Description
This volume presents comprehensive overviews of our current understanding of the encoding, representation, and transformation of acoustic information within the auditory nerve and the various regions of the brain subserving the sense of hearing. The first chapter provides a general overview of the volume, presenting the integrated view of auditory neurophysiology from the periphery to the auditory cortex. Other chapters focus specifically on the auditory nerve, the various auditory nuclei of the brainstem, and the auditory cortex. One chapter is devoted to the electrophysiology of the human auditory system.
The purpose of this volume is to enable students, clinicians, and hearing researchers to gain a fundamental understanding of the patterns of neural response to sound at various levels of the nervous system, and of the neural pathways and interactions responsible for these patterns. The companion Volume 1 has parallel chapters presenting the neuroanatomy of the mammalian auditory sytem. Other volumes of the series will deal specifically with the cochlea, human and animal psychoacoustics, development, and the auditory neurophysiology and neuroanatomy of nonmammals.
A listener’s perception of the world of sound is but an abstraction of physical reality. It is determined initially by the linear acoustic transformation performed by the head, pinnae and external ear canals, followed in turn by the nonlinear mechanoelectric transduction in the bilaterally placed receptor organs of Corti and the ever changing spatio-temporal discharge patterns in ensembles of first-order afferent fibers of the auditory nerve. Within the central auditory system, which faces the acoustic world only indirectly through this filtered and fluctuating afferent input, incoming information encoded in trains of all-or-none action potentials is received, transformed, and then redistributed over parallel pathways to higher centers in the brain. Sound perception involves a number of central auditory mechanisms operating in concert, and these mechanisms form recurrent themes that run through the chapters that follow. They include sensory coding, temporal and spatial transformation, divergent and convergent projection, parallel and serial processing, localization of function and neuronal plasticity. These themes are related to hearing in ways still not fully understood, although all of them have been thought about, discussed, and studied in one form or another for a century or more.
Many of the earliest studies of auditory system function were relatively primitive by today’s standards, but even with the limited experimental and analytical tools available at the time they provided a framework upon which much of today’s auditory research is based. Physician scientists who studied the nervous system during the latter part of the nineteenth century took a keen interest in central hearing and speech mechanisms. Broca’s paper of 1861 which described the deleterious effects on speech production caused by cortical damage of one of his patients, along with more thorough analyses of brain trauma by Broca ( 1865) and Wernicke (1874), had major impact on the ideas oflocalization of cerebral function and central mechanisms of speech perception and production. Around the same time Ferrier’s experiments on animals, in which he electrically stimulated or ablated localized areas of cerebral cortex of several mammalian species, provided some of the first experimental evidence for localization of auditory function within the central nervous system (see Heffner 1987). Certainly early anatomists studying the cellular and myeloarchitecture of cerebral cortex were well aware of the locations of acoustic receptive areas and, moreover, were able to postulate from their observations the serial nature of sensory processing (see Brugge 1982). To the present day, ablation/behavior studies and cyto- and myeloarchitectonic analyses, often combined with more modern methods such as neuroanatomical tracing of pathways, continue to contribute to our understanding of the function of the auditory system at all levels.
A revolution in functional studies of the auditory system began in the 1930s when it became possible to record electrical events from the brain and peripheral nerves. The historic work of Wever and Bray ( 1930) on the cochlear nerve and, later, of Woolsey and Walzl ( 1942) on the auditory cortex can, arguably, be considered points of departure for much of the subsequent research on the auditory system. Then, with the application ofmicroelectrode recording in the early 1940s (e.g., Galambos and Davis 1943), there began a long series of quantitative studies in many laboratories on neural encoding mechanisms of single neurons at all levels of the auditory system. This work accelerated considerably some two decades later with the introduction of digital computer techniques for reduction and analysis of electrophysiological data (Gerstein and Kiang 1960; Kiang et al. 1965). Microelectrode recording has also been applied to functional mapping of auditory fields by taking advantage of the relatively high spatial resolution of single-neuron or neuron-cluster recording (Imig et al. 1982). Biophysical studies of single auditory neurons, employing intracellular and patch-clamp recording techniques, have begun to reveal synaptic and membrane properties of neurons in the auditory brainstem, including the identity and distribution ion channels and of neurotransmitters and their receptors (Oertel et al. 1988). With few exceptions (e.g., M0ller and Jannetta 1982, 1983; M0ller et al. 1988), most of the experimental work carried out either in vivo or in vitro has necessarily been on animals or animal tissue.
With the advent of powerful, noninvasive (and presumably harmless) positron emission tomography (PET), regional cerebral blood flow imaging (rCBF), magnetic resonance imaging (MRI) and magnetic encephalographic (MEG) techniques, new information is being gained on central auditory processing in normal and hearing-impaired humans. PET requires a cyclotron, computer, and scanner to measure quantitatively physiological, biochemical, and pharmacological activities in the living brain. MRI, which uses large superconducting magnets, affords excellent definition ofbrain structure. In rCBF imaging an array of collimated detectors maps changes in blood flow associated with brain activity. MEG records the weak magnetic signals emitted by the human cerebral cortex in response to sound. Results derived from these approaches will continue to augment information gained about the central auditory pathways through more traditional evoked potential recordings from electrodes in contact with the scalp. This latter approach, though limited in the information it can provide about mechanisms of central auditory function, has proved very successful in the diagnosis and management of hearing and neurological impairments, as Kraus and McGee document thoroughly in their chapter on the subject (this volume).
Armed with new technology for acoustic signal processing and for collecting and analyzing large volumes of electrophysiological data, and with modern powerful neuroanatomical methods, auditory researchers have in recent years obtained a great amount of new information about the mechanisms of central auditory processing. The chapters that follow review these developments in great detail. In doing so, the central auditory system is portrayed in a hierarchical, serial fashion, proceeding from the auditory nerve to the auditory cortex. Whereas this arrangement may be convenient for presentation, we remember that central auditory processing involves both serial and parallel operations. In a companion volume Webster ( 1992) provides an overview of the structural organization of the auditory pathways with an emphasis on the human nervous system. The present chapter paints with a broad brush a picture of the functional organization of the central auditory system, incorporating where possible the themes described above while leaving many of the details to the chapters that follow.