Biomedical subjects
L Stark
Publications and source records attributed to L Stark.
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.
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.
Study of eccentric fixation with secondary visual feedback.
Secondary visual feedback (2VFB) is a visual signal derived from continuous measurement of eye position and provides an extra artificial indication of the point of gaze. 2VFB may be eccentrically displaced and subjects are able to visually superimpose 2VFB onto a visual target signal and thus achieve and maintain eccentric fixation. Initial transient patterns of movement depend upon training but even naive subjects can achieve eccentric fixation within the first 40 s of such a task. Individual strategies and idiosyncratic patterns are exaggerations of normal control and fixational eye movements. The variance of maintained fixation increases with eccentricity and appears to be related to visual acuity as well as to precision of ocular motor control.
Omnidirectional increase in threshold for image shifts during saccadic eye movements.
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Abnormal saccadic substitution during small-amplitude pursuit tracking in amblyopic eyes.
Small-amplitude, low-velocity, predictable triangular tracking was tested in patients having amblyopia without strabismus, intermittent strabismus, or constant strabismus amblyopia by means of a photoelectric eye-movement recording technique. In the majority of amblyopic patients, abnormal saccadic substitution was found; that is, abnormally large saccades rather than small-amplitude smooth movements were used by the amblyopic eye to follow a spot stimulus that moved horizontally with low to high frequencies. Pursuit for the same range of stimuli was normal for binocular tracking and for monocular tracking with the dominant eye, pointing to a sensory rather than motor basis for the defect. This abnormal saccadic substitution response appeared to be related to the presence of amblyopia rather than strabismus. Several possible mechanisms responsible for causing this unusual response are discussed, including impairment of direction sense over small central regions of the amblyopic eye.
Fixational eye movements in amblyopia and strabismus.
Horizontal eye position was monitored using a photoelectric method during monocular and binocular fixation in four patients having amblyopia without strabismus, thirteen patients having constant strabismus with amblyopia, and five patients having intermittent strabismus. Four abnormalities of fixation were found: increased drift, saccadic intrusions, manifest nystagmus, and latent nystagmus. Increased drift was related to the presence of amblyopia, while saccadic intrusions and nystagmus were related to the presence of strabismus. Understanding dynamic aspects of oculomotor control can provide insight into clinical assessment of fixation in amblyopia and strabismus.
Different rates of functional recovery of eye movements during orthoptics treatment in an adult amblyope.
Although it is common clinical knowledge that oculomotor control appears to normalize during the course of successful orthoptics therapy for amblyopia, reports providing a quantitative analysis of eye movements during extended periods of treatment are lacking. We provide for the first time such a report in an adult amblyope. Aspects of eye movement control that tended to normalize with therapy include drift amplitude and velocity, duration and frequency of steady fixation, and pursuit gain. These results suggest that smooth pursuit control can be modified, even in an adult amblyope. Aspects of eye movement control that remained abnormal throughout therapy, in spite of normalization of visual acuity and centralization of fixation, include increased saccadic latencies, use of large saccades during small-amplitude pursuit tracking, and static overshooting. These results suggest that certain aspects of saccadic and pursuit control could either no longer be modified or would require longer periods for this to occur.
Glissadic overshoots are due to pulse width errors.
Glissades are the slow, gliding eye movements often appended to the end of human saccadic eye movements. They have been used as an aid in diagnosing disease states, eg, multiple sclerosis and vascular lesions. Glissades are a consequence of a mismatch between the sizes of the pulse and step components of the pulse-step motoneuronal controller signals. This physiological and simulation study shows that glissadic overshoot is caused by pulse width errors and not by pulse height errors. This implies that the CNS can control the firing frequencies and recruitment of motoneurons more precisely than it can control the duration of the high-frequency motoneuronal saccadic burst.
Saccadic initiation time in multiple sclerosis.
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Sensory and motor mechanisms interact to control amplitude of pupil noise.
