Task Repetition Can Affect Functional Magnetic Resonance Imaging-Based Measures of Language Lateralization and Lead to Pseudoincreases in Bilaterality

Abstract

Repeated functional magnetic resonance imaging (fMRI) during learning and recovery can inform us about functional reorganization in the brain. We examined how, in the absence of reorganization, simple task repetition affects measures of fMRI activation. We studied fMRI activation over 10 consecutive sessions of silent word generation in a healthy subject. Additionally, we performed functional transcranial Doppler sonography (fTCD) to learn about the temporal pattern of corresponding changes in cerebral blood flow velocity (CBFV) and pulsatility. With repetition, word generation-associated increases in heart rate diminished steadily. Task repetition also led to a net increase in CBFV bilaterally and to a bilateral increase in the number of activated voxels on fMRI. As a result, whereas the absolute interhemispheric difference of activated voxels remained constant, there was a decrease in the standard fMRI index for language lateralization [LI_fMRI = 100 (activated voxels in left hemisphere - voxels in right)/(voxels in left + voxels in right)]. Thus task repetition can lead to changes in task-related autonomic drive and an augmentation of bihemispheric blood flow. This can mimic increasing bilaterality of brain activation.