Is reaction time an index of white matter connectivity during training?

Voelker et al. (this issue) discuss the idea of linking white matter (WM) plasticity to improved reaction time (RT) during training. While compelling, this argument has important confounds and should be taken with cautions. RT is constrained not only by the speed of signal transmission in WM, but also by the properties of synaptic and neural processing in cortical gray matter. It is still unclear to what extent RT variability could be explained by WM plasticity and cortical plasticity. Future studies should examine both WM plasticity and cortical plasticity in relation to RT changes, to fully understand the brain mechanisms underlying RT improvement during training.