J Am Acad Audiol 21:145–152 (2010)
Electrocochleography: A Review of Recording
Approaches, Clinical Applications, and New Findings
in Adults and Children
DOI: 10.3766/jaaa.21.3.2
John A. Ferraro*
Abstract
Research related to expanding and improving the clinical use of electrocochleography (ECochG) has
been ongoing for 25 yr at the University of Kansas Medical Center. This article presents highlights of
findings from our laboratory during this period that have contributed to current ECochG recording
approaches and clinical applications. A review of new data related to improving the sensitivity of ECochG
in the diagnosis of Ménière’s disease, the use of an ear canal recording approach for improving auditory
brain stem response testing in newborns, and technical aspects related to recording the cochlear microphonic in newborns also will be presented.
Key Words: Action potential, auditory brain stem response, auditory evoked potential, auditory
neuropathy, cochlear microphonic, condensation, electrocochleography, endolymphatic hydrops,
extratympanic, Ménière’s disease, rarefaction, summating potential, transtympanic, tympanic membrane
Abbreviations: ABR 5 auditory brain stem response; AEP 5 auditory evoked potential; AN 5 auditory
neuropathy; AP 5 action potential; C 5 condensation; CM 5 cochlear microphonic; ECochG 5
electrocochleography; ELH 5 endolymphatic hydrops; ET 5 extratympanic; MD 5 Ménière’s
disease; R 5 rarefaction; SP 5 summating potential; TM 5 tympanic membrane; TT 5 transtympanic
T
he ability to record the receptor potentials of the
cochlea and the whole nerve/compound auditory
nerve action potential (AP) in humans via electrocochleography (ECochG) has led to numerous investigations in the Hearing and Speech Department’s
auditory evoked potential laboratory at the University
of Kansas Medical Center (KUMC) during the past
25 yr. Some of this research has been done in collaboration with colleagues from other universities, most notably John Durrant (University of Pittsburgh) and Roger
Ruth (University of Virginia). In general these studies
have been designed to improve the techniques and approaches for recording ECochG noninvasively and
painlessly; identifying, modifying, and expanding its
clinical applications; and seeking ways to improve both
the sensitivity and the specificity of these measurements in the diagnosis, assessment, and management
of inner ear and auditory nerve disorders.
Although it has been available to the hearing scientist since the 1930s, ECochG’s emergence as a clinical
tool was due in part to the discovery and application
of the auditory brain stem response (ABR) over 40 yr
later. Utilization of improved signal-averaging approaches for recording the miniscule and extremely
early responses of auditory centers in the brain stem
helped to direct attention back to the periphery.
Another important factor that has facilitated the clinical popularity of ECochG, at least in the United States,
is the development and refinement of noninvasive recording techniques. Ruben and his coworkers (1960),
for example, measured the AP intraoperatively from
patients undergoing middle ear surgery. A few years
later, nonsurgical techniques that involved passing a
needle electrode through the tympanic membrane
(TM) to rest on the cochlear promontory were introduced (e.g., Yoshie et al, 1967; Aran and LeBert,
1968). This transtympanic (TT) recording approach to
ECochG is still used in Europe and other countries outside the United States. Extratympanic (ET) alternatives to TT recording methods began to appear in the
early to mid-1970s. Cullen and colleagues (1972) performed recordings from the lateral surface of the TM,
while Coats (1974) used a silver ball electrode glued
to a small plastic leaf to achieve recordings from the
*Hearing and Speech Department, Intercampus Program in Communicative Disorders, and School of Allied Health, University of Kansas Medical Center
John A. Ferraro, Ph.D., Information Hearing and Speech Department, University of Kansas Medical Center, 39th and Rainbow Blvd., Kansas City,
KS 66160-7605; Phone: (913) 588-5937; E-mail: jferraro@kumc.edu
145
Journal of the American Academy of Audiology/Volume 21, Number 3, 2010
ear canal. My initial research utilizing ECochG was
based on ear canal measurements made with the Coats
Mylar leaf electrode (Ferraro et al, 1983; Ferraro et al,
1985). In subsequent studies we employed a gold foil
electrode wrapped around a compressible foam earplug
(i.e., the TIPtrodeTM), which tended to produce less discomfort for subjects than the leaf electrode (e.g., Ruth,
Lambert, et al, 1988). Since around 1990, however, virtually all of our adult studies have utilized a modified
version of the electrode described by Stypulkowski
and Staller (1987), which directed and facilitated attention back to the TM as the primary site of choice for
ECochG recordings. Figure 1 is an illustration of the
“tymptrode” we currently use for our recordings.
