Download Sources of the Scalp-Recorded Amplitude

J Am Acad Audiol 13 : 188-204 (2002)
Sources of the Scalp-Recorded AmplitudeModulation Following Response
Shigeyuki Kuwada*
Julia S. Andersont
Ranjan Batrat
Douglas C. Fitzpatrick§
Natacha Teissier*
William R. D'Angelo*
Abstract
The scalp-recorded amplitude-modulation following response (AMFR),, is gaining
recognition as
an objective audiometric tool, but little is known about the neural sources
that underlie this potential . We hypothesized, based on our human studies and single-unit recordings in animals,
that the
scalp-recorded AMFR reflects the interaction of multiple sources . We tested this hypothesis using
an animal model, the unanesthetized rabbit. We compared AMFRs recorded from
the surface of
the brain at different locations and before and after the administration of
agents likely to enhance
or suppress neural generators . We also recorded AMFRs locally at several stations
along the auditory neuraxis. We conclude that the surface-recorded AMFR is indeed a composite response
from
multiple brain generators . Although the response at any modulation frequency can
reflect the activity of more than one generator, the AMFRs to low and high modulation frequencies
appear to
reflect a strong contribution from cortical and subcortical sources, respectively .
Key Words: Amplitude-modulation following response, auditory steady-state potentials,
electrical
audiometry, evoked potentials, temporal coding
Abbreviations : AMFR = amplitude-modulation following response ; ASSR = auditory
steady-state
response ; EFR = envelope following response ; GABA = y-aminobutyric acid ; HCI =
hydrochloride ;
KCI = potassium chloride ; MTF = modulation transfer function ; SAM = sinusoidally
amplitudemodulated ; SSEP = auditory steady-state evoked potential
Sumario
Las respuestas de seguimiento de amplitud modulada (AMFR)II registradas
en el craneo estan
ganando reconocimiento como una herramiento audiometrica objetiva, pero poco se
sabe sobre
los fuentes neurales que generan este potencial . Se plante6 la hip6tesis, basada en
nuestros estudios en humanos y en registros de unidad unica en animales, que los AMFR
registrados en el
craneo reflejan la interacci6n de multiples fuentes . Evaluamos esta hip6tesis utilizando un
modelo
animal : un conejo no anestesiado . Comparamos los AMFR registrados en diferentes
sitios sobre
la superficie del cerebro, asi como antes y despues de la administraci6n de agentes que
estimuIan o suprimen la acci6n de generadores neurales . Tambien registramos localmente
los AMFR en
diferentes estaciones a to largo del neureje auditivo . Concluimos que los AMFR registrados
en el
craneo son verdaderamente una respuesta compuesta de multiples generadores
cerebrales .
Aunque las respuestas a cualquier frecuencia de modulaci6n pueden reflejar
la actividad de mas
de un generador, los AMFR producidos ante frecuencias de baja o alta modulaci6n
parecen reflejar una fuerte contribuci6n de fuentes corticales o subcorticales, respectivamente .
Palabras Clave: Respuesta de seguimiento de amplitud modulada, potenciales auditivos
de
estado estable, audiometria electrica, potenciales evocados, codificaci6n temporal
*Department of Neuroscience, University of Connecticut Health Center, Farmington, Connecticut ;
tDepartment of
Anatomy, University of Connecticut Health Center, Farmington, Connecticut ; $Department of
Anatomy, University of Mississippi
Medical Center, Jackson, Mississippi ; §Department of Otolaryngology, University of North
Carolina, Chapel Hill, North Carolina
"The terms amplitude-modulation following response (AMFR), auditory steady-state response
(ASSR), steady-state
evoked potential (SSEP), and envelope following response (EFR) are used interchangeably in
the literature . Whereas other
authors in these special issues use the term ASSR, author Kuwada prefers AMFR . (Editor)
Reprint requests : Shigeyuki Kuwada, Department of Neuroscience, The University of
Connecticut Health Center, 263
Farmington Ave ., Farmington, CT 06030-3405
188
Sources of the Scalp-Recorded AMFR/Kuwada et al
Abreviaturas : AMFR = respuesta de seguimiento de amplitud modulada ; ASSR = respuestas
audifvas de estado estable ; EFR = respuesta de seguimiento de envolvente ; GABA = Acido gamaaminobutirico ; HCI = hidrocloruro ; KCI = cloruro de potasio ; MTF = fundbn de transferencia de
modulaci6n ; SAM = amplitud modulada sinusoidalmente ; SSEP = potential auditivo de estado
estable
he amplitude-modulation following
response (AMFR) is a neural potential
T that follows the envelope of complex
sounds . This potential is also known as the envelope following response (EFR ; Dolphin and
Mountain, 1992), auditory steady-state response
(ASSR, Picton et al, 1987), and auditory steadystate evoked potential (SSEP ; Rickards et al,
1994). The AMFR can be recorded from the scalp
in response to sinusoidally amplitude-modulated
(SAM) tones and can be used to assess hearing
on a frequency-by-frequency basis across the
audiometric range (250-8000 Hz) in adults
(Kuwada et al, 1986 ; Rees et al, 1986 ; Picton et
al, 1987 ; Griffiths and Chambers, 1991) and in
neonates and children (Levi et al, 1992 ; Rickards
et al, 1994 ; Aoyagi et al, 1996).
Despite the AMFR's growing reputation as
an objective method for hearing evaluation, we
know little about its neural generators . Scalprecorded AMFRs to SAM tones in humans
(Kuwada et al, 1986) and in gerbils (Dolphin
and Mountain, 1992) appear to reflect the activity of multiple generators . Different generators
appear to dominate over different ranges of modulation frequency. Generators with long activation delays dominate at low frequencies, whereas
generators with short activation delays dominate at high frequencies . The lengths of the
delays implicate cortical generators at low frequencies and subcortical generators at high frequencies . The idea that multiple sources underlie
the AMFR was supported by local recordings
made earlier in the auditory cortex and inferior
colliculus (Tielen et al, 1969). These recordings
found that local AMFRs in the auditory cortex had
maximum amplitude at low modulation frequencies (15-30 Hz) and were activated with
long delays, whereas AMFRs in the inferior colliculus could be recorded at higher modulation
frequencies and had shorter delays . However,
later studies that recorded AMFRs in animals
after ablation of the auditory cortex discredited
the idea that the primary auditory cortex was a
major source (Tsuzuku, 1993 ; Kiren et al, 1994).
To investigate the sources of the AMFR, we
employed an animal model, the unanesthetized
rabbit. The advantage of the rabbit is that record-
ings can be made without the confounds of anesthesia, and we have considerable data on the
responses of single neurons to SAM tones at
several levels along the auditory pathway. Using
behavioral, pharmacologic, and cortical inactivation techniques, we conclude that the surfacerecorded AMFR reflects the activity of multiple
generators .
METHOD
Preparatory Surgery
Six adult Dutch Belted rabbits (-2-2.5 kg)
with clean external ears were used . Three were
surgically prepared for surface-recorded AMFRs
(epidural) and three for locally recorded AMFRs
from the superior olivary complex, inferior colliculus, and auditory cortex . All surgical and
postoperative care followed the National Institutes of Health guidelines and was approved
by the Institutional Animal Care Committee at
the University of Connecticut Health Center .
