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Biomedical subjects

G K Hung

Publications and source records attributed to G K Hung.

At least 19 recordsLinked to original sources
PubMed1992

Symptoms related to abnormal tonic state: experimental results and computer simulations.

There have been numerous attempts to relate tonic accommodation and tonic vergence and their adaptive capabilities to a variety of parameters in vision science and clinical optometry. In this paper, investigations attempting to correlate the tonic state with nearwork symptoms will be reviewed. Results suggest that tonic vergence is more closely linked to near symptoms than is tonic accommodation. The results will be discussed in terms of a steady-state model of accommodation and vergence.

Accommodation, Ocular↗
PubMed1992

Quantitative analysis of associated and disassociated phorias: linear and nonlinear static models.

Ogle proposed two measures of oculomotor balance, called associated and disassociated phorias, which he assumed were equivalent. However, experimentally determined values of these phorias do not show a close correspondence. To analyze the rationale behind Ogle's assumption of equality, a linear static model was evaluated. It was found that indeed the linear model predicts an exact correspondence between associated and disassociated phorias. Thus, his assumption depended on the presence of a linear model. To account for the discrepancy between these two measures, a nonlinear static model, containing the dead space operators depth of field and Panum's fusional area, was evaluated. Four equations for fixation disparity were derived corresponding to the four combinations of deadspace operator outputs. It was found that only one of these four equally possible solutions for associated phoria corresponded to the disassociated phoria. This suggests that the variability in the four solutions may account for the scatter in the experimental data. The nonlinear model was analyzed further to determine its sensitivity to parameter changes and to show how such a model could generate the classical shape of the fixation disparity curve.

Accommodation, Ocular↗
PubMed1992

Adaptation model of accommodation and vergence.

Both accommodation and vergence have been shown to exhibit adaptation after extended near viewing. Normally, when the stimulus to accommodation is removed, the accommodation system returns rapidly towards its tonic position. However, if the stimulus is removed after an extended focusing effort, the decay is much slower. A similar effect can be observed in the vergence system. After prolonged wearing of horizontal prisms, blockage of one eye results in a much slower decay of the vergence output towards its tonic value. No previous models have been shown to simulate quantitatively these effects. An interactive dual-feedback model of accommodation and vergence was developed to simulate the adaptive behaviour found experimentally. The unique feature of the model is that the output of each controller drives a dynamic adaptive component whose output governs the time constant of the controller. The model was able to simulate the rapid and slow decays following short and long viewing intervals in each of the accommodative and vergence systems. It also simulated adaptation during alternate binocular and monocular viewing under the accommodation closed-loop condition. Thus, this model can serve as the basis for detailed quantitative evaluation of adaptive behaviour in the accommodation and vergence systems.

Accommodation, Ocular↗
PubMed1992

Accommodative responses to eccentric and laterally-oscillating targets.

There is little information on the accommodative response to stimuli having naturalistic motion on the retina. In the present experiment, the steady-state accommodative response at various dioptric levels was assessed with a Hartinger coincidence optometer as sinusoidal frequency of a small target was systematically altered across the horizontal foveal region (+/- 2 degrees). The steady-state accommodative response became less accurate as target oscillation frequency increased, approaching the tonic accommodation level when the frequency was between 0.5 and 1 Hz (corresponding to peak velocities of 6.3 and 12.6 deg s-1, respectively). These results suggest that the accommodation system is reasonably robust to naturally-occurring retinal-image motion.

Accommodation, Ocular↗
PubMed1991

The interpretation of kernels--an overview.

The kernel identification method is a powerful technique for mathematically representing the dynamic behavior of a nonlinear system. This technique has been applied to a number of physical and physiological systems. An important development which has enhanced the usefulness of the kernel method has been the interpretation of the internal structure of a system by examining the shapes of the higher-degree kernels. Examples of various nonlinear models with known structure are illustrated to show a repertoire of kernel shapes. Variations in parameters of these models result in well-defined changes in the shapes of the kernels. Also, examples are shown of kernels obtained from physiological systems to demonstrate how examination of kernel shapes can lead to accurate predictions of the dynamic behavior of the physiological system. Finally, limitations of the applicable range of the kernel identification method are discussed.

