Is white matter integrity associated with speed or variability in cognitive performance?

Intra-individual variability (IIV), that is the inconsistency of responding across multiple trials of a test, has been suggested as a measure of cognitive performance alternative to the commonly-used mean level of performance. IIV in reaction time (RT) is a trait-like characteristic of an individual and has been purported as an index of the central nervous system functioning. In this study we investigated whether white matter integrity explains individual differences in RT IIV. Volume and severity of white matter lesions were assessed from MRI brain scans of 671 73-year-olds. Individuals with greater volume of lesions and more severe lesions in frontal regions had higher RT IIV. However, these associations were eradicated by controlling for mean RT. We conclude that white matter integrity does not explain variance in RT IIV that is independent of mean RT.