NEUROREPORTNEUROPHYSIOLOGY, BASIC AND CLINICAL

Dynamic activation of distinct
cytoarchitectonic areas of the human SI
cortex after median nerve stimulation
Isao Hashimoto,1,2,CA Tomoaki Kimura,3 Yoshinobu Iguchi,4 Ryosuke Takino5 and Kensuke Sekihara6
1

Human Information Systems Laboratory (Tokyo Of®ce), Kanazawa Institute of Technology, 6-7-8 Akasaka, Minato-ku, Tokyo
107-0052; 2 Department of Integrative Physiology, National Institute for Physiological Sciences, Okazaki; 3 Department of
Acupuncture, Tsukuba College of Technology, Tsukuba; 4 Department of Psychophysiology, Tokyo Institute of Psychiatry, Tokyo
Metropolitan Organization for Medical Research, Tokyo; 5 Department of Psychology, Shiraume Gakuen College, Tokyo;
6
Department of Electronic Systems and Engineering, Tokyo Metropolitan Institute of Technology, Tokyo, Japan
CA,1

Corresponding Author and Address

Received 7 March 2001; accepted 11 April 2001

MEG recordings visualized non-invasively a serial mediolateral
activation of the human somatosensory 3b area followed by astationary activation of area 1 after median nerve stimulation.
Somatosensory evoked ®elds (SEFs) were recorded over thehand area contralateral to the right median nerve stimulation
at the wrist in six normal subjects. A newly developed MEG
vector beamformer technique applied to the SEFs revealed two
distinct sources (areas 3b and 1) in the primary somatosensory
cortex (SI) during the primary N20m-P22m response in all
subjects. The ®rst source was located in area 3b, which started
to move sequentially toward mediolateral direction 0.7 msprior to the peak of N20m and ended its movement 1.4 ms
after the peak with a total distance of 11.2 mm. We speculatethat the movement re¯ects a sequential mediolateral activationof the pyramidal cells in area 3b, which is mediated byhorizontal connections running parallel to the cortical surface.
The second source in area 1, located 5.6 mm medial and
4.2 mm posterior to the ®rst source, was active 1.0 ms after
the N20m peak. Then, the ®rst source became inactive and thesecond source was dominant. In sharp contrast with the ®rst
source, the second source was stationary. The different behavior of these two components (moving vs stationary) indicates
independent parallel inputs to area 3b and area 1 from thethalamus. NeuroReport 12:1891±1897 & 2001 Lippincott Williams & Wilkins.

Key words: Area 3b; Area 1; Horizontal ®bers; Median nerve; MEG vector beamformer technique; Moving source; N20m-P22m; Parallel
thalamocortical inputs; Somatosensory evoked ®elds; Stationary source; Two overlapping sources

INTRODUCTION

It has long been considered established that N20 primary
response in the somatosensory evoked potentials (SEPs)
after median nerve stimulation is generated in area 3b andis approximated by a ®xed dipole. However, in a recent
magnetoencephalographic (MEG) study by Hashimoto etal. [1], it was suggested that N20m equivalent current
dipole (ECD) is not stationary and moves sequentially frommedial to lateral direction in area 3b. With regard to thefollowing P22 component (corresponding to Allison's P25),
it is well established that one of the generators for thiscomponent is located in the anterior crown of the postcentral gyrus in area 1 of primary somatosensory cortex(SI) [2±5]. However, there has been a considerable debate
about the contribution of area 4 of primary motor cortex(M1) to this component [2,5,6]. Even if area 4 is active, coactivation of two anti-parallel dipoles, one in area 3b andanother in area 4 will mutually cancel out the magnetic®elds generated in respective areas.
The issue has been complicated further when source

