compensating for about half of the lens-induced pointing error (adaptation effect). Removal of the lenses after adaptation caused open-loop, overshooting pointing errors (adaptation after-effect). Four patients with remission of cerebellar signs showed normal visual-motor adaptive performance, evidence of ability to recalibrate gain. One patient with persisting cerebellar ataxia was unable to recalibrate gain during close-loop visual-motor training. His history of transient palatal myoclonus implicates a role for the cerebellar-olivary system in calibration of visual-motor gain.
Adaptation, Ocular↗
Saccadic intrusions in strabismus.
Fixational eye movements were studied to determine the presence of and to quantify saccadic intrusions under monocular and binocular viewing conditions in subjects with intermittent strabismus, amblyopia without stabismus, or constant strabismus amblyopia. Saccadic intrusions were present under most test conditions in intermittent strabismus, were rarely observed in amblyopia without strabismus, and were prominent during monocular fixation with the amblyopic eye in constant strabismus anblyopia. This suggests that the presence of saccadic intrusions was related to strabismus and not amblyopia. There was no relationship between saccadic intrusion amplitude and visual acuity. Two possible mechanisms for producing intrusions are abnormally rapid regional visual adaptation and strabismus-induced fixation degradation.
Adolescent↗
Absence of accommodation during perceptual reversal of Necker cubes.
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Accommodation, Ocular↗
The trajectories of saccadic eye movements.
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Action Potentials↗
Opposed adducting saccades in convergence-retraction nystagmus: a patient with sylvian aqueduct syndrome.
We report high resolution oculographic recordings on a patient with convergent-retraction nystagmus (Parinaud's Sign). Recordings show opposed adducting saccades which are slightly asynchronous (8 ms), in contrast to the conjugate horizontal saccades which are exactly synchronous (0.3 ms). Normal saccadic velocity of these opposed adductions necessarily implies reciprocal innervation of horizontal and vertical agonist-antagonist pairs. Lateral view cine films show absence of retraction, apparently as a consequence of normal orbital mechanics. We suggest that normal mechanisms for dynamic overshoot saccades are involved in the production of these abnormal opposed adducting saccades.
Adult↗
Objective assessment of accommodation orthoptics. I. Dynamic insufficiency.
Three young adult females with symptoms related to focusing difficulties at near were treated by standard orthoptic procedures, including jump focus, plus-and-minus lens flippers, and pencil pushups. Home training was done 20 minutes each day for 4 1/2--7 weeks. Objective measures of dynamic accommodation were made each week in our Neuro-optometry Clinic. Initially, these objective measures showed prolongations of time constants and latencies of accommodation. During treatment, the patients showed significant reductions in time constants and latencies that correlated well with elimination of subjective symptoms. Also, in all three patients, flipper rates increased and symptoms were either markedly diminished or no longer present at termination of therapy. These results clearly demonstrate that orthoptic treatment in our three adult patients resulted in objective improvement of accommodation function.
Accommodation, Ocular↗