Comparison of different baseline conditions in evaluating factors that influence motor cortex excitability

Identifying task-related changes in cortical excitability requires comparing motor evoked potentials (MEPs) measured under an experimental condition with that obtained in a baseline, control condition. The goal of this study was to compare two different procedures for measuring baseline that are commonly used in transcranial magnetic stimulation (TMS) studies. We hypothesized that baseline measurements obtained during task performance may be elevated due to an overall heightened state of arousal or task-specific fluctuations in excitability. Single-pulse TMS was used to elicit MEPs during an experimental task involving action observation. Baseline MEPs were recorded before (preblock) and during (intrablock) the experimental blocks. Intrablock baseline MEPs were modulated in a manner correlated with the effect of the experimental manipulation. Although there are advantages to obtaining baseline measurements during the experimental block, such measurements are biased by the experimental manipulation. Unbiased baseline measurements are best obtained between experimental blocks.