Asymmetry in the human primary somatosensory cortex and handedness

Abstract

Brain asymmetry is a phenomenon well known for handedness and language specialization and has also been studied in motor cortex. Less is known about hemispheric asymmetries in the somatosensory cortex. In the present study, we systematically investigated the representation of somatosensory function analyzing early subcortical and cortical somatosensory-evoked potentials (SEP) after electrical stimulation of the right and left median nerve. In 16 subjects, we compared thresholds, the peripheral neurogram at Erb point, and, using MRI-based EEG source analysis, the P14 brainstem component as well as N20 and P22, the earliest cortical responses from the primary sensorimotor cortex. Handedness was documented using the Edinburgh Inventory and a dichotic listening test was performed as a measure for language dominance. Whereas thresholds, Erb potential, and P14 were symmetrical, amplitudes of the cortical N20 showed significant hemispheric asymmetry. In the left hemisphere, the N20 amplitude was higher, its generator was located further medial, and it had a stronger dipole moment. There was no difference in dipole orientation. As a possible morphological correlate, the size of the left postcentral gyrus exceeded that of the right. The cortical P22 component showed a lower amplitude and a trend toward weaker dipole strength in the left hemisphere. Across subjects, there were no significant correlations between laterality indices of N20, the size of the postcentral gyrus, handedness, or ear advantage. These data show that asymmetry of median nerve SEP occurs at the cortical level, only. However, both functional and morphological cortical asymmetry of somatosensory representation appears to vary independently of motor and language functions.

Full text

Asymmetry in the human primary somatosensory cortex and handedness
Patrick Jung,
a
Ulf Baumga¨rtner,
a
Thomas Bauermann,
b
Walter Magerl,
a
Jochen Gawehn,
b
Peter Stoeter,
b
and Rolf-Detlef Treede
a,
*
a
Institute of Physiology and Pathophysiology, Johannes Gutenberg-University, Saarstr. 21, 55099 Mainz, Germany
b
Institute of Neuroradiology, Johannes Gutenberg-University, Mainz, Germany
Received 8 November 2002; revised 7 February 2003; accepted 17 March 2003
Abstract
Brain asymmetry is a phenomenon well known for handedness and language specialization and has also been studied in motor cortex.
Less is known about hemispheric asymmetries in the somatosensory cortex. In the present study, we systematically investigated the
representation of somatosensory function analyzing early subcortical and cortical somatosensory-evoked potentials (SEP) after electrical
stimulation of the right and left median nerve. In 16 subjects, we compared thresholds, the peripheral neurogram at Erb point, and, using
MRI-based EEG source analysis, the P14 brainstem component as well as N20 and P22, the earliest cortical responses from the primary
sensorimotor cortex. Handedness was documented using the Edinburgh Inventory and a dichotic listening test was performed as a measure
for language dominance. Whereas thresholds, Erb potential, and P14 were symmetrical, amplitudes of the cortical N20 showed significant
hemispheric asymmetry. In the left hemisphere, the N20 amplitude was higher, its generator was located further medial, and it had a stronger
dipole moment. There was no difference in dipole orientation. As a possible morphological correlate, the size of the left postcentral gyrus
exceeded that of the right. The cortical P22 component showed a lower amplitude and a trend toward weaker dipole strength in the left
hemisphere. Across subjects, there were no significant correlations between laterality indices of N20, the size of the postcentral gyrus,
handedness, or ear advantage. These data show that asymmetry of median nerve SEP occurs at the cortical level, only. However, both
functional and morphological cortical asymmetry of somatosensory representation appears to vary independently of motor and language
functions.
© 2003 Elsevier Science (USA). All rights reserved.
Keywords: Hemispheric asymmetry; Somatosensory cortex; SEP; Source analysis; Morphometry; Handedness
Introduction
Cortical asymmetries are an evolutionary peculiarity,
most distinctive in humans (Springer and Deutsch, 1997).
The most prominent examples for functional asymmetries
of the brain are language and handedness. Approximately
90% of our population show dominant speech processing in
the left hemisphere and a great majority prefers to use their
right hand (only about 10% are left-handers; Annett, 1973).
Language lateralization and handedness are suspected to be
related because left-handers exhibit an increased incidence
of bilateral or right hemisphere language dominance com-
pared to right-handers (Branch et al., 1964; He´caen et al.,
1981). Recently, the incidence of speech dominance in the
right hemisphere measured by functional transcranial Dopp-
ler sonography was found to increase linearly with the
degree of left-handedness (Knecht et al., 2000).
In the language system, the functional asymmetry of
cortical organization is mainly represented in Broca’s and
Wernicke’s areas (Broca, 1864; Wernicke, 1874). It is still
controversial if asymmetry of the planum temporale which
includes part of Wernicke’s area reflects an anatomical
substrate for language specialization (Geschwind and Lev-
itsky, 1968; Foundas et al., 1994; Ja¨ncke and Steinmetz,
1993; Moffat et al., 1998; Beaton, 1997).
