Comparison of earlobe and promontorium recording sites in electrocochleography

A compound cochlear action potential can be recorded in man from the promontorium, after perforation of the tympanic membrane. This same cochlear action potential (along with the responses of the brain stem auditory nuclei) can also be recorded by means of earlobe-vertex electrodes. Recordings were made in patients with chronic perforations of the tympanic membrane from the promontorium and simultaneously from the earlobe (-vertex) in order to see if there is a difference in response threshold. As click intensity was decreased, the responses disappeared in both recordings at about the same stimulus intensity (threshold). At near-threshold click intensities, the response seen in the promontorium recording was the compound cochlear action potential while in the earlobe vertex recording, the response often seen was seen was that from the inferior colliculus. Thus, in most cases, there is no real advantage to be gained by perforating the tympanic membrane in order to record from the promontorium since an identical response threshold can be obtained using surface electrodes.