Electroencephalography and clinical Neurophysiology, 90 (1994) 82-90
© 1994 Elsevier Science Ireland Ltd. 0013-4694/94/$07.00
The auditory evoked sustained field:
origin and frequency dependence
C. Pantev *, C. Eulitz, T. Elbert and M. Hake
Wilhelms-University ofMunster, Institute ofExperimentalAudiology, Centre ofBiomagnetism, Kardinal-von-Galen-Ring 10,
D-48129 Munster (Germany)
(Accepted for publication: 18 August 1993)
dependence of the magnetic field distribution of this "per-stimulatory" sustained field (SF) on tone frequency. Tone bursts of 2 sec duration and
60 dB nHL intensity were presented to 11 subjects at varying interstimulus intervals between 5 and 7 sec. The carrier frequencies of 250, 1000
and 4000 Hz varied randomly from trial to trial. The field distributions obtained are consistent with the view that the auditory evoked sustained
field activity originates in the supratemporal cortex. Differences in the locations of equivalent current dipoles of the SF from those of the MlOO
wave of the slow auditory evoked field are consistent across subjects. The SF source locations corresponding to stimulus frequencies over an
extended frequency range are arranged in a tonotopic manner and support the idea that the sources of the MlOO and the SF are current dipole
sheets located on the superior surface of the primary auditory cortex.
A sound lasting for several seconds is known to elicit a basline shift in electrical and magnetic records. We have studied the
Key words: Auditory evoked sustained field; Auditory evoked field; Auditory evoked sustained potential; Auditory evoked potential; EEG; MEG
A transient auditory stimulus elicits several transient
deflections (auditory evoked responses) following the
onset of the stimulus (Picton et al. 1974). If the stimu-
lus is of longer duration, the transient deflections of
the on-effect are followed by a stimulus-locked DC
shift that lasts as long as the stimulus is present. This
"per-stimulatory" DC response is called sustained po-
tential (SP) in electrical records, or sustained field (SF)
in magnetic records. It is usually followed by an off-re-
sponse which is time-locked to the end of the transient
auditory stimulus. Although evidence for the SP was
provided by K6hler et al. as early as 1952, using DC
recording from the auditory cortex of the cat, and by
K6hler and Wegener in 1955 in DC scalp records of
humans, the number of investigations of the SP and SF
during the following 40 years has remained small com-
pared to the number of studies of transient slow audi-
tory evoked potentials (AEP) and fields (AEF). One
reason for this difference may be technical difficulties,
as electrical records are contaminated by electrode
drifts and skin potentials. Such artifacts do not appear
in magnetic records. Experimental studies of auditory
evoked sustained potentials include those by David et
al. (1969), Keidel (1971), Jarvilehto and Fruhstorfer
(1973), Picton et al. (1977, 1978a,b) and Hari et al.
* Corresponding author.
SSDl 0013-4694(93)E0212- 0
(1979a,b). Sustained fields were investigated for the
first time by Hari (1980) and Hari et al. (1980, 1987).
Across the scalp, the auditory evoked sustained nega-
tive potential shift becomes largest over the fronto-
central regions. This is probably a consequence of
volume conducted activity generated in the supratem-
poral cortex. With respect to functional behaviour, the
auditory sustained potential is quite distinct from the
slow transient AEP that occurs at the onset and offset
of auditory stimuli. The excellent experimental study of
Picton et al. (1978a) revealed that, whereas the tran-
sient AEP can be attenuated by intermixing a series of
clicks and tonebursts, this manipulation leaves the SP
unchanged. This means that the two phenomena, the
slow AEP and the SP, have different refractory proper-
ties as well as generator mechanisms and, thus, physio-
logically distinct generators.
