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Relevant Paper Short Latency Auditory Evoked Potentials Tutorial
Short Latency
Auditory Evoked Potentials
Reference this material as: American Speech-Language-
Hearing Association. (1987). Short latency auditory evoked
potentials. Rockville, MD: Author.
Index terms: Audiology, electrocochleograpy, hearing as-
sessment, otoacoustic emissions, physiologic measures
Document type: Tutorials
Audiologic Evaluation Working Group on
Auditory Evoked Potential Measurements
Introduction
Background
The Working Group on Auditory Evoked Poten-
tial Measurements was constituted (a) to review evi-
dence and elevate the degree of consensus existing
with respect to the procedural variables and instrumen-
tation in the application of auditory sensitivity and (b)
to provide a report that is highly specific in nature and
intended to be a state-of-the-science update about meth-
odology.
In partial response to this mandate, the working
group elected to develop a basic overview or tutorial
focused on the short latency auditory evoked poten-
tials (AEPs). This class of AEPs encompasses the ar-
eas of electrocochleography (ECochG) and auditory
brainstem response (ABR) measurement. These poten-
tials represent sensory or neural responses from lower
levels of the auditory system. The term latency is used
to describe the time of occurrence of a given potential
that, for these potentials, generally falls within 10 ms
of stimulus onset. This restriction in scope was made
in view of the voluminous literature that has developed
concerning the short latency potentials. Although
rapid expansion of information continues, basic prin-
ciples can be drawn from research and clinical experi-
ence with these potentials.
Scope
Short latency AEPs are popular for the
electrophysiologic assessment of otology and neuro-
logic impairment. The stability of these potentials over
subject state, the relative ease with which they may be
recorded, and their sensitivity to dysfunctions of the
peripheral and brainstem auditory systems make them
well-suited for clinical application. However, clinical
application of AEP measurements requires an under-
standing of some procedural and subject variables.
The short latency potentials are small amplitude,
far field potentials; that is, they are recorded at some
distance from their sources. Sophisticated techniques
are needed to measure these potentials because they
are buried in a background of physical and physiologi-
cal noise. Additionally, variables such as the subjects
age, gender, and core temperature and the status of the
outer, middle, and inner ears may predictably affect
these responses. The ways in which these factors in-
fluence the measurement, analysis, and/or interpre-
tation of the short latency potentials are discussed in
this report.
The intent of this document is not to mandate a
set of standards for the measurement and evaluation
of short latency AEPs.
Rather the objective is to present
a background of information that the working group
believes to be requisite for a basic understanding of
these measures. The audiologist wishing to enter this
area of clinical study is encouraged to take appropri-
ate courses and seek supervised clinical experiences.
Additionally, several texts on this topic have appeared
that may be useful references (see Glattke, 1983; Hood
& Berlin, 1986; Jacobson, 1985; Moore, 1983).
The tutorial is divided into three major sections.
The first, InstrumentationBasic Principles, presents
instrumentation for both the stimulus generation and
the recording and analysis methods that are common
to noninvasive ECochG and ABR measurement. The
second section, Electrocochleography, details the re-
cording, stimulus, and subject variables relevant to this
topic. These sections purposefully precede the specific
treatment of the Measurement of Auditory Brainstem
Evoked Potentials (the last section) because the infor-
mation in the first two sections is basic to an under-
Relevant Paper II - 336 / 1987
ASHA 2002 Desk Reference Volume 2 Audiology
standing of the brainstem potentials. The reader is
urged strongly to read this document from beginning
to end because each section proceeds on the assump-
tion that previous sections have been read and under-
stood.
InstrumentationBasic Principles
An understanding of how evoked potentials (EPs)
are recorded and analyzed requires the grasp of cer-
tain principles of instrumentation. Some of these con-
cepts are addressed in the sections that follow.
Electrodes and Electrode Impedance
The human body is a field of ongoing electrical
activity. The sources of this activity may include muscle
contractions, sensory end organ responses, and neu-
ral events from the central and peripheral nervous
system. These electrical events are often conducted to
the bodys surface in an attenuated form and may be
recorded using appropriate methods and equipment.
