Effect of target size on critical flicker frequency in flicker perimetry

Vision Res. Vol. 3, pp. 523-529. PergamonPre~ 1963. Printed in Great Britain.

E F F E C T OF T A R G E T SIZE O N C R I T I C A L F L I C K E R FREQUENCY

IN FLICKER PERIMETRY t

ERNST WOLF and ROBERT J. VINCENT Department of Clinical Eye Research, Institute of Biological and Medical Sciences, Retina Foundation, Boston, Massachusetts (Received 20 October 1963)

Al~raet---Cdtical flicker frequencies (CFF) wcr¢ determined on the horizontal meridian between 90 ° nasal and 90 ° temporal with test fields subtending visual angles of 4 °, 2 °, 1°, ~°, ¼°, ~°, ~ ° , and ~ ° when luminance was 32 f t L and light-time/dark-time ratio was 1 : 1. C F F increases substantially when test field size varies between 2" and 1° or 2 ° of arc. With larger test fields, the increase in C F F is only slight. In flicker pcrimetry, an optimal response is therefore obtained w i t h test fields of about ! ° or 2 ° angular subtens¢.

R~,sam~--On d~termine la fr~luence critique de fusion claus le m&idien horizontal entre 90 ~ nasal et 90* temporal avee des tests de diam~tres apparents 4 °, 2°, I*, ½", ¼°, ~*, ~°, et ~o, la luminance ~tant 32 ftL et le rapport lumi,~'re:obscurit~ 1 : I. La fr&luence augmente netternent quand le d i a ~ apparent du test varie de 2" jusqu'~t 1° ou 2°. Pour les tests plus grands, il n'y a qu'un l~ger accroissement de fr~quence. En p&im~trie de papillotement, la r~ponse optimale est done obtenue avec des tests de diam~:tre apparent d'environ 1° ou 2°. Zusammeah~ag~Am horizontalen Meridian des Gesichtsfeldes wurden zwischen 90 ° auf der nasalen und 90 ° auf der ternporalen SeRe mit Priiffeldem, die Sehwinkel yon 4 °, 2 °, 1°, ½°, ¼°, ~r°, ~ ° , und ~ ° ausmachten, kritische Flimmeffrequenzen bestimmt. Die Helligkeit war 32 Fusskerzen und das Licht: Dunkelverh~tnis 1 : 1. Die kdtisch¢ Flimmcrfrequenz w~icbst his zu Pri.iffeldgr6ssen yon 1° bis 2 ° stark an und nur wenig mehr fiir Priiffeldgr6ssen fiber 2 °. Bei Flimmerpcrimetrie sind deshalb beste Resultate mit Priiffeldern yon I ° bis 2 ° Sehwinkel zu erwarten. THE purpose o f perimetry is to determine the extent o f the visual field and the size and location o f scotomata. In stationary perimetry, test objects are presented in fixed positions o f the visual field, while exposure time, target luminance, contrast between target and surround, and target size are at variance. Thus, relative sensitivity o f various retinal regions is studied by threshold determinations (SLOAN, 1947, 1961; HARMS, 1950; Z1GLER and WOLF, 1958; WOLF and ZIGLER, 1959; CAMPBELL e t aL, 1963). In dynamic perimetry, in addition to luminance, contrast and target size, the velocity and direction o f motion are parameters in field determination (GOLDMANN, 1945; MCCOLGIN, 1960). In flicker perimetry, targets are presented at various fixed positions o f the visual field, and flicker frequency is lowered from fusion to a level at which flicker becomes perceptible (MILES, 1950; KLEBERGER, 1955). Thus, critical flicker frequency values ( C F F ) are obtained within the entire field o f vision, and relationships between C F F and retinal position axe established (LovEKEN and CHANDLER, 1959; CAMPBELL and RITTLER, 1959). F o r each retinal location, C F F is found to be a function o f target luminance, contrast, target size, and light1 This investigation was supported by a research grant (B--1482) from the "National Institute of Neurological Diseases and Blindness of the National Institute of Health, Public Health Service. 523

