Auditory processing--speech, space and auditory objects

There have been recent developments in our understanding of the auditory neuroscience of non-human primates that, to a certain extent, can be integrated with findings from human functional neuroimaging studies. This framework can be used to consider the cortical basis of complex sound processing in humans, including implications for speech perception, spatial auditory processing and auditory scene segregation.     

Duration tuning in the auditory midbrain of echolocating and non-echolocating vertebrates

Neurons tuned for stimulus duration were first discovered in the auditory midbrain of frogs. Duration-tuned neurons (DTNs) have since been reported from the central auditory system of both echolocating and non-echolocating mammals, and from the central visual system of cats. We hypothesize that the functional significance of auditory duration tuning likely varies between species with different evolutionary histories, sensory ecologies, and bioacoustic constraints. For example, in non-echolocating animals such as frogs and mice the temporal filtering properties of auditory DTNs may function to discriminate species-specific communication sounds. In echolocating bats duration tuning may also be used to create cells with highly selective responses for specific rates of frequency modulation and/or pulse-echo delays. The ability to echolocate appears to have selected for high temporal acuity in the duration tuning curves of inferior colliculus neurons in bats. Our understanding of the neural mechanisms underlying sound duration selectivity has improved substantially since DTNs were first discovered almost 50 years ago, but additional research is required for a comprehensive understanding of the functional role and the behavioral significance that duration tuning plays in sensory systems.     

Psychophysiology and psychoacoustics of music: Perception of complex sound in normal subjects and psychiatric patients

Perception of complex sound is a process carried out in everyday life situations and contributes in the way one perceives reality. Attempting to explain sound perception and how it affects human beings is complicated. Physics of simple sound can be described as a function of frequency, amplitude and phase. Psychology of sound, also termed psychoacoustics, has its own distinct elements of pitch, intensity and tibre. An interconnection exists between physics and psychology of hearing.Music being a complex sound contributes to communication and conveys information with semantic and emotional elements. These elements indicate the involvement of the central nervous system through processes of integration and interpretation together with peripheral auditory processing.Effects of sound and music in human psychology and physiology are complicated. Psychological influences of listening to different types of music are based on the different characteristics of basic musical sounds. Attempting to explain music perception can be simpler if music is broken down to its basic auditory signals. Perception of auditory signals is analyzed by the science of psychoacoustics. Differences in complex sound perception have been found between normal subjects and psychiatric patients and between different types of psychopathologies.     

Encoding of amplitude modulations by auditory neurons of the locust: influence of modulation frequency,rise time,and modulation depth

Using modulation transfer functions (MTF), we investigated how sound patterns are processed within the auditory pathway of grasshoppers. Spike rates of auditory receptors and primary-like local neurons did not depend on modulation frequencies while other local and ascending neurons had lowpass, bandpass or bandstop properties. Local neurons exhibited broader dynamic ranges of their rate MTF that extended to higher modulation frequencies than those of most ascending neurons. We found no indication that a filter bank for modulation frequencies may exist in grasshoppers as has been proposed for the auditory system of mammals. The filter properties of half of the neurons changed to an allpass type with a 50% reduction of modulation depths. Contrasting to reports for mammals, the sensitivity to small modulation depths was not enhanced at higher processing stages. In ascending neurons, a focus on the range of low modulation frequencies was visible in the temporal MTFs, which describe the temporal locking of spikes to the signal envelope. To investigate the influence of stimulus rise time, we used rectangularly modulated stimuli instead of sinusoidally modulated ones. Unexpectedly, steep stimulus onsets had only small influence on the shape of MTF curves of 70% of neurons in our sample.     

