High-pass Resolution Targets in Peripheral Vision LARS FRISEN, MD, PhD

Abstract: Visual acuity was measured at 10° intervals on the horizontal merid­ian in two normal subjects, using high-pass spatial frequency filtered test tar­gets in a computer graphics display. The close similarity between detection and recognition thresholds resulted in quick and reliable measurements. Peripheral acuity was proportional to local retinal ganglion cell separation. High-pass tar­gets appear to be nearly ideal for clinical perimetry because of the easy test task and the possibility of interpreting results in terms of numbers of functional neuroretinal channels. The major limitation appears to be a somewhat fuzzy definition of small, circumscribed defects. [Key words: optotypes, perimetry, peripheral acuity, retinal ganglion cells .] Ophthalmology 94:1104-1108, 1987

Most clinical tests of peripheral vision assess the ca­pacity for discriminating a spot of light from a neutral background. The major advantage of this "differential light sensitivity" approach is its simplicity. The major disadvantage is that it is poorly understood how the measuring values reflect the state of the visual system. Resolution measurements are more informative in this regard because the results reflect the number of func­tional retinal output channels. I,2 A recent study of pe­ripheral acuity in glaucoma patients has indicated that such measurements are not only clinically feasible, but also appear to offer superior sensitivity and specificity.3

Unfortunately, measurements of peripheral acuity are even more demanding and time-consuming than ordi­nary perimetry. This is due to the fact that there are two visual thresholds associated with acuity targets: one for detection and one for resolution.4

,5 This is apparent al­ready in daily life. For example, when approaching a distant road sign, the presence of symbols can be appre­ciated long before identification is possible. This percep­tual dichotomy is easy to recognize in central vision but commands considerable effort in peripheral vision.

So-called vanishing resolution targets offer a solution to this problem, by virtue of their closely similar detec­tion and resolution thresholds. The vanishing attribute means that the targets are either invisible or resolvable.

From the Department of Ophthalmology, University of Goteborg, Swe· den.

Supported in part by De Blindas Forening, Goteborg, Sweden.

Reprint requests to Lars Fris€m, MD, Ogonkliniken, Sahlgrenska sjukhu· set, S-413 45 Goteborg, Sweden.

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This peculiar property is obtained by including two lu­minance levels in the test target, one above and the other below background level. Luminances are adjusted so that the space-average equals the background. Hence, when presented with a size below the resolution thresh­old, the target components blend invisibly into the background. This technique, originally developed by Friedenwald6 for an astigmatism chart, has been shown to be equivalent to filtering out low-spatial frequency components, or "high-pass" filtering. 4 Ring-shaped tar­gets of this type have been proposed to be particularly useful in measuring peripheral vision. 5 This was investi­gated in normal subjects, using a computer graphics technique for target generation.

SUBJECTS AND METHODS

The subjects had no history of eye disease, and results were normal on examination. Only one eye was tested, with a normal pupil. The other eye was covered. Subject A, a 31-year-old woman, was 0.50 sph myopic. Subject B, a 47-year-old man, was -0.75 cyl X 165. Both were well trained in visual discriminations of various types.

The tests were generated on an IBM Personal Com­puter combined with an extra graphics card (Tecmar Graphics Master [Solon, OR]), and a black and white video monitor (Electrohome EVM-1719, 17 -inch diago­nal, P4 low-persistence phosphor [Kitchener, Ontario, Canada]). Resolution was set to 520 X 400 picture ele­ments at four grey levels in noninterlaced mode. Refresh rate was 60 Hz.

Test distance was 1.00 m, except for 0° and 10° of eccentricity where it was increased to 5.00 m. Ametro-

FRISEN • HIGH-PASS RESOLUTION PERIMETRY

Fig 1. Photograph of video screen, showing part of target series. Con­trasts are not exactly identical to those of video display due to limita­tions in reproduction process. Notice how targets vanish one by one into background with an increase in viewing distance.

