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PubMed1998

The visual effects of head-mounted display (HMD) are not distinguishable from those of desk-top computer display.

Concerns about potentially harmful effects on the visual system due to the use of head mounted displays (HMDs) in general, and stereoscopic systems in particular, have been raised in the literature. Most of the concerns were based on studies measuring visual function changes following short-term use of HMDs. This study measured functional changes in binocular vision, accommodation, and resolution following 30 min use of HMD in both stereoscopic- and non-stereoscopic modes, and compared them to changes following the same task performed on a desk-top CRT display. No functional differences were found between HMD and CRT and most measured changes were too small to be considered clinically meaningful. An evaluation of subjective comfort found a statistically significant difference in the impression of comfort between the CRT and the HMD in stereoscopic mode, with the latter being less comfortable. It can be concluded that the functional changes reported following short term use of HMDs are not specific to stereoscopic presentation and do not differ from those caused by desk-top CRT display.

Accommodation, Ocular↗
PubMed1998

An infrared eccentric photo-optometer.

An objective infrared optometer has been designed, based on the optical principles of eccentric photorefraction. A CCD camera with an eccentric infrared light source images the subject's pupil through a Badal optometer. The slope of the light distribution across the pupil is continuously recorded. Accommodative state is measured by moving the camera behind the Badal lens until the slope is zero. This position corresponds to the case where the camera is conjugate with the retina of the observer. In this Badal optometer, the irradiance of light at the pupil plane, the sensitivity of the photorefractor, and the focal setting of the camera lens remain constant for all positions of the camera from the eye. The repeatability of a single measure of refractive state in a cyclopleged eye was less than 0.05 D. Static accommodative responses taken from 3 subjects in both closed and open loop conditions provided expected stimulus/response measures. The instrument can also be adapted to measure dynamic accommodation.

Accommodation, Ocular↗
PubMed1997

The development of accommodation.

We investigated the roles that blur, proximity and vergence cues play in the development of accommodation. Accommodative responses to targets incorporating one or more of these cues were measured for four adults and eight infants at 1.5 and 3 months of age using eccentric photorefraction. Adults showed accurate accommodation to blur cues, and variable accommodation with proximity cues alone. Some infants at both ages showed fixed accommodative responses to all stimulus conditions. Others responded consistently in the correct direction for pattern targets at different distances, but made poorer responses when blur was presented in conflict with distance. Binocular viewing improved the accommodative responses in only some infants.

Accommodation, Ocular↗
PubMed1998

Lens induced aniso-accommodation.

Despite the evidence for consensual accommodation in response to consensual accommodative stimuli, only a few studies have investigated the binocular accommodative response to unequal (aniso) accommodative stimuli. Past studies investigating an unequal binocular accommodative response (aniso-accommodation) to aniso-accommodative stimuli have been limited by viewing conditions and measurement technique making the results, which were equivocal, difficult to interpret. This investigation addressed these limitations by the following design parameters: (1) monocular dichoptic blur cuese were provided in the binocular stimulus target to provide subjects feedback on their aniso-accommodative response and to alert the investigator of a monocular blur suppression response; (2) a training period was provided; (3) in the subjective method, each eye's stigma was positioned near the dichoptic letter viewed by the other eye. By this method, a true aniso-accommodative response could be differentiated from successive consensual responses; (4) a large range of aniso-accommodative stimuli was used, 0.50-3.0 D, presented in incremental steps of 0.5 D, allowing measurement of an average 0.75 D aniso-accommodative response for the highest (3.0 D) aniso-accommodative stimulus; (5) aniso-accommodation was measured as a function of viewing distance. For four of seven subjects, the gain of the aniso-accommodative response was significantly greater at near than at far viewing distances; (6) aniso-accommodation was confirmed objectively with measures of the response to steady state and step aniso-accommodative stimuli, using a binocular SRI Dual Purkinje Eye Tracker Optometer System. The aniso-accommodative response to step stimuli showed a very long latency period (about 11 s) and a response time of 4.5 s. A potential benefit of aniso-accommodation would be to overcome small amounts of uncorrected anisometropic refractive error. This would preserve fine stereo acuity which is impaired by unequal intraocular image contrast. Aniso-accommodation also may provide an appropriate efferent feedback signal for each eye's unique refractive error which could be used to guide developmental isometropization (attainment of equal refractive error in the two eyes.).

Accommodation, Ocular↗
PubMed1998

Retinal image quality in the human eye as a function of the accommodation.

The changes in the retinal image quality with accommodation in the human eye were studied by using a near-infrared double-pass apparatus. A slightly better modulation transfer function (MTF) in the unaccommodated eye with respect to the accommodated eye was found when using an artificial pupil with a fixed diameter. The technique allows the estimation of the MTF of the accommodated eye discounting the effect of the accommodative defocus error. Most of the reduction found in the MTF with accommodation could be explained in terms of the accommodative defocusing error. However, the shape of the retinal images clearly changes with accommodation, indicating that other aberrations are also altered with accommodation. In general, the double-pass image for the accommodated eye tends to be more symmetric than that of the unaccommodated eye. This is probably due to either a decrease in the amount of coma-like aberrations with accommodation or to an increase of other symmetric aberrations, such as defocus or spherical aberration, that hide the asymmetries present in the retinal image of the unaccommodated eye.

