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Biomedical subjects

L Stark

Publications and source records attributed to L Stark.

At least 73 records · Page 4Linked to original sources
PubMed1983

Dynamic pupillary response controlled by the pupil size effect.

Pupillary escape has been described as an initial contraction followed by a slow redilatation, occurring in response to a step stimulus of low-intensity light. When the initial pupil size is small, the response to the same step stimulus is pupillary capture, a steady and sustained contraction. In this experiment a comparison was made between three modes of controlling pupil size and thereby of regulating the pupillary response: contralateral light background level, ipsilateral light background level, and accommodative level with which there is no change in retinal adaptation. All three level setting modes showed similar results in illustrating the pupil size effect. In addition, an inhibitory effect was found with both ipsilateral and contralateral light backgrounds that is independent of Weber's Law in the contralateral case. Our results lead to the formulation of a binocular model, featuring an internal parameter control whereby a signal dependent on the static pupil size regulates the gains of the parallel phasic and tonic pathways, the former responsive to transient changes of light, and the latter to background levels of light and accommodative levels. Our findings also raise interesting questions concerning the loci of these complex interactions in the simple neuroanatomy of the pupillary pathways.

Humans↗
PubMed1983

Pupillary escape intensified by large pupillary size.

Pupil responses to light are greatly influenced by initial pupil size. Small pupils, operating under photopic conditions, show tonic responses to step increases of light and high gains; thus the pupil is a good regulator of light. Large pupils, operating under mesopic or scotopic conditions show phasic responses, "pupillary escape", and smaller gains; the pupil only transiently influences retinal flux. By using accommodation level to set the size of the pupil, the mechanism of the "pupil size effect" is shown to be dependent on retinal light level only so far as retinal activity sets pupil size.

Accommodation, Ocular↗
PubMed1983

Abnormal patterns of normal eye movements in schizophrenia.

The multilevel control of eye movements for the researcher in the schizophrenic disease process includes the saccadic trajectory, the dual mode tracking system, and schematically directed eye movements in fixation, in scanpaths, and other looking strategies, and in reading. Coordination of eye movements with head movement in gaze and sensory consequences and concomitants of eye movements round out the picture. Recent studies with precise bioengineering instrumentation have defined two abnormal patterns of eye movements--saccadic intrusions and saccadic "smooth" pursuit. Although these signs are not pathognomonic, they raise interesting questions concerning the relationship of these eye movements to the schizophrenic disease process.

Attention↗
PubMed1983

Saccadic intrusions contributing to reading disability: a case report.

Horizontal eye movements were recorded during tracking and reading in a patient with probable neurological involvement who complained of periods of oscillopsia and severe asthenopia, especially during reading. Saccadic intrusions occurred during all tracking tasks. Single and multiple saccadic intrusions found while reading, frequently accompanied periods of visual disturbance, and we attribute part of the reading disability to the presence of such abnormal eye movements. Various therapeutic measures were attempted and achieved some degree of success.

Adult↗
PubMed1982

Oculomotor effects of intermittent conduction block in myasthenia gravis and Guillain-Barré syndrome. An oculographic study with computer simulations.

Five abnormal oculographic patterns were identified in eight patients with either myasthenia gravis or Guillain-Barré syndrome (GBS). These could be differentiated into three intrasaccadic and two postsaccadic abnormalities. From our studies of computer simulations, and considering the established pathophysiology of myasthenia gravis and GBS, we believe that our oculographic findings were a consequence of defects in peripheral neural and neuromuscular conduction, together with a simple adaptive increase in duration of the saccadic burst of central innervation. We conclude that the eye movement abnormalities we observed are explained by intermittent block of peripheral conduction, and suggest that any disease causing intermittent blockage of neural signals to extraocular muscles will produce similar abnormalities of eye movement.

Adolescent↗
PubMed1982

Identification of abnormal patterns in eye movements of schizophrenic patients.

Using precise infrared recording instrumentation, we measured the eye-movement patterns in six schizophrenic patients who were taking medication and found saccadic intrusions and saccadic smooth-pursuit tracking in three. This defined some of the eye-movement irregularities reported previously by many investigators. A variety of eye-movement functions in addition to smooth pursuit exhibited these abnormalities, including fixation, vergence, and head-eye pursuit. Saccadic intrusions or saccadic pursuit also occurs in a number of neuro-ophthalmologic disorders, and occasionally in healthy normal subjects. Thus, these eye-movement abnormalities do not occur exclusively in schizophrenia.

Adult↗
PubMed1982

Eye-head coordination in homonymous hemianopia.

Quantitative studies of latencies and trajectories of eye-head movements during visual search and fixation in patients with occipital hemianopia show that (1) the latency of head movement is bilaterally increased, with significantly greater delays in movements toward the blind side; (2) the compensatory eye movements (CEMs) during head movements toward the blind side have increased velocity; (3) these CEMs entail a non-vestibular anticipatory component; and (4) head movements toward the blind side are comprised of multiple steps similar to staircase eye movements documented in previous studies. Hemianopic patients seemingly simplify search and fixation strategies by minimizing or entirely eliminating head movements and relying on eye movements instead.

