Distortion-product otoacoustic emissions and their anaesthesia sensitivity in the European starling and the chicken.

The aim of the present experimental series was to provide further information on the distortion-product otoacoustic emissions (DP) of birds and contribute to a general understanding of DP generation. Basic characteristics of the DP 2f1-f2 and 2f2-f1 were measured in the ear canal of both awake and anaesthetized European Starlings and chickens. The effect of a third suppressive tone and the behaviour of the DP under anaesthesia were also studied. In general, the DP characteristics of both bird species resembled those of lizards and mammals, but first appeared at somewhat higher primary-tone levels. The best frequencies of third tones suppressing 2f1-f2 lay near the first primary tone (f1), but for 2f2-f1, the situation was more complex. Facilitation via a third tone was also seen for both DP, often at levels below those eliciting suppression. The DP 2f1-f2 disappeared completely at the onset of deep anaesthesia and recovered to its original magnitude when the anaesthesia was lightened, sometimes with a considerable delay. The compound action potential (CAP) was somewhat more sensitive to anaesthesia than the DP. Control experiments showed that the anaesthesia effect was not a result of hypoxia. Avian DP at low and intermediate sound levels are thus physiologically-sensitive manifestations of normal hair-cell function that are, in contrast to mammals, also anaesthesia-sensitive.