Influence of contralateral acoustic stimulation on distortion-product and spontaneous otoacoustic emissions in the barn owl

The avian auditory papilla provides an interesting object on which to study efferent influences, because whereas a significant population of hair cell s in birds is not afferently innervated, all hair cells are efferently inne rvated (Fischer, 1992, 1994a, b). Previous studies in mammals using contral ateral sound to stimulate the efferent system demonstrated a general suppre ssive effect on spontaneous and click-evoked, as well as on distortion-prod uct otoacoustic emissions (DPOAE). As little is known about the effects of contralateral stimulation on hearing in birds, we studied the effect of suc h stimuli (broadband noise, pure tones) on the amplitude of the DPOAE 2f(1) -f(2) and on spontaneous otoacoustic emissions (SOAE) in the barn owl, Tyro alba. For the DPOAE measurements, fixed primary-tone pairs [f(1) = 8.875 k Hz (ratio = 1.2), f(1) = 8.353 kHz (ratio = 1.15) and f(1) = 7.889 kHz (rat io = 1.1)] were presented and the DPOAE measured in the presence and absenc e of continuous contralateral stimulation. The DPOAE often declined in ampl itude but in some cases we observed DPOAE enhancement. The changes in ampli tude were as large as 9 dB. The influence of the contralateral noise change d over time, however, and the effects of contralateral tones were frequency -dependent. SOAE were suppressed in amplitude and shifted in frequency by c ontralateral broadband noise. Control measurements in animals after middle- ear muscle resection showed that these phenomena were not attributable to t he acoustic middle-ear reflex. The finding of DPOAE enhancement is interest ing, because a type of efferent fiber that suppressed its discharge rate du ring stimulation has been described in birds (Kaiser and Manley, 1994). (C) 1999 Elsevier Science B.V. All rights reserved.