hide
Free keywords:
-
Abstract:
We report a novel method to determine the site and size of stimulated cortical area in
TMS. Applied to the motor cortex, it allows to determine the likely cortical representation
of muscles. Up to now, the most common procedure for this is motor mapping. In
motor mapping, the obtained two-dimensional distribution of coil positions with associated
muscle responses is used to calculate a center of gravity on the skull. However,
classical mapping does not allow to determine the exact stimulation site on the cortex
and only rough estimates of its size are possible. Our method combines physiological
measurements with a physical model used to predict the electric field induced by the
TMScoi l to overcome these limitations. In four subjects motor responses in a small
hand muscle were mapped with 9 - 13 stimulation sites at the head perpendicular to the
central sulcus in order to keep the induced current direction constant in a given cortical
region of interest. Input-output functions from these head locations were used to determine
stimulator intensities that elicit half-maximal muscle responses. Based on these
stimulator intensities the field distribution on the individual cortical surface was calculated
as rendered from anatomical MR data. The region on the cortical surface in which
the different stimulation sites produced the same electric field strength (minimal variance
4.2 ± 0.8 . ) was determined as the most likely stimulation site on the cortex. In all
subjects, it was located at the lateral part of the hand knob in the motor cortex. Comparisons
of model calculations with the solutions obtained in this manner reveal that the
stimulated cortex area innervating the target muscle is substantially smaller than the size
of the electric field induced by the coil.
