Laplacian mapping of the somatosensory response of the median and tibial nerves

The somatosensory evoked response recorded using monopolar methods is influenced by activity at the reference electrode. The surface Laplacian operator applied to the evoked recordings removes nonlocal influences by providing a measure of the current flowing perpendicular to the skull near the recording electrodes. The accuracy of this measure is dependent on the density of the recording electrodes; thus a larger number is required. In this study Laplacian evoked response maps obtained from stimulation of several nerves are compared with corresponding potential maps. The influence of the number of electrodes and the ability of a Laplacian spatial map to distinguish between radial and tangential sources is also demonstrated. This is an extension of computer simulations and evoked potential studies that are being done in the authors´ laboratory.