Peripheral origin of BAEP wave II in a case with unilateral pontine pathology: a comparison of intracranial and scalp records.

In a patient with unilateral hearing impairment due to a predominantly ipsilateral pontine pathology, BAEP peak II in a vertex to ipsilateral mastoid derivation (BAEP-II) was preserved on stimulation of the affected ear. All later activity was lost. Intracranial recording from electrodes intraoperatively placed over both cochlear nuclei on the floor of the IVth ventricle showed potentials with size and wave form comparable to the scalp records. Thus only peripheral sources, distant from the ventricular electrode, seem to contribute to scalp BAEP-II. Upon stimulation of the unaffected ear considerably larger intracranial activity was seen with peak 2-4 wave shapes obviously different from the scalp BAEP II-IV. It is concluded that intracranial electrodes sense a complex field distribution predominantly originating in structures to which they are in close electrical connection. This activity, however, need not necessarily project to the scalp.