PET-Based Confirmation of Orientation Sensitivity of TMS-Induced Cortical Activation in Humans

Abstract

Background Currently, it is difficult to predict precise regions of cortical activation in response to transcranial magnetic stimulation (TMS). Most analytical approaches focus on applied magnetic field strength in the target region as the primary factor, placing activation on the gyral crowns. However, imaging studies support M1 targets being typically located in the sulcal banks. Objective/hypothesis To more thoroughly investigate this inconsistency, we sought to determine whether neocortical surface orientation was a critical determinant of regional activation. Methods MR images were used to construct cortical and scalp surfaces for 18 subjects. The angle (θ) between the cortical surface normal and its nearest scalp normal for ~50,000 cortical points per subject was used to quantify cortical location (i.e., gyral vs. sulcal). TMS-induced activations of primary motor cortex (M1) were compared to brain activations recorded during a finger-tapping task using concurrent positron emission tomographic (PET) imaging. Results Brain activations were primarily sulcal for both the TMS and task activations (P < 0.001 for both) compared to the overall cortical surface orientation. Also, the location of maximal blood flow in response to either TMS or finger-tapping correlated well using the cortical surface orientation angle or distance to scalp (P < 0.001 for both) as criteria for comparison between different neocortical activation modalities. Conclusion This study provides further evidence that a major factor in cortical activation using TMS is the orientation of the cortical surface with respect to the induced electric field. The results show that, despite the gyral crown of the cortex being subjected to a larger magnetic field magnitude, the sulcal bank of M1 had larger cerebral blood flow (CBF) responses during TMS.

A multi-scale computational model of the effects of TMS on motor cortex

“…In L5 PNs, activation thresholds were low in the sulcal walls, matching predictions based on the orthogonal component of the electric field. The excitation in the sulcal cortical surface was consistent with the well-established columnar neuronal orientation and functional organization of the cortex and functional imaging studies 41, 58 . The L3 PNs had different morphology with shorter axons than the L5 PNs such that they were located completely within the gray matter.…”

Simulation of transcranial magnetic stimulation in head model with morphologically-realistic cortical neurons

“…Previous publications hypothesized that the threshold for neural activation by TMS is inversely proportional to either the E-field magnitude 7 or its normal component relative to the cortical surface, based on the orientation of cortical columns 24,25 , thereby generating the lowest thresholds in either the gyral crown or the sulcal wall, respectively. Thresholds were lowest in the pre-and postcentral gyral crowns for all five layers, corresponding to regions of maximal E-field amplitude.…”

The Effect of a Transcranial Channel as a Skull/Brain Interface in High-Definition Transcranial Direct Current Stimulation—A Computational Study

“…3). We compared the spatial extents of polarization with the magnitude of the radial component of EFs, because the stimulation might contribute to neural activation when the EFs are perpendicular to the cerebrospinal fluid (CSF) and GM boundary242526. In Fig.…”