| Using mimic natural constant frequency-frequency modulation (CF-FM) echolocation signal as acoustic stimulus under free field stimulation condition, the effect of sound duration on pulse repetition rate (PRR) selectivity and following ability of inferior collicular (IC) neurons in leaf-nosed bats(Hipposideros armiger) were explored by extracellular recording method. The duration used in this experiment were 5+2 ms (CF,5 ms; FM,2 ms),15+3 ms (CF,15 ms; FM,3 ms) and 30+4 ms (CF,30 ms; FM,4 ms), respectively. And the PRR used in the experiment were 0.5,1,2,3.3,4,5,10,20,50 and 100 pps, respectively.122 IC neurons were obtained in 7 leaf-nosed bats. The range of best frequency (BF), minimum threshold (MT) and recording depth of these neurons were 11.1-81.4 (48.2±17.6) kHz,5-94 (59.8±22.1) dB SPL and 307-4497 (2656.4±847.9)μm, respectively. The responses of 91 neurons under different PRR stimulation conditions were measured. And the results were summarized as following:(1) The BF of IC neurons increased significantly with the recording depth increasing (r=0.155, p=0.089, n=122). But there was no significant correlation between BF and MT of IC neurons(r=0.155, p =0.089, n=122). (2) According to their PRR selectivity,91 IC neurons could be classified into 3 types, such as high pass (HP), band pass (BP) and multipeaked (MP) neurons. Under 5+2 ms sound stimulation condition, the proportion of HP, BP and MP neurons were 38.5%,40.6%and 20.9%, respectively. When stimulated by 15+3 ms sound stimulus, the proportion changed into 42.8%(HP),37.4%(BP) and 19.8%(MP), respectively, and then changed into 42.8%(HP),38.5%(BP) and 18.7%(MP), respectively, when stimulated by 30+4 ms sound stimulus. The results suggested that the sound duration could affect the proportion of HP, BP and MP neurons of leaf-nosed bats. (3) By calculating 50%PRR bandwidth of the PRR-discharge rate function curves, we found that 50%PRR bandwidth decreased significantly with sound duration increasing (P<0.001). (4) The IC neurons also could be classified into two types, duration-independent neurons (52.7%,48/91) and duration-dependent neurons (47.3%, 43/91). For duration-independent neurons, the proportion of HP, BP and MP neurons were 50.0%,41.7%and 8.3%, respectively. But for duration-dependent neurons, the proportion of these three types could change with the sound duration increasing. We also found the 50%PRR bandwidth of some duration-independent (79.2%) and duration-dependent (90.7%) neurons were affected by sound duration, and the affected proportion was very different. (6) To explore the effect of sound duration on PRR following ability in IC neurons,50%PRR ranges were compared among 3 sound durations. And the results showed that the following ability of IC neurons became weaker and weaker with the sound duration increasing.The results demonstrated that the PRR following ability of IC neurons enhanced with the pulse duration decreasing during the bat approaching the target. And the PRR following ability of IC neurons revealed a duration-dependent automatic control property, which matched the bats'behavior during preying on.
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