Figure 8.
The cortical column cosine (C3) model. The linear, scalp‐parallel E field (bold arrows) induced by a B‐shaped coil is illustrated. In this study, the average value of the E field applied at the scalp was 300 V/m. The average depth of M1 excitation by TMS was 31 mm. (A centimeter ruler is shown at right). The E field diminishes exponentially with distance from the scalp. The C3 model posits that activation is induced only by the column‐aligned component of E. The absolute E field vector (Eabs) is decomposed into two components: a column‐aligned component that is effective (Eeff) and column‐normal component that is ineffective. Otherwise stated, Eeff is the product of Eabs and the cosine of the angle between the column and the E vector (θ). Whereas the Eabs is greatest at the gyrus, the Eeff is least and activation fails to occur (A). In the sulcal depth (C), Eabs is least, but Eeff is greatest and cortical excitation threshold is achieved. On the shoulder of the gyrus (B), Eabs can be greater than in the sulcus (C), but Eeff may still fall below threshold. The bending angle (Φ) of the pyramidal cell axon as it exits cortex and enters the corticospinal tract is illustrated.