The first analysis (Groups [between] × RFP [within]) was conducted to evaluate differences between age groups for luminance-defined RFPs of varying number of curvatures (three, five, 10 RFs). From this analysis, a significant Group × RFP interaction was firstly identified,
F(2.76, 57.99) = 4.70,
p = 0.006,
η2partial = 0.18 (see
Figure 2). This interaction indicated that age group differences vary for differing number of RFPs. Simple main effects tests, conducted to evaluate group differences for three, five, 10 RFPs, revealed group differences for RFPs of three,
F(2, 42) = 11.41,
p < 0.001,
η2partial = 0.35, and five,
F(2, 42) = 5.60,
p = 0.007,
η2partial = 0.21, radial frequencies. No group difference was noticed for RFPs of ten radial frequencies,
F(2, 42) = 3.43,
p = 0.042,
η2partial = 0.14. Tukey post hoc tests were performed to evaluate group differences for RFPs of three and five radial frequencies. For both types of RFPs, children performed worse compared to adolescents (three RFP:
p < 0.001; five RFP:
p = 0.004) and adults (three RFP:
p < 0.001; five RFP:
p = 0.01); however, adolescents and adults performed similarly. These results are also evidenced when analyzing
Figure 2, which demonstrates that thresholds decreased systematically as the number of curvatures surrounding the contour of the RFP increased suggesting that global processing (required for three or five radial frequencies—Jeffrey, Wang, & Birch,
2002; Loffler, Wilson, & Wilkinson,
2003) is more difficult than local processing (required for ten radial frequencies—Jeffrey et al.,
2002; Loffler et al.,
2003). The identified interaction revealed that children performed worse for first-order RFPs when a global processing strategy is required. Finally, a significant main effect of group,
F(2, 42) = 12.44,
p < 0.001,
η2partial = 0.37, with Tukey post hoc tests revealing that children performed overall worse compared to adolescents (
p < 0.001) and adults (
p < 0.001). No significant difference between adolescents and adults was evidenced. There was also a significant main effect of RFP,
F(1.38, 57.99) = 114.63,
p < 0.001,
η2partial = 0.73. Tukey post hoc tests, calculated by hand using an alpha level at 0.01, revealed that it is more difficult to discriminate RFPs of three radial frequencies compared to five (
p < 0.01) and 10 (
p < 0.01) radial frequencies, and it is more difficult to discriminate RFPs of five radial frequencies compared to 10 radial frequencies (
p < 0.01). These main effects will not be discussed further, since a significant Group × RFP interaction was evidenced.