Both types of recordings involved attaching
a square brass rod to the skull to immobilize head
movement during recordings . Under anesthesia
(ketamine hydrochloride [HCll 35 mg/kg and
xylazine 5 mg/kg), the scalp was retracted to
expose the skull, and the brass rod was anchored
to the skull with screws and dental acrylic. The
rod was positioned parallel and to one side of the
midsagittal suture .
Surface Epidural Electrodes : During the
initial surgery, stainless steel screws (0-80 x 1/a°)
were threaded into the skull and contacted the
dural surface. Silver wire leads soldered to the
screws were led into a plastic strip connector, and
this assembly was also attached to the exposed
skull with dental acrylic. In almost all cases, one
active electrode was placed near the sagittal
suture on the side contralateral to the stimulated
ear about halfway between the coronal and the
lambdoid suture, and the reference electrode
was placed in the frontal sinus . A third skull
screw served as a ground . In one animal, two
additional screw electrodes were placed : one
over the auditory cortex and the other near the
midline and lambdoid suture .
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Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002
Local Microelectrodes: One animal was used
to record AMFRs from the superior olivary complex and inferior colliculus, and two animals
were used to record AMFRs from the auditory
cortex . A small hole (2 x 2 mm) in the skull just
rostral to the lambdoid suture and ~ 2 mm lateral to the midline allowed access to the inferior
colliculus and superior olivary complex. Access
to the auditory cortex was via a rectangular
hole in the skull (2 x 4 mm, ~ 3 mm posterior
to the bregma and - 11 mm lateral to the midline) that overlaid the dorsal part of the auditory
cortex . Recordings were made through glasscoated Pt/W microelectrodes (~1 MOhm). In one
animal, we used a microwire array to make local
AMFR recordings in the auditory cortex. This
array consisted of two rows of 12 wires each. Separation of the wires was 250 ~Lm both within
and between rows . Each wire was 50 p,m in
diameter (<200 kOhms impedance) and insulated except for the tip.
In two of the rabbits used for surface recordings, a chronic cannula was inserted into the
external jugular vein and fed into the subclavian
vein during the initial surgery. The other end of
the cannula was brought up subdermally and
fixed onto the skull surface with dental acrylic
for the convenient administration of drugs .
Heparinized saline (50 U/mL) was injected
(1 mL) daily to keep the catheter patent . In
another rabbit, a small hole (3 mm in diameter)
was drilled over each cortical hemisphere . These
holes were used to apply a potassium chloride
(KCI) (15% or 25% solution)-soaked cotton pledgett to the dural surface to induce cortical spreading depression .
Acoustic Stimulation
A custom-fitted ear mold for each rabbit
was constructed from ear impression compound
(Audilin) . A sound delivery tube was inserted
through the mold, and its tip was within 2 cm
of the tympanum . The mold was made while
the animal was anesthetized .
SAM tones were generated digitally using a University ofWisconsin-based system (Rhode, 1976) and
delivered through a Beyer DT-48 earphone connected
to the sound delivery tube in the custom-fitted ear mold.
Except where noted, the sounds were presented to the
ear contralateral to the recording site. Modulation
depth was either 80 or 100 percent. Where appropriate, stimuli were presented in a block-randomized
order.
Amplitude (dB SPL re : 20 ~tPA) and phase
at frequencies from 60 to 40,000 kHz were mea190
sured just prior to sacrificing the animal . The
earphone was connected to the sound delivery
tube, and, in most animals, the tip of a calibrated probe tube connected to a 1/2" microphone
(Bruel and Kjaer) was placed near the tympanum via a small sealed hole in the external bony
meatus . In one animal, the tip of the probe was
at the end of the sound delivery tube .
Recording Procedure
Each rabbit was tested, unanesthetized, over
a period of several months . Daily recording sessions were conducted in a double-walled soundproofed room . During data collection, the rabbit
was placed in a custom plexiglass cradle and its
body was restrained in a zippered Lycra sleeve .
The rabbit's head was clamped in a fixed position
via the head bar. The rabbit was monitored continuously by video camera. Each session usually
lasted 1 to 3 hours, but if the rabbit fidgeted, the
session was terminated. All rabbits were first
given preparatory sittings in which to become
accustomed to the recording environment.
Surface potentials were recorded differentially between active and reference electrodes,
with a third electrode used for ground . The
interelectrode resistance was < 12 kOhms. Two
amplifiers (EG&G Pare) in series were used to
record (total gain 20,000-50,000) and filter
(0 .3-3000Hz, 12 dB/octave rolloff) the scalp
potentials . The SAM tones were delivered continuously, and a pulse at the zero crossing of each
modulation cycle was used to trigger an A/D
converter to begin digitizing the scalp potentials (> 25 points/ modulation cycle) . Potentials
were averaged over 10 cycles of the modulation
frequency, in blocks of 50 to 2500 trials, and
several blocks at each modulation frequency
were usually collected. To ensure that there was
no electrical or magnetic contamination in the
AMFR from the earphones, we tested in each
animal the potentials to different carrier and
modulation frequencies when the sound delivery tube was plugged. We could not detect any
artifacts.
The local recordings were conducted in a
similar manner. Prior to these recordings, singleunit recordings were made to ensure that the
local recordings were from the auditory nuclei of
interest. This was later confirmed by histologic
examination of the relevant brain sections .
Recordings were made with an extracellular
amplifier (Dagan 2400) coupled to a conventional
amplifier (EG&G Pare) using the same filter
Sources of the Scalp-Recorded AMFR/Kuwada et al
settings (0 .3-3000 Hz) . Microwire recordings
were made with a different amplifier (TDT
Bioamp) and filters (5-3000 Hz) .
To provide a point of comparison to the rabbit, we included the AMFRs from one human
subject with normal hearing . Recording techniques were similar to those for surface recordings in the rabbit . The scalp potentials were
recorded differentially between the vertex and
the contralateral earlobe (50,000-100,000 gain ;
filters, 0 .3-3000 Hz), while the other earlobe
served as ground . The resistance between any
two electrodes was less than 5 kOhms . During
the recording, the subject read or sat quietly in
a comfortable chair in a soundproofed booth .
The SAM tones were delivered monaurally
through mu-metal shielded earphones calibrated
for both intensity and phase .
Pharmacologic Procedures
We tested the effects on the surface-recorded
AMFR of systemically administered cocaine,
sodium pentobarbital, and ketamine HCl/
xylazine and of KCI-induced cortical depression . The effect of cocaine was tested on two
rabbits, cocaine and sodium pentobarbital on
separate occasions in one rabbit, and ketamine
HCl/xylazine and KCI on separate occasions in
another rabbit . Different drug treatments in a
single animal were separated by at least 2 days .
Cocaine (1-1 .5 mg/kg) and sodium pentobarbital were injected through the indwelling catheter.
Since rabbits metabolize sodium pentobarbital
rather rapidly, we attempted to achieve a steady
anesthetic state by giving an initial bolus of
sodium pentobarbital (12-25 mg/kg) followed by
supplements (12 mg/kg) at approximately 20to 30-minute intervals. Ketamine HCI/xylazine
(35 mg/kg, 5 mg/kg) was injected intramuscularly
and lasted about 1 hour.
