'Magnetized' brains are slower: The cognitive effects of fMRI

Abstract

MRI is generally thought to have no impact on cognition. Although safety experiments have shown that MRI is not harmful, its finer effects have not been investigated. Because we repeatedly observed delayed response time during functional MRI (fMRI), we designed an experiment to confirm this effect and to identify its causal factor(s), including environment, noise, static magnetic field and/or gradient switch. Here we show that the participants had increased response times of +70 ms (up to +30%) in two different detection tasks, with most of this effect due to the 2 Tesla static magnetic field. The latter also specifically accounted for the longer time interval needed to detect two stimuli as occurring successively rather than simultaneously. These observations demonstrate that brain processes are slowed during fMRI, and that this slowing is caused by the static magnetic field. This may be the behavioral counterpart of the effects of static magnetic fields on neuronal excitability. Consequences on fMRI data should be taken into account especially considering the forthcoming very high field MRI. The ability of magnetic fields to modify brain activity suggest that they may be used for therapeutic purposes