Dipole orientation of receptive fields in the somatosensory cortex after stimulation of the posterior tibial nerve in humans.

The origins of the earliest evoked potentials and magnetic fields after tibial nerve electrical stimulation are still controversial. We focused on the initial activity from the gyrus area and analyzed the components for the coronal and sagittal planes. In 12 healthy right-handed subjects, electrical stimuli were delivered to the left posterior tibial nerve at the ankle. The cortical somatosensory evoked fields were recorded, and the equivalent current dipoles were calculated and separated into the sagittal plane (y-components) and coronal plane (x-components) components. In nine subjects, we observed two deflections (y1 and y2) in the y-component waveform and two deflections (x1 and x2) in the x-component waveform over 60 milliseconds; y1 was directed anteriorly, y2 posteriorly, x1 to the left, and x2 to the right. The y1 was originated in the anterior wall of the central sulcus. By focusing on the y-component, we elucidated the existence of the posteroanterior component, which may originate from the area 3b (gyrus) in tibial nerve somatosensory evoked fields and has the same quality as N20m for median nerve somatosensory evoked fields. This is the first report to suggest that the posteroanterior component in the tibial nerve is analogous to N20m in the median nerve using magnetoencephalography.