The function of the commissure of the inferior colliculus in auditory processing

耳朵对声音的反应产生的神经信号进入脑干两侧的两条镜像通道。虽然左右两个部分都接受来自两只耳朵的输入，但右耳主要接收来自听者左耳的声音信息，反之亦然。该应用提出声源的定位和识别依赖于一侧的活动被比较和参考另一侧。这种相互作用克服了单侧神经元的输出是模糊的问题，因为它们不专门对声音的一个属性做出反应，因此神经元放电的变化可以反映声音的响度变化，位置变化，或两者兼而有之。我们认为连接两个下丘（中脑的听觉中心，脑干通路的终点）的大束纤维为调节这种左右比较提供了基础。由于所有到达高级皮层中心的信号都是在下丘的途中进行处理的，因此下丘是促进这种相互作用的理想位置。此外，连接一个小丘和另一个小丘的纤维束（连接）是每个下小丘接收的最大输入之一，它以高度有组织的方式在小丘内终止。我们试图通过暂时停用大脑一侧的下丘，并研究这对另一侧下丘对声音的神经反应有何影响，来研究交联的功能作用。我们已经开发并测试了一种通过冷却来迅速使脑丘失活的方法。这是通过在其表面放置一个小的不锈钢管回路来实现的，我们在零度以下的温度下泵送冷却剂。我们可以通过一个微小的热电偶测量大脑温度来检查冷却效果是否局限于预期的结构，但作为双重检查，我们还将采用一种方法，使我们能够向下丘注入阻断神经活动的药物。我们也会研究直接切断连接纤维的效果。实验将在麻醉的豚鼠身上进行，这样我们就能记录下声音引起的神经活动。我们将测量大脑活动的不同方面，包括单个神经元的反应（同时记录在下丘的不同区域），以及代表大量神经元活动的电信号。通过比较下丘在另一个下丘失活之前、期间和之后对不同类型和组合的声音刺激的细粒反应和总结反应，我们将发现下丘在不同情况下如何影响另一个下丘。我们预测，阻断共同输入将影响声音处理的几个方面——这一观点得到了试点研究的支持。具体地说，我们期望观察到对声音特征的反应变化，这些特征表明了它的位置。不同位置的声音会在耳朵之间产生不同的声音水平和时间。最近关于大脑如何从这些差异中计算声音位置的发现表明，参照左右活动是很重要的。我们对下丘的连接组织所知甚少，对它在听觉中的作用就更少了。因此，这项研究的结果将为这些连接在听力中的作用提供重要的信息，并帮助我们更好地理解声音感知的机制。下丘也正在接受评估，以植入听觉-大脑假体，以造福那些因创伤或疾病而失去耳朵和大脑之间神经的患者。研制这种装置的成功与否取决于对下丘的进一步了解。

Neural signals generated by the ears in response to sounds feed into two mirror image pathways on either side of the brainstem. While both left and right divisions receive inputs from both ears, the right side is dominated by information about sounds on the listener's left side and vice versa. This application proposes that the localisation and recognition of sound sources depend on the activity in one side being compared and referenced to the other. This interaction overcomes the problem that the outputs of neurons on one side alone are ambiguous because they don't respond exclusively to one attribute of sound, thus a change in a neuron's firing can reflect a change in the loudness of a sound, a shift in its position, or both. We propose that the large bundle of fibres interconnecting the two inferior colliculi (the auditory centres in the midbrain where the brainstem pathways culminate) provides the basis for mediating this left- right comparison. Since all signals reaching the higher cortical centres are processed en route within the inferior colliculi they are ideally situated to facilitate such an interaction. Furthermore, the bundle of fibres (the commissure) that connects one colliculus to the other is one of the largest inputs each inferior colliculus receives, and it terminates within the colliculus in a highly organised fashion. We seek to investigate the functional role of the commissural connections by temporally deactivating the inferior colliculus on one side of the brain and investigating what effect this has on the neural responses to sounds in the other inferior colliculus. We have developed and tested a means of rapidly deactivating the colliculus by cooling it. This is achieved by placing a small loop of stainless steel tubing on its surface through which we pump a coolant at subzero temperature. We can check that that the cooling effect is limited to the intended structure by measuring brain temperature with a tiny thermocouple, but as a double check, will also apply a method that enables us to infuse drugs that block neural activity into the inferior colliculus. We will also examine the effect of cutting the commissural fibres directly. The experiments will be performed in the anaesthetised guinea pig so that we can record the neural activity elicited by sounds. We will measure different aspects of brain activity, including the responses of single neurons (recorded simultaneously in different regions of the inferior colliculus), and electrical signals that represent the activity of large populations of neurons. By comparing these fine grain and summary responses to different types and combinations of sound stimuli before, during, and after deactivation of the other inferior colliculus we will discover how one inferior colliculus influences the other in different situations. We predict, that blocking commissural input will influence several facets of sound processing - a view supported by pilot studies. Specifically, we expect to observe changes in responses to features of sound that signal its location. Sounds in different locations give rise to differences in the level and timing of the sound between the ears. Recent discoveries about how the brain computes sound position from these differences suggest that referencing of left and right activity is important.We know little about the organisation of commissure of the inferior colliculus, and even less about its function in hearing. The outcomes of this research will, therefore, provide information of fundamental importance about the role of these connections in hearing, and help us better to understand the mechanisms of sound perception. The inferior colliculus is also being assessed for implanting an auditory- brain prosthesis to benefit patients who, through trauma or disease, have lost the nerves between the ears and the brain. Success in developing such a device depends on a greater understanding of the inferior colliculus.