The ability to isolate and measure psychophysically the functional responses from specific subsets of retinal ganglion cells has led to widespread investigations of this kind, in the hope of finding improved detection methods for open-angle glaucoma.
1 2 Of particular interest in this large body of research is the selective light sensitivity loss of the short-wavelength system in the early stages of the disease,
3 4 5 which has resulted in the development and implementation of clinical tests such as short-wavelength automated perimetry (SWAP; for a review, see Sample
6 ), which measures the light increment sensitivity of the blue pathway that carries the response from the short-wavelength-sensitive (SWS) cones. This pathway is mediated by the small group of “blue-on” ganglion cells (comprising only 5% to 10% of the total ganglion cell population) which are commonly referred to by their morphologic appearance as the small-field, bistratified ganglion cell.
7 8 This selective chromatic light sensitivity loss can be supported by two main theories of ganglion cell loss in early glaucoma. First, the notion of selective death of certain ganglion cell subtypes, whereby ganglion cells with specific physical and functional properties, such as those that carry responses from the short-wavelength system, may preferentially die early in the disease process, would provide one explanation. Histologic evidence of selective damage to ganglion cells with larger cell bodies and axon diameters (which would include small bistratified cells
7 9 ) early in the disease process,
10 11 would lend weight to this argument, but is disputed by other recent reports.
12 13 14 However, this apparent selective loss of SWS ganglion cells may merely be the result of reduced redundancy.
15 Within this scenario, deficits in a subpopulation such as the SWS-driven pathway (or any other sparse population), which under white-on-white stimulation could be compensated for by cells from another population, remain exposed under SWS-isolating conditions. Evidence has been presented that several visual functions are affected in early glaucoma
16 and that damage does not appear to be exclusively selective for the SWS ganglion cell subpopulation early in the disease.
17 18 In view of these alternative theories, further psychophysical evidence for the existence or nonexistence of a selective reduction in the number of small bistratified ganglion cells in early to moderate glaucoma would be useful in understanding the disease and the development of methods for early diagnosis.