PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “FIXATION, OCULAR”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 415 records · Page 23Linked to original sources
PubMed1985

Accommodative hysteresis: relation to resting focus.

Seventeen subjects sustained focus for up to 8 min on targets placed either at the nearpoint (NP) or farpoint (FP) of accommodation. This was repeated in separate sessions from three to six times at each target distance and pre- to postexposure changes in dark focus (DF), i.e., accommodative hysteresis, were recorded. For eight subjects the probability (P) of exhibiting hysteresis effects was greater than or equal to 0.50 on both types of sessions. Five additional subjects showed P(NP) greater than or equal to 0.66 and P(FP) less than or equal to 0.33, whereas 4 subjects showed a reversed preference with P(NP) less than or equal to 0.17 and P(FP) greater than or equal to 0.83. Thus, strong individual differences emerged with respect to susceptibility to accommodative hysteresis. Linear regression analyses were performed on the individual data of the 13 subjects in condition NP and 12 in condition FP who exhibited hysteresis effects on greater than or equal to 50% of their sessions. Relative magnitudes of accommodative hysteresis resulting either from sustained NP or FP focusing were negatively related to the DF level measured directly before the period of sustained focus. Thus, for example, a relatively near initial DF correlated with a small inward shift of the DF after NP focusing, but with a large outward shift after FP focusing. Only in the FP condition was there evidence for a positive regression of hysteresis effects on the dioptric distance separating the sustained focus target and the initial DF level, viz., the less the separation, the smaller the hysteresis effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1997

Subjective depth-of-focus of the eye.

An experiment is described in which the subjective depth-of-focus (DOF) of the eye, defined as the range of focusing errors for which the image of the target appears to have the same clarity, contrast, and form as the optimal in-focus image, was measured as a function of the size of high contrast (99%) Snellen Es for 5 trained subjects under cycloplegia. Mean DOF increased by approximately 60% as the size of the letter detail increased from -0.2 to 0.87 log min arc (Snellen equivalent: 6/3.8 to 6/45), although there were considerable intersubject variations. DOF declined with increasing pupil diameter, the mean total DOFs being 0.86, 0.59, and 0.55 D for 2-, 4-, and 6-mm pupils, respectively. In a second experiment, use of low (21%) contrast letters with a 4-mm pupil and 4 subjects marginally increased the DOF (by 0.08 +/- 0.05 D); refraction also shifted in a myopic direction by a mean of 0.15 +/- 0.06 D compared with the high contrast letters. A third experiment with four less-experienced subjects demonstrated the importance of instruction and training in any measurement involving judgment of just-perceptible defocus blur. The clinical implications of the results for measurements of refraction and amplitude of accommodation are discussed.

Accommodation, Ocular↗
PubMed1998

Influences of the accommodative stimulus and aging on the accommodative microfluctuations.

The effects of the accommodative stimulus and age on the accommodative microfluctuations were investigated in human subjects. Accommodative responses were measured by using an infrared (IR) optometer, and accommodative microfluctuations were quantified by power spectrum analysis. Two frequency bands were determined: low frequency components (< 0.5 Hz, LFC) and high frequency components (1.3 to 2.2 Hz, HFC). Among younger subjects, the changes in the HFC in response to the accommodative stimuli differed between subjects. The activity of the HFC reached a peak in the center of the accommodation range, whereas the activity of the LFC was its maximum in the presence of darkness or blur in the over-accommodation range. Smaller changes in the magnitude of the HFC were observed in older subjects. Although the LFC may be controlled neurologically, the HFC seems to arise from mechanical and elastic properties of the lens, and may be modulated by other physiological rhythmic variations.

Accommodation, Ocular↗
PubMed1999

Blur sensitivity in myopes.

PURPOSE: This study compared the ability of myopes and emmetropes to detect subjectively the presence of retinal defocus. METHODS: Subjects (12 myopes, 12 emmetropes) were cyclopleged and monocularly viewed a bipartite target through an appropriate near addition lens via a 2-mm artificial pupil. One-half of the target remained fixed while the other half was alternatively moved forward or backward until subjects first reported a difference in clarity between the two halves of the target. RESULTS: The mean blur threshold for the emmetropes and myopes was +/-0.11 and +/-0.19 D, respectively (p = 0.0001). CONCLUSIONS: These results demonstrate that myopes are less sensitive to the presence of blur, and may at least partially explain why previous reports have demonstrated a larger lag of accommodation in this refractive group. Additionally, the hyperopic retinal defocus resulting from the increased accommodative error may play a significant role in myopia development and progression.

