PURPOSE. A recent morphological study has shown that in experimental glaucoma, the soma and dendritic field of parasol retinal ganglion cells decrease in diameter prior to cell death (Weber et al., Invest. Ophthalmol. Vis. Sci., 2304-2320, 1998). A psychophysical correlate of this finding could improve the ability to detect the early stages of glaucoma. In searching for such a psychophysical correlate, we focus on phase shifts between Long-wavelength (L-) and Middle-wavelength (M-) cone signals. Several studies indicate that phase shifts between L- and M- cone signals reflect the receptive field structure of magnocellular retinal ganglion cells. The aim of this study is to investigate the effect of spatial frequency on phase shifts in patients with glaucoma, to look for a functional correlate of altered dendritic field structure of magnocellular retinal ganglion cells. METHOD. We used a technique (Tsujimura et al., J. Opt. Soc. Am. A,
846-857, 2000), which could measure the phase shifts between L- and M-cone signals together with thresholds in cone-contrast space. Ten normal observers and nine patients with early glaucoma were tested.
RESULTS. Differences in contrast sensitivity between normal and patient groups were small (<1.6 dB). There was a significant difference in the effect of spatial frequency on phase shift for the normal group (ANOVA: F=41.254, p<.0005), while no significant difference was found for patients (F=0.995, 0<.333). The lack of a spatial frequency effect in patients is precisely what would be predicted by decreased dendritic field size.
CONCLUSIONS. The phase shift appears to provide a psychophysical correlate to the anatomical finding of altered dendritic fields of retinal ganglion cells. These data indicate that it is possible to measure functional changes in patients with glaucoma which are separate from the effects of cell death.