Accommodation responses and refractive error

Transcription

1 Accommodation responses and refractive error Richard J. Maddock, Michel Millodot,* Susan Leat,* and Chris A. Johnson Previous investigations have reported considerable individual differences in the resting point of accommodation. Recent independent studies in Davis, Calif., and Cardiff, Wales, indicate that part of these variations are related to refractive errors. The average dioptric value of the resting point of accommodation is lowest for corrected myopes, highest for corrected hyperopes, and intermediate for emmetropes. Another part of the variation in resting point is related to individual differences in the far point of accommodation. There appears to be no significant relationship between the resting point and near point of accommodation. Key words: accommodation, resting point, near point, far point, myopia, hyperopia, emmetropia Accommodation refers to the method of the eye for varying its refractive power to produce a focused image on the retina for different object distances. Accurate accommodation is dependent on several stimulus characteristics, including target luminance, 1 contrast, 2 and size. 3 In the absence of an appropriate stimulus, accomodation assumes an intermediate fixed-focus condition referred to as the "resting point" 4 or "dark focus" 5 of accommodation. The intermediate resting point of accommodation has been shown to be responsible for the occurrence of anomalous myopias, including night myopia, empty field or space myopia, and instrument myopia. 5 An interesting feature of the resting point of accommodation is that it exhibits consider- From the Department of Ophthalmology, University of California, Davis, and the *Laboratory of Experimental Optometry, UWIST, Cardiff, Wales, U.K. Supported in part by National Eye Institute Academic Investigator award KO7-EY95 (to C. A. J.). Submitted for publication Jan. 28, 198. Reprint requests: Richard J. Maddock, Department of Ophthalmology, University of California, Davis, Calif able variation from one individual to another, 6 although the value for a given individual is quite stable over time. 7 ' 8 The purpose of the present investigation was to determine whether the refractive properties of the eye are related to these individual differences in the resting point of accommodation. Independent studies were conducted in Davis, Calif., and Cardiff, Wales, to examine relationships between refractive status (myopia, hyperopia, emmetropia) and the resting point of accommodation, and relationships among the far point, near point, and resting point of accommodation. Methods Measurements of accommodation in both studies were obtained with a laser Badal optometer, 9 applying corrections for chromatic aberration and the position of the reference plane of stationarity. 1 Each subject exhibited a corrected visual acuity of /25 or better with no history of ocular abnormality other than refractive error. All but one of the subjects were below the age of 25. Prior to testing, the accuracy of each subject's spherical and astigmatic refractive correction was carefully checked using standard refraction techniques. Since the subjects wore their corrective lenses throughout the test, all dioptric values of accom /81/3387+5$.5/ 1981 Assoc. for Res. in Vis. and Ophthal., Inc. 387 Downloaded From: on 6/16/18

2 388 Macldock et al. Invest. Ophtltalmol. Vis. Sci. March 1981 CD c a> cc H yper. Emm. Myo. Fig. 1. Means and ±1 S.E.M. for the resting point of accommodation in five corrected hyperopes, nine "emmetropes" and nine corrected myopes in the Cardiff study. modation measurements incorporate their normal distance correction. Because the Cardiff and Davis studies were conducted independently and without prior collaboration, several procedural differences were present. The Cardiff subject population consisted of five hyperopes (refractive correction between +2 and +4 D), nine myopes (refractive correction between 2 and 7 D), and nine "emmetropes" (refractive correction <±2 D). Measurements of the resting point of accommodation were obtained in a completely darkened room. Subjects in the Davis study consisted of emmetropes (no refractive correction), 1 "low" myopes (refractive correction < 3 D), and 1 "high" myopes (refractive correction > 3 D). In addition to resting point determinations, the far point and near point of accommodation were measured for the Davis subjects. The far point target consisted of an illuminated Snellen chart (1.9 log cd/m 2, 9% contrast) at a distance of ft, and the near point target was a near acuity chart (Sloan letters, 1.6 log cd/m 2, 7% contrast). For both studies the criteria used to distinguish between different groups of refractive errors were somewhat arbitrary and were not based on any factors pertaining to the cause of refractive errors. The general intent of the criteria was to distinguish individuals with large amounts of refractive error from those with little or no refractive error. Results Cardiff study. The mean resting points of accommodation (and S.E.M.) for hyperopes, emmetropes, and myopes are presented in Fig. 1. These data showed a tendency for the mean resting point of accommodation to be greatest for corrected hyperopes and lowest for corrected myopes. A one-way analysis of variance 11 revealed that the relationship between refractive status and the resting point of accommodation was not statistically significant. However, this was probably due to the small sample sizes used in the study. Davis study. Summary data for all subjects in the Davis study revealed the following: resting point of accommodation, X = 1.43 D, S.D. = 1.3 D; far point,~x = JX28 D, S.D. =.41 D; and near point, X = 1.21 D, S.D. = 2.1 D. Resting point and far point measurements were significantly correlated (r = +.48, p <.1), indicating that part of the individual variation in the resting point of accommodation is related to individual differences in the far point of accommodation. Correlations between the resting point and near point of accommodation (r = +.8) and between far point and near point of accommodation (r = +.7) were not statistically significant. Distributions of the resting point, far point, and near point of accommodation are presented in Fig. 2 for emmetropes, corrected low myopes, and corrected high myopes. Note that the distribution of measurements for the resting point of accommodation was narrower in corrected high myopes than in emmetropes or corrected low myopes. In addition, the dioptric value of the resting point of accommodation in corrected high myopes was statistically significantly lower than in emmetropes (t' = 2.28, df = 27, p <.3; t' test for independent samples used the method of Welch, which does not assume equal variance in the two groups 11 ). No other between-group differences in resting point of accommodation were statistically significant. Determinations of the far point of accommodation were similar for all three groups of Downloaded From: on 6/16/18

