Contralateral suppression of non-linear click-evoked otoacoustic emissions

Click-evoked otoacoustic emissions from nominal 80 dB pSP (peak sound pressure) 80-microseconds pulses presented at 50 pulses per second were collected from the right ears of eleven normal hearing subjects using an ILO88 Otodynamic Analyzer in the non-linear mode. Clicks, pure tones, and narrow bands of noise were then presented to their left ears through insert earphones. The 80-microseconds contralateral clicks ranged in intensity from 80 dB pSP in 5 dB steps down to 60 dB pSP but data on only 10 of the subjects were collected successfully. The pure tones and narrow bands of noise centered at 250, 500, 1000, 2000, and 4000 Hz were also presented through insert phones at 20, 40, 60 and 80 dB HL (Hearing Level) to all 11 subjects. The mean overall 'echo amplitude' without contralateral stimuli was 11 dB SPL and underwent more than 3 dB of overall suppression in response to the noises which were the most effective of the contralateral suppressors. When we analyzed the echo suppression to noise in 2-ms segments, we found consistent contralateral suppression of 3-4 dB concentrated in the time zones after 8 ms. Time shifts of more than 200 microseconds between the control and experimental traces were also observed in the same zones. The clicks were the next most effective suppressors, but showed their amplitude and time effects in somewhat earlier time zones. The tones were the least effective suppressors suggesting that efferent effects we measured in the human system are not strongly tonotopic. Because 'non-linear' mode high intensity clicks were deliberately selected as stimuli to evoke the TEOAE's, the emissions and their suppression can represent neither the 'true' TEOAE nor all of the efferent system's suppression abilities.