Tuning of the auditory brainstem OFF responses is complementary to tuning of the auditory brainstem ON response.

Continuous masking studies show a complementary pattern of effects on the auditory brainstem responses (ABRs) which are generated by the onset and by the offset of a midfrequency tone. The masking profiles of the two responses are almost opposite with a probe stimulus frequency of 32 kHz (16-32 kHz is the midfrequency region for the CBA/J mouse). The Offset and Onset ABR tuning curves (TCs) also reveal very different properties at the midfrequencies of 16, 20, 24 and 32 kHz. The Offset TC is exquisitely sensitive to masking by very low intensity stimuli at a narrow range of frequencies which are lower than the probe stimulus frequency. Continuous masking produces a well-tuned low frequency tip to the Offset TC. For Offset TCs generated in response to midfrequency tones, the Q+10 dB of this tip averages 8.3. Masking at this low frequency tip of the Offset TC has no observable effect on the Onset ABR. The Offset ABR is also sensitive to masking by a narrow range of frequencies which are higher than the probe stimulus frequency. This occurs at an intensity which also has no observable effect on the Onset ABR. The Q+10 dB of this high frequency tip averages 9.2. The average frequencies where these Offset TC tips occur fit the cubic difference formula (2f1-f2), which describes a distortion product of two-tone suppression. At low probe stimulus frequencies, there is only a high frequency Offset TC tip; at high stimulus frequencies, only a low frequency tip. The high frequency tip has a higher threshold and appears more susceptible to metabolic disturbance. The Offset ABR TC also has a peak which corresponds to the probe stimulus frequency. Continuous masking with the stimulus frequency produces nonmonotonic enhancement of the Offset ABR, while it simultaneously reduces the magnitude of the Onset ABR. The tuning of this Offset TC peak (measured as Q-10 dB) is almost always much sharper than the corresponding Onset TC tip in the same mouse. These values for midfrequency stimuli average 6.2 for the Onset, and 13.6 for the Offset TCs. This fine tuning of the Offset TC at the probe stimulus frequency occurs at SPLs from 50 to more than 90 dB.