Humans↗
PubMed1991

Vergence control of central and peripheral disparities.

The results of previous studies using either small or large target configurations suggest that open-loop (OL) vergence response dynamics may depend on the peripheral extent of the target. To investigate systemically the effect of peripheral target extent on dynamic vergence control, OL vergence responses to central, peripheral, and central-plus-peripheral targets were recorded. Open-loop disparity stimuli included steps, ramps, and sinusoids. It was found that the OL step and ramp responses to the central target were more rapid and exhibited significantly more step and multiple-step movements than those for either the peripheral or the central-plus-peripheral targets. During OL ramp stimulation, the maximum disparity for which tracking could be maintained was largest for the peripheral target. Sinusoidal responses showed the greatest gain for the central target. For all three types of stimuli, responses to the central-plus-peripheral target showed characteristics that were somewhat between those for the central and the peripheral targets. These results resolve some previously conflicting findings about open-loop vergence dynamics and suggest that large peripheral disparity stimulation initiates slow fusion of a visual scene, whereas small central disparity stimulation produces more rapid movements for precise binocular tracking of targets moving in depth.

Convergence, Ocular↗
PubMed1991

Model of tonic accommodation after sustained near focus.

The effect of short-term near focus on tonic accommodation (TA) was previously investigated in 48 visually normal subjects. After sustained focus upon a near target for 10 min, all illumination was extinguished and the decay of accommodation was monitored for 20 min. We calculated from these results that accommodation decayed rapidly with a time constant of about 0.4 min, followed by a slower decay with a time constant of about 32 min. However, the asymptotic level remained slightly above the preadaptation level after 20 min in the dark. A feedback model with two parallel dynamic components (transient and decay), which summed with a "bias" element in the forward loop, was developed to account for the results. The gains of the dynamic elements were varied until the simulation curves matched the experimental adaptation and decay data. During the closed-loop adaptation phase, the outputs of the dynamic elements moved in opposite directions but combined to yield the overall accommodation step response. Moreover, when the model was subsequently open-looped in the dark, the transient component declined rapidly, whereas the decay component remained above the preadaptation level. When the 48 subjects were grouped equally into 4 refractive categories, somewhat different decay time courses could be simulated with changes in model parameter values.

Accommodation, Ocular↗
PubMed1991

Linear model of accommodation and vergence can account for discrepancies between AC/A measures using the fixation disparity and phoria methods.

The AC/A ratios obtained previously by the fixation disparity method were in most cases greater than those by the phoria method. An earlier study analysed a simplified linear model of the accommodation and vergence system. It was determined that the equations corresponding to the two methods were equal, indicating that such a model could not account for the discrepancy found experimentally. However, two important components--tonic accommodation and tonic vergence--were omitted from this simplified model. In the present study, an analysis of the complete linear model showed that the equations corresponding to the two methods are different. In addition, calculation of the error between the cross-link gain and the overall accommodative input--convergence output values indicated that the AC/A should be higher using the fixation disparity than the phoria method. Therefore, the predictions of the complete linear model are consistent with the data obtained experimentally.

Accommodation, Ocular↗
PubMed1991

The linearity of proximally induced accommodation and vergence.