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Fig 2. Minimum angle of resolution (MAR) at different contrast levels and loci along the horizontal meridian for subject B. Circles and bars designate 50% thresholds and represent 95% confidence intervals. Upper curve obtained with lowest contrast. Inset, contrast levels.

pia corrections were used only in the latter case. The video screen subtended 13° X 18° at the eye for the shorter distance. Head stability was ensured by a head­rest. Fixation marks were provided at 10° intervals along the horizontal meridian.

The computer was programmed to display one target at a time in the center of the monitor screen. The target was temporally alternated with a neutral background of the same space-average luminance in order to counter­act the Troxler fade-from-view effect. Display time was 165 ms each second; longer exposures do not improve acuity. 3 The targets were rings with black borders and white cores (Fig O. The width of the ring and the black borders was 1/5 and 1/20 of the ring diameter, respec­tively. Neighboring ring sizes differed by 0.05 log units (0.5 dB, about 1.12).

Target luminances were varied to produce different contrasts. Borders were set to either 10, 15, or 18 cd/m2, and cores to 30, 25, or 22 cd/m2, using a Hagner S-2 Universal Photometer (Solna, Sweden). These combina-

tions gave a space-average luminance identical to that of the photopic background (viz, 20 cd/m2). The corre­sponding within-target contrast levels were 0.50, 0.25 and 0.10, defining contrast as (max L - min L)/(max L + min L), where L is luminance. Ambient illumination was about 5 lux.

Thresholds were determined in a two-phase proce­dure: an initial exploratory phase under operator's con­trol and a computer-controlled phase for generating a frequency-of-seeing curve. The latter comprised ten pre­sentations each of five neighboring target sizes, in a de­sign balanced for fatigue and learning effects. The pre­sentation order was unpredictable to the subject. There was no limit on viewing time. The test task was identical to that of perimetry, namely to decide whether or not a target was present: actual recognition was not required. In practice, detection equalled recognition5 (see Com­ments section). The resulting data were probit analyzed to obtain 50% thresholds in minutes of arc (taking ring width as the critical detail) and 95% confidence inter­vals.

RESULTS

Visual acuity (expressed as minimum angle ofresolu­tion [MAR]) is shown as a function of target contrast and eccentricity for subject B in Figure 2. Not unexpec­tedly, acuity declined monotonically with increasing ec­centricity, and was better with higher target contrast. Results for subject A were closely similar (not shown). Generality of results was confirmed in another two but incompletely tested normal subjects.

Figure 3 shows the relationship between measured acuities at different contrasts and different eccentricities, and spatial separation between retinal ganglion cells at corresponding retinal loci. The latter data were obtained from Oppel's7 average cell counts from five normal eyes, corrected for visual field eccentricity and assuming an orthogonal distribution pattern, as described pre­viously.2 Ganglion cell separation for the fovea cannot be determined directly because of the perifoveal dis­placement, but it can be approximated by the cone sepa­ration under the assumption of one-to-one connec­tions.2,8,9 The predicted relationship is an exact propor­tionality (but for stochastic deviations) through the origin.2,8,9 Linear least squares regressions through the origin are represented by the broken regression lines in Figure 3. The solid lines represent the regressions ob­tained without the origin constraint. The regression pairs are apparently closely similar in all instances. Peri­metric differential light sensitivity has a quite different type of relationship to ganglion cell separation (Fig 4): this technique clearly measures a different visual func­tion.

If test targets of varying size are considered for use in clinical perimetry, it is important to know how well circumscribed visual field defects can be delineated. Therefore, measurements were made horizontally across the center of the blindspot, using a simple ascend-

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OPHTHALMOLOGY • SEPTEMBER 1987 • VOLUME 94 • NUMBER 9

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Fig 3. Relationships between retinal ganglion cell separation and MAR at corresponding eccentricities for subjects A (above) and 8, at different contrast levels. Solid lines represent linear least SQuares regressions, and broken lines represent regressions through the origin. Inset, contrast levels and regression parameters.

ing thresholding routine. Figure 5 compares the results with those of manual static perimetry.

