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的扬声器，语音音高和听力条件下的语音。然而，声音的声学波形（例如元音“ae”）将根据个体扬声器而显著变化，并且“ae”可能嵌入在我们经常嘈杂的环境中的其他背景声音的杂音中。尽管如此，我们还是能毫无困难地把“ae”识别为“ae”，这表明大脑能够形成一个对这些“讨厌的”变量不变的元音的表征。对于元音来说，音色或元音的同一性是由频谱包络决定的。通过嘴、嘴唇和舌头的过滤导致频谱中的能量峰或“共振峰”，并且正是这些共振峰的位置将元音声音彼此区分开。因此，事实上，我们能够区分'ae'从'ih'不考虑性别，年龄或口音的扬声器表明，我们能够形成一个不变的表示的共振峰关系独立的基频，房间混响，或空间位置在安静和嘈杂的条件。这项研究计划的目的是发现在哪里以及如何在中央听觉系统中出现这样的不变表示，以及如何在嘈杂的环境中保持这些表示。形成不变表征是感觉系统面临的最大挑战之一，理解这些表征在哪里以及如何读出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，因为它们的听觉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，我们能够记录它们在执行这种听力任务时神经细胞的活动。通过探测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不变性元音表征可能发生的位置。该项目的第二部分涉及通过冷却来可逆地沉默个别大脑区域。这项技术的原理与使用冰袋冷却一块瘀伤皮肤中的疼痛神经元大致相同。在经过训练的动物的听觉皮层上植入一个小的“冷冻环”，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关。本研究的最后一部分探讨视觉信息在听觉感知中的作用。众所周知，看到一个人说话时的嘴部动作可以提高你理解他们的能力-特别是当你在一个非常嘈杂的房间里听的时候。当你试图在嘈杂的背景中挑选出一个安静的声音时，知道这个声音什么时候可能出现也会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，这种视觉信息如何塑造我们的听觉感知是未知的。这项提案中的工作旨在研究视觉信息如何帮助受过训练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，了解信息性视觉刺激如何影响听觉皮层活动将提供有价值的见解，以了解感觉整合如何发生在健康的brain.Hearing受损的人最经常遭受无法有效地识别语音在嘈杂的环境中。了解神经元如何能够在各种听力条件和噪声环境下稳健地表示元音身份，将有助于提高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.