Identifying the signal in the noise: a systems approach for examining invariance in auditory cortex

我们能够识别和理解许多不同扬声器、音调和收听条件下的语音。然而，一个声音(例如，元音‘ae’)的声学波形会因说话人的不同而有很大的不同，而在我们经常嘈杂的环境中，‘ae’可能会嵌入其他背景声音的杂音中。尽管如此，我们可以毫不费力地将“ae”识别为“ae”，这表明大脑能够形成对这些“讨厌”变量不变的元音表示。对于元音，音色或元音身份由频谱包络决定。通过嘴巴、嘴唇和舌头进行过滤会产生能量峰值，也就是频谱中的共振峰，正是这些共振峰的位置将元音相互区分开来。因此，无论说话者的性别、年龄或口音如何，我们都能够区分‘ae’和‘ih’这一事实，这表明我们能够在安静和嘈杂的条件下形成不变的共振峰关系表示，而不受基频、房间混响或空间位置的影响。这项研究计划的目的是发现这种不变表征在中枢听觉系统中的位置和方式，以及这些表征如何在嘈杂的环境中保持。形成不变表征是感觉系统面临的最大挑战之一，而理解这种表征是在哪里以及如何读出的，对于任何神经假体设备的设计都至关重要。我们的研究使用雪貂，因为它们的听力范围与我们的频率范围非常相似。此外，雪貂的发音与人类的元音有许多相似之处。雪貂很快就学会了辨别元音，我们能够在它们执行此类听力任务时记录它们神经细胞的活动。通过探索雪貂能够辨别元音的情况，并测量神经活动，我们可以寻找听觉大脑中可能出现不变元音表示的地方。该项目的第二部分涉及通过冷却个别大脑区域来可逆地使它们沉默。这项技术的原理与使用冰袋冷却挫伤皮肤上的痛觉神经元大致相同。在受过训练的动物的听觉皮质上方植入了小的“冷冻环”。这项技术让我们能够测试特定的大脑区域是否与元音识别有关。这个项目的最后部分调查了视觉信息在听觉感知中的作用。众所周知，当一个人和你说话时，看到他们的嘴巴移动会增强你理解他们的能力--特别是当你在一个非常嘈杂的房间里听的时候。当试图在嘈杂的背景中辨别出安静的声音时，知道声音可能发生的时间也会增强你正确识别声音的能力。最近的研究表明，视觉信息被整合到最早的听觉皮质区域。然而，这种视觉信息是如何塑造我们的听觉知觉的还是个未知数。这项提案中的工作旨在研究视觉信息如何帮助经过训练的动物更准确地识别元音，同时检查视觉刺激如何影响听觉皮质神经元的行为。听觉和视觉信息的不适当整合被认为是精神分裂症症状的基础，了解信息性视觉刺激如何影响听觉皮质活动将为了解健康大脑中如何发生感觉整合提供有价值的见解。听力受损的人最常见的是在嘈杂环境中无法有效识别言语。了解神经元如何在各种听力条件和噪声环境中稳健地表示元音身份，将有助于增强助听器和人工耳蜗的设计。

We are able to recognize and understand speech across many different speakers, voice pitches and listening conditions. However, the acoustic waveform of a sound (e.g. the vowel 'ae') will vary considerably depending on the individual speaker, and the 'ae' may be embedded in a cacophony of other, background sounds in our often noisy environments. Despite this, we have no difficulty recognizing an 'ae' as an 'ae', suggesting that the brain is capable of forming a representation of the vowel sound which is invariant to these 'nuisance' variables. For vowel sounds, the timbre, or vowel identity, is determined by the spectral envelope. Filtering by the mouth, lips and tongue results in energy peaks, or 'formants' in the spectrum, and it is the location of these formants which differentiates vowel sounds from one another. Thus, the fact that we are able to discriminate 'ae' from 'ih' irrespective of the gender, age or accent of a speaker suggests that we are able to form an invariant representation of the formant relations independently of the fundamental frequency, room reverberations, or spatial location in both quiet and noisy conditions. The aim of this research program is to discover where and how such invariant representations arise in the central auditory system and how these representations are maintained in noisy environments. Forming invariant representations is one of the greatest challenges for sensory systems, and understanding where and how such representations are read out is crucial for the design of any neuroprosthetic device. Our research uses ferrets as their hearing range spans a very similar range of frequencies to ours. Moreover, ferret vocalizations share many similarities with human vowel sounds. Ferrets rapidly learn to discriminate vowel sounds and we are able to record the activity of their nerve cells whilst they perform such listening tasks. By probing the circumstances under which the ferret is able to discriminate vowel sounds, and measuring the neural activity, we can look for where in the auditory brain invariant vowel representation might occur. The second part of this project involves reversibly silencing individual brain areas by cooling them. The principle of this technique is much the same was as using an ice pack to cool pain neurons in a bruised piece of skin. Small 'cryoloops' are implanted above auditory cortex in trained animals.This technique allows us to test whether particular brain areas are causally involved in vowel discrimination. The final part of this project investigates the role of visual information in auditory perception. It is well known that seeing a persons mouth movements while they talk to you enhances your ability to understand them - especially if you are listening in a very noisy room. When trying to pick out a quiet sound in a noisy background knowing when the sound is likely to occur also enhances your ability to correctly identify it. It has recently been shown that visual information is integrated into the very earliest auditory cortical areas. However, quite how this visual information shapes our auditory perception is unknown. The work in this proposal seeks to examine how visual information helps a trained animal to identify vowel sounds more accurately, whilst simultaneously examining how the visual stimulus influences the behaviour of neurons in auditory cortex. Inappropriate integration of auditory and visual information is postulated to underlie schizophrenic symptoms and understanding how informative visual stimuli influence auditory cortical activity will provide valuable insight into how sensory integration occurs in the healthy brain.Hearing impaired individuals most frequently suffer from an inability to effectively identify speech in noisy environments. Understanding how neurons are able to represent vowel identity robustly across a variety of listening conditions and noise environments will enhance hearing aid and cochlear implant design.