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time dark-time ratio in each flicker cycle ( M¢ lzARLAND e,' al.. 1958: Wot. v and M c G o w x,.. 1963). While in stationary and dynamic perimetr', targets of small angular subtense often yield more revealing results than larger targets, in flicker perimetry targets subtending a visual angle of 2 ° or more are usually employed to obtain reliable and valid responses (H¥ LKESI.~. 1942). Studies on target size in relation to C F F in the central retina revealed a linear relationship between C F F and the logarithms of target area F = c log A -- d (G RAYI'r and H ARPE R, 1930). The linear relationship however has only limited validity ; over a wider range of target sizes. the relationship appears sigmoid (PI1ERON, 1955; KUGEL.~|ASSand LANDIS, 1955). C F F determinations outside the central retina with test fields of various sizes reveal the C F F rises to values higher than found in the center when test targets subtend angles of 2° or more. For targets smaller than 2 °, C F F drops immediately outside the fovea to values lower than those found at the center (GRANiT and HARPER, 1930; CREED and RUSH, 1932; HYLKEMA, 1942; KUGELStASS and LAND|S, 1955; W o L f and McGOwAN, 1963). When C F F was determined with very large test targets (between 7 ° and 50" angular subtense) it was found that C F F remained unaffected when a considerable central portion (up to two-thirds of the target area) was blocked out. C F F is therefore not determined by the total area of the retina stimulated, but only by that portion which is capable of the most effective spatial summation and best temporal resolution (ROEHRIG, 1959a, b). Similarly, it has been shown that when a large target area is sulxlivided by narrow cross-bars, C F F increases. The proximity of contours seems to enhance flicker perception, while in a large uniform flickering area, the center portion remains ineffective (CRozIER and WOLF, 1944a, b). In the following study, the size of a flickering test target was varied over a wide range while all other pertinent parameters were kept constant.

METHOD Flicker tests were carried out with a flicker perimeter as described in an earlier paper ( W o L f and M c G o w A N , 1963). The light source was a Sylvania Glow Modulator Tube (R 1131C) activated by a Grayson-Stadler Flicker Apparatus. The diameter of the light source was 2 ram. In order to obtain test fields of sufficiently large size at an object distance of I m a short-focus collimating lens was used to illuminate evenly a translucent screen which yields a circular test field of 2 ° angular subtense. By inserting diaphragms, the target size was reduced to 1°, ½°, ¼o, ~_- ~o, and ~.°. The smallest target subtends a visual angte of 1-9 rain which corresponds to 20./40 vision. To increase the test field size to 4 °, a special housing with a different optical arrangement was built and s ~ u t e d for the source ordinarily used. For all test f~ld sizes, luminJmee was held constant at 32 m L as measured from a steady source with a Macbeth l l l u m i n o ~ . This r ~ r e s e n t s the middle n m p o f photopic vision. In order to avoid excessive contrast belween tar~.t ~ surrouad, an ambient illumina~on of about 1 ftL was kept on in the laboratory. Tests were carried out on ten individuals, five males and five readies in the age range between sixteen and twenty-six years, except for one, E.W. Each ob~,s'ver was allowed to use his preferred eye, while the other was occluded. No corrective lenses were required.

Effect of Target Size on Critical Flicker Frequency in Flicker Perimetry

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The square waves of. light in these tests were used only at a light-time;'dark-time ratio of I 1, which according to earlier studies should yield C F F values near the maximum for each retinal location under the prevailing conditions of testing ( M c FARLAND et al., 1958; WOLF and McGowA~r, 1963). C F F determinations were made on the horizontal meridian between 902 temporal and 90 ° nasal from fixation at nineteen retinal positions. Test field size was varied from 4 ° to 0.03 ° . The observer's head was held in a fixed position by means of a head and chin rest so that his eye was at the same level and distance from the target for all test field positions. The observer was warned each time a C F F determination was to be made. Flicker frequency was gradually reduced from fusion (more than 80 c/s) to the point at which the observer indicated by tapping that he saw flicker. The critical flicker frequency value was read at this instance from a dial on the flicker apparatus. Imediately flicker frequency was raised again to a level above fusion in order to avoid undue fatigue from the flickering light. After a short interval, C F F was determined a second time. In the event that a difference of more than 2 c/s was obtained, additional readings were taken and the arithmetic mean was used as C F F for that point. RES U LTS

