BAEPs are responses of the auditory
nerve, brainstem, and, perhaps, higher
subcortical structures to acoustic
Most of its components appear to arise
from multiple sources, preventing a
simple one-to-one correspondence
between potential generators and
individual BAEP waves.
The term brainstem auditory evoked potentials,” is
somewhat inappropriate in that
(1) the first component of the “brainstem
auditory evoked potentials” does not arise in the
brainstem but in the auditory nerve; and
(2) the latest components may or may not
originate, at least in part, above the brainstem.
• Although the pathways is mainly along the
brainstem and some of the potentials are
assumed to be generated from it, so called BAEP.
The Auditory system
• In order to know about the potentials of
BAEPs one must aware about the
anatomy and physiology of auditory
system which is mainly about the
following two components.
• The Peripheral Auditory System (The
• The Central Auditory System (The Brain)
1. The Peripheral Auditory System (The
The components are,
Outer Ear: The folds of cartilage surrounding the ear canal
are called the pinna, which amplifies the sounds .
Middle Ear: The sound wave information travels across
the air-filled middle ear cavity via a series of delicate
• Malleus (Hammer), Incus (Anvil) and Stapes (Stirrup)
which are mainly responsible to convert the lowerpressure eardrum sound vibrations into higher-pressure
sound vibrations at another, smaller membrane called
the oval (or elliptical) window.
The inner ear consists of the cochlea and several non-auditory structures,
including Organ of Corti, which is located at the scala media (cochlear duct )
and transforms mechanical waves to electric signals in neurons.
Hair cell :
Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at
the top, for which they are named. There are two types of hair cells. Inner hair
cells are the mechanoreceptors for hearing: they transduce the vibration of
sound into electrical activity in nerve fibers, which is transmitted to the brain.
Outer hair cells are a motor structure. Sound energy causes changes in the
shape of these cells, which serves to amplify sound vibrations in a frequency
• Neurons (Hair cell neural connection)
Afferent neurons innervate cochlear inner hair cells, at synapses where the
neurotransmitter glutamate communicates signals from the hair cells to the
dendrites of the primary auditory neurons.
Types of AEPs
Short latency AEP
Middle latency AEP
Long latency AEP
The short latency AEP include peak of up to 10 msec
and amplitude of about 0.2uv, they are generated in
• The middle latency AEP have several variable peaks
with latency of 10-50 msec and with amplitude of about
1 uv, they probably reflect early cortical excitation.
• The long latency AEPs beginning after 50sec and having
peak of 1-10uv, represent later cortical excitation.
The Central Auditory System (The Brain)
• The sound information, re-encoded, travels down the
vestibulo-cochlear nerve, through intermediate
stations such as the cochlear nuclei and superior
olivary complex of the brainstem and the inferior
colliculus of the midbrain, being further processed at
each waypoint. The information eventually reaches
the thalamus, and from there it is relayed to the
cortex. In the human brain, the primary auditory
cortex is located in the temporal lobe. Associated
anatomical structures include
• Cochlear nucleus
• Trapezoid body
• Superior olivary complex
The cochlear nucleus is the first site of the neuronal processing
of the newly converted “digital” data from the inner ear. This
region is anatomically and physiologically split into two regions,
the dorsal cochlear nucleus (DCN), and ventral cochlear nucleus
The Trapezoid body is a bundle of decussating fibers in the
ventral pons that carry information used for binaural
computations in the brainstem.
Superior olivary complex
The superior olivary complex is located in the pons, and
receives projections predominantly from the ventral cochlear
nucleus, although the posterior cochlear nucleus projects there
as well, via the ventral acoustic stria.
The lateral lemniscus is a tract of axons in the brainstem
that carries information about sound from the cochlear
nucleus to various brainstem nuclei and ultimately the
contralateral inferior colliculus of the midbrain.
• Inferior colliculi
• The Inferior colliculi (IC) are located just below the visual
processing centers known as the superior colliculi. The
central nucleus of the IC is a nearly obligatory relay in the
ascending auditory system, & most likely acts to integrate
information (specifically regarding sound source
localization from the superior olivary complex & dorsal
cochlear nucleus) before sending it to the thalamus and
Medial geniculate nucleus
• The medial geniculate nucleus is part of the thalamic relay
• Primary auditory cortex
• The primary auditory cortex is the first region of cerebral
cortex to receive auditory input.
• Perception of sound is associated with the left posterior
superior temporal gyrus (STG). The superior temporal gyrus
contains several important structures of the brain,
including Brodmann areas 41 and 42, marking the location
of the primary auditory cortex, the cortical region
responsible for the sensation of basic characteristics of
sound such as pitch and rhythm
GENERATORS OF WAVEFORMS FROM 1-VII
Site of Neural Generator (peripheral portion of cranial nerve VIII)
Cochlear nerve (distal end)
Cochlear nerve ( proximal end)
Superior Olivary Complex/Nucleus
VI & VII
Presumed to be generated by the medial geniculate body and the
thalamocortical pathways respectively
STANDARDS AND GUIDELINES FOR/OF BAEP
ACCORDING TO ACNS:
• The ACNS guidelines for the standard parameters settings and
techniques for the Brain stem Auditory-Evoked Potentials (BAEPS)
are as followings.
