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。新的研究方法使人们能够发现大脑中与视觉和触觉有关的区域是如何影响行为的。我们希望第一次在听觉系统中使用这些方法，以研究听觉皮层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。其次，声音往往会在其音高和质量或“音色”上有所不同。声音的音色对我们理解语音至关重要，而音高使我们能够识别谁在说话。例如，元音/i/无论是由小孩还是男人说，都具有相似的音色，但孩子的声音很可能具有更高的音高。同样，钢琴和小提琴演奏相同的音符会产生相同音高的声音，但声音的音色差异意味着这两种乐器听起来非常不同。人们对大脑听觉区域的神经细胞活动如何使我们能够定位或识别声音知之甚少。一些研究人员认为，在视觉系统中，可能存在劳动分工，例如听觉皮层的某些区域参与提取声源的位置，而其他区域则更关心声源的身份。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，一个专门负责确定声音位置的皮层区域，可以对任何声音做出反应，而不管它的音高或音色如何，只要它是从适当的位置出现的。我们项目的一部分将涉及记录听觉皮层中数百个神经细胞的反应，以寻找它们对声音反应的差异。我们计划使用的声音与人类语言的主要元素之一元音有许多共同的特征，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如何区分这些特征之一不同的声音。与此同时，我们将训练雪貂进行一项行为任务，要求它们区分两种位置、音高或音色不同的声音。我们将测量动物做出正确选择的可能性与刺激的不同程度之间的关系。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反应获得的数据进行比较。这样，就有可能将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的选择是否遵循单个神经细胞的选择。由于大脑中有数十亿个神经细胞，我们还计划研究小群体细胞如何共同作用以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映射出来的，以及这些区域内细胞的放电如何有助于我们对世界的感知。这些实验不仅将帮助我们了解大脑这个人体最复杂的器官是如何工作的，而且还将指导助听器的设计，以恢复听力受损者的功能。

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.