Reverse correlation study of cochlear filtering in normal and pathological guinea pig ears.

The filtering properties of single cochlear fibres have been determined in normal and kanamycin-treated guinea pigs using the reverse correlation technique. This method allows investigation of filtering over a wide dynamic range. For normal guinea pig fibres, the near threshold filter functions obtained with this method correspond to the tone derived frequency threshold curves ((FTCs). The 10 dB bandwidth of the filter functions increased monotonically with increasing noise levels above threshold. Thus with noise levels at approximately 50 dB above threshold, the 10 dB bandwidth had increased by a factor of 1.3--3. The changes in 3 dB bandwidth with increasing levels were, for some fibres, different from those of the 10 dB bandwidths. For the pathological fibres, the derived filter functions corresponded to their tone determined FTCs, and were therefore comparatively broadly tuned. Their tuning (Q10dB) approximated to those of normal fibres when the latter were measured 60 dB or more above threshold (i.e., at similar levels of stimulus), and did not increase further with increase in level. The findings in the normal guinea pig are consistent with those obtained by others in rodents, but are not consistent with those from the cat, where normal filtering is more robust to high levels of stimulus noise.