0959-4965 & Lippincott Williams & Wilkins

localization for the activity of a relatively wide area is to be
estimated using a single dipole model. In the somatosensory hand area, the central sulcus takes a curved course
resembling the inverted letter Ù [7]. The cortical activation
after median nerve stimulation can be seen as an extended
sheet of dipoles and the ECD model gives us an estimateof the center of gravity of this activity. Given that thelateral part of the inverted Ù receives projection from themedian nerve, the center of gravity of this activation canwell be in the precentral wall (area 4) instead of in the
postcentral wall (area 3b) [8].
The MEG vector beamformer technique recently developed by Sekihara et al. [9,10] provides us with a uniquetool for the analysis of multiple sources located nearby,
which are activated simultaneously and/or successively. In
order to overcome the above dif®culties in previous studies, the N20m-P22m response for stimulation of median
nerve was characterized by applying the beamformer technique. We found that two distinct sources, one in area 3b
and another in area 1, were sequentially and simulta-

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NEUROREPORT
neously active during N20m-P22m response and evenduring a single peaked N20m response. The results using asingle dipole model were reported previously [1]. However, the study was con®ned to ECD estimations aroundthe peak of N20m and the ECDs for a later part of N20m
including the superimposed P22m were dif®cult to localize. In fact, we have often failed to estimate the ECD
localizations with the single dipole model.

MATERIALS AND METHODS

Six healthy adults (two females, four males; ages 26±33
years, mean 29) were studied. Informed consent was
obtained prior to the study. Brief electrical stimuli with
0.2 ms duration were delivered to the right median nerveat the wrist (cathode proximal) with a repetition rate of
4 Hz. The stimulus intensity was three times sensory
threshold and elicited a mild twitch of the abductor pollicis
brevis muscle. Magnetic recordings (bandpass 0.1±1200 Hz) were taken from the left hemisphere centering at
C3 of the international 10-20 system with a 37-channel
SQUID gradiometer (BTi Magnes, San Diego, CA) in amagnetically shielded room. The detection coils of thegradiometer are arranged in a uniform, concentric array on
a spherical surface with a radius of 122 mm; each coilmeasures 20 mm in diameter. The sensors are con®gured
as ®rst-order axial gradiometers with a baseline ofp50
mm.
The ®eld sensitivities of the sensors were 10 fT/ Hz or
better. An epoch of 50 ms duration (10 ms pre- and 40 ms
post-stimulus) was digitized at a 4000 Hz/channel sampling rate and 4000 responses were averaged off-line. DC
offset was based on the pre-stimulus period.
The origin of the head-based co-ordinate system wasde®ned as the midpoint between the pre-auricular points.
The x-axis pointed from the origin to the nasion, the y-axis
to the left pre-auricular point, and the z-axis to the vertex
in a direction perpendicular to the x±y plane. Magnetic
resonance imaging (MRI) was acquired with spherical lipid
markers (5 mm diameter) placed on the MEG ®duciarypoints to allow for superposition of source locations onto
the MRI slices. A local spherical model was ®tted to thedigitized head shape over the recording area for eachsubject.
Data analysis: The MEG vector beamformer technique
was applied to localize sources from the N20m-P22m
component. The results of the analysis were shown as
current±density maps for transverse, coronal and sagittal
sections of the generic head. Furthermore, the locations of
the highest intensities of the current sources in the two
activation areas (areas 1 and 3b) were overlaid on the MRI
slices. The details of this vector beamformer techniquehave been reported elsewhere [9,10], so it is only brie¯ydescribed here.
Let us de®ne the magnetic ®eld measured by the mth
sensor at time t as bm (t), and a column vectorb(t)  [b1 (t),...,bm (t)]T as a set of measured data. Here M isthe total number of sensors and the superscript T indicatesthe matrix transpose. A spatial location (x,y,z) is represented by a three-dimensional vector r: r  (x,y,z) The
source moment magnitude at r and time t is de®ned as athree-dimensional vector s(r,t)  [sx (r,t),sy (r,t),sz (r,t)]. The
estimate of the source moment is denoted as ^s(r,t). We

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I. HASHIMOTO ET AL.

