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At least 235 records · Page 13Linked to original sources
PubMed2003

Retinotopic accommodation responses in myopia.

PURPOSE: A reduced sensitivity to retinal image blur has been reported in myopes. Diminished blur detection reduces the error signal to the retinotopic (blur-induced) accommodation system and results in impaired accommodation responses under retinotopic conditions. This study was conducted to investigate retinotopic accommodation responses in emmetropia and myopia under dynamic conditions. METHODS: Static accommodation responses to a blur-only target with vergences of 0 to 4.5 D were measured with an optometer. Microfluctuations of accommodation were recorded with the subject viewing the target at a vergence of 4 D, and dynamic step responses were measured for step stimuli from 2.5 to 3.5 D and 2.0 to 4.0 D, with the optometer in dynamic recording mode. Measurements were obtained from a group of 32 visually normal emmetropes (EMMs) and subjects with progressing myopia. RESULTS: Stimulus-response curves were not significantly different between the refractive groups. Subjects with late-onset myopia (LOMs) demonstrated significantly larger accommodation microfluctuations compared with emmetropes and subjects with early-onset myopia (EOMs). Fourier analysis revealed that the increase in the magnitude of the fluctuations was mediated by the low-frequency components. Accommodation step responses revealed longer reaction times in LOMs. Further analysis showed that LOMs responded to accommodation step stimuli only between 43% and 64% of the time. In contrast, the other groups showed a response rate of almost 100%. CONCLUSIONS: The experiments demonstrate a reduction in retinotopic processing in LOMs, which results in an increased variability in their dynamic accommodation response to stationary near targets and reduced performance for dynamic step tasks. The results demonstrate a reduced blur appreciation under dynamic conditions in these refractive groups that may lead to periods of retinal image blur of varying magnitude during near work.

Accommodation, Ocular↗
PubMed1976

Effects of luminance and stimulus distance on accommodation and visual resolution.

Measurements of accommodation and visual resolution were obtained at a number of luminance levels and stimulus distances. With reductions in luminance the eye approached a "fixed-focus" condition of accommodation for intermediate distances, resulting in successively larger errors in accommodation for both near and far stimuli. The visual resolution values were initially affected by both the luminance and distance of the stimulus. Subsequent measurements of resolution, following the correction of accommodative errors, were found to be independent of the stimulus distance. The findings are discussed with regard to the problems of "night myopia" and variations in visual resolution with stimulus distance.

Accommodation, Ocular↗
PubMed1992

Contribution of accommodation and disparity-vergence to transient nearwork-induced myopic shifts.

Many investigations have attempted to demonstrate a relationship between sustained near-vision and the aetiology of environmentally-induced myopia, but it remains unclear whether myopic development relates to the actions of either accommodation or vergence. The present study investigated the effect of varying the disparity-vergence demand during a near-vision task while maintaining the accommodative stimulus constant. We required 20 young subjects to perform a near task (viewing distance = 20 cm) for 20 min through base-in, zero power and base-out prisms. The magnitude of the prisms for each individual corresponded to one-third their near vergence range. Pre- and post-task measurements of refractive state were determined objectively using an infrared optometer, with post-task readings being recorded over the initial 50 s following task completion. The results indicated no significant difference between the transient far-point shift for the three disparity-vergence conditions. However, combining data across conditions, a significant, mean shift in the myopic direction of 0.14 D was observed during the 10-20 s period immediately following task completion which dissipated within 20-50 s post-task. These findings suggest that the transient post-task myopic shift was not related to the output of disparity-vergence.

Accommodation, Ocular↗
PubMedDate not supplied

Ablation design in relation to spatial frequency, depth-of-focus, and age.

PURPOSE: To consider factors relevant to the design of wavefront-aberration-based customized ablations. METHODS: Review. RESULTS: Ablations that seek to eliminate all wavefront aberrations, both second- and higher-order, may not be optimal for all patients. This is particularly the case for presbyopes. Their main requirement will normally be for extended binocular depth-of-focus to yield adequate distance and near vision with good retinal contrast at lower spatial frequencies, rather than the highest levels of acuity and modulation transfer function at a single distance. For many presbyopes, this can be achieved by aiming for monovision correction or low myopic astigmatism, with reasonable but not necessarily complete correction of higher-order aberrations. This compromise allows a range of everyday tasks to be carried out, including face recognition and reading. CONCLUSION: The targeted correction of wavefront aberration should take into account the visual needs and preferences of the individual patient.

Accommodation, Ocular↗
PubMed2003

Suppression of the human vestibulo-ocular reflex by visual fixation or forced convergence in the dark, with a model interpretation.

Six normal humans experienced yaw axis steps of velocity at 120 degrees /s in the dark. During the post-rotary period, subjects either had a null-task (do nothing); an ocular motor task (forced convergence: crossing the eyes); or a visual task (fixating a head-stationary target against a background of 10 degrees light/dark bars). Tasks started at 3 s post-rotation, and lasted either 2, 5, 10, or 15 s. Ocular motor and visual tasks were tested on different days. Five repetitions of each task duration were recorded for each subject. A mean VOR gain of 0.52 was observed, which did not vary with experimental conditions. Both convergence and fixation markedly suppressed nystagmus; in fact, the VORs obtained with the two different tasks are superficially similar in appearance. However, mean null-task time-constants were 9.4 s for convergence days, but 8.4 s for fixation days, and there was a small but significant reduction in overall null-task VOR amplitude on fixation days. Also, post-convergence slow-phase velocities were slightly enhanced, while post-fixation velocities were significantly reduced. The time-constant of velocity storage was found to be 10.1 s for convergence responses and 8.2 s for fixation responses. These differences can be understood in terms of modifications in central velocity storage during visual fixation which do not occur with convergence. The mean fixation data were analyzed in the context of a VOR model well-established for monkey data. With appropriate choice of parameters, this model accurately reproduces most features of the human data. An estimate for the human cupula time-constant of 3.3 s is obtained. Compared with the monkey, fixation suppression is greater and post-fixation velocity reduction less. Retinal slip alone accounts well for this; "velocity dumping" by an integrator shunt must be slight if present at all. The model correctly represents the post-fixation VOR for all durations of fixation.

Convergence, Ocular↗
PubMed1998

Short-latency ocular following of motion by monkeys during a fixation task.

Short-latency ocular following responses in monkeys, induced by first- and second-order motion during a fixation task, were sensitive to stimulus size, retinal eccentricity and motion duration. Latency of eye movements was no different for first- or second-order motion and simply marked the occurrence of movement in the visual field. Second-order motion produced higher velocity eye movements than did first-order motion. The velocity of eye movements was influenced by the orientation of second-order pattern components as well as their conspicuous energy. Even when partially suppressed by a fixation task, involuntary eye movements during the initial pursuit remain sensitive to several properties of visual motion.

Analysis of Variance↗