| Using constant frequency (CF), frequency modulation (FM), and mimic natural CF-FM echolocation signals as acoustic stimuli under free field stimulation condition, the responses of inferior colliculus (IC) neurons of 12 leaf-nosed bats (Hipposideros armiger) with normal hearing were recorded by extracellular recording method. In this experiment, we studied the effect of different sound stimuli on response patterns, recovery cycles, and frequency tuning curves (FTCs) of IC neurons, and the results obtained were summarized as following:1. The effect of different sound stimuli on response pattern of IC neurons was studied. The results showed that discharge patterns measured by FM sound stimulus were consist with them obtained by CF sound stimulus. And neurons had only one on response both under CF and FM sound stimulation conditions. Under CF-FM sound stimulation condition,156 neurons appeared two types of responses, single-on (n=118) and double-on (n=38) responses. Those single-on neurons in IC basically processed CF-FM signal as a whole, while the double-on neurons in IC generated responses to CF and FM components of CF-FM sound stimulation, separately.2. Recovery cycles of 114 neurons were examined in present experiment. The results showed that recovery cycles of 85 single-on neurons under CF, FM, and CF-FM stimulation conditions were 44.4±33.0 (CF),28.4±18.9 (FM) and 44.1±30.3 (CF-FM), respectively (one-way ANOVA, P＜0.01). For 29 double-on neurons, their recovery cycles were 45.8±31.6 (CF),17.7±16.7 (FM) and 30.0±25.9 (CF-FM) ms, respectively (one-way ANOVA, P＜0.01). And double-on neurons'recovery cycles apparently shorter than single-on neurons'under FM and CF-FM sound stimulation conditions. It is more interesting that when two 7 ms CF-FM sound stimuli were overlapped, a special kind of recovery was observed from partial single-on and double-on neurons and these neurons didn't generate any response to the second sound stimulus under two overlapped 2 ms FM or 7 ms CF stimulation condition. Our data showed that the mean IPI of 50% recovery under CF-FM sound stimulation condition in special single-on and double-on neurons was the shortest among three stimuli. These results demonstrated that double-on neurons could more effectively analyze the echo than single-on neurons during bat approaching the target. And the specific recovery cycle of single-on and double-on neurons was possibly an evolutionary result of CF-FM bat auditory system for echo information processing.3. The characteristics of recovery cycles in IC neurons and effect of the recovery cycle on the following pulse repetition rate were studied using mimic CF-FM sound stimuli. The results showed that IC neurons (n=93) were classified into three types, i.e. long recovery (LR,47.4%), moderate recovery (MR,35.1%), and short recovery (SR,17.5%), according to their inter-pulse interval (IPI) (ms) of 50% recovery under two CF-FM sound stimulation condition. Each type of the neurons could also be categorized different subtypes according to changes induced by IPI increasing such as single-IPI response area neurons, multi-IPI response area neurons, and monotonic-IPI response neurons. Mean IPIs of 50% recovery of LR, MR, and SR neurons were 64.0±24.8,19.6±5.8, and 7.1±2.4 ms, respectively (P＜0.001). The calculated theoretically following pulse repetition rate (pulse per second, Hz) of LR, MR, and SR neurons by mean IPI of 50% recovery for each type were 18.2±7.0,55.4±15.7, and 171.3±102.9 Hz, respectively (P＜0.001). These three types of IC neurons were well corresponding to their three hunting phases, search, approach, and catch phases. The sub-types of single-IPI response area neurons and multi-IPI response area neurons had hunting phase selectivity, and sub-type of monotonic-IPI response neurons had well sensitivity to IPI change, but their hunting phase selectivity was of a sort. These results demonstrated that recovery cycle of IC neurons determined the ability to follow pulse repetition rate and matched with this bat's echolocation behavior.4.175 FTCs of IC neurons were obtained in present experiment. Under single CF sound stimulation condition, FTCs of IC neurons were classified into 8 types:V-Shaped; U-Shaped; Lower-tail-upper-sharp, LTUS; Upper-tail-lower-sharp, UTLS; Slant-lower, SL; Slant-upper, SU; Multipeaked and Closed. Appending a prior expected (CF-FM) or unexpected (CF) sound stimulus changed the types of FTCs in IC neurons, and there was an increase in the number of UTLS neurons. The present study also found that expected (CF-FM) or unexpected (CF) sound stimulus affected the sensitivity of frequency tuning curves. The mean value of Q20 value of IC neurons under Test, PulseCF-Test, and PulseCF-FM-Test stimulation conditions were 4.5±3.7,6.6±5.1 and 8.0±7.3, respectively. Paired t test demonstrated that the value of Q2o had significant difference between Test and PulseCF-Test or PulseCF-FM-Test sound stimuli, and Q2o value measured by PulseCF-FM-Test sound stimulus was higher than other two sound stimulation conditions. In addition, the mean value of FTC-slope on lower-side under 3 stimulation conditions were-6.4±-3.9 (Test),-8.2±-5.2 (PulseCF-Test) and-11.5±-8.1 (PulseCF-FM-Test) dB/kHz, respectively; and on upper-side were 11.2±4.9 (Test), 12.3±6.1 (PulseCF-Test) and 12.2±6.5 (PulseCF-FM-Test) dB/kHz, respectively. Paired t test demonstrated that the FTC-slope on both lower-side and upper-side had no significant difference between Test and PulseCF-Test stimuli (P＞0.05, n=33), but there was significant difference of FTC-slope on lower-side between Test and PulseCF-FM-Test stimuli (P< 0.01, n=33). These results suggested that expected CF-FM pulse could sharpen FTC more than unexpected CF pulse, and the sharpness mainly happened on the lower-side.
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