The purpose of this article is to provide a review of
recent research in our clinic/laboratory at the KUMC
related to the recording methods and clinical applications of ECochG. These studies include a 5 yr retrospective review of our patient database to evaluate the
sensitivity and specificity of the recording protocol we
use when ECochG is administered to help identify/rule
out Ménière’s disease/endolymphatic hydrops. Other
research utilizing an ECochG approach to record the
ABR and cochlear microphonic (CM) in newborns also
will be described. For a more complete review of current
ECochG procedures and applications that includes a
discussion of electrode and patient preparation, recording parameters, waveform interpretation, and normative values, the reader is referred to Ferraro and
Durrant (2006) and Ferraro (2007).
popular recording sites. I use the term TM ECochG for
the latter approach (Ferraro and Ferguson, 1989) even
though this procedure is still considered to be an ET
one. As my colleagues and I have reported numerous
times (e.g., Ferraro, 2000; Ferraro and Durrant,
2002, 2006), both TT and ET approaches to ECochG
have advantages and disadvantages. An advantage of
the TT approach is the proximity of the primary recording electrode to the response generators, which produces large components with relatively little signal
averaging. ET recordings require more signal averaging and are smaller in magnitude but do not require
the assistance of a physician to place the electrode or
a topical (usually) anesthetic to dull the pain of puncturing the TM. In 1994, my colleagues and I compared TT
and ET (TM) recordings from the same patients. Figure
2 illustrates our findings from this study (Ferraro et al,
1994). The ECochG tracings in the top panel were
recorded from the cochlear promontory of a patient with
suspected Ménière’s disease (MD), while the lower tracings were measured from the TM. Although the magnitude of the TT recordings is approximately four times
larger than the TM responses, both sets of waveforms
display an enlarged summating potential (SP)/AP
amplitude ratio, which was the primary diagnostic criterion we used for reporting a positive finding for MD.
In other words, the features of the waveform that were
essential to interpreting the electrocochleogram were
ECochG RECORDING METHOD
A
s indicated above, there are two, general recording
approaches for ECochG: TT and ET. The TT
method involves penetrating the TM with a needle electrode so that the tip rests on the cochlear promontory.
ET approaches utilize primary recording sites that are
peripheral to the tympanic cavity (thus the name
“extratympanic”). The skin of the ear canal and the
external surface of the TM have evolved to be the most
Figure 1. Illustration of the “tymptrode” used for recording electrocochleography from the tympanic membrane (from Ferraro,
2007, p. 401).
146
Figure 2. Abnormal electrocochleograph responses to clicks
recorded from the promontory (transtympanic [TT]) and tympanic
membrane (TM) of the affected ear of a patient with Ménière’s disease. Although the magnitudes of the TT responses are approximately 43 greater than the extratympanic recordings, both sets
of tracings display an enlarged summating potential (SP)/action
potential (AP) amplitude ratio, which is a positive finding for
Ménière’s disease. “Base” indicates the reference point for SP
and AP amplitude measurements. The amplitude scale is in microvolts; the time scale is in milliseconds. Stimulus onset was delayed
by 2 msec (from Ferraro et al, 1994, p. 27).
Electrocochleography Update/Ferraro
apparent in both TT and TM recordings. Furthermore,
Stypulkowski and Staller (1987) and Ferguson and I
(Ferraro and Ferguson, 1989) showed that ECochG
components recorded from the TM displayed magnitudes that were at least twice as large as corresponding
measurements made from the ear canal. Thus, the TM
offers a good and practical compromise between ear
canal and TT recording sites, producing components
with larger magnitudes than corresponding ET measurements while eliminating the discomfort and
medical-related requirements associated with TT approaches and, most importantly, preserving the features of the waveform essential to the diagnosis of MD.
It also should be noted that the technique of placing
the tymptrode is partially “blind” since the TM is
obscured by the electrode tip during this process. Proper
placement is verified by having the subject acknowledge
when he or she feels it touching the membrane and
monitoring of the electrical noise floor as the electrode
is inserted into the ear canal. The noise floor drops dramatically and becomes free of cyclic artifact and clipped
peaks when the tip makes proper contact with the TM.
