Innocuous-related sural nerve-evoked and finger-evoked potentials generated in the primary somatosensory and supplementary motor cortices

Objective: Our earlier work revealed two components of the somatosensory evoked potential, which we have labeled SP1 and SP4a, that appear to be generated by neurons involved in the innocuous aspects of somatosensation. The objective of the present study was to examine a hypothesis developed in our earlier work, namely that SP1 and SP4a are generated in the primary somatosensory cortex. Methods: The dipole source localization method was applied to SP1 and SP4a evoked by electrical stimulation of the fingers and of the sural nerve in 20 subjects. The subjects rated the subjective magnitude of each stimulus on a 9 point scale. Results: The finger-evoked and sural nerve-evoked SP1 were best-fit by single sources located in the primary somatosensory cortex (SI) hand and foot areas, respectively. Both the finger-evoked and the sural nerve-evoked SP4a, on the other hand, were best-fit by a single source located in the supplementary motor area (SMA). Conclusions: These results are consistent with our hypothesis that SP1 reflects the activity of SI neurons that are involved in innocuous somatosensation. SP4a is not generated in SI as we originally hypothesized, but rather in the SMA. The SP4a amplitude-stimulus intensity function and the dependence of the SP4a source location on the evoking stimulus site and not the hand registering the magnitude rating suggests that SP4a reflects the response of SMA neurons to afferent input from the innocuous somatosensory pathways. Hence, SP4a may be generated by SMA activity involved in the sensory-guided selection and/or generation of motor responses