Neuronal Mechanisms Of Hearing

In contrast to the level of interest which is paid to the orga nization of meetings about the structure and function of the audi tory periphery, the central auditory system has received little attention in the last several years. However, much recent data accu mulated during this period has provided auditory physiologists with new ideas about the function of the central auditory system. The successful exploration of new anatomical tracing techniques (triti ated aminoacids, horseradish peroxidase, 2-deoxyglucose) together with the collection of electrophysiological data obtained with intra cellular and extracellular recordings from the receptors and neurones in the auditory pathway have considerably deepened our understanding of central auditory function. Particular interest was concentrated upon the development of the auditory system under normal conditions and in conditions ofaudi tory deprivation. Although, from the methodological point of view, the conditions of reversible auditory deprivation are complicated, promising new data appeared in this field. Similarly the specific ability of the auditory system to encode communication signals and speech sounds has been examined in many laboratories allover the world. A very fruitful method. based upon the results of electrical stimulation of cochlear nerve fibres in experimental animals, is the application of neuroprostheses in deaf patients. At the present time, the method still does not meet all requirements and many improvements will be necessary. Undoubtedly the exploration of the results of recent physiological experiments may help in the further improvement of neuroprostheses.

Since the last symposium on "Neuronal Mechanisms of Hearing" held in Prague in 1980 and published in the volume of the same name (J. Syka and L. Aitkin, Eds. , Plenum Press, 1981), remarkable progress has been achieved in the understanding of the auditory system. A variety of new ideas and new methods have emerged. This progress can be easily documented by comparing the volume based on the 1980 Symposium with the program for the 1987 Symposium. For example, there were 45 contributions to auditory physiology in each symposium but there were 27 contributions focusing on anatomy in 1987 as compared to 7 in 1980, and perhaps most telling, there were 12 contributions to the neurochemistry of the system in 1987 while there were only 3 in 1980. In terms of percentages of contributions, neuroanatomy rose from 13% to 32% and neurochemistry (or chemical anatomy) rose from 5% in 1980 to 14% in 1987. These increases in the numbers and proportions of anatomical and neurochemical contributions undoubtedly reflects the increasing availabil ity and rising expertise in the new neuroanatomica1 and biochemical techniques most notably, tract-tracing by exploitation of axonal transport or by intracellular micro-injection methods, and neurotransmitter identifi cation by use of immunocytochemistry or receptor-binding techniques. New ideas have emerged on the function of cochlear hair cells particularly in connection with olivococh1ear bundle stimulation and supported by findings of contractile proteins in outer hair cells.

Basic Mechanisms in Hearing is a collection of papers that discusses the function of the auditory system covering its ultrastructure, physiology, and the mechanism's connection with experimental psychology. Papers review the mechanics, morphology, and physiology of the cochlear, including the physiology of individual hair cells and their synapses. One paper examines the combined physiological and anatomical studies of stimulus coding in the mammalian auditory nervous system. The results of these studies pertain to the latency, frequency selectivity, and time pattern of responses to short tone bursts. Other research compare the cochlear nerve, behavioral, and psychophysical frequency selectivity which show that frequency selectivity of the auditory system occurs at the level of the cochlear nerve, becoming downgraded in end-organ deafness. Other papers discuss neural coding at higher levels such as the feature extraction in the auditory system of bats. Some papers also analyze the specialized hearing mechanisms in animals, for example, the echolocation of bats and in some insects, the function of the swimbladder in fish hearing, as well as the "invertebrate frequency analyzer" in the locust ear. Physiologists, neurophysiologists, neurobiologists, general medical practioners, and EENT specialists will find this collection valuable.

The symposium that has provided the basis for this book, "Plasticity of the Central Auditory System and Processing of Complex Acoustic Signals" was held in Prague on July 7-10, 2003. This is the fourth in a series of seminal meetings summarizing the state of development of auditory system neuroscience that has been organized in that great world city. Books that have resulted from these meetings represent important benchmarks for auditory neuroscience over the past 25 years. A 1980 meeting, "Neuronal Mechanisms of Hearing" hosted the most distinguished hearing researchers focusing on underlying brain processes from this era. It resulted in a highly influential and widely subscribed and cited proceedings co-edited by professor Lindsay Aitkin. The subject of the 1987 meeting was the "Auditory Pathway - Structure and Function". It again resulted in another important update of hearing science research in a widely referenced book - edited by the late Bruce Masterton. While the original plan was to hold a meeting summarizing the state of auditory system neuroscience every 7 years, historical events connected with the disintegration of the Soviet Empire and return of freedom to Czechoslovakia resulted in an unavoidable delay of what was planned to be a 1994 meeting. It wasn't until 1996 that we were able to meet for the third time in Prague, at that time to review "Acoustical Signal Processing in the Central Auditory System".

Synaptic Mechanisms in the Auditory System will provide a basic reference for students, clinicians, and researchers on how synapses in the auditory system function to encode acoustic signals. These mechanisms are the groundwork for all auditory processing, and understanding them requires knowledge of the microphysiology of synapses, cellular biophysics, receptor pharmacology, and an appreciation for what these synapses must do for a living, what unique jobs they carry out.

The full power of combining experiment and theory has yet to be unleashed on studies of the neural mechanisms in the brain involved in acoustic information processing. In recent years, enormous amounts of physiological data have been generated in many laboratories around the world, characterizing electrical responses of neurons to a wide array of acoustic stimuli at all levels of the auditory neuroaxis. Modern approaches of cellular and molecular biology are leading to new understandings of synaptic transmission of acoustic information, while application of modern neuro-anatomical methods is giving us a fairly comprehensive view ofthe bewildering complexity of neural circuitry within and between the major nuclei of the central auditory pathways. Although there is still the need to gather more data at all levels of organization, a ma jor challenge in auditory neuroscience is to develop new frameworks within which existing and future data can be incorporated and unified, and which will guide future laboratory ex perimentation. Here the field can benefit greatly from neural modeling, which in the central auditory system is still in its infancy. Indeed, such an approach is essential if we are to address questions related to perception of complex sounds including human speech, to the many di mensions of spatial hearing, and to the mechanisms that underlie complex acoustico-motor behaviors.