In addition, although TM recordings can be made using
circumaural headphones, tubal earphones by comparison offer at least two important advantages: (1) the
speaker diaphragm is farther away from the ear, which
helps to reduce electromagnetic artifact in the recording, and (2) the compressible foam tip of the sound tube
helps to stabilize and hold the tymptrode in place.
ECochG IN THE DIAGNOSIS OF MÉNIÈRE’S
DISEASE/ENDOLYMPHATIC HYDROPS
A
s shown in Figure 2, and documented in the literature for over 30 yr, ECochG responses recorded
from patients with MD/endolymphatic hydrops (ELH)
often are characterized by an enlarged SP and SP/AP
amplitude ratio (e.g., Schmidt et al, 1974; Eggermont,
1976; Gibson et al, 1977; Morrison et al, 1980; Coats,
1981, 1986; Kitahara et al, 1981; Goin et al, 1982;
Kumagami et al, 1982; Ferraro et al, 1983; Ferraro
et al, 1985; Staller, 1986; Dauman et al, 1988; Ruth,
Lambert, et al, 1988; Ruth, 1990; Ferraro and
Krishnan, 1997). One rationale that has been offered
(but not verified) for this finding is that an increase
in endolymph volume creates mechanical biasing of
vibration of the organ of Corti that amplifies the SP
since at least some of its components represent nonlinearities in the transduction process. Whether the nature of this increased distortion is mechanical (Gibson
et al, 1977) and/or electrical (Durrant and Dallos, 1972;
Durrant and Gans, 1975) has not been resolved, and
other factors such as biochemical and/or vascular
changes may also be responsible (Eggermont, 1976;
Goin et al, 1982; Staller, 1986). Despite verification of
its bases, an enlarged SP/AP amplitude ratio has evolved
as the primary characteristic of an electrocochleogram
that is positive for MD.
While the specificity of the SP/AP amplitude ratio in
the diagnosis MD has been reported to be 90% or higher
(Ferraro et al, 1983; Pou et al, 1996; Murphy et al, 1997),
the sensitivity of this measurement in the general
Ménière’s population is only between 55 and 65% or less
(Gibson et al, 1977; Coats, 1981; Kitahara et al, 1981;
Kumagami et al, 1982; Campbell et al, 1992; Margolis
et al, 1995; Pou et al, 1996; Ferraro and Tibbils, 1999).
In other words, patients who display enlarged SP/AP
amplitude ratios are very likely to receive a positive diagnosis for MD, but only around half of the individuals with
MD have enlarged ratios. Fluctuations in symptoms,
deterioration of outer hair cells in more advanced stages
of the disease, differences in recording conditions and
techniques among researchers/clinicians, and other factors are possible reasons to account for this lack of sensitivity (Ferraro and Ruth, 1994). Nonetheless, there is
no question that this factor has limited the clinical usefulness of ECochG in the diagnosis of MD.
Attempts to improve the sensitivity of ECochG have
included studies designed to correlate positive findings
with symptoms at the time of testing (Ferraro et al,
1985) and manipulation of various recording parameters such as stimulus type, rate, and polarity (Durrant
and Ferraro, 1991; Levine et al, 1992; Margolis et al,
1992; Margolis et al, 1995; Orchik et al, 1998). Although
differences between MD and non-MD subjects were
reported under all of these conditions, data on whether
they led to an overall improvement of ECochG sensitivity were not reported.
Some of the above studies (Levine et al, 1992; Margolis
et al, 1992; Margolis et al, 1995; Orchik et al, 1998)
reported abnormal AP–N1 latency differences between
the electrocochleograms evoked by condensation versus
rarefaction clicks in MD patients. The rationale for this
finding was that the vibration of the cochlear partition
under hydropic conditions may be abnormally restricted
(or enhanced) in one direction over the other. As a result,
the velocity of the traveling wave (on which the AP–N1 is
dependent) will differ depending on whether the initial
deflection of the partition is in condensation or rarefaction phase. This relationship is obscured when stimuli
are presented in alternating polarity, giving way to an
SP–AP complex that appears to have a prolonged duration. Nearly 20 yr earlier, Morrison and colleagues (1980)
also observed a widened SP–AP duration in MD patients
but attributed this finding to a prolonged after-ringing of
the CM under hydropic conditions. Since this earlier
study employed alternating polarity stimuli, it may be
likely that Morrison and colleagues were observing
the same phenomenon as Levine and colleagues that
occurs when phase-disparate AP–N1 components from
two different waveforms are added together.
