Early scalp responses evoked by stimulation of the supraorbital nerve in man.

In 25 healthy volunteers the supraorbital nerve was stimulated and evoked potentials were recorded. Leads were placed on the scalp and along the ipsilateral eyebrow-mastoid line and were either referred to a non-cephalic reference (on the neck, or Cv7) or linked to form bipolar derivations. As template wave form was chosen the one obtained from derivation Cz-Cv7, which had an initial triphasic component with negative (SW1a), positive (SW1b), negative (SW1c) polarity (mean latencies 0.63, 0.95 and 1.43 msec), followed by 2 negative waves (SW2 and SW3, mean latencies of 2.20 and 2.89 msec). A final positive wave could be observed in most cases (SP4, mean latency of 4.08 msec). The records collected from the various derivations showed that each component (SW1, SW2, SW3 and SP4) had a different behaviour, thus suggesting separate origins. SW1 would originate from a volley travelling from the point of stimulation towards the mastoid, probably across the ophthalmic branch of the trigeminal nerve. The subsequent components would be generated by deeply situated structures: double pulse stimulation suggests that SW1, SW2 and SW3 are generated before the first synapse, whereas SP4 is a postsynaptic event. A strong similarity exists between the components evoked by stimulation of the supraorbital and the infraorbital nerves. Local anaesthetic block of the frontal nerve on the stimulated side and monitoring of the EMG activity of m. orbicularis oculi and m. frontalis ruled out any muscle contamination of the responses described in this paper.