O-12 Different contribution of muscular and cutaneous afferents to the scalp SEPs

Background Although somatosensory evoked potentials (SEPs) represent a largely diffused neurophysiological technique, the pathways generating the short-latency scalp components are not fully known. We aimed to investigate the SEPs recorded from the scalp and deep brain stimulation (DBS) leads to selective electrical stimulation of either muscular or cutaneous afferents. Material and methods SEPs were recorded in 6 patients suffering from advanced Parkinson’s disease who underwent electrode implantation in the pedunculopontine (PPTg) nucleus area. We compared SEPs recorded from the scalp and DBS electrode contacts to electrical stimulation of: (a) the median nerve at the wrist (mixed stimulation), (b) the abductor pollicis brevis motor point (pure stimulation of the muscle afferents), and (c) the distal phalanx of the thumb (pure cutaneous stimulation). High-frequency oscillations (HFOs) subtending the low-frequency SEPs were also analysed. Results The macroelectrode contacts recorded a triphasic (P1-N1-P2) potential and the scalp electrodes recorded all the classical SEP components, including the widespread N18 response, after both median nerve and pure cutaneous stimulation. After motor point stimulation, the scalp N18 component and the PPTg N1 response were not recorded. No difference in the HFOs between the stimulation modalities was found. The scalp N20 component was recorded to stimulation of any modality, even if with different latency and amplitude. Conclusions Considering that both the scalp N18 response and PPTg N1component reflect the activity of a pre-postsynaptic neural network in the cuneatus nucleus, our data suggest that the cuneate response is specifically evoked by cutaneous inputs, while muscle afferents are processed elsewhere.