EFFECT
OF DIOPTRICS ON PERIPHERAL VISUAL ACUITY
MICHEL MILLOLKIT,CHRIS A. JOHNSON,’ ANNE LAMONT’ and HERSCHEL W. LEIBOWITZ~ Department of Ophthalmic Optics. UWIST. Cardiff. Wales (Receieed
14 Jarurary
1975)
Abstract-The peripheral dioptrics of the eye displays a considerable error of refraction caused primarily by oblique-ray astigmatism. It was hypothesised that this refractive error might contribute to the well known reduction in visual acuity in the peripheral visual field. Independent experiments were carried out in two laboratories. using different methods and different targets; sinusoidal gratings and Landolt rings. A third experiment was run in which two subjects from one laboratory were tested in the other. Measurements of visual acuity were obtained between 0” and 60” of eccentricity along the horizonral meridian both with and without correction of refractive error. The results of all three experiments indicate that the existing errors in refraction do not influence peripheral visual acuity. EFFEC-I. OF
DIOPTRICS ON
PERIPHERAL VISUAL ACUITY It has been well established that acuity declines as one moves progressively away from the fovea into the periphery (literature reviews may be found in Kerr, 1971; LeGrand, 1967; Low, 1951; Millodot 1972; and Riggs, 1965). The basis for this marked degradation in visual acuity with increasing anpJe of eccentricity has most frequently been attributed to neural factors such as increased receptive field size and decline in cone density. However, the relationship between cone density (Osterberg 1935), and acuity breaks down in the far periphery, and large refractive errors of refraction also occur (Ferree and Rand, 1933; Ferree, Rand and Hardy, 1931; Lotmar and Lotmar, 1974; Millodot and Lamont, 1974a). Thus, it is difficult to interpret degraded visual acuity in the periphery solely in terms of neural properties. Investigations of peripheral visual acuity have generally ignored the possible influence of refractive errors. Albini (cited in Low, 1951) observed that placing lenses between the eye and peripherally presented stimuli altered visual acuity. However, this was a qualitative observation and no quantitative data are reported. Wcale (1956) measured visual acuity at 55” and 70”; Clarke and Belcher (1962) (gratings ai 20”) and Green (1970) (gratings and interference fringes up to So) all find that optical errors do not, to a first approximation, account for the decrease in peripheral acuity. The purpose of the present investigation was to determine the effect of correction of peripheral refractive error on visual acuity. To this end. both peripheral refractive error and visual acuity were assessed by different methods. To examine the possible influence of different refraction methods. psycho-physical
’ Present address: Department of Ophthalmology. University of Florida. ’ Laboratory of Experimental Optometry, University of Montreal. Quebec, Canada. ’ Department of Psychology. Pennsylvania State University.
testing technique, and individual differences, two of the subjects were tested under all experimental conditions. EXPERIMENT
WITH SINUSOIDAL
GRATINGS
Apparatus
The apparatus. shown schematically in Fig. I, was designed to present the test and surround fields in Maxwellian view. Tungsten ribbon filament lamps (6 V, 9 A) served as light sources (Sl, S2) for the test and surround channels. Lenses LI and L2 formed sharp filament images on opal
w Fig. I. Schematic representation of the apparatus employed in Experiment I. The symbols refer to the following: S1. SZ--tungsten ribbon filament lamps; H-heat absorbing glass; Al, AZ-apertures serving as secondary light sources; OGI, OG2Apal diffusing -glass; Ll, Li L3, L4. L5. Lblenses; SF-surround field: B-beamsolitting cube; G-sinusoidal grating test stimulus; FS-held stop; SH-electronic shutter; F-fixation stimulus; AP-artificial pupil.
1357
The fixatlon stimulus (F) cmpioyed during perlphcrai determinattons consisted of a Landolt C (I R’I cenwred within a circular background field (41 1 of 151 cd m’ luminance. Durmg successive determinations the fixation stimulus. located I m from the observer. was moved to eccentricities of 20‘. 40’ and 60’ for the nerioheral stlmuh The surround field annulus served as a’ fixation sttmulus for fovea1 determinations. The observer‘s head was held rlgld by a bite-board arrangement. which the observers were taught to properI> align to the apparatus. An artificial pupil (API. ?5mm dia. was located on a thin metal vertical strip which was sufficiently narrow to prevent occluding the fixation stlmulus during peripheral determinations. The artificial pupil was employed to control for changes m effective pupil size with eccentric viewing. thus maintaining retinal illumination constant for all determinations. Procedure Id
Fig. 2. Schematic representation of (a) the surround field only. (b) the test stimulus only and (c) the test stimulus superimposed on the surround field. diffusing glass sheets (OGI. OG2) masked off by apertures Al and A2. which acted as secondary light sources. The final source image formed in the plane of the artifical pupil was a 27 x 6,Omm vertical slit. The collimated portion of the surround field channel (between lenses L3 and L4) contained the annulus-surround field arrangement (SF) presented in Fig. 2(a). It consisted of an opaque ring centered within a larger clear circular portion of a thin glass sheet. The entire clear circular area subtended IO”. while the ring exhibited a separation of 2” between the inner edges and 3” between the outer edges. This arrangement was placed at an optical distance of I m from the observer. Luminance of the surround field was 524 cd/m’ (7658 td). The test stimulus consisted of a sinusoidal grating (G) placed within the collimated portion of the test channel (between lenses L5 and L6). The stimulus size (2”) was determined by a field stop (FS) attached to lens L6. The appearance of the stimulus is presented in Fig. 2(b). A beam-splitting cube (B) combined the test and surround channels to present the observer with the representation shown in Fig. 2(c). The space-average luminance of the grating stimulus was I 118.2 cd/m’ (1633.6 td). A series of 20 National Bureau of Standards sinusoidal gratings (modulation 0.658. maximum transmittance 0.664. minimum transmittance 0.137), successively increasing in spatial frequency by a factor of 1.26. served as test stimuli. Refined adjustments in spatial frequency were accomplished by rotation of the grating in the collimated light. about the horizontal axis of the grating lines. At large angular rotations, the spatial distribution of light transmitted from the grating test stimulus departs from a true sinusoidal representation. For this reason, angular rotations were limited to less than approx 65”. A circular table. equipped with a holder for securing the gratings horizontally. accomplished the stimulus rotations. The outer edge of the table contained a Vernier scale. allowing measurements of angular rotation accurate to within 5”. The dioptric difference between the upper and lower portions of the grating was less than 05 D for the largest stimulus rotation. The grating lest stimulus was superimposed on the continuously presented surround field For 250msec. controlled by an electronic shutter (SH). This brief exposure time was employed to control for possible influences of the field stop (FS) on accommodation during fovea1 determinations, and to minimize Troxler’s effect for peripheral measures.
