The source along the basilar membrane of the cochlear microphonic potential recorded by surface electrodes in man

In order to determine the region along the basilar membrane which contributes to the recorded CM, an attempt was made to record CM responses in subjects with normal hearing and in subjects with high frequency cochlear hearing losses (e.g. acoustic trauma). In such cases of high frequency hearing loss, one may assume that damage has occurred to the hair cells in the basal turn of the cochlea. The CM, elicited by one cycle sinusoidal stimuli of various frequencies, intensities, phases, etc., was recorded by means of earlobe and scalp vertex electrodes. The CM recorded in subjects with normal hearing to high frequency sounds showed (besides the previously reported properties of the CM) large amplitudes and short latencies in response to low frequency stimuli. On the other hand, the CM recorded in subjects with high frequency hearing loss were either prolonged in latency and small in amplitude or completely absent. A plot of the relationship between CM lastency to high intensity 500 HZ stimuli stimuli and audiometric hearing loss in 66 ears shows clearly that in those cases in which hearing was normal at frequencies up to about 8 kHZ, CM latency was very short. On the other hand, in those cases in which the high frequency hearing loss progressed to include more and more lower frequencies, CM latency was more prolonged and smaller in amplitude until CM could no longer be observed. These results indicate that the CM recorded in normally hearing subjects from skin electrodes in response to low frequency (500 HZ) stimuli is generated in the basal turn of the cochlea. This finding is probably a consequence of the form of the mechanical response of the basilar membrane (travelling wave) to low frequency stimuli.