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Depth-of-focus of the human eye: theory and clinical implications.

The depth-of-focus, or the perceptual tolerance of the human eye to retinal defocus, is important to and imbedded in many aspects of clinical refraction and physiological optics. Although the depth-of-focus is a common concept in classical optics, there is relatively little detailed discussion of its implications as related to normal vision function and to vision anomalies. With current advances in refractive surgery and ophthalmic lens design, the demand for knowledge in this topic is both timely and important. This review of our current understanding of the depth-of-focus should prove to be useful to clinicians, researchers, and students as an introduction to the subject. Two areas will be considered: 1) basic definitions of and factors affecting the depth-of-focus, and 2) its contemporary clinical implications.

Accommodation, Ocular↗
PubMed2004

Dynamic aspects of accommodation: age and presbyopia.

There has been no comprehensive study involving each of the primary dynamic components of accommodation in the same cohort as related to age and presbyopic onset; furthermore, the current findings are equivocal. Dynamic monocular components of accommodation (latency, time constant, peak velocity/amplitude relationship, and microfluctuations) were assessed objectively using an infrared optometer within the linear region of accommodation in 30 visually-normal human subjects aged 21-50 years. The time constant and the peak velocity/amplitude relationship did not change with age. However, latency progressively increased, and microfluctuation amplitude and frequency progressively decreased, with increasing age. The invariance in time constant suggests that the gross biomechanical aspects of the lens and related structures in the remaining linear region are relatively unaffected by age. In contrast, the decrease in microfluctuation activity with age suggests more subtle alterations in the biomechanical aspects of the lens to these very small perturbations, such as a response amplitude non-linearity. With respect to neurologic control, the progressive latency increase suggests a processing delay of the blur input, and this is consistent with age-related changes in reaction time measures. The lack of any age-related changes in the peak velocity/amplitude relationship implies normalcy of central and peripheral neuromotor control, as well as grossly normal first-order lens biomechanics, in this linear response region. The results are consistent with the Hess-Gullstrand theory of presbyopia.

Accommodation, Ocular↗
PubMed2003

Amplitude dependent accommodative dynamics in humans.

Dynamics of accommodation (far-to-near focus) and disaccommodation (near-to-far focus) are described as a function of response amplitude. Accommodative responses to step stimuli of various amplitudes presented in real space were measured in eight 20-30 year old subjects. Responses were fitted with exponential functions to determine amplitude, time constant and peak velocity. Despite the intersubject variability, the results show that time constants of accommodation and peak velocity of disaccommodation increase with amplitude in all subjects. The dynamics of accommodation and disaccommodation are dependent on amplitude, but have different properties in each case.

Accommodation, Ocular↗
PubMed2004

The role of short-wavelength sensitive cones and chromatic aberration in the response to stationary and step accommodation stimuli.

The aim of the experiment was to test for a contribution from short-wavelength sensitive cones to the static and step accommodation response, to compare responses from short and long- plus middle-wavelength sensitive cone types, and to examine the contribution of a signal from longitudinal chromatic aberration to the accommodation response. Accommodation was monitored continuously (eight subjects) to a square-wave grating (2.2 c/d; 0.57 contrast) in a Badal optometer. The grating stepped (1.00 D) randomly towards or away from the eye from a starting position of 2.00 D. Five illumination conditions were used to isolate cone responses, and combine them with or without longitudinal chromatic aberration. Accuracy of the response before the step, step amplitude, latencies and time-constants, were compared between conditions using single factor ANOVA and t-test comparisons. Both S-cones and LM-cones mediated static and step accommodation responses. S-cone contrast drives "static" accommodation for near, but the S-cone response is too slow to influence step dynamics when LM-cones participate.

Accommodation, Ocular↗
PubMed2004

Depth-of-focus of the human eye in the near retinal periphery.

Although the depth-of-focus in the foveal region has been well investigated, knowledge regarding the effect of retinal eccentricity on blur detection and sensitivity is limited. In the present study, the depth-of-focus at the fovea and in the near retinal periphery (0 degrees -8 degrees ) was assessed psychophysically in 7 human subjects using a 5 mm artificial pupil with accommodation paralyzed. The group mean total depth-of-focus progressively increased linearly from 0.89 D at the fovea to 3.51 D at a retinal eccentricity of 8 degrees at the rate of 0.29 D/degree, with response variability (S.E.M.) remaining relatively constant (+/-0.17 D). We speculate that the reduced detection and sensitivity to blur in the near periphery may be attributed to retinal topography, sharpness overconstancy, optical aberrations, and visual attention in peripheral vision.

