The Auditory System and Human Sound-Localization Behavior

John van Opstal

• Publisher: Academic Press
• Year: 2016
• Print ISBN: 9780128015292
• eBook ISBN: 9780128017258
• Topic: Biological Sciences
• Subtopic: Neuroscience
• Index: Biological Sciences

The Auditory System and Human Sound-Localization Behavior provides a comprehensive account of the full action-perception cycle underlying spatial hearing. It highlights the interesting properties of the auditory system, such as its organization in azimuth and elevation coordinates. Readers will appreciate that sound localization is inherently a neuro-computational process (it needs to process on implicit and independent acoustic cues). The localization problem of which sound location gave rise to a particular sensory acoustic input cannot be uniquely solved, and therefore requires some clever strategies to cope with everyday situations. The reader is guided through the full interdisciplinary repertoire of the natural sciences: not only neurobiology, but also physics and mathematics, and current theories on sensorimotor integration (e.g. Bayesian approaches to deal with uncertain information) and neural encoding.- Quantitative, model-driven approaches to the full action-perception cycle of sound-localization behavior and eye-head gaze control- Comprehensive introduction to acoustics, systems analysis, computational models, and neurophysiology of the auditory system- Full account of gaze-control paradigms that probe the acoustic action-perception cycle, including multisensory integration, auditory plasticity, and hearing impaired

Chapter 1

A Brief Introduction to the Topic

Abstract

The human auditory system has evolved to extract two essential aspects of sound sources in the environment: what are they (identification), and where are they (localization)? Because the sound field may consist of a superposition of multiple sounds, auditory identification, and localization problems are essentially ill posed. As a result, unique and unambiguous solutions do not exist, as infinitely many sound–source combinations may produce identical sound fields. This chapter introduces some of the strategies that the auditory system is thought to employ in order to deal with such problems. The chapter ends with a brief overview of this book, and offers some useful suggestions to the reader, as well as to students and lecturers who intend to use this book as a psychophysics course.

Keywords

sound identification

sound localization

ill-posed problem

spectrogram

top–down selection

eye movement

audiovisual integration

Bayesian inference

1.1. Two tasks for the auditory system

Whenever a detectable sound wave reaches our ears, the brain will try to assign meaning to the acoustic event. In a split second, the auditory system succeeds in identifying the nature of the sound source out of a virtually unlimited number of possibilities: is it noise (the wind, the rain, the sea, a sigh)? Was it perhaps a familiar or an unfamiliar human voice? Was it an animal vocalization? Maybe it was a car, some other man-made machine, a musical instrument, or an orchestra? Perhaps, the sound was caused by the ticking of an object (a fork?) against another object (a dinner plate?), etc.

At the same time, the auditory system localizes the sound source. But just like source identification, the seemingly simple localization task could refer to multiple possibilities: where is the sound source located in “external space,” that is, in the world around us, through which we navigate? Or: where is the sound relative to the ears or head? Where is it relative to other landmarks in the environment? Surprisingly, as we will see later, the brain seems to be particularly interested in determining where the sound source is located relative to your eyes! Thus, the auditory system has evolved to perform the following major tasks on the acoustic input:

Auditory task	It answers
Identification	What?
Localization	Where?

Obviously, the ability to rapidly identify and localize sound sources is vital for survival. In any case, it was crucial when in a not too distant past, we as hominids, had to struggle fiercely to stay alive, as food sources (good for us) and predators (very bad for us) had to be identified and localized as fast as possible. It is therefore not surprising that throughout evolution, the auditory systems of virtually all the animal species have developed dedicated neural circuits to efficiently and accurately solve identification and localization tasks.

Although simply formulated, these tasks are in fact astonishingly difficult to perform. Current technological advances, despite the tremendous increase in computer speed and memory storage over the last decades, are still not able to execute these tasks with the same accuracy, speed, flexibility, and efficiency as biological auditory systems. Indeed, the auditory system seems to carry a bag loaded with sophisticated tricks (neural algorithms) in order to do what it is supposed to.

