Effects on grating detection and grating resolution automated perimetry
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AGE AND ECCENTRICITY EFFECTS ON GRATING DETECTION AND GRATING RESOLUTION AUTOMATED PERIMETRY SHABAN DEMIREL1, CHRIS A. JOHNSON2 and LARRY N. THIBOS1 of Optometry, Indiana University, Bloomington, IN; 2Discoveries in Sight Research Laboratory, Devers Eye Institute, Portland, OR; USA
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Abstract The link between thresholds measured with standard automated perimetry and retinal ganglion cell density is not well defined at the present time. However, sampling theory predicts that the density of retinal ganglion cells can be estimated via the finest grating that can be accurately resolved by the array (see Wang et al.16). The purpose of this study was to develop a perimetric method of measuring detection and resolution acuity in normal observers and patients with glaucomatous visual field loss. This was accomplished using four, 21inch, high-resolution monochrome monitors combined optically to produce seamless coverage of the central 30°. The resulting pixel density allowed square wave gratings to be generated that were fine enough to measure detection and resolution acuities at all locations except the fovea. Stimuli were square grating patches, 4° across, arranged in a manner identical to a Humphrey 24-2 pattern. Stimulus duration was 640 msec. Results in normal observers showed an expected decline in acuity with eccentricity. Average detection and resolution acuities were 0.38 logMAD (minimum angle of detection) and 0.55 logMAR (minimum angle of resolution), respectively, inside 10°, increasing to 0.67 logMAD and 0.77 logMAR outside 20°. Resolution acuity was poorer than detection acuity in all visual field quadrants and eccentricity zones examined. Inside 10°, the age-related fall-off in detection and resolution acuities was 0.04 logMAD and 0.03 logMAR per decade, respectively. Outside 20°, these values were 0.05 logMAD and 0.03 logMAR per decade. Detection acuity, which is limited by the eye’s optics, tends to show a slightly greater aging effect than resolution acuity, which is limited by retinal ganglion cell density. Average normal data and several glaucoma cases are presented. In conclusion, the perimetric measurement of detection and resolution acuities in normals and glaucoma patients suggest that this procedure may be a viable means of estimating local ganglion cell density.
Introduction The most common method of assessing the visual field, using an increment detection task, is not based on a theoretical framework that allows direct estimation of ganglion cell density from threshold measurements. This applies equally to achromatic and chromatic
Address for correspondence: Chris A. Johnson, PhD, Devers Eye Institute, 1040 NM 22nd Avenue, #200, Portland, OR 97210, USA Email:
[email protected].
Perimetry Update 1998/1999, pp. 229–239 Proceedings of the XIIIth International Perimetric Society Meeting, Gardone Riviera (BS), Italy, September 6–9, 1998 edited by M. Wall and J.M. Wild © 1999 Kugler Publications, The Hague, The Netherlands back
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increment detection tasks such as white-on-white perimetry and SWAP. A similar argument can be made for flicker perimetry; without invoking some sort of selective loss hypothesis, it is difficult to explain how temporal resolution might become abnormal as ganglion cells die. Even if selective loss of ganglion cell subtypes does occur, the concept of a relative loss of increment sensitivity is difficult to explain without invoking arguments based on probability summation. Otherwise, sensitivity should be near normal as long as a given field location has a few remaining ganglion cells with relatively normal responses to the stimulus. Empirical studies performed by Quigley et al.1, and more recently by Harwerth et al.2, have correlated the level of threshold reduction with the proportion of ganglion cell loss, and have suggested that there can be substantial reductions in ganglion cell density prior to the appearance of visual field defects with standard automated perimetry. This empirical correlation, however, does not provide a theoretical link between the density of ganglion cells and increment detection thresholds; it simply confirms the knowledge that loss of ganglion cells will cause a reduction of increment sensitivity. Furthermore, the use of increment detection is complicated by the fact that this task is very sensitive to optical and media factors such as blur and cataract. There are several psychophysical tasks, each of which has been implemented as a clinical test, that have a theoretical