results.71 Nevertheless, for most accurate results, refractive errors should be corrected where possible.56 Frequency-doubling perimetry seems to be affected only a little bit by aging;72 the small aging effects are incorporated as part of the normative database included with both models of the device.The normative database allows for similar analysis as seen with SAP, including mean deviation, pattern deviation, glaucoma hemifield test and reliability indices. Also, the second eye tested is less sensitive than the first eye tested, and this is also incorporated into the analysis procedures.73

As with SAP, the presence of media opacities or pupil constriction which reduces retinal illumination may confound the results of FDP, causing the expected reduction in mean deviation but also possibly masking some local abnormalities.74,75 Removing a cataract generally improves the mean deviation with little effect on the pattern deviation, although, at least in one study, the pattern devia-

tion worsened after cataract surgery suggesting that media opacity may mask some localizing defects.76,77

Because of its relative small size, making it the most portable of threshold devices, FDP has been found to be a very useful device for community screening for glaucoma.78–82 In an English-speaking

population, the screening C-20 program has only a modest learning effect.83,84 The screening algorithm of the FDP has acceptable

sensitivity compared to SAP to justify its use in a mass screening to detect moderate to advanced glaucoma.85 The FDP in screening mode is superior to the Damato campimeter in both sensitivity and specificity.86 However, in a non-English speaking developing country, FDP may not be relied upon as the sole screening device since it has relatively low sensitivity when compared to expert optic nerve evaluation, although specificity can be improved by repeat testing.87,88 Children with learning disabilities have significant difficulty with FDP; however, adults with relatively mild reading disabilities have no greater problems performing reliable FDPs than adults without reading disabilities.89 The preceding study used college students, so it is unkown whether adults who had reading disabilities that interfered with their ability to attend college would have enough difficulty with FDP that other screening methods for glaucoma would be more effective in this particular population.

While progression of some defects has been shown with FDP, its ability to detect progression as a routine monitoring tool has not been demonstrated.90

While no single test has been shown to detect all glaucoma, FDP has become a most useful screening tool for finding glaucoma and detecting it in its relatively early phases.91 Careful studies have suggested that each of the functional tests detect a subset of early glaucomatous changes and that some combination of functional tests (such as SWAP and FDP) may be better than any single functional test at finding the earliest functional changes in glaucoma.92 Usually, structural changes precede the functional ones, and these structural changes, when they finally correlate with functional ones, may affect different ganglion cell populations in different patients.

Other psychophysical tests

High-pass resolution perimetry

High-pass resolution perimetry (HRP) was first described by Frisen in 1987 and is the measurement of resolution over the extent of

the visual field.93 It appears to be measuring the function of the ganglion cells.94,95 In this technique, rings with a bright core and a

dark border of various sizes (although other target types have been used) are projected onto the visual field while the subject fixates on a central target and the subject indicates when the ring is perceived.

The process is a modification of acuity perimetry first described by Johnson et al in 1979 and later by Phelps in the 1980s.96,97 The

smaller the ring that can be perceived at a given location, the higher is the resolution of that part of the retina. While contrast could be varied, in the test described by Frisen, contrast is held constant and the size of the target is varied. As one gets further from fixation, as might be expected, the rings need to be larger in order to be perceived. Furthermore, the bowl screen is only 167 mm away at the center but has a convex curve so that the more peripheral targets are actually further away than the central fixation target. Compare this to SAP where the bowl is at 330 mm and is curved in a concave way so that, in general, the test targets are all equidistant from the retina. The computer, therefore, needs to adjust the targets for both size and shape to maintain constancy of angular size.

The typical test involves 50 locations within the central 30° where the majority of ganglion cells lie and, in that way, is similar to the 30–2 of light-sense perimetry.98 Because of the close distance, a six diopter optical correction must be added to the distance correction.The test is easier on subjects because it is quicker and discriminations are more positive than standard automated perimetry, but it is still a subjective test and may be susceptible to many of the errors

of any psychophysical test. High-pass resolution perimetry has less variability than SAP.98,99 The HRP probably depends on the parvo-

cellular channels, since that appears to be the only system within the ganglion cell family that carries resolution information; parvocellular ganglion cells are the largest group of ganglion cells representing about 80% of the total population.100 Another study showed a good relationship between HRP and the number of midget ganglion cells, although the number of patients was quite small.101 However, not all studies support the selective action of HRP.102

The technique is not affected by anti-glaucoma medications and seems suitable for both diagnosis and monitoring.103

Like SAP, the response to HRP declines with age, but, unlike SAP, this decline is proportional to the ‘normal’ age-related loss of ganglion cells with a direct correlation with the known age-related ganglion cell loss.104 In addition to glaucoma, defects are found in intracranial

hypertension, optic neuritis, and other neuro-ophthalmologic conditions that affect the visual fields.105,106 Also like SAP, HRP is affected

by media opacities.107

High-pass resolution perimetry is most useful in ocular hypertension and glaucoma.There is an overall general reduction in sensitivity as well as location-specific defects.108 Sensitivity for glaucomatous

defects is probably slightly better than or similar to full-threshold SAP.98,109 Using age-related probability plots, sensitivity and specifi-

city for early glaucoma were about 85%.110 High-pass resolution perimetry also seems useful in monitoring glaucoma eyes over time.111

In fact, one prospective and one cross-sectional study suggested that HRP can detect progression before it is evident on SAP.112,113 In

another study, the sensitivity and specificity of HRP correlated well with that obtained with FDT.114 High-pass resolution perimetry

may be more sensitive to intraocular pressure-induced damage than

SAP.115,116

The location of defects correlates well with SAP.117 Abnormalities

on HRP correlate well with neuroretinal rim area as well as with other measures of optic nerve structure.118,119 High-pass resolution

perimetry also correlates well with nerve fiber layer thickness measurements, although there appears better correlation with the higher pressure forms of glaucoma.120 The test takes about one-third less