There were no significant differences in the age of participants between groups (t-test between DS and control groups, t = 1.21, P = 0.23, and between CP and control groups, t = −0.92, P = 0.36). Data were found to be normally distributed for the DS (P = 0.13) and CP groups (P = 0.55), but not normally distributed for the control group (P = 0.02); therefore nonparametric statistics were used for comparisons between control and DS/CP data.
There was no significant association between acuity and age in either the DS or the CP group (linear regression analyses, R 2 between 0.02 and 0.08, P > 0.1). For the CP group, inspection of high-contrast VA according to level of motor impairment (by GMFCS) indicated a reduction in acuity with increasing GMFCS, but a one-way analysis of variance comparing acuities between GMFCS classifications did not reach significance (F (4,33) = 2.3, P = 0.07). There was no significant difference in high-contrast VA measures between different CP subtypes (F (2,35) = 1.7, P = 0.19). Within the control group, there was a significant improvement in high-contrast VA with age (linear regression analysis, R 2 = 0.18, F (1,206) = 45.6, P < 0.0001).