Accommodation, Ocular↗
PubMed1998

A model of the contribution of oculomotor and optical factors to emmetropization and myopia.

The purpose of this work was to investigate quantitatively the interactions between accommodation, vergence and a mechanism of emmetropization driven by optical blur within the retinal image with a view to developing a model that provides an explanation of both normal emmetropization and near-work associated myopia. The simulations of the change in the refractive state of the eye over time that derive from this model indicate that optical regulation of eye growth can result in emmetropization, i.e. a progressive reduction in refractive errors over time leading towards emmetropia. This occurs when viewing conditions involve a preponderance of distance work. With increasing near work, the model predicts that the refraction of the eyes will converge towards myopia. In keeping with the previously reported associations of myopia with esophoria, poor accommodation function and high AC/A ratios, these conditions increase the amount of myopia produced under intensive near viewing conditions but do not lead to myopia during mainly distance viewing. This model provides quantitative validation of the hypothesis that the epidemiological association between myopia and increased nearwork may be caused by a disturbance of normal emmetropization by steady state errors of accommodation. The same model can explain normal emmetropization, increasing myopia with increasing nearwork demands and the currently recognised oculomotor associations that have been reported to precede the development of myopia.

Accommodation, Ocular↗
PubMed1999

Optics of the developing fish eye: comparisons of Matthiessen's ratio and the focal length of the lens in the black bream Acanthopagrus butcheri (Sparidae, Teleostei).

Matthiessen's ratio (distance from centre of lens to retina:lens radius) was measured in developing black bream, Acanthopagrus butcheri (Sparidae, Teleostei). The value decreased over the first 10 days post-hatch from 3.6 to 2.3 along the nasal and from four to 2.6 along temporal axis. Coincidentally, there was a decrease in the focal ratio of the lens (focal length:lens radius). Morphologically, the accommodatory retractor lentis muscle appeared to become functional between 10-12 days post-hatch. The results suggest that a higher focal ratio compensates for the relatively high Matthiessen's ratio brought about by constraints of small eye size during early development. Combined with differences in axial length, this provides a means for larval fish to focus images from different distances prior to the ability to accommodate.

Accommodation, Ocular↗
PubMed1999

Characteristics of accommodation toward apparent depth.

This paper deals with characteristics of accommodation evoked by perceived depth sensation and the dynamic relationship between accommodation and vergence, applying newly developed optical measurement apparatuses. A total of five subjects looked at three different two-dimensional stimuli and two different three-dimensional stimuli; namely a real image and a stereoscopic image. With regard to the two-dimensional stimuli, a manifest accommodation without any accompanying vergence was found because of an apparent depth sensation even though the target distance was kept constant. With regard to the three-dimensional stimuli, larger accommodation and clear vergence were evoked because of binocular parallax and a stronger depth sensation. As for the stereoscopic image, a manifest overshoot (the accommodation peaked first and receded considerably) was found while the vergence remained constant. On the other hand, the overshoot of accommodation was smaller when subjects were watching the real image. These results reveal that brain depth perception has a higher effect on accommodation than expected. The relationship of accommodation and vergence toward the stereoscopic image suggests a reason why severe visual fatigue is commonly experienced by many viewers using stereoscopic displays. It has also paved the way for the numerical analysis of the oculomotor triad system.

Accommodation, Ocular↗
PubMed1999

Mechanics of accommodation of the human eye.

The classical Helmholtz theory of accommodation has, over the years, not gone unchallenged and most recently has been opposed by Schachar at al. (1993) (Annals of Ophthalmology, 25 (1) 5-9) who suggest that increasing the zonular tension increases rather than decreases the power of the lens. This view is supported by a numerical analysis of the lens based on a linearised form of the governing equations. We propose in this paper an alternative numerical model in which the geometric non-linear behaviour of the lens is explicitly included. Our results differ from those of Schachar et al. (1993) and are consistent with the classical Helmholtz mechanism.

Accommodation, Ocular↗
PubMed1999

The growing eye: an autofocus system that works on very poor images.

It is unknown which retinal image features are analyzed to control axial eye growth and refractive development. On the other hand, identification of these features is fundamental for the understanding of visually acquired refractive errors. Cyclopleged chicks were individually kept in the center of a drum with only one viewing distance possible. Defocusing spectacle lenses were used to stimulate the retina with defined defocus of similar magnitude but different sign. If spatial frequency content and contrast were the only cues analyzed by the retina, all chicks should have become myopic. However, compensatory eye growth was still always in the right direction. The most likely cues for emmetropization, spatial frequency content and image contrast, do therefore not correlate with the elongation of the eye. Rather, the sign of defocus was extracted even from very poor images.

