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Biomedical subjects

L Stark

Publications and source records attributed to L Stark.

At least 127 records · Page 7Linked to original sources
PubMed1977

Oblique saccadic eye movements. Independence of horizontal and vertical channels.

Horizontal and vertical components of oblique saccadic eye movements are dynamically independent. That is, they have independent dynamic trajectories determined by either the presence, or the absence, as well as the magnitudes of dynamic overshoot, glissades, overlapping saccades, and closely spaced saccades. Temporally, oblique movements manifest varying degrees of independence, for the two components can begin and end either together or separately. Purely horizontal saccades (ie, between two points on a horizontal line) usually show crosstalk demonstrated by extraneous, transient, vertical components. Therefore, saccades are very seldom linear or straight; the trajectories are usually curved.

Eye Movements↗
PubMed1977

Normal saccadic structure of voluntary nystagmus.

Infrared oculographic recordings in three cases showed that voluntary nystagmus is composed of rapidly alternating small-amplitude saccades with normal velocity-amplitude characteristics. These bursts of reciprocating saccades are routinely associated with oscillopsia. This means in terms of oculomotor control theory that the brain stem mechanism mediating voluntary nystagmus must bypass the level in the oculomotor path where the saccadic corollary discharge (efferent copy) is initiated.

Adult↗
PubMed1977

Paralysis of accommodation in infectious mononucleosis.

A case report of a 22-year-old patient with accommodative paralysis is presented including (1) the five-year history beginning with infectious mononucleosis; (2) recent clinical examination showing accomodative paralysis and reduced pupilary responses to light and near; (3) objective recordings confirming both the absence of any accommodation and the presence of pupillary responses to monocular and binocular near stimuli and to light, the latter with pupillary escape; and finally (4) neuropharmacological tests showing 7-diopter accommodative responses to pilocarpine (an acetylcholine substitute acting directly on the ciliary muscle receptor sites) and absent responses to demecarium bromide (a cholinesterase blocking agent which potentiates neurally released acetylcholine). Infectious mononucleosis includes ocular signs and symptoms. In young persons with accommodative difficulties, infectious mononucleosis should be suspected.

Accommodation, Ocular↗
PubMed1977

Dynamic measures of vergence accommodation.

Dynamic measures of vergence accommodation (VG-ACC) are necessary for complete analysis of the triadic near response but are difficult to obtain because eyeball rotation affects accommodative measures. A dynamic optometer that uses the third Purkinje image is shown to be insensitive to small eye rotations and permits dynamic measures of VG-ACC responses. Such measures are presented to demonstrate the feasibility of this method. Average latency measures of 260 msec for VG-ACC fall between the latency of 160 msec for disparity vergence and the latency of 380 msec for accommodation. Additional dynamic features of VG-ACC resemble those of other components of the near triad.

Accommodation, Ocular↗
PubMed1977

Dynamics of the human eyeblink.

High-speed photography of eyeblinks in 4 subjects provided displacement and velocity time functions. The results from dynamic measurements made with a mechanical transducer connected physically to the upper eyelid gave estimates of the passive spring constant (Kp = 1.5 gmf/mm) and the passive viscosity (Bp = 0.09 gmf - sec/mm) of the eyelid. Noting the similarity between the reciprocal-innervation mechanisms in the eyelid and the eye-movement systems, a mathematical eyelid model was derived based on the well-known eye-movement model and using similar ideal mechanical-element representations. The model-simulation time course shows the essential characteristics of an eyeblink and suggests that the force program consists of reciprocally acting pulse forces during the downblink, and pulse-step forces during the upblink. When 1 mm of lid displacement is equated with 5 deg of eyeball rotation and when the main sequences for lid movements and for time-optimal saccadic eye movements are compared, the eyeblinks are not found to be time optimal for their various amplitudes.

Eyelids↗
PubMed1977

Ketamine self-administration by the rhesus monkey.

Intravenously administered ketamine served as a reinforcer of self-administration behavior in rhesus monkeys during daily 2-hour sessions. When the dose of ketamine was varied over a wide range at fixed-ratio schedules of reinforcement of 1, 8 and 64, the response rate was an inverted U-shaped function of the dose. Maximal response rates occurred at progressively higher doses as the fixed-ratio size was increased; drug intake per session was positively related to the dose. When the ketamine dose was held constant and the fixed-ratio was increased in a geometric series, the response rate increased as the fixed-ratio was increased to FR 128 or FR 256; additional increases in fixed-ratio size produced abrupt decreases in response rates. Since the response rate increased linearly as the fixed-ratio was increased geometrically, drug intake was a decreasing function of the response requirement. With respect to magnitude of reinforcement and fixed-ratio response requirement, the data show that ketamine maintains self-administration behavior in a manner similar to that of a number of other drugs and conventional reinforcers.

Animals↗
PubMed1977

Neuro-optometry: an evolving specialty clinic.