3 Volume Number 3 Accommodation responses and refractive error 389 o 5 S o 3 Ul CT> ^ ^ 1 Ul ) or or or O *- O c O O U. - o >- u - o UJ O c Ul or u_ Emmetropes Low Myopes High Myopes S..= 1.15 x = 1.54 S.D.= 1.12 x «.94 S.O. = ii =.26 S.D. =.44 x.28 S.D : x = 1.28 S.D.« DIOPTRIC VALUE Fig. 2. Frequency distributions of the resting point, far point, and near point of accommodation in emmetropes, 1 corrected low myopes, and 1 corrected high myopes in the Davis study. Note that the abscissas of the far point, resting point, and near point are scaled differently Emmetropes Low Myopes High Myopes x = 1.36 S.D. = 1.1 x = 1.24 S.D. =.99 x =.66 S.D. = O.3I DIOPTRIC VALUE Fig. 3. Frequency distributions of the amplitude of far accommodation (resting point minus far point of accommodation) in emmetropes, 1 corrected low myopes, and 1 corrected high myopes in the Davis study. subjects. This result is not surprising because all myopes wore their appropriate refractive correction for distance during testing. For measurements at the near point of accommodation, corrected low myopes (X = 11.6 D) exhibited a greater dioptric value than emmetropes (X = 9.5 D). This difference was statistically significant (f = 2.85, df = 19; p <.1). The near point of accomodation in corrected high myopes was not significantly different from emmetropes or corrected low myopes. We define amplitude of far accommodation as the difference between resting point and far point of accommodation. This has also been referred to as "negative accommodation." 12 In view of the significant correlation between the far point and resting point of Downloaded From: on 6/16/18