The refractive errors of the four observers were determined for the four eccentricities tested. fovea] (0). and 20’. 40” and 60” in the temporal field, by an experienced optometrist using static retinoscopy. The observers were given instruction and practice in proper alignment to the apparatus, especially for the alignment procedures necessary for peripheral determinations, followed by IO practice sessions. each lasting approx 1.5 hr, prior to the collection of data. This was to control for the marked improvements with practice known to occur for visual tasks in the periphery (Abernethy and Leibowitz, 1971: Johnson and Leibowitz, 1974; Low, 1946. 1951; Saugstad and Lie, 1964). Each session consisted of three successive determinations of the resolution threshold at each of the eccentricities. An interleaved double staircase method (Cornsweet. 1962) with variable step size was employed. The observers were instructed to report whether they could resolve the lines of the grating. The mean point about which the staircases varied was the position at which the observers reported the presence of lines 50% of the time. For half of the sessions, testing began at 0” of eccentricity and continued out to 60”. This order was reversed for the other half of the sessions. The starting point for an individual session was randomly determined. Four experimental sessions were conducted after the practice sessions. Two of the sessions were without correction of the peripheral refractive error while the other two were with appropriate correction, the order of these sessions being randomly determined. During sessions using no correction, the grating test stimulus was placed al an optical distance of 1m, the same distance as the surround field and the fixation stimulus. For the sessions with correction of refractive error. this procedure differed in the manner described below. Since the test stimulus consisted entirely of horizontal lines. only the horizontal meridian was corrected. This was done by adjusting the distance between the grating and lens L6 to compensate for horizontal refractive error. The grating was moved between L5 and L6 to adjust the optical distance of the grating so that it was in focus when the observers fixated a target at a distance of I m. This may be considered as analogous to a Badal optimf system as the angular dimensions of the grating remained constant with changes in the optical distance of the grating. Brief rest periods were given between each threshold determination and, in addition. if the observers reported loss of fixation, eye movements, or momentary attentional lapses the trials were discarded. RESULTS
The correction required in the horizontal meridian at each stimulus eccentricity is presented. in Table 1. Individual differences in both the amount and direc-
Elects of dioptrics on peripheral visual acuity Table I. Amount of correction (in d@tersl requii+edin the horizontal meridian for observers JG. FO. RM and CJ. in sinusoidat gratings experiment
Obsarver
0
20
JO.
-ff.so
-
IO,
-4.25
RM.
cr.
5,7s
70
60
c 2.75
t 4.25
+ 0.25
i
1.00
+ 1.50
- 1.50
* 1.50
+ 0.25
+ 0.25
-
m 0.25
- 0.25
+ 6.00
0.50
tion of the xcessaq correction can be readily observed In general greater corrections are necessary with increasing eccentricity. The data are reported in terms of the reciprocal of the minimum angle of resolution (angle subtended
by one half-cycle) in minutes of arc. The rest&s for each of the four observers are presented in Fig. 3 which presents vised acuity on a Iogarithmic scale as a function of stimulus eccentricity. The solid lines represent the data obtained without correction of refractive error. and the dashed lines represent the functions obtained when refractive error was corrected. Each point on both of the curves represents the average of six threshold determinati0nS.
1359
opt&l S$WIII add psycho-physical ..method similar to the present study. The values reported by Mandelbaum and Sloan (1947) for a Landolt C test-object. are much lower. Although correction of refractive error improves acuity for the fovea. it has little or no cfTect in the periphery. For one subject (CJ) who has a large error at 60”. peripheral refractive error correction had a slight effect. The peripheral data contrast sharply with
those for the fovea for which smaft refractive errors have a marked e&et on acuity.
EXPERIMENT
WlTH LANDOLT
RINGS
The apparatus for measuring acuity consisted of an optical system which focussed a black negative of a Landolt ring on a white screen. It is basically the same as that previously described by Millodot and Lamont (1974b). The gap of the ring could be orientated in the four major directions and the size of the break could be varied in discrete steps between 44“ and 19.5’.The projector remained stationary and the Iocation of its projected image was varied by means of a rotating mirror pjaced at right angles to its optical axis. The projection screen was adjusted at various angular positions from a central fixation point along the horizontal (temporal) plane and was kept equidistant from the subject’s eye for all eccentricities. The luminance of the adapting background was 86cd/ m’ and the projected circular area containing ihe Landoft ring was 245&n’. The contrast of the Landolt ring was 55%. Target exposure duration was constant at 0.12 sec.
in line with previous investigations, acuity is degraded in the periphery, with the most rapid drop occurring between 0” and 20” of eccentricity. ‘These The experiment was divided into two parts. Experiment acuity values are in good agreement with those of II and validation. Peripheral refraction in the first part Kerr (1971) who used square-wave gratings and an was determined to eccentricities of 50” on three subjects
I
+-With
cometion
Fig 3. Log visuaf acuity, with and without correction of refractive error, as a function of eccentricity for observers JG. FO. RM and CJ, using gratings.