Mean C F F values obtained from ten observers at nineteen positions along the horizontal meridian from 90 ° nasal to 90 ° temporal from fixation and with test fields subtending visual angles of 4 °, 2 °, 1°, ½°, ¼°, ~r°, -~°, and ~ ° are presented in Fig. 1. With the exception of the 42 curve, the highest C F F values are found at the center of the visual field and decline steadily toward the periphery. For the 4 ° field the highest C F F values are found 10° nasal. The curves are not symmetrical; C F F decreases more rapidly in the nasal field than in the temporal field. The temporal field is projected on the phylogenetically older nasal retina

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which in lower vertebrates serves primarily for the perception of motion and intermittent stimulation. While visual acuity decreases very rapidly outside the fovea, C F F declines muc[~ more slowly in the periphery and it may be assumed that the flicker response depends besides upon density and neural linkage of elements on the specific effects of repetitive photic excitation. As target size becomes smaller, CFF changes little in the temporal periphery beyond 30" or 40 °, whereas in the nasal field the decline in C F F is more continuous beyond 30 ° . 60

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FIG. 2. Relationship between arithmetic mean values of Critical Flicker Frequencies (CFF=) and visual angle subtended by test field ( d e l r e ~ ) at seven locations of the horizontal meridian of the visual field. Up to 1° an~,dar subtense CFF rises steeply; above I° increase is only slight. When the flicker target is fixated foveatly, it appears clear and sharp. Outside t l ~ fovea, especially beyond 30 °, the target contours seem blurred, and byond 60°, the outline becomes so hazy that one has only the impression of a light spot of indistinct shape. Only when flicker frequency is lowered to the critical level has one a clear impression of the target during each individual flash. With decreasing target size, perception becomes increa~ngly more difficult until one is aware of the presence of the test target only when flicker frequency is reduced to the critical level. O f particular interest is the observation that with the exeel~ion o f the 4 ° test field, C F F is highest at the center. Others have reported an increase in C F F to a maximum 100 or 20 ~ from fixation (MILES, 1950; C^MPiVLL and RnrrLER, 1959). A detailed analysis of our individual data shows for some o b ~ r v ¢ ~ a paraeontral rise with test ~ l d s o f 4°; 2 °, a n d at times 1° angular subtense. Some show art a t m o ~ ~lat central section t o 20° or 30° f r o m fixation. With all smaller test fields a raT~id d r o p to much lower C F F values is alrctady in some individuals, and has never been observed with smaller targets.

Effect of Target Size on Critical Flicker Frequency in Flicker Perimetry

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In Fig. 2 the relationship between target size and C F F is shown for seven retinal positions, i.e. for the fovea, 30 °, 60 °, 90 ° temporal and 30 °, 60 °, and 90 ° nasal. Hyperbolic curves are obtained which rise sharply when test field size increases from 2' to 1°, then approach asymptotically a m a x i m u m when test field size increases to 2 ° and 4 °. At the fovea, C F F slightly more than doubles, while test field diameter increases thirty times from 2' to 1°. Increasing field diameter from 1° to 4 ° yields a small additional increase in C F F by a factor of 1-08. In the temporal field at 30 °, 60 °, and 90 ° C F F increases about 3.2 times while test field size varies between 2' and 1°, and 1-3 times when test field size varies between 1° and 4 °. In the nasal field, C F F increases at 30 °, 60 °, and 90 ° by a factor of 3.6, 4.5, and 10 times respectively, when test field size varies between 2' and 1°, and by a factor of 1.3 when test field size increases from 1° to 4 ° at all three nasal field positions. Thus, a marked change in C F F occurs when test field size is reduced below 1o and only a very small change when testfields subtend angles larger than 1°. In practical flicker perimetry it seems therefore that nearly maximal response is obtained with test fields subtending visual angles of 1° or 2 °, while larger targets will not enhance the flicker response and smaller targets will yield decidedly lower C F F values. I

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In Fig. 3, the relationship between C F F and the logarithm of test field area is shown for the same retinal positions as in Fig. 2. A set of seven S-shaped curves was obtained. In the central retina a smooth curve is found which tends to flatten at the upper end when test fields are large. At the low end, when test fields are very small, a change in slope is also indicated. When test fields are presented 30 ° nasally and 30 ° and 60 ° temporally, the initial rise of the curves is not as steep, and a change in slope at the top of the curves is not as clearly indicated as in the central curve. 90 ° temporally and 60 ° and 90 ° nasally almost symmetrical S-shaped curves are obtained. The CFF-log Area relationship therefore appears sigmoid at all retinal locations within the range of test field sizes employed in this study. A linear relationship (Granit-Harper Law) may be assumed only within a rather limited range o f test field sizes.

