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Biomedical subjects

G K Hung

Publications and source records attributed to G K Hung.

52 records · Page 3Linked to original sources
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↗
PubMed1986

Static vergence and accommodation: population norms and orthoptics effects.

The steady-state characteristics of the accommodation and vergence systems can be described by a model with six major oculomotor parameters. These include the system biases (tonic vergence and accommodation) and forward-loop gains (vergence and accommodative gains), as well as the interactive system gains (AC/A and CA/C ratios). We investigated these parameters in two populations: (1) 22 visually-normal asymptomatic individuals, and (2) 21 visually-abnormal symptomatic individuals before and after conventional orthoptic therapy. Two parameters related to system gain differentiated between the symptomatic and asymptomatic individuals: the slope of the fixation disparity curve with accommodation open-looped and the slope of the accommodative response/stimulus curve. Following orthoptic therapy, 4 static model parameters and 1 dynamic clinical parameter showed changes toward the normal mean; this included tonic accommodation, slope of the fixation disparity curve with accommodation closed-loop (2.5D), slope of the accommodative response/stimulus curve, the CA/C ratio, and the +/- 2D monocular accommodative flipper rate.

Accommodation, Ocular↗
PubMed1986

Quantitative assessment of disparity vergence components.

Earlier evidence suggests qualitatively that at least two control modes may mediate a single vergence response. Thus, in a vergence response to step disparity, the transient component drives the initial fast dynamic portion of the response, while the sustained component maintains the latter slower portion of the response. The authors extended this hypothesis by quantitatively defining the stimulus pattern and dynamics that elicit this dichotomous behavior. The disparity target consisted of vertical lines 2 deg in height presented to each eye. Ramp disparity velocities ranged from 0.7 deg/sec to 36 deg/sec with amplitude of up to 4 deg. Photoelectric recordings of eye movements from both eyes were subtracted to give the vergence response. Fast and slow ramp stimulus velocities were found to elicit transient and sustained component responses respectively. In addition, the finding of staircase-like responses to fast ramp stimuli has strong implications on control mechanisms, indicating a sampling process in the transient component of the disparity vergence system.

Eye Movements↗
PubMed1984

Accommodative stimulus/response function in human amblyopia.

Three parameters are essential to describe static accommodative behavior in a comprehensive, quantitative manner: the slope of the stimulus/response curve, the depth of focus, and the tonic response. These parameters were obtained in amblyopes, former amblyopes, strabismus without amblyopia, and normals. Results showed that the accommodative response in the amblyopic eye was characterized by a reduction in the slope of the stimulus/response curve and increased depth of focus. Similar abnormalities but of lesser magnitude were found in the non-dominant eye of some former amblyopes and some strabismus without amblyopia. Orthoptic therapy always increased the slope of the stimulus/response curve in the amblyopic eye. We believe that the reduced accommodative responses found in amblyopic eyes reflect a primary sensory loss over the central retinal region that occurs as a result of prolonged, early, abnormal visual experience associated with the presence of strabismus and/or anisometropia.

Accommodation, Ocular↗
PubMed1983

Static aspects of accommodation in human amblyopia.

Static aspects of accommodation in human amblyopia were investigated. Abnormalities uncovered included decrease in accommodative controller gain, decrease in slope of the accommodative stimulus/response curve, decrease in accommodative amplitude, and increase in depth of focus. Orthoptic therapy improved accommodative function in the amblyopic eye. Similar defects, but of lesser magnitude, were frequently found in the nondominant eyes of subjects in related test groups. This included former amblyopes who had received successful orthoptic therapy in their youth, which suggested lack of complete and/or maintained recovery of accommodative function, and strabismics without amblyopia, which suggested that the effects of strabismic suppression contributed to the accommodative deficits found in some strabismic amblyopes. The accommodative abnormalities found in our amblyopes were attributed to the effects of early, prolonged, abnormal visual experience on the sensory visual system.

Accommodation, Ocular↗
PubMed1983

Identification of accommodative vergence contribution to the near response using response variance.

An experimental method was developed to isolate accommodative and disparity vergence contributions to coordinated near vergence motor responses. The variability normally associated with the neural control signal was used as an identifying marker or tag. Using this approach, the results showed an increased variability associated with the blur-driven, accommodative vergence component that is particularly noticeable during the latter half of the movement. This indicated that accommodative vergence provides a moderate contribution to the near vergence response primarily in the late and post-transient period when the movement is essentially complete. Presence of this accommodative contribution lends further support to the "dual interactive model" of near triad control.

Accommodation, Ocular↗
PubMed1980

Binocular interactions of vergence components.

Classical theories describing the interaction between blur-driven and disparity-driven components of the visual motor near triad are examined and the underlying assumptions explicitly stated. Experimental evidence is presented that refutes one necessary assumption behind the traditional structuring of vergence components proposed by Maddox. An alternative theory of binocular interaction is outlined, and experimental support for the new theory is provided.

Accommodation, Ocular↗
PubMedDate not supplied

Differential retinal-defocus magnitude during eye growth provides the appropriate direction signal.

INTRODUCTION: The rate of ocular growth can be modified by the imposition of either plus or minus lenses before the eyes, which creates changes in retinal defocus. Several hypotheses with relatively complicated mechanisms have been proposed to explain these changes, but the underlying mechanism has remained elusive. MATERIAL AND METHODS: Our new analysis using schematic models, however, provides a relatively simple and logically-consistent explanation of how retinal defocus magnitude alone during ocular growth gives the requisite sign for an appropriate change in ocular growth rate. RESULTS: During a normal genetically-determined incremental increase in axial length, the presence of either an imposed plus or minus lens results in an increase or decrease, respectively, of the blur circle magnitude. Neuromodulators in the retina are proposed to regulate the sensitivity to retinal-image contrast by means of a local feedback mechanism, and the alteration in retinal-image contrast associated with the change in blur circle causes an increase or decrease, respectively, in the rate of release of neuromodulators as well as the rate of proteoglycan synthesis, the latter being associated with the structural integrity of the sclera. CONCLUSION: This provides the critical sign, as well as amplitude, information needed to modulate appropriately the rate of eye growth, to result in a decrease or increase, respectively, in the rate of ocular growth.

Accommodation, Ocular↗