Distinct fronto-central N60 and supra-sylvian N70 middle-latency components of the median nerve SEPs as assessed by scalp topographic analysis, dipolar source modelling and depth recordings

Objectives: To investigate the possible contribution of the second somatosensory (SII) area in the generation of the N60 somatosensory evoked potential (SEP). Methods: In 7 epileptic patients and in 6 healthy subjects scalp SEPs were recorded by 19 electrodes placed according to the 10–20 system. All epileptic patients but one were also investigated using depth electrodes chronically implanted in the parieto-rolandic opercular cortex. Scalp SEPs underwent brain electrical source analysis. Results: In both epileptic patients and healthy subjects, scalp recordings showed two middle-latency components clearly distinguishable on the basis of latency and scalp distribution: a fronto-central N60 potential contralateral to stimulation and a later bilateral temporal N70 response. SEP dipolar source modelling showed that a contralateral perisylvian dipole was activated in the scalp N70 latency range whereas separate perirolandic and frontal sources were activated at the scalp N60 latency. Depth electrodes recorded a biphasic N60/P90 response in the parieto-rolandic opercular regions contra- and ipsilateral to stimulation. Conclusions: Two different middle-latency SEP components N60 and N70 can be distinguished by topographic analysis and source modelling of scalp recordings , the sources of which are located in the fronto-central cortex contralateral to stimulation and in the supra-sylvian cortex on both sides, respectively. The source location of the scalp N70 in the SII area is strongly supported by its spatio-temporal similarities with SEPs directly recorded in the supra-sylvian opercular cortex.