Fig. 1.

Comparison of electrical field modelling for montages directing current across and along the cortical surface.

Electric field orientation on the cortex as applied by electrodes along (A) or across (B) the motor strip. Note: The streamlines and arrows have two separate colorscales. Starting outside the motor-strip, streamlines trace the direction of current density from high to low voltage (red to blue), anode to cathode. The streamlines confirm that current flows down the voltage gradient established by the electrodes and shaped by the head anatomy. On the motor strip, arrows illustrate the direction of electric field passing through the motor cortex surface. Arrow colour represents normal electric field where red is inward and blue is outward. Inward field corresponds to expected pyramidal soma depolarization and outward corresponds to expected pyramidal soma hyper-polarization (though we note that the polarization of axons and terminals must also be considered). It is important to not confuse the red/blue of voltage with the red/blue of electric field/polarization since the two are not simply related; none-the-less, this representation allows correlation of macro-scale current flow patterns set by electrode montage with gyri-scale current flow pattern which determine cellular polarization. These modelling results show how current can be directed to flow more uniformly through M1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)