听觉中枢神经元对声信号的识别和处理

人类听觉的基本特性和机制与其他哺乳动物相似,因此,利用动物所作的听觉研究和获得的结果,有助于认识人类自身的听觉.围绕听觉中枢神经元对不同模式的声信号的识别和处理,简要综述了这方面的研究.声信号和声模式识别在听觉中枢对声信号的感受和加工中具有重要意义.听神经元作为声模式识别的结构和功能基础,对不同的声刺激模式产生不同反应,甚至是在同一声刺激模式下,改变其中的某个声参数,神经元的反应也会发生相应改变,而其反应的特性和机制均需要更多研究来解答.另外,声信号作为声信息的载体,不同的声信息寓于不同的声参数和声特征之中,研究发现,听觉中枢神经元存在相应的声信息甄别和选择的神经基础,能对动态变化的声频率、幅度和时程等进行反应和编码,并且,在不同种类动物上获得的研究结果极为相似,表明听觉中枢对不同声信号和声刺激模式的识别、分析和加工,具有共同性和普遍性.     

Global processing of spectrally complex sounds in macaques (Macaca mullata) and humans

How nonhuman species perceive the world is a biological question of fundamental importance, and has major significance for establishing the validity and possible limitations of animal models of human sensory function and perception. Studies in comparative hearing have revealed that almost all animals, including monkeys, are worse than humans at discriminating tone frequencies. Less is known, however, about comparative differences in discriminating more spectrally complex sounds. We compared the capacity of macaques and humans to discriminate complex sound patterns by measuring spectral-contrast sensitivity using stimuli having sine-modulated power spectra, analogous to sine-wave gratings used in visual studies. We found that the auditory system of the macaque is far less sensitive than the human system over the sine-profile frequency range tested (0.5-2.0 cycles/octave). These results indicate that rhesus macaques hear at least some spectrally complex sounds with less fidelity than do humans, and demonstrate large differences in primate species' abilities to process low-resolution spectral patterns. These results cannot be accounted for by traditional, narrowband peripheral filter models of spectral analysis, but instead, imply the involvement of a central, frequency integration process that may differ significantly across species.     

Auditory processing in birds

Over the past year, much progress has been achieved in the study of both the peripheral and the central auditory systems of birds. Significant advances have been made in the study of hair cells, including elucidation of the mechanisms of selectivity for sound frequency, functional differentiation, efferent innervation, and regeneration. Most of the studies of central auditory neurones have concerned the developmental and physiological correlates of vocal learning in songbirds and sound localisation in owls.     

A generalized linear model for estimating spectrotemporal receptive fields from responses to natural sounds

In the auditory system, the stimulus-response properties of single neurons are often described in terms of the spectrotemporal receptive field (STRF), a linear kernel relating the spectrogram of the sound stimulus to the instantaneous firing rate of the neuron. Several algorithms have been used to estimate STRFs from responses to natural stimuli; these algorithms differ in their functional models, cost functions, and regularization methods. Here, we characterize the stimulus-response function of auditory neurons using a generalized linear model (GLM). In this model, each cell's input is described by: 1) a stimulus filter (STRF); and 2) a post-spike filter, which captures dependencies on the neuron's spiking history. The output of the model is given by a series of spike trains rather than instantaneous firing rate, allowing the prediction of spike train responses to novel stimuli. We fit the model by maximum penalized likelihood to the spiking activity of zebra finch auditory midbrain neurons in response to conspecific vocalizations (songs) and modulation limited (ml) noise. We compare this model to normalized reverse correlation (NRC), the traditional method for STRF estimation, in terms of predictive power and the basic tuning properties of the estimated STRFs. We find that a GLM with a sparse prior predicts novel responses to both stimulus classes significantly better than NRC. Importantly, we find that STRFs from the two models derived from the same responses can differ substantially and that GLM STRFs are more consistent between stimulus classes than NRC STRFs. These results suggest that a GLM with a sparse prior provides a more accurate characterization of spectrotemporal tuning than does the NRC method when responses to complex sounds are studied in these neurons.     

Auditory processing of complex sounds: an overview.

