Intraindividual variability in daily cognitive performance is associated with white matter organization of the superior longitudinal fasciculus in healthy adults

Behavioural studies have traditionally focused on mean differences in cognitive performance between individuals, often regarding fluctuations within individuals over time as measurement error. However, recent research emphasizes that cognition is inherently dynamic, exhibiting high within-person variability over time. This intraindividual variability (IIV) has been suggested as an important marker of attentional control and brain functioning. Stability of attentional control is supported by the organisation of fronto-parietal tracts, such as the superior longitudinal fasciculus (SLF). In the present study, we investigate the relationship between IIV in daily cognitive performance and fibre-specific white matter characteristics of the SLF in 30 healthy adults (aged 21 to 62 years, 19 females). Participants underwent a diffusion MRI scan followed by daily ecological momentary assessment (EMA) of working memory and processing speed over a 30-day period. Using the fixel-based analysis (FBA) framework, we investigated the association between IIV in cognitive performance and microstructural fibre density (FD) and macrostructural fibre cross section (FC) of the SLF. Our results revealed a high degree of IIV across each task, particularly for working memory tasks. Moreover, higher IIV was negatively associated with average performance for working memory but not processing speed tasks. Finally, higher IIV and lower mean performance for visual working memory were negatively associated with decreased white matter fibre microstructure and macrostructure. Our findings indicate that variation in micro/macrostructure of a fronto-parietal pathway may play a role in supporting the stability of cognitive performance over time, highlighting its potential as a marker for understanding cognitive dynamics in healthy and clinical populations.