Accommodative Accuracy Research Articles

Minus Lens Stimulated Accommodative Lag as a Function of Age

Previous studies in children report reduced accommodative responses with minus lens-stimulated demands compared with proximal demands. This study seeks to identify age-related changes in accommodative lag with minus lens-stimulated demands in subjects from preschoolers to adults. Accommodative responses were measured in 101 subjects (3 to 40 years) with at least 10 subjects in each 5-year age bin. Subjects monocularly viewed a high-contrast target at 33.3 cm on the near-point rod of the Grand Seiko autorefractor. Measurements of refraction were taken as the subject viewed the target. Accommodative lag was defined as the difference between demand and measured response. Four additional demands were tested by introducing minus lenses [-1 to -4 diopter (D)] in the spectacle plane of the viewing eye. Maximum accommodative amplitudes were determined by presenting additional lenses until the measured response plateaued or peaked. Accommodative demands and responses were adjusted to the corneal plane. RESULTS.: Accommodative lag showed a significant linear decrease with age for subjects 3 to 20 years for each of the first four demands (3 D, 3.92 D, 4.80 D, 5.67 D, p <or= 0.013) and approached significance for the largest demand (6.52 D, p = 0.053). For the entire group, accommodative lag increased with increasing stimulus demand, with the largest increase occurring for subjects aged 30 to 40 years as stimulus demands approached the subjects' maximum amplitude. For subjects aged 3 to 20 years, multilevel modeling analysis revealed a significant relationship between age and lag (p < 0.0001) and a significant relationship between maximum amplitude and the increase in lag per unit increase in stimulus demand (p = 0.0032). These findings suggest the accuracy of accommodation to minus lens-stimulated accommodation improves throughout the school years and that the degree to which lag increases with increasing demand is related to maximum accommodative amplitude rather than age.

Measurement of Visual Strain in Radiologists

The authors hypothesized that the current practice of radiology produces oculomotor fatigue that reduces diagnostic accuracy. Testing this hypothesis required an ability to measure eyestrain. This capability was developed by measuring the visual accommodation of radiologists before and after diagnostic viewing work using an autorefractor that was capable of making multiple measurements of accommodation per second. Three radiologists and three residents focused on a simple target placed at near to far distances while accommodation was measured. The target distances varied from 20 to 183 cm from the eye. The data were collected prior to and after a day of digital diagnostic viewing. The results indicated that accommodation at near distances was significantly worse overall compared to far distances and was significantly worse after a day of digital reading at all distances. Because diagnostic image interpretation is performed at near viewing distances, this inability to maintain focus on an image could affect diagnostic accuracy. As expected, younger residents had better accommodative accuracy than older radiologists.

The effect of positive‐lens addition and base‐in prism on accommodation accuracy and near horizontal phoria in Chinese myopic children

The effect of positive-lens addition (0, +0.75, +1.50, +2.25, +3.00 D each eye) and base-in prism power (0, 1.5, 3 Delta each eye) on both near focusing errors and latent horizontal deviations was evaluated in 29 Chinese myopic children (age: 10.3 +/- 1.9 years, refractive error: -2.73 +/- 1.31 D). Accommodation response and phoria were measured by the Shin-Nippon auto-refractor (right eye) and Howell-Dwyer near phoria card at 33 cm with each of the 15 lens/prism combinations in random order. The initial accommodative error was -0.96 +/- 0.67 D (lag) and near phoria was -0.8 +/- 5.0 Delta (exophoria). The positive-lens addition decreased the accommodative lag but increased the exophoria as the power increased (e.g. up to -9.1 +/- 4.1 Delta with +3 D). A 6-Delta base-in prism totally controlled the exophoria induced by a +1.50 D addition (-0.3 +/- 4.3 Delta), but the accommodative lag was still considerable (-0.69 +/- 0.54 D). In the graphical analysis of the data, a lens addition of +2.25 D combined with a 6-Delta base-in prism minimized both the lag and lens-induced exophoria to -0.33 D and -2.4 Delta respectively (regression analysis). This lens and prism combination decreased the lens-induced exophoria by 4.5 Delta compared with that measured with +2.25 D alone (-2.4 Delta vs -6.9 Delta). The results suggest that incorporating near base-in prism when prescribing bifocal lenses for young progressing myopes with exophoria could reduce the positive lens-induced oculomotor imbalance.