The objection of Jarvilehto and Fruhstorfer (1973)
that the SP is probably a type of contingent negative
variation (CNV), associated with subjective uncertainty
about the timing of stimulus onset (rather than a
sensory response to the auditory stimulus), has been
disproved by the results of Picton et al. (1978a) which
show that, in contrast to the CNV, an SP can be
recorded even in the absence of any attention to the
stimulus. The auditory SP can be recorded during the
waking state as well as during sleep, as opposed to the
CNV, which requires a sufficient degree of attentive
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First publ. in: Electroencephalography and Clinical Neurophysiology 90 (1994), 1, pp. 82-90
AUDITORY SUSTAINED FIELD
expectancy. Therefore it seems that the auditory SP
and CNV are independently generated phenomena
that can be concurrently recorded if an appropriate
stimulus paradigm is used (Picton et al. 1978a). This
view receives support from the localization results for
the SP reported by Scherg et al. (1989) and Scherg and
Picton (1991), and the localization of the SF reported
by Hari (1980) and Hari et al. (1980, 1987). These
authors found the sources of the SP and the SF to be
located in the supratemporal cortex near the sources of
the major component of the slow AEF (M100), having
a latency of about 100 msec after the stimulus onset,
which has been repeatedly shown to indicate the loca-
tion of the auditory cortex and association areas
(Yamamoto et al. 1988; Pantev et al. 1990; Papanico-
laou et al. 1990; Pantev 1992). On the other hand, the
magnetic counterpart of the CNV, the contingent mag-
netic variation (CMV), cannot be accounted for by a
single equivalent dipole (Elbert et al. 1993).
Tonotopicity is one of the general principles of the
functional organization of the auditory system. That
tonotopic organization is maintained throughout the
central auditory pathway, including the primary audi-
tory cortex, is known not only from animal studies
(Merzenich and Brugge 1973; Merzenich et al. 1975,
1976; Schreiner 1991) but also from human studies
(Elberling et al. 1982; Romani et al. 1982; Lauter et al.
1985; Pantev et al. 1988, 1989, 1991a; Yamamoto et al.
1988; Bertrand et al. 1991). The depth of the source of
the MlOO component of the slow AEF indicates the
tonotopic organization of the auditory cortex: the
higher the stimulus frequency, the deeper the equiva-
lent source of the MlOO component. Moreover, in most
subjects the depth of the MlOO equivalent source in-
creases linearly with the logarithm of the stimulus
Existing information concerning the origin of the
auditory evoked sustained response is still insufficient:
there exists practically no information about the be-
haviour of the source of the sustained activity as the
stimulus frequency changes. Picton et al. (1978b), using
the technique of Butler (1968), investigated the fre-
quency specificity of the refractory period of the SP to
gain further knowledge of the physiological character-
istics of the sustained potential generator. Their results
were easily interpreted in terms of overlapping recep-
tive fields for the generator processes at each fre-
quency. Whereas the NlOO component of the slow
AEP has a well-defined receptive field, the expression
of frequency specificity in the SP was much more
complex. The results suggested that there might be two
distinct generator processes for the auditory SP, one
non-specific in its frequency receptive field, and one
specific (Picton et al. 1978b).
This paper reports the results of a neuromagnetic
study designed to investigate the nature and the sources
of the auditory SF as well as their tonotopic organiza-
tion, compared with the corresponding tonotopic orga-
nization of the sources of the MlOO component of the
slow AEF. Based on our experimental findings it ap-
pears that the auditory evoked sustained activity, or at
least the part generated by the tangentially oriented
sources which can be recorded magnetically, is indeed
supratemporal cortex. The sources of the SF are quite
distinct from those of the slow AEF. The source loca-
tions corresponding to stimulus frequencies over an
extended frequency range differ in space. These results
support the idea that the sources of the MlOO and the
SF are current dipole sheets located at the superior
surface of the primary auditory cortex (Hari 1980).
Six male and 5 female subjects with no history of
otological or neurological disorders and normal audio-
logical status (air conduction and bone conduction
thresholds in the range from 250 to 8000 Hz no more
than 10 dB hearing level), aged between 21 and 33
years (mean age 25.8 years), participated in this study.
All subjects were right-handed as determined with a
modified handedness questionnaire of Annett (1967).
