title: Unexpected latency shifts of the stationary P9 somatosensory evoked potential far field with changes in shoulder position.
abstract: Early somatosensory evoked potential (SEP) components to median nerve stimulation were recorded with non-cephalic reference from scalp, neck and oesophagus in normal young adults. The nerve volley was recorded from the arm and Erb's point. The latencies of the stationary P9 far field were increased significantly by supporting the shoulder in a high position. This manoeuvre was shown in X-rays to change the axis of the nerve at levels between lateral and middle parts of the clavicle. This suggests that the far field is not related to inhomogeneities of the medium surrounding the nerve whereby abrupt changes in extracellular current flow might be produced as the volley arrives at a certain level. Rather, the axial orientation of the propagated dipole appears as a major factor determining the features of the stationary P9 far field. The reversible effect described does not significantly influence the conduction of the volley itself nor the latencies of the subsequent SEP components.
mesh_terms: D000328:Adult; D001921:Brain / Q000502:physiology*; D004569:Electroencephalography; D005073:Evoked Potentials, Somatosensory; D005260:Female; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000502:physiology; D011187:Posture; D011930:Reaction Time; D012782:Shoulder / Q000502:physiology