using Zeiss ~f~to~try~ retinoscopy and subjective method. The values obtained with these three techniques were used as correction and placed at 90” to the line of sight at each eccentricity tested. The individual data which were actually used have been reported elsewhere (Millodot and Lament, 1974a). In the validation experiment a retinofocometre (built by EsseI, Paris) was used to measure refraction at 0“. 20’, 40” and 55”. Three subjects participated in Experiment II. Two additional subjects who had also participated in Experiment I. served in the validation experiment. All subjects were familiar with visual experimentation. The subject was seated at the centre of a screen arc with his head resting on a headrest and adapted to the background fuminance for 7 min. Then he was warned by a ready signal to maintain central fixation and a Landoit ring was exposed at a given eccentricity. Fixation was not monitored throughout the experiment but some runs in the experimental group were carried out using a photo-electric eve movement recorder (Eve-Trdc). The subject indicated’verbaliy in which direction he saw the gap: The targets were presented at a rate of about one every 10sec. Twelve presentations of the ring were made. i.e. 3 times in each direction. but the sequence was haphazard. The method of constant stimuli was used and a percentage of 50% correct responses was considered as threshold. Judgment was made monocularly with the right eye. The order of presentation was varied between starting with and without peripheral correction, above and below threshold. at O”-W eccentricity. Each session was Iimited to 35-40 min so as to avoid fatigue which easily overcame the subjects as they concentrated on a peripheral target. Most sessions
51.
1360
1
0.5 r c;
0.6 0.4
03 f 02
MILLWOT.
01
“U0
A.
JOHWW.
50
60
ANQ
LAMOST
and
H.
M’.
L~IIKN’IT,’
\ 4, \\ \
\
\
4
C.
IO V~sunl
20 field
30
‘Y -\
40
eccentricity,
0
deg
Fig. 4. Visual acuity (in decimal value), as a function of eccentricity (temporal visual field.) H represents the data with correction of peripheral refractive error; O--O represents the data without correction. Each data point represents the mean of three subjects. Background luminance of the Landolt ring, 245 cd/m*.
were repeated on several other occasions in order to assess the effect of habituation which is known to play a role in peripheral acuity. The entire procedure was repeated on all three subjects at a lower luminance with an ND 2 filter patched in front of the viewing eye. Results
The mean optical correction of the peripheral diog tries for the three subjects has been reported elsewhere (Millodot and Lamont. 1974a). It is evident that peripheral correction due principally to obliqueray astigmatism becomes very large with visual field eccentricity. The measurements of visual acuity with retinal eccentricity obtained with three subjects at the higher luminance are given in Fig. 4. The well-known decrease in visual acuity with eccentricity is apparent in these data. Each data point is the mean of three subjects. The open circles represent the findings with no peripheral correction and the closed circles the values with the peripheral correction. It is apparent that with the appropriate peripheral correction in place at each eccentricity investigated, there is no appreciable improvement in peripheral visual acuity. The same trend was found for each of the three subjects. At the lower luminance (Fig. 5) the overall visual acuity is slightly reduced in the central part of the retina and hardly affected towards the periphery as compared to the data obtained with the same subjects at the higher luminance (Fig. 4). The lack of a relationship between peripheral acuity and luminace is in good accord with Mandelbaum and Sloan (1947) and Kerr (1971). The correction of the peripheral optics yields no appreciable improvement in visual
IO V~swl
20 frld
M
40
eccentncity.
50
60
deg
Fig. 5. Visual acuity (in decimal value) as a function of eccentricity (temporal visual field). W represents the data with correction of peripheral refractive error; O--O represents the data without correction. Each data point represents the mean of three subjects. Background luminance of the Landolt ring, 2.45 cd/m?.
performance as can be noted in Fig. 5. The same trend was found for each of the three subjects. Validatiort experiment The results with the two subjects who had previously served in Experiment 1 are presented in Fig. 6. Individual data for each subiect are plotted with and without peripheral correction. At 20” and 55’ eccentricity the results are the same whether the eye is corrected or not for both subjects. At 40” acuity Corrected 0 Uncorrected FO~Cwrected 0 Uncorrected
cJ.
0.6 -
2.
s ::
0.4-
s .L? > 0.2 -
o-
I
I
0
I
I
I
I
I
I
IO
20
30
40
50
60
Visual
field
eccentrbclty,
deg
Fig. 6. Visual acuity (in decimal value) with and wrthout correction of peripheral refractive errors. as a function of eccentricity (temporal visual field) for observers CJ and FO using Landolt rings.
