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Biomedical subjects

A T Bahill

Publications and source records attributed to A T Bahill.

At least 19 recordsLinked to original sources
PubMed1985

Learning to track predictable target waveforms without a time delay.

Humans can learn to overcome the 150 msec delay of the eye movement system and track predictable targets with no latency. The mean squared error between the target and eye position was used as a measure of the goodness of tracking. For typical subjects, this error decreased from 0.5 deg2 to 0.1 deg2 after 100-200 sec of viewing the target. Professional athletes had much smaller mean squared errors at the beginning of the learning period.

Adult↗
PubMed1983

Model emulates human smooth pursuit system producing zero-latency target tracking.

Humans can overcome the 150 ms time delay of the smooth pursuit eye movement system and track smoothly moving visual targets with zero-latency. Our target-selective adaptive control model can also overcome an inherent time delay and produce zero-latency tracking. No other model or man-made system can do this. Our model is physically realizable and physiologically realistic. The technique used in our model should be useful for analyzing other time-delay systems, such as man-machine systems and robots.

Bionics↗
PubMed1983

Smooth pursuit eye movements in response to predictable target motions.

The human smooth pursuit eye movement system has a latency of about 150 msec. However, this study shows that humans can learn to perform zero-latency tracking of targets that move with continuous velocity and amplitude-limited acceleration. Superposition of eye velocity and target velocity records, for our unique target waveforms, demonstrated that the subject was using the correct waveform and not just approximating it with a sinusoid or some other simple waveform. Calculation of the mean square error between target and eye position gave a quantitative measure of how well the human can track. The mean square error between target and eye position was 0.32 deg2 for one thousand seconds of steady-state tracking by seven subjects. For several cycles at a time all subjects were able to reduce this error to less than 0.1 deg2.

Adult↗
PubMed1982

Frequency limitations of the two-point central difference differentiation algorithm.

A two-point central difference algorithm is often used to calculate the derivative of a function. This estimate is only valid over a limited frequency range. Therefore, the algorithm can be modeled as an ideal differentiator in series with a low-pass filter. The filter cutoff frequency is a function of the time between the points. We discuss the accuracy and limitations of using this algorithm on human saccadic eye movement data. To calculate the velocity of saccadic eye movements the algorithm should have a cutoff frequency of 74 Hz or above.

Eye Movements↗
PubMed1978

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.

Computers↗
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

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

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↗
PubMed1975

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.

Computers↗