J Am Acad Audiol 2 : 105-114 (1991)
Reliability of Evoked Responses to
High-Frequency (8-14 kHz) Tone Bursts
Stephen A. Fausti,* B. Z. Rappaport,t Richard H. Frey,* James A. Henry,t David S. Phillips,*
Curtin R. Mitchell,§ and Deanna J. Olson*
Abstract
Instrumentation to evaluate the auditory brainstem response to high-frequency ( 8-14 kHz)
tone bursts has been developed in the Auditory Research Laboratory, Portland, Oregon VA
Medical Center . This system is intended to monitor the audition of patients receiving ototoxic
drugs who are unresponsive to behavioral test procedures . The reliability of responses obtained with the high-frequency tone-burst system was studied in 30 normal ears . Intrasubject
variability of intersession data from response waves I, III, and V to tone bursts of frequencies
8, 10, 12, and 14 kHz was not significantly different from click response variability. The
results of this study demonstrate the reliability of the ABR to these high-frequency tone-burst
stimuli. This technique may provide early identification of hearing loss in unresponsive subjects receiving treatment with potentially ototoxic agents, thus allowing alternative treatments
to minimize or prevent communicative handicap .
Key Words: Auditory brainstem response, high-frequency tone-burst, response reliability,
early ototoxicity identification
atients administered aminoglycoside
antibiotics and cisplatinum are at risk
for developing hearing loss due to
ototoxicity . It has been documented that such
ototoxic loss is initially observed at the highest
conventional frequencies tested, eventually
progressing into lower frequencies (Fee, 1980).
There is no generally accepted definition of
what constitutes hearing loss due to ototoxicity .
The following operational definition has been
adopted by this laboratory : loss of 20 dB or
greater at any single frequency ; loss of 10 dB
or more at any two consecutively tested frequencies ; or no response at three consecutively
tested frequencies where responses were previously recorded . Using behavioral high-fre-
P
`Director of Auditory Research, Chief of Audiology,
Portland Veterans Affairs Medical Center (PVAMC) and
Associate Professor, Oregon Health Sciences University,
Portland, Oregon
tChief, A/SP, New Mexico Regional Federal Medical
Center, Albuquerque, New Mexico
$PVAMC Auditory Research Lab, Portland, Oregon
§Oregon Hearing Research Center, Portland, Oregon
Reprint requests : Stephen A . Fausti, Portland VA
Medical Center (151J), P .O . Box 1034, Portland, OR
97207
quency pure-tone audiometry (>_ 8 kHz), we
have observed ototoxic loss at a rate of approximately 20 percent in a veteran subject
sample being administered aminoglycosides
(Fausti et al, 1984b), and 60 percent in a sample
receiving cisplatinum treatment (Fausti et al,
1984c) . The reliability and validity of the behavioral monitoring technique, however,
depend on the active cooperation and attention
of the patient. Based on a survey of hospitalized
patients at PVAMC, we estimate that 30 to 35
percent of this population are too ill to provide
valid behavioral responses . These unresponsive
patients are highly susceptible to significant
hearing loss because they may be unaware of,
or unable to report, ototoxic symptoms .
A technique that does not rely on the
patient's active participation would be useful in
monitoring auditory effects produced by potentially ototoxic agents in unresponsive individuals. The Auditory Brainstem Response
(ABR) is one such objective technique. The ABR
is a neural, electrophysiologic response that
reflects, in part, cochlear function . Asubject with
substantially decreased hearing should reveal
some differences of ABR threshold, waveform
morphology, and/or latency with respect to nor-
Journal of the American Academy of Audiology/Volume 2, Number 2, April 1991
mal-hearing individuals . Thus, it is logical to expect that drug-induced changes in hearing can
be detected with the ABR technique.
The most common stimulus used to elicit the
ABR is the unfiltered click. The spectral properties of a click, although broad-band, are shaped
by the resonant properties of the earphone and
its coupling to the ear, with typical concentration
in the 2000 to 4000 Hz region (Mitchell et al,
1989). In 1978, Jerger and Mauldin reported a
correlation between ABR threshold and behavioral hearing threshold at 4 kHz .