EtTectsof dioptrics on peripheral visual acuity is slightly better with the peripheral correction for subject CJ and the reverse for subject FO. These values are in good accord with those obtained on the same subjects with a grating in Experiment I. This experiment provides further evidence showing that peripheral correction of the eye does not improve peripheral acuity for either a grating or Landolt ring target. DiSCUsSION
The data of these experiments demonstrate that the poor quality of the peripheral dioptrics cannot account for the degradation of acuity of the adult eye in the peripheral visual field. It must be noted that refractive error is usually measured along the axis of the eye. However, whether the eye requires a correction or not along the visual axis, it is nevertheless affected by oblique-ray astigmatism as any other optical system (Fincham, 1956). This obliqueray astigmatism induces a rather substantial peripheral correction which increases with retinal eccentricity (Ferree et al., 1931; Dupuy, 1967; Millodot and Lamont, 1974a). It could have been thought. therefore, that such off-axis refractive error might limit visual acuity but our data do not support this hypothesis. It must also be noted that although retinal illumination is affected by oblique Iight incidence this effect is the same with and without the correction of peripheral dioptrics, and therefore it does not alter the difference obtained in the two conditions. The present results are in good agreement with those found by Weale (1956). Clarke and Belcher (1962) and Green (1970). As in central vision the optical quality of the eye does not necessarily limit visual acuity (Westheimer, 1960; Riggs, f965) provided that the focus is appropr~te to the st~ulus distance and for average pupil sizes. This conclusion applies, at least, to the adult subjects used in this experiment. Rather visual acuity is apparently limited by the neurophysiological organisation of the visual pathway from the retina to the cortex. It could be argued, though, that in the early years of life the poor quality of the peripheral dioptrics leads to a degradation of visual acuity. This loss of function would be due to inappropriate visual stimulation, the visual deprivation being a consequence of exposure of the retina to defocussed images. It has been shown (Freeman. Mitchell and Millodot, 1972; Mitchell, Freeman, Millodot and Haegerstrom, 1973) that individuals with large amounts of astigmatism early in life develop a deficit that persists even after the astigmatism is corrected as an adult. It appears that there is a critical period for the proper development of the “feature detectors” and the presence of a blurred retinal image can, at least, in one eye and in central vision. have a marked effect on the development of these anaiysers. It is possible that an analogous deficit occurs in the periphery. This suggestion could be tested by correcting the peripheral oblique-ray astigmatism of the eye at birth and measuring the peripheral visual acuity after the critical period of development. It is also interesting to note that Kerr (1971) found acuity to be less affected by luminan~ as the angle of eccentricity was increased. In the fovea1 regions. visual acuity as we11as aimost all other psycho-physical
1361
functions, are greatly affected by luminance level. However, for the two subjects in Kerr’s investi~tion, visual acuity changed only slightly over a mnge of over 6 log units of luminance at an eccentricity of 30”. This independence of luminance and peripheral visual acuity is also apparent in the findings of Mandelbaum and Sloan (1947) and those of Experiment II in which acuity was measured at two luminance levels. Thus, two fun&men~l variables. luminan~ and refractive error, which represent limiting factors in central vision are relatively unimportant for discrimination of detail in the peripheral visual fields. The results of this experiment contrast with previous work on motion thresholds in the periphery (Johnson and Leibowitz, 1974; Leibowitz. Johnson and Isabelle, 1972) peripheral increment thresholds (Fankhauser and Enoch, 1962) and Westheimer function determination in the periphery (Enoch, Sunga and Bachman, 1970; Sunga and Enoch. 1970). It is difficult to interpret this discrepancy. However, the notion of “two visual systems” (Ingle. 1967; Tevarthan, 1968; Held, 1968, 1970; Schneider. 1967. 1969) may be relevant. Basically this approach states that there are two independent modes of processing visual information; a localization or orientation made for locating objects in the visual field and becoming aware of their presence; and an identification mode, which is mainly responsible for the discrimination of details and form. This may be thought of as a “where” and a “what” system. It is possible that these systems may be related to the X-Y classification of ganglion cell responses (Cleland, Dubin and Levick, 1971). Finally it may be worth noting that the decline of visual acuity from the fovea to 60” of eccentricity differs for the sinewave grating and the Landolt ring. Acuity diminishes by a factor of 10 for the sinewave grating and a factor of 15 for the Landolt ring. Kerr (1971) also found that the decrease in peripheral visual acuity was much lower with a square wave grating in the periphery than the data of Mandelbaum and Sloan (1947) obtained with Landolt rings. It could have been argued that the difference was due to differences in subject’s responses since these experiments use very small samples. However, the fact that in the present study two of the subjects served under both conditions suggest that the difference is attributable to the test object. Ackrlowk~dgemerlts-The authors are grateful to G. 8. Stein. for his skill in carrying out the peripherai refraction and for the support received in part from Grant MH ON61 from NIMH and CAFIR Research Fund of the University of Montreal. REFERENCES
Ahernethy C. N. and Leibowitz H. W. (1971) The effect of feedback on luminance th~shoIds for periphemlly presented stimuli. Prrcqt. Fs~c~Io~/J~s.10. 172-l 74. Clarke F. J. J. and Belcher S. J. (1962) On the localisation of Troxler’s effect in the visual pathway. VisiuctRex 2.
53-68. Cleland B. G., Dubin M. W. and Levick W. R. (1971) SUStained and transient neurones in the cat retina and lateral geniculate nucleus. J. Pltysioi.. Lo&. 217. 473497. Cornsweet T. N. (1962) The staircase method in psychoohvsics. Am. J. Pslrhol. 75. 485391. . ,
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M. MILLODOT.C..A. JOHNSON. ANNE LAMOST and H. W. LLIIWUXTL
Fincham W. H. A. (1956) Optics, 6th ed. Hatton, London. Freeman R. D.. Mitchell D. E. and Millodot M. (19721 A neural effect of partial visual deprivation in hu’mans: Science 175. 1384 1386. Green D. G. (1970) Regional variations in the visual acuity for interference fringes on the retina. J. Physiol., Lord. 207, 351-356. Held R. (1968) Dissociation of visual functions by deprivation and rearrangement. Psychol. Forsch. 31. 338-348. Held R. (1970) Two modes of processing spatially distributed visual stimulation. In The Neurosciences. Second Study Program (Edited by Schmitt F. 0.). Rockfeller Univ. Press, New York. Ingle D. (1967) Two visual mechanisms underlying the behaviour of fish. Psychol. Forsch. 31. 44-51. Johnson C. A. and Leibowitz H. W. (1974) Practice, refractive error, and feedback as factors influencing peripheral motion thresholds. Percept. Psychophys. IS. 276-j80. Kerr J. L. (1971) Visual resolution in the DeriDherV. . . < Per-
in the human eye: experimental data and theoretlcai model predictions. J. op!. Sot. .4rn. 64. 51&51? Low F. N. (1946) Some characteristics of peripheral visua! performance. Am. J. Physiol. 146. 573-584. Low F. N. (1951) Some peripheral visual acuit!. .4.nf..4 Archs Ophthal. 45. 80-99. Mandelbaum J. and Sloan L. L. (1947) Peripheral visual acuity. Am J. Ophthal. 30. 581-588. Millodot M. (1972) Variation of visual acult) in the central region of the retina. Br. J. ph!xiol. Opt. 27. 1?- ‘8. Millodot M. and Lamont A. (1974a) RefractIon of the periphery of the eye. J. opt. Sot. Am 64. 1l&l I I Millodot M. and Lamont A. (1974b) Peripheral vtsual acuity in the vertical plane. Visioll Rex 14. 1497-1498. Mitchell D. E.. Freeman R. D., Millodot M. and HaegerStrom G. (1973) Meridional amblyopia: evidence for modification of the human visual system by early visual experience. Vision Res. 13. 535-558. Osterberg G. A. (1935) Topography of the layer of rods and cones in the human ;et&a. Acta ophthal.~ Suppl. VI. RLiggs L. A. (1965) Visual acuity. In Vision and Visual Perception (Edited by Graham C. H.). Wiley. New York. Ronchi L. (1971) Absolute threshold before and after correction of oblique-ray astigmatism. J. opt. Sot. A,,I. 61. 1705-1709. Saugstad P. and Lie I. (1964) Training of peripheral visual acuity. ScarId. J. Psychol. 5. 218-224. Schneider G. E. (1967) Contrasting visuomotor functions of tectum and cortex in the golden hamster. Psycho/. Forsch. 31. 52-62. Schneider G. E. (1969) Two visual systems. Science 163. 895-902. Sunga R. N. and Enoch J. M. (1970) Further perimetric analysis of patients with lesions of the visual pathways.