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Journal of the American Academy of Audiology/Volume 21, Number 3, 2010
Regardless of the rationale for a widened SP–AP complex, the above studies motivated us to begin measuring
both the amplitude and the duration of these components to possibly improve the sensitivity of ECochG
in the diagnosis of MD. We accomplished our area measurements using a special software routine from Nicolet
designed to measure the area under a curve defined by a
straight line connecting two cursors placed at selected
points on the ECochG waveform. Figure 3 from Devaiah
and colleagues (2003, p. 548) illustrates this method.
The results of our measurements yielded two studies
(Ferraro and Tibbils, 1999; Devaiah et al, 2003) that
showed a significant improvement in ECochG’s sensitivity to MD in the relatively small populations that
were studied when area values are included in the
assessment. After 5 yr of collecting data that included
area measurements we recently performed a retrospective chart review of our patient database to evaluate the
sensitivity and specificity of all of the ECochG parameters we measure from suspected MD patients (Ferraro
and Al-Momani, 2008; Al-Momani et al, 2009). In this
study, ECochG results from 178 suspected MD patients
were compared to the eventual diagnoses these individ-
uals received from their physicians. Our protocol
included recording ECochG to broadband clicks presented in alternating polarity and measuring the amplitudes and areas of the SP and AP (from which SP/AP
amplitude and area ratios are derived), the absolute latency of the AP–N1, and the amplitude of the SP to 1000
and 2000 Hz tone bursts (two-cycle rise–fall, 10-cycle
plateau). The results of this study revealed the sensitivity and specificity of our recording protocol leading to
the diagnosis of MD to be 92% and 84%, respectively.
The sensitivity value in particular is considerably higher
than previously reported and was attributable to the
inclusion of area values in our measurements, especially the SP/AP area ratio. Thus, when measuring
the area of the SP–AP complex is included in the testing
protocol, ECochG is both a highly specific and sensitive
tool for diagnosing MD.
EFFECTS OF HEARING LOSS
I
n our experience, my colleagues and I have found
that patients with sensorineural hearing loss greater
than 40–50 dB HL in the 1000–4000 Hz range are not
good candidates for ECochG. First, the relationship
between the SP and AP is altered as threshold increases
(Asai and Mori, 1989; Mori et al, 1993), which could
affect both amplitude and area values. Second, the
reduction of component amplitudes that tends to accompany increased hearing loss makes both the SP and AP
difficult to identify and separate from each other in
extratympanic recordings. Finally, when hearing loss
is greater than 60 db HL above 500 Hz both components
may be unrecordable from the TM or at least so poorly
defined as to render ECochG ineffective for diagnosing
MD/ELH. Thus, ECochG is best applied in suspected
MD/ELH patients in the earlier stages of the disorder,
when hearing (at least in the higher frequencies) may
still be normal, or in the mild to low moderate loss range,
and not “after the fact,” when the disease and its accompanying hearing loss have progressed over time.
PEDIATRIC APPLICATIONS OF ECochG
W
Figure 3. Measurement of the summating potential (SP)/action
potential (AP) area ratio. On 5 onset of SP (baseline); N1s 5 onset
of the AP–N1; N1e 5 offset of AP–N1; bl 5 next point on the waveform where amplitude returns to baseline. Shaded area of the tracing in the top panel (A) represents the total area of the SP and AP,
which is divided by the area of the AP–N1 component (shaded portion of the tracing in the lower panel [B]) to derive the SP/AP area
ratio (from Devaiah et al, 2003, p. 548).