In the sensorimotor system, up to now the neurobiolog-
ical basis for handedness remains widely unclear. Several
studies demonstrated a relationship between handedness
and anatomical asymmetries, e.g., in the sizes of the frontal
* Corresponding author. Fax: ⫹49-6131-3925902.
E-mail address: treede@uni-mainz.de (R.-D. Treede).
NeuroImage 19 (2003) 913–923 www.elsevier.com/locate/ynimg
1053-8119/03/$ – see front matter © 2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S1053-8119(03)00164-2

and occipital petalia (Bear et al., 1986; Weinberger et al.,
1982; LeMay, 1992) and in the surfaces of the plana tem-
poralia and plana parietalia (Steinmetz et al., 1991; Foundas
et al., 1995; Ja¨ncke et al., 1994), but only few studies
detected asymmetries in brain regions that are directly re-
lated to the motor system (White et al., 1994; Amunts et al.,
1996, 1997a, b, 2000; Dassonville et al., 1997; Volkmann et
al., 1998). In the somatosensory system, even less is known
about lateralization than in the motor system. Pneumatic
stimulation of the first and fifth digit of both hands revealed
a larger distance between dipole positions of the two digits
in the left primary somatosensory cortex (SI) in right-hand-
ers. A significant correlation between handedness and sizes
of hand representation areas was not determined (So¨ro¨s et
al., 1999). Furthermore, in a magnetoencephalographic
study equivalent current dipoles of the somatosensory-
evoked field (SEF) components N20m and P30m were
stronger in the left SI of both right- and left-handers
(Rossini et al., 1994).
A previous EEG study (Buchner et al., 1995) described
higher amplitudes of the N20 median nerve somatosensory-
evoked potential (SEP; generator: area 3b of primary so-
matosensory cortex; Allison et al., 1989) over the left scalp
in approximately 70% of the exclusively right-handed sub-
jects. The intention of the present study was to clarify if this
N20 asymmetry is due to side different subcortical input or
to side differences of N20 source locations, source orienta-
tions, and/or source strengths by means of dipole source
analysis. To verify the hypothesis that functional hemi-
spheric asymmetry is associated with interhemispheric dif-
ferences in brain structure, the intrasulcal length of the
rostral surface of the left and right postcentral gyrus was
determined, and its asymmetry was correlated with handed-
ness and N20 asymmetry. Finally, we assessed the relation-
ship of auditory lateralization (speech dominance), handed-
ness, and asymmetry of the N20 SEP component. Some
results of the study have been published in abstract form
(Jung et al., 2002).
Materials and methods
Subjects
Sixteen subjects (eight females and eight males, ages
ranged from 23 to 28 years, mean age: 24.3 years) were
included in the study which was approved by the local
ethics committee. All subjects gave their written informed
consent, according to the Declaration of Helsinki, and were
paid for participation. None of them suffered from any
neurological or psychiatric diseases.
Subjects were comfortably seated in an electrically
shielded, noise- and light-reduced room which had a con-
stant temperature of 24°C. They were instructed to relax and
keep their eyes open and fixed to a visual target.
Stimuli
The left and right median nerves were stimulated sepa-
rately at the wrist with constant-current square-wave pulses
of 0.2-ms duration. The stimulus intensity was set at the
sum of the individual sensory and motor threshold. Median
nerve SEPs were recorded using a 32-channel EEG montage
within a time window of 300 ms including a prestimulus
interval of 50 ms, bandpass filtered (0.16 –500 Hz) and
digitized at 2.5 kHz. In order to record the subcortical
components N10 and P14 adequately, we additionally chose
a 3-channel EEG montage (ipsilateral Erb point-Fz, Fz-
ipsilateral A, contralateral P-Fz) with a sampling rate of 10
kHz. In this case, the bandpass was 0.16 –3000 Hz and a
time window of ⫺7 to 30 ms was analyzed. The interstimu-
lus interval (ISI) was 0.531 s. Subjects were instructed to
mentally count the electrical stimuli.
The order of the recording conditions (32-channel vs
3-channel montage, left vs right median nerve stimulation)
was counterbalanced across subjects. For each recording
condition, two runs of 250 responses were averaged and
checked for reproducibility. Data from these two runs were
collapsed yielding averages of 500 responses for each con-
dition to achieve a maximal signal-to-noise ratio for ampli-
tude measurements and dipole source analysis.
EEG recording
The EEG was recorded with a 32-channel amplifier
(Neurotop, Nihon Kohden) using Ag/AgCl electrodes (5
mm diameter) attached to the scalp with electrode cream
(EC2, Grass). A ground electrode was fixed to the forearm
ipsilateral to stimulation. All electrode/skin impedances
were below 5 k⍀. Electrodes were positioned according to
the 10 –20 system (Pivik et al., 1993). In addition to the 19
standard positions, the montage included frontocentral
(FC5, FC1, FC2, FC6) and centroparietal (CP5, CP1, CP2,
CP6) electrodes. We also added electrodes on the zygomatic
arch (F9, F10), the preauricular points (T9, T10), and the
mastoids (P9, P10) in order to cover the lower parts of the
head. All electrodes were referenced to Fz.
