Presence of two subcomponents in P9 far-field potential following stimulation of the median nerve

Close scrutiny of scalp recorded P9 far-field potentials following stimulation of the median nerve often revealed dilobed wave forms. We observed that the P9 became 2 distinct peaks (P9a and P9b) when the arm was flexed 90° forward at the shoulder and that it became a pointed single peak with 90–170° lateral abduction of the arm. A simultaneously recorded stationary negative peak (N9), registered over the stimulated arm with the use of a distant reference, also showed similar changes, a dilobed configuration (N9a and N9b) with forward flexion and a single peak with lateral abduction. The latencies of the scalp recorded P9a and P9b and arm recorded N9a and N9b were close but not exactly the same. Nevertheless, the latencies of the scalp-positive and arm-negative peaks shifted in nearly a parallel fashion by changing the arm positions.These findings suggest that the change of axial orientation of the propagating nerve impulse plays an important role for the rise of P9a and that the change of volume geometry surrounding the nerve contributes to the P9b generation. Also, the scalp recorded P9 and arm recorded N9 are one and the same, and oriented with dipole fields extending from the arm, body and to the scalp.