Using unfiltered click stimuli, Bernard et
al (1980) reported significant alteration of the
ABR wave V latency in aminoglycoside-treated
neonates . Also with unfiltered click stimuli,
Piek et al (1985) reported latency changes in
comatose adult patients receiving aminoglycosides .
Frequency-specific stimuli for eliciting the
ABR have been suggested by Don and Eggermont (1978), and by Gorga and Worthington
(1983) . The rationale for achieving frequency
specificity in the ABR evolves from the need to
provide a better estimate of hearing sensitivity
in difficult-to-test subjects than can be provided
by click stimuli. The need for high-frequencyspecific stimuli is to identify significant shifts
in high-frequency (8-20 kHz) thresholds
produced by ototoxic agents before the speech
frequency range is affected . Such early identification can allow the health care provider to
examine treatment alternatives which may
preserve, or reduce deficit in, communicative
abilities.
Tone-burst stimuli for frequencies above 8
kHz were not available in commercially produced ABR systems. The Auditory Research
Laboratory at the Veterans Affairs Medical
Center, Portland, Oregon (PVAMC) developed
laboratory instrumentation to provide high-frequency tone-burst stimuli (Portland Auditory
Research/Veterans Affairs-Tone Burst
[PARVA-TB])(Fausti et al, 1991).
Reliable ABRs to click stimuli have been
documented over repeated sessions when
stimulus, measurement, and subject variables
are held constant (Rosenhamer et al, 1978 ;
Chiappa et al, 1979 ; Edwards et al, 1982 ;
Schwartz and Berry, 1985 ; Lauter and Loomis,
1986). Lower frequency-specific stimuli (<_ 8 kHz)
have also been shown to produce reliable ABRs .
Gorga et al (1988) reported data for wave Vlatencies using 0.5, 2, and 8 kHz tone-bursts, revealing highly reliable intrasubject wave V latencies.
106
Although it has been demonstrated that
high-frequency (>- 8 kHz) tone-bursts are capable
of producing measurable responses in normalhearing persons (Fausti et al, 1984a; Rappaport
et al, 1985 ; Gorga et al, 1987), response
reliability has not been documented for all waves
of the ABR to frequency-specific stimuli above 8
kHz. Prior to using the PARVA-TB as a tool for
serial monitoring of hearing, it was necessary to
document the reliability of intrasubject ABRs to
these high-frequency tone-burst stimuli.
The purpose of this study, therefore, was to
investigate the variability of ABRs to high-frequency (>_ 8 kHz) tone bursts over repeated sessions in a group of normal-hearing subjects .
Waves I, III, and V, in response to tone-burst
stimuli at frequencies of 8-14 kHz, were
evaluated for intrasubject, intra- and intersession peak latency stability. Results were compared to stability of click responses obtained
under identical testing conditions .
METHOD
Subjects
Thirty ears from normal-hearing young
adults were evaluated (21 ears from 17 females
and nine ears from five males) . Subject ages
ranged from 19 to 33 years with a mean of 25 .9
years. Acceptance criteria for subjects were
based upon no history of ear disease, normal
aural immittance results (Wiley et al, 1987 ;
Shanks et al, 1988), conventional frequency
(0 .25-8 kHz) hearing thresholds no greater
than 15 dB HL (re: ANSI, 1989), and high-frequency thresholds no greater than one standard
deviation from the mean thresholds reported by
Schechter et al (1986) .
Instrumentation
For all testing conditions, subjects were
seated in a reclining chair in an Acoustic Systems 19701A double-walled, R-F shielded,
sound-treated booth. Middle-ear function
screening was conducted with a Virtual 310 immittance system .
Behavioral
Behavioral pure-tone air conduction thresholds for conventional frequencies (octave in-
High-Frequency Tone-Burst ABR Reliability/Fausti et al
tervals from 0.25 to 8 kHz) were obtained with
a Grason-Stadler 1701 (G-S 1701) clinical
audiometer using Telephonics TDH 49 earphone transducers mounted in MX 41/AR
cushions . Hearing in the high-frequency range
was assessed on the Portland Auditory Research/Veterans Affairs-High Frequency
(PARVA-HF) laboratory audiometer (Fausti et
al, 1979 ; Fausti et a1,1990) using matched Koss
HV/lA earphone transducers .
