Neural responses to simple simulated echoes in the auditory brain stem of the unanesthetized rabbit
Neural responses to simple simulated echoes in the auditory brain stem of the unanesthetized rabbit
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DOI：
10.1152/jn.1995.74.6.2469
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发表时间：
1995-12-01
影响因子：
2.5
通讯作者：
Trahiotis, C
中科院分区：
文献类型：
--
作者：
Fitzpatrick, DC;Kuwada, S;Trahiotis, C
1.在大多数自然环境中，来自单一来源的声波将通过直接路径和反射路径到达听众。在直达路径上传播的声音最先到达，并确定源的感知位置，尽管存在来自许多不同位置的反射。这种现象被称为“第一波前定律”或“优先效应”。反射第一次被感知为可单独定位的声音的时间定义了优先窗口的结束，并被称为“回声阈值”。优先效应代表了听觉系统的一个重要特性，其神经基础直到最近才开始被研究。本文报道了未麻醉兔下丘(IC)和上橄榄复合体(SOC)中单个神经元对声音及其模拟反射的反应。刺激是通过耳机发出的一对单耳或双耳滴答声。前导滴答声或调节器模拟的是直接声音，滞后的滴答声或探头模拟的是反射。在双耳调节器和探头中引入耳间时间差(ITD)来调整它们的模拟位置。探头总是设置在神经元最好的ITD，而条件反射设置在神经元最好的ITD或最差的ITD。为了测量调节剂对探测器影响的时间进程，我们检查了作为调节器-探测器间隔(CPI)的函数的探测器的反应。当使用条件化处理和设置在神经元最佳ITD的探针测试IC神经元时，根据CPI的函数，对探针的反应有两种形式之一：早低或早高。在早期低值神经元中，对探针的反应最初受到抑制，但在较长的CPI时单调恢复。早期兴奋神经元呈现非单调的恢复模式。在这些神经元中，最大的抑制并不发生在最短的CPIs，而是在一段较少抑制的时期之后。在这一点之后，恢复类似于早期低水平神经元。早期兴奋神经元的存在意味着整个种群永远不会完全受到抑制，即使在很短的CPI下也是如此。从整体上看，神经元对探头反应的50%恢复的CPI值在1到毫秒之间，中位数接近6毫秒。上述结果与优先效应的时间进程是一致的，原因如下。1)在任何CPI上没有完全抑制与行为结果相一致，该结果表明即使在短的CPI上，当探针不能单独定位时，也可以检测到它的存在。2)在与人类听者的回声阈值相对应的CPI处(类似于4ms CPI)，对探头有相当大的响应，这与在该CPI处听到的作为单独可定位的声音是一致的。3)所有神经元的完全恢复所需的时间比优先效应所需的时间长得多。这与空调和探头同样响亮所需的较长时间是一致的。使用具有最好的ITD或最差的ITD的调节器来确定ITD对探头反应的影响。这两种ITD引起的抑制量在不同的神经元之间是不同的。一些神经元被两种类型的条件反射抑制得差不多，另一些则被ITD最好的条件反射抑制得更严重，还有一些被ITD最差的条件反射抑制得更严重。因为最好的ITD和最差的ITD可能激活不同的通路，这些结果表明不同的神经元从不同的来源得到不同的抑制平衡。对ITDS不敏感的神经元恢复功能与对ITD敏感的神经元相似。这表明，在不同的IC人群中，抑制的时间过程可能是共同的。我们还研究了SOC中的神经元。尽管许多患者表现出双耳交互作用，但没有一例对ITDS敏感。因此，这一群体的反应可能不能直接与IC中对ITD敏感的神经元的反应相比较。一般来说，SOC神经元对探针的反应恢复得比IC神经元快。50%恢复的CPI的中位数与SOC的2ms相似，而IC的中位数CPI类似于6ms。这些数据表明，IC中较长时间抑制的机制在SOC之后。
1. In most natural environments, sound waves from a single source will reach a listener through both direct and reflected paths. Sound traveling the direct path arrives first, and determines the perceived location of the source despite the presence of reflections from many different locations. This phenomenon is called the ''law of the first wavefront'' or ''precedence effect.'' The time at which the reflection is first perceived as a separately localizable sound defines the end of the precedence window and is called ''echo threshold.'' The precedence effect represents an important property of the auditory system, the neural basis for which has only recently begun to be examined. Here we report the responses of single neurons in the inferior colliculus (IC) and superior olivary complex (SOC) of the unanesthetized rabbit to a sound and its simulated reflection.2. Stimuli were pairs of monaural or binaural clicks delivered through earphones. The leading click, or conditioner, simulated a direct sound, and the lagging click, or probe, simulated a reflection. Interaural time differences (ITDs) were introduced in the binaural conditioners and probes to adjust their simulated locations. The probe was always set at the neuron's best ITD, whereas the conditioner was set at the neuron's best ITD or its worst ITD. To measure the time course of the effects of the conditioner on the probe, we examined the response to the probe as a function of the conditioner-probe interval (CPI).3. When IC neurons were tested with conditioners and probes set at the neuron's best ITD, the response to the probe as a function of CPI had one of two forms: early-low or early-high. In early-low neurons the response to the probe was initially suppressed but recovered monotonically at longer CPIs. Early-high neurons showed a nonmonotonic recovery pattern. In these neurons the maximal suppression did not occur at the shortest CPIs, but rather after a period of less suppression. Beyond this point, recovery was similar to that of early-low neurons. The presence of early-high neurons meant that the overall population was never entirely suppressed, even at short CPIs. Taken as a whole, CPIs for 50% recovery of the response to the probe among neurons ranged from 1 to 64 ms with a median of similar to 6 ms.4. The above results are consistent with the time course of the precedence effect for the following reasons. 1) The lack of complete suppression at any CPI is compatible with behavioral results that show the presence of a probe can be detected even at short CPIs when it is not separately localizable. 2) At a CPI corresponding to echo threshold for human listeners (similar to 4 ms CPI) there was a considerable response to the probe, consistent with it being heard as a separately localizable sound at this CPI. 3) Full recovery for all neurons required a period much longer than that associated with the precedence effect. This is consistent with the relatively long time required for conditioners and probes to be heard with equal loudness.5. Conditioners with either the best ITD or worst ITD were used to determine the effect of ITD on the response to the probe. The relative amounts of suppression caused by the two ITDs varied among neurons. Some neurons were suppressed about equally by both types of conditioners, others were suppressed more by a conditioner with the best ITD, and still others by a conditioner with the worst ITD. Because the best ITD and worst ITD presumably activate different pathways, these results suggest that different neurons receive a different balance of inhibition from different sources.6. The recovery functions of neurons not sensitive to ITDs were similar to those of ITD-sensitive neurons. This suggests that the time course of suppression may be common among different IC populations.7. We also studied neurons in the SOC. Although many showed binaural interactions, none were sensitive to ITDs. Thus the response of this population may not be directly comparable with the responses of ITD-sensitive neurons in the IC. In general, the responses of SOC neurons to the probe recovered faster than those of IC neurons. The median CPI for 50% recovery was similar to 2 ms for the SOC, compared with similar to 6 ms for the IC. These data imply that the mechanisms responsible for the longer suppression in the IC lie subsequent to the SOC.