A Checklist to Reduce Response Variability in Studies Using Transcranial Magnetic Stimulation for Assessment of Corticospinal Excitability: A Systematic Review of the Literature

Response variability between individuals (inter-individual variability) and within individuals (intra-individual variability) is an important issue in the transcranial magnetic stimulation (TMS) literature. This has raised questions of the validity of TMS to assess changes in corticospinal excitability (CSE) in a predictable and reliable manner. Several participant-specific factors contribute to this observed response variability with a current lack of consensus on the degree each factor contributes. This highlights a need for consistency and structure in reporting study designs and methodologies. Currently, there is no summarised review of the participant-specific factors that can be controlled and may contribute to response variability. This systematic review aimed to develop a checklist of methodological measures taken by previously published research to increase the homogeneity of participant selection criteria, preparation of participants prior to experimental testing, participant scheduling and the instructions given to participants throughout experimental testing to minimise their effect on response variability. Seven databases were searched in full. Studies were included if CSE was measured via TMS and included methodological measures to increase the homogeneity of the participants. Eighty-four studies were included. Twenty-three included measures to increase participant selection homogeneity, twenty-one included measures to increase participant preparation homogeneity, while sixty-one included measures to increase participant scheduling and instructions during experimental testing homogeneity. These methodological measures were summarised into a user-friendly checklist with considerations, suggestions and rationale/justification for their inclusion. This may provide the framework for further insights into ways to reduce response variability in TMS research.