by Millodot M. and Heath G. G.). Indiana Univ. Press, Bloomington. Leibowitz H. W., Johnson C. A. and Isabelle E. (1972) Peripheral motion detection and refractive error. Sciertce 177. I 207- 1208. Lotmar W. and Lotmar T. (1974) Peripheral astigmatism
Trevarthan C. B. (1968) Two mechanisms of vision in primates. Psycho/. Forsch. 31. 299-337. Weale R. A. (1956) Problems of peripheral vision. Br. J. Ophthal. 40. 392-415. Westheimer G. (1960) Modulation thresholds for sinusoidal light distributions on the retina. J. Physiol.. Land. 152. 67-74.
Dupup 0. C1967) Mesure de la puissance de I’oeil en dehors de I’axe. Recw Opr. 46. 79-83.
Enoch J. M., Sunga R. and Bachman E. (1970) A static perimetric technique believed to test receptive field properties: l-Extension of the Westheimer experiments on spatial interaction. Ant. J. Ophti~ul. 70. 113-116. Fankhauser F. and Enoch J. M. (1962) The effects of blur on perimetric thresholds. A.M.A. Archs Ophthal. 68. 240251. Ferree C. E. and Rand G. (1933) Interpretation of refractive conditions in the peripheral field of vision. A.M.A. Archs Ophthal. 9. 925-938.
Ferree C. E.. Rand G. and Hardy C. (1931) Refraction for the peripheral field of vision. A.M.A. Archs Ophthnl. 5. 717-731.
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Am J. Ophthol. 70. 403-422.
OF DIOPTRICS ON PERIPHERAL VISUAL ACUITY
MICHEL MILLOLKIT,CHRIS A. JOHNSON,’ ANNE LAMONT’ and HERSCHEL W. LEIBOWITZ~ Department of Ophthalmic Optics. UWIST. Cardiff. Wales (Receieed
14 Jarurary
1975)
Abstract-The peripheral dioptrics of the eye displays a considerable error of refraction caused primarily by oblique-ray astigmatism. It was hypothesised that this refractive error might contribute to the well known reduction in visual acuity in the peripheral visual field. Independent experiments were carried out in two laboratories. using different methods and different targets; sinusoidal gratings and Landolt rings. A third experiment was run in which two subjects from one laboratory were tested in the other. Measurements of visual acuity were obtained between 0” and 60” of eccentricity along the horizonral meridian both with and without correction of refractive error. The results of all three experiments indicate that the existing errors in refraction do not influence peripheral visual acuity. EFFEC-I. OF
DIOPTRICS ON
PERIPHERAL VISUAL ACUITY It has been well established that acuity declines as one moves progressively away from the fovea into the periphery (literature reviews may be found in Kerr, 1971; LeGrand, 1967; Low, 1951; Millodot 1972; and Riggs, 1965). The basis for this marked degradation in visual acuity with increasing anpJe of eccentricity has most frequently been attributed to neural factors such as increased receptive field size and decline in cone density. However, the relationship between cone density (Osterberg 1935), and acuity breaks down in the far periphery, and large refractive errors of refraction also occur (Ferree and Rand, 1933; Ferree, Rand and Hardy, 1931; Lotmar and Lotmar, 1974; Millodot and Lamont, 1974a). Thus, it is difficult to interpret degraded visual acuity in the periphery solely in terms of neural properties. Investigations of peripheral visual acuity have generally ignored the possible influence of refractive errors. Albini (cited in Low, 1951) observed that placing lenses between the eye and peripherally presented stimuli altered visual acuity. However, this was a qualitative observation and no quantitative data are reported. Wcale (1956) measured visual acuity at 55” and 70”; Clarke and Belcher (1962) (gratings ai 20”) and Green (1970) (gratings and interference fringes up to So) all find that optical errors do not, to a first approximation, account for the decrease in peripheral acuity. The purpose of the present investigation was to determine the effect of correction of peripheral refractive error on visual acuity. To this end. both peripheral refractive error and visual acuity were assessed by different methods. To examine the possible influence of different refraction methods. psycho-physical
’ Present address: Department of Ophthalmology. University of Florida. ’ Laboratory of Experimental Optometry, University of Montreal. Quebec, Canada. ’ Department of Psychology. Pennsylvania State University.
testing technique, and individual differences, two of the subjects were tested under all experimental conditions. EXPERIMENT
WITH SINUSOIDAL
GRATINGS
Apparatus
The apparatus. shown schematically in Fig. I, was designed to present the test and surround fields in Maxwellian view. Tungsten ribbon filament lamps (6 V, 9 A) served as light sources (Sl, S2) for the test and surround channels. Lenses LI and L2 formed sharp filament images on opal
w Fig. I. Schematic representation of the apparatus employed in Experiment I. The symbols refer to the following: S1. SZ--tungsten ribbon filament lamps; H-heat absorbing glass; Al, AZ-apertures serving as secondary light sources; OGI, OG2Apal diffusing -glass; Ll, Li L3, L4. L5. Lblenses; SF-surround field: B-beamsolitting cube; G-sinusoidal grating test stimulus; FS-held stop; SH-electronic shutter; F-fixation stimulus; AP-artificial pupil.
1357
The fixatlon stimulus (F) cmpioyed during perlphcrai determinattons consisted of a Landolt C (I R’I cenwred within a circular background field (41 1 of 151 cd m’ luminance. Durmg successive determinations the fixation stimulus. located I m from the observer. was moved to eccentricities of 20‘. 40’ and 60’ for the nerioheral stlmuh The surround field annulus served as a’ fixation sttmulus for fovea1 determinations. The observer‘s head was held rlgld by a bite-board arrangement. which the observers were taught to properI> align to the apparatus. An artificial pupil (API. ?5mm dia. was located on a thin metal vertical strip which was sufficiently narrow to prevent occluding the fixation stlmulus during peripheral determinations. The artificial pupil was employed to control for changes m effective pupil size with eccentric viewing. thus maintaining retinal illumination constant for all determinations. Procedure Id
Fig. 2. Schematic representation of (a) the surround field only. (b) the test stimulus only and (c) the test stimulus superimposed on the surround field. diffusing glass sheets (OGI. OG2) masked off by apertures Al and A2. which acted as secondary light sources. The final source image formed in the plane of the artifical pupil was a 27 x 6,Omm vertical slit. The collimated portion of the surround field channel (between lenses L3 and L4) contained the annulus-surround field arrangement (SF) presented in Fig. 2(a). It consisted of an opaque ring centered within a larger clear circular portion of a thin glass sheet. The entire clear circular area subtended IO”. while the ring exhibited a separation of 2” between the inner edges and 3” between the outer edges. This arrangement was placed at an optical distance of I m from the observer. Luminance of the surround field was 524 cd/m’ (7658 td). The test stimulus consisted of a sinusoidal grating (G) placed within the collimated portion of the test channel (between lenses L5 and L6). The stimulus size (2”) was determined by a field stop (FS) attached to lens L6. The appearance of the stimulus is presented in Fig. 2(b). A beam-splitting cube (B) combined the test and surround channels to present the observer with the representation shown in Fig. 2(c). The space-average luminance of the grating stimulus was I 118.2 cd/m’ (1633.6 td). A series of 20 National Bureau of Standards sinusoidal gratings (modulation 0.658. maximum transmittance 0.664. minimum transmittance 0.137), successively increasing in spatial frequency by a factor of 1.26. served as test stimuli. Refined adjustments in spatial frequency were accomplished by rotation of the grating in the collimated light. about the horizontal axis of the grating lines. At large angular rotations, the spatial distribution of light transmitted from the grating test stimulus departs from a true sinusoidal representation. For this reason, angular rotations were limited to less than approx 65”. A circular table. equipped with a holder for securing the gratings horizontally. accomplished the stimulus rotations. The outer edge of the table contained a Vernier scale. allowing measurements of angular rotation accurate to within 5”. The dioptric difference between the upper and lower portions of the grating was less than 05 D for the largest stimulus rotation. The grating lest stimulus was superimposed on the continuously presented surround field For 250msec. controlled by an electronic shutter (SH). This brief exposure time was employed to control for possible influences of the field stop (FS) on accommodation during fovea1 determinations, and to minimize Troxler’s effect for peripheral measures.
The refractive errors of the four observers were determined for the four eccentricities tested. fovea] (0). and 20’. 40” and 60” in the temporal field, by an experienced optometrist using static retinoscopy. The observers were given instruction and practice in proper alignment to the apparatus, especially for the alignment procedures necessary for peripheral determinations, followed by IO practice sessions. each lasting approx 1.5 hr, prior to the collection of data. This was to control for the marked improvements with practice known to occur for visual tasks in the periphery (Abernethy and Leibowitz, 1971: Johnson and Leibowitz, 1974; Low, 1946. 1951; Saugstad and Lie, 1964). Each session consisted of three successive determinations of the resolution threshold at each of the eccentricities. An interleaved double staircase method (Cornsweet. 1962) with variable step size was employed. The observers were instructed to report whether they could resolve the lines of the grating. The mean point about which the staircases varied was the position at which the observers reported the presence of lines 50% of the time. For half of the sessions, testing began at 0” of eccentricity and continued out to 60”. This order was reversed for the other half of the sessions. The starting point for an individual session was randomly determined. Four experimental sessions were conducted after the practice sessions. Two of the sessions were without correction of the peripheral refractive error while the other two were with appropriate correction, the order of these sessions being randomly determined. During sessions using no correction, the grating test stimulus was placed al an optical distance of 1m, the same distance as the surround field and the fixation stimulus. For the sessions with correction of refractive error. this procedure differed in the manner described below. Since the test stimulus consisted entirely of horizontal lines. only the horizontal meridian was corrected. This was done by adjusting the distance between the grating and lens L6 to compensate for horizontal refractive error. The grating was moved between L5 and L6 to adjust the optical distance of the grating so that it was in focus when the observers fixated a target at a distance of I m. This may be considered as analogous to a Badal optimf system as the angular dimensions of the grating remained constant with changes in the optical distance of the grating. Brief rest periods were given between each threshold determination and, in addition. if the observers reported loss of fixation, eye movements, or momentary attentional lapses the trials were discarded. RESULTS
The correction required in the horizontal meridian at each stimulus eccentricity is presented. in Table 1. Individual differences in both the amount and direc-
Elects of dioptrics on peripheral visual acuity Table I. Amount of correction (in d@tersl requii+edin the horizontal meridian for observers JG. FO. RM and CJ. in sinusoidat gratings experiment
Obsarver
0
20
JO.
-ff.so
-
IO,
-4.25
RM.
cr.
