## 3 Results
### Picture Priming Task  
Partial least squares (PLS) analysis revealed a significant overall relationship between brain age gap (BAG) and cognitive performance on one specific cognitive test, the Picture Priming Task (overall \(p<0.05\)). Figure 1 displays the results of two separate PLS analyses for this task, one for BAG derived from baseline cross-frequency coupling (CFC) metrics (left column) and another for BAG derived from capacity CFC metrics (right column). In each column, the lower plot shows the overall correlation coefficients between BAG and individual cognitive scales from the Picture Priming Task, while the upper plot shows the corresponding bootstrap ratios (Z-scores) for each functional network's contribution to this relationship.  
For both baseline-based and capacity-based BAG analyses, the distributions of network Z-scores (Figure 1, upper plots) were predominantly positive, indicating a consistent directionality of network contributions to the overall PLS effect. We focused on networks with Z-scores exceeding 2, signifying robust contributions. Four such networks were identified for the baseline-CFC BAG, and four for the capacity-CFC BAG. The consistently positive Z-scores allow for a direct interpretation of the brain-behaviour relationships based on the sign of the overall correlation coefficients (Figure 1, lower plots).  
Examining the baseline CFC-derived BAG (Figure 1, left column), the pattern of correlations with cognitive scales from the Picture Priming Task was mixed. For instance, BAG positively correlated with lower semantic priming difference scores but negatively correlated with lower phonetic priming difference scores. This heterogeneity in the direction of effects did not yield a clear, unidirectional relationship between baseline-BAG  
Figure 1: Partial least squares (PLS) Analysis of the brain age gap (BAG) and Picture Priming Task Performance. This figure presents results from two separate partial least squares (PLS) analyses examining the relationship between cognitive performance on the Picture Priming Task and brain age gap (BAG). The left column displays results for BAG derived from baseline cross-frequency coupling (CFC) metrics (“baseline-based BAG”). In contrast, the right column shows results for BAG derived from capacity CFC metrics (“capacity-based BAG”). The lower plot illustrates the overall correlation coefficients between individual cognitive scales and the BAG derived from all functional networks for each BAG type. The upper plot displays the corresponding bootstrap ratios (z-scores) for each functional network, indicating the reliability of each network’s contribution to the observed brain–behaviour correlation. Positive Z-scores and positive correlations in the lower plots would suggest that a higher BAG (interpreted as relatively slower brain ageing) is associated with better cognitive performance, and vice versa.  
and cognitive performance, suggesting that both positive (slower ageing) and negative (accelerated ageing) BAG values could be associated with better performance on different aspects of the task.  
In contrast, the capacity CFC-derived BAG (Figure 1, right column) exhibited a qualitatively distinct and more consistent pattern. Specifically, BAG derived from CFC capacity measures showed positive correlations with five out of the seven cognitive scales of the Picture Priming Task. Given that a positive BAG indicates a brain appearing younger than its chronological age (i.e., slower ageing) and positive values on these cognitive scales represent better performance, this pattern consistently suggests that slower brain ageing, as indexed by capacity-BAG, is associated with enhanced cognitive function on this task. This consistent relationship was supported by the predominantly positive network-level Z-scores.