de®ne lx j (r), ly j (r) and 1z j (r) as the jth sensor output whena single source exists at r with the unit moment directed in
the x, y, and z directions, respectively. We then de®ne thelead ®eld matrix L(r) as a matrix whose jth row is equal to[lx j (r),ly j (r),lz j (r)]; this lead ®eld matrix represents thesensitivity of the whole sensor array at r.
The vector beamformer technique reconstructs thesource moment by applying the following linear operation:^s(r, t)  WT (r)b(t)(1)

where the matrix W(r) is a weight matrix de®ned as
W(r)  [wx (r),wy (r),wz (r)], and the weight vectors
wx (r),wy (r) and wz (r) detect the x, y, and z components ofthe source moment, respectively. Let us de®ne the measurement covariance matrix as D, and de®ne the threedimensional unit column vector as fx , fy , and fz . The
weight vectors in an intermediate stage are ®rst obtainedusingqwì (r)  Dÿ1 L(r)[LT (r)Dÿ1 L(r)]fì = fTì Ùfì
where ì  x, y, or z, andÙ  [LT (r)Dÿ1 L(r)]ÿ1 LT (r)D2 L(r)[LT (r)Dÿ1 L(r)]ÿ1(2)

Let us de®ne Es as a matrix whose columns consist of thesignal-level eigenvectors of D. Then, the ®nal form of theweight vectors are obtained by projecting the weight vector
wì onto the signal subspace of the measurement covariance matrix, i.e.
wì (r)  Es E Ts wì (r):(3)

Then the source moment at r is estimated using Eqn (1)with the weight vectors obtained using Eqns (2) and (3).
Note that when calculating the above weight vectors, thelocation r is a controllable parameter. Therefore, thesource-moment distribution can be reconstructed by scanning the beamformer output over the region of interest in acompletely post-processing manner. Because this reconstruction is performed at each instant in time, the spatiotemporal reconstruction of the source activities can beobtained.

RESULTS

The top traces in Fig. 1 illustrate N20m-P22m response inan extended time scale recorded over hand SI in the left
hemisphere. The magnetic signals start 15 ms after right
median nerve stimulation. The N20m peak occurs at
18.5 ms. The current±density maps produced by application of the beamformer technique to the MEG data
revealed initial activation in area 3b of SI hand area at
17.4 ms (a selected sampled point shown by a red dottedline on the superimposed SEF traces). Other sampled
points in the SEFs were indicated by green (18.5 ms), blue
(19.6 ms) and black (21.1 ms) dotted lines (Fig. 1). These
maps clearly show that the ®rst source moves seriallytoward lateral and inferior direction around the peak of
N20m (compare the maps at 17.4 ms post-stimulus in the
top row and those at 18.5 ms and at 19.6 ms in the second
and third rows). Just before termination of the movement

NEUROREPORT

amplitude (fT)

DYNAMIC ACTIVATION OF AREAS 3b AND 1 DURING N20m-P22m PRIMARY RESPONSE

200
0
2200
10

35

12
R

L

8

(cm)

(cm)

8
17.4 ms

4

4

R
L

0

12
R

L

8

(cm)

(cm)

12

4

25

8
18.5 ms

4

R
L

5
0

12
R

L

8

(cm)

(cm)

12

4

25

8
19.6 ms

4

R
L

5

(cm)

12

0

12
R

L

8
4

25
25

R
0
5
(cm)

10

210

(cm)

(cm)

0

5
(cm)

30

12

25

(cm)

20
25
latency (ms)

L

5

(cm)

15

8
21.1 ms

4
25
0
(cm)

5

25

0
5
(cm)

10

Fig. 1. Current-density maps at selected time points during N20m response. The top traces show N20m response recorded over hand S1 in the left
hemisphere after median nerve stimulation. The contour maps show reconstructed source magnitude distributions at four different latencies. The
maximum-intensity projections onto the axial (left column), coronal (middle column), and sagittal (right column) directions are shown. The letters L and
R in the coronal views indicate the left and right hemispheres, respectively. The circles depicting a human head show the projections of the sphere used
for the forward modeling. The colors of contours represent the relative intensity of the source magnitude; the relationship between the colors andrelative intensities is indicated by the color bar. The maps illustrate that the ®rst source moves sequentially toward lateral direction around the peak of
N20m. Near the end of the movement of the ®rst source, the second source located postero-medial to the ®rst source begins to be active. The secondsource is stationary throughout its activation.

