Scalp distribution of electrical fields related to blink reflex

In 20 subjects the supraorbital nerve was stimulated and R1 recorded from electrodes placed over the ipsilateral orbicularis oculi muscle and from locations Fz, F8, F7, Cz, C6, C5, Pz, T4, and T3 on the scalp. The latter were referred either to an extracranial electrode or to Fz. In five subjects an artificial dipole was set at three different positions on the eyebrow and records were taken from the same derivations on the scalp to study the distribution of fields of known intensity originating from known locations. It was found that R1 could be easily detected from all scalp locations. According to its scalp distribution, three patterns were identified, which matched those of the artificial dipole. Conversely from what had been believed by previous authors, the amplitude of R1 could be larger on the contralateral scalp, according to the reference used or to the location of its origin. Therefore, it is remarked that larger amplitude contralateral to the stimulus cannot anymore be considered an exclusive feature of responses arising from the cortex. The evidence we have provided recommends a highly cautious approach in interpreting results describing trigeminal scalp responses in the latency range of R1.