The past 30 years has seen a remarkable development in our understanding of how the auditory system--particularly the peripheral system--processes complex sounds. Perhaps the most significant has been our understanding of the mechanisms underlying auditory frequency selectivity and their importance for normal and impaired auditory processing. Physiologically vulnerable cochlear filtering can account for many aspects of our normal and impaired psychophysical frequency selectivity with important consequences for the perception of complex sounds. For normal hearing, remarkable mechanisms in the organ of Corti, involving enhancement of mechanical tuning (in mammals probably by feedback of electro-mechanically generated energy from the hair cells), produce exquisite tuning, reflected in the tuning properties of cochlear nerve fibres. Recent comparisons of physiological (cochlear nerve) and psychophysical frequency selectivity in the same species indicate that the ear's overall frequency selectivity can be accounted for by this cochlear filtering, at least in bandwidth terms. Because this cochlear filtering is physiologically vulnerable, it deteriorates in deleterious conditions of the cochlea--hypoxia, disease, drugs, noise overexposure, mechanical disturbance--and is reflected in impaired psychophysical frequency selectivity. This is a fundamental feature of sensorineural hearing loss of cochlear origin, and is of diagnostic value. This cochlear filtering, particularly as reflected in the temporal patterns of cochlear fibres to complex sounds, is remarkably robust over a wide range of stimulus levels. Furthermore, cochlear filtering properties are a prime determinant of the 'place' and 'time' coding of frequency at the cochlear nerve level, both of which appear to be involved in pitch perception. The problem of how the place and time coding of complex sounds is effected over the ear's remarkably wide dynamic range is briefly addressed. In the auditory brainstem, particularly the dorsal cochlear nucleus, are inhibitory mechanisms responsible for enhancing the spectral and temporal contrasts in complex sounds. These mechanisms are now being dissected neuropharmacologically. At the cortical level, mechanisms are evident that are capable of abstracting biologically relevant features of complex sounds. Fundamental studies of how the auditory system encodes and processes complex sounds are vital to promising recent applications in the diagnosis and rehabilitation of the hearing impaired.     

Neural representation of spectral and temporal information in speech

Speech is the most interesting and one of the most complex sounds dealt with by the auditory system. The neural representation of speech needs to capture those features of the signal on which the brain depends in language communication. Here we describe the representation of speech in the auditory nerve and in a few sites in the central nervous system from the perspective of the neural coding of important aspects of the signal. The representation is tonotopic, meaning that the speech signal is decomposed by frequency and different frequency components are represented in different populations of neurons. Essential to the representation are the properties of frequency tuning and nonlinear suppression. Tuning creates the decomposition of the signal by frequency, and nonlinear suppression is essential for maintaining the representation across sound levels. The representation changes in central auditory neurons by becoming more robust against changes in stimulus intensity and more transient. However, it is probable that the form of the representation at the auditory cortex is fundamentally different from that at lower levels, in that stimulus features other than the distribution of energy across frequency are analysed.     

Biological Markers of Auditory Gap Detection in Young,Middle-Aged,and Older Adults

The capability of processing rapid fluctuations in the temporal envelope of sound declines with age and this contributes to older adults'' difficulties in understanding speech. Although, changes in central auditory processing during aging have been proposed as cause for communication deficits, an open question remains which stage of processing is mostly affected by age related changes. We investigated auditory temporal resolution in young, middle-aged, and older listeners with neuromagnetic evoked responses to gap stimuli with different leading marker and gap durations. Signal components specific for processing the physical details of sound stimuli as well as the auditory objects as a whole were derived from the evoked activity and served as biological markers for temporal processing at different cortical levels. Early oscillatory 40-Hz responses were elicited by the onsets of leading and lagging markers and indicated central registration of the gap with similar amplitude in all three age groups. High-gamma responses were predominantly related to the duration of no-gap stimuli or to the duration of gaps when present, and decreased in amplitude and phase locking with increasing age. Correspondingly, low-frequency activity around 200 ms and later was reduced in middle aged and older participants. High-gamma band, and long-latency low-frequency responses were interpreted as reflecting higher order processes related to the grouping of sound items into auditory objects and updating of memory for these objects. The observed effects indicate that age-related changes in auditory acuity have more to do with higher-order brain functions than previously thought.     