4 39 Maddock et al. Invest. Ophtlialmol. Vis. Sci. March 1981 accommodation, the amplitude of far accommodation was compared among the three subject groups in the Davis study. This comparison permits examination of the relationship between refractive status and the resting point that are independent of variations in the far point of accommodation. These data are presented in Fig. 3. The amplitude of far accommodation in corrected high myopes (X =.66 D) was significantly smaller (t' = 2.83; df^= 25; p <.1) than for emmetropes (X = 1.36 D). Corrected low myopes were not significantly different from emmetropes or corrected high myopes for their amplitude of far accommodation. The range of accommodation for each subject was calculated as the far point minus the near point of accommodation. There was no significant correlation between resting point and range of accommodation. We defined amplitude of near accommodation as the difference between the near point and resting point of accommodation. Corrected low myopes had a greater_range of accommodation than emmetropes (X, 11.2 D vs. X = 9.3 D, t' = 2.58, df = 19, p <.2) and a_greater amplitude of near accommodation (X = 1. D vs. X = 7. D, t' = 2.64, df = 18, p <.2). Corrected high myopes were not significantly different from corrected low myopes or emmetropes. At the present time we find it difficult to interpret these unexpected findings. Discussion The main findings of this study include (1) substantial variation in the resting point of accommodation across all subjects; (2) lower average dioptric values of the resting point of accommodation for corrected myopes, higher average values for corrected hyperopes, and intermediate average values for emmetropes; (3) a significant, positive correlation between resting point and far point of accommodation; and (4) no relationship between the resting point and near point of accommodation. The distributions of resting points for our subject populations are similar to the one reported by Leibowitz and Owens 6 (mean = 1.52 D, S.D. =.77 D, and range = -.4 to 4. D; N = 2). The Cardiff results show a mean of.95 D, S.D. of.64 D, and range of -.3 to 3. D (N = 23), whereas the Davis findings reveal a mean of 1.43 D, S.D. of 1.3 D, a range of -.2 to 4.4 D (N = ). Differences in the absolute values of resting point data from Cardiff and Davis may be due to either slight methodologic differences, the small sample sizes in both studies, or variations in the general subject populations in Wales and California. Results of the Davis study indicate that individual differences in resting point are associated with variations in far point. The simplest interpretation of this strong positive correlation is that as the far point shifts (whether due to intrinsic refractive changes or corrective lenses), all points on the accommodative spectrum shift with it. In this regard, the slope of the regression equation relating resting point to far point in the Davis study has a value near unity (1.19). However, the range of individual variation in far point is much smaller than the range for resting point, suggesting that differences in the far point only account for a portion of the variation in resting point of accommodation. Some of the individual differences in resting point are related to differences in refractive status. The results of both studies indicate a progressive increase in the average dioptric value of the resting point of accommodation from corrected myopes to emmetropes to corrected hyperopes, implying a concomitant increase in the amplitude of far accommodation. The underlying basis for these differences among various groups of refractive error poses an interesting question. Is it related to some intrinsic difference in the ability to accommodate to far objects (i.e., physiologically based limitations on the amplitude of far accommodation)? It has been reported that the normal physiologic tone of the ciliary muscle in hyperopes is greater than in myopes. 13 Alternatively, might it represent an active functional response to refractive error as part of an attempted physiologic compensation? In this view it is interesting to note that in the Davis myopes there was a significant negative correlation be- Downloaded From: on 6/16/18

5 Volume Number 3 Accommodation responses and refractive error 391 tween the number of years since the initial diagnosis of myopia and both resting point (r = -.53, p <.2) and the amplitude of far accommodation (r =.58, p <.1). The questions raised by these findings require further studies of resting point and refractive characteristics. In addition, factors such as stress 14 and mood changes 15 may be related to individual differences in the resting point of accommodation. We are grateful to Dr. D. Alfred Owens for his helpful comments and suggestions on the manuscript. REFERENCES 1. Johnson CA: Effects of luminance and stimulus distance on accommodation and visual resolution. J Opt Soc Am 66:138, Owens DA: A comparison of accommodative responses and contrast sensitivity for sinusoidal gratings. Vision Res :159, Owens DA and Leibowitz HW: The fixation point as a stimulus for accommodation. Vision Res 15:1161, Toates FM: Accommodation function of the human eye. Physiol Rev 52:828, Leibowitz HW and Owens DA: Anomalous myopias and the intermediate dark focus of accommodation. Science 189:646, Leibowitz HW and Owens DA: New evidence for the intermediate position of relaxed accommodation. Doc Ophthalmol 46:133, Mershon DH and Amerson TL: Stability of measures of the dark focus of accommodation. INVEST OPHTHALMOL VIS SCI 19:217, Miller RJ: Temporal stability of the dark focus of accommodation. Am J Optom Physiol Opt 55:447, Hennessy RT and Leibowitz HW: Laser optometer incorporating the Badal principle. Behavioral Research Methods and Instrumentation 4:237, Charman WN: On the position of the plane of stationarity in laser refraction. Am J Optom Physiol Opt 51:832, Winer BJ: Statistical Principles in Experimental Design. McGraw-Hill, 1962, New York. 12. LeGrand Y: Sur l'existence chez certains sujets dune accommodation negative. CR Acad Sci Paris 23:1422, Duke-Elder S and Abrams D: System of Ophthalmology. Vol. V, Ophthalmic Optics and Refraction. London, 197, Henry Kimpton, p Leibowitz HW: Visual perception and stress. In Physical Work and Effort, Borg G, editor. New York, 1976, Pergamon Press. 15. Miller RJ: Mood changes and the dark focus of accommodation. Perception Psychophysics 24:437, Downloaded From: on 6/16/18