Comparison of representational maps using functional magnetic resonance imaging and transcranial magnetic stimulation

Comparison of functional magnetic resonance imaging (fMRI) representational maps, that were generated during voluntary thumb abduction, hand dorsiflexion and foot elevation to amplitude maps of motor-evoked potentials (MEPs) elicited by single transcranial magnetic stimulation (TMS) administered to cortical motor representation areas of the muscles of the thenar eminence, extensor carpi radialis and tibialis anterior muscles.

Stimulus locations that produced maximal motor-evoked potential amplitudes were compared to fMRI activation maxima in three-dimensional (3D)-space and in a 2D-projection using a novel technique that allowed fMRI activation sites to be projected onto the surface of the brain.

When analyzing pooled data from all target muscles, the location of projected fMRI and TMS activation maxima on the cortical surface differed by an average 13.9 mm. The differences in 3D distances were particularly large for representation areas of lower leg muscles. 3D distances between fMRI activation maxima and highest MEP site in TMS correlated significantly with higher TMS thresholds. These observations strongly suggest that higher TMS excitation thresholds and lower MEP amplitudes are largely due to the absolute distance between the stimulation site and the excitable cortical tissue targeting this muscle. After the projection 4 out of 5 representation sites as evaluated by TMS were located anterior to the fMRI activation maxima, an observation which may due to the orientation of the magnetic field induced by the current in the coil. The representation sites as evaluated with both methods were specific for the type of movement: distances between representation maxima of the same movements were significantly smaller than those within different movements. Nevertheless, fMRI and TMS provide complementary information, which is discussed on the basis of the functional map observed with both methods.