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 2f(1)-f(2) distortion product otoacoustic emission (DPOAE). The present study investigates the frequency and level dependence of this ipsilateral OC effect in the anesthetized gui nea pig, compares its magnitude and sign to OC effects elicited by contrala teral sound ("contralateral" OC effect), and characterizes the influence of such activity on steady-state DPOAE amplitude. DPOAEs were measured with f ine time resolution in response to primary stimuli varied systematically in frequency and level. DPOAEs showed rapid and remarkably stereotyped post-o nset amplitude alterations. These ipsilateral OC effects were greater for h igh (8-12 kHz) than for low (2-4 kHz) f(2) primary frequencies and for high er primary levels (70-80 dB SPL). For any f(2)/f(1) pair, the sign as well as the magnitude of the ipsilateral effects varied with primary level ratio . For example, with L-1 fixed at 75 and L-2 varied in 1-dB steps from 60 to 75 dB SPL, DPOAE amplitude underwent a stereotyped progresssion from post- onset increases at the lowest levels of the f(2) primary to post-onset decr eases at the highest levels. At intermediate levels, near the region of sig n change (L-2 = 5-10 dB below L-1), post-onset effects were often particula rly large (as great as 20 dB). These large ipsilateral OC effects were alwa ys associated with "dips" in the DPOAE amplitude vs. level functions, and b oth disappeared after OC section. Although smaller in magnitude, contralate ral OC effects were identical to ipsilateral effects in frequency and level dependence and in form.