Pure-tone conventional frequency stimuli
from the G-S 1701, presented at a 50 percent
duty cycle, were of 200 msec duration with
rise-fall times of 50 msec . Pulsed pure-tone
high-frequency stimuli, of 300 msec duration
and rise-fall times of 25 msec, were also
presented at a 50 percent duty cycle with the
PARVA-HF . Five-dB steps were utilized in obtaining all threshold results with the clinically
accepted, modified Hughson-Westlake ascending-descending audiometric test technique
(Carhart and Jerger, 1959).
Click stimuli were presented by a Nicolet
1170 evoked-potential signal averager using
Telephonics TDH 49 earphone transducers
with MX 41/AR cushions mounted in Amplivox
circumaural Audiocups. High-frequency toneburst stimuli were generated by the PARVA-TB
(Fausti et al, 1991). Tone bursts centered at
frequencies of 8, 10, 12, and 14 kHz were gated
at zero-crossing with rise times of 0.1 msec
(linear between the 10% and 90% on-condition).
Duration between zero voltage points was 2.0
msec . Stimulus artifact was reduced by alternating stimulus polarity . Tone-burst stimuli
were delivered through Koss HV/lA earphone
transducers. An active amplifier/filter network* (Fausti et al, 1979) was utilized to match
earphone transducer input impedance, improve
the signal-to-noise ratio, lower the sidebands
and narrow the bandwidth in order to provide
a signal with exceptionally sharp cutoff slopes
and low background noise (as described below) .
The PARVA-TB was connected to the external
From stimulus triggering (electrical onset) to stimulus
arrival at the earphone transducer (acoustic onset), a
constant time delay will occur from passing the stimulus
through a filter . In this network, the delay constant
equals 0.27 msec across frequency. Data can be corrected for this time delay.
trigger input of the Nicolet 1170 for synchronization with signal averaging.
Calibration
The acoustic spectra of the tone-burst
stimuli (shown in Fausti et al, 1991) were
measured with a Hewlett-Packard 3561A
Dynamic Signal Analyzer, using a rectangular
window (0 - 20 kHz) and peak hold mode . Signal
output was measured through the high-frequency transducer (Koss HV/1A) centered on a
flat-plate coupler with a Bruel & Kjaer (B&K)
1/2" pressure condenser microphone as reported by Fausti et al (1979) . Although stimuli
were not generated by digital methods, the
acoustic spectra are comparable to those
reported by others (Dolan and Klein, 1987 ;
Gorga et al, 1988). The roll-off, the bandwidths
measured at 20, 40, and 60 dB down, and the
noise floor were all comparable to, or more
sharply defined than, reported digitally
generated signals.
The acoustic output for each click and tone
burst was displayed on a Tektronix 7633 digital
oscilloscope to determine the peak equivalent
SPL. For clicks, a continuous pure tone of 3500
Hz (maximum peak of spectrum) was matched
to the maximum scope displacement value
(peak-peak) of the waveform . For tonebursts, a
continuous pure tone at the frequency of the
tone burst was matched in the same manner .
A Hewlett-Packard 3400A True RMS Voltmeter, calibrated at 94 dB sound pressure level
(SPL) via a B&K 4930 1 kHz microphone
calibrator, was then used to determine dB SPL
of the continuous pure tone . This value was
then assigned as the peak equivalent sound
pressure level (peSPL) of the click or tone burst .
ABR Testing Procedures
Responses were obtained from each subject
in two sessions of approximately 60 minutes
each . During each session, behavioral thresholds to pulsed pure tones and tone bursts were
obtained . Two ABR averages were obtained per
session for each of the five stimuli. To prevent
an order effect, the stimuli were presented in a
counter-balanced, pseudorandom order. Each
ABR average was the sum of 1024 stimulus
presentations within a response window of
10 .24 msec . Each of the five stimulus conditions
107
Journal of the American Academy of Audiology/Volume 2, Number 2, April 1991
(four high-frequency tone-burst stimuli and
clicks) was presented in two separate sessions.
Thus, a total of four ABRs was obtained for each
stimulus condition. This test format allowed
reliability to be determined within a session as
well as across sessions.