Amplitude, facility and accuracy of accommodation in a primary school population

Vision screening at an early age in children is important as it can identify aspects in the visual system that may need to be managed to enable a child to function optimally at school. The National School Vision Screening Programme was discontinued in many provinces of South Africa, often due to a lack of financial resources or adequately trained personnel. This action has resulted in the majority of children not having a visual examination during their school career. In a few instances where vision screenings are performed, these are usually limited to visual acuity (VA) evaluation alone; an endeavour that may miss many significant visual problems. The purpose of this article is to highlight the need for vision screening to be conducted in schools and for the screening protocols to include the various accommodative tests. A retrospective analysis of the amplitudes of accommodation, accommodative facility and accuracy of accommodation findingsfrom a primary school vision screening of 264 children between 6 and 13 years was undertaken in this study. Data was captured and analysed with Microsoft Excel. The ages of the children ranged from 6 to 13 years with a mean of 9.38 years (SD = 1.85). One hundred and thirty eight (52.3%) were males and 126 (47.7%) females. A significant number of the children failed the monocular accommodative amplitude tests (24%), binocular accommodative amplitude test (26%), the accommodative facility (30%) and the MEM test (27%). These results highlight the need for a more comprehensive vision screening exercise rather than VA alone as this approach would have missed more than a quarter of the children who had other visual problems that could impact on their ability to perform optimally at school.

The stability of steady state accommodation in human infants

Retinal image quality in infants is largely determined by the accuracy and the stability of their accommodative responses. Although the accuracy of infants' accommodation has been investigated previously, little is known about the stability of their responses. We performed two experiments that characterized the stability of infants' steady state accommodation. Analyses were performed in the time domain (root mean square [RMS] deviation) and in the frequency domain (spectral analysis). In Experiment 1, accommodation responses were recorded for a period of 3 s from the left eye of four groups of infants (8-10, 11-13, 14-19, and 20-30 weeks of age) and eight prepresbyopic adults while they focused on a small toy placed at a dioptric viewing distance of 1.0 D (at 1 m). In Experiment 2, accommodation responses were recorded for a period of 14 s from the left eye of a group of 8- to 12-week-old infants and six prepresbyopic adults while they focused on a cartoon image placed at three different dioptric viewing distances (1.25, 2.0, and 3.0 D). The data, collected using a photorefractor sampling at 25 Hz, showed two important characteristics. First, the RMS deviations and the power were quantitatively similar across different infant age groups, and they were significantly larger in infants than in adults. Second, the overall and relative power also increased with the dioptric viewing distance both in infants and adults. At all three dioptric viewing distances, the measures of power were larger in infants than in adults. These data demonstrate that infants' accommodative responses contain instabilities that are qualitatively very similar to those observed in adults. However, the larger RMS deviations suggest that infants are likely to experience larger fluctuations in retinal image quality than adults.

To Emmetropize or Not to Emmetropize? The Question for Hyperopic Development

Emmetropization appears to be a rapid process, occurring in the first year of life. Failure to emmetropize leaves about 2 to 8% of children with potentially clinically significant hyperopia after infancy. Uncorrected hyperopia in childhood has a negative impact on distance acuity and the accuracy of the accommodative response for some unknown number of children. The clinical "gray zone" for these problems as judged by distance refractive error alone might begin somewhere around +2.00 to +3.00 D. Use of a refractive correction seems to improve distance acuity and the accuracy of accommodation. Clinicians' reluctance to prescribe hyperopic corrections to children to improve visual performance might be unwarranted. If emmetropization is largely complete, if defocus has only a minor effect on the development of refractive error in infancy or childhood, and if the hyperopic eye is already growing longer but not moving toward emmetropia, then there may be little reason to either wait or be concerned about interfering with emmetropization that may never happen. The immediate visual benefit may outweigh these concerns.