Accommodation, Ocular↗
PubMed1988

Effect of a mental arithmetic task on dark focus of accommodation.

Three subjective techniques for measuring accommodation (polarized vernier optometer, laser optometer, and a hand optometer based on the vernier principle) were used to measure subjects' dark focus while they performed a short-term mental arithmetic task. Compared with the initial values measured without the arithmetic task, significant changes in dark focus occurred when the task was performed. The effects depended on the initial dark focus of the subjects: in far-dark-focus subjects, inward shifts were generally observed; in near-dark-focus subjects, the dark focus shifted in different directions when different measuring techniques were used. The individual changes in dark focus due to the mental task were reproducible when the mean dark focus of the group shifted inward and when the same technique for measuring accommodation was used. The results have implications for the interpretation of dark focus shifts that are found following prolonged visual work at close distances, e.g. at visual display terminals.

Accommodation, Ocular↗
PubMed1993

Accommodation and chromatic aberration: effect of spatial frequency.

When subjects view an edge in white light, a colour fringe, produced by longitudinal chromatic aberration (LCA) of the eye, is formed at the edge. The colour fringe changes with changes in focus, and serves as a complex colour-coded cue for reflex accommodation. Fincham found that 60% of his subjects failed to accommodate appropriately when the colour fringe was removed with an achromatizing lens or by the use of monochromatic light. Our experiment sought to determine the spatial frequencies at which LCA is most effective. We monitored accommodation in 10 subjects while they viewed sinusoidally moving sine-wave gratings (1-3 D at 0.2 Hz; 1-10.5 c/deg) in a Badal optometer. The targets were 'white' gratings with LCA normal, doubled, neutralized or reversed. Doubling the aberration has minimal effect, removing the aberration reduces gain and increases phase-lag, and reversing the aberration severely disrupts accommodation. Sensitivity to these chromatic cues exists at all spatial frequencies tested, but is most prominent between 3 and 5 c/deg. These results support the view that the system monitors focus by comparing contrast in red-green and perhaps blue-yellow colour-opponent mechanisms.

Accommodation, Ocular↗
PubMed2002

The dependence of binocular contrast sensitivities on binocular single vision in normal and amblyopic human subjects.

We have measured monocular and binocular contrast sensitivities in response to medium to high spatial frequencies of vertical sinusoidal grating patterns in normal subjects, anisometropic amblyopes, strabismic amblyopes and non-amblyopic esotropes. On binocular viewing, contrast sensitivities were slightly but significantly increased in normal subjects, markedly increased in anisometropes and esotropes with anomalous binocular single vision (BSV) and significantly reduced in esotropes and exotropes without BSV. Application of a prismatic correction to the strabismic eye in order to achieve bifoveal stimulation resulted in a significant reduction in contrast sensitivity in esotropes with and without anomalous BSV, in exotropes and in non-amblyopic esotropes. Control experiments in normal subjects with monocular viewing showed that degradative effects of the prism occurred only with high prism powers and at high spatial frequencies, thus establishing that the reduced contrast sensitivities were the consequence of bifoveal stimulation rather than optical degradation. Displacement of the image of the grating pattern by 2 deg. in normal subjects and anisometropes by a dichoptic method to simulate a small angle esotropia had no effect on the contrast sensitivities recorded through the companion eye. By contrast, esotropes showed similar reductions in contrast sensitivity to those obtained with the prism experiments, confirming a fundamental difference between subjects with normal and abnormal ocular alignments. The results have thus established a suppressive action of the fovea of the amblyopic eye acting on the companion, non-amblyopic eye and indicate that correction of ocular misalignments in adult esotropes may be disadvantageous to binocular visual performance.

Accommodation, Ocular↗
PubMed2002

Effects of voluntary blinks on saccades, vergence eye movements, and saccade-vergence interactions in humans.