Spreading depression is confined to the
hemisphere where the electrical, mechanical,
or chemical (KCI) stimulation is applied and is
marked by the absence of the electroencephalogram . It creates a deep, reversible inhibition of
all cortical functions (Leao, 1944 ; Bures et al,
1974). It is long lasting (2-3 hours) and does not
appear to cause irreversible neuronal injury
outside the area of KCI application (Nedergard
and Hansen, 1988). In one rabbit, we reversibly
inactivated the cortex by applying KCI (15% or
25% solution) to the dural surface to induce cortical spreading depression . Acotton pellet soaked
in KCI or control solution of saline (0 .9%) was
applied to the dural surface. Recordings were initiated about 7 minutes after application of the
KCI . After each session, the dura was flushed
with saline, filled with a dab of antibacterial ointment (Bacitracin), and capped with an
elastopolymer (Rolyan) .
Analysis of the AMFR
Figure lA illustrates a surface-recorded
AMFR from a rabbit to a SAM tone (modulation
= 67 Hz). In this example, the AMFR represents
an average of 1000 trials, each trial being
10 modulation cycles in length . The spectrum of
the AMFR was analyzed with a discrete Fourier
transform (Fig . 1B). This yielded an estimate of
the amplitude (peak to peak) at the modulation
frequency, which was taken to be the amplitude
of the AMFR . The Fourier transform also yielded
A
h
to Ftv
0
s
10
Modulation Cycles
0
a
a
AMFR = 5.3 pV, Phase = 0.58 cycles
Noise = 0.42 ^ SD = 0 .14 )tV
Criterion = (AMFR-Noise)/SD = 34 .8
w
E
a
III
III
e7
134
201
Modulation Frequency (Hz)
Figure 1 Waveform (A) and spectrum (B) of a surfacerecorded amplitude-modulation following response
(AMFR) from a rabbit to a sinusoidally amplitude-modulated tone (modulation frequency = 67 Hz, carrier frequency = 2000 Hz, level = 80 dB SPL) . A, Waveform of
the AMFR displayed over 10 cycles ofthe modulation frequency (1000 averages). B, Discrete Fourier transform of
the waveform in A yields the amplitude (5 .3 p V) and phase
(0 .58 cycles) at the modulation frequency and an estimate
of the noise in the three frequency bins (arrows; mean =
0 .42 pLV) surrounding the modulation frequency (excluding the immediately adjacent bins). The criterion for the
presence of an AMFR is that the amplitude at the modulation frequency (5 .3 p V) minus the mean amplitude of
the noise (0 .42 wV) divided by the standard deviation of
the noise (0 .14 p.V) must exceed 3. In this example, this
calculation (34.8) far exceeds our criterion (>3) . p-p = peakto-peak amplitude.
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Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002
estimates of the amplitude and standard deviation of the surrounding noise . The noise estimates were derived from the three flanking
bins, excluding the immediately adjacent bin,
around the modulation frequency (arrows) . We
considered the AMFR to be present if the amplitude at the modulation frequency minus the
amplitude of the noise divided by the standard
deviation of the noise was > 3. The present
example far exceeds this criterion (34.8). The discrete Fourier transform also provides an estimate
of the phase of the AMFR. We use the wellknown relationship between phase and frequency in a linear system to estimate the neural
delay. If the relationship between the phase of
the AMFR and modulation frequency is linear
(our criterion is that r > .97), then the slope of
this function provides an estimate of the neural
delay.
The neural delay estimates were corrected for
the influence of the recording system (i .e ., delays
due to filtering) .
RESULTS
AMFR as a Function
of Modulation Frequency
The amplitude and timing ofAMFRs in the
rabbit were similar to those in humans . Figure
2, A and B, shows the modulation transfer functions (MTF) (i.e ., amplitude and phase of the
AMFR as a function of modulation frequency)
from a rabbit (dashed lines indicate the noise
level; see Fig. 1) . The amplitude displays a series
of peaks and valleys . The larger two peaks are
at low modulation frequencies (15-70 Hz),
whereas the smaller two peaks are at high modulation frequencies. The presence of peaks and
valleys suggests the interaction of different generators, with the peaks being the regions where
the responses from different sources are in phase
and the valleys the regions where the responses
are out of phase. In Figure 213, the phase of the
AMFRs in the peak regions, excluding those in
Rabbit
"I
-9 -criterion >3
0 criterion <3
1 .5, C
noise
Human
0
100
m
v
300
2 .7 msec
8 msec
N
t
a
de 12 msec
0
0
200
400
600
800
0
,/26 msec
0
50
100
Modulation Frequency (Hz)
150
200
250
300
Figure 2 The modulation transfer function (MTF) of a rabbit and a human both display peaks and valleys. Rabbit
surface recording was from a near midline screw placed midway between the bregma and the lambdoid suture (i .e .,
vertex) referenced to a screw in the frontal sinus. Human scalp recordings were from the vertex referred to the earlobe
contralateral to the stimulated ear. A, MTF for a rabbit to sinusoidally amplitude-modulated (SAM) tones (carrier =
5000 Hz, level = 90 dB SPL) across a range of modulation frequencies (14-799 Hz). B, Phase versus modulation plot
displays four linear segments . C, MTF for a human adult with normal hearing to SAM tones (carrier = 1 kHz, level =
70 dB SPL) across a range of modulation frequencies (26-261 Hz). In both A and C, solid circles indicate AMFRs with
criterion >3 and open circles criterion <3 ; the dashed line indicates the mean noise at each modulation frequency. D,
The slope of the phase versus modulation frequency plot shows three linear segments (three different neural delays),
each associated with a particular range of modulation frequencies . The neural delays decrease with modulation frequency. To illustrate linearity of the phase plots, modulation frequency is plotted on a linear scale, whereas the peakto-peak (p-p) amplitude is plotted on the conventional log scale.
192
Sources of the Scalp-Recorded AMFR/Kuwada et al
and near the valleys, was fitted with linear
regression . The slope of the fit yielded an estimate of neural delay. The phases formed orderly
linear segments : the two segments below -70 Hz
both had a long neural delay (27 msec), the next
segment (90-275 Hz) had a shorter delay
(5 msec), and the last segment (450-700 Hz)
had an even shorter delay (3 msec).
Figure 2C illustrates the MTFs from a
human. Like that of the rabbit, the human MTF
shows a series of peaks, with the largest peak associated with the lower modulation frequencies .
Although there were some differences, the segment below 46 Hz had a long neural delay
(27 msec), the next segment (80-100 Hz) had a
shorter delay (12 msec), and the last segment
(160-260 Hz) had the shortest delay (8 msec ;
Fig. 2D). Note that the highest modulation region
tested in the rabbit (> 400 Hz) was not tested in
the human. The neural delays in the human over
a comparable frequency range were, in general,
slightly longer than those in the rabbit . This may
reflect, in part, differences in brain size and
shape.
Note that the peak amplitudes of the AMFR
from the rabbit were larger than those for the
human . This is probably due to the closer proximity of the active electrode to the auditory generators and to the brain because the rabbit's
electrode was in contact with dura, whereas for
the human, the electrode was placed on the scalp .