Amplitudes were measured baseline to peak. For base-
line correction, the mean amplitude of the 5- to 12-ms
poststimulus interval was substracted (for the derivation
“Erb-Fz”: 4 to 8 ms). In order to eliminate low-frequency
baseline shifts contaminating EEG signals, we used a high-
pass filter (20 Hz, 12 dB/octave, forward filter; Gobbele´et
al., 1999). The 32 EEG channels were rereferenced off-line
versus the average reference, excluding electrodes contam-
inated by artifacts.
Dipole source analysis
After recording, each electrode was replaced by a 8-mm-
diameter oil capsule (cf. Lagerlund et al., 1993). MR images
of each subject were obtained with a 1.5-T scanner (Sie-
914 P. Jung et al. / NeuroImage 19 (2003) 913–923

mens Magnetom Vision; FLASH 3D, repetition time 14 ms,
echo time 4 ms, flip angle 25°, 1-mm slices, 256-mm field
of view, 256 ⫻256 matrix). Subsequently, MR images were
aligned to the anterior commissure (AC)–posterior commis-
sure (PC) plane and transformed into Talairach space using
AFNI software (Cox, 1996). Individual electrode coordi-
nates were taken as basis for source analysis calculations.
For simplification, we designate equivalent current dipoles
evaluated by means of source analysis as “dipoles.”
For dipole source modeling, we used the Brain Electrical
Source Analysis software (BESA; Scherg, 1992) which first
calculated the best-fitting ellipsoid of each subject on the
basis of the individual electrode coordinates and then ad-
justed dipole sources using a 4-shell ellipsoidal head model.
Source analysis was initially performed on grand mean
EEG data since this provides the best signal-to-noise ratio
(16 ⫻500 averaged responses for each condition). EEG
segments were time-locked to the mean N10 latency (Erb
potential) to minimize the jitter due to differences in pe-
ripheral conduction time. As time window for the dipole
fits, onset to peak of the corresponding peaks (P14, N20,
P22) in the global field power (i.e., the spatial standard
deviation of amplitudes in the different EEG channels as a
function of time) was chosen. Dipole sources were fitted in
the order of appearance resulting in a 3-dipole model (P14,
N20, P22) for each stimulus side with a goodness of fit
value (GoF) of ⬎95% in the 10- to 35-ms poststimulus
interval.
The same fit procedure was then applied to individual
data. In most of the subjects (n⫽9), an individual fit of all
three sources was possible. In subjects where the fit of N20
did not result in stable solutions for both stimulus sides, the
time window for P30 was chosen (5 cases), assuming that
N20 and P30 are generated in similar locations but inverted
dipole orientation (e.g., Allison et al., 1989). If the P30 time
window did not lead to stable solutions with an explicit N20
peak in the source waveform, we utilized grand mean loca-
tion coordinates to fit the N20 source (2 cases). For instable
P22 fits, grand mean coordinates were used (3 cases). In that
way, stable dipole solutions for every single subject were
acquired with a mean GoF of ⬎80% for all three fitted
dipole sources in their corresponding time window.
Dipole orientations were determined by BESA’s angles
␽
and
␸
.
␽
corresponds to the angle deviation of dipoles
from the vertical axis running from the center of the head to
the vertex, and
␸
expresses the angle deviation of dipoles
from the axis passing through the preauricular points. Di-
pole orientations, coordinates, and source waveforms (di-
pole strengths) of P14, N20, and P22 were then used for
statistical analysis.
Evaluation of functional asymmetries
After excluding any hearing impairments by means of
audiometry, each subject performed a dichotic listening test
(Ja¨ncke, 1992). Dichotic listening tests are regarded as non-
invasive methods that yield valid estimates of hemisphere
speech dominance (Geffen et al., 1981; Zatorre et al., 1989;
Hugdahl et al., 1997). A right ear advantage in the dichotic
listening test indicates language dominance of the left hemi-
sphere and vice versa. The auditory stimuli were presented
to the subjects via headphones from an audio CD. The
dichotic stimuli consisted of 6 stop consonants (/b/, /d/, /g/,
/p/, /t/ and /k/) that were paired with the vowel /a/ to form
6 consonant-vowel syllables whose volume and basic fre-
quency were equalized. At every dichotic trial, both ears
were simultaneously presented a different syllable. Four
blocks were presented, each consisting of 30 dichotic trials
in a different sequence. Each block was listened to twice,
with the headphones reversed the second time to balance
headphone asymmetries. The subjects were instructed to
mark the consonant of the dichotically presented syllables
which was perceived louder or more distinct. Thus, the
score for one ear resulted from the sum of the marks indi-
cating a preferential perception of the syllable by the re-
spective ear. From the scores for each ear, a laterality index
(LI) was calculated (see “Data evaluation”).
Both the direction and the degree of handedness of each
subject were determined on the basis of the 10-item version
of the Edinburgh Inventory (Oldfield, 1971). For each item
(e.g., usage of scissors), the preference was recorded as
right only (2– 0), left only (0 –2), right rather than left (1– 0),
left rather than right (0 –1), or either hand (1–1). On this
basis, laterality indices were evaluated (see Data evalua-
tion). Additionally, the subjects were assigned to two hand-
edness groups (LI ⬎80 were classified as “extreme right-
handers” and LI ⱕ80 were considered as “non-right-
handers”) according to previous suggestion (LeMay, 1992;
Habib et al., 1995).