Blinks are known to change the kinematic properties of horizontal saccades, probably by influencing the saccadic premotor circuit. The neuronal basis of this effect could be explained by changes in the activity of omnipause neurons in the nucleus raphe interpositus or in the saccade-related burst neurons of the superior colliculus. Omnipause neurons cease discharge during both saccades and vergence movements. Because eyelid blinks can influence both sets of neurons, we hypothesized that blinks would influence the kinematic parameters of saccades in all directions, vergence, and saccade-vergence interactions. To test this hypothesis, we investigated binocular eye and lid movements in five normal healthy subjects with the magnetic search coil technique. The subjects performed conjugate horizontal and vertical saccades from gaze straight ahead to targets at 20 degrees up, down, right, or left while either attempting not to blink or voluntarily blinking. While following the same blink instruction, subjects made horizontal vergence eye movements of 7 degrees and combined saccade-vergence movements with a version amplitude of 20 degrees. The movements were performed back and forth from two targets simultaneously presented nearby (38 cm) and more distant (145 cm). Small vertical saccades accompanied most vergence movements. These results show that blinks change the kinematics (saccade duration, peak velocity, peak acceleration, peak deceleration) of not only horizontal but also of vertical saccades, of horizontal vergence eye movements, and of combined saccade-vergence eye movements. Peak velocity, acceleration, and deceleration of eye movements were decreased on the average by 30%, and their duration increased by 43% on the average when they were accompanied by blinks. The blink effect was time dependent with respect to saccade and vergence onset: the greatest effect occurred 100 ms prior to saccade onset, whereas there was no effect when the blink started after saccade onset. The effects of blinks on saccades and vergence, which are tightly coupled to latency, support the hypothesis that blinks cause profound spatiotemporal perturbations of the eye movements by interfering with the normal saccade/vergence premotor circuits. However, the measured effect may to a certain degree but not exclusively be explained by mechanical interference.

Adult↗
PubMedDate not supplied

Decline of vertical gaze and convergence with aging.

BACKGROUND: Disturbance of vertical eye movement and ocular convergence is often observed in elderly people, but little is known about its frequency. OBJECTIVE: The purpose of this study was to investigate age-associated changes in vertical eye movement and convergence in healthy elderly people, using a digital video camera system. METHODS: We analyzed vertical eye movements and convergence in 113 neurologically normal elderly subjects (mean age 70 years) in comparison with 20 healthy young controls (mean age 32 years). The range of vertical eye movement was analyzed quantitatively and convergence was analyzed qualitatively. RESULTS: In the elderly subjects, the angle of vertical gaze decreased with advancing age and it was significantly smaller than that of the younger subjects. The mean angle of upward gaze was significantly smaller than that of downward gaze for both young and elderly subjects. Upward gaze impairment became apparent in subjects in their 70s, and downward gaze impairment in subjects in their 60s. Disturbance in convergence also increased with advancing age, and was found in 40.7% of the elderly subjects. CONCLUSION: These findings indicate that the mechanisms of age-related change are different for upward and downward vertical gaze. Digital video camera monitoring was useful for assessing and monitoring eye movements.

Adult↗
PubMed2001

Dynamics of the eye's wave aberration.

It is well known that the eye's optics exhibit temporal instability in the form of microfluctuations in focus; however, almost nothing is known of the temporal properties of the eye's other aberrations. We constructed a real-time Hartmann-Shack (HS) wave-front sensor to measure these dynamics at frequencies as high as 60 Hz. To reduce spatial inhomogeneities in the short-exposure HS images, we used a low-coherence source and a scanning system. HS images were collected on three normal subjects with natural and paralyzed accommodation. Average temporal power spectra were computed for the wave-front rms, the Seidel aberrations, and each of 32 Zernike coefficients. The results indicate the presence of fluctuations in all of the eye's aberration, not just defocus. Fluctuations in higher-order aberrations share similar spectra and bandwidths both within and between subjects, dropping at a rate of approximately 4 dB per octave in temporal frequency. The spectrum shape for higher-order aberrations is generally different from that for microfluctuations of accommodation. The origin of these measured fluctuations is not known, and both corneal/lenticular and retinal causes are considered. Under the assumption that they are purely corneal or lenticular, calculations suggest that a perfect adaptive optics system with a closed-loop bandwidth of 1-2 Hz could correct these aberrations well enough to achieve diffraction-limited imaging over a dilated pupil.

Accommodation, Ocular↗
PubMed2003

Dynamic accommodation response in the presence of astigmatism.

