Electrocochleography potentials evoked by condensation and rarefaction clicks independently derived by a new numerical filtering approach

The cochlear microphonic potential (CM) and the compound action potential ( CAP) cannot be measured separately but only in combination. In the literatu re their individual estimates are conventionally recovered by the so-called CM cancellation technique. This method averages the potential obtained in response to rarefaction and condensation clicks under the assumption that c hanging the polarity of the clicks only affects the CM sign and does not al ter the CAP in any way. However, both theory and evidence suggest that thes e hypotheses can be critical. In addition, recent contributions in the elec trocochleography (ECochG) literature suggested that assessing the influence of stimulus polarity on the evoked CAP may constitute an indicator of clin ical usefulness which the CM cancellation method cannot supply. In this wor k we propose a new algorithm to estimate the cochlear potentials evoked fro m positive clicks, CAP+ and CM+, and those evoked from negative clicks, CAP - and CM-, by processing the same kind and amount of data employed in the C M cancellation method. The application to real data taken from 3 subjects e xhibiting quantitatively and qualitatively different ECochG responses at va rious levels of stimulation intensity is presented. In addition, simulated problems where the true CAP and CM are known are studied to permit a fair a ssessment of the proposed technique. Results suggest that the new algorithm is potentially able to point out small differences between CAP+ and CAP-, This encourages its further employment on a larger scale. Copyright (C) 200 0 S. Karger AG, Basel.