Adult↗
PubMed1982

A model for nonlinear stochastic behavior of the pupil.

In this paper we present new experimental results that show pupillary noise to be multiplicative in a particular fashion with greatest variance in midrange and smaller variance at high and low ranges. This confirms the finding of multiplicative noise by Stanten and Stark (1966), but modifies and extends the relationship they suggested between standard deviation and mean pupil diameter. We propose a parametric model of the iris muscle which not only describes the static characteristics of pupil response to given stimuli, but also explains its random fluctuations in terms of probability density functions. We emphasize the point that the range nonlinearity is not due to decreased gain at the extrema of the pupil range, but is operational over a wide portion of the pupillary behavioral range, hence its name--"expansive range nonlinearity". We conclude that noise amplitude, which is a function of the pupil diameter, closely parallels the changes in deterministic gain. Thus pupil noise can be simply considered as cross-talk additive Gaussian noise injected into the pupil system at midbrain level.

Humans↗
PubMed1982

Neural control of head rotation: electromyographic evidence.

Electromyograms, representing samples of neurological control signals, can be used to predict head rotation that can be either time optimal or non-time optimal, depending upon a given subject's intent. Kinematic factors such as operating levels in terms of horizontal and vertical position of the head from which horizontal rotations are made, importantly influence the tonic and phasic aspects of the EMG signal. Pairs of head-rotating muscles, splenius and sternocleidomastoideus muscles, demonstrate reciprocal innervation, particularly for time optimal head rotations. Head rotations of the same amplitude but different velocities correlate with pulse height and pulse width of envelopes of the rectified EMG. Rise of the agonistic EMG correlates in detail with rise of the acceleration transient for a given fast or slow head rotation. Our data demonstrate both the optimal control of details of head movements and the influence of the stretch reflex that strongly relates to initial head position and the velocity of the movements around the primary position. The results support the idea of higher level programmed control of head rotation in human subjects.

Electromyography↗
PubMed1982

Saccadic eye movements of schizophrenic patients measured by reflected light technique.

In order to evaluate the integrity of their saccadic system, we have examined saccadic eye movements and smooth pursuit in a small group of schizophrenic patients, using precise infrared recording instrumentation. There were no significant differences between mean saccadic latencies of patients and controls. Both groups had mean latencies within the normal range of 200-250 msec. Saccades of schizophrenic patients appeared to have normal trajectories and dynamic characteristics. Three of the six schizophrenic patients and one of the three normal controls had smooth pursuit tracking characterized by saccadic intrusions and saccadic smooth pursuit tracking. Occasional saccadic intrusions and saccadic smooth pursuit tracking appeared in the records of all other subjects, but with considerably lower frequency. These results, emerging from reflected light methods, confirm a previous report of normal saccadic latencies in schizophrenic patients using an electrooculographic technique.

Adult↗
PubMed1981

Saccadic eye movement strategies in patients with homonymous hemianopia.

Infrared oculographic recordings from three patients with hemianopia due to an occipital lesion showed that these patients employed a consistent set of (presumably unconscious) compensatory strategies to find and fixate objects. For targets in the blind hemifield, patients at first used a staircase strategy consisting of a series of stepwise saccadic search movements. This is safe but slow. When retested later, one patient had adopted a more efficient strategy employing one large saccade calculated to overshoot the target. Other strategies for finding targets in the blind hemifield were employed in response to specific situations presented by our experiments: a predictive strategy using past experience to anticipate where the target would be found, and special strategies for recovering a lost target and for awaiting the reappearance of the target. To fixate targets in the seeing hemifield, our subjects undershot the target to prevent losing it in the blind hemifield, then held it off-fovea on the seeing side of the macula.

Adult↗
PubMed1981

Simulation of head movement trajectories: model and fit to main sequence.

A sixth order nonlinear model for horizontal head rotations in humans is presented and investigated using experimental results on head movement trajectories and neck muscle EMG. The controller signals, structured in accordance with time optimal control theory, are parameterized, and controller signal parameter variations show a dominating influence on different aspects of the head movement trajectory. The model fits the common head acceleration types over a wide range of amplitudes, and also less common (dynamic overshoot) trajectories.

Electromyography↗
PubMed1981

Sensitivity analysis and optimization for a head movement model.

A sixth order nonlinear model for horizontal head rotations in humans is analyzed using an extended parameter sensitivity analysis and a global optimization algorithm. The sensitivity analysis is used in both the direct sense, as a model fitting tool, and in the indirect sense, as a guide to experimental design. Resolution is defined in terms of the sensitivity table, and is used to interpret the sensitivity results. Using sensitivity analyses, the head and eye movement systems are compared and contrasted. Controller signal parameters are the most influential. Their variations and effects on head movement trajectories and accelerations are investigated, and the conclusions are compared with clinical neurological findings. The global optimization algorithm, in addition to automating the fitting of various types of data, is combined with time optimality theory to give theoretical time-optimal inputs to the model.

Computers↗