Twenty-four trajectories per fundus photograph were traced, one per half clock hour.
Figure 1 shows an example of a traced photograph. The fitting process has been described before.
6,7 In short, the trajectories were fitted in a modified polar coordinate system (
r,
φ), with
r representing the distance from the center of the optic nerve head (ONH) and
φ the corresponding angle. In this coordinate system, the trajectories were described by:
where
φ0 =
φ(
r =
r0) is the angular position of the trajectory at its starting point at a circle with radius
r0 around the center of the ONH,
b a real number, and
c a positive real number. Parameter
c determines the location of the curvature (punctum maximum of curvature close to the disc for
c < 1 and farther away from the disc for
c > 1) while
b determines the amount of curvature. The required nonlinear fitting was solved by performing a two-stage fitting process. In the first stage, the relationship between
c and
φ0 was evaluated and substituted in
Equation 1. The second stage of the fitting process yielded an ln
b (superior half of the retina) or ln(−
b) (inferior half) value for each trajectory. The deviation of a trajectory from the previously published model was defined as the difference between the ln
b or ln(−
b) value of the trajectory and the corresponding ln
b or ln(−
b) value as predicted by the model.
6 The average difference within a region of interest was depicted by the variable “mean departure.”
7 The mean departure was determined for each individual, for the superior-temporal (right eye clock hours 9 to 1) and the inferior-temporal (clock hours 5 to 9) region separately. The left column of
Figure 2 shows, for the superior-temporal region, the original model (middle row) and the model ± 1 standard deviation of mean departure (upper and lower row, respectively). The right column of
Figure 2 presents the corresponding data for the inferior-temporal region. See legend to
Figure 2 for details. The standard deviation was 0.2 for both regions.