It has been suggested that in the presence of astigmatism some individuals make cyclic changes in focus over the astigmatic interval to obtain better visual performance. The aim in the present study was to identify such cyclic accommodative behavior and to characterize the variability of the response in the presence of astigmatism. The dynamic accommodation response in the presence of induced astigmatism was recorded objectively with an infrared optometer in seven young adults. Astigmatism led directly to increased accommodative variability in certain individuals. In two of seven participants there was evidence for aperiodic cyclic accommodative responses between different portions of the astigmatic interval. However, the amplitude of these tracking responses was much smaller than the astigmatic interval.

Accommodation, Ocular↗
PubMed2005

Visual adjustments to temporal blur.

After observers have adapted to an edge that is spatially blurred or sharpened, a focused edge appears too sharp or blurred, respectively. These adjustments to blur may play an important role in calibrating spatial sensitivity. We examined whether similar adjustments influence the perception of temporal edges, by measuring the appearance of a step change in the luminance of a uniform field after adapting to blurred or sharpened transitions. Stimuli were square-wave alternations (at 1 to 8 Hz) filtered by changing the slope of the amplitude spectrum. A two-alternative-forced-choice task was used to adjust the slope until it appeared as a step change, or until it matched the perceived transitions in a reference stimulus. Observers could accurately set the waveform to a square wave, but only at the slower alternation rates. However, these settings were strongly biased by prior adaptation to filtered stimuli, or when the stimuli were viewed within temporally filtered surrounds. Control experiments suggest that the latter induction effects result directly from the temporal blur and are not simply a consequence of brightness induction in the fields. These results suggest that adaptation and induction adjust visual coding so that images are focused not only in space but also in time.

Adaptation, Ocular↗
PubMed2006

[Near triad measurement--accommodative pupillometry with horizontal eye tracking and its qualitative analysis].

PURPOSE: Recording of "near triad responses" was done by static accommodative stimulation to reveal the qualitative analysis of asthenopia. METHOD: Twelve healthy subjects in agegroups of 20, 30, and 40 years were examined before and after several hours of visual display terminal (VDT) work after informed consent. Three trials of forward <-> backward movement of focusing on a target were carried out. Response curves of horizontal eye tracking and pupillary diameter, that is, convergence with miosis and divergence with mydriasis, were recorded continuously, bilaterally and simultaneously by TriIRIS. RESULTS: 1) Normal patterns of vergence and pupillary responses with constant amplitudes were recorded in five subjects and were stable. 2 ) Gradual decline of the mydriatic peak was observed in two subjects, suggesting spastic accommodation. Increasing mydriatic peak or notching of mydriatic phase was seen in one eye each, suggesting tonic disaccommodation. These abnormal patterns were reversed by adequate refractive correction, and classified as an eye-fatigue group. 3) Abruption of miotic phase was observed in two cases of exophoria. 4) Low and declining amplitudes despite normal light response were observed in two subjects, suggesting debilitated accommodation, and they were classified as an eye-strain group. 5) Post-traumatic mydriatic anisocoria responded in parallel, suggesting paretic accommodation. CONCLUSIONS: Coordinated dynamics of near triad responses revealed remarkable characteristics of accommodative iris movements with vergence. These objective findings revealed the different patterns of functional disorder of accommodation.

Accommodation, Ocular↗
PubMed1990

Effects of stimulus distance on measurements of dark convergence.

Dark convergence is often measured psychophysically by aligning two dichoptic stimuli, which are briefly flashed in a dark surround. In the present study, the effect of different stimulus parameters on dark convergence measurements was investigated. Chief among these was the effect of viewing distance (50, 100 and 500 cm) of the stimulus. Measured dark convergence was biased towards the actual viewing distance of the stimuli; this effect was stronger with bar stimuli than with point-and-line stimuli. Measured dark convergence depended on the viewing distance to a lesser extent if bar stimuli were observed through pin-holes (open accommodative feedback loop). The subject's knowledge of the viewing distance had a small but significant effect; the size of the nonius bars had none. To reduce artefacts due to the stimulus distance on psychophysical measurement of dark convergence, point-and-line stimuli should be used at a viewing distance of about 1 m. In this case, the effect of stimulus distance can be about 0.25 meter angle (ma), averaged over subjects, but it may be as large as 0.5 ms in some individuals.

Adolescent↗