31: The Auditory Central Nervous System

Introduction

• Multiple Types of Information Are Present in Sounds

• The Neural Representation of Sound Begins in the Cochlear Nuclei

  • The Cochlear Nerve Imposes a Tonotopic Organization on the Cochlear Nuclei and Distributes Acoustic Information into Parallel Pathways

  • The Ventral Cochlear Nucleus Extracts Information About the Temporal and Spectral Structure of Sounds

  • The Dorsal Cochlear Nucleus Integrates Acoustic with Somatosensory Information in Making Use of Spectral Cues for Localizing Sounds

• The Superior Olivary Complex of Mammals Contains Separate Circuits for Detecting Interaural Time and Intensity Differences

  • The Medial Superior Olive Generates a Map of Interaural Time Differences

  • The Lateral Superior Olive Detects Interaural Intensity Differences

• Efferent Signals from the Superior Olivary Complex Provide Feedback to the Cochlea

  • Brain Stem Pathways Converge in the Inferior Colliculus

  • Sound Location Information from the Inferior Colliculus Creates a Spatial Map of Sound in the Superior Colliculus

  • Midbrain Sound-Localization Pathways Are Sensitive to Experience in Early Life

• The Inferior Colliculus Transmits Auditory Information to the Cerebral Cortex

  • The Auditory Cortex Maps Numerous Aspects of Sound

  • Auditory Information Is Processed in Multiple Cortical Areas

  • Insectivorous Bats Have Cortical Areas Specialized for Behaviorally Relevant Features of Sound

  • A Second Sound-Localization Pathway from the Inferior Colliculus Involves the Cerebral Cortex in Gaze Control

  • Auditory Circuits in the Cerebral Cortex Are Segregated into Separate Processing Streams

  • The Cerebral Cortex Modulates Processing in Subcortical Auditory Areas

• Hearing Is Crucial for Vocal Learning and Production in Both Humans and Songbirds

  • Normal Vocal Behavior Cannot Be Learned in Isolation

  • Vocal Learning Is Optimal During a Sensitive Period

  • Both Humans and Songbirds Possess Specialized Neural Networks for Vocalization

  • Songbirds Have Feature Detectors for Learned Vocalizations

• An Overall View

Because of its role in the understanding and production of speech, auditory perception is one of the most important sensory modalities in humans. In most animals hearing is crucial for localizing and identifying sounds; for some species, hearing additionally guides the learning of vocal behavior.

Once sounds have been transformed into electrical responses in the cochlea, a rich hierarchy of auditory circuits analyzes and processes these signals to give rise to auditory perception. The auditory system differs from most other sensory systems in that the location of stimuli in space is not conveyed by the spatial arrangement of the afferent pathways. Instead, the localization and identification of sounds is constructed from patterns of frequencies mapped at the two ears as well as from their relative intensity and timing. The auditory system is also notable for its temporal sensitivity; time differences as small as 10 μs can be detected. Auditory pathways resemble other sensory systems, however, in that different features of acoustic information are processed in discrete circuits that eventually converge to form complex representations of sound.

In addition to studies of primates and mammals such as cats and rodents, research on animals with especially acute or specialized auditory capacities—frogs, bats, barn owls, and songbirds—has provided a wealth of information ...