Selectively eliminating cochlear microphonic contamination from the frequency-following response

The frequency-following response (FFR) is the scalp recorded response to low frequency stimuli. As an electrophysiological method for determining auditory threshold, it has application in both clinical and research settings. However, the response is often contaminated with the cochlear microphonic (CM), reflecting the response of outer hair cells, rather than neural generators (i.e., auditory nerve, cochlear nucleus, superior olivary nucleus, inferior colliculus, etc.). The FFR needs to be a purely neural response to establish its clinical and experimental usefulness. The methods applied to date have failed to accomplish this. The present study demonstrates a method of deriving a pure neural response by subtracting a forward masked FFR, which contains only CM, from an unmasked FFR. To confirm that the residual response after forward masking is solely CM, one needs to record two forward masked responses with opposite phase probe stimuli. When the responses are added they will sum to zero only if the residual response with forward masking is pure CM. This study demonstrates that the traditional method for removing CM from FFR, by simple summation of unmasked responses recorded with stimuli of opposite phase, does not accurately reflect the amplitude or frequency of the FFR, while the proposed method provides an accurate assessment of the FFR amplitude free of CM contamination.