148
e have not confined the utilization of electrocochleographic recording approaches to adult populations in our clinic/laboratory. Several years ago we
began studying the feasibility of ear canal recordings
in newborns and infants, primarily as a way to improve
the amplitude and thus detectability of ABR components measured during pediatric hearing screening/
assessment procedures. These studies were motivated
by previous investigations in adult populations showing
that such improvements can be achieved by the use of
various ear canal electrodes in combination with more
conventional ABR recording procedures (Coats, 1974;
Harder and Arlinger, 1981; Lang et al, 1981; Walter
Electrocochleography Update/Ferraro
and Blevgad, 1981; Yanz and Dodds, 1985; Durrant,
1986; Ruth, Mills, et al, 1988; Ferraro and Ferguson,
1989; Bauch and Olsen, 1990). In all of these studies,
the amplitude of wave I was substantially larger in both
normal- and hard-of-hearing subjects when the ABR
was recorded using an ear canal (vs. a mastoid or earlobe) site. Ruth, Mills, et al (1988) compared scalp and
ear canal ABR recordings in adults and demonstrated
more sensitive wave I thresholds when the secondary
electrode was seated in the ear canal. In 1989, Ferguson
and I found no differences between the thresholds of
waves I and V utilizing a forehead-to-TM electrode configuration (Ferraro and Ferguson, 1989). Furthermore,
both waves I and V were measurable in subjects with
hearing loss who did not display a wave I when the
ABR was recorded using a forehead-to-mastoid approach.
Our experiments with ear canal recordings in newborns began in 1994, when Bealer and colleagues
utilized gold foil wrapped around a pediatric sounddelivery tube to record the ABR from the ear canal in
a small sample of newborns. Comparison between forehead (1)-to-ear canal (–) and forehead (1)-to-mastoid
(–) recordings were made at 60 and 30 dB nHL, which
are the two levels most commonly used in newborn
screening protocols. Wave I was consistently larger at
both stimulus levels and identifiable more frequently
at 30 dB nHL in the ear canal recordings. Another interesting aspect of this study was that once constructed,
the ear canal electrodes were easier and took less time
to apply than the scalp electrodes. Furthermore, because preparation of the ear canal merely involved
slight cleansing of the outer portion with a cotton swab
without scrubbing the site with water/alcohol/abrasive
compound, sleeping infants were less likely to awaken
during preparation.
More recently, funding from the Hall Foundation of
Kansas City and the Deafness Research Foundation
supported two studies in our clinic/laboratory involving
ear canal recordings from newborns/infants. Gaddam
and I (2008) used a modified (i.e., shortened) TIPtrode
to compare ear canal and scalp ABRs in 45 newborns
who passed their newborn hearing screening. Figure
4 from this study is an illustration of the modified electrode in place. Figure 5 displays a summary of our findings for wave I amplitude. As can be seen from this
graph, wave I from the ear canal is almost twice as large
as its mastoid counterpart at sound levels of 80 and
60 dB nHL. Although the amplitude difference at
40 dB was not as dramatic, it was nonetheless statistically significant. At 20 dB, the differences were not significantly different between the two approaches,
primarily because many of the ABRs did not display
a wave I at this level. When present, however, ear
canal–recorded wave I’s always had larger amplitudes
than their scalp-recorded counterparts. We are currently applying the ear canal approach to ABR record-
Figure 4. Illustration of the shortened TIPtrode seated in a newborn’s ear canal. The electrode cable from the evoked potential
unit attaches to the gold foil of the TIPtrode via a micro–alligator
clip (from Gaddam and Ferraro, 2008, p. 501).
ings in our clinical pediatric populations and have found
it to be particularly helpful for identifying the I–III and
I–V interwave intervals in children with suspected
retrocochlear disorder (with or without hearing loss).
Additional research in our laboratory/clinic utilizing
an ear canal approach for pediatric auditory evoked
potential (AEP) testing has been directed at recording
the CM from this site. Riazi and I (2008) performed ear
canal CM recordings in both newborns and adults in an
effort to optimize recording parameters for this potential when it is used to help diagnose auditory neuropathy (AN). Normative data were collected from seven
full-term newborns and four adults with no known risk
factors for cochlear or retrocochlear disorders. Figure 6
displays the two-channel recordings from one of the
Figure 5. Summary of wave I amplitude differences between
high forehead (1)-to-mastoid (–) versus high forehead (1)-to-ear
canal (–) auditory brain stem response recordings in 45 normally
hearing newborns at stimulus levels of 80, 60, and 40 dB nHL.
Wave I amplitude was significantly larger in the ear canal recordings (p , .01) at all three levels. Bars indicate 11 SD (from
Gaddam and Ferraro, 2008, p. 502).
149
Journal of the American Academy of Audiology/Volume 21, Number 3, 2010
as in the diagnosis of AN). We currently are in the process of assessing the validity of this claim by comparing
ear canal versus scalp recordings of CM in children with
suspected/diagnosed AN. Our database remains too
limited at this time to report any findings.