昆虫脑中央复合体的结构与功能研究进展

中央复合体是昆虫脑内具有显著特征的一个重要结构,它位于昆虫脑的中央,主要包括四个亚结构,相互间形成高度组织化的网络连接。中央复合体通过大范围神经元与多种感觉神经元和运动神经元相连,是一个控制脑的高级功能的中心。近年来的研究表明中央复合体参与了记忆的形成、运动的协调与控制以及处理偏振光进行导航等多种功能。揭示中央复合体参与以及调控这些复杂功能的神经机制,必将会极大地促进我们在神经回路层次上理解脑的高级复杂功能。     

Coding of communication calls in the subcortical and cortical structures of the auditory system

The processing of species-specific communication signals in the auditory system represents an important aspect of animal behavior and is crucial for its social interactions, reproduction, and survival. In this article the neuronal mechanisms underlying the processing of communication signals in the higher centers of the auditory system--inferior colliculus (IC), medial geniculate body (MGB) and auditory cortex (AC)--are reviewed, with particular attention to the guinea pig. The selectivity of neuronal responses for individual calls in these auditory centers in the guinea pig is usually low--most neurons respond to calls as well as to artificial sounds; the coding of complex sounds in the central auditory nuclei is apparently based on the representation of temporal and spectral features of acoustical stimuli in neural networks. Neuronal response patterns in the IC reliably match the sound envelope for calls characterized by one or more short impulses, but do not exactly fit the envelope for long calls. Also, the main spectral peaks are represented by neuronal firing rates in the IC. In comparison to the IC, response patterns in the MGB and AC demonstrate a less precise representation of the sound envelope, especially in the case of longer calls. The spectral representation is worse in the case of low-frequency calls, but not in the case of broad-band calls. The emotional content of the call may influence neuronal responses in the auditory pathway, which can be demonstrated by stimulation with time-reversed calls or by measurements performed under different levels of anesthesia. The investigation of the principles of the neural coding of species-specific vocalizations offers some keys for understanding the neural mechanisms underlying human speech perception.     

Clinical psychoacoustics in Alzheimer's disease central auditory processing disorders and speech deterioration

BACKGROUND: Difficulty in speech understanding in the presence of background noise or competing auditory signals is typically present in central auditory processing disorders. These disorders may be diagnosed in Alzheimer's disease as a result of degeneration in the central auditory system. In addition perception and processing of speech may be affected. MATERIAL AND METHODS: A MEDLINE research was conducted in order to answer the question whether there is a central auditory processing disorder involved in Alzheimer's disease. A second question to be investigated was what, if any is the connection, between central auditory processing disorders and speech deterioration?Articles were retrieved from the Medline to find relevance of Alzheimer's dis ease with central auditory processing disorders, they summed up to 34. Twelve papers were studied that contained testing for CAPD through psychoacoustic investigation. An additional search using the keywords 'speech production' and 'AD' produced a result of 33 articles, of them 14 are thoroughly discussed in this review as they have references concerning CAPD. The rest do not contain any relavent information on the central auditory system. RESULTS: Psychoacoustic tests reveal significantly lower scores in patients with Alzheimer's disease compared with normal subjects. Tests concerning sound localization and perception of tones as well as phoneme discrimination and tonal memory reveal deficits in Alzheimer's disease. Central auditory processing disorders may exist several years before the onset of clinical diagnosis of Alzheimer's disease. Segmental characteristics of speech are normal. Deficits exist concerning the supra-segmental components of speech. CONCLUSIONS: Central auditory processing disorders have been found in many cases when patients with Alzheimer's disease are tested. They may present as an early manifestation of Alzheimer's disease, preceding the disease by a minimum of 5 and a maximum of 10 years. During these years changes in the central auditory system, starting in the temporal lobe, may produce deficits in speech processing and production as hearing and speech are highly connected human functions. Another theory may be that spread of degeneration of the central nervous system has as a consequence, speech deterioration. Further research and central auditory processing disorders testing in the elderly population are needed to validate one theory over the other.     