The waveforms of the AMFRs near the peaks
of the MTF differed in shape from the waveforms
near the valleys (Fig. 3A) . Near the peaks (Fig .
3, B-D), the waveforms over one cycle of modulation had one peak (Fig . 3, C and D) or one
peak that was substantially larger than any
other (see Fig. 3B). In contrast, near the valleys
(Fig . 3, E-G), the waveforms of the AMFR over
one cycle of modulation exhibited multiple peaks.
Each of these peaks may correspond to a different
generator of the AMFR, with the different generators summing out of phase in the valleys to
reduce the overall amplitude of the AMFR.
AMFR as a Function
of Electrode Placement
The position of the active electrode influenced the amplitude of the AMFR and its delay
as inferred from the phases of the MTF (Fig . 4) .
When the recording site was near the auditory
cortex (Fig . 4A), the amplitude of the AMFR
was large at low frequencies, exhibited a second
maximum near 70 Hz, declined above this frequency, and had a small but constant amplitude above -100 Hz . Despite the presence of two
peaks at low modulation frequencies (< 100 Hz),
the phases in this range were well fit by a single line, which indicated a long delay (33 msec).
The delay associated with the small AMFR at
high modulation frequencies was extremely
t criterion >3
O
criterion <3
625 Hz
800
B
25 Hz
i1
.
Ij
C
h
U
D 625 Hz (3 x gain)
62 Hz
U
Figure 2. B to D, waveforms near the
peaks of the MTF. E to G, Waveforms
near the nulls or valleys of the MTF.
p-p = peak-to-peak amplitude.
y
G 325 Hz (3 x gain)
F 45 Hz
E 20 Hz
Figure 3 The waveforms of the
AMFR near the peaks of the modulation transfer function (MTF) (A) are
relatively simple compared with those
near the nulls or valleys of the MTF.
Responses are from the same rabbit in
W
0
Modulation Cycles
5
193
Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002
A
Near Auditory Cortex
Figure 4 Different surface recording sites yield different modulation
transfer functions (MTFs) and neural
2 .6 msec
33 msec
0
B
delays . The amplitude (left column)
and phase (right column) MTFs at
different surface recording sites in
one rabbit (A-C). In all cases, the carrier frequency (2000 Hz, 80 dB SPL)
and the position of the reference electrode (frontal sinus) were the same .
Solid circles indicate amplitudemodulation following responses with
criterion >3 and open circles with criterion <3 . A, Recordings from a surface
Near Midline
4.4 msec
a
a
6_1C
Near Midbrain
electrode placed over the dorsal part
of the auditory cortex (see Methods).
B, Recordings from a surface electrode
placed near the midline, midway
between the bregma and the lambddoid suture . C, Recordings from a
surface electrode placed near the lamboid suture, - 2 mm from midline. pp = peak-to-peak amplitude.
4
m
>=L
d
v
4)
N
w
L
a
10
100
0 i
0
-~ ~
Modulation Frequency (Hz)
100
short (2 .6 msec). Near the midline, medial to the
auditory cortex (Fig. 4B), the MTF was bimodal,
with two peaks of roughly equal amplitude separated by a valley. The phases associated with
the low-frequency peak were well fit by a straight
line, and the delay was shorter than over the
auditory cortex but was still relatively long
(22 msec). The phases associated with the peak
at higher modulation frequencies were also well
fit by a straight line, but the delay was longer
(4 .4 msec) than at similar frequencies over the
auditory cortex . Note that the amplitudes at
high frequencies near midline were greater than
over the auditory cortex and that the peak at
these frequencies extended below 100 Hz, into
the low-frequency region over the auditory cortex. Finally, at the third, more caudal recording
site (Fig . 4C, near midbrain), the MTF was similar to that medial to the auditory cortex, except
that the peak at low frequencies was now smaller
than the peak at high modulation frequencies.
The shifts in amplitude and delay at low and
high frequencies support the idea that multiple
generators are present. At high frequencies,
there appear to be two generators . One generator is associated with an extremely short delay
(2 .6 msec) and is picked up by an electrode over
the auditory cortex. At more medial locations, the
194
200
00
signal from this generator is either absent or is
overshadowed by a stronger signal from a generator with a slightly longer delay (-4.5 msec).
At low frequencies, amplitudes decline as the
electrode is moved away from the auditory cortex, suggesting that this is the chief source at
these frequencies. The shifts in delay suggest an
additional source, but these could also be a
result of inadequate sampling of the relatively
large volume of the auditory cortex .
Effects of Behavioral and Pharmacologic
Stimulation on the AMFR
If the AMFR reflects contributions from different sources, then sources higher in the pathway might be more affected by behavioral state
than sources lower in the pathway. Since low
and high modulation frequencies are associated
with long and short delays, respectively, they are
also likely to be associated with centers higher and
lower in the auditory pathway. We tested this idea
by measuring the AMFR at low and high modulation frequencies, with and without behavioral
stimulation that consisted of gently touching the
rabbit at irregular intervals as an attempt to
keep it alert. The AMFRs to 52 and 201 Hz, before
and during behavioral stimulation, are shown
Sources of the Scalp-Recorded AMFR/Kuwada et al
A
B
Control
Behavioral Stimulation
J
0
o
frequency (201 Hz). At this modulation
frequency, behavioral stimulation had
little or no effect on the AMFR .
s
Modulation Cycles
in Figure 5 . Behavioral stimulation increased
the amplitude of the AMFR at low modulation
rates by about a factor of two (Fig. 5, A and B :
3 .9 vs 7 .6 V,V), but at high modulation rates, a
change was barely noticeable (Fig . 5, C and D :
0 .98 vs 1 .03 wV) . Thus, AMFRs at low modulation frequencies are influenced by behavioral
stimulation, whereas those at high modulation frequencies are not . This supports the idea that the
AMFRs over different ranges of frequency are
dominated by different sources, with the source
at low modulation frequencies lying in a higher
center such as the cortex, which is more likely
influenced by behavioral stimulation.
Using the same rationale, we tested the
effects of a stimulant (cocaine) at a low (62 Hz)
A
D
Control
E
F
Modulation (cycles)
and a high modulation frequency (200 Hz). Figure 6 shows that as with behavioral stimulation,
cocaine caused a marked increase in the AMFR
at a low modulation frequency (Fig . 6, A-B) but
no change at the high modulation frequency
(Fig. 6, D-E). Approximately 80 minutes after
the injection of cocaine, the AMFRs at low modulation frequencies returned to their control
levels (Fig . 6C).
Since stimulation (behavioral or cocaine)
elevated the AMFRs to low modulation frequencies, we reasoned that a depressant (sodium
pentobarbital) should attenuate the AMFRs at
these frequencies . Pentobarbital eliminated the
two lowest-frequency peaks of the MTF, replacing them with a broad plateau of roughly con-
Figure 6 Cocaine enhanced the
amplitude-modulation following
response (AMFR) at a low modulation frequency but not at a high modulation frequency. Organization
similar to Figure 5. Carrier frequency
was 5000 Hz at 90 dB SPL. The
responses under cocaine were taken
within a 10-minute interval immediately after an intravenous injection
Cocaine
0
Figure 5 Behavioral stimulation
enhanced the amplitude-modulation
following response (AMFR) at a low
modulation frequency but not at a
high modulation frequency. Carrier
frequency was 2000 Hz at 80 dB SPL.