A single-channel differential electrode
recording montage was utilized . The electrode to
the noninverting amplifier input was placed on
the vertex, with electrodes to the inverting and
common amplifier inputs at the ipsilateral and
contralateral mastoids, respectively . Absolute
impedance did not exceed 2 k.Q and the interelectrode impedance differences were at or below
1 kQ . Bioamplifier filter settings were 150 and
1500 Hz .
Stimuli were presented at a 60 dB sensation level (SL) with respect to behavioral click
and tone-burst threshold, at a rate of 11 .1 per
second. A moderate suprathreshold intensity
level (60 dB SL) was chosen to increase the
likelihood of obtaining all response waves (1,111,
and V) . Additionally, as clicks, and 8, 10, 12,
and 14 kHz tone bursts, were to be tested on a
repeated basis, a single presentation level was
selected in the interest of reducing overall testing time . The Koss HV/lA earphone utilized to
deliver high-frequency tone-burst stimuli has
an interaural attenuation of about 35 dB (Rappaport et al, 1982). Therefore, band-pass masking (7 .5 -25 kHz) was presented contralaterally
at an intensity level 30 dB less than the toneburst signal SPL to prevent potential transcranial interference . The reclining chair
remained upright for all behavioral measures,
and was lowered to full reclining position (45
degrees from horizontal) for all evoked potential
measures .
Wave identification techniques used with
ABR click stimuli (Chiappa et al, 1979 ; Beattie
et al, 1986 ; Picton et al, 1988) were employed
as a guideline for peak picking with high-frequency tone-burst stimuli. For all stimulus conditions, reliability of high-frequency tone-burst
responses for each ABR wave was evaluated
using analysis of variance (ANOVA) procedures
(Collyer and Enns, 1986).
RESULTS
igure 1 shows the latencies of each subject's
Fresponses for each wave and stimulus condition . Thus, all individual raw data points are
displayed, although many points are obscured
due to overlapping of symbols. This graphic
108
provides an overall picture of the results of this
study in terms of the consistency of intra- and
intersubject responses .
In each subject, four ABR averages (runs),
two in the first session and two in the second,
were obtained for each stimulus . The intersubject means of the four runs were compared using
a repeated measures ANOVA (Table 1) . No sigTable 1 Mean Intersubject, Intra-run Latencies
(in msec)'
Stimulus
Run 1
Run 2
Run 3
Run 4
8 kHz
10 kHz
1 .96
1 .86
1 .95
1 .89
1 .96
1 .85
1 .98
1 .86
8 kHz
Wave III 10 kHz
12 kHz
4 .15
4 .15
4 .09
4.14
4 .19
4.08
4.17
4.10
4.14
4.18
4.12
4.09
6 .04
6 .10
6 .10
6 .10
6 .07
6 .10
6 .09
6 .16
6 .10
6 .08
6 .10
6 .11
6 .07
6 .07
6 .11
6 .13
Wave l
12 kHz
14 kHz
14 kHz
Wave V
8
10
12
14
kHz
kHz
kHz
kHz
1 .81
1 .72
4 .02
1 .76
1 .72
4 .04
1 .78
1 .70
4 .07
1 .78
1 .72
4 .06
`Runs 1 and 2 represent ABR trials 1 and 2 within session 1, while runs 3 and 4 represent trials 1 and 2 within
session 2; for each stimulus and wave condition, no significant differences were found between means of runs
(p > 0 .05) .
nificant difference was detected either intra- or
intersession for any of the three waves at any
of the four tone-burst conditions (p > 0.05) .
Reliability of intrasession high-frequency
tone-burst responses was demonstrated by calculating intrasubject latency differences between runs with each session. For each subject,
the first run within each session was subtracted
from the second run resulting in intrasubject,
intrasession latency differences . These differences were then summed across subjects to
determine mean intrasession differences . Absolute values of the differences reflect magnitude of the mean latency differences, and, as
shown in Table 2, are minimal. Actual values
of the differences reflect magnitude and direction of the mean latency differences, and are
shown in Table 3 . A positive value indicates
that, on average, the latencies of the second
runs within a session were longer than those of
the first runs . Examination of Table 3 indicates
no preponderance for the second runs within a
session being either longer or shorter than the
first runs .