of the ®rst source in the area 3b, the second source in anarea posterior-medial to the ®rst source became active(19.6 ms). In sharp contrast with the ®rst source, the secondsource was stationary throughout N20m-P22m response
and only the signal strength increased until the peak of
P22m. However, in this subject, there was no P22m distinct
from this underlying N20m and thus the primary response
was single peaked. Nevertheless, the second source, clearly
separated from the ®rst source, was detected (21.1 ms). The®rst source began to disappear at the peak of the secondsource. Figure 2 shows the results of co-registration of two
MEG sources of area 3b and area 1 onto the subject MRI.
The temporal and spatial relations between the twosources were summarized in Table 1, Table 2 and Table 3.

In order to facilitate comparison between individual subjects with different lengths of conduction pathways, thetime origin was set at the peak of N20m (Table 1).
Similarly, the spatial origin was set at the location of the®rst source at the peak of N20m with the highest currentsource density (Table 1, Table 2). With these normalization
processes interindividual anatomical variations documented for the central sulcus with respect to the head coordinate system can be minimized. The latency of N20m
peak ranged between 17.5 ms and 20.9 ms with the mean of
18.4 ms after median nerve stimulation (Table 1). The
shortest latency when the ®rst source started to movetoward lateral direction was 2.1 ms prior to the N20m peak
and the longest latency was 0.2 ms after the N20m peakVol 12 No 9 3 July 2001

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NEUROREPORTI. HASHIMOTO ET AL.

Fig. 2. Locations of the ®rst and second sources superimposed on the axial (left), coronal (middle) and sagittal (right) sections of the subject's MRI.
The location of the ®rst source is shown in the upper images, and the location of the second source is shown in the lower images. The location of the®rst source is represented by that of the source at the peak vertex of N20m.

Table 1. Latencies (ms) of the ®rst and second sources during N20m-P22m primary responsea .
Subject

N20m peak

1st source
movement starts

1st source
movement ®nishes

2nd source
turns on

1
2
3
4
5
6
Mean
s.d

18.00
17.50
18.50
17.50
20.90
18.00
18.40
1.28

ÿ0.70
ÿ1.20
ÿ0.26
ÿ2.14
0.20
ÿ0.20
ÿ0.72
0.85

0.70
1.40
1.18
0.50
3.10
1.20
1.35
0.92

0.24
0.74
0.94
0.00
2.80
0.96
0.95
0.99

a

Values normalized with respect to the peak latrencies of N20m.

with the mean of 0.7 ms before the peak. Similarly, thelatency when the movement of the ®rst source terminatedranged from 0.5 ms to 3.1 ms after the N20m peak with themean of 1.4 ms post-peak. The second source started toappear between 0 ms (the peak of N20m) and 2.8 ms postpeak with the mean of 1.0 ms following the N20m peak.
The localizations of the ®rst source at the initial and ®nal
position, and of the second source are summarized inTable 2. The ®rst source at the initial position was located1894

Vol 12 No 9 3 July 2001

close to the original position. There was a robust trend forthe initial activation to move sequentially toward lateraland inferior direction in area 3b with the mean ®nal
position at 6.4 mm lateral and 5.2 mm inferior from theoriginal position. This was followed by activation in area 1:
4.2 mm posterior, 5.6 mm medial and 8.0 mm higher to theinitial activation at the peak of N20m. ANOVA showedstatistical signi®cance for each coordinate for the threepositions and the difference was signi®cant for all coordi-

NEUROREPORTDYNAMIC ACTIVATION OF AREAS 3b AND 1 DURING N20m-P22m PRIMARY RESPONSE
Table 2. Localization of the ®rst and second sources during N20m-P22m responsea .
Subject

First initial (A)

1
2
3
4
5
6
Mean
s.d.