High-order SNP combinations associated with complex diseases: efficient discovery, statistical power and functional interactions

There has been increased interest in discovering combinations of single-nucleotide polymorphisms (SNPs) that are strongly associated with a phenotype even if each SNP has little individual effect. Efficient approaches have been proposed for searching two-locus combinations from genome-wide datasets. However, for high-order combinations, existing methods either adopt a brute-force search which only handles a small number of SNPs (up to few hundreds), or use heuristic search that may miss informative combinations. In addition, existing approaches lack statistical power because of the use of statistics with high degrees-of-freedom and the huge number of hypotheses tested during combinatorial search. Due to these challenges, functional interactions in high-order combinations have not been systematically explored. We leverage discriminative-pattern-mining algorithms from the data-mining community to search for high-order combinations in case-control datasets. The substantially improved efficiency and scalability demonstrated on synthetic and real datasets with several thousands of SNPs allows the study of several important mathematical and statistical properties of SNP combinations with order as high as eleven. We further explore functional interactions in high-order combinations and reveal a general connection between the increase in discriminative power of a combination over its subsets and the functional coherence among the genes comprising the combination, supported by multiple datasets. Finally, we study several significant high-order combinations discovered from a lung-cancer dataset and a kidney-transplant-rejection dataset in detail to provide novel insights on the complex diseases. Interestingly, many of these associations involve combinations of common variations that occur in small fractions of population. Thus, our approach is an alternative methodology for exploring the genetics of rare diseases for which the current focus is on individually rare variations.     

后掩蔽效应对大棕蝠下丘神经元声反应的影响

以回声定位蝙蝠为模式动物,采用在体动物细胞外单位记录法,研究了后掩蔽效应对下丘神经元声反应的影响。结果显示,部分神经元(38％,12／31)对测试声刺激的反应明显受到掩蔽声的抑制,其后掩蔽效应强弱与掩蔽声和测试声的相对强度差(inter-stimulus level difference,SLD),以及测试声与掩蔽声之间的间隔时间(inter-stimulus onset asynchrony,SOA)有关：当掩蔽声强度升高或测试声强度降低时,后掩蔽效应增强;而SOA的缩短,亦可见后掩蔽效应增强。另外,相当数量的神经元(52%,16／31)对测试声刺激的反应并不受掩蔽声的影响,其中有的神经元只有在特定SLD和SOA时,才表现出后掩蔽效应。而少数下丘神经元(10％,3／31)在特定SLD和SOA时,掩蔽声对测试声反应有易化作用。上述结果表明,部分下丘神经元参与了声认知活动中的后掩蔽形成过程,推测下丘神经元在定型声反应特性中,对掩蔽声诱导的兴奋前抑制性输入与测试声诱导的兴奋性输入之间的时相性动态整合起关键作用。     

Processing of complex sounds in the auditory system

The coding of complex sounds in the early auditory system has a 'standard model' based on the known physiology of the cochlea and main brainstem pathways. This model accounts for a wide range of perceptual capabilities. It is generally accepted that high cortical areas encode abstract qualities such as spatial location or speech sound identity. Between the early and late auditory system, the role of primary auditory cortex (A1) is still debated. A1 is clearly much more than a 'whiteboard' of acoustic information-neurons in A1 have complex response properties, showing sensitivity to both low-level and high-level features of sounds.     

Representation of spectrotemporal sound information in the ascending auditory pathway

The representation of sound information in the central nervous system relies on the analysis of time-varying features in communication and other environmental sounds. How are auditory physiologists and theoreticians to choose an appropriate method for characterizing spectral and temporal acoustic feature representations in single neurons and neural populations? A brief survey of currently available scientific methods and their potential usefulness is given, with a focus on the strengths and weaknesses of using noise analysis techniques for approximating spectrotemporal response fields (STRFs). Noise analysis has been used to foster several conceptual advances in describing neural acoustic feature representation in a variety of species and auditory nuclei. STRFs have been used to quantitatively assess spectral and temporal transformations across mutually connected auditory nuclei, to identify neuronal interactions between spectral and temporal sound dimensions, and to compare linear vs. nonlinear response properties through state-dependent comparisons. We propose that noise analysis techniques used in combination with novel stimulus paradigms and parametric experiment designs will provide powerful means of exploring acoustic feature representations in the central nervous system.