Informed consent was obtained from each subject after
the nature of the study was fully explained to her or
him. The subjects were paid for their participation.
Stimuli were delivered to the right ear, and AEFs
were recorded from the left hemisphere, since previous
studies have shown that stronger AEFs are recorded
over the hemisphere contralateral to the side of hand-
edness (Elberling et al. 1981; Hoke 1988), and con-
tralateral to the side of stimulation (Elberling et al.
1981; Reite et al. 1981 Pantev et al. 1986). Tone bursts
of 2 sec duration (10 msec rise and decay time, cosine
function) and 60 dB nHL (normative hearing level)
were presented to the subject's right ear (contralateral
to the investigated hemisphere) with an interstimulus
interval randomized between 5 and 7 sec. The carrier
frequencies of 250, 1000 and 4000 Hz were varied
randomly, and each frequency appeared a total of 128
times within the 2 blocks of 192 stimuli each. The
stimuli were presented through a non-magnetic and
echo-free stimulus delivery system with an almost lin-
ear frequency characteristic (deviations less than ±4
dB in the range between 200 and 4000 Hz). During
stimulus presentation the subjects were asked to keep
their eyes open, to stay awake, and to minimize eye
movements and eye blinks during stimulus presenta-
c. PANTEV ET AL.
..............-·--..---·-·----.- Ch 26r
Fig. 1. a: set of 37 averaged wave forms of 2400 msec length
(N = 128), obtained in the passband between DC and 100 Hz from
the left hemisphere of one subject in response to a 2 sec, 1000 Hz
tone burst, schematically shown at the bottom of part b. The wave
forms are projected onto a sketch of the head (not precisely in scale).
b: enlarged presentation of the magnetic wave forms in the region
marked by dotted lines in a, connecting the minimum and the
maximum of the sustained field pattern. Clear extrema and polarity
reversal are evident for both the M100 of the slow AEF, in the on-
and off-responses and also for the sustained field. c: isofield contour
map of the sustained field at a latency of 1300 msec after stimulus
onset. Outward going flux is marked by solid lines in the contour
plot, inward going flux by dotted lines. Spacing between two adjacent
isocontour lines is 50 fT. d: isofield contour map of the MIOO at
latency of 94 msec after the stimulus onset. Plot parameters are the
same as in c.
position indicator system) was the midpoint of the line
between the preauricular points. The x-axis joined the
origin to the nasion; the y-axis passed between the
preauricular points with positive values towards the left
preauricular point. The z-axis was perpendicular to the
x-y plane. Correlations between the theoretical field
generated by the model and the observed field were
used to estimate the goodness of fit of the model
parameters. The x, y and z coordinates of the dipole
location with their confidence volumes were calculated
in the above described head frame-based Cartesian
coordinate system. Only estimates with a goodness of
fit above 0.90 were considered for further analysis.
The source locations of the M100 were estimated
after the usual criteria for best matching of: the aver-
aged M100 peak latency in the recording channels
around the maximum and the minimum of the mea-
sured field distribution, the maximal RMS, the maxi-
mal dipole moment and the maximal goodness of fit in
the time range of ±10 msec around the averaged
M100 peak (Pantev et al. 1991a,b). The estimation of
(d) Data analysis
Wide-band responses to each of the two blocks were
first selectively averaged after artifact rejection, which
epochs due to eye blinks or muscle activity. The peak-
to-peak amplitude threshold criterion was set to 3000
fT. Since the wave forms of the averaged responses
recorded in the two blocks were reasonably similar for
all subjects (test/retest criterion), the average re-
sponse across both blocks was used for further evalua-
tion. The wide-band responses were digitally filtered
using a second-order zero phase-shift Butterworth fil-
ter (12 dB/ oct). For the analysis of the SF, the low-pass
was set to 5 Hz. For the analysis of the MlOO compo-
nent, responses were filtered using a low-pass of 20 Hz.