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DISCUSSION An increase in test field size enlarges the number of retinal elements stimulated. A substantial increase in C F F , however, was seen only with test fields up to 1° angular subtensc. while it became much smaller for test fields larger than 1 ~. In the flicker tests described here luminances are far above threshold and the responses observed depends u p o n the availability o f elements for excitation by successive flashes o f light at a given critical flicker rate (CROZIER and WOLF, 1944a, b). Both photochemical processes in the retinal receptors and the transmission o f impulses to the C N S are involved. Excitation from a certain number o f elements stimulated by a test target of a given size reaches in cortical representation an area corresponding in size and location to the peripheral area stimulated. If the test field is made larger, the marginal elements may lie too far from the center, and their excitations may not add to the excitation conveyed t h r o u g h a " c o m m o n p a t h " to the same cortical area (SHERRINGTON, 1906). Recordings from optic nerve fibers and single cells o f the visual cortex have given information on the transmission o f impulses from specific retinal regions to specific cortical cells (HARTLINE. 1938; BARLOW. 1953; KUFFLER, 1953). It was found that the retina consists of well-defined areas which, when stimulated, cause activity of specific cortical units. Such "receptive fields" are usually composed o f antagonistic excitatory and inhibitory regions. Their size increases with distance from the fovea. In the cat, it vanes between a fraction o f a square degree and 16-32 square degrees. The greatest number is in the range between 1 and 8 square degrees (HUBEL and WIESEL, 1959, t962). In the Spider Monkey, the centers of receptive fields surrounded by inhibitory areas vary between 0"05 ° and 2 °, increasing in size with distance from the fovea (HUBEL and W|ESEL, t960). The similarity o f receptive field size and optimally effective target size in flicker perimetry might be coincidental. But one might surmise that the failure of increasing C F F substantially with fields larger than 2 ° could be due to the overlap of test fields into inhibitory areas o f retinal receptive fields. REFERENCES BARLOW, H. B. (1953). Summation and inhibition in the frog's retina. J. Physiol. 119, 69-88. CAMPBELL, C. J. and R.ITTLER,M. C. (1959). The diagnostic value of flicker l~rimetry in chronic simple glaucoma. Trans. Amer. Aead. Op,~tkaL Oto-laryng. 63, 89-97. CAMPBELL, C. J., RITTLER, M. C, and KRAMER, W. G. (1963). A new projection adaptometer. Arch. Ophthal., IV. Y. 69, 564-570. CREED, R. S. and RUSH, T. C. (1932). Regional variation in sensitivity to flicker. J. PhysioL 74, 407-423. CROZIER, W. J. and WOLF, E. (1944a). Theory and measurement of visual mechanisms. X. Mo~4tCation of the flicker response contour, and the significance of the avian pecten. J. sen. l~nysioL 27, 287-313. CROZXER,W. J. and WOLF, E, (1944b). XI. On flicker with subdivid~l ftclds. J. gcn. Pkysiol. 27, 401-432. GRANtT, R. and HARPER, P. (1930) Comcarative studies on the peripheral and central retina ii. Synapse •reaettons in the eye. Amer. J. PhysioL 93, 21 !-228. GOLDMANN,H. (19, HARMS, H. (1950). HARTLINE, H. K. (1 the retina. Amet HumtL, D. H. and Wt~seL, T. N~ (1959). Receptive fields of sinsl¢ n~arones in the cat's st,~' t¢ coctcx. 2. Physiol. 148, 574-580. HUeEL, D. H. and Wt~s~L, T. N. (1960). Receptive fields of optic nerve fibers in the spider monkey. J. Physiol. 1~4, 572-580. Huett., D. H. and WtHEL, T. N. (4962). Receptive fields, binocular imcraction and functional architecture in the cat's visual cortex. ,/. Phalli. 1 ~ 106--t54. HYLKtMA, B. S. (1942). Fusion frequency with intermittent light under various c i r c l e s . Acta Ophthal.. Kbh. 20, 159-180.

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