[Neurophysiologic study of 2 cases of hemianesthesia as a result of subcortical lesions. Results of recording far-field somatosensory evoked potentials].

The volume-conducted responses of the lemniscal pathways to median nerve stimulation at wrist may be recorded on the scalp (far-field potentials). These positive far-field SEPs components are widely distributed on the scalp and their peaking latencies vary between 9 and 15 milliseconds. In normal adults a maximum of 4 far-field potentials (P9, P11, P13 and P14) may be individualized; two of them (P9 and P14) are constant. These SEPs were studied in two patients with lateralized somatosensory loss; one with a cervico-medullary traumatic lesion, the other with a thalamic infarct. These observations allow the following conclusions 1) the P9 component takes origin in the proximal part of the brachial plexus roots; 2) the P14 potential has a brainstem origin; 3) the contralateral N20 potential is generated in (or close to) the primary somato-sensory cortex (SI). Thus it is possible with a single channel to record the activity of the somatosensory pathways from dorsal roots up the parietal cortex.