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Binocular eye movements during accommodative vergence.
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Eye movements during the viewing of Necker cubes.
Eye movements were recorded while subjects viewed a Kopfermann-like series of Necker cubes and signaled perceptual reversals. At the instant of reversal, subjects tend to fixate the vicinity of the externally appearing corner. These fixations at the instant of reversal tend to have longer duration than those immediately before or after. The longer fixation times associated with pperceptual reversal probably reflect the time required to construct the alternate three-dimensional interpretation of the cube. After construction of this new model, the subject then fixates the vicinity of the newly interpreted externally appearing corner.
Processing delays in amblyopic eyes: evidence from saccadic latencies.
Saccadic latencies were measured in amblyopes with constant strabismus, amblyopes without strabismus, and intermittent strabismics with or without amblyopia. Subjects tracked a small spot of light, either monocularly or binocularly, which moved with random horizontal step displacements of 0.25--8.5 deg over the central field. Increased saccadic latencies were observed in the amblyopic eyes of 6 of 11 subjects, with or without strabismus; saccadic latencies were similar in each eye of 2 subjects having intermittent strabismus without amblyopia. Amblyopia was a necessary condition for increased saccadic latencies and not strabismus. Evidence for normal motor control of eye movements in amblyopic subjects is as follows: normal saccadic durations in the amblyopic eyes, normal saccadic-latency distribution curves for binocular tracking and monocular tracking with the nonamblyopic eyes, and synchronous movements of the 2 eyes. Our results are interpreted in terms of a processing delay in the sensory pathways leading from the central region of the amblyopic eye to the centers involved in saccadic initiation.
Eye movements, scanpaths, and dyslexia.
The oculomotor performance of 25 dyslexic and 19 normal children was evaluated to determine whether or not dyslexia involves deficits in oculomotor function or visual perception. When the children were required to follow a meaningless target or to solve pictorial tasks, the two age-matched groups could not be differentiated. When they were required to read selections commensurate with their measured reading level, the dyslexic children were markedly different on a number of parameters. It appears that the dyslexic's difficulty lies beyond visual perception, perhaps in the language area itself. The dyslexic's characteristic deficit seems to involve the integration of visual input into the language-acquisition function.
Accommodation dynamics I. Range nonlinearity.
We show the presence of a new nonlinearity in accommodation dynamics. Positive accommodation is slower when the eye is operating in the near range; conversely, relaxation of accommodation is apparently slower during work in the far range. Latency of accommodation is unaffected by working range. This new dynamic range nonlinearity has been defined in connection with a continuing research study of accommodation responses in normal subjects and in clinic patients. Instrumentation and analysis procedures are described.
Increased saccadic latencies in amblyopic eyes.
Increased saccadic latencies were measured in the amblyopic eyes of subjects having amblyopia without strabismus, constant strabismus amblyopia, and intermittent strabismus. The subjects tracked a small, bright spot of light moving with random, horizontal step displacements of 0.25 to 8.5 degrees over the central retina. Normal saccadic latencies were generally found during monocular tracking with the nonamblyopic eye as well as during binocular tracking. Studies of eye-hand reaction time in amblyopic eyes have shown delays to occur over the central retina; our new finding establishes this for saccadic initiation. Normal trajectories found for all tracking saccades, normal saccadic latencies measured when the nonamblyopic eye was utilized for tracking, and synchronous movement of the eyes under all test conditions point to a sensory rather than motor basis underlying these delays. Our results are interpreted in terms of a processing delay in the sensory pathways leading from the central region of the amblyopic eye to those centers involved in saccadic initiation.
Blur: a sufficient accommodative stimulus.
Experiments under a variety of open and closed loop feedback configurations demonstrate that accommodative responses to target blur are equivalent to those to defocus blur; this supports blur as the 'sufficient' neurological stimulus to accommodations. The hunting action of accommodation compensates for the even error aspect of blur and also adaptively minimizes any close loop error components while finally accepting open loop components.