Basis and some diagnostic implications of electrocochleography

Electrocochleography (ECoG) involves the recording of electrical responses to sound from the vicinity of the cochlea. The technique, as we practice it, utilizes a trans-tympanic recording needle situated on the promontory of the middle ear. Filtered clicks in the frequency range between 500 and 8,000 Hz are presented to the ear under test at a rate of 10/sec. Repetitive clicks of a given frequency are first presented at high intensity and the responses summed (averaged) in a computer. The click is systematically lowered in intensity, and an average is collected at each level until the normal dynamic range of hearing has been explored. Two major electrical indices are present in the recording: these are 1. the whole-nerve action potential (AP) derived from the first-order auditory neurons, and 2. the cochlear microphonic (CM) derived from the hair cells. Inspection of the whole nerve AP as intensity is lowered allows the estimation of the response threshold, which correlates well with behavioral threshold. Inspection of the relation between the CM and the AP allows qualitative differentiation to be made between pathology arising in the hair cell (sensory) and in the nerve (neural).