COMMENTS

Measuring peripheral acuity with vanishing targets proved to be a simple and rapid procedure. It was per­ceived to be somewhat more demanding with low con­trast targets, but results correlated well with those ob­tained with higher contrast (Fig 3, Table 1). One of the subjects, who had participated in an earlier peripheral acuity study using both interference fringes and sinusoi­dal gratings,'O found the test task easier in the new tech-

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nique. The latter also generated threshold estimates with narrower confidence intervals, attesting to a higher con­fidence of discrimination (Fig 6).

The relationship between the sinusoidal grating mea­surements obtained previously for subject B and his current high-pass results appears to be nonlinear, grat­ings providing higher acuity estimates in the central vi­sual field, and vice versa in the periphery (Fig 6). This is attributable to methodologic differences, particularly the fixed test field size used with the gratings.8 The fixed window causes both the number and the relative length of grating elements to vary with the spatial frequency. This results in an inconstant spatial interaction ("crowding") between the elements. 1 1 This undesirable

FRISEN • HIGH-PASS RESOLUTION PERIMETRY

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effect can be circumvented by the use oflarge windows, at the cost of uncertainty concerning the locus of mea­surement, or variable windows. Using a variant of the latter technique out to 60° of eccentricity, Rovamo and Virsu9 found an exact proportionality between acuity and ganglion cell separation. This concordance of re­sults indicates that thresholds for detection of high-pass rings are equivalent to resolution thresholds. The ab-

Fig 5. Left, definition of horizontal blindspot diame­ter in subject A, using van­ishing targets at three differ­ent contrasts. Right, the same in static perimetry, using two target sizes (Goldmann designation).

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Table 1. Correlation Coefficients

Compared Parameters

MAR versus ganglion cell separation Contrast

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MAR for high-pass targets at different contrasts

Contrast 0.25 versus 0.50 0.10 versus 0.50

MAR for different targets High-pass versus sinusoidal

gratings High-pass versus interference

fringes

MAR = minimum angle of resolution.

Subject

A

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0.974 0.982

B

0.945 0.955 0.939

0.995 0.988

0.972

0.976

Average

0.974 0.941 0.937

0.994 0.979

sence of window effects for high-pass rings is advanta­geous from a practical point of view. Incidentally, mea­surements with high-pass rings also seem to be less in­fluenced by training effects than grating acuity measurements.

The appearance of the test rings requires a comment because of the unusual perception. Rings close to thresh­old size were usually perceived as rings of a vague, light­grey shade. Slightly smaller targets were usually com­pletely invisible. For rings slightly larger than threshold size, the bright core was the dominant feature, even if the dark borders also could be discerned. Hence, even with the small step factor used here (0.05 log units, or

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OPHTHALMOLOGY • SEPTEMBER 1987 • VOLUME 94 • NUMBER 9

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Fig 6. Comparison of results obtained at different eccentricities with sinusoidal gratings lO (dominant wavelength, 633 nm; test field, 4°) and vanishing targets in subject B. Contrast levels were 0.92 and 0.5, space-average luminances were II and 20 cd/m2, viewing distances were infinity and I m , and pupils were artificial and natural, respec­tively. Diagonal represents perfect agreement. Bars designate 95% confidence intervals.

0.5 dB), it was subjectively easy to decide whether a target was seen or not, and to discern its shape. Much larger changes are required to traverse the threshold zone in ordinary perimetry, where step factors of 1 to 3 dB are normal. Incidentally, plain ring-shaped targets have been tried previously in an ordinary perimetric setting. 12 Without high-pass spatial frequency filtering, it is difficult to predict any advantages over ordinary cir­cular targets, and none was found.

Test targets devoid oflow spatial frequency detail can be given many configurations. The reason for selecting a ring shape was that rings show a particularly close appo­sition of detection and resolution thresholds.5 Rings also probe visual function over a more extensive area than do more compact targets: this should further contribute to a desirable reduction in examination time in perimet­ric applications. The trade-off would be an underesti­mate of depth of small, circumscribed defects. The blindspot, for instance, can never be shown to be an absolute scotoma with the current technique: at best, it can be shown to be blind to a target that precisely fits inside its borders (Fig 5). Larger targets will spill over the borders, and will be visible. It remains to be seen whether this limitation has clinical importance.