Accommodation, Ocular↗
PubMed2004

Small foveal targets for studies of accommodation and the Stiles-Crawford effect.

The properties of small monochromatic targets as accommodative stimuli are not well understood. We used a dynamic optometer to record accommodation responses to monochromatic disc targets (1.0-27.3 min arc) and to a Maltese cross. Accommodation responded adequately to points as small as 13.6 min arc. The response to these small targets is relevant to the question of whether the Stiles-Crawford (SC) effect could provide a stimulus to accommodation. Previous studies have used pupil apodizing filters to neutralise the natural SC function and so determine how visual performance or accommodation is influenced by the SC effect. However, these filters cannot correct for known inhomogeneities in the SC function across the retina for extended targets. Therefore, we calculated the SC function inhomogeneities across the retinal image of a smaller 13.6-min arc target. Unfortunately, even this small target is too large to permit a homogenous SC function across its extent. Alternatives to the apodizing filter approach are discussed.

Accommodation, Ocular↗
PubMed2004

Accommodation responses to stimuli in cone contrast space.

The aim was to identify the cone contributions and pathways for reflex accommodation. Twelve illumination conditions were used to test specified locations in cone-contrast space. Accommodation was monitored continuously in a Badal optometer while the grating stimulus (2.2 c/d sine-wave; 0.27 modulation) moved sinusoidally (0.195 Hz) towards and away from the eye from a mean position of 2.00 D (+/-1.00 D). Mean accommodation level and dynamic gain and phase at 0.195 Hz were calculated. Mean accommodation level varied significantly when the long- and middle-wavelength cone contrast ratio was altered in both the luminance and chromatic quadrants of cone-contrast space. This experiment indicates that L- and M-cones contribute to luminance and chromatic signals that produce the accommodation response, most likely through magno-cellular and parvo-cellular pathways, respectively. The L:M cone weighting to the luminance pathway that mediates accommodation is 1.63:1. The amplitude and direction of the response depends on changes in chromatic contrast and luminance contrast signals that result from longitudinal chromatic aberration and defocus of the image.

Accommodation, Ocular↗
PubMed2004

Accommodation as a function of age and the linearity of the response dynamics.

The changes with age in the accommodation responses to dynamic stimuli can reveal useful information on the factors underlying presbyopia development. Analysis of the monocular accommodation responses of 19 normal observers (ages 18-49 years) to stimuli whose vergence varied sinusoidally with time at different temporal frequencies (peak-to-peak stimulus 1.33-2.38 D, at 0.05-1.00 Hz) showed that at all ages both the gain and phase of the response were essentially linear functions of the temporal frequency. Extrapolation of least-squares, regression line fits to the gain data for each subject gave the gain at zero frequency, G0, and the cut-off frequency, fc, at which the gain fell to zero. G0 reduced with age but fc remained essentially constant at about 1.7 Hz, up to at least the age of about 40. The magnitude of the response to step stimuli covering the same stimulus range was well correlated with the value of G0. The linear changes in phase lag with temporal frequency corresponded to simple time delays td. The time lag varied from close to zero for the youngest subjects to about 0.5 s for the subjects in their late forties. There was substantial variation between the responses of subjects of similar age: those subjects with high values of G0 also tended to have low values of td, both effects probably being due to the superior ability of some individuals to predict the sinusoidal changes in the accommodation stimulus. Comparison of theoretical step responses, derived by applying linear theory to the parameters obtained from the results for the sinusoidal stimuli, with the actual responses to unpredictable steps for the same subjects supports the view that prediction effects and other possible factors make linear theory inapplicable to this type of data. The results are discussed in the context of current ideas on the development of presbyopia: it is suggested that the constancy of fc with age is the result of the ciliary body maintaining its efficiency, whereas the fall in G0 and increase in td result from increases in lens rigidity.

Accommodation, Ocular↗
PubMed2005

Change in shape of the aging human crystalline lens with accommodation.