5,7s
70
60
c 2.75
t 4.25
+ 0.25
i
1.00
+ 1.50
- 1.50
* 1.50
+ 0.25
+ 0.25
-
m 0.25
- 0.25
+ 6.00
0.50
tion of the xcessaq correction can be readily observed In general greater corrections are necessary with increasing eccentricity. The data are reported in terms of the reciprocal of the minimum angle of resolution (angle subtended
by one half-cycle) in minutes of arc. The rest&s for each of the four observers are presented in Fig. 3 which presents vised acuity on a Iogarithmic scale as a function of stimulus eccentricity. The solid lines represent the data obtained without correction of refractive error. and the dashed lines represent the functions obtained when refractive error was corrected. Each point on both of the curves represents the average of six threshold determinati0nS.
1359
opt&l S$WIII add psycho-physical ..method similar to the present study. The values reported by Mandelbaum and Sloan (1947) for a Landolt C test-object. are much lower. Although correction of refractive error improves acuity for the fovea. it has little or no cfTect in the periphery. For one subject (CJ) who has a large error at 60”. peripheral refractive error correction had a slight effect. The peripheral data contrast sharply with
those for the fovea for which smaft refractive errors have a marked e&et on acuity.
EXPERIMENT
WlTH LANDOLT
RINGS
The apparatus for measuring acuity consisted of an optical system which focussed a black negative of a Landolt ring on a white screen. It is basically the same as that previously described by Millodot and Lamont (1974b). The gap of the ring could be orientated in the four major directions and the size of the break could be varied in discrete steps between 44“ and 19.5’.The projector remained stationary and the Iocation of its projected image was varied by means of a rotating mirror pjaced at right angles to its optical axis. The projection screen was adjusted at various angular positions from a central fixation point along the horizontal (temporal) plane and was kept equidistant from the subject’s eye for all eccentricities. The luminance of the adapting background was 86cd/ m’ and the projected circular area containing ihe Landoft ring was 245&n’. The contrast of the Landolt ring was 55%. Target exposure duration was constant at 0.12 sec.
in line with previous investigations, acuity is degraded in the periphery, with the most rapid drop occurring between 0” and 20” of eccentricity. ‘These The experiment was divided into two parts. Experiment acuity values are in good agreement with those of II and validation. Peripheral refraction in the first part Kerr (1971) who used square-wave gratings and an was determined to eccentricities of 50” on three subjects
I
+-With
cometion
Fig 3. Log visuaf acuity, with and without correction of refractive error, as a function of eccentricity for observers JG. FO. RM and CJ, using gratings.
using Zeiss ~f~to~try~ retinoscopy and subjective method. The values obtained with these three techniques were used as correction and placed at 90” to the line of sight at each eccentricity tested. The individual data which were actually used have been reported elsewhere (Millodot and Lament, 1974a). In the validation experiment a retinofocometre (built by EsseI, Paris) was used to measure refraction at 0“. 20’, 40” and 55”. Three subjects participated in Experiment II. Two additional subjects who had also participated in Experiment I. served in the validation experiment. All subjects were familiar with visual experimentation. The subject was seated at the centre of a screen arc with his head resting on a headrest and adapted to the background fuminance for 7 min. Then he was warned by a ready signal to maintain central fixation and a Landoit ring was exposed at a given eccentricity. Fixation was not monitored throughout the experiment but some runs in the experimental group were carried out using a photo-electric eve movement recorder (Eve-Trdc). The subject indicated’verbaliy in which direction he saw the gap: The targets were presented at a rate of about one every 10sec. Twelve presentations of the ring were made. i.e. 3 times in each direction. but the sequence was haphazard. The method of constant stimuli was used and a percentage of 50% correct responses was considered as threshold. Judgment was made monocularly with the right eye. The order of presentation was varied between starting with and without peripheral correction, above and below threshold. at O”-W eccentricity. Each session was Iimited to 35-40 min so as to avoid fatigue which easily overcame the subjects as they concentrated on a peripheral target. Most sessions
51.
1360
1
0.5 r c;
0.6 0.4
03 f 02
MILLWOT.
01
“U0
A.
JOHWW.
50
60
ANQ
LAMOST
and
H.
M’.
L~IIKN’IT,’
\ 4, \\ \
\
\
4
C.
IO V~sunl
20 field
30
‘Y -\
40
eccentricity,
0
deg
Fig. 4. Visual acuity (in decimal value), as a function of eccentricity (temporal visual field.) H represents the data with correction of peripheral refractive error; O--O represents the data without correction. Each data point represents the mean of three subjects. Background luminance of the Landolt ring, 245 cd/m*.
were repeated on several other occasions in order to assess the effect of habituation which is known to play a role in peripheral acuity. The entire procedure was repeated on all three subjects at a lower luminance with an ND 2 filter patched in front of the viewing eye. Results
The mean optical correction of the peripheral diog tries for the three subjects has been reported elsewhere (Millodot and Lamont. 1974a). It is evident that peripheral correction due principally to obliqueray astigmatism becomes very large with visual field eccentricity. The measurements of visual acuity with retinal eccentricity obtained with three subjects at the higher luminance are given in Fig. 4. The well-known decrease in visual acuity with eccentricity is apparent in these data. Each data point is the mean of three subjects. The open circles represent the findings with no peripheral correction and the closed circles the values with the peripheral correction. It is apparent that with the appropriate peripheral correction in place at each eccentricity investigated, there is no appreciable improvement in peripheral visual acuity. The same trend was found for each of the three subjects. At the lower luminance (Fig. 5) the overall visual acuity is slightly reduced in the central part of the retina and hardly affected towards the periphery as compared to the data obtained with the same subjects at the higher luminance (Fig. 4). The lack of a relationship between peripheral acuity and luminace is in good accord with Mandelbaum and Sloan (1947) and Kerr (1971). The correction of the peripheral optics yields no appreciable improvement in visual
IO V~swl
20 frld
M
40
eccentncity.