Unlike other resolution targets, rings should not be influenced by the orientational preferences that occur in peripheral vision. 13 On the other hand, they are like most other targets susceptible to peripheral ametropia. Only interference fringes are robust in this regard, which explains why fringes produce higher acuity results than targets seen in natural imagery. 10 In practice, this ad­vantage may be balanced out by the complicated equip­ment and the tedious examinations.3

, 10 Generating van­ishing targets in computer graphics is much easier, and has the potential of a quick and powerful visual field test. 14, 15

The most compelling reason for considering a replace­ment of the differential light sensitivity measurements

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of perimetry with acuity measurements is that the latter more directly reflect the state of the visual system. Acuity measurements can be used to estimate the sepa­ration between functional retinal ganglion cells in the corresponding locus, by virtue of the direct proportion­ality relationships shown in Figure 3.2,8,9 The separation between functional ganglion cells increases with all le­sions of the eye and the visual pathways which directly interfere with ganglion cell function, or disconnect these cells from contact with higher visual centers. The more severe the lesion, the greater becomes the distance be­tween remaining and functional ganglion cells. This causes a proportional elevation of local acuity thresh-01ds. 16 Hence, acuity measurements allow an actual nu­merical estimation of the number of functional neuro­retinal channels (i.e., a truly quantitative view of the state of the visual system). A similar interpretation model remains to be defined for ordinary perimetry. 17

REFERENCES

1. Frisen L. Visual acuity and visual field tests: psychophysical versus pathophysical objectives. In : Rose FC, Kennard C, eds. Physiology in Clinical Neuro-ophthalmology. London: Chapman and Hall, (in press).

2. Frisen L, Frisen M. A simple relationship between the probability distribution of visual acuity and the density of retinal output channels. Acta Opthalmol 1976; 54:437-44.

3. Phelps CD. Acuity perimetry and glaucoma. Trans Am Ophthalmol Soc 1984; 82:753-91.

4. Howland B, Ginsburg A, Campbell F. High-pass spatial frequency letters as clinical optotypes. Vis Res 1978; 18:1063-6.

5. Frisen L. Vanishing optotypes: new type of acuity test letters. Arch Ophthalmol1986; 104:1194-8.

6. Friedenwald JS. A new astigmatic chart. Am J Ophthalmol 1924; 7:8-15.

7. Oppel O. Untersuchung uber der Verteilung und Zahl der retinalen Gangliezellen beim Menschen. Graefes Arch Klin Exp Ophthalmol 1967; 172:1-22.

8. Drasdo N. The neural representation of visual space. Nature 1977; 266:554-6.

9. Rovamo J, Virsu V. An estimation and application of the human cortical magnification factor. Exp Brain Res 1979; 37:495-510.

10. Frisen L, Glansholm A. Optical and neural resolution in peripheral vision. Invest Ophthalmol Vis Sci 1975; 14:528-36.

11 . Howell ER, Hess RF. The functional area for summations to threshold for sinusoidal gratings. Vis Res 1978; 18:369-74.

12. Francois J, Verriest G, Ortiz-Olmedo A. Resultats en pathologie ocu­laire d'une perimetrie effectuee a I'aide d'objets pleins et d 'objects annulaires de memes surfaces. Ann Oculist 1970; 203:109-30.

13. Rovamo J, Virsu V, Laurinen P, Hyvarinen L. Resolution of gratings oriented along and across meridians in peripheral vision. Invest Oph­thalmol Vis Sci 1982; 23:666-70.

14. Frisen L. A computer-graphics visual field screener using high-pass spatial frequency resolution targets and multiple feedback devices. Doc Opthalmol Proc Ser (in press).

15. Wanger P, Persson HE. Pattern-reversal electroretinograms and high-pass resolution perimetry in suspected or early glaucoma. Oph­thalmology (in press).

16. Frisen L, Quigley HA. Visual acuity in optic atrophy: a quantitative clinicopathological analysis. Graefes Arch Klin Exp Ophthalmol1984; 222:71-4.

17. Wood JM, Wild JM, Drasdo N, Crews SJ. Peri metric profiles and cortical representation. Ophthalmic Res 1986; 18:301-8.