Evaluation of anatomical asymmetries
The intrasulcal length of the anterior wall of the left and
right postcentral gyrus was measured as an indicator of the
hand representation area in SI in every second of 41 hori-
zontal MR slices (leading to 21 analyzed slices). The ap-
plied method has been previously described for the primary
motor cortex (MI) by Amunts et al. (1996). In contrast to
this study, MR data were not transformed into Talairach
space to exclude contortions due to possibly unusual asym-
metries or symmetries in the Talairach reference brain. In
the present study, morphometric analysis was performed in
the AC-PC aligned MR format. In conformity with the
Talairach coordinate system, AC was defined as x⫽0, y⫽
0, and z⫽0. Negative values indicate locations left (x),
caudal (y), and ventral (z) to AC. Thus, horizontal sections
extended from z⫽69 to 29 according to reports that
proclaimed an extent of hand representation in MI and SI up
to 40 mm in the dorsoventral direction (Sanes et al., 1995;
Hlusˇtı´k et al., 2001). In each horizontal section, the intra-
sulcal contour of the anterior wall of the postcentral gyrus
was traced from the most medial point of the central sulcus
915P. Jung et al. / NeuroImage 19 (2003) 913–923

to the most lateral point on the convexity of the postcentral
gyrus. In a final step, the entire area (for z⫽69 to 29) of the
anterior wall of the left and right postcentral gyrus was
estimated by summing the trapezoidal areas between adja-
cent sections according to the formula,
A⫽⌬z
冘
i⫽1
n⫺1
共Li⫹Li⫹1兲/ 2, (1)
where Ais the estimated total area, ⌬zis the section interval
(2 mm), and L
i
and L
i
⫹1 are the measured lengths of the
rostral wall of the postcentral gyrus in adjacent sections.
Moreover, the resultant laterality indices (see Data evalua-
tion) were calculated. To evaluate the relative position of
N20 source localizations to the rostral surface of the post-
central gyrus, N20 source locations were retransformed
from Talairach space to the original AC-PC aligned format
using AFNI software.
Data evaluation
To assess the direction and the degree of auditory later-
alization (language dominance), handedness, functional and
morphological asymmetry of the somatosensory system,
laterality indices (LI) were calculated due to the formula,
LI ⫽共R⫺L兲/共R⫹L兲⫻100, (2)
where Ris right ear score, right-hand score, mean ampli-
tudes of SEP components following right-sided median
nerve stimulation or area of the anterior wall of the left
postcentral gyrus, Lis left ear score, left-hand score, mean
amplitudes of SEP components following left-sided median
nerve stimulation or area of the anterior wall of the right
postcentral gyrus. Consequently, positive values indicate
right ear advantage, hand preference of the right hand,
predominant somatosensory processing in the left hemi-
sphere or greater extension of area 3b of SI in the left
hemisphere.
All data are presented as mean ⫾SEM. Data were
analyzed using Student’s two-tailed paired (to assess side
differences) and unpaired (to evaluate gender differences
and differences between handedness groups) ttests. Pval-
ues ⬍0.05 were considered significant.
Results
Early subcortical and cortical SEP components
Neither sensory (1.3 ⫾0.1 mA vs 1.2 ⫾0.1 mA) nor
motor thresholds (3.6 ⫾0.2 mA vs 3.6 ⫾0.1 mA) were
different after right and left median nerve stimulation. Con-
sequently, stimulus intensities were identical for the right
and left median nerve, both in absolute current application
and in relation to thresholds.
In the 3-channel EEG montage, N10 amplitudes, derived
over the distal part of the brachial plexus (Erb point), were
also symmetrical after right and left median nerve stimula-
tion (Fig. 1A). Thus, the central nervous system (CNS)
received the same input on both sides. In the CNS, ampli-
tudes of the subcortical P14 component, generated in the
brainstem (medial lemniscus; Desmedt, 1985), did not differ
in side comparison but on the cortical level asymmetries of
amplitudes became apparent. The N20 potential showed on
average 18 ⫾5 % higher amplitudes after right median
nerve stimulation (P⬍0.01). No differences between mean
peak latencies after right- and left-sided stimulation were
found for all investigated SEP components (Table 1).
In the 32-channel EEG montage, the cortical components
N20 and P22 appeared contralateral to stimulation only.
Significantly higher amplitudes of the first cortical response
N20 were demonstrated for the left-sided electrode posi-
tions P3, CP1, and O1 compared to their right-sided coun-
terparts (P4, CP2, O2; all P⬍0.05). In the EEG channels
with maximal N20 deflection (P3 and P4, respectively), the
N20 amplitude was on average 31 ⫾5 % higher after right
median nerve stimulation (P⬍0.001, Fig. 1B and Fig. 3A).