High-Frequency Tone-Burst ABR Reliability/Fausti et al
Table 2 Mean Intrasubject, Intrasession Latency Differences (in msec) (Absolute Values)'
Wave I
Clicks
8 kHz
10 kHz
12 kHz
14 kHz
Session 1
08
Combined
11
09
08
10
09
08
09
08
09
09
09
06
10
08
Session 2
Wave III
Session 1
Session 2
Combined
03
03
03
10
10
10
13
07
10
11
09
10
Wave V
Session 1
Session 2
Combined
05
05
05
10
07
08
12
11
11
10
09
09
10
12
10
12
12
12
Differences calculated by subtracting run 1 from run 2 for each subject .
Table 3 Mean Intrasubject, Intrasession Latency Differences (in msec) (Actual Values)'
Clicks
Session 1
Wave I
00
01
01
00
01
00
03
00
Combined
Wave V
10 kHz
- .02
Session 2
Wave III
8 kHz
Session 1
Session 2
Combined
00
00
00
- .01
06
00
04
Session 1
00
Combined
01
02
-.02
Session 2
00
-.08
00
- .04
00
14 kHz
00
02
01
05
01
-.01
- .02
- .01
- .01
07
- .01
00
04
00
03
12 kHz
-.02
00
01
00
02
`Differences calculated by subtracting run 1 from run 2 for each subject .
Intrasubject intersession reliability was
evaluated. For each subject's two identical runs
within a session, the mean latency was computed. For the same stimulus and wave, the
mean of the first session was subtracted from
the mean of the second session. These latency
differences were then summed across subjects
and analyzed for intersession reliability of
latencies. Means of the absolute values of these
differences are shown in Table 4. A repeated
Table 4 Mean Intrasubject, Intersession
Latency Differences (in msec)
(Absolute Values)"
Wave I
Wave 111
Wave V
Clicks
8 kHz
10 kHz
12 kHz
14 kHz
13
08
10
12
13
13
12
13
10
11
13
10
11
17
13
`Differences calculated by subtracting session 1 from
session 2 for each subject; no significant differences were
found between clicks and any tone-burst frequency (p >
0 .05), except for wave III between clicks and 14 kHz (p <
0.05) .
measures ANOVA was performed to determine
if there were significant differences between
clicks and high-frequency tone-bursts when intersession latency differences were compared .
No significant differences were found for waves
I and V (p < 0.05) . Additionally, no significant
differences were found for wave III (p < 0.05)
except between clicks and 14 kHz (p < 0.05) .
Intersession latency differences were also
analyzed to determine if there was a tendency
for the second session latencies to be either consistently shorter or longer than those collected
during the first sessions . Table 5 shows the actual values of mean intersession differences .
Range values of intersession differences are also
included in Table 5, which show the extreme individual values of actual differences between
sessions . Examination of this table shows no
directional preponderance between sessions .
Table 6 shows means and standard deviations (± 1) of high-frequency tone-burst latencies across subjects . Mean values for each
stimulus condition and wave were calculated
by determining intrasession means for each
subject, averaging the means of the two ses109
Journal of the American Academy of Audiology/Volume 2, Number 2, April 1991
Table 5 Mean Intrasubject, Intersession Latency Differences (in msec) (Actual Values)'
Stimulus
Clicks
8 kHz
10 kHz
12 kHz
14 kHz
- .04
03
- .02
00
00
(- .38/.28)
(- .28/.26)
(- .34/.30)
(- .26/:28)
(- .26/.34)
- .03
04
- .05
04
04
(- .22/ .10)
(- .22/ .38)
(- .36/ .34)
(- .26/ .36)
(- .36/ .42)
03
04
- .03
02
01
(- .23/ .60)
(- .42/ .30)
(- .36/ .36)
(- .18/ .30)
(- .26/ .44)
'Differences calculated by subtracting session 1 from session 2 for each subject .
sions, and then collapsing across subjects to
determine the means shown in Table 6. Significant differences were found between frequencies (p < 0 .01) for wave I, so StudentNewman-Keuls paired comparisons tests (Collyer and Enns, 1986) were done . Between any
two frequencies for wave I, mean latencies were
significantly shorter for the higher frequencies
(p < 0.01 for each comparison except 12 versus
14 kHz significant at p < 0 . 05) . The same
analysis was performed for wave III, revealing
significantly longer mean latencies only for 8
and 10 kHz compared to 14 kHz (p < 0.05) .
