Electrocochleography

Abstract

The tympanic membrane was anesthetized with a lidocaine (Xylocaine) spray. A fine electrode insulated to within 5 mm of the tip was inserted through the tympanic membrane in the posteroinferior segment of the membrane several millimeters from the umbo. The electrode was held in contact with the promontory by a taut rubber band which passed over its forked distal end. The ends of the band were attached to a 'donut' placed around the ear. The acoustic stimulus was a click of 0.1 msec width delivered at a rate of 10/sec. The AP response to a single click is small, measuring only a few microvolts; however, with use of a computer, the response to a 1,000 clicks (made up of 500 rare faction and 500 condensation clicks) are summated and averaged. This response is written out on an XY recorder. The procedure was performed by using an acoustic stimulus of 90 dB then progressively decreasing the sound level of the stimulus in 10-15 dB steps until there was no recordable response. The latency and amplitude of the responses were measured and charted. The amplitude of the response was expressed as a percentage of the maximum amplitude. The sound level corresponding to the minimum measurable threshold was compared to the average pure tone threshold (ie, the average of the 500, 1,000, and 2,000 Hz thresholds re ISO norms) and the speech reception threshold.