The neural basis of auditory pitch timbre and location judgments

神经科学领域的一个重要问题是大脑中神经元的活动如何影响我们感知环境的方式。关于大脑中与视觉和触觉有关的区域是如何影响行为的，新方法使人们有了令人兴奋的发现。我们希望首次在听觉系统中使用这些方法，以研究听觉皮层中神经元的放电与我们“听到”声音的方式之间的关系。其他人的声音，其他物种的叫声和环境的声音都可以沿着几个维度来描述。首先，我们能够判断声音来源的位置，即它来自外部世界的哪里。其次，声音的音高和音质或“音色”通常会有所不同。声音的音色对我们理解语言至关重要，而音高使我们能够识别谁在说话。例如，无论是小孩还是大人说的元音/i/音色都差不多，但小孩的声音可能会高得多。同样，钢琴和小提琴演奏相同的音符会发出相同的音高，但音色的差异意味着这两种乐器听起来非常不同。关于大脑听觉区神经细胞的活动如何让我们定位或识别声音，我们知之甚少。一些研究人员认为，就像在视觉系统中一样，听觉皮层的某些区域可能参与提取声源的位置，而其他区域则更关注其身份。虽然有一些证据支持这一点，但对这些研究的解释是有争议的。我们将研究雪貂大脑听觉皮层内的神经细胞如何对不同声音的位置、音高和音色做出反应。例如，大脑皮层中一个专门负责识别声音所在位置的区域，可能会对任何声音做出反应，而不管它的音高或音色，只要它出现在适当的位置。我们项目的一部分将包括记录听觉皮层中数百个神经细胞的反应，以寻找它们对声音的反应方式的差异。我们计划使用的声音具有元音的许多特征，元音是人类语言的主要元素之一，并且具有我们可以轻松独立地改变其音高，音色或位置的优势。我们将测量单个神经细胞区分这些不同特征的声音的能力。与此同时，我们将训练雪貂进行一项行为任务，要求它们区分两种不同位置、音高或音色的声音。我们将测量动物做出正确选择的可能性和刺激物的差异之间的关系。我们将把这些行为测量结果与通过记录听觉皮层神经细胞的反应获得的数据进行比较。这样，就有可能把单个神经细胞区分两种声音的能力与动物的整体能力联系起来。换句话说，我们将能够判断动物做出的选择是否遵循单个神经细胞的选择。因为大脑中有数十亿的神经细胞，我们也计划研究一小群细胞如何共同作用来代表听觉世界的不同方面。因此，这项研究的结果将告诉我们声音的不同方面是如何在大脑中被绘制出来的，以及这些区域内细胞的放电如何影响我们对世界的感知。这些实验不仅将帮助我们了解大脑这个人体中最复杂的器官是如何工作的，而且还将指导帮助听力受损者恢复功能的辅助设备的设计。

Summary An important question in the field of neuroscience is how the activity of neurons within the brain contributes to the way in which we perceive our environment. New methods have enabled exciting discoveries to be made as to how areas of the brain concerned with vision and touch may contribute to behaviour. We wish to use these methods, for the first time, in the auditory system in order to investigate how the firing of neurons in auditory cortex relates to the way in which we 'hear' sounds. The voices of other people, the calls of other species, and environmental sounds can all be described along several dimensions. Firstly, we are able to judge the location of the source of the sound, i.e. where it came from in the outside world. Secondly, the sound will often differ in its pitch and its quality or 'timbre'. The timbre of the sound is critical for our understanding of speech, while the pitch enables us to identify who is speaking. For example the vowel /i/ has a similar timbre whether said by a small child or a man, but the child's voice will most likely have a much higher pitch. Similarly, a piano and a violin playing the same note will produce a sound of identical pitch, but differences in the timbre of the sound mean that the two instruments sound very different. Little is known about how the activity of nerve cells in the hearing areas of the brain allow us to localize or identify sounds. Some researchers have argued that, as in the visual system, there may be a division of labour such that some regions of the auditory cortex are involved in extracting the location of the sound source, while others are more concerned with its identity. While there is some evidence to support this, the interpretation of these studies is controversial. We will investigate how nerve cells within the auditory cortex of the ferret brain respond to the location, pitch and timbre of different sounds. A cortical area devoted to working out where the sound is, for example, might be expected to respond to any sound regardless of its pitch or timbre, as long as it is presented from the appropriate location. Part of our project will involve recording the responses of hundreds of nerve cells in the auditory cortex in order to look for differences in the way that they respond to sounds. The sounds that we plan to use share many characteristics of vowels, one of the main elements of human speech, and which have the advantage that we can easily and independently change their pitch, timbre or location. We will measure how well individual nerve cells can tell apart sounds that differ in one of these features. At the same time, we will train ferrets in a behavioural task that requires them to distinguish between two sounds that differ in their location, pitch or timbre. We will measure the relationship between how likely the animal is to make a correct choice and how different the stimuli are. We will compare these behavioural measurements to the data obtained by recording the responses of nerve cells in auditory cortex. In this way, it will be possible to relate how well an individual nerve cell can distinguish between two sounds to the overall ability of the animal. In other words, we will be able to tell whether the choice made by the animal follows that of individual nerve cells. Because there are billions of nerve cells in the brain, we also plan to study how small populations of cells act together to represent different aspects of the auditory world. The results of this study will therefore tell us how different aspects of a sound are mapped out within the brain and how the firing of the cells within those areas contributes to our perception of the world. These experiments will not only help us to understand how the brain, the most complex organ in the body, works, but will also guide the design of aids that restore function to the hearing impaired.