| During the flight, constant frequency-frequency modulation (CF-FM) bat gradually adjusts pulse CF frequency to compensate for the target echo frequency shift caused by relative flying speed, this unique behavior of CF-FM bat is called as Doppler-shift compensation (DSC) behavior. The multiple parameters of echolocation signals can influence DSC behavior, pulse-echo intensity difference and pulse-echo overlap is considered to be the two most important factors. Inferior colliculus (IC) is an important relay nuclei in auditory pathway, and IC also involved in the DSC process. How pulse-echo intensity difference effect IC neurons of Pratt’s roundleaf bats to process DSC signal has been investigated previously, and found that various types of IC neurons have the best ability to process DSC signals at its most preferred pulse-echo intensity difference. In this study, using in vivo extracellular recording combined with in vivo intracellular recording methods, we continued to investigate the effect of pulse-echo overlap on IC neurons of Pratt’s roundleaf bats to process DSC signal and its possible synaptic mechanisms. The results obtained were summarized as following:1. In this experiment, recovery cycles of 116 neurons under different compensation values were measured. We compared recovery cycle types and the inter pulse intervals (IPIs) when recovery rate reached 50% of IC neurons under uncompensation and best compensation pulse-echo stimulus conditions. Results showed that recovery cycle curves of IC neurons could be divided into four types in both stimulus conditions:unrecovered, monotonic IPI response (monotonic IPI R), with single IPI response area (with single IPI RA) and with multi-IPI response area (with multi-IPI RA), but their proportions changed. In the state of uncompensation stimulus, recovery cycle curves of IC neurons are mostly of with single IPI RA and monotonic R type, while for stimulus in best compensation, with multi-IPI RA is prominent. Additional, when stimulus turned from uncompensation into best compensation condition,50% IPI values of IC neurons shortened and concentrated in short recovery (SR) area, wherein with multi-IPI RA neurons almost exclusively located in the SR area. These results suggest that DSC behavior may prompt bats use more with multi-IPI RA neurons to judgment insect changing flight paths or trans-phase analysis to improve the ability of tracking targets. And DSC behavior is benefit for central auditory neurons to respond quickly to echo signal, which happened to match the behavior of bat gradually increased pulse repetition rate as close to the target in predation.2. We also calculated the area of IPI response range in 106 IC neurons under uncompensation and best compensation conditions, and compare the changes at different degree of pulse-echo overlap. We found that when stimulus turned from uncompensation into best compensation condition, the area of pulse-echo overlap IPI response range (IPI range is 0-2 ms,2-5 ms and 5-7 ms) increased significantly (P<0.001), the area of pulse-echo non-overlap IPI response range (IPI range is 7-100 ms) has no significant difference (P> 0.05). The experimental result indicated that the ability of IC neurons process DSC signal affected by pulse-echo overlap, only when pulse-echo overlap IC neurons can process DSC signal best.3. We also explored the possible synaptic mechanisms of pulse-echo overlap enhanced the ability of IC neurons process DSC signal:(1) For IC neurons that respond stimuli with a long duration hyperpolarization followed by action potential (AP), the duration and amplitude of hyperpolarization that evoked by simulated pulse alone shortened after DSC. So after pulse frequency compensated, it can activate different neural circuits which may results IC neurons receive lower duration and amplitude of integrating inhibitory input, thereby, the ability of IC neurons process DSC signal enhanced when pulse-echo overlap. (2) For IC neurons that respond stimuli with a short duration hyperpolarization followed by AP, the duration and amplitude of post-spike hyperpolarization that evoked by simulated pulse alone have no difference between uncompensation and best compensation conditions, but post-spike hyperpolarization that evoked by simulated pulse alone in advance after DSC, then the suppression effect of hyperpolarization on subsequent simulated echo response is weakened when simulated pulse-echo overlap under best compensation condition, which make the recovery rate of the echo response was significantly increased. (3) For IC neurons that did not respond to simulated pulse after compensation when pulse-echo overlap, these neurons may not pay attention to simulated pulse but only pay attention to simulated echo to against interference between pulse and echo, so the recovery rate of the simulated echo response was significantly increased. |
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