The Strategy For Echolocating FM Bats To Extract Information From Weak Echoes By Suppressing Particular Frequencies Revealed By The Cochlear Microphonics

Echolocating bats have a highly developed biosonar system.By comparing emitted pulses with returning echoes,bats can instantaneously recognize physical properties of the environment.Coming back weak echoes containing the information of wing-beat of insect are interfered with surrounding noise,However,bats are able to extract useful information and catch preys.What's more,when mass of bats flied together,they can easily identify the returning echoes corresponding to their own pulses among the similar echoes from conspecific bats.The cochlear microphonics(CMs)produced by the outer hell cells are an important indicator of mechanical movement of the basilar membrane.So,we wonder whether there is a refined mechanism to control the basilar membrane movement in order to reduce disturbance of noise surrounding terminal frequency(TF),which creates a good signal-noise ratio and helps bat to extract useful detailed information.Compound action potentials(CAPs)created by the cochlear nerve fibers reliably reflect the function of amplification.The weak echoes should be amplified considerably so that the useful information can be sensed by the cochlear nerve fibers.The high SPL sounds which can be heard easily should not be amplified many times,so the function of amplification should be non-linear.Bats may adopt those two strategies to analyze returning weak echoes to survive in the dark night.As a result,well-designed experiments were conducted to explore the jamming avoidance of bat's hearing system by examining relationship between TF and CMs and identify the nonlinear amplification by comparing CAP peak-to-peak amplitude.The structure of pulses is a crucial element to search insects.FM bats change structure of pulse by extending the duration.Thus,they emit with pseudo-constant frequency(the terminal frequency of FM sweep)to detect wing-beats of flying insects by frequency modulation when the distance between insects and bats is far away.The terminal frequency of Pipistrellus we measured was about 42 kHz.Its audiogram shows that the most sensitive frequency region is around 40 kHz and half of neurons in the inferior colliculus are tuned to the frequency around 40 kHz.Eptesicus is another species of FM bat,whose terminal frequency is around 25 kHz.We recorded CMs responding to tone bursts of 4 to 85 kHz for Eptesicus and 4 to 75 kHz for Pipistrellus by placing a metal electrode around the cochlear nucleus.Data show the CMs of Eptesicus and Pipistrellus demonstrate huge suppression at specific frequencies at high sound pressure levels(SPL),while for low SPL suppression would not operate at all.In other words,both species of FM bats would select such frequencies for suppression.For Eptesicus,the suppression frequency is repeated at a certain frequency interval of 15 kHz.The first deep notch appears around 15 kHz and the following one is found around 30 kHz.The terminal frequency is located between these two deep notches(15 kHz and 30 kHz).Also,we have found for Pipistrellus,especially at its terminal frequency of 42 kHz,which we recorded,is exactly between the two deep notches(40 kHz and 45 kHz).In addition,we collected CAPs responding to 4 kHz to 85 kHz stimuli of tone pips at 20 dB to 80 dB SPL for both of Eptesicus and Pipistrellus.CAP lines were plotted by using SPL serving as parameter.With comparison to CAPs under different sound pressure levels,we found the spaces between CAPs with equal SPL appear to be wide for low SPLs while those for high SPLs tend to be narrow.The findings suggest that CAP is amplified much for low SPLs but much less amplified for high SPLs.The generation of CMs is prerequisite of producing CAPs.Hence strong suppression in CMs should cause effects on CAPs which synchronous created by neurons.By comparing with CAP and CM lines,we found there also are obvious notches which precise corresponding to the frequency of CM notches.That means,when the outer hell cells stop generation CMs,fewer of cochlear neurons are excited and notches also occur at CAP lines.The relationship of notches between CM and CAP reveals that the strong suppressions do occur and help bats to obtain more detailed information of wing-beat insect contained in TF.