Biomedical subjects
M Rosenfield
Publications and source records attributed to M Rosenfield.
Effect of near-vision addition lenses on the accuracy of the accommodative response.
BACKGROUND: Addition lenses (adds) are frequently used to increase the accuracy of the near-vision focusing response, and may also play a role in the prevention or retardation of refractive error development. However, following the introduction of such a lens, if the accommodative response is reduced by an amount equal to the magnitude of the near add, then the resulting accommodative error (and degree of retinal defocus) would remain unchanged. METHODS: This study measured the accommodative response in 28 subjects while they viewed a near target binocularly, either through their distance refractive correction alone or this correction combined with a +0.75 D, +1.50 D, +2.00 D, or +2.50 D near add. RESULTS: The mean findings demonstrated a small (0.03 D) lead of accommodation to the near target through the distance correction, and monotonically increasing leads of accommodation with larger adds. Further, the additional lens power required to reduce the accommodative error to zero correlated significantly with the initial accommodative error. CONCLUSIONS: Near addition lenses may actually increase the degree of retinai defocus for individuals who manifest small accommodative errors.
Vergence adaptation in children and its clinical significance.
BACKGROUND AND PURPOSE: Vergence adaptation has not been well investigated in children even though it may contribute to binocular dysfunction and near work induced asthenopia. METHODS: We compared vergence adaptation in 18 children and 18 young adult subjects by assessing tonic vergence (TV) before and immediately after a period of sustained near fixation, by measuring heterophoria with a synoptophore through 0.5 mm binocular pinholes. Adaptation was induced by a reading task at 15 cm for a continuous 5 minute period. RESULTS: Mean pre-task TV values of 0.70 MA (Meter Angles) and 0.20 MA were observed for the children and young adults, respectively (p = 0.08). The initial mean vergence adaptation for children and adults was +0.45 MA and +0.11 MA, respectively (p = 0.001). CONCLUSION: The greater vergence adaptation observed in children may impact upon the clinical assessment of their binocular vision, especially heterophoria measurement which may require longer periods of dissociation than previously recommended, and might also ultimately be partly responsible for the predominant development of esodeviations during childhood
Vergence adaptation and the clinical AC/A ratio.
The accuracy of the gradient technique for measuring the clinical accommodative convergence to accommodation (AC/A) ratio is dependent upon obtaining veridical heterophoria measurements. However, previous studies have demonstrated that the sustained output of slow fusional vergence, which may take several minutes or even hours to decay, can bias heterophoria assessment. In the clinical setting, it is usual to estimate the AC/A ratio after just a few seconds of dissociation. This study investigated whether the slow fusional vergence response alters this crosslink ratio by comparing values of AC/A measured both before and immediately after a 1-hr period of continuous monocular occlusion. Sustained occlusion produced a significant change in near heterophoria in 10 out of the 21 subjects examined, but no significant change in AC/A was observed in either the adapting or non-adapting subgroups. Accordingly, while the sustained output of slow fusional vergence will influence clinical measurements of heterophoria, its presence does not alter the stimulus AC/A ratio significantly.
Accommodative error, adaptation and myopia.
Accommodative adaptation, resulting from the sustained output of slow blur-driven accommodation during the course of a sustained near-vision task, has generally been assessed under open-loop conditions. This study examined whether adaptation influences closed-loop accommodation during the course of a sustained near-vision task. Accommodative adaptation was assessed in 18 fully-corrected subjects by comparing pre- and post-task values of dark accommodation recorded objectively with an infra-red optometer. Subjects performed a continuous 10 min binocular near-vision task at a viewing distance of 33 cm, with the within-task accommodative response being assessed at 1 min intervals during this period. Subjects were categorized into adaptors (N = 11) and non-adaptors (N = 7) on the basis of whether their initial 10 sec post-task adaptation exceeded +0.30D. The adapting group exhibited a significant decline in the lag of accommodation during the first 3 min of the near-task, whereas no significant change in the within-task response over time was observed in the non-adapting group. These results indicate that accommodative adaptation increases the accuracy of the within-task, closed-loop accommodative response. Furthermore, we speculate that a deficit in accommodative adaptation, being accompanied by increased retinal defocus during near fixation, may contribute to the development of nearwork-induced myopia.