• I. Stimulus
• It is recommended that “broad-band” clicks, the acoustic energy of
which is spread over a wide range of audio frequencies, be used for
the neurologic applications of auditory evoked potentials. These
clicks should be generated by driving with a 100usec rectangular
pulse (single monophasic square wave), a standard audiometric
earspeaker having a relative flat frequency spectrum.
• Many other types of acoustic stimuli are used for eliciting BAEPs,
such as tone bursts, tone pips, filtered clicks, single-cycle clicks, etc.
II. Stimulus Polarity
The polarity of the first and most prominent wave of the acoustic waveform of
the click determines whether a negative or positive pressure is applied in front
of the ear speaker diaphragm. Those clicks in which the first and major
acoustic wave applies negative pressure in front of the ear-speaker diaphragm
are referred to as rarefaction clicks.
Those clicks in which the first and most prominent acoustic wave applies a
positive pressure in front of the ear-speaker diaphragm are referred to as
In certain pathologic conditions associated with severe, steep high-frequency
hearing loss, BAEPs elicited by rarefaction clicks may differ in latency and, to a
degree, in morphology from BAEPs evoked by condensation clicks. In these
circumstances, using clicks of alternating polarity results in poorer resolution
of the response than using either rarefaction or condensation clicks alone. This
problem is obviated by using rarefaction only, condensation only, or separate
rarefaction and condensation clicks.
III. Stimulus Rate
• Stimulus rates employed vary widely from 5 to 200/s. depending on
test applications. Waves I,II, VI, and VII are particularly reduced in
amplitude at rates higher than 10/s. Thus, stimulus rates of 8-10/s
are especially suited to resolve these peaks.
• IV. Stimulus Intensity
• It is recommended that click intensity be acoustically calibrated in
“decibels peak-equivalent sound pressure level” (dB pe SPL).
Stimulus intensities employed generally range between 40 and 120
dB pe SPL.( sound pressure level). Intensity of stimulus should
always be well defined to ensure :
Maximum release of neurotransmitters from the hair cells, so the
nerve fires promptly.
- Less intensity will result in increase in
Wave I latency.
V. Monaural Versus Binaural Stimulation
Click should be delivered monaurally, i.e., to one
ear at a time.
It isAlthough not necessary in every situation, it is
recommended that contralateral masking be
included in the routine test protocol to avoid its
inadvertent omission when it is required.
recommended that the contralateral
(nonstimulated) ear be masked by white noise at 60
dB SPL to eliminate “crossover” responses, i.e.,
bone-conducted responses originating in this ear.
• The recommended system bandpass for BAEP recording is
10-30 to 2,500-3,000 Hz with a filter rolloff not exceeding
12 dB/octave for the low frequencies and 24 dB/octave for
the high frequencies. Whenever this test is performed in
the presence of irreducible EMG and mechanical artifacts,
the low-frequency cutoff may be raised to 100-200 Hz.
• VII. Stimulus Artifact
• The use of properly electrostatically and
electromagnetically shielded stimulus delivery systems is
suggested to attenuate or eliminate the stimulus artifact,
especially when using rarefaction-only or condensationonly clicks.
VIII. Analysis Time
• An analysis time of 10-15 ms from stimulus onset is
suggested. An analysis time of no less than 15 ms is
sometimes required to demonstrate extremely delayed
responses in certain pathologic conditions. Analysis times
of 15 ms are also essential for neonatal and
• IX. Filters Setting
• Low frequency filter setting is 10-30Hz but may be
increased to 100Hz.BAEP consist of multiple high
frequency components reaching a frequency close to
1000Hz.Thus the high frequency filter should not be less
X. Number of Trials to be Averaged
• It is suggested that about 1,000-4,000 individual trials be averaged
until good waveform resolution has been achieved. Two or more
responses must be obtained and superimposed to demonstrate
replicability or lack of replicability of their components.
• XI. Electrode Placement
• It is recommended that recording electrodes be placed as follows:
(1) on the scalp at the vertex (Cz position of the 10-20 International
System of EEG electrode placement) and
• (2) over the left and right earlobes (auricular) A1 and A2 positions of
the 10-20 System) or the left and right mastoid processes (M1 and
• The ground electrode may be placed anywhere on the body. For
convenience, it is recommended that it be placed on the head, for
instance, on the scalp in a midline frontal location (position Fz of the
10-20 System). Electrode impedances must be < 5 KOhms.
