The similarity between the left and right panels of
Figure 2 is interesting, because previous studies have pointed out that the use of BCEA to quantify fixation stability assumes that the eye positions during fixation follow a normal distribution,
39 whereas the isoline method does not make this assumption.
34,35 Therefore, one would expect that the results from the two methods do not necessarily correlate or agree with one another. Contrary to the expectation, the values obtained using the two methods are highly correlated (
r = 0.99) with one another, as shown in the left panel of
Figure 3. The slope (on log–log axes) of the regression line is very close to 1 (0.99) but there is a constant offset from the 1:1 line (gray dashed line), implying that the isoline values are smaller than the BCEA by a similar amount across observers (normally sighted or with macular disease). Considering that a high correlation does not imply a high agreement, we also examined how well the values obtained using the two methods agree with one another, as shown in a Bland-Altman plot
40 (
Fig. 3, right). Given that all except one data point fall within ±95% limits of agreement, we conclude that the BCEA and the isoline methods agree reasonably well in representing fixation stability, with the caveat that the isoline yields slightly better stability (the mean difference between the log values of the two methods = 0.138 log deg
2 , which is statistically different from a null effect [no difference],
P < 0.0001). The result that the values obtained using the two methods are highly correlated and in excellent agreement with one another is unexpected, and there are two noteworthy points in relation to these findings. First, none of our observers, with or without macular disease, showed evidence of using more than one retinal location as their fixation locus, as revealed by the recorded SLO video files. Second, none of our observers had eye positions during fixation that followed a normal distribution, based on the Kolmogorov-Smirnov test to test for normality in the data. Although this was expected for people with macular disease, our finding showed that this was also true for older adults with normal vision, at least using our method of eye movement measurements. We believe these two points are likely to account for the high correlation and agreement observed between the values obtained using the two methods, but more detailed analyses and further studies are necessary for us to fully understand why the values from the two methods are so similar. The important point in relation to the purpose of this study is that using the two different methods to quantify fixation stability, observers with macular disease consistently demonstrated much higher fixation instability than older adults with normal vision.