The control response was taken with
the rabbit alone in the soundproofed
room, whereas behavioral stimulation was taken while the experimenter
gently tapped the rabbit's head and
nose . A and B, AMFR to a low modulation frequency (52 Hz). The amplitude ofthe AMFR increased by a factor
of 2 during behavioral stimulation. C
and D, AMFR to a high modulation
5
of cocaine (1 .5 mg/kg) and the postcocaine response (C and F) -80 minutes after the injection of cocaine. A to
C, AMFR to a low modulation frequency (62 Hz) . The amplitude
increased by a factor of ~3 immediately after injection of cocaine (B) and
returned to near control levels by -80
minutes (C). D to F, AMFR to a high
modulation frequency (200 Hz). At
this modulation frequency, cocaine
had little or no effect on the AMFR.
Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002
1
0
1 00
300
400
600
500
600
Modulation Frequency (Hz)
Figure 7 Sodium pentobarbital attenuated the amplitude of the AMFR and shortened the neural delay at low
modulation frequencies, whereas the responses to high
modulation frequencies were relatively unaffected . Control responses are the same as in Figure 2. Sodium pentobarbital was administered via an indwelling intravenous
catheter. Recordings were made during and after an initial bolus of sodium pentobarbital (25 mg/kg) followed by
supplements (12 mg/kg) at approximately 20- to 30minute intervals. A, Control and sodium pentobarbital
modulation transfer functions (MTFs) . B, Control and
sodium pentobarbital phase versus frequency plots. Corresponding frequency regions in the MTF and phase
plots are displayed as dotted vertical lines. p-p = peakto-peak amplitude.
stant amplitude (Fig. 7A). Thus, the amplitude
decreased at the peaks but increased at frequencies where valleys were present. At intermediate frequencies (region 3), the amplitude
decrease was small but consistent, and at the
highest frequencies (region 4), they hardly
changed at all. The phase portion of the MTF
also changed under pentobarbital (Fig. 7B). At
low frequencies, the slope of the fitted regression line decreased (Fig. 7B), whereas at higher
frequencies, there was barely any change . Furthermore, the slope at low frequencies (region
2) under pentobarbital was 5 msec, which
matched the slope associated with the third
peak observed in the control (region 3) . All of the
changes are consistent with the suppression of
a generator at low frequencies that had been
activated with a long delay. The remaining signal at these frequencies under pentobarbital
appears to be generated by a source with a
shorter delay.
Further evidence for source interactions
was the changes in the AMFR waveform under
196
sodium pentobarbital, cocaine, and ketamine .
Figure 8A compares the AMFR waveform under
sodium pentobarbital and cocaine at a modulation frequency (45 Hz) that corresponded to a valley in the MTF (see Fig. 3) . Where two peaks per
modulation cycle were present prior to administration of pentobarbital (control), one narrow
and one broad, under pentobarbital, the broad
peak was attenuated, leaving only the narrow
peak . In contrast, under cocaine, a single, large
,peak was present at the phase where the broad
peak had been previously. The amplitude of the
single peak under cocaine was over 5 times the
amplitude of the broad peak prior to cocaine
administration. The changes under pentobarbital
and cocaine are consistent with the presence of
two generators, one of which was selectively
depressed by pentobarbital and enhanced by
cocaine . Along with Figure 6, the effects of
cocaine are consistent with the presence of two
generators, which are active over different but
overlapping ranges of frequency.
6
Modulation Cycles
5
Figure S Sodium pentobarbital, cocaine, and ketamine selectively altered the waveform of the amplitudemodulation following response at or near the nulls in their
modulation transfer functons . Control, sodium pentobarbital, cocaine, and ketamine HCI/xylazine were taken
in separate sessions in the same rabbit using the same
recording configuration (vertex referenced to frontal
sinus) . A, Control, pentobarbital, and cocaine responses
to a 45-Hz sinusoidally amplitude-modulated (SAM) tone
(carrier = 5000 Hz, 90 dB SPL). B, Control and ketamine
responses to a 47-Hz SAM tone (carrier = 2000 Hz, 80 dB
SPL) . In both A and B, as a visual aid, vertical dashed
lines are positioned at the initial peak of the control
response.
Sources of the Scalp-Recorded AMFR/Kuwada et al
Figure 8B compares the AMFR, with and
without ketamine at a slightly different modulation frequency (47 Hz) than that used in Figure 8A (45 Hz) but still near a valley in the
MTF (see Fig. 3) . Here the waveform over one
cycle of modulation displayed three peaks in
the unanesthetized state (control). Under ketamine, the waveform was simplified . It appeared
that ketamine did not affect the source associated with the initial peak in the cycle but
depressed the activity of the sources associated
with the subsequent peaks.
Identification of Sources of the AMFR
The AMFRs at low frequencies appeared to
be activated at long delays and also appeared to
be selectively modulated by a depressant and a
stimulant . To investigate the possibility that
AMFRs at these frequencies were cortical in
origin, we reversibly inactivated the cortex using
KCI-induced spreading depression while recording the AMFR from a surface electrode . Figure
9A shows the amplitude (left panel) and phase
(right panel) MTFs to a range of low (23-37 Hz)
and high (101-201 Hz) modulation frequencies
in the unanesthetized animal . As usual, the
responses at low modulation frequencies were
large and their delays long relative to the higher
modulation frequencies . Applying KCI to the
cortex ipsilateral to the stimulated ear had minimal, if any, effect on the AMFR (Fig. 9B) . This
suggests that the sources of the AMFR do not
reside in the cortex on the same side as the
stimulated ear. However, KCI applied to the
cortex contralateral to the stimulated ear (Fig .
9C) selectively attenuated the AMFRs at low
modulation frequencies, whereas those at high
modulation frequencies remained relatively
unchanged . Furthermore, after KCI application,
the phase slopes at the low modulation frequencies appeared to align with those at the
high modulation frequencies . These changes in
the surface-recordedAMFR suggest that a major
source at low modulation frequencies is located
in the cortex opposite to the stimulated ear . In
contrast, the AMFRs at high modulation frequencies were relatively unaffected by cortical
depression, suggesting that their origin lies in
subcortical regions .
To determine if the different delays observed
in the surface-recorded AMFR over different
ranges of modulation frequency corresponded to
delays in structures along the auditory pathway,
we made local recordings of the AMFR in the
auditory cortex (Fig. 10A), inferior colliculus
(Fig . 10B), and superior olivary complex (Fig .
10C) . The MTFs show that the peak modulation
frequency decreases from the superior olivary
complex to the auditory cortex (Fig . 10, left
Figure 9
A
Control
4 msec
22 msec
B Ipsilateral KCI
a 41 C
n
Contralateral KCI
3 msec
27 msec
2-1
4 msec
A
200
0
Modulation (Hz)
SPL, and recording configuration was
vertex referenced to frontal sinus. Right
and left columns display the amplitude and phase modulation transfer
functions, respectively. Owing to time
constraints imposed by cortical inactivation, we tested only a limited range
oflow modulation (23-37 Hz) and high
modulation frequencies (101-201 Hz).
