Intermodulation distortion in the cochlea as shown by offset action potential (AP) masking curves.

In their recent article "Offset AP masker tuning curve and the FFT of the stimulus" [J. Acoust. Soc. Am. 84, 1354-1362 (1988)], Henry and Lewis demonstrated that the tuning curve obtained by the simultaneous masking of the whole nerve action potential (AP) could have two tips when the AP is generated at the offset of the envelope of a high-level probe. The primary tip falls below the probe frequency, whereas the secondary tip falls above the probe frequency. Curves obtained for the onset response with either forward or simultaneous masking did not show the secondary peak, nor did curves obtained for the offset response with forward masking. Henry and Lewis discussed various reasons for the secondary tip, but came to no conclusion as to the underlying mechanisms. Here, it is reasoned that the secondary tip of the offset curve can be simply explained by the generation within the cochlea of intermodulation distortion (IMD), which acts as a forward masker to the offset response. The IMD is dominated by the cubic component (2f1-f2) and arises from the interaction of the probe tone and the simultaneous masker. Finally, it is reasoned that the lower sideband of the frequency splatter present at probe offset is the primary stimulus for the evoked neural response under probe offset conditions. Thus the offset curve will always have a primary tip that is lower in frequency than that of the respective onset curve. These hypotheses are supported by single-fiber data.