This study sought to determine the linearity and amplitude of both proximally induced accommodation (PIA) and vergence (PIV) by comparing the open-loop accommodation and vergence responses, respectively, to individual stimuli located at viewing distances ranging from 0.20 m to 1,500 m. For the measurement of PIA, the vergence and accommodation loops were opened by subjects (N = 10) monocularly viewing the target through a 0.5-mm pinhole, while accommodation was assessed concurrently using an infrared optometer. In the assessment of PIV, the accommodation loop was opened either by subjects (N = 10) viewing a low-frequency difference of Gaussian (DOG) grating or by viewing the targets binocularly through 0.5-mm pinholes. For both PIV methods, the vergence loop was opened by vertical dissociating prisms while the heterophoria was assessed for the various target distances using the von Graefe technique. The results demonstrated significant changes in both PIA and PIV for stimuli located closer than 3 m. However, both proximal components attained a minimum level and remained constant for targets at or beyond 3 m. Furthermore, the magnitudes of PIA and PIV were linearly related to the target distance measured in diopters and meter angles, respectively. These findings clearly demonstrate the influence of target proximity on the oculomotor system.

Accommodation, Ocular↗
PubMed1990

Suppression of sensitivity to change in target disparity during vergence eye movements.

It has been demonstrated recently in human psychophysical experiments that sensitivity to surround displacement is suppressed during convergence eye movements. To determine whether sensitivity to changes in target disparity is also reduced, responses to test disparities that were superimposed on standard 4 degrees step disparities were investigated. The test disparities consisted of brief (20 ms) positive and negative pulses as well as steps (in the range of +/- 0.6 degrees). A two-alternative forced-choice procedure was used in which the test disparity was presented in either the first or the second portion of a trial. The results showed that suppression of both test pulse and step disparities began before the start of the convergence movement and continued during the movement. Maximum suppression was about 0.50 to 0.85 log units and occurred between 150 ms before to 50 ms after convergence onset. The differences in sensitivity curves for pulse and step stimuli suggest the presence of different central and peripheral neural factors during vergence eye movements.

Eye Movements↗
PubMed1990

Application of the root locus technique to the closed-loop SO2 pacemaker-cardiovascular system.

A previously developed nonlinear model of the pacemaker-cardiovascular system contained a term of the form 1/CO in the forward loop. CO, the cardiac output, is a linear function of heart rate and exercise level. To evaluate the dynamic behavior of the system over a range of pacemaker controller gains, a piecewise linear model was developed. The model was linearized by taking the first two terms of the Taylor's series expansion of 1/CO about the steady state heart rate. A root locus algorithm, with the unique feature of a time delay element in the forward loop, was used to calculate the linear model closed-loop poles as a function of pacemaker controller gain for exercise levels of 25, 50, and 100 W. Step response simulation of the complete nonlinear model showed dynamics close to that predicted from the pole location in the root locus plot. Thus, the root locus technique can be used to quantify systematically the dynamics of the pacemaker-cardiovascular system for different exercise levels and over a wide range of controller gain values.

Cardiac Output↗
PubMed1989

Suppression of sensitivity to surround displacement during vergence eye movements.

Suppression was investigated psychophysically in three human observers by measuring their loss of sensitivity to brief (20 ms) simultaneous vertical displacement (up to 0.5 degrees) of horizontal lines during 4 degrees convergence eye movements. A two-alternative forced choice procedure was used in which the stimulus was presented either in the first or second portion of a trial. The amplitude of the displacement pulse, the time of the pulse relative to convergence onset, and the portion of a trial in which the stimulus was presented were randomized. The results showed that suppression began about 200 ms before, and continued until 350 ms after, convergence onset with maximum loss occurring at 25 to 125 ms after convergence onset. The maximum sensitivity loss was about 0.25 to 0.30 log units. Since peripheral factors were minimized by the use of a brief stimulus presentation and an eccentrically placed surround, the suppression found was primarily attributed to central neural mechanisms. Finally, the suppression of sensitivity to pulse displacement during the initial phase of the vergence movement is consistent with a recently developed dual-mode model of the vergence system, in which the initial transient portion of a step response is preprogrammed whereas the final sustained portion is maintained by continuous feedback control.

Choice Behavior↗
PubMed1989

Reduced vergence response velocities in dyslexics: a preliminary report.