After subtracting the prestimulus baseline, root-mean-
square (RMS) field values over the 37 recording chan-
nels were calculated for every sampling point.
Source analyses using a single moving dipole model
were applied to both obtained field distributions: DC-5
Hz and DC-20 Hz. A spherical model was fitted to the
digitized head shape for each subject, and the location
(x, y, z positions), orientation, and amplitude of a
best-fitting equivalent current dipole (Sarvas 1987) were
estimated for each time point. The origin of the head-
based coordinate system (determined by the sensor
(c) Neuromagnetic recording
Recording was carried out in a magnetically shielded
room (Vacuumschmelze) using a 37-channel biomagne-
tometer (Magnes™, Biomagnetic Technologies). The
detection coils of the biomagnetometer are arranged in
a circular concave array 144 mm in diameter, with a
spherical radius of 122 mm. The axes of the detection
coils are normal to the surface of the sensor array. The
distance between the centres of two adjacent coils is 22
mm; each coil itself measures 20 mm in diameter. The
sensors are configurated as first-order axial gradiome-
ters with a baseline of 50 mm. Each coil is connected
to a superconducting quantum interference device
(SQUID) that produces a voltage proportional to the
field radial to the coil. The spectral density of the
intrinsic noise of each channel was between 5 and 7
fT/ JHZ. The subjects were in a right lateral position
with their head and neck supported by a specially
fabricated vacuum cushion. A sensor position indicator
system determined the spatial locations of the sensors
relative to the head and also indicated if head move-
ments occurred during the recording. The sensor array
was centred over a point lying about 1.5 cm superior to
the position T3 of the 10-20 system for electrode
placement, as near as possible to the subjects head.
Using a bandwidth from DC to 100 Hz, stimulus-re-
lated epochs of 2400 msec (including 200 msec prestim-
ulus baseline) were recorded and stored for later analy-
AUDITORY SUSTAINED FIELD
the equivalent source locations of the evoked sustained
field proved to be more difficult. This assessment was
based on the equivalent current dipole parameters in
the time range of 800-2000 msec after the stimulus
onset, which was assumed to be free of overlapping
transient event-related components. For the calcula-
tion of the averaged source location of the SF we used
only those data points which had RMS values of at
least 3 times higher than the average RMS during the
prestimulus baseline. A second criterion for acceptance
required that the goodness of fit of the data points
within the range of 800-2000 msec had to be greater
than 0.9. If these conditions were not met, the corre-
sponding data points were excluded from further anal-
ysis. The averaged x, y, z values of the equivalent
current dipole in 3 subdivisions of 400 msec of the SF
range of 1200 msec (800-2000 msec) were calculated
for the remaining data points. If these values were
similar they were averaged. Finally, these averaged
values were taken to represent the space locations of
the equivalent SF sources and so were submitted to
further statistical evaluation using a multivariate analy-
sis of variance (MANOVA) with the 3 different tone
frequencies (250, 1000 and 4000 Hz), comprising one
within-subject factor, and the two components (MlOO
and SF) another within-subject factor.
Fig. la shows one representative set of data of one
subject in response to the 1000 Hz tone burst. Traces
are projected onto a sketch of the head. The well-de-
fined dipolar structures of the SF and the slow AEF
(on- and off-response) can be easily recognized (the
results on the off-response will be presented in a
Details of the wave forms appear in the enlarged
presentation of the region marked by dotted lines in
Fig. la. It shows magnetic activity in the supratemporal
cortex recorded from channels located along the line
connecting the minimum and the maximum of the
field, a line which corresponds well with the course of
the Sylvian fissure (Fig. lb). Clear extrema and polarity
reversals are evident for the pronounced MlOO (at
stimulus onset and offset) and also for the sustained
field. Fig. lc presents the isofield contour map of the
sustained field at a latency of 1300 msec, Fig. Id the
isofield contour map of the MlOO peak. Both field
distributions resemble the theoretical field distribution
of a current dipole, so they can be described in terms
of the parameters of an equivalent current dipole.