However, it is difficult to measure the AEPs because
they are small in amplitude and buried in a back-
ground of electrical noise. Added to these problems are
the electrically insulative characteristics of the skin,
particularly the outermost layer, the corneum stratum
or the dead skin layer. There also is a fundamental
difference between biological and physical electricity.
In physical systems, electrical current is mediated via
electrons, whereas in biological systems it is mediated
via ions, that is, atoms/molecules with a net positive
or negative valence. Applying an electrode, a metal
conductor, to the skin constitutes a barrier over which
there can be no net charge transfer. Such an interface
opposes, or impedes, current flow. Impedance varies
with frequency: in the present context the impedance
varies inversely with frequency because the electrode-
skin interface acts like a capacitor (Geddes, 1972). For
applications discussed in this tutorial, impedance is
generally assessed at one frequency within the range
of approximately 101000 Hz.
Electrode impedance is a product of the electrode
material and surface area, the skin, muscle, or mucosa
to which it is interfaced and anything in between (e.g.,
oil, dirt, fluid, etc.). Silver, gold, and platinum have
lower impedances and half-cell potentials than most
other metals. The half-cell potential is a voltage that
results from the tendency for charge to build up on
each side of the electrode interface, much as the elec-
trode of a battery. The half-cell potential will be desta-
bilized by mechanical movement, so a large half-cell
potential makes the recording of bioelectric potentials
much more vulnerable to movement artifact. Silver is
an especially useful material for constructing elec-
trodes because it also can be plated with salt, forming
a silver-silver chloride (Ag-AgCl) electrode, which has
an even lower impedance but requires rechloriding on
a regular basis. Unlike electrodes made of silver or
other pure metals or alloys, the Ag-AgCl electrode is
reversible or nonpolarized. This means that it can be
used to record (or pass) direct current (dc) and thus
performs well at very low frequencies. Impedance is
also lowest when the electrode makes direct contact
with body fluids, even just under the skins surface.
Needle electrodes provide such contact but are not at-
tractive for routine clinical work because the skin must
be punctured.
Good electrical contact can be achieved using sur-
face electrodes. The skin must be cleansed thoroughly
to remove dirt, oil, and superficial dead skin. An elec-
trolyte gel, paste, or cream is applied to improve the
conductivity of the dead skin layer, give contact sta-
bility, and effectively increase the electrode surface
area. Numerous techniques for achieving low imped-
ances are found in texts in electroencephalography
(EEG; e.g., Binnie, Rowan, & Gutter, 1982).
Interelectrode impedances, which are the impedances
between each possible pair of electrodes, should be
measured routinely and, as a rule, should not exceed
5 kohms.
Analysis
The amplitude of surface-recorded AEPs is small
in relation to the amplitude of background electro-
physiological activity and electrical noise; therefore,
it is necessary to improve the signal-to-noise ratio
(SNR). Routine EP evaluations have become possible
primarily through the advent and availability of rela-
tively small and inexpensive digital computers that
can efficiently perform signal averaging. Computer-
ized signal averaging reduces the background noise
and the variance in the sound-elicited potential. The
recorded signal, which is a continuous function of
time, is represented as an ensemble of discrete samples
to the computer, as illustrated in Figure la. The sam-
pling of the signal is accomplished through a process
known as analog-to-digital (A-D) conversion, wherein
the amplitude of the signal at a given point in time is
translated into a binary value that can be manipulated
by the computer.
The accuracy with which a computer represents
the fine structure, and therefore frequency content, is
determined, in part, by the number of sampled points
on the waveform (see Figures la and 1b). This number
depends on the maximum sampling rate of the A-D
conversion process, which is inversely related to how
long each conversion takes. The amount of time re-
quired for the A-D converter and computer to sample
each point is called the dwell time. The sampling rate 1987 / II - 337
Relevant Paper Short Latency Auditory Evoked Potentials Tutorial
Figure 1.
(a) Digital sam