Michael R. Clark

The main sequence, a tool for studying human eye movements

Abstract The astronomical term “main sequence” has been applied to the relationships between duration, peak velocity, and magnitude of human saccades over a thousandfold range of magnitude. Infrared photodiodes aimed at the iris-sclera border and a digital computer were used in experiments to derive the main sequence curves. In the pulse width modulation model, the duration of the controller signal pulse determines saccadic amplitude and peak velocity. The high-frequency burst of the oculomotoneurons needs to be only one-half the duration of the saccade, because of the “apparent inertia” of the eyeball.

Dynamic overshoot in saccadic eye movements is caused by neurological control signal reversals

Three quite different types of overshoot occur in saccadic eye movements; each has unique characteristics determined by distinct neuronal control patterns. Most saccades have dynamic overshoot; it is more prevalent among, and more prominent in, small saccades. Dynamic overshoot is caused by nonrandom reversals of the neuronal control signals. It is a monocular phenomenon. The return velocities for dynamic overshoot are equal to saccadic velocities and are much larger than vergence velocities.

Glissades-Eye Movements Generated by Mismatched Components of the Saccadic Motoneuronal Control Signal

Abstract Human saccadic eye movements have three types of overshoot: dynamic overshoot, lasting 10–30 ms; glissadic overshoot, lasting 30–500 ms; and static overshoot, which is amended—after a delay of about 200 ms—by a subsequent corrective saccade. Glissades are the slow drifting eye movements occasionally seen at the end of saccadic eye movements. Glissades are hypothecated to be produced by mismatches in the pulse and step components of the motoneuronal controller signals. Glissades are not vergence eye movements, although the dynamics are similar.

Control of human eye movements: II. a model for the extraocular plant mechanism

Abstract Control of eye movements is essential in accomplishing visual or perceptive tasks. The brain and central nervous system process retinal information and send nervous signals to the extraocular muscles, which exert forces that cause the eye to move. A model for the human extraocular plant, which consists of the nervous input signals, the extraocular muscles, the orbit and the globe, is proposed. The derivation is based on anatomical and physiological data as well as experiments concerned with a variety of eye movements under normal and abnormal conditions. The nervous activity controlling eye movements was estimated from electromyography and single unit studies of the extraocular nuclei. The equations describing muscle properties were discussed in a previous paper by the authors; these results were incorporated into the present model. The characteristics of the isolated globe and its visco-elastic interaction with the orbit were computed from length- tension curves and isotonic experiments. Simulations using the resulting representation accurately depicted the isotonic experiments on the isolated globe and on the total extraocular plant, the isometric forces during three different types of eye movements, and the weighted globe experiment. A future paper will show that the model accurately simulates normal eye movements of different types and amplitudes.

Control of human eye movements: I. modelling of extraocular muscle

Abstract The properties of extraocular muscle are important in consideration of the control of human eye movements. A proposed model for human extraocular muscle is based on the anatomical and physiological evidence; it considers both the static and dynamic properties of active and passive muscle. The passive parallel elasticity was determined from the length-tension curves for passive muscle, while the active series elasticity was defined utilizing quick stretch results for active muscle. The characteristics of active muscle as the tension generator were computed from length-tension data; the force-velocity relationship was used to describe the viscosity of active muscle. Simulations using the muscle model accurately depicted the quick stretch experiments of both active and passive muscle as well as the isometric development of muscle force to a state of tentanus. The model will be incorporated into an overall representation of the extraocular plant mechanism in the immediately suceeding paper.

Control of human eye movements: III. dynamic characteristics of the eye tracking mechanism

Abstract A model for the human horizontal eye tracking system is proposed which is based on the dynamic characteristics of a variety of eye movements as well as the anatomical and physiological aspects of the extraocular mechanism. It includes the nonlinear force-velocity relationship of active muscle and considers separately the distinct asymmetrical properties of the agonist-antagonist muscle pair. The inertial and visco-elastic forces of the globe and orbit on the muscles is also discussed; the nervous activity which causes the muscles to move the eye was approximated from studies of the extraocular motor nuclei. Responses of the model accurately depict position, velocity and acceleration of saccades over a large variation of amplitude while faithfully reproducing the “main sequence” of saccadic velocity and duration versus amplitude. Smooth pursuit and vergence eye movements were also successfully simulated.

Decreased urinary beta-defensin-1 expression as a biomarker of response to arsenic.

Ingestion of arsenic (As) through contaminated drinking water results in increased risks of skin, lung, kidney, and bladder cancers. Due to its association with kidney and bladder cancers, we hypothesized that analysis of the urinary proteome could provide insight into the mechanisms of As toxicity. Urine from participants in a cross-sectional As biomarker study conducted in Nevada, classified as having either high (>or= 100 microg total urinary As/l) or low exposure (< 100 microg total urinary As/l) was analyzed by surface-enhanced laser desorption/ionization time-of-flight mass spectrometry. Two polypeptides, 2.21 and 4.37 kDa, were significantly decreased in the high exposure group (p < 0.05) and were limited to men when stratified by sex. To replicate these findings, urine from participants in a second As study in Chile was analyzed and results confirmed the decrease of the 4.37 kDa polypeptide as well as a 4.76 kDa polypeptide among highly exposed men. These peaks were identified and confirmed as human beta-defensin-1 (HBD-1) peptides. In a separate in vitro experiment, gene expression analysis of As-treated cell lines demonstrated reduced HBD1 mRNA confirming that the observed decrease in HBD-1 resulted from As exposure. HBD-1 is an antimicrobial peptide constitutively expressed in multiple tissues including epithelial cells of the respiratory and urogenital systems. Recent studies support its role as a tumor suppressor gene for urological cancers suggesting that decreased HBD-1 levels may play a role in the development of cancers associated with As exposure. Further studies are warranted to investigate the role of HBD-1 in As-related toxicity.

Computer simulation of overshoot in saccadic eye movements

The human horizontal eye movement system produces quick, precise, conjugate eye movements called saccades. These are important in normal vision. For example, reading tasks exclusively utilize saccadic eye movements. The majority of saccades have dynamic overshoot. The amplitude of this overshoot is independent of saccadic amplitude, and is such that it places the image of the stimulus within the retinal region of maximum acuity within a minimum of time. A computer based model of the saccadic mechanisms was used to study the origin of this overshoot. It was discussed that dynamic overshoot cannot be attributed to biomechanism properites of the eye movement mechanism, but must instead be explained by variations in the controlling nervous activity. The form of this neural controller signal is very similar to that required for a time optimal response of an inertial system.

Sensitivity of control parameters in a model of saccadic eye tracking and estimation of resultant nervous activity

A model for the extraocular plant of the human visual eye tracking mechanisms is discussed. Its sensitivity to variation of controller signal nervous activity is studied in order to determine the type of activity that yields realistic simulations characteristic of typical saccadic eye movements.