Abstract and Keywords
This article deals with the perception of sound; that is, how humans and other animals experience the auditory world. It is important to study perception of sound since physiological mechanisms need to be understood in terms of the perceptual functions that they perform. Auditory psychophysics, or psychoacoustics, is the study of the relations between the physical characteristics of sounds and the evoked sensations. The boundaries between psychoacoustics and physiology are blurred by the use of human brain imaging techniques in combination with behavioural measures. These techniques include electroencephalography (EEG) and magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI). Finally, this article states that while considering human hearing it is important to encode and analyse the different physical characteristics of sounds, separate the acoustic information from different sources, and identify and extract meaning from the sounds produced by a particular source.
The final volume in the series considers the perception of sound; that is, how humans and other animals experience the auditory world. Without a study of perception there would be little meaningful auditory science, since physiological mechanisms need to be understood in terms of the perceptual functions that they perform. For example, we want to understand the physiological mechanisms of frequency selectivity to explain the remarkable ability of listeners to separate and identify sounds.
Auditory psychophysics, or psychoacoustics, is the study of the relations between the physical characteristics of sounds and the evoked sensations. However, as well as providing mathematical formulae (and qualitative and quantitative models) that describe these relations without assuming any particular biological implementation, psychoacoustic research is increasingly directed to an understanding of perceptual phenomena in terms of the underlying physiology. Psychoacoustic research and physiological research are therefore strongly co-dependent, and throughout this volume you will read examples of psychoacoustic results being interpreted in terms of, and indeed informing, our understanding of the underlying biomechanical and neural mechanisms.
Traditionally, perceptions have been measured using psychoacoustic techniques in which participants are required to make behavioural responses (often by pressing a button on a response box) to indicate their experience of a sound. These techniques allow researchers to estimate basic subjective quantities such as the magnitude of a sensation, or to measure the limits of discrimination performance, for example the smallest frequency difference that is perceptible. However, the boundaries between psychoacoustics and physiology are becoming blurred by the use of human brain imaging techniques in combination with behavioural measures. These techniques include electroencephalography (EEG) and magnetoencephalography (MEG), which measure the electrical activity of neurons in the brain and have good temporal resolution (of the order of milliseconds) but poor spatial resolution; and functional magnetic resonance imaging (fMRI), that accurately localizes activity in the brain by measuring the increased blood oxygen levels in active regions, but has poor temporal resolution (of the order of seconds). By using brain imaging in combination with behavioural measures it is possible to link neural activity to sensation for individual listeners. In addition, brain imaging provides a connection between psychoacoustics and animal neurophysiology, bridging the gap between our understanding of human perception and our understanding of the responses of individual neurons. Being educated as a hardcore psychophysicist, I was surprised (pleasantly) by how many authors in this volume included brain imaging research in their discussions.
When considering human hearing it is important not to lose sight of the end goal of perception, which is to provide meaningful information about the world around us. To do this, the auditory system has to encode and analyse the different physical characteristics of sounds, separate the acoustic information from different sources, and identify and extract meaning from the sounds (p. 2) produced by a particular source, be it an inanimate object or the human voice. These topics of encoding, separation, and meaning provide a broad framework for the chapters in this volume.
1.2 Chapter summary
The overall flow of Chapters 2–11 in this volume is from our perception of the primary physical characteristics of sounds such as frequency and intensity, and basic measures of auditory sensation such as loudness and pitch, to the perception of complex sounds such as speech and music, and high-level, more cognitive, functions such as attention. In tandem, the discussions vary from processes that can be understood in terms of basic auditory physiology, to those that are dependent on vastly complex neural interactions in the cerebral hemispheres. The final chapters describe related topics including cross-modal interactions, auditory development, hearing and language disorders, and environmental sound.
Masking is a fundamental auditory phenomenon, and Chapter 2 (Oxenham and Wojtczak) considers the processes that determine whether one sound will obscure or mask another sound, with particular regard to the remarkable frequency selective mechanisms of the cochlea. Despite being one of the first areas of psychoacoustic investigation, there is currently considerable excitement in the field, as new behavioural techniques have allowed accurate measures of the complex non-linear response properties of the basilar membrane.
Chapter 3 (Epstein and Marozeau) is concerned with the auditory representation of sound intensity, and its perceptual correlate of loudness. The authors discuss intensity discrimination and the dynamic range of hearing, techniques for measuring loudness, and the complex relation of loudness to the physical characteristics of sounds. Important new developments in loudness modelling are described.
