Because RGC density declines with distance from the optic nerve, the percentage of decline in RGC density was estimated by the following method. For each half retina of the control and experimental eyes of each rat, the mean density of the three fields equidistant from the disc was calculated. These areas were designated levels 1 to 4, where level 1 was closest to the disc and level 4 was most peripheral
(Fig. 1) . Mean densities in each level of the superior and inferior control retinas were pooled by averaging the data from each rat within a given treatment group at that level location. For each rat, the mean densities for levels in the superior and inferior experimental retinas were compared with the pooled control densities at the same level location. From these, the absolute number of cells lost and the proportion of cell loss per experimental level were calculated. The results were used to calculate an average proportion lost in the inferior, superior and whole retinas of each rat. The data were then combined for an average percentage lost in the inferior, superior, and whole retinas in a given treatment group.
RGC mean density differences between pooled control and treatment eyes were tested for significance in whole, superior, and inferior retinas. To verify that significant degeneration occurred after partial transection of the superior 10% to 30% of the retina, the mean (±SD) percentage of RGC loss was tested for whole, superior, and inferior retinas at each time point of each experimental group, by using the null hypothesis of no loss (one-sample, two-tailed t-test). Differences in change in RGC densities were compared among the three experimental groups in the whole, superior, and inferior retinas, at both the 1- and 4-week time points in the partial transection groups (one-way analysis of variance; ANOVA), and at the 1- and 2-week time points in the full transection groups (unpaired two-tailed t-test). Significance was set as P < 0.05. Statistical analysis, including power calculations, was performed on computer (InStat ver. 3.01 for Windows 95/NT; GraphPad Software, San Diego, CA, www.graphpad.com).