Far-field potentials generated by thalamocortical fibers in median nerve SEPs

We previously suggested that P15 in median nerve SEPs is generated by thalamocortical fibers by the mechanism of the junctional potential. In this study, we examined five normal subjects using long-distance bipolar leads over the scalp and aimed to further elucidate the nature of the P15 potential as a junctional potential. As a result, negative potentials were detected in all the sagittal (fronto-occipital) bipolar leads, which coincided with the P15 latency. In contrast, we found no far-field potential in any coronal bipolar lead. Accordingly, a dipolar distribution, frontal negative and occipital positive, is suggested at this latency, indicating the presence of a junctional potential. We suggested that the mechanism of this junctional potential is the bending of the thalamocortical fibers. Computer simulations revealed that the change of the direction of neural propagation can produce a junctional potential. Thalamocortical fibers are angled backwards anatomically. The expected polarity of the dipole generated by this kinking coincides with the observed distribution.