Association Between Accommodative Accuracy, Hypermetropia, and Strabismus in Children with Down’s Syndrome

A significant proportion of children with Down's syndrome have been shown to have reduced accommodation. The purpose of this study was to investigate any association between reduced accommodation and refractive error, strabismus, visual acuity, and other ocular parameters. Subjects were children with Down's syndrome enrolled in a longitudinal cohort to monitor visual development. Twenty-seven children with accurate accommodation were age-matched to children with reduced accommodation based on their most recent assessment for which a full, reliable data set was available. Each child was used only once for matching. Cross-sectional ocular and visual data were analyzed using chi or Fisher's exact test, or the Mann-Whitney U test for (non-normally distributed) quantitative data. Children with under-accommodation were statistically more likely to have moderate/high hypermetropia (> or = +3.00 D) and to be strabismic (most with esotropia). No significant difference between the groups was found for any other ocular parameters. This study demonstrates the marked association between under-accommodation, hypermetropia, and strabismus in children with Down's syndrome. No causal relation can be demonstrated with these data, but findings suggest that the link between under-accommodation and hypermetropia (and between accurate accommodation and emmetropia) is present in early infancy.

Accuracy of accommodation in heterophoric patients: testing an interaction model in a large clinical sample

A model of the cross-link interactions between accommodation and convergence predicted that heterophoria can induce large accommodation errors (Schor, Ophthalmic Physiol. Opt. 1999;19:134-150). In 99 consecutive patients with intermittent tropia or decompensated phoria, we tested these interactions by comparing their accommodative responses to a 2.50-D target under binocular fused conditions (BFC) and monocular occluded conditions (MOC). The accommodative response in BFC frequently differed from that in MOC. The magnitude of the accommodative errors in BFC, ranging from an accommodative lag of 1.80 D (in an esophoric patient) to an accommodative lead of 1.56 D (in an exophoric patient), was correlated with distance heterophoria and uncorrected refractive errors. These results indicate that heterophoria affects the accuracy of accommodation to various degrees, as the model predicted, and that an accommodative error larger than the depth of focus of the eye occurs in exchange for binocular single vision in some heterophoric patients.

The Effect of Common Reductions in Letter Size and Contrast on Accommodation Responses in Young Adult Myopes and Emmetropes

Accommodation errors during reading and the subsequent near work-induced transient myopia (NITM) that occurs have been implicated in the development and progression of myopia. This study investigated the effects of two letter variables, size and contrast, on accommodation accuracy during the near task and on NITM and its subsequent decay. These were varied so as to mimic what might occur when students photocopy and reduce reading materials. Based on their refractive errors, young adult subjects (18-25 years) were classified into three groups: emmetropes (n = 19), stable myopes (n = 17), and progressing myopes (n = 17). Three print sizes (N4, N6, and N8) and two print contrasts (90% and 60%) were used to give six different reading targets. Targets were presented in random order at 25 cm (4 D demand) and the text read for comprehension for 3 minutes. For each target, accommodation accuracy and NITM and its decay were measured using the free space Shin-Nippon SRW-5000 autorefractor. When data for all subjects were pooled, there was a significant effect of letter size (p = 0.030) but not contrast (p = 0.898) on accommodation accuracy; however, differences were small and unlikely to be clinically relevant. NITM (p = 0.033) and its decay (p = 0.012) also varied with letter size. NITM was greater and decay longer for larger letters. We found no effect of refractive error group on accommodation accuracy. In contrast, there was a significant difference in the magnitude of NITM and its decay for emmetropic and myopic subjects (although no effect of progression status); myopes had larger NITM values and longer decay times to baseline than emmetropes (NITM myopes: 0.37 +/- 0.14D vs. emmetropes: 0.19 +/- 0.17 D, p = 0.005; decay time myopes: 15.12 +/- 6.58 seconds vs. emmetropes 7.10 +/- 4.82 seconds, p = 0.0045). The differences in NITM and its decay between the two refractive groups were of similar magnitude for all six combinations of letter size and contrast. Our data do not support the suggestion that common reductions in letter size or contrast of reading material (as might occur for photocopied reading materials) cause greater accommodation inaccuracy and greater near work-induced adaptation effects that would exacerbate myopia development in young adults.

Convergence accommodation to convergence (CA/C) ratio in patients with intermittent exotropia and decompensated exophoria.