Inspection of the isocontour plots of the MlOO and the
SF demonstrates not only a difference in density of the
121 250 Hz
III 1000 Hz
Fig. 2. Differences between the estimated source locations (x, y, z) of the SF relative to MlOO for each subject and each frequency investigated.
The differences are presented in terms of the postero-anterior, medio-Iateral and infero-superior dimensions of the head frame. The 3
frequencies used (250, 1000, and 4000 Hz) are indicated by different fill patterns.
c. PANTEV ET AL.
isocontour lines, but also a difference in location of the
underlying current sources. It appears from these plots
that the equivalent source of the SF is located more
anterior and deeper relative to the MlOO source.
Using the source localization procedure and the
evaluation criteria described in Methods, the equiva-
lent source locations for SF and MI00 were deter-
mined for each stimulus frequency. The equivalent
current dipole location of the MI00 for each subject at
each frequency can be used as an internal marker of
the location of the subjects auditory cortex (Pantev
1992). Therefore these displays become largely inde-
pendent of the individual neuroanatomy. The differ-
ences between the estimated source locations of the SF
relative to the MlOO are presented for each subject and
separately for every frequency in Fig. 2 for the
postero-anterior, medio-Iateral and infero-superior di-
rections of the head frame.
Fig. 2 demonstrates consistent differences between
the SF and MlOO locations in antero-posterior and
medio-Iateral directions. The MANOVA results con-
firm that, over the whole frequency range, the equiva-
lent sources of the SF are located more anterior -
about 6 mm (F 0, 9) = 15.7, P <o.on and more me-
dial - about 9 mm (F 0, 9) = 23.45, P <o.oon than
the corresponding equivalent sources of the MlOO.
Figs. 3 and 4 show the dependence of the estimated
distance of the source location of the MI00 and the SF
from the origin of the 3-dimensional (3-D) Cartesian
coordinate system in the medio-Iateral direction for
each test subject investigated. This value represents
indirectly the depth of the corresponding equivalent
source. Fig. 3 shows on a semi-logarithmic scale the
results of MlOO obtained for each subject.
The absolute values on the y-axes of the 11 scatter-
plots may vary across subjects. The difference between
the highest and the lowest values, however, amounts to
3 cm in all subjects.
In scatterplots for which tonotopic organization for
the source locations was obvious, filled circles were
used. In 2 out of the 11 subjects (SS and SS) tonotopic-
ity of the supratemporal cortex remained obscure (filled
squares); in one other case (S9), the tonotopic repre-
sentation was weak. The thick lines inserted into the
scatterplots represent logarithmic fits. The coefficients
of determination (squared correlation coefficients) of
3 . 0 0 + - i - - + + T t ! i l t - - + - + + + + ~
1001000100001000 10000 100010000 100010000
.~ - - .
- - - - ~
' " = p : ~
r I ~ ~ \
6 . 0 0 ' - - - T ' " ' T ' " r " " " " " ~ i ~ S " " ' 9 : " r n 1 :
5 . 0 0 ~ 1 4 l o l ~ b . . o . l
Fig. 3. Dependence of the estimated distance of the source location of the MlOO from the origin of the coordinate system of the head frame in
the medio-lateral direction for each subject investigated. This value represents indirectly the depth of the corresponding equivalent source and is
shown on semi-logarithmic scale. The abscissa represents the stimulus frequency, and the ordinate the y coordinate of the estimated source
location. Although the values on the y-axes vary across subjects, the difference between the highest and the lowest values amounts to a constant 3
cm. The thick lines represent logarithmic function fits to the y values. The coefficients of determination (squared correlation coefficients) of
these functions are noted in the lower left part of each plot.
AUDITORY SUSTAINED FIELD
: ::: "T.-.,..
4.00 ~ U
3 . 0 0 + - r 2 + - ; = 0 ~ 9 , . t i ~ ; , l ; " i i f - - i , . . . - i - + + + + . i i
... ~ , . . . , . , . , , , . , . ! i r l ..-,-..-.,..""
