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

我们能够识别和理解不同说话者、不同音高和不同听力条件下的讲话。然而，一个声音的声波波形（例如元音“ae”）会因说话者的不同而有很大的不同，在我们经常嘈杂的环境中，“ae”可能会被嵌入到其他不和谐的背景声音中。尽管如此，我们还是毫不费力地将“ae”识别为“ae”，这表明大脑有能力形成对这些“讨厌的”变量不变的元音表示。对于元音，音色或元音身份是由谱包络决定的。通过嘴、嘴唇和舌头的过滤会产生能量峰，或频谱中的“共振峰”，正是这些共振峰的位置将元音彼此区分开来。因此，我们能够区分“ae”和“ih”，而不考虑说话者的性别、年龄或口音，这一事实表明，我们能够独立于基本频率、房间混响或安静和嘈杂条件下的空间位置，形成形成峰关系的不变表示。本研究计划的目的是发现这种不变表征在中央听觉系统中出现的位置和方式，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，理解这种表征在哪里以及如何被读出对于任何神经假肢装置的设计都是至关重要的。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，雪貂的发声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，我们可以记录下它们在听音时神经细胞的活动。通过探索雪貂能够辨别元音的环境，并测量神经活动，我们可以寻找听觉大脑中可能发生不变元音表征的地方。这个项目的第二部分涉及通过冷却单个大脑区域来可逆地沉默它们。这项技术的原理与用冰袋冷却受伤皮肤上的疼痛神经元非常相似。小的“冷冻环”被植入训练有素的动物的听觉皮层上方。这项技术使我们能够测试特定的大脑区域是否与元音辨别有因果关系。本项目的最后一部分将探讨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，观察他的嘴部运动可以提高你理解他的能力——尤其是当你在一个非常嘈杂的房间里听的时候。当你试图在嘈杂的背景中找出一个安静的声音时，知道这个声音可能出现的时间也能提高你正确识别它的能力。最近的研究表明，视觉信息被整合到最早的听觉皮层区域。然而，这些视觉信息是如何塑造我们的听觉感知的还不得而知。这项研究旨在研究视觉信息如何帮助训练有素的动物更准确地识别元音，同时研究视觉刺激如何影响听觉皮层神经元的行为。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，了解信息丰富的视觉刺激如何影响听觉皮层活动将为了解感觉整合如何在健康大脑中发生提供有价值的见解。听力受损的人通常无法在嘈杂的环境中有效地识别语言。了解神经元如何能够在各种听力条件和噪声环境中稳健地表示元音身份，将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.