Dyslexia is a developmental disorder characterized by a reduced ability to recognize or comprehend written or printed words. Two clinically diagnosed dyslexics and two normals served as subjects. Both dyslexic subjects showed reduced static accommodative and vergence ranges compared with normals and numerous regressions and fixation pauses during reading and target tracking tasks. The subjects were then presented with convergent and divergent ramp disparity stimuli ranging in speed from 1.33 to 32.0 deg/s and steps of either 2 or 4 degrees amplitude. The dyslexic subjects showed reduced vergence velocities and fewer step-ramp and multiple step responses than normals. These preliminary findings suggest that certain vergence control functions may be diminished or absent in dyslexics. However, the vergence deficit may parallel dyslexia and may not be a causative factor. Thus vergence eye movement measurements may be used to provide additional insight into the components of the central deficits associated with dyslexia.

Accommodation, Ocular↗
PubMed1988

Human pupillary response to ramp changes in light intensity.

The dynamic behavior of the pupillary system was explored using different velocity ramp changes in light intensity. Ramp stimuli ranged in velocity from 0.02 to 8.00 log units/s and the responses were recorded from three subjects. For ramp velocity below 0.066 log units/s, the responses were mainly ramps. Above 1.0 log units/s ramp velocity, the responses were mostly steps. In the stimulus velocity range of 0.066 to 1.0 log units/s, the primary responses were step-ramps. The amplitudes of the initial step movement in step-ramp responses were found to be proportional to stimulus ramp velocity. Thus the pupillary system appears either to convert the total energy in a time interval or the rate of change of intensity of the ramp stimulus into a step signal for driving the pupillary plant output.

Humans↗
PubMed1988

Auditory biofeedback to control vertical and horizontal eye movements in the dark.

Although there is evidence demonstrating the effectiveness of oculomotor auditory biofeedback on the control of the horizontal component of gaze in the dark, the oculomotor behavior of the horizontal and vertical components under such conditions remained unknown. Horizontal and vertical fixational eye movements were measured monocularly using an infrared limbal eye tracker in three normal subjects under three conditions: in the light, total darkness, and total darkness with two-dimensional auditory biofeedback of the eye movement. With fixation in the light, all subjects showed small drifts and corrective movements of up to about 0.5 degrees horizontally and vertically. With fixation in total darkness, the eye movements generally exhibited drifts and saccades of well over 2 degrees. However, with auditory biofeedback added during fixation in total darkness, the drifts and corrective saccades were reduced to levels more similar to those found with fixation in the light. The percent time on target in the light, dark and dark plus auditory biofeedback conditions was about 100, 50 and 80%, respectively, for both horizontal and vertical eye position. These results demonstrate that incorporation of two-dimensional oculomotor auditory biofeedback alone is sufficient to maintain fixation accuracy of both horizontal and vertical eye movements in total darkness close to that found during normal fixation in the light.

Adolescent↗
PubMed1988

Dual-mode behaviour in the human accommodation system.

The human accommodation system was investigated using a dynamic infra-red optometer to determine whether its feedback control process exhibited dual-mode behaviour. Ramp changes of target accommodative demand presented monocularly at various velocities (0.5-0.6 D s-1) with a fixed 2 D amplitude elicited two modes of behaviour. Smooth ramp movements were observed for low-velocity stimuli, whereas step responses predominated for high-velocity stimuli. At intermediate velocities, combined step-ramp responses were more prevalent. The characteristic of these step-ramp responses showed two important properties that suggest a pre-programming mechanism. The amplitude of the step portion of a step-ramp response approximated the value of the ramp stimulus at the time corresponding to the end of the step movement. Also, the initial step amplitude increased with increasing target ramp velocity. It appears that the amplitude of the preprogrammed accommodative movement was based on an estimate of the expected ramp target position at the end of the response. These findings are consistent with a recent unifying theory of oculomotor control in which the various oculomotor subsystems (versional and vergence eye movements and accommodation) each has fast and slow components. The fast component exhibits preprogramming, while the slow component shows continuous feedback control.

Accommodation, Ocular↗