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has also focused on the development of new, ‘‘unconventional’’ perimetric techniques for diagnosing and monitoring the earliest glaucomatous changes. Although a number of techniques are now available (short-wavelength automated perimetry, SWAP, or blue-on-yellow perimetry; fre- quency-doubling technology perimetry, FDT; motion-automated perimetry, MAP; high-pass resolution perimetry, HPRP), the most largely used in clinical settings are SWAP and FDT. The aim of this article is to review the rationale, the most relevant clinical data available in literature, and the tasks required to improve the clinical usefulness of these two instruments.

Retinal ganglion cells: anatomy and function

In the human visual system, RGCs project to relay cells in the layers of the dorsal lateral geniculate nucleus (LGN), which project to the primary visual cortex. All retinal and optic nerve head (ONH) diseases determine the death of the first axon of this pathway (in glaucoma through apoptosis). The primary damage causes a secondary atrophy of the LGN (Weinreb et al., 1994) and, ultimately, a loss of information projected to the visual cortex (Yucel et al., 2003; Gupta and Yucel, 2007). A full comprehension of the anatomy and the physiology of RGC is therefore required to understand the mechanisms by which the disease may induce changes in the visual function.

Recent electrophysiology studies on primates provided evidence that the retina is endowed with three primary pathways: parvocellular (P-cells), magnocellular (M-cells), and koniocellular (K-cells) (Kaplan, 2004; Callaway, 2005). This subdivision into different pathways is maintained through the LGN (Kaplan, 2004; Callaway, 2005), while the projections to the cortex are both anatomically and functionally much less segregated (Dobkins and Albright, 2004; Kaplan, 2004). In addition to these pathways, a number of other ganglion cell types connect to the LGN (Polyak, 1941) whose functional properties and postsynaptic targets still remain unidentified (Callaway, 2005).

Table 1 summarizes the main features of the subgroups of RGCs. Although this classification is

almost universally adopted, it must be remembered that the fiber diameter is influenced not only by the ganglion cell type but also by eccentricity: fibers within the central retina are thinner than those projecting from the peripheral retina. As a consequence, larger diameter fibers are not exclusively magnocellular: some eccentric parvocellular RGC axons may be even larger than more central magnocellular ones.

P-cells represent nearly 80% of RGC. They are generally small in size and located in the whole retina, although they have a very high concentration in the macular region; their receptive fields are much smaller than M- and K-pathways and they have substantial overlapping. Conducting velocity of P-cells is slower than K- and M-cells. P-cells are responsible for detecting, encoding, and transmitting information about colored, high-contrast, low temporal frequency (i.e., static) stimuli, although M- and K-cells can detect the same stimuli, albeit with a lower sensitivity (Solomon et al., 2002). An example of a selective stimulation of P-neurons is represented by the projection to the retina of the smallest letters of a standard Snellen chart used to test visual acuity.

P-cells can be classified as central and peripheral. Central P-cells are sensitive to color vision, and they can be subdivided in red-ON, red-OFF, green-ON, green-OFF, and, probably, blue-OFF. Peripheral P-cells comprise two groups (luminance-ON and -OFF) and they are sensitive to luminance (Callaway, 2005). P-cells can be also classified as type I and II on the basis of their receptive organization (type I has a centre–surround organization; type II is less diffuse and has coextensive ON and OFF regions) (Callaway, 2005).

Within the same retina region (see above), M-cells are larger than the other RGCs; they represent about 10% of all RGCs and they are endowed with low redundancy (Sample et al., 2000a, b). M-cells are located in the peripheral retina and they have large receptive fields with very limited overlapping. Within RGCs, M-cells have the fastest conduction of the stimulus. They are sensitive to low-contrast, high-temporal frequency (i.e., motion) stimuli (Solomon et al., 2005); for example, a black car rapidly passing by a driver’s side window at night would selectively stimulate M-pathway neurons.

It has been speculated that a portion of the M-cell population, the My cells, serves as the primary basis for the frequency-doubling phenomenon (Maddess and Henry, 1992). The response of this subgroup of cells is supposed to be independent by the wavelength of the stimulus, a feature that differentiates them from all the other RGCs, which show a biphasic response at the variation of stimulus wavelength (Solomon et al., 2005).

The third subgroup of RGCs is represented by koniocells. Literally, ‘‘koniocells’’ means ‘‘cells as small as dust.’’ This term was used because, due to their small size, it was very difficult to detect them in the context of the peripheral retina, where they are located. Studies on animal models recently improved our knowledge of K-cells: They represent a small subgroup (about 9%) of RGCs of small size (though they are larger than P-cells) and little redundancy (DeMonasterio, 1979); they are sparse in location and their receptive field is very large with no surrounds (Callaway, 2005); they are heterogeneous both in structure and function, and different subtypes have been identified. An important reduction of the subtype of koniocells expressing CaMKII-a has been shown in a model of glaucoma in monkeys compared to controls (10,45678770 neurons in glaucoma vs. 73,3037 15,776 in controls) (Yucel et al., 2003).

