Topography of the initial cortical component of the median nerve somatosensory evoked potential. Relationship to central sulcus anatomy.

The initial cortical component of the median nerve somatosensory evoked potential (SSEP), the parietal N20, is generated in the posterior bank of the central sulcus and inverts in polarity across the sulcus. The inversion is used to identify the central sulcus. The precentral P20 is sometimes not identifiable in scalp recordings, and this has been attributed to a dipole orientation that directs the maximum positivity downward, into the brain. The authors mapped cortical SSEPs during resection of an arteriovenous malformation in the left sensorimotor area. Preoperative scalp SSEPs over the right hemisphere were normal with a frontal P20, but those over the left hemisphere had an unusual topography with a frontal N20. Intraoperative cortical surface recordings demonstrated an N20-P20 inversion in the inferior-superior rather than the usual posterior-anterior direction. This was a result of the trajectory of the central sulcus over the surface of the brain. The section containing the hand representation was coursing in an anterior-posterior direction. This anatomic variant is an additional cause of absent frontal P20 in scalp recordings. Variations in central sulcus anatomy may cause unusual SSEP topographies, but two-dimensional SSEP mapping and correlation with the sulcal anatomy can still permit localization of the central sulcus in such cases.