..:: , ~ .
: .:: ........I!I .....HHHI
i ' = 0 ~ 5 "
::: + - · · ' ~ ~ ~ · ; ; ~ . H ~ " ' . ' l l r r = l :
I[ ii, ::: < ~
I ! ~
4.00! I; ..
3.00 + - + - + - ~ i f - - i r t +
Fig. 4. Same as Fig. 3, but for the SF.
these functions are noted in the lower left part of each
scatterplot. The calculated mean coefficient of deter-
mination amounts to a value of 0.95.
The corresponding results obtained for the equiva-
lent sources of the sustained field are illustrated in Fig.
Compared to the MlOO data, the tonotopic organi-
zation of the source locations for the 3 stimulus fre-
quencies is less evident in the SF data. The effect,
however, can be seen in 5 out of the 11 subjects. In 4
cases where a tonotopic representation is obvious in
the MlOO data (SI, S2, S6 and Sl1) it cannot be
detected in the SF data. The main reason for this
failure is that the value of the y coordinate of the
source location for the 250 Hz stimulus frequency is
smaller than the respective value at 1000 Hz. In subject
S9, the dipole fit calculations for the 4000 Hz stimulus
frequency failed. In subject S5, neither the MlOO nor
the SF data showed a tonotopic organization for equiv-
alent cortical sources corresponding to different stimu-
lus frequencies. In S8, for whom no signs of MlOO
tonotopicity are present, a clear tonotopic representa-
tion is shown for the SF with a coefficient of determi-
nation of 1. Regression analysis on y coordinates of the
source locations of the MlOO and SF over the fre-
quency range between 250 and 4000 Hz showed similar
slope coefficients of the corresponding regression lines
of - 4 and - 5 mm, respectively.
Finally, the individual x, y and z coordinates of the
estimated source locations of the MIOO and SF for the
stimulus frequencies 250, 1000 and 4000 Hz were aver-
aged across all subjects. These averaged values, pre-
sented as a 3-D plot in Fig. 5, are intended to illustrate
"centres of activity" of the excited neuronal popula-
O> ... SF.1000Hz
;~ ~ ~ ~ ~ ~ ~ ~ S ~
. . . o ~
( I l l ~ r '
Fig. 5. Mean estimated source locations of the M100 and the SF for
the stimulus frequencies 250, 1000 and 4000 Hz. The empty symbols
represent the averaged source locations of the M100; filled symbols
correspond to the averaged source locations of the SF. The 3
stimulus frequencies are coded with different symbols. Antero-pos-
terior, medio-lateral and infero-superior axes of the 3-D plot refer to
the head-based coordinate system described in the Methods section
of the text.
tions generating the two evoked responses: the MlOO
of the slow AEF, and the SF.
Statistical evaluation of the data (MANOVA, F (1,
9) = 17.5, P <o.on confirmed the visual impression
that there is a significant difference between the source
locations of the MlOO and the SF.
Differences between the origins of SF and MlOO
The auditory evoked sustained response reflects an
important aspect of central auditory processing, indi-
cating cortical activity that occurs throughout the sen-
sation of the sound rather than in response to its onset
or offset. Unfortunately, there is limited information
about the origin of the sustained response. The nature
of this evoked response, as well as its underlying brain
functions, are poorly understood. It may derive from
both the primary auditory areas and the frontal associ-
ation areas (Peronnet and Michel 1977). The apparent
activity of these two regions during the appearance of
the sustained potential indicates the functioning of
extensive connections between temporal association ar-
eas and prefrontal regions of the brain. Although Pic-
ton et al. (1978c) found the maxima of the sustained
potentials at the vertex and at frontal head regions,
Hari et al. (1980) did not observe any magnetic SF
above the fronto-central midline. This can easily be
explained by a configuration of the sources of the
sustained response being predominantly tangentially
oriented in the supratemporal cortex, but radially ori-
ented in the frontal association areas and thus invisible
in magnetic records. The results of Picton et al.