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.
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.
Tonic vergence and vergence adaptation.
In the absence of an adequate visual stimulus, the eyes are typically converged by approximately 0.25 to 0.75 meter angles (MA). This vergence response (VR) was believed to reflect the level of tonic innervation to the extraocular muscles, and accordingly has been termed tonic vergence (TV). However, this estimation fails to consider the magnitude of the anatomical position of rest. The true typical value of TV is approximately 23 degrees. This paper will consider various aspects of this parameter, including both clinical and laboratory methods of measurement, and the relationship between TV and the distance heterophoria. In addition, the role of vergence (or prism) adaptation, i.e., the apparent change in TV after periods of sustained fixation, is discussed. This shift appears to result from the relatively prolonged decay of the slow fusional vergence response (VR), with no evidence for a change in the level of tonic innervation. On occasion, the decay of slow fusional vergence may take hours or even days to reach completion. This extended rate of decay will have a significant impact upon the clinical measurement of a number of binocular parameters, most notably the assessment of heterophoria under truly dissociated conditions (i.e., in the absence of any fusional VR). Furthermore, both the magnitude and rate of decay of vergence adaptation appear to vary with age, as well as the presence of oculomotor imbalance. It is concluded that the output of the slow fusional vergence mechanism, as reflected by the degree of vergence adaptation, makes a major contribution to the aggregate, sustained VR in most visually normal patients.
Effect of contralateral fog during refractive error assessment.
BACKGROUND: When assessing refractive error using static retinoscopy, it is conventional to fog the contralateral eye by approximately 2.00 D to prevent a blur-driven accommodative response stimulating consensual accommodation in the tested eye. However, the effect of higher amounts of contralateral fog (e.g., in a moderate-to-high uncorrected myopic individual) during refractive error assessment is unclear. METHODS: We assessed the refractive state in 16 visually normal myopic subjects while fogging the contralateral eye between zero and 6.00 D, in 1.00 D increments. Retinoscopy was simulated by shining a streak retinoscope light into the right eye, while simultaneously measuring the refractive state of this eye objectively. RESULTS: No significant change in mean refractive state was observed for up to 5.00 D of contralateral fog. But, 6.00 D of contralateral fog produced a significant mean increase in the myopic direction of 0.13 D. Also, in three subjects, a myopic shift of approximately 0.60 D was recorded after the introduction of 6.00 D of contralateral fog. Nevertheless, the magnitude of these largest shifts in refractive error are still smaller than the previously reported degree of repeatability of static retinoscopy. CONCLUSIONS: Since large amounts of contralateral fog produced only small and clinically insignificant changes in the refractive state, the practitioner merely needs to ensure that the nontested eye is indeed fogged. The magnitude of fog present will have only a minimal effect on the final result.
The AC/A ratio, age and presbyopia.
Previous reports concerning the effect of age on the AC/A ratio have been equivocal. Therefore, the present study investigated both the stimulus (AC/As) and response (AC/Ar) ratios using a subjective haploscope-optometer in a relatively large sample of subjects (n = 42) over a wide range of ages (22-65 years). The AC/As showed a small but significant decrease with age (approximately 0.04 delta/D/year). When the older subjects (> 45 years of age) were excluded, however, there was no systematic age effect. The AC/Ar exhibited a small but significant increase with age (approximately 0.08 delta/D/year) for subjects under 45 years of age. However, when the older pre-presbyopes and younger presbyopes (35-44 years of age) were excluded, there was no systematic age effect. In subjects 45 years of age and older, the AC/Ar could not be reliably assessed. This was attributed to physiological and instrumentation noise as a result of the minimal change in accommodative response. In the mid-aged subjects (35-44 years of age), the apparent increase in AC/Ar with age was speculated to be due to neural adaptation of the crosslink gain from the accommodative to the vergence system and/or slight intrusion into the upper non-linear response region of accommodation with the measurements. The finding of AC/Ar constancy with age when mid-aged pre-presbyopes and early presbyopes were excluded supports the non-linearity hypothesis, and thus there appears to be no real change in AC/Ar with increased age. The results support the Hess-Gullstrand theory of presbyopia.