First ®nal (B)Second (C)

x1

y1

z1

x2

y2

z2

x3

y3

z3

ÿ2.5
0
ÿ0.4
ÿ3.4
ÿ0.4
0
ÿ1.1b
1.5

ÿ5.0
0.8
ÿ0.8
3.4
0
0
ÿ0.3c
2.7

ÿ2.1
1.7
0
4.2
0
0.8
0.8d
2.1

2.1
3.4
0
0
1.7
0.8
1.3
1.3

5.0
10.0
5.0
0.8
3.4
14.0
6.4
4.8

1.3
ÿ14.7
ÿ2.5
ÿ0.8
ÿ4.2
ÿ10.0
ÿ5.2
6.1

ÿ9.2
0.4
ÿ9.2
ÿ0.3
ÿ4.6
0.4
ÿ4.2e
4.3

ÿ7.6
ÿ0.4
2.1
ÿ16.0
ÿ11.0
ÿ0.8
ÿ5.6f
7.1

22.0
2.0
11.7
13.0
ÿ2.5
2.0
8.0g
9.1

a
The values were normalized with respect to those for the ®rst source at the peak of N20m, and the values are expressed in mm.
ANOVA for b x ( p  0.012), c y ( p  0.004) and d z ( p  0.10) coordinate. Multiple comparison was assessed using ScheffeÂ's test: signi®cance between B and C at e p  0.012,fp  0.004 and g p  0.010 for x, y and z coordinates, respectively.

Table 3. Distances (mm) from the origin (location of the ®rst sourceat the peak of N20m) to the second source (0-C) and among the ®rst
source at the initial (A) and ®nal (B) position, and the second source(C)a .
Subject

A-B

B-C

A-C

0-C

1
2
3
4
5.
6.
Meanb
s.d.

11.5
19.1
6.3
6.6
5.8
17.7
11.2
6.0

26.7
19.9
17.2
21.9
15.8
19.1
20.1
3.9

25.1
1.3
14.9
21.3
12.0
1.5
12.7
9.9

25.0
15.0
15.0
20.8
12.2
2.2
12.9
9.5

a

The distance of A-B, for example, is de®ned asin Table 2.
b
ANOVA, p  0.096 (not signi®cant).

p(x1ÿx2)2  (y1ÿy2)2  (z1ÿz2)2

nates between the ®rst source at the ®nal position and thesecond source (Table 2). The distances between the initialand ®nal locations of the ®rst source distributed from
5.8 mm to 19.1 mm with the mean of 11.2 mm (Table 3).
The mean total distance between the ®rst source at theN20m peak and the second source was 12.9 mm. Comparison between the ®rst source at the initial location and thesecond source revealed a similar distance (12.7 mm).
Although ANOVA showed no statistical signi®cance, thedistance between the ®rst source at the ®nal position andthe second source (20.1 mm) was longer.

DISCUSSION

The present study demonstrates that the two sources areactive during the N20m-P22m response to median nerve
stimulation. The ®rst source moved from medial to lateral
direction in area 3b around the peak of N20m. The ®nding
supports our previous study showing a robust movement
toward the same direction using a single moving ECD
model [1]. This ECD movement was further con®rmed by
SEF recordings to stimulation of the index ®nger [11],ruling out the possible sequential activation of thalamocortical projection ®bers from the ring, middle and index®ngers, to thumb all innervated by the median nerve [2].
Another possibility is that the movement of the sourcere¯ects sequential activation of different projection areasby the thalamocortical afferents with different conduction