The objective was to measure the change in shape of the aging human crystalline eye lens in vivo during accommodation. Scheimpflug images were made of 65 subjects between 16 and 51 years of age, who were able to accommodate at least 1D. The Scheimpflug images were corrected for distortion due to the geometry of the camera and the refraction of the cornea and anterior lens surface, which is necessary to determine the real shape of the lens. To ensure accurate correction for the refraction of the anterior lens surface, the refractive index of the crystalline lens must be determined. Therefore, axial length was also measured, which made it possible to calculate the equivalent refractive index of the lens and possible changes in this index during accommodation. The results show that during accommodation there is a decrease in both the anterior and the posterior radius of the lens, although the change in mm per diopter of the latter is much smaller. The increase in lens thickness with accommodation is higher than the decrease in the anterior chamber depth, indicating that the posterior lens surface moves backwards with accommodation. During accommodation the anterior lens surface becomes more hyperbolic. Furthermore, an increase in the equivalent refractive index during accommodation was determined.

Accommodation, Ocular↗
PubMed2005

Dynamics of accommodative fatigue in rhesus monkeys and humans.

Changes in accommodative dynamics with repeated accommodation were studied in three anesthetized rhesus monkeys and two conscious humans. Maximum accommodation was centrally stimulated via the Edinger-Westphal nucleus in monkeys with a 4 s on, 4 s off paradigm (4 x 4) for 17 min, 4 x 1.5 for 27 min and 2 x 1 for 16 min. Humans accommodated repeatedly to visual targets (5 x 5; 5D and 2 x 2; 6D) for 30 min. In all cases, accommodation was sustained throughout. The anesthetized monkeys showed inter-individual variability in the extent of changes in accommodative dynamics over time while no systematic changes were detected in the human accommodative responses. Little accommodative fatigue was found compared to previous studies which have reported a complete loss of accommodation after 5 min of repeated stimulation in monkeys.

Accommodation, Ocular↗
PubMed2005

Interocular suppression is gated by interocular feature matching.

We present a new form of contrast masking in which the target is a patch of low spatial frequency grating (0.46 c/deg) and the mask is a dark thin ring that surrounds the centre of the target patch. In matching and detection experiments we found little or no effect for binocular presentation of mask and test stimuli. But when mask and test were presented briefly (33 or 200 ms) to different eyes (dichoptic presentation), masking was substantial. In a 'half-binocular' condition the test stimulus was presented to one eye, but the mask stimulus was presented to both eyes with zero-disparity. This produced masking effects intermediate to those found in dichoptic and full-binocular conditions. We suggest that interocular feature matching can attenuate the potency of interocular suppression, but unlike in previous work (McKee, S. P., Bravo, M. J., Taylor, D. G., & Legge, G. E. (1994) Stereo matching precedes dichoptic masking. Vision Research, 34, 1047) we do not invoke a special role for depth perception.

Accommodation, Ocular↗
PubMed2005

Accommodative state of young adults using reading spectacles.

We examined the accommodative state of young adults wearing +2D and +3D reading spectacles under normal conditions and with the elimination of accommodative cues. Subjects' refractions were measured with an infrared PowerRefractor. Power of the vertical meridian was recorded for subjects viewing far and near targets in free space and through a Badal lens apparatus with and without reading spectacles. Additionally, refractive measurements were taken after subjects wore +2D reading spectacles for 30 min (post-adaptation). In free viewing and viewing through the Badal lens, subjects uniformly over-accommodated relative to the target while wearing reading spectacles (i.e., with the spectacles, they focused at a plane in front of the target). Subjects in the first post-adaptation test showed no significant difference in accommodation between viewing a near target with and without +2D spectacles after having read with them for 30 min, though they had without post-adaptation. Subjects in the second post-adaptation test were not significantly differently accommodated before and after reading when binocularly viewing a near target with +2D reading spectacles. The results imply that no adaptation of the subjects' accommodative postures while viewing visual targets occurred as a result of a 1/2 h near work task with the spectacles. The over-accommodation of subjects using reading spectacles while they are performing visual tasks shows the necessity of measurement if their true accommodative posture is to be determined.

Accommodation, Ocular↗
PubMed2005

The association of wavefront aberration and accommodative lag in myopes.

Accommodative lags, induced by a target at 33 cm (distance-induced condition) and by a -3.0 D lens (lens-induced condition), and wavefront aberrations were measured in 27 young myopic eyes. The accommodative lags and Strehl ratios derived from the wavefront aberrations in myopes were compared with those from 57 emmetropes. Accommodation was measured using a Canon R-1 autorefractor, while aberrations were measured using a psychophysical ray-tracing technique. In accord with previous results, larger accommodative lags were found for the myopes than the emmetropes in both the lens-induced and distance-induced conditions. The mean Strehl ratio was smaller in the myopes (0.079) than the emmetropes (0.091); this difference approached significance (p = 0.055). In addition, for myopes the accommodative lag was significantly correlated with the Strehl ratio in the lens-induced condition (r = -0.45, p < 0.02) and approached significance in the distance-induced condition (r = -0.35, p = 0.07). No significant correlations were found for emmetropes. Possible reasons to account for these results are discussed.