50
60
deg
Fig. 5. Visual acuity (in decimal value) as a function of eccentricity (temporal visual field). W represents the data with correction of peripheral refractive error; O--O represents the data without correction. Each data point represents the mean of three subjects. Background luminance of the Landolt ring, 2.45 cd/m?.
performance as can be noted in Fig. 5. The same trend was found for each of the three subjects. Validatiort experiment The results with the two subjects who had previously served in Experiment 1 are presented in Fig. 6. Individual data for each subiect are plotted with and without peripheral correction. At 20” and 55’ eccentricity the results are the same whether the eye is corrected or not for both subjects. At 40” acuity Corrected 0 Uncorrected FO~Cwrected 0 Uncorrected
cJ.
0.6 -
2.
s ::
0.4-
s .L? > 0.2 -
o-
I
I
0
I
I
I
I
I
I
IO
20
30
40
50
60
Visual
field
eccentrbclty,
deg
Fig. 6. Visual acuity (in decimal value) with and wrthout correction of peripheral refractive errors. as a function of eccentricity (temporal visual field) for observers CJ and FO using Landolt rings.
EtTectsof dioptrics on peripheral visual acuity is slightly better with the peripheral correction for subject CJ and the reverse for subject FO. These values are in good accord with those obtained on the same subjects with a grating in Experiment I. This experiment provides further evidence showing that peripheral correction of the eye does not improve peripheral acuity for either a grating or Landolt ring target. DiSCUsSION
The data of these experiments demonstrate that the poor quality of the peripheral dioptrics cannot account for the degradation of acuity of the adult eye in the peripheral visual field. It must be noted that refractive error is usually measured along the axis of the eye. However, whether the eye requires a correction or not along the visual axis, it is nevertheless affected by oblique-ray astigmatism as any other optical system (Fincham, 1956). This obliqueray astigmatism induces a rather substantial peripheral correction which increases with retinal eccentricity (Ferree et al., 1931; Dupuy, 1967; Millodot and Lamont, 1974a). It could have been thought. therefore, that such off-axis refractive error might limit visual acuity but our data do not support this hypothesis. It must also be noted that although retinal illumination is affected by oblique Iight incidence this effect is the same with and without the correction of peripheral dioptrics, and therefore it does not alter the difference obtained in the two conditions. The present results are in good agreement with those found by Weale (1956). Clarke and Belcher (1962) and Green (1970). As in central vision the optical quality of the eye does not necessarily limit visual acuity (Westheimer, 1960; Riggs, f965) provided that the focus is appropr~te to the st~ulus distance and for average pupil sizes. This conclusion applies, at least, to the adult subjects used in this experiment. Rather visual acuity is apparently limited by the neurophysiological organisation of the visual pathway from the retina to the cortex. It could be argued, though, that in the early years of life the poor quality of the peripheral dioptrics leads to a degradation of visual acuity. This loss of function would be due to inappropriate visual stimulation, the visual deprivation being a consequence of exposure of the retina to defocussed images. It has been shown (Freeman. Mitchell and Millodot, 1972; Mitchell, Freeman, Millodot and Haegerstrom, 1973) that individuals with large amounts of astigmatism early in life develop a deficit that persists even after the astigmatism is corrected as an adult. It appears that there is a critical period for the proper development of the “feature detectors” and the presence of a blurred retinal image can, at least, in one eye and in central vision. have a marked effect on the development of these anaiysers. It is possible that an analogous deficit occurs in the periphery. This suggestion could be tested by correcting the peripheral oblique-ray astigmatism of the eye at birth and measuring the peripheral visual acuity after the critical period of development. It is also interesting to note that Kerr (1971) found acuity to be less affected by luminan~ as the angle of eccentricity was increased. In the fovea1 regions. visual acuity as we11as aimost all other psycho-physical
1361
functions, are greatly affected by luminance level. However, for the two subjects in Kerr’s investi~tion, visual acuity changed only slightly over a mnge of over 6 log units of luminance at an eccentricity of 30”. This independence of luminance and peripheral visual acuity is also apparent in the findings of Mandelbaum and Sloan (1947) and those of Experiment II in which acuity was measured at two luminance levels. Thus, two fun&men~l variables. luminan~ and refractive error, which represent limiting factors in central vision are relatively unimportant for discrimination of detail in the peripheral visual fields. The results of this experiment contrast with previous work on motion thresholds in the periphery (Johnson and Leibowitz, 1974; Leibowitz. Johnson and Isabelle, 1972) peripheral increment thresholds (Fankhauser and Enoch, 1962) and Westheimer function determination in the periphery (Enoch, Sunga and Bachman, 1970; Sunga and Enoch. 1970). It is difficult to interpret this discrepancy. However, the notion of “two visual systems” (Ingle. 1967; Tevarthan, 1968; Held, 1968, 1970; Schneider. 1967. 1969) may be relevant. Basically this approach states that there are two independent modes of processing visual information; a localization or orientation made for locating objects in the visual field and becoming aware of their presence; and an identification mode, which is mainly responsible for the discrimination of details and form. This may be thought of as a “where” and a “what” system. It is possible that these systems may be related to the X-Y classification of ganglion cell responses (Cleland, Dubin and Levick, 1971). Finally it may be worth noting that the decline of visual acuity from the fovea to 60” of eccentricity differs for the sinewave grating and the Landolt ring. Acuity diminishes by a factor of 10 for the sinewave grating and a factor of 15 for the Landolt ring. Kerr (1971) also found that the decrease in peripheral visual acuity was much lower with a square wave grating in the periphery than the data of Mandelbaum and Sloan (1947) obtained with Landolt rings. It could have been argued that the difference was due to differences in subject’s responses since these experiments use very small samples. However, the fact that in the present study two of the subjects served under both conditions suggest that the difference is attributable to the test object. Ackrlowk~dgemerlts-The authors are grateful to G. 8. Stein. for his skill in carrying out the peripherai refraction and for the support received in part from Grant MH ON61 from NIMH and CAFIR Research Fund of the University of Montreal. REFERENCES
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