velocities. Again, there have been no published data tosuggest that the thalamocortical afferents project to different regions of area 3b in accordance with the different
diameters of the thalamocortical ®bers.
A movement of a single source such as observed in ourreconstruction has been often interpreted as a result ofactivity of two ®xed sources. This is because when weapply the single-dipole search to the magnetic ®eld generated from two sequentially active ®xed sources, the singledipole method sometimes gives a false solution where asingle source seems to move according to the phase ofactivation of the two sources. That is, this spurious source
movement is caused by a wrong source model, i.e., byattempting to ®t a magnetic ®eld generated from a single
source to that from two sources. Moreover, the chances ofthis movement seem to be slim, because the goodness-of-®twill necessarily be worse if two source are approximatedby a single source. Our beamformer method, however, canprovide the reconstruction of neural source activities without using any explicit source model. Therefore, it is not
likely that the method provides a false reconstruction of amoving single source when sequentially active two ®xed
sources exist. Taken together, it is highly probable that the
movement re¯ects a real activation pattern of pyramidalcells in area 3b. We suggest that the sequential mediolateral activation of the pyramidal cells in area 3b is mediatedby horizontal connections cunning parallel to the corticalsurface [1,11].
The P37m component after posterior tibial nerve stimulation is the primary cortical response, consisting of two
components. The ®rst source is located in area 3b in themesial wall of the hemisphere and the second source is12.5 mm posterior to the ®rst source, presumably in area 1[12]. The ®rst source begins to be active during the initialslope of the P37m and the second source is active after the
peak of P37m. The ®rst source is analogous to the N20m
for median nerve stimulation [13]. It is likely that thesecond source corresponds to P22m on the basis of similar
spatial and temporal features [14]. However, in the majority of cases, the ®rst source of P37m is stationary in sharp
contrast with the moving source of N20m. It may beargued that this simply re¯ects the size of the projection
area from the hand and foot; the foot area is much smaller
than the hand area [15], making it dif®cult to detect the
movement of the source, if present. In fact, P37 amplitude

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NEUROREPORTof the posterior tibial nerve SEPs and SEFs is much smaller
than N20 amplitude for median nerve stimulation[13,14,16,17].
Fine coordination of hand movement is accomplished bya sensorimotor integration. The cortical efferent zones in
MI receive sensory information from the skin of the limbthat is close to the muscle to which they project [18,19].
This sensory feedback inputs were fed into area 4 in M1
via areas 3b and 1 and played a role in monitoring ofongoing hand movements [20,21]. However, the activity ofarea 4 was not observed during N20m-P22m primary
response in the present study. This is either due ®rstly tocancellation of two anti-parallel dipoles located very closewith each other, one in area 3b and another in area 4, orsecondly to temporal dispersion of activity as a result of an
extra synaptic transmission in areas 3b and 1, or both.
It is obvious that hand movements are more complex
than foot movements. The human hand has uniquely
evolved functionally and morphologically [22±24]. It seems
reasonable to expect that the human brain has also evolved
in relation to the important roles played by the human
hand, and that somatosensory information from the hand
would be processed adaptably for a ®ne motor control of
the hand. We speculate that the dynamic movement of the®rst source in the median nerve SEFs re¯ects integration of
the somatosensory information from the hand, which has
been ®rst preprocessed by the columnar organization withvertical, translaminar connectivity of intrinsic circuitry [25].
This integration is mediated by horizontal ®bers connecting
nearby columns and these intracortical connections could
form the basis of convergence in spatial as well as nonspatial information at an early stage of somatosensory
information processing [26].
Most pyramidal cells in layer 3 have 1±3 major collaterals with long horizontal trajectories (6 mm or more) inlayers 3 and 4 [27±29]. Indeed, the horizontal intracorticalconnection is the principal route for horizontal spread of
excitation in neocortex [26,30]. In monkey somatosensory
cortex, it has been shown that the metabolic labeling
patterns with 2-deoxyglucose to stimulation of a single®nger are spatially extensive and complex [31,32]. The
prominent characteristic of the distribution is a focal patchnear the 3b-1 border in the most medial sections withmultiple patches extending 4 mm in length from posteromedial to anterolateral direction in area 3b. The mediolateral extent of somatosensory cortex is about 3.5 cm inmonkeys and about 10 cm in humans [2]. Assuming asimilar projection of the body surface onto area 3b in both
species, the estimated distance between the most medial
and lateral activation in humans would be 11.4 mm, very
similar to the average distance (11.2 mm) between theinitial and ®nal location of the moving source in area 3b inthe present study or in the previous studies (8.4±8.7 mm)
using a single dipole model [1,11]. However, the metaboliclabeling patterns can offer no clue on the timing ofactivation of each patch. Thus, our previous and thepresent studies are the ®rst to demonstrate a dynamic
sequence of activation of the area 3b unavailable withmetabolic or intracranial SEP studies.
Near the end of the ®rst source movement, the secondsource, located medial and posterior to the ®rst source inarea 1, begins to be active. The second source is character-1896

