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Editors Selection IGR 14-3

Examination methods: Electro-opthalmo diagnosis: Magno and parvo cellular pathways

Comment by Lyne Racette on:

18183 Contrast sensitivity changes due to glaucoma and normal aging: low-spatial-frequency losses in both magnocellular and parvocellular pathways, McKendrick AM; Sampson GP; Walland MJ et al., Investigative Ophthalmology and Visual Science, 2007; 48: 2115-2122

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Function-specific tests have been shown to be more sensitive to glaucoma than standard automated perimetry. Each of these testsisolates a sub-population of retinal ganglion cells (RGCs) by presenting targets optimized for each cell type. Several function-specific tests have been developed for glaucoma, including short-wavelength automated perimetry, frequency-doubling technology, and high-pass resolution perimetry. Comparisons between these tests, however, are not straightforward because different stimuli, presentation patterns and thresholding algorithms are used in each test. In a carefully designed study, McKendrick et al. (545)measured the low-spatial-frequency-sensitive components of both the magno- and parvo-cellular pathways using the same stimuli under two different conditions: a steady and a pulsed pedestal. The stimuli consisted of a Gabor patch (sine-wave grating presented in a Gaussian envelope) superimposed onto a square (pedestal). The steady pedestal condition, in which participants adapted to the pedestal before and after the presentation of the Gabor, favored the magno-cellular pathway. In the pulsed pedestal condition, the pedestal and the Gabor were presented simultaneously, favoring the parvo-cellular pathway. The isolation of each pathway was carefully demonstrated and observed for Gabor patches with spatial frequencies of 0.25 and 0.50 cycles per degree. The results of this study show a non-selective reduction in contrast sensitivity for the magno- and parvo-cellular pathways in normal aging. In glaucoma patients, both pathways were also equally affected but a larger reduction in contrast sensitivity was observed,highlighting the potential of this test as a clinical tool. From a research standpoint, an important strength of this method is the use of the same stimulus configuration to isolate the two pathways. While this paper thoroughly reviews the chronological impact of glaucoma on the different types of RGCs, the results are consistent with the theory of reduced redundancy. This theory suggests that the increased sensitivity to glaucoma of function-specific tests occurs through the isolation of specific sub-populations of RGCs. Further evaluation will be required to determine the diagnostic accuracy of this interesting new method.

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