Middle latency somatosensory evoked potentials: noninvasive source analysis.

The sources of the components of short latency somatosensory evoked potential in humans have been identified using electrical source localization, magnetoencephalography, and electrocorticography, but the sources of the middle latency components are less understood. The purpose of this study was to identify the source of the middle latency component N60 of the electrically elicited somatosensory evoked potential (SEP). Noninvasive equivalent electrical multiple dipole source localization technique (BESA) was used. SEPs were recorded from nine healthy subjects with 30 electrodes placed on the scalp. Median nerve at wrist (bipolar electrode), index, and little fingers (ring electrodes) were successively stimulated with 2.1 Hz frequency. A window of 100 ms was analyzed. Spatiotemporal equivalent electrical dipole models, each consisting of four dipoles, explained the three different SEPs, from median, index, and little fingers, with residual variances below 9%. The sources generating N20, P30, and N60 all appeared to locate in the posterior bank of the central sulcus contralateral to the stimulated hand. The location of the dipole number 2, most active around N20 component, differed between the index and little finger models, suggesting that somatotopic organization of the primary somatosensory area of the digits was demonstrated by these models. A radial source (dipole 4) was most active around 60 ms in all three models, thus explaining the N60 component. The location of this dipole did not suggest somatotopic organization specific to the stimulated finger for the 60 ms component.