MULTIMODAL OUTPUT MAPPING OF HUMAN CENTRAL MOTOR REPRESENTATION ON DIFFERENT SPATIAL SCALES

1. Non-invasive mapping by focal transcranial magnetic stimulation (TM S) is frequently used to investigate cortical motor function in the in tact and injured human brain. We examined how TMS-derived maps relate to the underlying cortical anatomy and to cortical maps generated by f unctional imaging studies. 2. The centres of gravity (COGs) of TMS map s of the first dorsal intersosseus muscle (FDI) were integrated into 3 -D magnetic resonance imaging (MRI) data sets in eleven subjects. In s even of these subjects the TMS-derived COGs were compared with the COG of regional cerebral blood flow increases using positron emission tom ography (PET) in an index finger flexion protocol. 3. Mean TMS-derived COG projections were located on the posterior lip of the precentral g yrus and TMS-derived COG projections were in close proximity to the me an PET-derived COG, suggesting that the two methods reflect activity o f similar cortical elements. 4. Criteria for a reliable assessment of the COG; and the number of positions with a minimum amplitude of two-t hirds of the maximum motor-evoked potential (T3Ps) were determined as a function of the number of stimuli and extension of the stimulation f ield. COGs and T3Ps were compared with an estimate of the size of the human motor cortex targeting a-motoneurons of forearm muscles. This co mparison suggests that TMS can retrieve spatial information on cortica l organization below the macroanatomic scale of cortical regions. 5. F inally, we studied the cortical representation of hand muscles in rela tion to facial and foot muscle representations and investigated hemisp herical asymmetries. We did not find any evidence for a different ipsi - or contralateral representation of the mentalis muscle. Also, no dif ference was found between FDI representations on the dominant versus t he non-dominant hemisphere.