Pitch is the main perceptual dimension of western music, and has important roles in speech perception and in sound segregation. In Chapter 4, de Cheveigné describes how the sensation depends on the spectral, temporal, and binaural properties of sounds, and provides an overview of models of pitch perception. The author shows how our perceptions may relate to the physiological properties of auditory neurons, particularly their ability to ‘phase lock’ to periodic sounds, and describes recent hypotheses regarding neural pitch processing in the auditory brain.
The auditory system shows remarkable temporal resolution, having the ability to detect changes in sounds lasting just a few milliseconds (much faster than the visual system!). This acuity is necessary since information in sounds is often conveyed as a rapid sequence (e.g. speech). In Chapter 5 (Verhey) the main characteristics of auditory temporal resolution are described, as well as the specific mechanisms that process fluctuations in temporal envelope. The author also explains how the auditory system combines information over time in order to improve performance.
Frequency, intensity, periodicity, and temporal envelope can be regarded as fundamental physical characteristics of sounds. To these we might add spatial location as a fundamental characteristic of a sound source. Chapter 6 (Culling and Akeroyd) concerns spatial hearing; how we localize sound sources using binaural and monaural cues, and how we use echoes and reverberation to tell us about the nature of the listening space.
This volume, along with most perceptual research, focuses on human hearing. However, H. sapiens is not the only species to make use of acoustic information, and many insights have been provided from behavioural measurements on other animals. These advances are described in Chapter 7 (Shofner and Niemiec). As explained by the authors, this research provides a conceptual bridge between human psychophysics and animal neurophysiology, and allows us to understand human hearing in an evolutionary context.
Chapter 8 (Dyson) addresses the huge topic of auditory organization: How the auditory system separates out the sounds from different sources, and groups together the sounds from the same (p. 3) source into a coherent auditory stream. This vital stage in hearing, a prerequisite of sound identification, can be regarded as one of the most complex processing tasks accomplished by the auditory brain. The author shows how behavioural results are being combined with advances in neuroimaging to shed new light on the brain mechanisms involved.
Oral communication is perhaps the most important use of hearing for humans, allowing us to convey our thoughts and emotions with comparative ease. Chapter 9 (Darwin) considers many aspects of speech perception, including speech production, the representation of speech in the auditory system, grouping mechanisms, and speech recognition. It is shown how the auditory system copes remarkably well with severe distortions of the speech signal and the huge variability in speech across different contexts.
Music is arguably second only to speech as a means of acoustic communication (some might say higher than second!), and is an extremely rich and diverse product of human creativity. In Chapter 10, Dowling describes the dimensions of the music experience, how we organize the components of music, and how tension and relaxation are generated. The chapter emphasizes the importance of expectation and familiarity to our understanding and appreciation of a musical work.
Auditory attention is discussed in Chapter 11 (Spence and Santangelo). Through a series of ingenious experiments, the authors describe the limitations on our ability to analyse multiple streams of auditory information, how we direct our auditory attention, and how neuroimaging results have identified the brain regions involved in these complex processes.
Chapter 12 (Spence and Soto-Faraco) takes us beyond the purely acoustic world, and describes how auditory information combines and interacts with that from vision. What we hear is determined by much more than the sounds entering our ears, and the authors show how real-world perception needs to be understood as a multisensory experience.
In Chapter 13 (Mattock, Amitay, and Moore), the changing nature of auditory abilities is addressed. The authors describe how the developing auditory system learns how to make sense of the complex waveforms arriving at the ears, from the fetal stage, to infancy, and through childhood. The chapter also considers auditory learning; how the auditory brain changes and adapts to experience throughout life.
Hearing impairment is a very common disability, and improving the diagnosis and treatment of hearing disorders is the most important application for auditory research. In Chapter 14, Zeng and Djalilian describe the wide range of hearing disorders, and their underlying physiological and ‘system level’ bases. The authors also summarize the different treatments for hearing loss, including exciting recent developments.
Chapter 15 (Halliday and Moore) concerns the effects of hearing disorders, some of which may be quite subtle, on language and cognitive development. The links between auditory function and the development of higher-level abilities are still poorly understood, and present a considerable challenge to researchers. The authors present a balanced view of the evidence, and argue for an approach based on ‘risk factors’ rather than simplistic causal links.
In the final chapter (Chapter 16), Davies describes the mathematics and acoustics of sound production and propagation in the contemporary environment, including the different types of sound source, and the effects of human structures, including room acoustics. He also discusses the perception of environmental sound, and the recently reinvigorated concept of the ‘soundscape’.
I am deeply indebted to the chapter authors for contributing their time and expertise to this volume. Together they have produced a wide-ranging, current, and stimulating account of auditory perception that should be an invaluable resource for students, educators, and researchers in the field. (p. 4)