Stability of maps of human motor cortex made with transcranial magnetic stimulation

Cortical representation maps derived by transcranial magnetic stimulation (TMS) are often used, inter alia, in studying the plasticity of the brain. Parameters such as map area, map volume, optimal stimulation site and centre of gravity are commonly used to quantify changes in the topography of the motor cortex. However, reports on the stability of these parameters over time has not been conclusive. In the present study, the areas of the scalp from which responses were evoked from corticospinal cells projecting to three intrinsic hand muscles were systematically mapped with TMS at intervals of 24 hours, one week and two weeks from eight normal subjects. The area, volume and centre of gravity of these maps did not change significantly over this period. It is concluded that mapping with TMS is suitable for studies which aim to study the effect of various interventions on the cortical representation of individual muscles in human subjects.