How do we know all this, and how do we study, understand, and model the different aspects of sound processing in human and animal brains? Can we learn something essential from this, and perhaps implement this knowledge in future sound-recognition technologies, including healthcare applications, such as improved hearing aids and implants? This monograph forms my personal account of an exciting line of research on sound localization behavior in humans and nonhuman primates, which has kept me busy for well over 20 years, and is likely to keep me busy for the next decade as well. Some of the questions raised here form the central topic of this book.

To persuade the reader that sound processing in the brain is an interesting topic, wholly worthy of study, and above all, intellectually challenging and rewarding, here I would like to briefly illustrate, in a very general way, the sound–source identification problem as it presents itself to the auditory system.

1.2. An ill-posed problem

The fundamental problem faced by audition has been particularly nicely formulated and illustrated by Albert Bregman (1990) in his “man-at-the-lake” analogy (Fig. 1.1). Imagine this guy, lying at the shore of a large lake. Although the story doesn’t tell, we may assume that the man is either deaf, or deafened by earplugs, so he can’t hear. He just dug two narrow, parallel channels that fill themselves with water from the lake, and has then draped and fastened two thin plastic sheets onto the water surface of each channel.

Figure 1.1 Our hero at the lake is allowed to only look at the movements of the two thin sheets to identify the sources that caused the water waves on the lake. (Courtesy: Carmela Espadinha.)

Then the game starts: the man is allowed to only look at the up and down movements of the two sheets. Meanwhile, the lake’s water surface is continuously perturbed by all kinds of objects (toy boats, swimming and splattering kids, dropped stones, landing ducks and geese, wind, etc.) that each cause their own specific water waves to travel at some fixed speed, from some direction, along the surface of the lake. Of course, the man doesn’t know all this, since he is not allowed to look at the lake, and he can’t hear either.

However, a small part of these traveling waves will at some moment, enter each of his two channels. The challenge for our guy is to decide, only on the basis of the motion patterns of the two sheets (which he may assume to oscillate without any energy loss with the water motion, and which he may analyze in every possible way), what exactly is happening on the lake.

Clearly, the two channels symbolize our ear canals, while the lake is the air, set in vibration by different sound sources; the two plastic sheets represent our eardrums. The “guy” in this story represents the homunculus within our auditory system that can only “look” at the one-dimensional temporal vibrations of the two eardrums to analyze acoustic input.

One doesn’t have to be very imaginative to recognize that this is a formidable, if not an unsolvable, challenge for the auditory system! Indeed, mathematics tells us that such a problem is in fact, ill posed (Kabanikhin, 2008). This means there is no unique solution to the problem as in reality there are infinitely many mathematically valid solutions! How can we appreciate the severity of this problem?

Recall the “Hitchhiker’s Guide to the Galaxy” by Douglas Adams, in which the earthlings have been told the answer (which is “42”), but have to guess the correct question in order to save Planet Earth from total destruction. Was the question: how old is your wife’s sister? How many hairs does your adolescent son have on his chest? How much is 6 × 7, or 43.5 × (42/43.5)? How many presidents had governed the USA when G.W. Bush took office in 2001? Clearly, infinitely many questions can be formulated, all with the same correct answer: “42.”

A similar mind-boggling problem bugs the auditory system. Think about it: the sound wave that reaches the ears is a linear superposition of all the so...

Table of contents

• Cover
• Title page
• Table of Contents
• Copyright
• Dedication
• List of Abbreviations
• Chapter 1: A Brief Introduction to the Topic
• Chapter 2: The Nature of Sound
• Chapter 3: Linear Systems
• Chapter 4: Nonlinear Systems
• Chapter 5: The Cochlea
• Chapter 6: The Auditory Nerve
• Chapter 7: Acoustic Localization Cues
• Chapter 8: Assessing Auditory Spatial Performance
• Chapter 9: The Gaze-Orienting System
• Chapter 10: The Midbrain Colliculus
• Chapter 11: Coordinate Transformations
• Chapter 12: Sound Localization Plasticity
• Chapter 13: Multisensory Integration
• Chapter 14: Impaired Hearing and Sound Localization
• Subject Index