SOUND AND VIBRATION SENSITIVITY OF 8TH NERVE-FIBERS IN THE GRASSFROG,RANA-TEMPORARIA

We have studied the sound and vibration sensitivity of 164 amphibian p apilla fibers in the VIIIth nerve of the grassfrog, Rana Temporaria. T he VIIIth nerve was exposed using a dorsal approach. The frogs were pl aced in a natural sitting posture and stimulated by free-field sound. Furthermore, the animals were stimulated with dorso-ventral vibrations , and the sound-induced vertical vibrations in the setup could be canc eled by emitting vibrations in antiphase from the vibration exciter. A ll low-frequency fibers responded to both sound and vibration with sou nd thresholds from 23 dB SPL and vibration thresholds frolic 0.02 cm/s (2). The sound and vibration sensitivity was compared for each fiber u sing the offset between the rate-level curves for sound and vibration stimulation as a measure of relative vibration sensitivity, When measu red in this way relative vibration sensitivity decreases with frequenc y from 42 dB at 100 Hz to 25 dB at 400 Hz. Since sound thresholds decr ease from 72. dB SPL at 100 Hz to 50 dB SPL at 400 Hz the decrease in relative vibration sensitivity reflects all increase in sound sensitiv ity with frequency. probably due to enhanced tympanic sensitivity at h igher frequencies. In contrast, absolute vibration sensitivity is cons tant in most of the frequency range studied. Only small effects result from the cancellation of sound-induced vibrations. The reason a for t his probably is that the maximal induced vibrations in the present set up are 6-10 dB below the fibers' vibration threshold at the threshold for sound. However, these results ale only valid for the present physi cal configuration of the setup and the high vibration-sensitivities of the fibers warrant caution whenever the auditory fibers are stimulate d with free-field sound. Thus, the experiments suggest that the low-fr equency sound sensitivity is not caused by sound-induced vertical vibr ations. Instead, the low-frequency sound sensitivity is either tympani c or mediated through bone conduction or sound-induced pulsations of t he lungs.