The distances between the ⍀region (cortical landmark that
represents approximately the hand area of the primary mo-
tor cortex; Yousry et al., 1995) and the derivation points P3
and P4 did not differ on both sides (Fig. 1B). Hence, the
measured side differences of N20 amplitudes were not an
artifact due to unequal electrode placements with reference
to landmarks in the sensorimotor region. Amplitudes of the
second cortical response (P22) had maximal deflections in
the EEG channels C3 and C4, respectively, and were also
lateralized. In contrast to N20 asymmetry, significantly
higher P22 amplitudes were found after left median nerve
Fig. 1. (A) Grand mean SEP waveforms recorded with the 3-channel EEG montage. Waveforms after right (red line) and left (blue line) median nerve
stimulation are illustrated. There were no side differences for the subcortical SEP components N10 (derivation: ipsilateral Erb–Fz; generator: brachial plexus)
and P14 (Fz–ipsilateral A; medial lemniscus) while the first cortical SEP component N20 (contralateral P–Fz; area 3b) exhibited significantly higher
amplitudes after right compared to left median nerve stimulation (P⬍0.01). (B) The N20 amplitude asymmetry was also significant in the 32 channel EEG
montage (P⬍0.001). Distances between P3 and P4 recording positions and cortical landmarks in the hand areas of primary motor cortices (⍀regions; Yousry
et al., 1995) were not significantly different (n⫽16). Thus, the illustrated asymmetry of N20 amplitudes may not be explained by asymmetric electrode
placements.
Fig. 2. Three-dipole source model of early subcortical and cortical SEP components based on grand mean EEG data after right (A) and left (B) median nerve
stimulation. The spatial configuration of the sources in the BESA head model is demonstrated. Time courses of goodness of fit (GoF, gray line), global field
power (GFP, black line), and dipole strengths are shown. There were no side differences of the brainstem source (1, blue line) and P22 source (3, green line)
activities at peak latencies of 14 and 22 ms, respectively. However, the source waveform of the N20 source (2, red line) was significantly more pronounced
at 20-ms peak latency after right compared to left median nerve stimulation (P⬍0.001).
916 P. Jung et al. / NeuroImage 19 (2003) 913–923

917P. Jung et al. / NeuroImage 19 (2003) 913–923

stimulation (side difference of 21 ⫾8%,P⬍0.05). No P22
latency differences were detected after right- and left-sided
stimulation of the median nerve.
Dipole source analysis
The fit procedure led to stable solutions for the grand
mean and for every individual containing a brainstem
source (P14) located near midline, the first cortical response
(N20) in the contralateral hand area of the primary somato-
sensory cortex and a second cortical response (P22) located
slightly more frontally (Fig. 2).
The dipole location of the P22 source was significantly
more anterior (6.3 ⫾2.3 mm, P⬍0.05) and superior (16.7
⫾2.0 mm, P⬍0.001) in the left hemisphere than the
ipsilateral N20 source location. In the right hemisphere,
significant differences of P22 and N20 localizations could
not be stated.
Table 2 outlines the interhemispheric comparison of
mean dipole localizations, mean dipole strengths, and mean
dipole orientations. For dipole locations, no significant side
differences were found for the P14 and P22 sources. How-
ever, the N20 dipole was localized 7.7 ⫾2.5 mm more
medial (P⬍0.01) and therefore less eccentric (P⬍0.05,
Fig. 3C) in the left hemisphere. The P22 source was located
slightly more medial (4.2 ⫾2.6 mm) in the left sensorimo-
tor region, too, but this difference did not reach significance.
Comparing dipole strengths in both hemispheres, no asym-
metry was found for the subcortical P14 source whereas at
the cortical level hemispheric asymmetry was detected. N20
dipole strengths were considerably more pronounced in the
left hemisphere. On average, N20 source activity was 31 ⫾
6 % stronger in the left SI cortex compared to the right (P
⬍0.001). This clear left hemisphere preponderance of N20
dipole strengths properly reflected the asymmetric N20 am-
plitudes (Figs. 3A and B) which was supported by the
highly significant correlation between the asymmetry of
N20 dipole moments and N20 amplitudes across all subjects
(Pearson’s r⫽0.71, P⬍0.01). In contrast, P22 source
strengths approached significance with higher dipole mo-
ments in the right hemisphere.
Comparing side differences in dipole orientations, no
side different angle deviation from midsagittal plane in the
frontal plane (
␽
) and in the horizontal plane (
␸
) was dem-
onstrated for all fitted sources after right- and left-sided
median nerve stimulation.
SI morphometry
From z⫽69 to 29 in the dorsoventral direction (Fig. 4),
15 out of 16 brains showed a greater total area of the
anterior wall of the left postcentral gyrus in side compari-
son; this left hemispheric preponderance was significant in
8 brains. Across all subjects, the rostral area of the postcen-
tral gyrus was significantly larger in the left hemisphere
(17.34 ⫾0.41 cm
2
vs 15.74 ⫾0.34 cm
2
,P⬍0.001, Fig.
3D), mainly because of a further medial extension of the left
central sulcus (CS). For z⫽61 to 29, the left CS ended
constantly closer to the interhemispheric fissure compared
to the right. On the contrary, the most lateral point of the
postcentral gyrus did not differ between the hemispheres in
any of the analyzed horizontal slices.
