Separate generators with distinct orientations for N20 and P22 somatosensory evoked potentials to finger stimulation?

Sequential spatial maps of scalp potentials, obtained with a 16-channel montage, were used in 12 healthy subjects in order to assess the temporal and spatial distribution of early cortical SEPs to single finger stimulation. It was found that when the contralateral parietal N20 negativity peaks there is a synchronous frontal P20 positivity, supporting the view of a tangentially orientated dipolar generator for this couple of scalp SEPs components. It was not possible to show a distribution of N20 peak on the scalp that would parallel the somatotopic finger representations in area S1; however, the orientation of the putative dipolar source of the N20/P20 complex was found to change according to the finger stimulated. A central P22 component was also constantly obtained without any synchronous negativity on the scalp surface corresponding to the electrode array; a clear somatotopic organisation was found for P22. These features favour the hypothesis that this latter component has a radially orientated generator situated in the prerolandic motor cortex, close to the scalp surface. Because of overlapping between the P20 and P22 components, the determination of P22 onset latency was hazardous in some cases, and spatial mapping was then essential to identify this component. The conclusion that the contralateral parietal N20 and central P22 could be generated by separate dipolar generators with distinct orientations is supported by recent data from combined electrical and magnetic field recording.