A, Control responses (unanesthetized).
'9-criterion >3
-o--criterion <3
100
Cortical inactivation of the
cerebral cortex contralateral to the
stimulated ear attenuated the amplitude of the amplitude-modulation following response and shortened the
neural delay at low modulation frequencies, whereas inactivation to the
ipsilateral cortex had little if any effect
at all on modulation frequencies . Carrier frequency was 2000 Hz at 80 dB
so
_
10
15o 0
zoo
B, Responses under potassium chloride (KCI)-induced inactivation of the
cortex ipsilateral to the stimulated ear.
C, Responses under KCI-induced inac-
tivation of the cortex contralateral to
the stimulated ear. p-p = peak-to-peak
amplitude.
197
Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002
A
Superior Olivary Complex
2 .9 msec
3.2 msec
B
_~
Inferior Colliculus
A
C
1,
d
a
Figure 10
The peaks of locally
recorded amplitude-modulation following responses shift to lower mod-
ulation frequencies and the phase
slopes increase at higher stations
along the auditory pathway. Right
and left columns display the amplitude and phase modulation transfer
functions, respectively. A, Local
recordings in the superior olivary
complex to sinusoidally amplitude
modulated (SAM) tones (carrier =
6000 Hz, 70 dB SPL). B, Local recordings in inferior colliculus to SAM
tones (carrier = 8000 Hz, 55 dB SPL) .
C, Local recordings in auditory cortex to SAM tones (carrier = 2000 Hz,
70 dB SPL) . Recordings are single
ended.
Auditory Cortex
3
.a
N
E
V
d
v
d
a
N
N
A
a
E
`o
z 0
10
100
0
1 27 3 msec
T
1000
100
200
Modulation Frequency (Hz)
panel) . It is approximately 250 Hz for the superior olivary complex, 90 Hz for the inferior colliculus, and 20 Hz for the auditory cortex . The
corresponding phase plots also show an increase
in neural delay from the superior olivary complex to the auditory cortex . In addition, each
Figure 11 Local recordings in the
auditory cortex can display similar
nulls and similar neural delays .
C
Recordings are from a subset of adjacent microwires (eight) in an array
chronically implanted in the auditory
cortex (see Methods) . Carrier frequency was 4000 Hz at 50 dB SPL.
Each wire was referenced to the same
microwire in the array that did not
show amplitude-modulation following response activity. A, Normalized
amplitude modulation transfer functions (MTFs) from the eight
microwires . Although peak amplitudes
occurred at different modulation frequencies, all MTFs showed a null at
71 Hz . B, Mean MTF and standard
errors of the responses in A. C, Phase
plots from the eight microwires . For
illustrative purposes, the phase plots
from each wire have been incremented
by one cycle. D, Mean phase plots and
standard error of the nonincremented
phase plots in C.
p
msec
y
d
0
N
U)
t
a
m
8-
19
~-
-
lomsec
24
11 msec
4
15
19
2
11 msec
-
11 msec
1
11 msec
2-
0
11 msec
~ 16
6-
0
msec
20
20 40
60 80 100 120 140 160 180 200
0 i--i--r--r
0 20 40 60
Modulation Frequency (Hz)
198
80 100 120 140 160 180 200
Sources of the Scalp-Recorded AMFR/Kuwada et al
A Best Modulation Frequency
2s a
r--,
phase plot displays two linear segments corresponding to different neural delays . In each
structure, the shorter delays are associated
with higher modulation frequencies, where the
amplitude is smaller. The longer delays may
reflect the activity of the neurons intrinsic to the
structure, and the shorter delays may reflect the
n1 ~I II II II II II II II Ir-,n~
activity of the inputs to that structure . If this
is so, then the long delay at low modulation
frequencies observed in surface-recorded AMFRs
(-27 msec) may correspond to the activity of neurons in the auditory cortex. However, the shorter
delays observed in surface recordings (-5 and
-3 msec) do not appear to correspond to the
delays of neurons in the inferior colliculus or the
superior olivary complex ; instead, they seem to
represent the delays of neurons feeding into
these structures .
We noticed considerable variability in the
locally recorded AMFRs within a structure . This
is probably because the electrode tips were small
and sampled a small area surrounding the electrode tip . To sample the AMFRs in the largest
structure more effectively, we made local recordings in the primary auditory cortex using a
microwire array. As expected, different electrodes
displayed different MTF shapes . However, a subset of the array (eight of the microwires) in close
proximity to each other (four in one row and four
in the other row) displayed remarkable similarities in their MTFs (Fig . 11, A and B) and phase
plots (Fig . 11, C and D) . The carrier frequency
(4 kHz) and level (50 dB SPL) were the same for
all of the recordings shown . Note that although
the position of the peaks varies among the wires,
all had a common null near 70 Hz (see Fig. 11A) .
The mean MTF shows a plateau between 20 and
40 Hz and a peak near 120 Hz, separated by a null
near 70 Hz (Fig . 12B) . The remaining microwires
(16/24) did not display this null . The phase plots
indicate that the lower modulation frequencies
(21-61 Hz) are associated with longer delays
compared with those at higher modulation fre-
quencies (91-181 Hz) .
Do the locally recorded AMFRs in the auditory cortex match the activity of neurons in this
structure? Figure 12 displays the distribution of
the best modulation frequency (A) and the mean
synchrony (B) of single cortical neurons recorded
extracellularly with a microelectrode . The best
modulation frequency and the highest synchrony
are both about 10 to 15 Hz . This corresponds, in
general, to the modulation frequency associated
with the peak amplitude of locally recorded
AMFRs from the auditory cortex (see Figs . 10C
and 11) .
1-2
4-6
6-12
16-24
32-48
64-96 128-192
z
0
r
U
fI
Modulation Frequency (Hz)
Figure 12 Neurons in the auditory cortex prefer low
modulation frequencies . The cortical neurons (n = 114)
were usually studied with the same range of modulation
frequencies (1-400 Hz in 0 .5-octave steps) using carriers
at their best pure-tone frequency (-2-20 kHz) and usually at a stimulus level of 70 dB SPL. Responses are all
to sinusoidally amplitude-modulated tones presented to
the contralateral ear (re: recording site). A, Distribution
of best modulation frequency (i .e ., the modulation frequency that evoked the most spikes and had significant
synchrony (p < .001, Rayleigh coefficient) . B, Mean synchrony and standard error across modulation frequency.
For each neuron, only responses that showed significant
synchrony (p < .001, Rayleigh coefficient) were included .
The synchrony across neurons was averaged at each
modulation frequency. See Kuwada and Batra (1999) for
details of synchrony and best modulation analysis and
Fitzpatrick and colleagues (2000) for details of cortical
recordings .
DISCUSSION
B
y analyzing waveforms, varying electrode
placement, performing pharmacologic
manipulations, and recording both local AMFRs
and single units, we have provided evidence
that the surface-recorded AMFR reflects contributions from multiple sources. We will, in
turn, discuss the neural substrates of the AMFR,
compare our results with other studies investigating the sources of the AMFR, and discuss
the use of the AMFR as an audiometric tool .
