Auditory Sensing Systems: Overview
KeywordsHair Cell Auditory Cortex Auditory System Auditory Processing Sound Localization
Auditory sensing, or the sense of hearing, is concerned with detecting and extracting information from pressure waves in the surrounding medium, typically air or water. Since waves are generated by movements or collisions, this primarily tells the perceiver about things happening in the environment. In addition, since pressure waves can be reflected, absorbed, and refracted by other objects, these pressure waves also contain a great deal of contextual information about the environment and the objects in it. A fundamental challenge for the auditory system is to segregate the contributions of individual sound sources to the sound pressure waves received by the sensors as these are made up of a combination of all concurrent sources and their various reflections. Sounds unfold in time, so modellers of auditory processing cannot ignore time and the need to process signals within time; this becomes especially challenging when considering the multiscale nature of the information contained within sounds.
Possibly due to the complexity of the problem, many aspects of auditory processing have not yet been modeled at a detailed neurocomputational level. In addition, a great deal of processing occurs even before the incoming signals reach cortex; with the result that there are more models related to subcortical processing, than to higher level, putatively cortical, functions. The articles commissioned for the auditory sensing systems section therefore span a range of topics from qualitatively different point of view, with a strong focus on the functionality of the auditory system. Together they provide an interesting and insightful view of current understanding of auditory processing, with a great deal of useful information for modellers regarding the neuroanatomy and neurophysiology of the auditory system, methodological approaches to studying the auditory system, and functional requirements and perceptual constraints on auditory processing.
A well-illustrated overview of the anatomy and physiology of the auditory system, including the extensive but poorly understood efferent system is covered by “Anatomy and Physiology of the Mammalian Auditory System.” A detailed account of efferent control of the auditory sensor, the cochlea, is presented in the article on the “Physiology and Function of Cochlear Efferents.” It is in the cochlea that the biological system begins to analyze the pressure waves over multiple time scales and to do so within the time constraints of the ongoing multiscale information flow. Another point of major transformation occurs at the gateway to the cortex, described in the article “Auditory Thalamo-cortical Transformations.” Three further overview articles dealing with electrophysiological correlates of auditory perception: “Auditory Event Related Potentials,” “Auditory Evoked Brainstem Responses,” and “Electrophysiological Indices of Speech Processing” document methodological approaches to studying high-level perceptual functions.
Specific aspects of the functional neurophysiology of the thalamocortical auditory system are addressed in a number of articles. The articles “Associations and Rewards in Auditory Cortex” and “Context Dependent Processing in Auditory Cortex” demonstrate that the auditory cortical code needs to be viewed in more complex terms than simple acoustic feature representations. For example, neural correlates of reward are found even within primary auditory cortex (“Associations and Rewards in Auditory Cortex”). In contrast, the articles “Spectro-Temporal Receptive Fields,” “Stimulus-Specific Information,” and “Stimulus Reconstruction from Cortical Responses” show how auditory cortical activity can in some circumstances be usefully interpreted in terms of acoustic feature combinations.
An overview of theories and models of higher level aspects of auditory perception are presented in “Auditory Perceptual Organisation,” “Music Processing in the Brain,” “Neural Coding of Speech Sounds,” “Pulse Resonance Sounds,” “Auditory Memory,” “Acoustic Timbre Recognition,” “Pitch Perception, Models,” “Rhythm Perception: Pulse and Meter,” and “Sound Localization and Experience-Dependent Learning.” These diverse topics emphasize the different ways in which sound is interpreted by the brain, while the article “Tinnitus, Models” shows how modelling may help advance understanding of a perceptual phenomenon that plagues 10–15 % of the population.
Finally, the section contains articles which present more detailed neurocomputational models and theories more closely related to the biology. These tend either to relate to very specific functions (“Sound Localization in Mammals, Models,” “Stimulus-Specific Adaptation, Models,” “Auditory Precedence Effect,” and “Masking and Masking Release”), or relate to processes at or near the sensor (“Auditory Nerve Response, Afferent Signals,” “Cochlear Distortion Products,” “Cochlear Inner Hair Cell, Model,” and “Cochlear Outer Hair Cell, Model”).