Brain Stem

Auditory Brain Stem: Overview

The auditory brain stem serves as a primary link in the path in which signals from the peripheral auditory system (the structures of the ear, from the outer ear—the pinna—up through the inner ear—the cochlea) travel to the cortex. Important structures in the auditory brain stem include the inferior colliculus and superior olivary complex. The inferior colliculus (IC) receives inputs from a number of other neural structures, notably, the cochlear nuclei. The IC is not merely a passive, afferent transmitter of signals to the cortex, however; it is responsible for a significant amount of signal integration in both the frequency and time domains, and receives somatosensory signals as well as more purely auditory ones. Thus, the IC is centrally involved in many sensory processes. In addition to receiving, processing, and transferring neural information upstream to the cortex, the IC receives efferent neural signals back from cortical regions. The superior olivary complex (SOC) receives its primary inputs from the cochlear nuclei and is important in processing binaural information. The SOC is crucial for processing of interaural time differences, at resolutions on the order of microseconds, and also interaural amplitude differences. These two acoustic features are some of the most important for perceptual lateralization of sound, a key aspect of sound localization; the IC is also involved in sound lateralization.

Methods of Investigating Brain Stem Function

Brain stem investigations use different methods for different species. For nonhuman studies, intercellular recordings may be used; however, with human subjects, scalp electrodes are used to noninvasively record the auditory brain stem response, or ABR. The ABR can be recorded easily in passive listening conditions (even while subjects are asleep); a common practice is to present the auditory stimuli of interest while participants are watching a subtitled film with the audio turned off. Although a cortical montage based on the 10–20 system may be used, the ABR can be recorded with only a few electrodes, and optimal recording parameters for ABR and cortical signals are quite different. One common question that emerges in discussions of ABR recordings is determining the source of the signal to be analyzed (cortical versus subcortical). Neurons in the brain stem phase lock to acoustic signals in a distinctive manner, allowing differentiation of brain stem and cortical responses.

The signals recorded from the ABR can be related to the acoustic signals that stimulated [Page 151]them with a high degree of fidelity (although there is significant high frequency roll-off). This means that the ABR can be compared with the originating acoustic signal in both time and frequency domains to examine the way in which the neural system transduces acoustic inputs. A number of types of acoustic signals have been studied with respect to the ABR. The first is a simple impulse sound, or “click.” ABRs to click signals have been used for decades and are commonly used in clinical settings for assessment of hearing disorders, particularly in infants and children; pure tones may also be used for these purposes. More complex stimuli can also be used, including speech sounds, whether synthesized (for example, a Klatt-synthesized syllable such as “dah”) or natural (such as spoken phrases or a baby's cry), and musical sounds such as instrumental tones, melodies, or chords.

...