Neural Substrates of the AMFR
At every level of the auditory system, neurons can temporally follow the envelopes of mod-
199
Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002
ulated signals. However, the upper limit of modulation frequencies that neurons can follow
decreases as the information ascends along the
auditory pathway (see Frisina, 2001, for a
review). In the auditory nerve, the MTF is lowpass in shape, with a corner cutoff frequency of
about 800 Hz for neurons tuned above - 2kHz
(Joris and Yin, 1992). For most neurons in the
inferior colliculus, the MTFs are bandpass in
shape, with the bulk of best modulation frequencies below - 100 Hz (Rees and Moller, 1983 ;
Batra et al, 1989 ; Krishna and Semple, 2000).
Cortical neurons also display bandpass MTFs,
and the bulk of their best modulation frequencies are below - 20 Hz (Schreiner and Urbas,
1986, 1988 ; see also Fig. 12). Our locally recorded
AMFRs also showed a similar decrease in the
ability to follow high-frequency envelopes at
progressively higher stations along the auditory pathway. Since neurons in all structures can
follow low modulation frequencies, the surfacerecorded AMFR to these frequencies can reflect
contributions from all levels of the auditory
pathway. In contrast, responses to higher and
higher modulation frequencies can only reflect
contributions from lower and lower auditory
structures .
Neurons in the same structure can process
envelopes differently. For example, in the cochlear nucleus, neurons with an onset discharge
pattern show higher synchrony to envelopes
than those with a sustained discharge pattern
(Rhode and Greenberg, 1994). In the superior olivary complex, the phase of the response of sustained neurons to envelopes is often 180 degrees
out of phase to that of neurons with an offdischarge pattern (Kuwada and Batra, 1999).
Even the same neurons can show different
responses under different circumstances . In the
cochlear nucleus and at higher stations, the
MTFs of some neurons can change from lowpass
to bandpass depending on the sound level
(Moller, 1974 ; Rees and Moller, 1983 ; Frisina,
2001 ; Krishna and Semple, 2000) . Moreover,
their MTFs often show two peaks separated by
a suppressive region (Krishna and Semple,
2000). Thus, the valleys and associated complex waveforms (doubling or tripling ; e.g ., Fig.
3) in the surface-recorded AMFRs could represent the heterogeneous response of neurons
within a structure, as well as the different
responses between structures .
Neurons in the cortex of the rabbit can
follow envelopes up to - 200 Hz . However, most
cortical neurons have their best modulation frequencies (amplitude and synchrony) below
200
-15 Hz (see Fig. 12). A similar distribution has
been reported in the auditory cortex of the cat
(Schreiner and Urbas, 1988). Furthermore, this
best modulation frequency corresponds to the
peak frequency for locally recorded AMFRs in the
auditory cortex of the rabbit (e .g ., see Figs . 10C
and 11). In the midbrain, local recordings of the
AMFR show a peak around -100 Hz (see Fig.
11B). However, the most prominent peak in the
surface-recorded AMFR of the rabbit usually
occurs neither at the peak activity of the cortex
nor at the peak activity in the inferior colliculus. Thus, it is unlikely that the surface-recorded
AMFR is a simple reflection of a single generator but instead is likely to be the result of multiple generators sensed by the surface electrode.
Similar events may underlie the circa 40-Hz
potentials in humans . That is, the large circa
40-Hz potential associated with human AMFRs
or with click trains at that frequency (Galambos
et al, 1981) is likely to be attributable to source
interactions that reinforce over that frequency
range rather than to entrainment of underlying
40-Hz neural oscillators .
Identifying the Generators in
the Surface-Recorded AMFR
Our results indicate that the surfacerecorded AMFR has multiple generators . Furthermore, different generators appear to
dominate over different ranges of modulation frequency and in different recording configurations . At low modulation frequencies (<80 Hz),
the cortex appears to be a major source . At
higher modulation frequencies, there appears to
be at least two generators that are likely subcortical. One generator has a delay of - 5 msec
and corresponds to perhaps a midbrain or pontine source . The other has a delay of - 3 msec
and corresponds perhaps to the superior olivary
complex or the cochlear nucleus .
The scalp or surface-recorded AMFR reflects
both near- and far-field sources ; therefore, the
contributions of these sources depend on brain
size, in general, and the distance from the recording electrode, in particular. For example, the
surface-recorded MTF in the rabbit has more
peaks than that in the human, suggesting more
source summation, especially at low modulation frequencies. This suggests that the generators in the rabbit are in closer proximity to
the surface electrode than in humans . As another
example, the scalp-recorded AMFRs in gerbils
(Dolphin and Mountain, 1992) display not only
synchrony to envelopes but also robust syn-
Sources of the Scalp-Recorded AMFR/Kuwada et al
chrony to the carrier frequency. Robust synchrony to carrier frequencies is not present in
neurons at or above the level of the inferior colliculus in rabbits (Stanford et al, 1992) and is not
detectable to carrier frequencies much above
--500 Hz in the surface-recorded AMFRs of rabbits and humans (Batra et al, 1986) . This suggests that surface recordings in the gerbil detect
almost equally contributions from structures
below and above the midbrain . Moreover, they
reported a consistent valley between 200 and
300 Hz, above which estimates of neural delays
became highly unreliable . The likely explanation
is the smaller brain size and, consequently, the
smaller distance of the recording electrode from
the generators in the gerbil compared with that
of the rabbit and human . Thus, the enumeration
of sources of the AMFR with different delays is
simplified in larger brains with widely spaced
generators .
The cortex appears to be optimally activated by low modulation frequencies and
subcortical structures by high modulation frequencies . Compared with low modulation
frequencies, at high modulation frequencies
(>150 Hz), the contributions of the cortex are
minimal based on the preferred modulation fre-
quencies of local cortical AMFRs and of cortical
neurons . Moreover, the AMFRs to low modulation frequencies have long neural delays,
whereas those to higher modulations have
shorter neural delays . Finally, the AMFRs at low
modulation frequencies are markedly affected by
behavioral and pharmacologic interventions,
whereas those at high modulation frequencies
are not .
In general, since inhibition cumulatively
increases along the auditory neuraxis, agents
that potentiate inhibition should have an increasing effect at higher and higher structures . A
major inhibitory transmitter is y-aminobutyric
acid (GABA), and sodium pentobarbital is known
to potentiate GABA-mediated inhibition (Barker
and Ransom, 1978). Sodium pentobarbital
depresses the activity of neurons in the inferior
colliculus (Kuwada et al, 1989) and auditory
cortex (Fitzpatrick et al, 2000). Consistent with
our reasoning, sodium pentobarbital had little,
if any, effect on the responses to the highest
modulation frequencies (see Fig. 7, region 4),
which had the shortest delays and are likely to
be generated by lower brainstem sources (e .g.,
cochlear nucleus or superior olivary complex) .
However, it reduced the AMFR at intermediate
modulation frequencies (see Fig. 7, region 3),
which had a longer delay and are likely to be gen-
erated by sources in the midbrain or pons, and
caused a profound reduction in amplitude at
the lowest frequencies (see Fig. 7, regions 1 and
2), which had the longest delays and are likely
generated by cortical sources.
