| The echolocation behavior of bats is one of the most important discoveries to adapt to the environment. Among them, there is a unique sensory-motor control behavior during the echolocation of the constant frequency - frequency modulation(CF-FM) bats,namely doppler-shift compensation (DSC) behavior, which is mainly manifested in the process of CF-FM bats’ prey. In order to eliminate the influence of Doppler shift,the bat will keep its echo frequency constant by adjusting its pulse frequency continuously, to ensure that the echo frequency is always maintained within the most sensitive frequency of the hearing system, and this special behavior of the bat ensures accurate extraction of the echo information. Then how the auditory of bats to process the echo after DS, and what is the adaptive physiological mechanism? We are not clear. In this study, we sitmulated the echolocation signal of the CF-FM bat in the DSC behavior, research the characteristics and physiological mechanism of the inferior colliculus (IC) neurons in processing the DSC information. The main results are as follows:(1) In the experiment,we recorded 117 IC neurons which sensitived to the echolocation signal. When stimulated with the CF-FM sounds, neurons either only discharged impulses to the onset of CF-FM sounds (single-on neurons, n=83) or to the onset of both CF and FM components of CF-FM sounds (double-on neuron, n=34). By analyzing the basic parameters of the two types of neurons, we found that the BF, MT and recording depth of the two types of neurons were not significantly different (P> 0.05)(45.4 ± 10.9 VS 45.1 ± 10.1 kHz; 50.6 ± 20.6 vs 46.1 ± 18.1 dB SPL; 2080.8 ± 692.8 vs 2158.4 ± 56.7 μm ). However, the BF of the SO neurons were mainly distributed in the second harmonic (48-60 kHz) frequency range, less than half of DO neurons’ BF were distributed in the second harmonic frequency range. The experiment also found that SO neurons have more U-type frequency tuning patterns than DO neurons, and the frequency tuning curve(FTC) is more narrow (Q20 and Q30 values are larger). The above results suggest that both types of neurons are topologically distributed along the dorsal-ventral axis in the IC, and the SO neurons ,which the BF were mainly distributed in the second harmonic frequency range and adapted to distinguish the dominant frequency, may be more conducive to process the echo information after DS.(2) In the experiment, we compaired the best compensation value distribution of the 61 SO and 25 DO neurons which sensitived to DSC signals. It is found that SO neurons are evenly distributed in the compensation values of -4 ～4 kHz, and the best compensation values of DO neurons are concentrated in the negative compensation range,and most of them are at 4 kHz. The above results suggest that the IC neurons of the CF-FM bat have the selective ability to the shifts of pulse, and the SO neurons, which were evenly distributed in the compensation values range and were specific to the CF component, may be more suitable for processing the DSC signal to determine the prey velocity information, and the DO neurons that sensitived to the CF and FM components may be more concerned with the FM component during positive DSC behavior to determine the distance of the prey.(3) In the experiment, we found that whatever bat conduct the positive DSC behavior or negative behavior, the interpulse interval values of SO and DO neurons to echo response recovery to 50% were changed with the change of compensation condition.When the acoustic stimulation is transformed from the uncompensated condition to the best compensation condition, The 50% IPI of the two types of neurons were concentrated in the short-term recovery (SR) region, and the average 50% IPI value was significantly shortened (P <0.001), However, the number of neurons with 50%IPI shortening rate of 70% of SO was more than DO neurons and the mean DSC selective range of SO neurons is also significantly wider than DO neurons. The above results suggest that SO neurons in the IC may make better use of bat’s DSC behavior than DO neurons to improve the recovery ability to the echo to maximize the acquisition of prey information and to accurately determine the relative velocity with the prey.(4) In addition, under the uncompensation conditions, the recovery cycle of DO neurons in CF component was significantly longer than that of FM component (P<0.05);while in the best compensation conditions, the recovery cycle of DO neurons of the CF component was significantly shorter than that of FM component (P<0.05). The results of the study suggest that, in order to adapt to the DSC behavior of bats in flight, DO neurons will increase the recovery of the echo in the CF component and more access to relevant velocity and the prey’s fluttering information, and during the final stage of prey,and when no need of DSC behavior, DO neurons were focused on the analysis of FM component information, to accurately know the texture of prey.In summary, in the long-term natural selection ,CF-FM bat auditory center evolved the different reaction patterns of SO and DO neurons to acoustic stimulation which related to the behavior, and assumed that play different roles in different echolocation tasks, this is the adaptive evolution of the auditory system on the behavior of echolocation. |
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