Table 1
Side comparison of SEP peak latencies and amplitudes
(mean ⫾SEM, n⫽16)
rMed lMed Pvalues
a
Peak latencies [ms]
N10 10.1 ⫾0.2 10.2 ⫾0.2 n.s.
P14 14.1 ⫾0.2 14.1 ⫾0.2 n.s.
N20 19.3 ⫾0.2 19.2 ⫾0.2 n.s.
Amplitudes [
␮
V]
N10 ⫺4.13 ⫾0.38 ⫺4.00 ⫾0.33 n.s.
P14 1.16 ⫾0.07 1.18 ⫾0.06 n.s.
N20 ⫺2.02 ⫾0.29 ⫺1.68 ⫾0.29 ⬍0.01
Values for median nerve SEP components are referred to the 3-channel
EEG montage. rMed, right median nerve stimulation; lMed, left median
nerve stimulation.
a
Student’s paired ttest.
Table 2
Side comparison of dipole source locations, dipole orientations and dipole source strengths.
P14 N20 P22
rMed lMed Pvalues
a
rMed IMed Pvalues
a
lMed IMed Pvalues
a
tlrc x[mm] ⫺7.6 ⫾2.0 ⫺4.2 ⫾2.2 n.s. ⫺37.2 ⫾2.1 45.0 ⫾2.4 ⬍0.01 ⫺37.8 ⫾1.6 42.0 ⫾1.9 n.s.
tlrc y[mm] ⫺33.5 ⫾3.0 ⫺37.2 ⫾3.7 n.s. ⫺23.7 ⫾2.1 ⫺21.7 ⫾1.9 n.s. ⫺17.4 ⫾1.8 ⫺18.8 ⫾2.2 n.s.
tlrc z[mm] ⫺17.1 ⫾5.8 ⫺28.1 ⫾6.2 n.s. 52.9 ⫾1.5 57.3 ⫾2.3 n.s. 69.6 ⫾2.3 64.4 ⫾3.2 n.s.
␽
⫺35.3 ⫾2.4 30.1 ⫾3.1 n.s. 74.7 ⫾3.3 ⫺70.2 ⫾4.3 n.s. ⫺88.3 ⫾4.8 79.0 ⫾6.2 n.s.
␸
⫺82.2 ⫾4.5 79.8 ⫾5.4 n.s. 68.2 ⫾3.0 ⫺65.3 ⫾4.2 n.s. 45.4 ⫾7.2 ⫺65.4 ⫾11.3 n.s.
dm [nAm] 13.4 ⫾1.3 13.6 ⫾1.4 n.s. 15.7 ⫾1.5 10.6 ⫾1.5 ⬍0.001 11.6 ⫾2.0 14.8 ⫾2.9 n.s.
b
Values are indicated as mean ⫾SEM (n⫽16). rMed, right median nerve stimulation; lMed, left median nerve stimulation; tlrc, talairach head coordinates;
␽
, angle deviation from the vertical axis running from the center of the head to the vertex;
␸
, angle deviation from the axis passing through the preauricular
points; dm, dipole moments.
a
Student’s paired t-test (absolute values were compared for tlrc x,
␽
,
␸
)
b
P⫽0.06
918 P. Jung et al. / NeuroImage 19 (2003) 913–923

The projection of both N20 dipole sources and the rostral
wall of the left and right postcentral gyrus on the frontal
plane distinctly illustrated that the left N20 source was
located significantly closer to the medial border of the
postcentral gyrus than the right N20 source (Fig. 4).
Laterality indices
According to the Edinburgh Inventory, two of the sub-
jects (one male, one female) were considered as left-handed
with a score ⬍0. Thus, the distribution of left-handedness
was representative for the general population. The average
handedness score was 70 ⫾12; 70 ⫾9 for men and 70 ⫾
15 for women, entailing no gender differences. In the di-
chotic listening test, none of the subjects revealed a left ear
advantage. The mean laterality index was 26 ⫾6. The
performances differed significantly between men and
women (11 ⫾4vs41⫾5, P⬍0.01) in which females
exhibited a more pronounced right ear advantage than
males.