CONCLUSIONS
E
Figure 6. Two-channel cochlear microphonic (CM) recordings
from a full-term newborn to a 1000 Hz tone burst presented at
70 dB nHL in rarefaction (R) and condensation (C) polarities.
The top four tracings (channel 1) were recorded using a high forehead (1)-to-test ear canal (TEC) (–) electrode configuration. The
lower four tracings (channel 2) utilized the test ear mastoid
(TEM) as the site for the inverting (–) electrode. The lower tracings
in each set of two (i.e., A5, B1, A6, and B2) were recorded when the
sound tube was pinched shut, indicating the presence of stimulus
electromagnetic artifact in all conditions except tracing A7 (from
Riazi and Ferraro, 2008, p. 50).
newborns in this study to a 70 dB nHL, 1000 Hz tone
burst presented in both rarefaction (R) and condensation (C) polarities. The top four tracings (channel 1)
were recorded using a high forehead (1)-to-test ear
canal (–) electrode configuration, while the site of the
inverting (–) electrode for the bottom tracings (channel
2) was the test ear mastoid. Once again, the ear canal
recordings displayed larger amplitudes than their mastoid counterparts, but an even more important finding
from this study is apparent in this figure. Namely, CM
is present only in record A7. In each set of two tracings
(R R and C C), the lower waveforms were recorded with
the sound tube pinched closed. Thus, what appears to be
CM in these tracings actually is stimulus electromagnetic artifact. To eliminate this artifact, we found it
necessary to apply strict recording and shielding/
grounding techniques that included enclosing the stimulus transducer in a Mu-metal case, grounding this
case, and using a shielded and grounded electrode
cable. In addition, separating CM from stimulus artifact was easier to do utilizing an ear canal recording site
and toneburst stimuli. Thus, our results indicated that
unless the above conditions are applied, considerable
caution must be exercised when recording and interpreting the CM in humans for clinical purposes (such
150
CochG has been a topic of considerable interest at
the University of Kansas Medical Center for over
25 yr. During this time our research in this area has
focused on ways to optimize noninvasive recording protocols, improve the sensitivity and specificity of this tool
in the diagnosis of Ménière’s disease and other otoneurological disorders, and expand its clinical applications to include pediatric populations. Among other
things, our studies have identified the TM as the optimal noninvasive ECochG recording site for adults. We
have used the TM for virtually all of our research studies and clinical recordings since 1990 and accomplish
our measurements with minimal/no discomfort to our
subjects/patients without ever damaging the membrane.
The use of SP and AP area measurements has significantly improved the sensitivity of ECochG in the
diagnosis of MD while maintaining high specificity.
Unfortunately, the software for performing these measurements is not yet commercially available. The AEP
unit we have used for several years that was equipped
with this routine (i.e., the Nicolet Spirit) has been discontinued, and we have devised our own program that
allows us to make these measurements from an ASCII
file. At the time of this writing, however, two different
manufacturers of AEP test instruments are planning to
add area-measuring software to their newer units.
Our recent research employing an electrocochleographic (i.e., ear canal) approach to AEP recordings in
children has yielded two important findings. First, newborn ABR recordings can be obtained using the ear canal
as a recording site, resulting in significantly larger wave
I components than observed in more conventional (i.e.,
scalp) recordings. Second, ear canal recordings and the
use of toneburst stimuli facilitate the measurement of
the CM in newborns and young children when this component is of interest (such as in the diagnosis of AN).
However, strict grounding/shielding approaches must
be applied to identify CM from stimulus artifact under
these conditions. Without such precautions, it is very
likely that stimulus artifact will be mistaken for CM,
leading to false-positive diagnoses.
Acknowledgments. I extend my grateful appreciation to
the myriad students who have completed research projects,
master’s theses, and doctoral dissertations in my laboratory
over the past 25 yr. Their work has led to important findings
regarding the recording and clinical applications of electrocochleography (ECochG). Recent funding from the Hall
Electrocochleography Update/Ferraro
Foundation of Kansas City and the Deafness Research Foundation has supported our research related to the pediatric
applications of ECochG. I dedicate this essay to the memory
of Dr. Roger Ruth, a former student, close friend, and collaborator who recently passed away.
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