The variability in the delay of the AMFR at
low frequencies, which we believe to be cortical
in origin, may have been due to the variation in
the processing of amplitude-modulated sounds
in different cortical areas . The cortical contributions to the AMFR are complex because there
are several cortical areas that code sound
envelopes (e .g ., Schreiner and Urbas, 1986,
1988) . Although neurons in all areas respond to
low modulation frequencies, some areas in the
cat's cortex are tuned to very low frequencies
(<7 Hz, secondary auditory cortex, posterior
auditory field, ventral posterior auditory field),
others to intermediate frequencies (--14 Hz, primary auditory cortex), and still others to higher
frequencies (- 30 Hz, anterior auditory field) .
Furthermore, the neural latencies in these areas
can differ substantially . For example, neurons
in the primary auditory cortex show latencies
between - 10 and 20 msec, whereas those in the
posterior auditory field show latencies between
20 and 100 msec (e .g ., Schreiner and Urbas,
1988). In general, our surface- and locally
recorded AMFRs displayed features consistent
with the heterogeneity of such cortical sources.
For example, the surface-recorded neural delays
to low modulation frequencies ( :580 Hz) ranged
between 19 and 33 msec, and those from local
cortical recordings ranged from 15 to 27 msec .
The peaks and valleys in this range (e .g., Fig. 7)
suggest that they are the result of contributions
from multiple sources within the cortex .
Comparison with Other Studies
There are studies that have investigated
the sources of event-related potentials (middle
latency response and the possibly related 40-Hz
potential), and most have used an anesthetized
animal . Since we have shown that anesthesia has
a major effect at low modulation frequencies, we
consider here only studies that have used an
unanesthetized preparation. Our results are in
disagreement with studies that did use an
unanesthetized animal model to investigate the
sources of the surface-recorded AMFR . Kiren
and colleagues (1994) measured the phase coherence of the surface-recorded AMFR to SAM tones
in cats and reported that bilateral ablation of the
auditory cortex had little effect on this measure, whereas lesions of the inferior colliculus
201
Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002
contralateral to the sound stimulation made
the AMFR undetectable at all modulation frequencies (20-200 Hz) . They concluded that the
inferior colliculus was the prominent generator
and that the auditory cortex was not a generator for the surface-recorded AMFR . At first
glance, their results are not in accord with our
findings . However, they based their conclusion
solely on changes in the phase coherence, which
is a measure of detectability, and provided no
information about amplitude . If that had been
our only measure, we also would have concluded
that the cortex was not a generator of the AMFR
as the AMFR was still detectable at most low frequencies . However, at these frequencies, the
amplitude of the AMFR was markedly attenuated after inactivation of the contralateral cortex, and its neural delay was considerably
shorter and similar to that at higher modulation
frequencies (see Fig. 9) . A study similar to that
of Kiren and colleagues (1994) investigated the
effects of cortical and collicular lesions on the
40-Hz click-evoked steady-state response
(Tsuzuku, 1993) . Although focal ablation of the
primary auditory cortices caused only a small
reduction (-10%), bilateral lesions of the inferior
colliculi produced a marked reduction (-40%) in
the 40-Hz response . The small reduction after
ablation of the primary auditory cortex is in
contrast to the marked reduction we observed
under cortical inactivation . One resolution may
be that areas of the cortex other than the primary
auditory cortex may be the major contributors
to the surface-recorded AMFR and 40-Hz
response . Another possibility is that the 40-Hz
response evoked by clicks (a broad-band stimulus) activates different generators than the
AMFRs to SAM tones . Support for this latter possibility is the finding that contralateral ablation
of the inferior colliculus abolished the AMFR to
SAM tones (Kiren et al, 1994), whereas the
40-Hz click response was still detectable, albeit
reduced (Tsuzuku, 1993) .
Use of the AMFR as an Audiometric Tool
Although the AMFR is most robust at low
modulation frequencies in humans, its use as an
audiometric tool at these frequencies is problematic because the amplitude changes with
state. For example, Cohen and colleagues (1991)
showed that the AMFRs to modulation frequencies around 40 Hz were attenuated in sleeping
adults . If behavioral state is controlled by tasks
such as reading, then the AMFR at low modulation frequencies can be used as an audiometric tool
202
(Kuwada et al, 1986). However, the use of electrical audiometry is most important in assessing
hearing in infants, and it is difficult to control their
behavioral state. For this reason, the AMFR at
low modulation frequencies has been viewed as
unsuitable for electrical audiometry.
Another reason for the disfavor of the circa
40-Hz AMFR is that it is difficult to detect in
sleeping and sedated infants. This is consistent
with our findings that, in rabbits, anesthesia
markedly attenuates the AMFR to these modulation frequencies. Furthermore, cortical inactivation through KCl-induced spreading depression
had similar attenuating effects to sodium pentobarbital . Since sleep or sedation is known to
dramatically reduce cortical activity, it is not
surprising that the circa 40-Hz AMFR was difficult to measure in sleeping or sedated infants.
So, the skepticism about the utility of the AMFR
was partly due to its variability with behavioral
state and also partly because it was tested in
sedated or sleeping children .
Later, it was discovered that AMFRs to
higher modulation frequencies (80-110 Hz) were
relatively unaffected by sleep in adults (Cohen
et al, 1991), and soon thereafter, a similar observation was made in sedated or sleeping infants
(Aoyagi et al, 1992 ; Levi et al, 1992). Although
the amplitudes at these frequencies are smaller
than those at low modulation frequencies, the
noise level is also lower so that the detection of
the AMFR is not compromised . We have also
found that the AMFRs in the rabbit were less
affected by behavioral state, anesthesia, stimulants, and cortical inactivation for higher modulation frequencies .
The reasons for the superiority of the circa
80-Hz AMFRs over the circa 40-Hz AMFRs
as an audiometric tool may be attributable to
several factors . First, this higher modulation
frequency may optimally drive subcortical
sources that are relatively unaffected by
behavioral state and anesthesia . Second, this
frequency eliminates the cortical contributions because cortical neurons are optimally
driven by lower modulation frequencies . Third,
sedation or sleep also eliminates cortical
sources and may reduce extraneous noise.
Fourth, the smaller size of the infant's brain
may increase the detectability of subcortical
activity compared with the larger adult brain.
Fifth, since the detectability of the AMFR
depends on the number of averages, twice the
time is needed for the same number of averages for a 40-Hz response compared with an
80-Hz response . In this way, less time is
Sources of the Scalp-Recorded AMFR/Kuwada et al
required to detect high than low modulation
frequencies . Finally, the developing cortex
may be poor at encoding any modulation frequencies, whereas subcortical structures may
be closer to their adult abilities .
Acknowledgment . We thank Dr. Ahmed Khan for participation in the behavioral stimulation experiments and
Dr. Sean J. O'Connor and Ms . Nancy Kluck for partici-
pation in the cocaine experiments. This work was
supported by National Institutes of Health grant R01
DC 01366 to S. Kuwada . The data in this manuscript
were presented orally at the International Evoked
Response Audiometry Study Group conference held in
Vancouver, British Columbia, July 22-27, 2001.
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