To evaluate the convergence accommodation to convergence (CA/C) ratio in strabismic patients and to clarify its clinical implications. Seventy-eight consecutive patients (mean age: 12.9 +/- 6.0 years) with intermittent exotropia and decompensated exophoria who showed binocular fusion at least at near viewing were recruited. The CA/C ratio was estimated by measuring accommodative responses induced by horizontal prisms with different magnitudes under accommodation feedback open-loop conditions. The CA/C ratios were compared with accommodative convergence to accommodation (AC/A) ratios and other clinical parameters. A linear regression analysis indicated that the mean (+/-SD) CA/C ratio was 0.080 +/- 0.043 D/prism diopter or 0.48 +/- 0.26 D/meter angle. There was no inverse or reciprocal relationship between CA/C and AC/A ratios. The patients with lower CA/C ratios tended to have smaller tonic accommodation under binocular viewing conditions and larger exodeviation at near viewing. The CA/C ratio, like the AC/A ratio, is an independent parameter that characterizes clinical features. A lower CA/C may be beneficial for the vergence control system to compensate for ocular misalignment with minimum degradation of accommodation accuracy.

The autonomic control of accommodation and implications for human myopia development: a review.

Prolonged nearwork has long been associated with myopia development, however, there is no well described linking mechanism. One theory suggests that if accommodation accuracy during nearwork is not maintained, the defocused retinal image leads to myopia development. Here we review the findings of research aimed at determining whether the autonomic inputs to the ciliary smooth muscle are involved in this type of environmental myopia. We examine whether an autonomic imbalance could be a precursor to axial elongation and the resulting myopia. Accommodation responses, such as tonic accommodation and nearwork-induced accommodative adaptation, as a function of refractive error, are described in relation to an autonomic imbalance model. The collective results of this research point to anomalous accommodation responses, possibly as a result of underlying anomalous autonomic input to the ciliary muscle, being involved in myopia development and progression.

Refractive error, cognitive demand and nearwork-induced transient myopia

Purpose. Whereas many previous studies have identified the association between sustained near work and myopia, few have assessed the influence of concomitant levels of cognitive effort. This study investigates the effect of cognitive effort on near-work induced transient myopia (NITM). Methods. Subjects comprised of six early onset myopes (EOM; mean age 23.7 yrs; mean onset 10.8 yrs), six late-onset myopes (LOM; mean age 23.2 yrs; mean onset 20.0 yrs) and six emmetropes (EMM; mean age 23.8 yrs). Dynamic, monocular, ocular accommodation was measured with the Shin-Nippon SRW-5000 autorefractor. Subjects engaged passively or actively in a 5 minute arithmetic sum checking task presented monocularly on an LCD monitor via a Badal optical system. In all conditions the task was initially located at near (4.50 D) and immediately following the task instantaneously changed to far (0.00 D) for a further 5 minutes. The combinations of active (A) and passive (P) cognition were randomly allocated as P:P; A:P; A:A; P:A. Results. For the initial near task, LOMs were shown to have a significantly less accurate accommodative response than either EOMs or EMMs (p < 0.001). For the far task, post hoc analyses for refraction identified EOMs as demonstrating significant NITM compared to LOMs (p < 0.05), who in turn showed greater NITM than EMMs (p < 0.001). The data show that for EOMs the level of cognitive activity operating during the near and far tasks determines the persistence of NITM; persistence being maximal when active cognition at near is followed by passive cognition at far. Conclusions. Compared with EMMs, EOMs and LOMs are particularly susceptible to NITM such that sustained near vision reduces subsequent accommodative accuracy for far vision. It is speculated that the marked NITM found in EOM may be a consequence of the crystalline lens thinning shown to be a developmental feature of EOM. Whereas the role of small amounts of retinal defocus in myopigenesis remains equivocal, the results show that account needs to be taken of cognitive demand in assessing phenomena such as NITM.

Does accommodation inaccuracy matter in children with Down syndrome?