Neuro-optometry is evolving as an optometric clinical specialty focusing on neurological dysfunctions of the visual system. Initially, we focused upon abnormalities of ocular movements, and our investigations have now broadened to include static and dynamic measurements of eye movements, accommodation, and the pupil. We feel the clinic serves three fundamental purposes: (1) to provide service to the patient, (2) to perform clinical research, and (3) to broaden the scope of the students' clinical experience. Operation of the clinic, technical methods of measurement, the testing protocol, and examples of interesting clinical recordings are described.

Accommodation, Ocular↗
PubMed1976

Parametric sensitivity analysis of a homeomorphic model for saccadic and vergence eye movements.

A non-linear sixth order homeomorphic model, fitted with parameters based on eye movements and physiological data, was tuned so that it provided good simulations for the shapes of the magnitude, velocity and acceleration trajectories. Excellent quantitative agreement was obtained in terms of the Main Sequence diagrams for human eye movements. Parametric sensitivity analysis was done for a saccade of ten degree amplitude, a physiologically normal magnitude at which experimental data is both abundant and relatively noise free. Among the many useful results of this sensitivity analysis are that pulse width (PW) and pulse height (PH) were confirmed as the two controlling parameters for the human eye movement model. Output behavior was relatively insensitive to variations of the passive elements of the plant. This analysis also pointed out that more physiological data are needed to understand the role of the non-linear force-velocity relationship of the extraocular muscles.

Computers↗
PubMed1976

Disorders in cerebellar ocular motor control. I. Saccadic overshoot dysmetria. An oculographic, control system and clinico-anatomical analysis.

Photoelectric eye movement recording in 9 patients with cerebellar disorders defined three features of saccadic overshoot dysmetria: (i) saccades were hypermetric and successively diminished in amplitude; (ii) saccadic initiation interval averaged 173 ms: and (iii) eye position was constant during the intersaccadic period. These characteristics indicated that the visually evoked saccades subserving foveation had increased gain, and were modelled by computer simulations using a sampled-data control model with increased feed-forward gain. Eight patients with saccadic overshoot dysmetria had cerebellar neoplasms, vermis-splitting surgical procedures and mid-line cerebellar signs. This clinical evidence suggests that vermian dysfunction is responsible for saccadic overshoot dysmetria. Normally, the cerebellar vermis appears to play an adaptive role by continuously adjusting gain of the direct visual motor pathway. When cerebellar disorder exists, adaptive gain modulation is lost, and, if gain then increases, saccadic overshoot dysmetria is a result.

Astrocytoma↗
PubMed1976

Disorders in cerebellar ocular motor control. II. Macrosaccadic oscillation. An oculographic, control system and clinico-anatomical analysis.

A distinctive cerebellar ocular motor disorder, macrosaccadic oscillation, evolved simultaneously with an acute cerebellar syndrome in 4 patients, 2 with haemorrhagic metastatic melanoma deep in the vermis, a third with a presumed cerebellar haematoma and a fourth with focal demyelinating disease. Ocular oscillations were conjugate, horizontal, symmetrical, occurred in bursts of several seconds duration, had amplitudes of 30 degrees to 50 degrees, and were evoked whenever the patient attempted to shift visual fixation or pursue a moving target. Photo-electric recordings in one patient with tumour defined features of this disorder of saccadic eye movement: (i) oscillation was composed of saccades, (ii) frequency was 2 Hz, (iii) bursts occurred with amplitude first increasing and then decreasing, (iv) intervals between beginnings of saccades averaged 260 ms and (v) eye position did not exhibit systematic drift during the intersaccadic period. These features documented the inreased gain and instability of the visually guided saccadic system. By using increased feed-forward gain in a sampled-data control model we simulated the pattern of macrosaccadic oscillation. We belive that the acute loss of the calibrator function of the cerebellum accounts for the gain abnormality underlying macrosaccadic oscillation.

Adult↗
PubMed1976

Eye movements during reading: case reports.

Since the time of Javal, it has been well established that normal reading eye movement patterns have 3 principal components: (1) small saccades that move the eyes from word to word, (2) large saccades that return the eyes to the beginning of the next line, and (3) fixation pauses between each saccade for information processing. We discuss the vision analysis results and show the quantitative reading eye movement records, measured with the infrared photoelectric method, of 5 patients examined in the Neuro-optometry Clinic. The reading records showed a wide variety of behavior: 1 patient performed normal reading movements, 1 "slow reader" manifested an excessive number of fixations as well as extended fixational durations, another "slow" reader only exhibited an excessive number of fixations, a patient with dyslexia performed backward reading movements, and 1 patient exhibited nystagmus superimposed upon the reading pattern.

Adult↗
PubMed1976

Dynamic and static violations of Hering's law of equal innervation.

Hering's Law of Equal Innervation treats the double eye as a single organ. Normal humans often execute saccadic eye movements that are dynamic violations of Hering's Law. These infractions are produced by differences in the neural controller signals sent to each eye and are exemplified by monocular movements, such as dynamic overshoot, glissades, and double saccades; these dynamic violations occur more frequently in fatigued subjects. In contrast to dynamic violations, static violations of Hering's Law are usually indicative of pathological conditions.

Aged↗