Effect of prolonged dissociation on the subjective measurement of near heterophoria.
Previous studies have demonstrated that vergence adaptation resulting from the prolonged decay of slow fusional vergence may prevent the accurate assessment of oculomotor deviations. Continuing changes in heterophoria have been reported after 27 days of monocular occlusion. However, since most slow fusional vergence will decay within the first few minutes of dissociation, a clinical assessment of both vergence adaptation and heterophoria could be made more rapidly. This report documents two investigations which examined changes in near horizontal heterophoria during 30 min and 180 min of continuous dissociation, respectively. Seven out of the 16 subjects exhibited significant changes in heterophoria during the 30 min dissociation period, with a mean increase in exophoria of 3.4 delta. The decay of slow fusional vergence took approximately 25 min to reach completion. Thus in the clinical setting, both the degree of vergence adaptation and a more accurate assessment of heterophoria may be obtained by maintaining dissociation for 25 min. Furthermore, the presence of vergence adaptation can be assessed after just 5 min of dissociation.
Comparison of clinical techniques to assess the near accommodative response.
Assessment of the accommodative response (AR) is an essential part of the optometric examination. This study compared a number of clinical procedures for determining the AR with the findings of an objective infrared autorefractor. First, the AR to a 2.5 D binocular stimulus was measured by dynamic retinoscopy (neutralization being obtained both with lenses and by varying the working distance), dynamic cross-cylinder (with and without fogging lenses), and a near red-green duochrome test. Second, the response to a monocular 2.5 D stimulus was examined using dynamic retinoscopy (neutralizing with lenses), dynamic cross-cylinder, and the red-green duochrome. Third, the monocular AR was examined with both red-green and blue-yellow near duochrome tests. Under binocular conditions, the mean ARs for all the tests were clinically equivalent. However, comparison with the findings from the objective autorefractor indicated that dynamic retinoscopy (where neutralization was obtained by varying the working distance) showed the closest agreement, whereas the two dynamic cross-cylinder procedures exhibited the greatest variability. For the monocular condition, the mean lag of accommodation observed with the autorefractor was significantly less than that observed with dynamic retinoscopy, the dynamic cross-cylinder, or the red-green duochrome. However, the blue-yellow near duochrome test did not provide a valid estimation of the AR. It is concluded that the technique of dynamic retinoscopy where the working distance is varied to obtain a neutral reflex should be the method of choice for the clinical assessment of the AR, because this procedure does not require the introduction of supplementary lenses, which may themselves alter the AR.
Proximal contribution to a linear static model of accommodation and vergence.
To determine the influence of target proximity on accommodation and vergence under both open- (OL) and closed-loop (CL) viewing conditions, a static interactive feedback model, which included proximal accommodation (PA) and proximal vergence (PV) inputs, was developed and analysed quantitatively. It was based on an earlier static dual-interactive feedback model of accommodation and vergence. The proximal inputs were added to both the accommodative and vergence loops at the output of the respective controllers. The values of the PA and PV gains were obtained from experimental dual-OL data. The model equations were analysed over a stimulus range of 1 to 6 D (or MA). It was found that under the dual-OL condition, the contribution of PA to the overall accommodative output ranged from 42.5 to 81.6%, whereas the contribution of PV to the overall vergence output ranged from 56.1 to 88.5%. In contrast, under all other stimulus conditions, with the exception of the PA contribution to the accommodative output under the Aol, Vcl condition (26.5-41.0%), the relative contributions were much smaller, ranging from 0.04 to approximately 7.0%. In particular, under the dual-CL condition, representing the normal binocular visual feedback condition, the relative contributions were only 4.0 and 0.04% for PA and PV, respectively. Thus, although the relative contributions of PA and PV were large under the dual-OL condition, they were generally very small under the various CL conditions that simulated more naturalistic viewing situations. Nevertheless, proximity may still play an important role by providing cues for attaining coordinated and harmonious motor responses under specific viewing conditions.
