There were several striking similarities between WT and MT-KO retinas in their response to HBO treatment. In both the normal and KO mice, HBO treatment for 3 and 5 weeks caused a dramatic loss of PRs from the central retinas, measured as a thinning of the ONL (see
Table 1 ). The loss of PRs was regionally selective, with large decreases in ONL thickness occurring exclusively in the central retinas of WT and MT-KO animals (
Table 1 ,
Fig. 2 , bins 1–6), but not in the periphery (
Fig. 2 , bins 7–12). To compare the WT and MT-KO mice, we took values in
Table 1and pooled them across eccentricities 1 to 3 and 4 to 6 to obtain an average for the central retinas. Specifically, in central retinas (bins 1–6) of WT animals, HBO exposure caused a significant reduction in the number of rows of PR at 3 weeks (38%,
P ≤ 0.0001) and 5 weeks (34%,
P ≤ 0.002), compared with untreated control retinas. The effect sizes were large, corresponding to 0.90 and 0.81, respectively. Similarly, in the central retinas (bins 1–6) of MT-KO mice, the number of rows of PR nuclei was 28% lower (
P ≤ 0.0001) at 3 weeks and 34% lower after 5 weeks of HBO exposure than in retinas not exposed to HBO. The effect sizes for these differences were also large, corresponding to 1.01 and 1.24, respectively. In contrast, no PR cell loss was detected in the peripheral retinas of either WT or MT-KO mice, even after 5 weeks of HBO exposure (
Table 1 ;
Fig. 2 , bins 7–12). Shorter HBO exposures of 1 week did not produce any measurable PR loss, even in central retinas of either normal or KO mice, compared to unexposed (0 weeks) control retinas (
Fig. 2 , bins 1–6).