Abstract
A model to explain the orientation selectivity of the neurophysiologic effects of electricโfield transients applied to cerebral cortex is proposed and supported with neuroimaging evidence. Although it is well known that transcranial magnetic stimulation (TMS) excites cerebral cortex in an orientationโselective manner, a neurophysiologically compelling explanation of this phenomenon has been lacking. It is generally presumed that TMSโinduced excitation is mediated by horizontal fibers in the cortical surfaces nearest to the stimulating coil, i.e., at the gyral crowns. No evidence exists, however, that horizontal fibers are orientation selective either anatomically or physiologically. We used positron emission tomography to demonstrate that TMSโinduced cortical activation is selectively sulcal. This observation allows the wellโestablished columnar organization of cerebral cortex to be invoked to explain the observed orientation selectivity. In addition, Rushton's cosine principle can used to model stimulation efficacy for an electrical field applied at any cortical site at any intensity and in any orientation. Hum. Brain Mapp. 22:1โ16, 2004. ยฉ 2004 WileyโLiss, Inc.