Multi-meridional keratometry.
During conventional keratometry, the two principal meridia are identified and measurements of the anterior corneal curvature determined along these orientations. However, in cases of corneal irregularity, the principal meridia may be difficult to identify. Accordingly, the present study sought to evaluate two alternative procedures which do not require identification of the principal meridia. Instead, measurements of anterior corneal curvature were determined along either three or four preselected meridia and subsequently converted into the standard format for measurements of anterior corneal curvature. The study was performed on 25 visually normal subjects and the results compared with conventional two meridian keratometry findings. No significant differences were observed between the mean findings for the three and four meridian techniques. Furthermore, both procedures gave comparable results to the conventional two meridian method. Accordingly, multi-meridional keratometry may be valuable in cases of irregular astigmatism, for example produced by keratoconus or scarred corneas, since measurements can be obtained without the necessity for identification of the principal meridia.
Repeatability of clinical measurements of the amplitude of accommodation.
In order to determine whether a significant change has occurred in a clinical parameter, it is essential to assess the repeatability of the measurement procedure. Accordingly, the present study has examined the repeatability of three techniques for evaluating the subjective amplitude of accommodation. Clinical amplitudes were determined using the push-up, push-down and minus lens procedures in 13 young individuals (mean age: 24.3 years) on five separate occasions. The standard deviation (SD) of the five examinations was calculated for each individual and the mean values for the population sample determined. The mean SDs for the push-up, push-down and minus lens techniques were equivalent, with 95% confidence limits of approximately 1.4 D being observed for all three procedures. Furthermore, the mean amplitude recorded using the push-up technique was significantly higher than that observed for the other two procedures. It is concluded that a change of at least +/- 1.50 D should be adopted as the minimum significant shift in amplitude of accommodation for this group.
Push-up amplitude of accommodation and target size.
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Vergence adaptation and the order of clinical vergence range testing.
When measuring horizontal relative vergence ranges, the conventional clinical procedure is to assess the base-in (or divergence) range before the base-out (or convergence) determination. The rationale for this order of testing is that the convergence responses stimulated during the base-out measurements may produce vergence adaptation (i.e., a fusional aftereffect), which may temporarily bias the subsequent base-in values in the base-out direction. However, the effect of reversing this order of testing (i.e, base-out before base-in) has not been directly evaluated. Accordingly, the present study compared distance base-in vergence ranges measured both before and after base-out testing at separate test sessions. Additionally, tonic vergence (TV) was assessed before and after each vergence range measurement. The results indicated that base-out vergence range testing stimulated vergence adaptation, and furthermore produced a statistically significant reduction in the subsequently measured base-in recovery value. However, the magnitude of this shift was less than the estimated degree of repeatability of the vergence range measurement. Therefore, a more appropriate order of testing would be to assess the heterophoria compensating range first in order to obtain accurate results during this more critical measurement.
Do changes in pupil size and ambient illumination affect the duochrome test?
Vague and conflicting suggestions currently exist within the literature regarding the appropriate level of ambient illumination to use while conducting the clinical duochrome (bichrome) test. One proposal is that reduced illumination should be adopted since this allows the pupils to dilate, thereby increasing the chromatic interval and resultant test sensitivity. The aim of the present study was to determine whether changes in pupil size and ambient illumination do indeed alter the subjective duochrome response. Accordingly, eight young, visually normal subjects were dilated with 2.5 percent phenylephrine and tested while monocularly viewing the duochrome chart through 1, 3, 5 and 7mm artificial pupils. Additionally, the effects of performing the test through a 1.0 log unit neutral density filter and varying the overall room illumination were examined. There were no significant differences between the mean responses under all test conditions. The variability of individual responses decreased with increasing pupil size; however, a 4mm change in pupil diameter was required to alter the response variability significantly. Therefore it is recommended that the test be performed under the minimum practical ambient illumination conditions in order to minimize this variability.