[A new electrode (HN-5) for CM measurement in extra-tympanic electrocochleography]

A new electrode (HN-5) for CM measurement in extra-tympanic electrocochleography (ECochG) has been developed. The electrode consists of a silver wire 2.0 mm in diameter and 5 cm in length connected to a low noise cable. The electrode impedance values were low and the frequency characteristics were excellent. In order to explore various artifacts, the electrode was inserted into a glass cup containing 100 ml of tap water, which served as a dummy ear. A sound stimulus with an intensity of 90 dBnHL was delivered by a shielded loudspeaker located 50 cm from the dummy ear (glass cup). In this dummy test, contamination with measurable artifacts, such as electromagnetic inductions generated by a sound transducer and the CM-like mechanical vibrations which activated the dummy ear acoustically, was kept within the noise levels inherent in the recording system. The CMs were recorded from subjects with normal hearing by placing an electrode adjacent to the postero-inferior rim of the tympanic membrane (annulus tympanicus). Short tone bursts were employed as acousti stimuli and delivered by the shielded loudspeaker. CM detection thresholds were very low and CM-like responses with delayed onset time (delayed CM) were clearly observed at lower stimulus levels. The input-output curves of the CMs and delayed CMs were non-linear. These findings from the CMs did not differ from those obtained by the transtympanic needle electrode technique. Responses obtained had satisfactory S/N ratios and resulted in almost the same average calculations as those with transtympanic ECochG.(ABSTRACT TRUNCATED AT 250 WORDS)