Purpose: Almost 57% of children with Down syndrome have significant refractive error (hypermetropia ≥+3.00 D, myopia ≥−1.00 D) (Woodhouse et al., 1997); they are 17 times more likely to have significant long or short sight than their typically developing peers. Also, for the age range 4.5 months to 11.1 years, 34% of children with Down syndrome have strabismus, compared with 4–7.5% of control children (British College of Optometrists, 1991–94). In addition, 82% of our cohort of children with Down syndrome (n = 100) have reduced accommodation at near. In order to investigate the influence of accommodation accuracy on refractive error and strabismus, this study compared the above parameters in children with Down syndrome who accommodated accurately with those who did not.Methods: Accommodation was measured using dynamic retinoscopy technique at testing distances of 25, 16.6 and 10 cm. An accommodative lag of plano to 0.75 D is normal for typically developing children (Rouse et al., 1984). Accurate accommodation for our cohort was therefore defined as ≤0.75 D lag of accommodation (at a minimum of two of three testing distances). Children with Down syndrome who accommodated accurately were age matched with a child with inaccurate accommodation, based on their most recent visit for which a full data set was available.Results: Chi‐square analysis of the data for the two groups revealed that there was a greater number of children with significant hypermetropia (p = 0.003) and strabismus (p = 0.003, Fisher's exact test) in the under‐accommodating group.Conclusion: Children with Down syndrome who under‐accommodate are more likely to be strabismic and have significant hypermetropia.

Accommodation responses in late‐onset myopia are improved by spatiotopic stimulation

Purpose: Inaccurate accommodation responses to retinotopic stimuli have been shown in late‐onset myopia (LOM). The aim of this experiment was to determine whether spatiotopic (perceived proximity) and binocular stimulation would improve the quality of accommodation responses in LOM subjects.Methods: Ten emmetropic subjects (mean age 22.3 ± 2.3 years) and 10 LOM subjects (mean age 21.8 ± 1.5 years) participated. A specially modified Canon R‐1 infrared optometer was used to measure accommodation responses. Three conditions were presented: (1) Monocular blur‐only stimulus, (2) Monocular free‐space stimulus, and (3) Binocular free‐space stimulus. Each condition was presented on a separate day to avoid fatigue effects.Results: Accommodation stimulus–response curves in the LOM group showed improved accuracy with the spatiotopic stimulus. Microfluctuations were significantly (p &lt; 0.01) smaller in magnitude in the LOM group with the spatiotopic stimulus, because of reductions in the low frequency components of the fluctuations. A significant (p &lt; 0.01) increase in the percentage of correct step accommodation responses for the spatiotopic stimulus was found in the LOM group. However, step response times in the LOM group with the spatiotopic stimulus were not significantly different than those found with the retinotopic stimulus, and were significantly (p &lt; 0.01) longer than those found in the emmetropic group.Conclusions: (1) Spatiotopic stimulation produces significant improvements in static accommodative accuracy in LOM subjects compared with retinotopic stimulation. (2) The quality of dynamic accommodation responses in LOM subjects was found to improve significantly with spatiotopic stimulation. (3) LOM subjects appear to rely upon proximity driven stimuli to improve accommodative response accuracy.

Irlen lenses do not improve accommodative accuracy at near.

The purpose of the experiment was to determine the influence of Irlen lenses on steady-state accommodation in successful users, as there is speculation that near accommodative dysfunction may be a factor in these patients. Monocular steady-state accommodation in six successful Irlen patients was assessed for near blur stimuli (2, 3, and 4 D) either with nonfiltered spectacle refractive correction or their spectrally broad-band filtered spectacle refractive correction. Accommodation was measured subjectively using a Hartinger coincidence optometer. There was no significant difference in mean level of accommodation between the two conditions. Additionally, there was a small but significant increase in accommodative variability with the filtered prescription. The use of Irlen correction did not have any positive effect on monocular steady-state accommodation at near.

Influence of Background Appearance on Visual Accommodation

Many experimental and real-world viewing situations provide a context in which the target stimulus is displayed against a background set at a different but determinate distance. Conversely, other situations occur where the background distance is indeterminate, i.e., a textureless background. There has been evidence accumulating over the past two decades to suggest that the assumption of accurate visual accommodation will not be sustained under all these circumstances. Although earlier assumptions held that the centrally located stimulus would determine the level of accommodation, this experiment tests that assumption by varying the cues to background distance (well-textured, lighted, distant background and the same background unilluminated) and the distance to the target stimulus. Two groups of six participants observed targets (2 deg.) at six distances (0.9, 1.8, 3.7, 7.3, 14.6 and 29.3 m) and their visual accommodation was measured with a laser optometer. Results indicated that the group viewing the visible distant background evidenced a more distant accommodative response with the typical lag of accommodation. These results indicate that conditions of accommodation in the natural environment may have a profound effect on accommodative accuracy. In turn, this inaccuracy has been shown by others to correlate with inaccuracies in the perception of size and distance. Inaccurate accommodation has been found to delay target detection appreciably as well. Ameliorative approaches are discussed.

