Effects of activation of the efferent system on psychophysical tuning curves as a function of signal frequency.

It has been shown that electrical stimulation of the efferent auditory system can influence neural tuning curves in animals. Here, we examined a psychophysical analog of this effect in humans. All of the 19 normally hearing subjects showed a reduction in the amplitude of otoacoustic emissions in one ear when contralateral broadband noise was presented, indicating a functioning efferent system. Psychophysical tuning curves (PTCs) were measured in simultaneous masking in the absence and presence of contralateral stimulation (CS). The CS was a continuous narrowband noise centered at the signal frequency and presented at a level of 50 or 60 dB SL. The CS had no consistent effect on the masker level at the tips of the PTCs. For the two highest signal frequencies (2000 and 4000 Hz), the CS reduced the masker level required for threshold on both the low- and high-frequency sides of the PTCs, and the sharpness of tuning, as measured by Q10, decreased significantly. For the two lowest signal frequencies (500 and 1000 Hz), the masker level required for threshold on the low-frequency sides of the PTCs increased with CS, and the Q10 values increased significantly. The general pattern of the results was consistent with that observed for electrical stimulation of the efferent system in animals.