Accommodation, Ocular↗
PubMed2004

Detection of the depth order of defocused images.

The sign of an accommodative response is provided by differences in chromatic aberration between under- and over-accommodated images. We asked whether these differences enable people to judge the depth order of two stimuli in the absence of other depth cues. Two vertical edges separated by an illuminated gap were presented at random relative distances. Exposure was brief, or prolonged with fixed or changing accommodation. The gap was illuminated with tungsten light or monochromatic light. Subjects could detect image blur with brief exposure for both types of light. But they could detect depth order only in tungsten light with long exposure, with or without changes in accommodation.

Accommodation, Ocular↗
PubMed2004

A pulse-step model of accommodation dynamics in the aging eye.

We have developed a dynamic model of accommodation that combines independent phasic-velocity and tonic-position neural signals to control position, velocity and acceleration properties of accommodative step responses. Phasic and tonic signals were obtained from neural integration of a fixed-height acceleration-pulse and variable-height velocity-step respectively to control independent acceleration and velocity properties of the step response. Duration and amplitude of the acceleration-pulse are increased with age to compensate for age-related increases of visco-elastic properties of the lens to maintain youthful velocity. The model illustrates a neural control strategy that is similar to the classical neural control model of step changes by the saccadic and vergence systems.

Accommodation, Ocular↗
PubMed2004

Accommodation with and without short-wavelength-sensitive cones and chromatic aberration.

Accommodation was monitored while observers (23) viewed a square-wave grating (2.2 cycles/deg; 0.53 contrast) in a Badal optometer. The grating moved sinusoidally (0.2 Hz) to provide a stimulus between -1.00 D and -3.00 D during trials lasting 40.96 s. There were three illumination conditions: 1. Monochromatic 550 nm light to stimulate long-wavelength-sensitive cones (L-cones) and medium-wavelength-sensitive cones (M-cones) without chromatic aberration; 2. Monochromatic 550 nm light+420 nm light to stimulate long-, medium- and short-wavelength-sensitive cones (S-cones) with longitudinal chromatic aberration (LCA); 3. Monochromatic 550 nm light+420 nm light to stimulate L-, M- and S-cones viewed through an achromatizing lens. In the presence of LCA mean dynamic gain decreased (p=0.0003; ANOVA) and mean accommodation level was reduced (p=0.001; ANOVA). The reduction in gain and increased lag of accommodation in the presence of LCA could result from a blue-yellow chromatic signal or from a larger depth-of-focus.

Accommodation, Ocular↗
PubMed2005

Sympathetic innervation of ciliary muscle and oculomotor function in emmetropic and myopic young adults.

PURPOSE: Evidence exists for an additional inhibitory accommodative control system mediated by the sympathetic branch of the autonomic nervous system (ANS). This work aims to show the relative prevalence of sympathetic inhibition in young emmetropic and myopic adults, and to evaluate the effect of sympathetic facility on accommodative and oculomotor function. METHODS: Profiling of ciliary muscle innervation was carried out in 58 young adult subjects (30 emmetropes, 14 early onset myopes, 14 late onset myopes) by examining post-task open-loop accommodation responses, recorded continuously by a modified open-view infrared optometer. Measurements of amplitude of accommodation, tonic accommodation, accommodative lag at near, AC/A ratio, and heterophoria at distance and near were made to establish a profile of oculomotor function. RESULTS: Evidence of sympathetic inhibitory facility in ciliary smooth muscle was observed in 27% of emmetropes, 21% of early-onset myopes and 29% of late-onset myopes. Twenty-six percent of all subjects demonstrated access to sympathetic facility. Closed-loop oculomotor function did not differ significantly between subjects with sympathetic facility, and those with sympathetic deficit. CONCLUSIONS: Emmetropic and myopic groups cannot be distinguished in terms of the relative proportions having access to sympathetic inhibition. Presence of sympathetic innervation does not have a significant effect on accommodative function under closed-loop viewing conditions.

Accommodation, Ocular↗