Accommodation and flicker: evidence of a role for temporal cues in accommodation control?

Static accommodation responses to sinusoidal grating stimuli that displayed temporal modulations in luminance contrast (i.e. contrast flicker) were measured with a laser speckle optometer. The effects of a variety of temporal waveforms were investigated including square-wave modulations, sinusoidal modulations, and band-pass filtered noise. The effects of altering both the amplitude and the temporal frequency (0.4-30 Hz) of the contrast flicker and the spatial frequency of the stimulus (0.77-9.2 c/deg) were also examined. All the flicker waveforms investigated (square wave, sinusoidal and band-pass noise) reduced accommodative accuracy, the effect being most apparent at lower spatial frequencies (0.77-1.15 c/deg). With band-pass filtered noise the effects of flicker were most apparent with frequencies in the range 1-4 Hz, at both lower (0.4 Hz) and higher flicker frequencies (8-32 Hz) accommodations was less affected. It was found that flicker impaired accommodation under conditions where the contrast was at all times suprathreshold. This is incompatible with the proposal that flicker reduces accommodation responses because for part of each flicker cycle the stimulus was below threshold. However, these results are compatible with the alternative hypothesis that flicker impairs the ability of the accommodation system to utilize temporal cues such as those derived from the higher frequency component (1-2 Hz) of accommodative oscillations.

Maintenance of Correct Accommodations in Red, Green, and white Light Environments

This project explored the practical importance of ambient color as a concern for maintaining human visual accommodation. Correct accommodation and regression toward resting point accommodation were considered in broadband red, broadband green and white environments. The involvement of voluntary control of accommodation was manipulated by requiring extended performance on a difficult visual task across four light levels. Declining light levels and increased time-on-task were found to degrade the accuracy of accommodation, while manipulation of ambient color produced differences attributable to chromatic aberration. Differential abilities associated with red, green or white conditions were not apparent, as no statistically significant interactions were evidenced. Results of these and other related findings generally suggest that, allowing for chromatic aberration of the lens, the human eye maintains visual accommodation equally well across varied color conditions. Maintenance of correct accommodation and regression to the resting point of accommodation do not appear to be influenced by ambient color.

The effect of optical defocus on the accommodative accuracy for chromatic displays.

Blur, probably arising from focusing inaccuracies, has been cited in ergonomic and vision literature as one of the causes of visual stress and decreased task efficiency in operation of video display units (VDUs). With the advent of colour coding in electro-optical displays, the need for a detailed quantification of focusing responses to chromatic stimuli is particularly important because of the influence of the chromatic aberration present in ocular optics on the focusing response of the eye. In this regard, we used a computer-aided laser speckle optometer system to measure the accommodative responses of 20 visually normal subjects, to brightness-matched monochromatic and multichromatic stimuli displayed on a high-resolution RGB monitor. We also investigated the effect of target size, target clarity, and viewing distance on the response accuracy of the focusing system of the eye. Our results show that while the accommodative responses are systematically influenced by target chromaticity, they are relatively independent of target size. There was no evidence of any anomalous focusing responses resulting from either target chromaticity or defocus that could account for the asthenopia frequently reported by VDU users. Furthermore, moderate levels of optical defocus did not drive accommodation into any visible hunting pattern for optimal focus or towards its tonic resting position. Implications of these findings in display designs are presented.

Accommodative accuracy to harmonically related complex grating patterns and their components.

Monocular steady-state accommodation was measured to harmonically related complex grating patterns and their components to determine the contribution of the various spatial waveforms to accommodative accuracy. Accommodative accuracy was greater for those waveforms containing summed odd-harmonic spatial components than for those waveforms with specific spatial components either removed or isolated. The results suggest that the contrast gradient produced by the summed waveforms, rather than the presence of any particular subset of spatial frequency components per se, was a critical factor for accurate accommodation.