Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man.

Somatosensory evoked potentials (SEPs) to median or fingers or posterior tibial nerve stimulation were recorded with earlobe reference in normal young adults. A system of 16 electrodes on the scalp served to create bit-mapped images of the potential fields at 1 msec intervals. The P14 (median SEP) or P30 (tibial SEP) far fields thought to reflect the afferent volley in the medial lemniscus produced widespread positivity over the scalp. Subsequent components had a characteristic focal distribution suggesting that they reflected one or more generators in cortical areas. For the median SEP, the parietal N20 and the prerolandic P22 showed differences in onset and offset times as well as distribution that precluded their being related to the same generator. While N20 was contralateral, P22 extended ipsilaterally. P22 may be generated in the motor area 4 and the supplementary motor area. P22 was also distinct from the P27 field restricted to the contralateral parietal region. The frontal N30 had a bilateral distribution and the P45 presented variable features. For the tibial SEP, no phase reversal was confirmed between the parietal P38 (midline-ipsilateral focus) and N33 (contralateral focus). N37 over the contralateral prerolandic region might reflect a generator in the motor region. P58 was more symmetrically distributed than P38, possibly because it reflected generators more posteriorly on the parietal convexity. N75 had a widespread field with focus on the ipsilateral side of midline.

The cognitive P40, N60 and P100 components of somatosensory evoked potentials and the earliest electrical signs of sensory processing in man.

Selective attention tasks involving random sequences of electrical stimulation of fingers were designed to compare cerebral potentials to identical stimuli (for example to the left thumb) when they are 'infrequent target' signals or 'frequent-neglected' signals in the series. The experiments were carried out in normal adult subjects. The early SEP components were analyzed for the earliest cortical electrical sign of information processing stages. Two new components, P40 (with onset at 26 msec) and N60, precede the processing positivity P100 which may be more significant in somatosensory processing than the negativities recorded in auditory or visual tasks.

Bit-mapped color imaging of human evoked potentials with reference to the N20, P22, P27 and N30 somatosensory responses.

Bit-mapped color imaging of scalp potential fields evoked by sensory stimulation in humans disclosed significant features not identified by mere inspection of multichannel traces. Methodological problems are considered in detail for early cortical SEPs which include several components with sharp rise times occurring at spatially distinct scalp locations. A manageable yet efficient imaging system requires recording electrodes in adequate number and scalp locations, bandpass fidelity to resolve slow and fast components, consistency of bioelectric input data, optimal interpolation and mapping algorithms, and consistent color scaling. Critical steps in these procedures were investigated in conjunction with new evidence on the scalp topography and neural generators of the N20, P20, P22, P27 and N30 SEP components. It is concluded that N20-P20 reflect a tangential equivalent dipole in parietal area 3b while P22 reflects a radial equivalent dipole in motor area 4.