Fifteen out of 16 subjects demonstrated a predomi-
nance of N20 amplitudes over the left scalp which was
expressed by a laterality index of 19.6 ⫾3.8 across all
subjects (P⬍0.001). Concerning N20 source strengths,
all subjects showed stronger dipole moments in the left
hemisphere (20.8 ⫾4.7, P⬍0.001). Concordantly, the
area of the anterior wall of the postcentral gyrus was
larger in the left hemisphere in 15 out of 16 subjects (LI
⫽4.8 ⫾0.8, P⬍0.001). No gender differences for these
asymmetry coefficients were detected. When examining
the relationship between handedness scores and laterality
indices of N20 amplitudes (Spearman’s rank order cor-
relation coefficient
␳
⫽⫺0.04), N20 source strengths (
␳
⫽⫺0.09), N20 eccentricity (
␳
⫽0.24), or SI morphology
(
␳
⫽0.28), no significant correlation could be deter-
mined. Moreover, correlation between morphological SI
asymmetry and N20 laterality (
␳
⫽⫺0.07 for amplitudes,
␳
⫽⫺0.16 for source strengths,
␳
⫽⫺0.04 for eccen-
tricity) was not significant. Ten subjects were classified
as “extreme right-handers,” six were considered as “non-
right-handers” (Habib et al., 1995). There were no evi-
dent differences in N20 laterality scores and in macro-
morphological SI asymmetry between the two groups
(Fig. 5). Likewise, no significant correlations between ear
advantage and handedness scores (
␳
⫽0.03) and between
ear advantage and the laterality indices of N20 eccentric-
ity (
␳
⫽0.13) or SI morphometry (
␳
⫽⫺0.07) were
found. The two left-handed subjects (handedness scores:
⫺13 and ⫺80) both had evidence for a left hemisphere
dominance in the dichotic listening test (ear advantage:
10 for the weak and 60 for the strong left-hander). The
highest, but still not significant, correlations were de-
tected between ear advantage and laterality indices of
N20 amplitudes (
␳
⫽0.35, P⬍0.2) and N20 source
strengths (
␳
⫽0.46, P⬍0.1).
Discussion
Our data documented that asymmetry of median nerve
SEP occurs at the cortical level only. No asymmetric ten-
dencies were found neither for the brachial plexus compo-
nent N10 nor for the brainstem potential P14. In contrast,
the first response at the cortical level (N20) was clearly
lateralized. The present study clarified that this previously
observed cortical asymmetry with higher N20 amplitudes
over the left scalp (Buchner et al., 1995) is based on asym-
metric dipole strengths and not side different dipole loca-
tions or dipole orientations. If N20 sources had equal dipole
strengths in both hemispheres, a more eccentric, i.e., less
deep, N20 dipole localization in the left hemisphere would
entail higher N20 amplitudes over the left scalp. In our data,
however, higher N20 amplitudes over the left scalp were
determined even though N20 sources were located deeper in
the left hemisphere. In contrast, a previous EEG study
showed that smaller P40 amplitudes of the tibial nerve SEP
in the left compared to the right hemisphere could be at-
Fig. 3. Side comparison of N20 amplitudes, N20 source strengths, N20
source locations, and SI morphometry (mean ⫾SEM, n⫽16). (A) N20
amplitudes were higher over the left hemisphere (P3-AR) after right me-
dian nerve stimulation compared to the right hemisphere (P4-AR) after left
median nerve stimulation. (B) N20 source strengths were larger in the left
compared to the right hemisphere after contralateral median nerve stimu-
lation. (C) N20 source localizations were less eccentric in the left compared
to the right cortex, i.e., left N20 sources were located deeper in the brain.
(D) The rostral surface of the postcentral gyrus was significantly larger in
the left hemisphere as revealed by morphometric analysis of 21 horizontal
MR slices from z⫽69 to 29. rMed, right median nerve stimulation; lMed,
left median nerve stimulation; lHem, left hemisphere; rHem, right hemi-
sphere; *P⬍0.05, **P⬍0.01, ***P⬍0.001
919P. Jung et al. / NeuroImage 19 (2003) 913–923

tributed to deeper located left P40 dipole sources with a
more tangential orientation but not to weaker source
strengths (Baumga¨rtner et al., 1998). In general, source
analysis studies exploring cerebral asymmetries with high
time resolution are possible with EEG or MEG. To distin-
guish the influence of different dipole strengths from that of
different dipole orientations, it is essential to use EEG for it
is sensitive to current flows in all orientations but not MEG
which is insensitive to radial current flow (e.g., Schneider et
al., 2002).
The leftward N20 asymmetry was distinctive to such an
extent that it is of clinical relevance and of importance for
cortical reorganization studies. A side difference of more
than 50% of N20 amplitudes is commonly considered
pathological. In our sample, two healthy subjects showed
N20 amplitude side differences of 58.2% and 65.5%, re-
spectively, because of a distinct prevalence in the left cor-
tex. Their median nerve SEP would have been misleadingly
classified as pathological by the commonly used criterion.
It is noteworthy that the preponderant N20 response in
the left hemisphere is followed by a leftward weaker re-
sponse, labeled P22. Source activities of P22 in the right
hemisphere only tended to be stronger than in the left
sensorimotor cortex although P22 amplitudes were signifi-
cantly higher over the right half of the scalp. This is plau-
sible against the background that P22 dipoles of two-thirds
of our subjects showed more radial orientations in the right
sensorimotor cortex which entails that P22 electrical poten-
tials were projected more concentrated onto the right half of
the scalp. Compared to the first cortical response, reversed
Fig. 4. (A) Frontal projection of medial and lateral borders of the postcentral gyrus in 21 horizontal MR slices (z⫽69 to 29) and locations of N20 sources
in the left (red dots) and right (blue dots) hemisphere (mean ⫾SEM, n⫽16). In side comparison, N20 sources in the left hemisphere were located 4.2 ⫾
2.6 mm further ventral (n.s.) and 7.7 ⫾2.5 mm further medial (P⬍0.01). The postcentral gyrus was significantly larger in the left than in the right hemisphere
(P⬍0.001). For z⫽61 to 29, medial borders of the left postcentral gyrus lay constantly closer to the midsagittal plane (P⬍0.05 for z⫽51, 49, 39, 35,
33, 31) whereas lateral borders were symmetrical in all horizontal sections. (B) Reprojection of the region sampled in (A) onto a coronal MR image.
