inferior colliculus. ICC central nucleus of the inferior colliculus. IE inhibitory ...... Over 50 years ago Jeffress (1948) published a short seminal paper that proposed ...
4 Neural Mechanisms of Encoding Binaural Localization Cues in the Auditory Brainstem Tom C.T. YIN
1. Introduction: the Importance of Sound Localization When an animal hears a sound in its environment, there are several important tasks that the auditory system must try to accomplish. Two major jobs are to determine what it was that produced the sound and where it comes from. Understanding how the nervous system can accomplish these tasks is a major goal of modern auditory neurobiological research. In this book, we explore what is known about these questions at several different levels of the auditory system. The purpose of this chapter is to review the anatomical and physiological mechanisms in the auditory brainstem of mammals that encode where a sound originates. Specifically, this chapter examines the two binaural localization cues: interaural time disparities (ITDs) and interaural level disparities (ILDs) (For abbreviations, see Table 1). The neural mechanisms of sound localization are of particular interest since the location of a stimulus is not represented in the sensory epithelium, as it is in the visual or somatosensory systems, but must be computed by combining input from the two ears in the central auditory system. To a large degree, we understand how these cues are encoded by single cells at this level of the auditory system. Indeed, it appears that certain cells in the auditory brainstem are highly specialized to facilitate the encoding of these cues, and more is known about the central processing of sound localization cues than of any other auditory function (e.g., pitch perception, vowel discrimination). It is easy to appreciate the importance to an animal of identifying the location of a sound source. For example, imagine hearing a snapping twig in the middle of the woods on a dark night. The ability to ascertain whether the sound originated from the left or right side may determine whether the animal survives predation or starvation. For Homo sapiens living in modern society with few predators outside of our own species, sound localization is perhaps not as critical, except for occasions like an American stepping off the curb on a busy London thoroughfare and hearing the sudden blaring horn of a rapidly oncoming cab from an unexpected direction. 99
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TABLE 4.1. AM AVCN CD CF CP DNLL EE El EM EPSP GABA GBC HRP IC ICC IE ILD IPD ITD IPSP LNTB LSO MNTB MSO NMDA PL PLn PST SBC SOC SPL VNLL VNTB
Abbreviations amplitude modulated anteroventral cochlear nucleus characteristic delay characteristic frequency characteristic phase dorsal nucleus of the lateral lemniscus excitatory (contra)/excitatory (ipsi) excitatory (contra)/inhibitory (ipsi) electron micrograph excitatory post-synaptic potential gamma aminobutyric acid globular bushy cell horseradish peroxidase inferior colliculus central nucleus of the inferior colliculus inhibitory (contra)/excitatory (ipsi) interaural level difference interaural phase difference interaural time difference inhibitory post-synaptic potential lateral nucleus of the trapezoid body lateral superior olive medial nucleus of the trapezoid body medial superior olive N-methyl-d-aspartate primary-like primary-like-with-notch post-stimulus time spherical bushy cell superior olivary complex sound pressure level ventral nucleus of the lateral lemniscus ventral nucleus of the trapezoid body
Despite the relatively minor importance of sound localization, per se, for the survival of humans, the neural circuitry that has evolved for localization is nonetheless significant for other tasks facing modern human beings. It is likely that the ability to detect and discriminate sounds in a noisy environment, the so-called cocktail party problem, also relies heavily on the same neural circuits. Cherry (1953) noted that one of the major factors in solving the cocktail party problem was the spatial separation of the sound sources. Related observations were made a few years earlier by Licklider (1948) and Hirsch (1948) that detection of a signal in a background of noise is much easier when the signal has a different interaural time difference than that of the noise, e.g., if the signal is inverted in phase to the two ears while the noise masker is in phase. Since interaural time differences are related to the spatial cues for azimuthal localization, this binaural masking level difference appears to be associated with the cocktail party problem.
4. Neural Mechanisms of Encoding Binaural Localization Cues
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1. 1 Acoustic Cues for Sound Localization The classical "duplex theory of sound localization" was first formalized by Thompson (1882) and Rayleigh (1907). According to this theory, the primary cues for sound localization are interaural level differences (ILDs) and interaural time differences (ITDs; for abbreviations see Table 4.1). Rayleigh recognized that substantial ILDs will be generated only at high frequencies (in humans, above 2-3 kHz) where the wavelength is short and the head can act as an effective acoustic shadow. To demonstrate the ability to encode ITDs, Rayleigh repeated an experiment by Thompson (1882) using the perception of what is now called "binaural beats," where two tones of slightly different frequencies (
