RESPONSE THRESHOLDS IN ELECTROCOCHLEOGRAPHY AND THEIR RELATION TO THEPURE-TONE AUDIOGRAM

Objective: The relations between response thresholds obtained by 1) tr anstympanic (TT) and 2) extratympanic (ET) electrocochleography, and b y 3) pure tone audiometry, were investigated in a population of patien ts with cochlear hearing loss of various types and degrees, with the p rediction of audiometric from electrocochleographic thresholds as the primary focus. Design: Analyses were made of the relation between TT r esponse thresholds for tone bursts with octave frequencies from 500 to 8000 Hz and audiometric thresholds in 148 ears. Similar analyses of E T thresholds were given for a subset of 30 ears in which TT and ET phy siological responses were simultaneously recorded. Results: TT electro cochleographic thresholds are highly correlated with audiometric thres holds. Linear regression analysis shows that audiometric thresholds ca n be predicted from physiological thresholds with an error in the esti mate of 11 dB. ET electrocochleography permits similar predictions but with a larger uncertainty of 16 dB. Both methods demonstrate a clear frequency specificity. As a remarkable secondary observation, it appea rs that electrocochleographic thresholds increase slightly less with i ncreasing cochlear dysfunction than do pure tone thresholds. The hypot hesis is elaborated that this is due to the different stimulus duratio ns on which the two threshold measurements are based in interaction wi th the difference in temporal integration between normal and pathologi cal ears. Conclusion: Electrocochleography is a powerful method for th e objective and frequency-specific determination of cochlear response thresholds. TT recording is the preferred method, but ET recording is suitable with a larger uncertainty in the estimate. Electrocochleograp hic thresholds directly measure the cochlear sensitivity as such, wher eas pure tone thresholds measure the combination of loss of cochlear s ensitivity and of reduced temporal integration.