Suprathreshold comparisons of derived and enhanced compound action potentials.

The Derived cochlear nerve compound action potential (CAP) and the Enhanced CAP are both measures which demonstrate the ability of a forward masker to increase the amplitude of the CAP produced by a probe stimulus. Enhancement occurs whenever the amplitude of a masked CAP is larger than that of a nonmasked CAP, whereas the derived CAP is produced by the subtraction of the entire masked CAP waveform from that of the nonmasked waveform. Therefore, a derived CAP is created whenever the masker produces a difference of amplitude, latency, and/or waveform shape. The present experiments compare these two measures by observing the effects of 13 kHz maskers varying from levels of -10 to +70 db SPL on CAPs produced by 50 or 60 dB SPL, 13 kHz probe stimuli. Enhancement is characterized by a nonmonotonic increase of CAP amplitude (and sometimes a decrease of latency) as a function of increasing levels of the forward masker, whereas this pattern seldom occurs with the derived CAP. Enhancement is typically seen with forward making, but seldom seen with simultaneous masking, whereas the derived CAP is very similar under these two types of masking.