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I. HASHIMOTO ET AL.

ized by the steady, ®xed position throughout its activation
period. The different behavior of the two sources (moving
vs stationary) suggests that P22 re¯ects activation of area 1
through parallel inputs from the thalamus and not throughserial inputs from area 3b. It is rather surprising that thesecond source in area 1 was visualized in all the subjects
examined, because P22 component of SEPs has been considered to re¯ect the radial current ¯ow and MEG is blind
to radial sources. The ®ndings suggest that, although thecurrents are mainly radial, there is also a robust tangential
component in P22 as re¯ected in P22m [3,4]. It is likely thatthe anterior part of area 1 within the central sulcus or at
the border between the sulcus and the gyrus may be thegenerator of P22m. The distance between the ®rst source atthe peak of N20m and the second source (the mean of
12.9 mm) is similar to the distance measured in intracranial
surface recordings of SEPs in which P22 with the largest
amplitude on the postcentral gyrus (area 1) is located 1 cm
medial to the focus of N20 component [2]. The time coursesof activation of the ®rst and second sources are also in linewith those in cortical SEP recordings [2].
However, the cortical SEP recordings failed to detect therobust serial movement of the ®rst source in area 3b
demonstrated by magnetic recordings. In the study ofAllison et al. [2], cortical SEPs were usually recorded froma 64-electrode array (8 3 8 grid) with 5 mm interelectrodespacing. Provided that the moving source is directly underthe electrodes, this electrode density will give nearly
optimal spatial resolution of voltage gradient of the currents for the ®rst source. However, area 3b lies within thecentral sulcus, the depth of which ranges from 16 to 18 mm[33,34] to 20 mm [35], making direct recordings of activity
in area 3b dif®cult with the surface electrode array overthe gyri near the central sulcus. It is likely that, in thissituation, extracellular volume currents are recorded moreor less as far-®elds and not as near-®elds even in cortical
recordings, resulting in smearing of spatial resolution (see[2,36] for intracranial distribution of auditory and somatosensory evoked potentials).
Thus, the discrepancy between the cortical EEG recordings and the magnetic recordings outside the head is one
excellent example of the advantages of MEG over EEG.
This is mainly due to the fact that, ®rst, the tissues insideand outside the brain do not signi®cantly distort themagnetic ®eld patterns generated by brain currents andrecorded outside the head. Secondly, MEG source localization is better in the direction orthogonal to the dipole
orientation [37] and the movement of the ®rst source isorthogonal to the N20m ECD orientation [1,11]. This posesa serious question, because interpretation of MEG data has
been heavily dependent on direct cortical recordings of theelectric counterpart for the MEG signals, which have been
considered to provide a 'gold standard' for 'real' brain
activation. Conversely, it is possible that MEG recordings
may, in favorable conditions, provide a touchstone leading
to new ®ndings in the physiological activity of the humanbrain non-invasively.

CONCLUSION

The MEG vector beamformer technique visualized noninvasively a serial mediolateral activation of the humansomatosensory 3b area followed by a stationary activation

DYNAMIC ACTIVATION OF AREAS 3b AND 1 DURING N20m-P22m PRIMARY RESPONSE

of area 1 after median nerve stimulation. The ®rst source
located in area 3b started to move sequentially toward
mediolateral direction prior to the peak of N20m. We
speculate that the movement re¯ects a sequential activationof the pyramidal cells in area 3b, which is mediated byhorizontal connections running parallel to the cortical surface [1,11]. The second source in area 1, located medial andposterior to the ®rst source, was active after the N20m
peak. In sharp contrast with the ®rst source, the secondsource was stationary, suggesting independent parallel
inputs to area 3b and area 1 from the thalamus.

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Acknowledgements: This research was supported by a Grant-in-Aid for Scienti®c Research from the Ministry of Education,
Science and Culture of Japan (09877185) and a Grant-in-Aid of the Japan Medical Association (1998). One of the authors (K.S.)
was supported by a research project `Mind Articulation' sponsored by Japan Science and Technology Corporation.

Vol 12 No 9 3 July 2001

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