Reporting matters: Brain mapping with transcranial magnetic stimulation

Abstract

Transcranial magnetic stimulation (TMS) allows researchers to noninvasively probe the human brain. In a recent issue of Human Brain Mapping, Massé-Alarie, Bergin, Schneider, Schabrun, and Hodges (2017) used this technique to investigate the task-specific organization of the primary motor cortex for control of human forearm muscles. Specifically, TMS was used to create cortical topographical maps of four forearm muscles at rest, and, in one of these muscles, during isometric wrist extension and isometric grip. The authors were interested in how these maps differ between muscles, how they overlap, and how they change with different motor tasks. Key to their approach was the use of indwelling fine-wire electrodes to record motor evoked potentials elicited by magnetic stimulation, which revealed the size of cortical maps is grossly overestimated when evoked potentials are recorded from electrodes placed on the skin surface. In their paper, Massé-Alarie et al. (2017) set (statistical) significance at p < 0.05. Yet the authors interpret several p values above this threshold as statistical trends. Post hoc analyses were even performed for main effects that were not statistically significant: