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Electrocochleography From the Promontory and via a Cochlear Implant
A.1
Introduction
Electrocochleography (ECochG) is the recording of stimulus-related potentials generated in the human cochlea and the auditory nerve. These potentials are the cochlear microphonics (CM), the summating potentials (SPs), and the compound action potential (CAP). There are three important applications of ECochG: (1) correlation of physiological and psychoacoustic properties, (2) investigation of certain diseases, and (3) objective diagnosis of individual cases of deafness ( ). Currently, auditory brainstem response (ABR) recording has taken over the role of ECochG for items 1 and 3. ECochG remains as an excellent tool for diagnosing and studying among others Ménière’s disease and various forms of auditory neuropathy (see chapter: Types of Hearing Loss ). In addition the use of neural response telemetry in cochlear implants resulting in electrically evoked CAPs (eCAPs) is used for testing electrode impedance and neural survival (see chapter: Cochlear Implants ). We will compare these two recording methods in Section A.5 but first present a brief introduction to ECochG.
A.2
Methods
A.2.1
Stimuli
Sound stimuli capable of evoking CAPs from the auditory nerve have to be rapidly changing in time since a well-defined triggering of the auditory nerve fiber activity is required. ECochG with clicks provides a general impression of cochlear functioning rather quickly. In cases of a flat hearing loss, one obtains an exact threshold determination and considerable information about the suprathreshold behavior of the cochlea. , however, used short trapezoidal tone bursts, which results in good threshold frequency specificity. In addition, the longer duration and the flat plateau of the tone burst considerably facilitates the quantification of the CM and SP. These pure tone bursts had two periods of the sine wave during the rise time and fall time and at least six periods during the plateau. This makes the duration of the tone burst dependent on the stimulus frequency. However, the shortest rise time used was 0.33 ms and the plateau duration was never made less than 4 ms to assure reliable CM and SP measurements.
When tone bursts are used, canceling of the CM becomes important. Two methods can be employed: rolling (random) phase stimulation or presentation of the tone bursts alternately in phase and counter-phase. Both methods have advantages. In rolling phase stimulation, the even harmonics present in the CM at high intensity levels are also canceled, which is not the case using alternating phase stimuli; however, the CM information is completely lost. In alternating-phase presentation ( Fig. A.1 ), the CM can also be obtained free of CAP by additionally commutating the responses recorded and averaging them separately.
A.2.2
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