Tympanic and extratympanic sound transmission in the leopard frog, Rana pipiens

The relative efficiency of sound transmission to the inner ear of the leopard frog by tympanic vibration and by extratympanic stimulation (presumably via tissue conduction) was investigated by recording the thresholds of eighth nerve auditory fibers at various frequencies within their excitatory tuning curves. The animals were placed on a vibration‐isolating table. The eighth cranial nerve was exposed through the roof of the mouth, and a clay cap, molded to mimic the mouth spaces into which the eustachian tubes open, was inserted and sealed with silicon grease, leaving the surgical exposure accessible. An earphone was sealed around one tympanum; the contralateral tympanum and external nares were covered with grease. A loudspeaker was placed 0.8 m from the frog for free‐field stimulation. Thresholds of single eighth nerve fibers to sound presented alternately through the earphone and through the loudspeaker were compared. For frequencies below 0.9 kHz, thresholds for the two modes of stimulation were comparable, and generally within 5 dB. For higher frequencies, threshold separation was greater: at 1.7–2.0 kHz, thresholds to sounds presented via the speaker were 15–20 dB higher than those presented through the earphone. These results suggest that at low frequencies the frog's inner ear can be stimulated by both tympanic vibration and by a nontympanic mechanism with nearly equal sensitivity. [Supported by NIH 06237 and NSF BNS 7706803.]