linkage between threshold and ganglion cell density. Motion detection thresholds3-8 may be linked to ganglion cell density if it is assumed that a stimulus must travel across a receptive field border for the perception of motion. This may or may not be the case with good evidence to suggest that motion can be detected by some neurons when a stimulus moves entirely within its own receptive field. Resolution acuity9,10 and high-pass resolution perimetry11,12 have a very well-developed model relating ganglion cell density and measured acuities. The theory behind pattern discrimination perimetry13-15 predicts that the amount of degeneracy of a detectable pattern must fall as ganglion cell density decreases. These stimulus modalities have been used for the detection of early glaucomatous visual field damage. This study applies the theoretical linkage between ganglion cell density and resolution acuity. This theory has been fully developed previously16. Basically, the sampling density of an array of ganglion cells sensitive to a given spatial pattern can be estimated from the finest spatial frequency accurately represented by the array. This frequency is known as the Nyquist frequency. Above the Nyquist frequency, the grating may still be detected, but will be perceived as an alias with a spatial appearance unlike that physically present in the stimulus. The highest spatial frequency that can be detected is limited by contrast and therefore is very dependent on optical factors such as defocus. The highest spatial frequency that can be resolved is limited by sampling density, so long as contrast is suprathreshold, and moderate levels of optical blur will not affect the measured resolution acuity. The local Nyquist frequency for a square wave grating can be used to estimate the local density of parvocellular (P cell) ganglion cells at that retinal location. This is because P cells have the highest density and corresponding highest Nyquist frequency of all cell types sensitive to luminance gratings. Given the attractive theoretical basis relating resolution acuities and local ganglion cell density, we implemented this measure in an automated perimetric device and evaluated its feasibility as a clinical test procedure by testing 100 normal subjects between the ages of 20 and 85 years. We report the effect of age on normal detection and resolution acuities, together with normal averages and confidence intervals. Several clinical cases of glaucoma are also presented.
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Methods Equipment To generate the stimuli for this investigation, a custom video perimetry system was constructed. The system consisted of four, 21-inch, high-resolution monochrome monitors, all driven by a host Macintosh computer running custom software. The monitors were mounted within a metal frame and combined optically using front surface mirrors to produce almost seamless coverage of the central 30°. Figure 1 is a schematic drawing of the equipment set-up. Three of the monitors were viewed via mirrors placed at 45° to the frontal plane (monitors 1,2 and 4) and one monitor was viewed directly (monitor 3). All monitors were at the same optical distance from the observer who saw an almost continuous field when properly aligned. The resulting horizontal and vertical pixel density allowed us to generate square-wave gratings fine enough to measure detection and resolution acuity at all locations except the fovea. Stimuli were 4° square grating patches, arranged in a manner identical to the Humphrey 24-2 pattern. Stimulus duration was 640 msec with square temporal onset and offset. Since foveal acuities were not assessed, the central 1° was used to provide a fixation stimulus and image the anterior eye via a video system, for the purposes of fixation monitoring.
Fig. 1. A not-to-scale schematic drawing of the experimental set-up. Four monitors were mounted on a scaffold and optically combined using three mirrors. Monitor 3 was viewed directly, whereas monitors 1, 2 and 4 were viewed via mirrors at 45° to the subject. Mirror 1 was angled back going from bottom to top, mirror 2 was angled back going from left to right, and mirror 4 was angled back going from right to left.
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Subjects The primary cohort for this investigation consisted of 100 normal subjects aged between 20 and 85 years. All subjects were considered to have normal findings for age during ophthalmic examination (including slit-lamp biomicroscopy and direct ophthalmoscopy), with no ocular or systemic conditions that might adversely affect their visual field sensitivity. All normal subjects also had IOP 20° (multiple linear regression, p
