The mean threshold decreased very slightly from 8.13±0.35 dots to 7.97±0.39 dots (blue line: decreased by 2%; paired-t = 1.336, p = 0.2524) and 7.84±0.21 dots (red line: decreased by 4%; paired-t = 1.796, p = 0.1468) for the lowered stimulus brightness and the optical blurring conditions, respectively (Fig. 5A).
Mean goodness of fit for the blur condition, averaged across participants was, R2 = 0.93; for the noise condition, mean R2 = 0.81.
In the blur condition, we find a significant shift in the psychometric function between older and younger observers (t(28.9) = 3.57, p = 0.001, d = 1.26, BF 10 = 25.89).
The difference between the two age groups was not significant at any of the remaining blur levels, including the no-blur condition (all p values > 0.08, BF 10 < 1.15).
However, while we find a significant effect of age in the blur condition, in both the low and high ambient illumination, we fail to replicate the effect in the noise condition in both the low and high ambient illumination conditions.
Therefore, in order to equate performance between younger and older participants in the blur condition, the Gaussian kernel SD would need to be increased by 1.48 arcmin for the stimuli presented to the 20 29 age group.
In the blur condition, there was a significant main effect of age group (F 1,30) = 15.51, p < 0.001, η 2 = 0.34, BF 10 = 47.97), with older participants responding more slowly than younger observers (620.8 ms and 399.6 ms, respectively).
In the blur condition, we observed a trending difference in the 90% threshold between the two age groups, with older observers' performance dropping to 90% at a lower level of blur compared to younger observers (1.04 versus 2.16 armin, t(29.28) = 2.0, p = 0.06, d = 0.71, BF 10 = 1.47).
Within the blur condition, there was no significant effect of age group (F 1,30) = 1.98 × 10−5, p = 0.996, η 2 = 6.59 × 10−7, BF 10 = 0.38), with similar slope estimates between the two age groups (1.755 versus 1.757 for younger and older participants, respectively).
In the blur condition, an ANOVA on percent correct responses showed a significant main effect of age group (F 1,30) = 12.13, p = 0.002, η 2 = 0.29, BF 10 = 10.63), with lower accuracy in the 60 69 age group than in the 20 29 age group (63.0 versus 67.8%, respectively).