Table 6 Mean Intersubject Latencies (in msec)
and Standard Deviations (±1) for
High-Frequency Tone Bursts'
Stimulus
8 kHz
10 kHz
12 kHz
14 kHz
Wave l
1 .95± .16
1 .87 ± .13
1 .78 ± .12
1 .71 ± .12
Wave ///
Wave V
4 .16± .21
4 .14 ± .21
6 .07 ± .28
4.09 ± .26
4 .03 ± .24
6 .09± .27
6.10+ .29
6.13 ± .25
`Data for 30 ears collapsed within and between sessions .
Figure 1 Raw latency values
from 30 ears used in this study.
Each point represents a single
ABR average from a specific
stimulus condition (clicks, 8, 10,
12, and 14 kHz) and wave (waves
I, III, and V) . Reliability of
responses can be observed in the
patterns seen within and between subjects .
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Wave / S - Session / R = Run
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High-Frequency Tone-Burst ABR Reliability/Fausti et al
Figure 2 Cumulated percentage
distributions ofabsolute values of
latency differences between sessions for each of 30 ears . For each
wave and stimulus condition, differences were calculated by finding the intrasession means, and
subtracting the mean of session 1
from the mean of session 2. Any
given intersession latency difference can be indexed against
these normal ears, providing an
estimate of the percentage of the
population having larger or
smaller differences .
o Wave I
o Wave III
e Wave V
14 kHz
.6
.4
.2
0
0
mro~
20
40
60
80
100
0
Percentile
20
There were no significant latency differences
between frequencies for wave V.
Figure 2 shows cumulated percentage distributions for absolute values of intersession
latency differences for each wave and stimulus
condition. These graphs provide a method to
determine, for any given stimulus condition
(click or tone-burst) and any given wave (1, 111,
or V), where the magnitudes of an individual's
intersession latency differences fall in relation
to the group of 30 normal ears . Thus, each intersession difference value can be indexed relative
to the normal group in terms of the percentage
of subjects having smaller differences, and the
percentage having larger differences . Figure 3
is analogous to Figure 2 except that actual values
of differences are displayed, allowing further
comparison of latency differences when direction of change is taken into account.
40
60
8o
100
DISCUSSION
A
BR variability has been described primarily in groups of subjects as betweensubject variability (Thornton, 1975 ; Kendall
and Lawes, 1978 ; Rosenhamer et al, 1978 ; Sohmer et al, 1978 ; Chiappa et al, 1979 ; Kjaer,
1979 ; Stockard et al, 1979 ; Spreng, 1979 ; Bergholtz, 1981 ; Rosenhamer and Holmkvist,
1982 ; Lauter and Loomis, 1986 ; Gorga et al,
1987 ; Picton et al, 1988). By comparison, only
a few studies have reported within-subject variability (Chiappa et al, 1979 ; Edwards et al,
1982 ; Lauter and Loomis, 1986 ; Gorga et al,
1988). The within-subject measures are of two
types: within a single session and across sessions. Most studies have used clicks with only
a few using tone bursts as stimuli.
Journal of the American Academy of Audiology/Volume 2, Number 2, April 1991
Figure 3 Cumulated percentage
distributions of actual values of
latency differences between sessions for each of 30 ears . Difference values were calculated in
the same way as in Figure 2, except that the actual differences
were used, resulting in positive
and negative values . A negative
value indicates that the second
session's latency was shorter
than the first. Positive values indicate longer latencies during the
second session and, depending on
degree, may reflect hearing loss .
o
o W-.
Wave III
,
v Wave V
B kHz
U
C
J
60
100
100
Percentile
In these studies different measures of
variability have been used . The standard deviation is commonly reported while the difference,
difference squared, and coefficient of variation
are occasionally reported . The clinical use of
variability has only occasionally been discussed
(Jerger et al, 1985 ; Oken, 1990 ; Sklare, 1990).
Since there is no accepted standard of ABR
reliability, standard deviations and differences
were chosen in this study to best describe
reliability .