The main function of K-cells is to process blue– yellow color vision (Dacey and Lee, 1994).

Information on the blue–yellow axis is captured by blue-ON receptors, projected by koniocells within and between the principal layers of LGN; the stimulus is conducted with intermediate velocity. K-cells also respond to stimuli with moderate spatial resolution (i.e., moderate contrast).

Is glaucoma damage selective for any subgroup of RGCs?

Over the last decades, a strong debate arose on the RGC-selectivity of glaucoma damage. Scientists dealing with unconventional perimetries supported the hypothesis that glaucoma damage was selective for the subgroups of RGCs isolated by those visual field techniques (Maddess and Henry, 1992). Yet the hypothesis of the selectivity of glaucoma damage was brilliantly confuted by Harwerth et al. (1999). In their experimental study in primate animal models of glaucoma, evaluated with psychophysics, electrophysiology, anatomy, and histochemistry, the authors showed that glaucomatous atrophy causes a nonselective reduction of metabolism of magnocellular and parvocellular neurons in the afferent visual pathway. Such findings were confirmed by Yucel et al. (2003), who showed the absence of selective cell loss within the LGN in experimental glaucoma models. Although few scientists still argue that not all glaucoma cases behave in the same way (with

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individuals showing damage first to K-cells, others to M-cells, and others to P-cells), after these findings the nonselectivity of glaucoma damage seems demonstrated: as M-, K-, and P-cells are equally affected by the disease, a hypothetical 10,000-fiber loss would determine the loss of 8000 P-cells, 1000 M-cells, 900 K-cells, and 100 non-P, -M, -K-cells.

As a corollary to the assumption of nonselectivity of glaucoma damage, one would expect that perimetric techniques which use stimuli that are detected by all ganglion subsets (such as SAP) would have the same diagnostic power than those which selectively test a single pathway (such as SWAP and FDT), a fact that has been refuted by many clinical studies (see below). How can this discrepancy be possible? In order to answer this question, some functional aspects of the visual system must be considered in more detail: they are segregation, isolation, receptive field organization, and redundancy.

Segregation

We previously stated that visual pathways have a well-defined anatomical segregation up to the LGN. Functional segregation is present as well, but probably only for M- and K-cells. In the presence of damage to these cells, the visual system has a reduced ability to use other subsets of RGCs to compensate their information (Kaplan, 2004; Callaway, 2005). On the opposite end, both M- and K-cells are also sensitive to the colored, highcontrast, static stimuli, and they can therefore substitute P-cells on the transmission of information to the visual cortex in the case of damage to the P-pathway.

This only partial functional segregation may explain by itself how, though all subtypes of RGCs are damaged in glaucoma, tests that favor detection of a stimulus by one visual pathway (for example, FDT for M-cells, and SWAP for K-cells) reduce the ability of the visual system to use other pathways to compensate for the damaged RGC type (Kaplan, 2004; Callaway, 2005). When visual function is tested by SAP, such a compensation would occur.

Isolation

In a visual pathway, isolation defines the amount of sensitivity, which has to be lost before another cell type could assist in responding to the stimulus.

Up to now, the amount of isolation is unknown for retinal pathways, except for K-cells stimulated by blue-on-yellow targets, which provide approximately 15 dB of isolation. This means that the blue–yellow ganglion cell system would have to lose 15 dB of sensitivity before another cell type could assist in responding to the SWAP stimulus (Sample et al., 1996). Together with segregation, isolation reduces the likelihood of other visual pathways to compensate for initial damage (at least for K-cells), thus confirming the potential diagnostic superiority of techniques which selectively test one visual pathway.

Receptive field structure and redundancy

The receptive field is the area of the visual space where presentation or withdrawal of light causes changes in the action potential firing of the visual responsive unit (Hartline, 1940; Polyak, 1941; Kuffler, 1953; Hubel and Wiesel, 1959; Solomon et al., 2002). Visual responsive units therefore represent the functional units of the peripheral visual system; each unit is composed of a variable number of retinal receptors, intraretinal cells, and a single RGC, which brings information to the brain. Intraretinal cells downand up-regulate the excitability of the whole functional unit and of the neighboring ones, and they create a variable overlapping between contiguous receptive fields; they comprise bipolar cells (which connect receptors to RGCs), horizontal cells (which interconnect receptors inside and outside the receptive field), and amacrine cells (which create a network of connections between contiguous RGCs).

Receptive fields are circular and their dimensions are proportional to the number of retinal receptors, which are connected to a single RGC. The size of the receptive fields increases with eccentricity. In the fovea, the ratio between receptors and RGCs is about 16:1, while in the peripheral retina, nearly 1500 receptors project to a single RGC. As a consequence, central vision has