Source generators of the early somatosensory evoked potentials to tibial nerve stimulation: an intracerebral and scalp recording study

Objective: To investigate the location of the cerebral generators of the ea rly scalp somatosensory evoked potentials (SEPs) after tibial nerve stimula tion, Methods: Tibial nerve SEPs were recorded in 15 patients, suffering from Par kinson's disease, who underwent implantation of intracerebral (IC) electrod es in the subthalamic nucleus, in the globus pallidum or in the thalamic ve ntralis intermediate nucleus. SEPs were recorded both from the scalp surfac e and from the IC leads. Results: The lemniscal P30 response was recorded by all the electrodes. The IC waveforms included a negative N40IC response, followed by a positive (P 50IC) and a negative (N60IC) potential, The N40IC, the P50IC and the N60IC potentials did not differ in latency from the P40, the N50 and the P60 resp onses recorded by the Cz electrode. In 6 patients, in which SEPs were recor ded also during the voluntary movement of the stimulated foot (active gatin g), an amplitude reduction of the SEP components following the P30 potentia l was observed during movement at the vertex and in the IC traces. Instead, in the contralateral temporal traces the SEP components (N40temp and P50te mp) were not modified by active gating, and in the ipsilateral parietal tra ces only the positive potentials at about 60 ms of latency was decreased. Conclusions: Two differently oriented generators are active in the contrala teral hemisphere at both 40 and 50 ms of latency after tibial nerve stimula tion. One source is oriented perpendicularly to the mesial hemispheric surf ace and generates the potentials recorded by the contralateral temporal and the ipsilateral parietal leads; the other dipolar source is radial to the hemispheric convexity, and generates the potentials at the vertex and those recorded by the IC electrodes. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.