OTOACOUSTIC EMISSIONS FROM THE COCHLEA OF THE CONSTANT FREQUENCY BATS, PTERONOTUS-PARNELLII AND RHINOLOPHUS-ROUXI

During stimulation with continuous pure tones, the cochlea of each ind ividual of the mustached bat, Pteronotus parnellii, produces a strong evoked stimulus-frequency otoacoustic emission (SFOAE) at about 62 kHz . The SFOAEs were on average 480 Hz above the dominant constant freque ncy component of the echolocation call (resting frequency, RF). In two out of nine individuals of Pteronotus the SFOAEs changed into spontan eous otoacoustic emissions of 25-40 dB SPL. In the rufuous horseshoe b at, Rhinolophus rouxi spontaneous emissions were not detected and only in two out of seven animals were there weak SFOAEs about 300 Hz above the RF of 78 kHz. This difference may be due to a stronger damping of underlying resonant processes in Rhinolophus (Henson et al., 1985a). Acoustic distortion products behaved quite similar in both species. Th e first lower sideband distortion 2f(1)-f(2) was measurable over a wid e frequency range between 10 and 100 kHz. The optimum frequency separa tion Delta f of the two primary tones to evoke maximum 2f(1)-f(2) dist ortion was 0.8 to 5.8 kHz in Pteronotus and 1 to 7 kHz in Rhinolophus for frequencies outside the range of the constant frequency components of the call. This corresponds to ratios f(2)/f(1) of about 1.03 to 1. 2. At the frequency of the SFOAE in Pteronotus (480 Hz above the RF) a nd about 300 Hz above the RF in Rhinolophus the optimum Delta f decrea sed sharply to values of 31-63 Hz in Pteronotus (ratio f(2)/f(1) of 1. 0005-1.001), and to 39-590 Hz in Rhinolophus (ratio f(2)/f(1) of 1.000 5-1.007). In Pteronotus a second minimum of Delta f was found at about 90 kHz (values of 180-620 Hz, ratios f(2)/f(1) of 1.002-1.007). In bo th bat species, the respective minima of Delta f are located at or clo se to frequencies where neuronal tuning sharpness is exceptionally hig h. This indicates a mechanical origin of enhanced tuning. After adjust ing the frequency of f(2) to match the optimum Delta fs, 2f(1)-f(2) th reshold curves were obtained. The distortion product threshold approxi mately parallels neuronal data and is in both species characterized by a pronounced insensitivity at the RF followed by a steep threshold mi nimum at frequencies 0.3-3 kHz above the RF. These features may be inv olved in reducing the cochlear response to the call such that the bats are able to focus on the Doppler-shifted echos which are slightly hig her in frequency and thus within the range of the treshold minimum.