(1978c,d) showed that the SP extended less posteriorly
than the NI00 component of the transient evoked
potential. More obvious are the source analysis results
of the NI00 and the SP reported by Scherg et al. (1989)
and Scherg and Picton (1991). These authors found the
source of the SP to be located anteriorly to the NlOO
source. Another major distinction between the sources
of the NlOO and the SP was their different orienta-
tions, SP being inclined more anteriorly and medially.
These findings are in line with both the results of the
neuromagnetic study reported by Hari et al. (1987a,b)
and with the present results, in which the source of the
SF was found to be located significantly more anterior
than the source of the MlOO. We found an average
difference between the two source locations of about 6
mm in the antero-posterior direction, in line with the
observation of Hari et al. (1987). Additionally, the
source of the SF was found to be located 9 mm more
medial than the corresponding source of the MI00.
Tonotopic organization of the human auditory cortex
The most characteristic features of the tonotopic
representation of stimulus frequency in the location of
c. PANTEV ET AL.
the equivalent source of the MlOO component in the
supratemporal cortex are that lower frequencies are
usually located more superficially than higher ones,
and that the depth of the MlOO equivalent source
increases linearly with the logarithm of the stimulus
frequency (Romani et al. 1982; Pantev et al. 1988;
Yamamoto et al. 1988). In this comparative study be-
tween the source locations of the SF and slow AEF
tonotopicity was checked for a larger group of normally
hearing subjects. Tonotopicity was better seen in the
behaviour of the MlOO source than in that of the SF.
In most of the subjects in whom the tonotopic repre-
sentation of the magnetic MlOO and SF sources could
not be seen, the reason was mainly the behaviour of
the equivalent source for the 250 Hz stimulus, which
was located more superficially than the source for 1000
Hz. Although we are not able to explain this result
satisfactorily, a probable explanation may be found in
the differences of individual neuroanatomy. The indi-
vidual neuroanatomy may cause such local orientation
of the cortical surface embedding the equivalent neu-
ronal sources for 250 Hz that the cortical activity
cannot be properly detected by the magnetic sensors,
which are mainly sensitive to sources oriented tangen-
tially to the head surface. Likewise, activity arising
from the presumed second fronto-central generator
may distort the computed source locations, depending
on the individual neuroanatomy, assuming that it may
include tangentialy oriented current sources in the
Results of the regression analysis for the y coordi-
nate of the MlOO and of the SF source locations show
that the depth of their equivalent current sources in-
creases by about 2-3 mm when stimulus frequency
increases by one octave. This confirms previous results
obtained by Elberling et al. (1982), Romani et al.
(1982), Pantev et al. (1988) and Yamamoto et al. (1988),
and it seems to be a reasonable value compared to the
size of the area of the primary auditory cortex and
integration region, which have a combined volume
estimated to be approximately 2.5-3.5 cm3(Talairach
and Tournoux 1988).
The tonotopic organization of the auditory cortex
along isofrequency strips comprising frequency specific
neurones is well known from animal experiments
(Schreiner 1991). The source locations of the MlOO
and the SF were obtained over a wide range of fre-
quencies (250 and 4000 Hz) and averaged across 11
subjects. They illustrate the "centres of activity" of the
excited neuronal populations which generate both
evoked responses. It appears that these "centres of
activity", represented by the centres of the estimated
equivalent current dipoles, lie on two different sheets
of the cortical surface. Thus the idea of spatial organi-
zation of anatomical substrates in the auditory cortex,
corresponding to different electrophysiological phe-
AUDITORY SUSTAINED FIELD
nomena (Hari et al. 1980), is nicely supported by the
results of this study.
This workwassupportedby grantsfromtheDeutsche
Forschungsgemeinschaft (Klinische Forschergruppe "Biomagnetis-
mus und Biosignalanalyse").
The authors thank Drs. R. Hari, TW. Picton and S. Makeig for
Annett, M. The binomial distribution of right, mixed and left-handed.
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