Offset tuning curves generated by simultaneous masking are more finely tuned than those generated by forward masking

The rapid ending of a tone produces an evoked potential which has different properties than that which is produced by the sudden onset of a tone. At the level of the round window, the offset N1 N2 follows the ending of the cochlear microphonic (CM) by approximately the same amount of time as does the onset N1 N2 to the onset of the CM. Both onset and offset responses are abolished with cochlear lesion. Continuous masking was used to generate tuning curves (TCs) from the NI-PI component of the evoked potential recorded from the round window of the gerbil. Those evoked potentials generated in response to the tone onset were complementary in appearance to those generated in response to the tone offset. TCs generated by continuous masking of the NI-PII component of the auditory brainstem response (ABR) of the gerbil show the same pattern. When it is generated by simultaneous masking, the midfrequency offset TC in the gerbil and mouse is W-shaped. It has two well tuned tips which occur at frequencies below and above that of the probe stimulus used to generate the TC. It also has an even better tuned peak occurring at or slightly above the probe stimulus frequency, which becomes sharper as the masker sound pressure level (SPL) is increased from 50 to over 80 dB. Because the midfrequency onset response is approximately 40 dB lower than the midfrequency offset response, probe stimuli for onset TCs are generally set at lower SPLs. When the onset probe stimulus is set to the same level as that of the offset probe, the Q10 dB of the offset TC may be up to 10 times the value of the Q10 dB of the onset TC. The offset TC generated in the CBA/J mouse by forward masking is quite different from that produced by simultaneous masking. Both forward and simultaneous conditions utilized a 40 ms duration tone to mask the PI-NI component of offset and onset ABRs of the mouse which were evoked by a 10 ms duration, 32 kHz tone, presented at an interstimulus interval of 160 ms. Forward masking (when compared with simultaneous masking) resulted in a more sharply tuned onset TC. But the offset TC was much less sharply tuned in the forward masking condition. This suggests that the offset response may reflect functions which are involved with fine tuning at moderate to high intensities in the presence of simultaneous sounds of similar spectral characteristics.(ABSTRACT TRUNCATED AT 400 WORDS)