Temporal factors associated with cochlear nerve tuning to dual and single tones: a qualitative study

The simultaneous presentation of a 10- and 10.86-kHz tone produces an 860-Hz cochlear nerve difference tone (DT) response in the gerbil which persists for the duration of the stimulus. Forward masking shows this response is generated by neurons sharply tuned to the stimulus frequencies. When compared with the DT response, the cochlear nerve compound action potential (CAP) to a single tone is smaller in amplitude, has a higher nonmasked threshold, and produces a less sensitive tuning curve (TC). Forward maskers can also produce amplitude enhancement of the CAP, but this was not observed for the onset portion of the DT response. The CAP TC is as sharply tuned as the TC of either the DT onset response or the entire DT response. A comparison was made of tuning of the DT response to the onset, the first half and second half of the 23-ms duration probe stimulus, using either a 5- or 15-ms masker-probe interval. An increase of the tip threshold of the TC to all three portions of the stimulus occurred as the interval was increased between the end of the masker and the midpoint of the portion of the stimulus under question. The 15-ms masker-probe interval produced sharper TCs.