1 Introduction to Auditory Evoked Potentials

Samuel R. Atcherson and Tina M. Stoody

1.1 Introduction

The study of auditory evoked potentials (AEPs) has a long and rich history, and both scientific and clinical exploration of the auditory system will continue in the years to come. With some basic knowledge and clinical training, the recording of AEPs can be relatively straightforward, and the goal of this text is to provide fast clinical information about a variety of AEPs commonly used in clinics, as well as those that have strong potential for future clinical use. Therefore, the purpose of this chapter is to provide an orientation to this textbook, an overview and classification of AEPs, some history, and a discussion of clinical competencies. These topics should help set the stage for the entire textbook. Because of obvious limitations of space and the underlying goal of this text, not all topics are reviewed. Clinicians desiring more detail or assistance with protocol design and development must consult other excellent texts and are highly encouraged to do so. Much information is presented in this textbook, with the information divided into easily digestible sections.

1.2 What’s in this Text?

For clinicians new to AEPs (and this second edition of the book), Section I provides a fast summary of information related to instrumentation, stimulus and recording parameters, and neuroanatomy and neurophysiology related to AEPs. It is assumed that the reader will have some prior background knowledge in anatomy and physiology, hearing science, psychoacoustics, and basic diagnostics.

Section II is a compendium of literature reviews of major classes (or types) of AEPs. Readers are encouraged to explore these chapters to better understand the description of various AEPs, their neurologic or myogenic origin, and some of their recording and analysis considerations. Clinicians requiring a crash course may skip this section for now and concentrate on Section III.

Section III gets the reader right in the clinical trenches with specific information about current clinical protocols, equipment setup, patient-related factors, common clinical interpretations, and case studies, which should help both new and returning clinicians. The AEPs that have made it into Section III represent those that appear to be in regular clinical use at the time of this writing. AEPs such as the frequency-following response (FFR) and the P1/N1/P2 complex and mismatch negativity (MMN) of the event-related potentials (ERPs) have strong possibilities for clinical application; however, they are largely still in the realm of research for the study of the central auditory nervous system (CANS) with respect to spectrotemporal processing, maturation, and aging.

Section IV contains information that would not quite align well with the themes of the other sections or chapters but is considered clinically useful and practical “how to” applications and concepts relevant to AEPs.

1.3 Overview of Auditory Evoked Potentials

From a simplistic point of view, AEPs are electrophysiologic responses arising from one or more sources within the peripheral and/or central auditory system structures in response to the presentation of acoustic stimuli. These responses, recorded from specific sites on the scalp, on the ears, or within the ears, appear as waveforms with both positive and negative voltage deflections at successive time points after the presentation of stimuli. These deflections are often referred to as waves, peaks, or components. The height or depth of a peak provides amplitude information, whereas the time of appearance of a peak provides latency information. One or both of these general measures may be of interest to the clinician. Other AEPs, however, are analyzed in the frequency domain, particularly if the AEP has the ability to follow or phase-lock to a repetitive pattern in a stimulus.

In a normal functioning brain, there is spontaneous neurophysiologic activity that is present whether or not there is direct external stimulation; this is apparent on an electroencephalogram (EEG). The EEG measures brain activity; it can depict one of several states of alertness, such as a drowsy brain oscillating around 10 Hz, a sleeping brain oscillating around 3 Hz, or an alert brain oscillating at or above 20 Hz. When recording from electrodes placed on the scalp, the EEG activity is recorded along with the AEPs. Compared with typical AEPs, EEGs are on the order of several times larger in amplitude. As a result, one cannot actually “see” the AEPs unless the EEG is submitted to a signal averaging process made possible through the use of digital signal processing (DSP). Through the signal averaging process, the otherwise quasi-random EEG signal begins to cancel itself out, whereas the AEP “sums” together. Although signal averaging is clearly an important piece for the recording of AEPs, several other technical considerations must be followed as well, such as filtering and amplification.

Another signal that can be recorded is large-amplitude myogenic (muscle) activity that can occur with or without direct external stimulation. Myogenic activity is often unwanted because it can contaminate AEP recordings. Blinking of the eyes, jaw clenching, swallowing, and tension in the neck are some of the most common forms of unwanted myogenic activity, but they can often be reduced or eliminated with proper electrode placement and/or instruction to the patient. Other unwanted myogenic activity may in fact be an auditory-sensitive response to loud acoustic stimulation, such as that seen in the postauricular muscle (PAM) behind the ear or the temporalis muscle just above the ear. Over the past couple of decades, research and clinical interest in the vestibular evoked myogenic potential (VEMP) has grown substantially. The VEMP test assesses the functions of the saccule and utricle and corresponding reflexes, which have been shown to be abnormal in a variety of vestibular and some cochlear disorders. In this case, the vestibular response is of interest and not a hearing-related response. We include the VEMP in this text because it is evoked by auditory stimuli, and its assessment is well within the scope of practice of audiologists.

As we live in the world of electricity, there will always be subtle electromagnetic signals that can also be picked up in AEP or VEMP recordings. One common example is the 60-Hz line noise (50 Hz in some countries) of electrical outlets in the wall or from light sources in the room. Through signal averaging, good patient control, and firm protocols, quality recordings can commence with the primary goal of reducing or eliminating background noise and capturing the desired, small-amplitude AEP. In effect, there would be an increase in the signal-to-noise ratio (SNR).

1.4 Classification of Auditory Evoked Potentials

AEPS can be classified in several different ways. Usually, classifications are handled by latency, anatomical site of generation, and relationship to the stimulus ( ▶ Table 1.1  ). The most popular means of AEP classification is by latency, which includes short latency potentials (< 15 milliseconds), middle latency potentials (15–80 milliseconds), and late, long, or slow latency potentials (> 80 milliseconds). Another classification of AEPs is whether they are exogenous, endogenous, or both. Exogenous potentials are obligatory, sensory evoked potentials largely elicited and subsequently affected by the physical parameters of the stimulus, ▶ [1] such as stimulus intensity, rise time, frequency, and duration. These potentials generally comprise all of the AEPs up to the late or long latency AEPs (also called cortical AEPs). The auditory brainstem response (ABR) is a classic example of an exogenous potential for a resting or sedated patient.