Effects of olivocochlear feedback on distortion product otoacoustic emissions in guinea pig.

Activation of ipsilaterally responsive olivocochlear (OC) neurons by sound produces rapid, post-onset alterations in the 2f1-f2 distortion product otoacoustic emission (DPOAE). The present study investigates the frequency and level dependence of this ipsilateral OC effect in the anesthetized guinea pig, compares its magnitude and sign to OC effects elicited by contralateral sound ("contralateral" OC effect), and characterizes the influence of such activity on steady-state DPOAE amplitude. DPOAEs were measured with fine time resolution in response to primary stimuli varied systematically in frequency and level. DPOAEs showed rapid and remarkably stereotyped post-onset amplitude alterations. These ipsilateral OC effects were greater for high (8-12 kHz) than for low (2-4 kHz) f2 primary frequencies and for higher primary levels (70-80 dB SPL). For any f2/f1 pair, the sign as well as the magnitude of the ipsilateral effects varied with. primary level ratio. For example, with L1 fixed at 75 and L2 varied in 1-dB steps from 60 to 75 dB SPL, DPOAE amplitude underwent a stereotyped progression from post-onset increases at the lowest levels of the f2 primary to post-onset decreases at the highest levels. At intermediate levels, near the region of sign change (L2 = 5-10 dB below L1), post-onset effects were often particularly large (as great as 20 dB). These large ipsilateral OC effects were always associated with "dips" in the DPOAE amplitude vs. level functions, and both disappeared after OC section. Although smaller in magnitude, contralateral OC effects were identical to ipsilateral effects in frequency and level dependence and in form.