In this investigation, the intrasubject
reliability of auditory brainstem responses to
high-frequency tone bursts was evaluated using
clicks as the standard . Results indicated clinically acceptable reliability of intra- and intersession ABRs to high-frequency tone bursts when
compared to clicks (see Tables 2-5) . Intrasubject,
intersession variability of high-frequency tone
bursts was not significantly different from
variability of click stimuli. Reliability was
demonstrated for waves I, III, and V using 8, 10,
12, and 14 kHz tone bursts .
There are two (progressive) effects that
may be seen in Table 6. First, variability (stand112
and deviation) increases from wave I to wave V
for all stimuli used . This progressive increase
in the standard deviation from waves I through
V was also found by Lauter and Loomis (1986)
and Fausti et al (1991) . Second, latencies
changed as a function of frequency for some
waves. That is, latency decreased as frequency
increased for wave I. The same effect occurred
for wave III, but to a lesser degree, while no
effect was seen for wave V. These results replicate and extend similar findings in a previous
study of rise time (Fausti et al, 1991).
The measures of variability shown in
Tables 4 and 6 might, at first, appear to be
contradictory . Waves I and V show similar
variability in Table 4, as demonstrated by intersession mean latency differences . Wave I
shows less variability than wave V in Table 6,
as demonstrated by standard deviations. Table
4 reflects the variability within subjects while
Table 6 describes the variability between subjects. Because the standard deviations in Table
6 are calculated across subjects, the progressive
increase in the standard deviation is probably
due to an increase in the intersubject variability
High-Frequency Tone-Burst ABR Reliability/Fausti et al
of higher auditory pathways . However, in contrast to these individual differences, Table 4
demonstrates no increase in variability within
subjects as the evoked activity progresses from
the cochlea through the brainstem.
In serial monitoring of patients receiving
ototoxic agents, intersession change, or evidence
of hearing loss, is based upon each subject's own
response variability . That is, each subject serves
as his/her own control. Occasionally, however,
subjects cannot produce stable baselines. Subjects demonstrating elevated intrasubject variability may not be good candidates for this
method of testing. The majority of subjects, however, produce a tight pattern of responses, and,
with these subjects, this method can be expected
to perform as an effective monitoring tool .
Figures 2 and 3 present cumulated percentages of subjects versus absolute values of differences and actual differences, respectively, in
latency between sessions . For a given wave and
stimulus, one can determine what percentage
of the difference scores were greater than or
less than a given value. The intrasubject, intersession latency differences are similar for
waves I, 111, and V (Tables 4 and 5 and Figs . 2
and 3), and similar criteria could be used to
detect a change in latency for each wave . On
the basis of the cumulative percent data in
Figures 2 and 3 it is possible to determine the
false-positive rate for a chosen criterion. Figure
2 could be used for any change, i.e ., latency
delay or advance between sessions . Figure 3
would be more useful for detecting ototoxic
change, where the relevant change would be a
delayed latency relative to the initial (or
baseline) session, resulting in a positive difference value. For example, if, during a later
testing session, a subject's latency is 0.2 msec
longer than the baseline session, the intersession difference of 0.2 msec would be compared
to the normal group (see Fig. 3) . If a delay of
0.2 msec for wave I, for the 8 kHz tone burst,
is chosen as the criterion, the expected falsepositive rate would be about 15 percent. Any
difference of 0.2 msec or greater would indicate
change had taken place at the 85 percent confidence level. If a delay of 0.3 msec is chosen,
the expected false-positive rate would be about
9 percent, resulting in a 91 percent confidence
level.
Our results, thus far, demonstrate that
high-frequency-specific tone bursts can be used
to evoke ABRs in order to assess high-frequency
auditory function objectively . The data we have
presented support the reliability of this procedure . We expect that this technique can be used
to assess high-frequency auditory function in
patients receiving ototoxic drugs who are too ill
to respond to behavioral audiometry .
Acknowledgments. The authors wish to acknowledge
significant contributions to this manuscript made by Drs .
Thomas Dolan, Cynthia Fowler, and David Lilly.
Funding for this study provided by Medical Research Ser;
vice, Department of Veterans Affairs.
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