Accommodation, Ocular↗
PubMed1999

A wide-angle gradient index optical model of the crystalline lens and eye of the octopus.

Cephalopods and fish have had no common ancestor since the Cambrian, and their eyes are a classic example of convergent evolution. The octopus has no cornea, and immerson renders the trout cornea optically ineffective. As a result, the nearly spherical lens is responsible for all refraction in these eyes. In spite of the fact that the octopus lens consists of two joined parts, while the trout lens consists of one part, we show here that their optical properties are very similar. An index gradient bends rays within these lenses, adding power and correcting spherical aberration. High spherical symmetry in both lenses strongly reduces other monochromatic aberrations and yields a wide field of vision, advantageous in attack and evasion. The octopus Mattheissen's ratio, 2.83, an inverse measure of light-gathering power, lies above the trout value of 2.38 but within the range of values reported for fish. Strong uncorrected longitudinal chromatic aberration is nearly identical in both animals as a result of similar lens protein optical properties, and will limit resolution. We discuss how animal lifestyle requirements and lens material properties influence the design of these eyes.

Accommodation, Ocular↗
PubMed1999

Adaptation to temporal modulation can enhance differential speed sensitivity.

During adaptation to a moving pattern, perceived speed decreases. Thus we know that the adapted visual system does not simply code the absolute speed of a stimulus. We hypothesised that adaptation to a moving stimulus serves to optimise coding of changes in speed at the expense of maintaining an accurate representation of absolute speed. In this case we would expect discrimination of speeds around the adapted level to be preserved or enhanced by motion adaptation. Speed discrimination thresholds were measured for sinusoidal gratings (1.25 cpd; 12.5 Hz; 40% contrast) with and without prior adaptation to moving, static, and flickering stimuli. After adaptation to motion in the same direction as the test, seven of eight subjects showed a reduction of perceived speed in the adapted region, and seven showed enhanced discrimination. Similar effects were found for adaptation to motion in the opposite direction to the test and to counter-phase flicker, suggesting that adaptation is driven by temporal modulation rather than by motion per se. We conclude that motion adaptation preserves or enhances differential speed sensitivity at the expense of an accurate representation of absolute speed.

Accommodation, Ocular↗
PubMed2000

Monochromatic aberrations in the accommodated human eye.

The wave-front aberration of the human eye was measured for eight subjects using a spatially resolved refractometer (a psychophysical ray-tracing test). The eyes were undilated and presented with accommodative stimuli varying from 0 to -6 diopters. Monochromatic wave-front aberrations tend to increase with increasing levels of accommodation, although there are substantial individual variations in the actual change in the wave-front aberration. While spherical aberration always decreased with increasing accommodation, it did not change from positive to negative for every observer. The direction and amount of change in fourth order aberrations varied between observers. Aberrations with orders higher than fourth are at a minimum near the resting state of accommodation. The accommodation induced change in wavefront aberration was not strongly related to the total amount of aberration in the eight eyes studied.

Accommodation, Ocular↗
PubMed2000

Spontaneous fluctuations in pupil size are not triggered by lens accommodation.

Fluctuations in pupil size and lens accommodation are measured concurrently under open loop conditions, constant illumination and far fixation. In 12/17 trials no correlation was measured between the fluctuations in pupil size and lens accommodation. For the remaining 5/17 trials no lag was observed between the changes in pupil size and lens accommodation indicating that this correlation does not arise as a consequence of a near response. These observations suggest that under conditions of constant illumination and far fixation, the supranuclear centers controlling the near response are not active.

Accommodation, Ocular↗
PubMed2000

Image formation by bifocal lenses in a trilobite eye?

In this work we report on a unique and ancient type of eye, in which the lower surface of the upper calcite lens units possessed an enigmatic central bulge making the dioptric apparatus similar to a bifocal lens. This eye belonged to the trilobite Dalmanitina socialis, which became extinct several hundred million years ago. As far as we know, image formation by bifocal lenses of this kind did/does not occur in any other ancient or modern animal visual system. We suggest that the function of these bifocal lenses may be to enable the trilobite to see simultaneously both very near (e.g. floating food particles and tiny preys) and far (e.g. sea floor, conspecifics, or approaching enemies) in the optical environment through the central and peripheral lens region, respectively. This was the only reasonable function we could find to explain the puzzling lens shape. We admit that it is not clear whether bifocality was necessary for the animal studied. We show that the misleading and accidental resemblance of an erroneous correcting lens surface (designed by René DesCartes in 1637 [DesCartes, R. (1637). Oeuvres de DesCartes. La Géometrie. Livre 2. pp. 134. J. Maire, Leyden] to the correcting interface in the compound Dalmanitina lens may be the reason why the earlier students of the Dalmanitina lens did not recognize its possible bifocality.

Accommodation, Ocular↗