Active enhancement found in responses of the cochlear nerve to detect weak echoes created by the auditory periphery in Japanese house bat, Pipistrelus abramus

Characteristics of frequency tunings related to echolocation were investigated in an FM bat, Japanese house bat (Pipistrellus abramus). A particular attention was paid on their terminal frequency of FM sweep. When search for preys in the field, they stretch duration of echolocation pulses and reduce FM sweeping range, resulting in pseudo constant frequency pulses. Distribution of best frequency in the neurons in the inferior colliculus indicated that about 50% of neurons appeared to be tuned to the pseudo constant frequency, around 40 kHz. The compound action potentials were recorded from medulla oblongata through a metal telectrode. We hypothesized that the bat has a peripheral system to enhance frequency component of 40 kHz to be detected easier than other frequency components. Active enhancement of 40 kHz frequency component by the outer hair cell and overwhelming number of cochlear nerves in charge of 40 kHz would create low threshold and high resolution sensitivity for frequency range around 40 kHz. Results of the present study show that compound action potential of the cochlear nerve (N1) generates nonlinearly enhanced amplification in frequency range of 40 kHz but linear amplification in lower frequency ranges.