Within‐subject reliability of concurrent TMS‐fMRI during a single session

Concurrent transcranial magnetic stimulation with functional MRI (concurrent TMS‐fMRI) allows real‐time causative probing of brain connectivity. However, technical challenges, safety, and tolerability may limit the number of trials employed during a concurrent TMS‐fMRI experiment. We leveraged an existing data set with 100 trials of active TMS compared to a sub‐threshold control condition to assess the reliability of the evoked BOLD response during concurrent TMS‐fMRI. This data will permit an analysis of the minimum number of trials that should be employed in a concurrent TMS‐fMRI protocol in order to achieve reliable spatial changes in activity. Single‐subject maps of brain activity were created by splitting the trials within the same experimental session into groups of 50, 40, 30, 25, 20, 15, or 10 trials, correlations (R) between t‐maps derived from paired subsets of trials within the same individual were calculated as reliability. R was moderate‐high for 50 trials (mean R = .695) and decreased as the number of trials decreased. Consistent with previous findings of high individual variability in the spatial patterns of evoked neuronal changes following a TMS pulse, the spatial pattern of Rs differed across participants, but regional R was correlated with the magnitude of TMS‐evoked activity. These results demonstrate concurrent TMS‐fMRI produces a reliable pattern of activity at the individual level at higher trial numbers, particularly within localized regions. The spatial pattern of reliability is individually idiosyncratic and related to the individual pattern of evoked changes. Reliability has become a central issue in fMRI research. While TMS‐fMRI can be a powerful tool to probe brain function, the reliability of the evoked signal has not yet been assessed. Within session reliability of the fMRI‐evoked response following TMS was assessed. Reliability was moderate‐high with 50 trials and moderate with 30 trials. This is higher than typical fMRI measurements, which has important implications for study design and interpretation.