T.L. Alvarez

Measurement of intraocular pressure in pig's eyes using a new tonometer prototype

We have developed a new tonometer prototype that may prove useful for noninvasive self-tonometry. The device measures the IOP by recording the force required to deform the cornea as function of distance. The goal of this experiment was to measure the IOP on porcine eyes with the new tonometer prototype. The IOP was a measured directly from the cornea of porcines eyes with different IOPS. These preliminary data demonstrate the clinical potential of home self-tonometry. Intraocular pressure (IOP) monitoring has revealed that diurnal fluctuations in IOP are a significant independent risk for glaucoma progression. The development of a noninvasive self-tonometer and a clinical method for its use by the patient at home will help in the management of glaucoma progression.

Saccade correlation to adaptation of progressive lens amongst presbyopes

This studys aim is to ascertain whether the amount of saccades during convergence has any relation to a persons preference of progressive additive lenses (PALs). Since patients who are better adapters to PALs have faster vergence dynamics, we presume those who do not adapt well must have an alternative to compensate for their slower vergence system. The goal of this experiment is to compare the eye movements of those who adapt and those who do not adapt when performing a double-step stimulus experiment. Of the presbyopes studied, eight subjects wear PALs daily, while the other seven could not adapt to PALs. Results concluded that presbyopes who could not adapt to PALs have an increased amount of saccades when performing the convergence experiment as compared to that of those who could adapt.

Interfacing a tonometer with a microcontroller to monitor diurnal intraocular pressure variations

Glaucoma is the degeneration of the optic nerve head caused by elevated intraocular pressure (IOP) and diurnal variations. It is the second leading cause of blindness. Proper treatment requires IOP to be monitored and maintained. Goldmann applanation tonometry is the clinical gold standard measured in the physicians office quarterly for those who are glaucoma suspects. It requires a local anesthetic administered to the cornea. Hence, this procedure cannot be utilized for patient home monitoring. The proposed tonometer to be used by a patient at home will use a sensor that measures force through the eye lid (transpalpebral) with a portable microcontroller device. The microcontroller will digitize data to be stored and transferred for offline analysis as well as display a single reading to an LCD display.

Independent Component Analysis of Divergence Eye Movements

Vergence eye movements are the inward (convergence) or outward (divergence) turning of the eyes which allows humans to view objects in depth. Previous studies on the dynamics of convergence and divergence eye movements have produced varied, sometimes contradictory, results. Four subjects were studied and tracked four degree disparity step changes for convergence and divergence at different initial target positions. Here we report that the dynamics of divergence movements not only differ from convergence movement, but depend on the initial vergence position. Velocities of divergence eye movements in response to targets that were initially near to the subject were approximately twice that of responses to initially distant targets. Hence, while convergence responses are fairly similar irrespective of the initial position, divergence dynamic properties are dependent on the initial stimulus position. It is speculated that the differences observed in divergence may be the result of a difference in the underlying neural controller potentially a difference in the magnitude of the fusion initiating component. Preliminary data using independent component analysis (ICA) shows the divergence system contains a step and pulse where the amplitude of the pulse is dependent on the initial stimulus position

Effects of a single green flash versus a white flash of light on saccadic oculomotor metrics

White light encompasses all wavelengths of the visible optical spectrum while variations of green light cover only a fraction. Saccades comprise a considerable portion of ocular activity and have been used for research in neurology, cognitive processing, reading, and weaponry design. The goal of this experiment was to study how different energies of light affect the saccadic oculomotor system. This was tested by white and green photic stimulation in eleven and eight subjects respectively as they visually attempted to locate a target. The subject was presented with a target: 15 degrees to either the right or left from the midline with no photic stimulus (control), 15 degrees to the right or left accompanied by a photic stimulus at: midline, 15 degrees to the left, or 15 degrees to the right. Data were collected using the Skalar infrared limbic tracking system and a custom LabVIEW program. Dynamics were quantified with a latency analysis and the time to acquire /spl plusmn/ one degree of the target analysis using MatLAB. Results show that an increase in latency occurs during target location accompanied by a photic stimulus compared to target location accompanied by no photic stimulus, and that green light has a more robust effect on saccadic metrics than white light.

Transmission of light in a synthetic fog medium

The primary goal of this project was to study the propagation of visible light through a synthetic fog medium as a function of wavelength. First, a controlled environment was necessary to deliver and remove fog. A system for the delivery of light and its measurement was developed. Transmission data in the fog medium were compared with data collected in ambient air. As expected there was an inversely proportional relationship between light intensity and distance for all wavelengths studied. The light intensity was also inversely related to the fog density of the environment. These data demonstrate that light transmission through fog is equally degraded at all wavelengths tested. Thus, synthetic fog seems to be a uniform obscurant. The study of synthetic fog has many biomedical applications such as: traffic and airline safety, security, and human health safety.

The dynamics of convergence insufficiency

Many people are hindered by vision dysfunctions. One of the most prominent vergence dysfunctions in society is convergence insufficiency (CI) affecting 7% of the population. This study inspects the dynamics of convergence movements in both CI and normal binocular vergence subjects. Two stimulus types, a four degree and six degree step, were presented to an subjects. The goal was to determine if the dynamics of CI subjects were different compared to subjects with normal binocular vergence. Using the main sequence analysis, data show the dynamics of CI subjects were in the normal range as those who had normal binocular vergence, but were on the lower end. This study shows the dynamics of Cl subjects, whom have not had vision therapy, are slower when initiating a convergence movement.

Short-term adaptation in disparity vergence eye-movements

The high-velocity initial component of disparity vergence eye movements is modified by stimuli that generate large transient disparities. Modification was observed in all three subjects studied. After modification, the peak velocities were substantially higher than in normal baseline responses. The temporal distribution of the peak velocities for a step stimulus as a function of number of trial revealed the existence of a very rapid adaptive process in the vergence system. Plots of main sequence, however, showed that the first-order dynamic characteristics of all initial components were the same for both preand post adaptive responses. A process of recovery or de-adaptation was also observed.

The correlation between change in near-dissociated phoria and vergence dynamics

Visual fatigue is defined as weariness or discomfort localized in either the head or the eye after a visually demanding task. The etiology of visual fatigue is insufficiently understood in vision research. This study investigates the correlation between changes of near-dissociated phoria versus vergence dynamics defined as the ratio of convergence average peak velocity to divergence average peak velocity. Four degree convergence and divergence eye movements as well as phoria levels were recorded using an infrared eye movement monitor from five subjects. Preliminary data show a high correlation between the changes in phoria versus the change in vergence dynamics. Results suggest that the amount of changes in the two parameters, phoria and vergence dynamics, are one of many factors which may infer why some people are more prone to visual fatigue than others.

Neural control in vergence eye movements

Vergence eye movements are the inward and outward turning of the eyes and contemporary views believe these movements are mediated by a preprogramming and feedback controlled mechanism. The individual left and right eye responses have been reported to have large dynamic asymmetries between the individual eye movements. One hypothesis to explain the observed asymmetry is the preprogrammed transient component is monocularly controlled. A visual stimulus was created to stimulate a single step change to one eye and a double step change to the other eye using a haploscope. Results show that in some responses a double high-velocity component was observed in one response with a single high-velocity response in the other eye. This behavior suggests the preprogrammed component within a vergence response may be monocularly controlled.