The bottom row shows that increasing
n results in a steeper slope of the ascending portion of the TvI curve (left panel). The right panel shows that the subjects with XLRS had slope values (
n) that ranged from normal to elevated, but the elevation appears greater at more central field locations (fovea, parafovea, perifovea). The mean value of
n for the subjects with XLRS was relatively constant across the visual field: 0.73, 0.67, 0.77, 0.80 for measurements in the fovea, parafovea, perifovea, and periphery. In contrast, the value of
n increased systematically from the fovea to the periphery for the controls: 0.24, 0.35, 0.55, 0.65 for measurements in the fovea, parafovea, perifovea, and periphery. ANOVA was performed to compare
n between the XLRS and control groups across the four visual field locations. ANOVA showed a significant effect of subject group (
F = 35.13,
P < 0.001) and location (
F = 25.32,
P < 0.001); there was a significant interaction between group and location (
F = 9.79,
P < 0.001). Holm-Sidak pairwise comparisons indicated statistically significant differences in
n between the XLRS and control groups at all visual field locations (all
t > 2.50;
P < 0.02). When fitting TvI functions, it is sometimes assumed that
n = 1. Although this is often a valid assumption, the data of
Figure 4 indicate that this is not always the case for our experimental conditions. This can also be seen in the supplementary data (
Supplementary Fig. S2), which compares the fits of
Equation 1 under conditions in which
n is a free parameter (black) and when it is set to 1.0 (red). Allowing
n to vary improved the fits for the control subjects but had little effect on the fits for the subjects with XLRS, because their data generally followed a slope of approximately 1.0 throughout the field (cf.,
Fig. 4, bottom).