• A montage consisting of the following derivations is suggested for
• Channel 1: Vertex-ipsilateral earlobe or mastoid (Cz-Ai or Mi)
• Channel 2: Vertex-contralateral earlobe or mastoid (Cz-Ac or Mc)
• XIII. State of Consciousness
• BAEPs can be obtained during either wakefulness or sleep. Sedation
may occasionally be indicated with very young or tense patients, but
now requires special provisions in most facilities. In recording
patients who are comatose or are undergoing surgery, consideration
must be given to the fact that hypothermia may produce BAEP
alterations indistinguishable from those caused by structural lesions
of the auditory pathways
• XIV. Analysis of Results
• Records are analyzed primarily for the presence of waves I, III, and V.
Technical modifications to improve waveforms identification
1. If Stimulus artifact is too large and obscures wave I.
Decrease the impedance of recording and ground electrodes
Adjust the location of input cables and stimulus cables separate them
Decrease the stimulus intensity
Replace the ear phone
Change the polarity
2. If Wave I is not identified:
Increase stimulus intensity
Change click polarity
Decrease stimulus rate
Use ear canal electrodes
3. If Wave V is difficult to distinguish from wave IV
Decrease stimulus intensity
Use contra lateral ear reference recording
4.Wave V is difficult to differentiate from wave IV or VI:
Decrease stimulus intensity. When stimulus intensity is progressively decreased , wave V
is the last wave to remain.
Not all normal recording contain all BAEP peaks.
Wave V present most often.
Wave I and III can usually also be identified.
Wave II is often absent and wave IV may merge more of less completely with wave V.
• Wave I may be enhanced by increasing the stimulus intensity and decreasing the
• Recording a BAEP to condensation clicks in addition to to the BAEP rarefaction
clicks may help to distinguish wave I from mechanical and electrical stimulus
• Wave II, although often absent in normal subjects, may be of clinical significance if
it is show prolong absolute latency of IPL I-II.
• Wave III may be normally splits into two peaks, its latency is then measured to the
first peak or to the middle between the two peaks. splitting may disappear if the
condensation clicks are used instead of rarefaction clicks and vice versa.
• Wave IV normally fuse with wave V.
• Wave V is the most realible peak. It may be identified by its low threashold, its
persistance during repetative stimulation up to 100/sec and by large negativity
that commonly follow it.
Application of BEAPS in Specific
BEAPs are widely used for evaluation of
• Acoustic neuromas
• Degenerative diseases
• Brain tumor and stroke
• Multiple sclerosis
• Reversibility of comas
• Hearing assessment in children
Why Wave V
is used for Interpretation
• Wave V is used as the indicator in HTT because
1. lowest threshold for stimulation
2.highest amplitude of BAEP waves
3.Actual & consistent later wave in all subjects
which indicates the integrity of the pathway
(peripheral to central).
Analysis of Results
Measurements must include the following:
(1) wave I peak latency;
(2) wave III peak latency;
(3) wave V peak latency;
(4) I-III interpeak interval;
(5) III-V interpeak interval;
(6) I-V interpeak interval;
(7) wave I amplitude;
(8) wave V amplitude; and
(9) wave IV-V/I amplitude ratio.
Criteria for Clinically Significant
• Abnormal BAEP measures do not necessarily imply
altered retrocochlear function. At present, criteria
for retrocochlear dysfunction include the following.
• 1. Absence of all BAEP waves I through V.
unexplained by extreme hearing loss determined by
formal audiometric testing.
• 2. Absence of all waves following waves I, II, or III.
• 3. Abnormal prolongation of I-III, III-V. and I-V
interpeak intervals. I-III or III-V intervals can
sometimes be abnormally prolonged even in the face
of a normal I-V interval
Minimal Test Protocol
• It is recommended that, for neurologic
applications, minimal BAEP testing should
consist of responses to rarefaction,
condensation, or summated separate
rarefaction and condensation clicks delivered
monaurally at intensities of 90—120 dB pe
SPL, preferably 115 or 120 dB pe SPL and at
rates preferably below 25/s. The contralateral
ear should be masked by white noise at 60 dB
Recording at High Stimulus Rates
• Recording BAEPs at stimulus rates of 50—70/s
facilitates the rapid identification of wave V in
screening studies of neonates and infants as well as
• 4. Abnormal diminution of the IV-V/I amplitude ratio,
especially when accompanied by other abnormalities.
• 5. Abnormally increased differences between the two
ears (interaural differences) as regards the I-III, III-V,
and I-V interpeak intervals, when not explained by
unilateral or asymmetric middle and/or ear
dysfunction determined by appropriate audiometric
Common Abnormal Finding
• Normal absolute latency of wave I but delayed
absolute latencies of corresponding waveform
• Prolonged interpeak latencies
• Prolonged absolute latency of wave I with
prolonged corresponding absolute latencies of
the remaining waveform but normal interpeak