摘要
在自由声场刺激条件下,采用恒频(constant frequency, CF)、调频(frequency modulation,FM)以及模拟大蹄蝠(Hipposideros armiger)自然状态下的恒频-调频(constant frequency-frequency modulation, CF-FM)回声定位信号为声刺激,在12只听力正常的大蹄蝠上观察和细胞外记录了下丘(inferior colliculus, IC)神经元的声反应,研究不同声刺激模式对IC神经元声反应类型(response pattern)、恢复周期(recovery cycle )、以及频率调谐(frequency tuning)等反应特性的影响。主要研究结果如下：
1.通过研究不同声刺激模式对IC神经元声反应类型的影响,发现IC神经元在CF和FM声刺激下的发放类型基本一致,均只产生一个on反应。而在CF-FM声刺激下,156个IC神经元表现出两类不同的反应,Single-on (n=118)和Double-on反应(n=38)。Single-on神经元将CF-FM声信号作为一个整体来处理,而Double-on神经元将CF-FM声信号中的CF和FM成分分别处理。
2.测量了114个IC神经元的恢复周期曲线。结果显示,85个Single-on神经元在CF、FM和CF-FM三种双声刺激模式下的平均恢复周期分别为44.4±33.0 (CF),28.4±18.9 (FM)和44.1±30.3 (CF-FM) ms (one-way ANOVA,P＜0.01).29个Double-on神经元在三种声刺激模式下的平均恢复周期分别为45.8±31.6 (CF),17.7±16.7 (FM)和30.0±25.9 (CF-FM) ms (one-way ANOVA, P＜0.01).此外,在FM和CF-FM声刺激下,Double-on神经元恢复周期明显短于Single-on神经元。实验结果还显示部分特殊的Single-on和Double-on神经元在CF-FM双声刺激重叠时表现出一种特殊的恢复能力,而在CF或FM双声刺激重叠时却对第二个声刺激不产生反应。另外,与CF和FM声刺激相比,特殊类型的神经元在CF-FM声刺激下的恢复周期最短。以上结果提示Double-on神经元可能较Single-on神经元在蝙蝠靠近及捕获靶物的过程中发挥更重要的作用,而特殊类型的Single-on和Double-on神经元则可能是CF-FM蝙蝠的听觉系统为处理CF-FM回声定位信号而演化和适应的结果。
3.采用模拟的CF-FM声信号为声刺激,研究了下丘神经元恢复周期特点及其对声脉冲跟随率的影响。结果发现根据IC神经元(n=93)恢复率达50%时的双声刺激间隔(interpulse interval, IPI, onset-onset),可将其分为长时恢复型(long recovery, LR; 47.4%)、中等时间恢复型(moderaterecovery, MR; 35.1%)和短时恢复型(short recovery, SR; 17.5%)。每种类型依据其恢复率随IPI增加而呈现的不同变化又可进一步分为单IPI反应区神经元,多IPI反应区神经元,以及单调IPI反应神经元。LR, MR和SR型神经元恢复率达50%时的平均IPI分别为64.0±24.8,19.6±5.8和7.1±2.4 ms(P＜0.001),相对应的平均理论每秒声脉冲数分别为18.2±7.0,55.4±15.7和171.3±102.9Hz(p＜0.001)。结果提示单IPI和多IPI反应区神经元具有特殊IPI反应特性,能对蝙蝠捕食和巡航期间所处的时相做出准确判断；而单调IPI反应神经元对IPI变化的敏感性较强,但时相判断性较差。另外LR,MR和SR型神经元恢复周期和理论脉冲跟随率的平均结果均能与这种蝙蝠回声定位期间3个时相的发声行为相匹配,且神经元恢复周期参与决定声脉冲跟随率,满足了蝙蝠巡航、捕食的行为学需要。
4.获得了175个IC声敏感神经元的频率调谐曲线(frequency tuning curve, FTC)。在单声刺激下,神经元频率调谐曲线可分为8种类型：V型；U型；高频边锐化型；低频边锐化型；低频倾向型；高频倾向型；多峰型和封闭型。在期待(CF-FM)或非期待(CF)的声刺激加入后,神经元频率调谐类型发生转变,且主要表现为低频边锐化型神经元增多。研究还发现期待(CF-FM)和非期待(CF)的声刺激对蝙蝠IC神经元频率调谐敏感性产生影响。在不同声脉冲模式下,神经元平均Q20值(Mean±1/2SD)分别为4.5±3.7 (Test),6.6±5.1 (PulseCF-Test)和8.0±7.3(Pulse CF-FM-Test),配对t检验显示期待和非期待声刺激加入后,神经元Q20值均较单声刺激对照有显著性差异(P＜0.01),且CF-FM声脉冲刺激加入后,神经元FTCQ20值最大。此外,不同声脉冲刺激下,IC神经元低频边斜率平均值(Mean±1/2SD)分别为-6.4±-3.9(Test),-8.2±-5.2(PulseCF-Test)和-11.5±-8.1 (PulseCF-FM-Test) dB/kHz,高频边斜率平均值(Mean±1/2SD)分别为11.2±4.9 (Test),12.3±6.1 (PulseCF-Test)和12.2±6.5 (PulseCF-FM-Test) dB/kHz,配对t检验显示非期待(CF)的声脉冲加入后,神经元高、低频边斜率较单声对照均无显著性差异(P＞0.05,n=33),而期待的(CF-FM)声脉冲加入后, FTC低频边斜率显著升高(P＜0.01,n=33)。以上结果提示大蹄蝠所期待的CF-FM声脉冲刺激较CF更能锐化神经元频率调谐曲线,且锐化主要发生在低频边。
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.
引文
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