M. Kossl, OTOACOUSTIC EMISSIONS FROM THE COCHLEA OF THE CONSTANT FREQUENCY BATS, PTERONOTUS-PARNELLII AND RHINOLOPHUS-ROUXI, Hearing research, 72(1-2), 1994, pp. 59-72
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.