2.5 Statistical Analysis
For SORT performance, we utilized repeated measures general linear models (GLM; IBM SPSS Statistics 28.0) with group as a between-subject factor (two group comparisons: YA vs HA, or HA vs MCI) and conditions as a within subject factor (retrieval vs non-retrieval). RT and accuracy data were analyzed separately. Post-hoc comparisons were adjusted with Bonferroni correction. Significant results were reported when p< 0.05.
EEG data were analyzed for theta (4– 8 Hz), alpha (8–12 Hz), low beta (12–20 Hz), and high beta (20-30 Hz) frequency bands with mean values calculated from each frequency range, based on canonical classification from prior EEG studies (Weiss & Mueller, 2012; Poulisse et al., 2020; Markiewicz et al., 2021). In order to examine differences in individual alpha frequency (IAF), given different age groups, we performed peak frequency extraction within the frequency range 7-14 Hz from epochs between -750 to 2500 ms to stimulus onset, for each individual across conditions (Klimesch, 1999). We ran a repeated measures GLM and did not find any significant effects (no effects of group, condition, or group by condition interaction), thus ruling out that individual alpha differences affected our analysis, and supporting the use of the standard window.
Permutation tests (N = 2000) were performed to compare group data, for difference between retrieval and non-retrieval conditions, based on two-tailed independent t-tests. These tests were performed to assess (1) normal aging effects by comparing YA to HA groups and (2) pathological aging effects by comparing HA to MCI groups, for all electrodes (N = 62) and all windows (N = 26) spanning from 188 to 1105 ms post stimulus onset. We chose this time window given that prior studies have shown effects within this time window on the SORT task (Chiang et al., 2014, 2015, 2016, 2020). A False Discovery Rate (FDR) < 0.05 from permutation tests was considered significant, which is a robust statistical approach commonly adopted in functional imaging studies to avoid excessive false negative results, so further corrections for multiple comparisons were not applied. Additionally, we tested conditional effects (retrieval vs non-retrieval conditions) within each group separately, based on permutation tests (N = 2000) using two-tailed paired-t tests, thresholded at FDR < 0.05. These results are presented in the supplementary materials since condition effects are not related to our primary research questions.
Post hoc analysis was performed based on the mean power scores extracted within the time window at the electrodes showing significant results, using 2-tailed paired-t tests to compare conditions (retrieval vs non-retrievals). Significant results were reported if p< 0.05.