Fig. 5. Distribution of laterality indices (degree of lateralization) of N20
source strengths (A) and SI morphometry (B) of extreme right-handers
(RH, handedness score ⬎80, n⫽10) and non-right-handers (nonRH,
handedness score ⱕ80, n⫽6). Red symbols in the group of non-right-
handers are assigned to left-handers (E, handedness score ⫽⫺13, F:
handedness score ⫽⫺80). Positive values indicate preponderance of the
left hemisphere, i.e., higher N20 responses after right median nerve stim-
ulation and a larger rostral surface of the postcentral gyrus in the left
cortex, respectively. No significant differences between handedness groups
were found for both N20 and SI lateralization.
920 P. Jung et al. / NeuroImage 19 (2003) 913–923

brain asymmetry and slightly more frontal location of the
second cortical component are in accordance with a previ-
ous study by Buchner et al. (1995) who labeled this com-
ponent as P25. Thus, somatosensory function is not simply
processed predominantly in the left hemisphere. Asymmet-
ric somatosensory processing is rather dependent on differ-
ent SEP components. However, asymmetry of the N20
component was far more pronounced than P22 asymmetry.
In this study, only hemispheric asymmetry of the first
two cortical SEP responses, presumably generated in the
primary somatosensory cortex, was demonstrated. Knowl-
edge about cortical asymmetry of later SEP components is
still insufficient. But there are already indications for a
predominant sensory processing in the left secondary so-
matosensory cortex (Forss et al., 1994; Kany et al., 1997).
The rostral area of the postcentral gyrus was more ex-
tended in the left than in the right hemisphere within the
dorsoventral coordinates z⫽69 to 29. This asymmetry of
the postcentral gyrus seems to be predominantly caused by
a larger hand representation area in the left primary somato-
sensory cortex by the following lines of evidence: Accord-
ing to previous reports, the region of hand representation
accounts for the main portion of the primary sensorimotor
cortex in the range of z⫽69 to 29 (Sanes et al., 1995;
Hlusˇtı´k et al., 2001). This was supported by N20 source
localizations in both hemispheres that ranged from z⫽67 to
41 in our study. Furthermore, a more pronounced hand
representation area in the left SI has been previously de-
scribed on the basis of a larger distance between dipole
localizations of the first and fifth digit in the left compared
to the right SI (So¨ro¨s et al., 1999). In the present study, the
larger hand representation area in the left SI was possibly
reflected by leftward stronger N20 source strengths after
median nerve stimulation.
The distance between the lateral border of the postcentral
gyrus and midsagittal plane did not differ in the left and
right cortex whereas the medial border lay closer to mid-
sagittal plane in the left hemisphere (Fig. 4). Thus, other
functional SI areas could only have been shifted in medio-
ventral or mediodorsal direction by a larger hand area size
in the left primary somatosensory cortex. As a consequence,
functional displacements of SI areas which lie dorsal to the
hand area (e.g., forearm, foot) would be shifted along the
course of the postcentral gyrus if its lateroventral-to-me-
diodorsal course were taken into account (Fig. 4). In fact, a
deeper location along the interhemispheric fissure was noted
for the SI foot representation area of the left hemisphere
(Baumga¨rtner et al., 1998). However, functional SI regions
with a more ventral location in reference to the hand rep-
resentation area (e.g., face) would be shifted in medioven-
tral direction. For the lateroventral course of the postcentral
gyrus runs opposite to a displacement in medioventral di-
rection, such a shift should result in a macrostructural al-
teration, e.g., a further extension of the rostral wall of the
postcentral gyrus toward the interhemispheric fissure. In the
present study, the left postcentral gyrus consistently ex-
panded closer to midsagittal plane from z⫽61 to 29.
Actually, this further extension of the left postcentral gyrus
toward midline was most pronounced in horizontal MR
slices with z⫽39, 35, 33, 31, i.e., sections beneath the most
ventral N20 source localization (z⬍41). To our knowledge,
no interhemispheric comparison of functional SI areas
which lie somatotopically ventral to hand representation
(e.g., face) has been presented so far. In accordance with our
prediction, the face representation area in the left SI should
be located further medioventral than in the right SI due to
the larger SI hand representation area in the left hemisphere.
Unlike N20 sources, P22 sources were situated in the
prerolandic cortex (Fig. 2). In the literature, the origin of the
P22 generator still remains controversial. According to our
data, the primary motor cortex (area 4) is suggested to be the
generator of the P22 potential which is consistent with
several studies (Mauguie`re et al., 1983; Mauguie`re and
Desmedt, 1991; Desmedt and Bourguet, 1985; Deiber et al.,
1986; Kawamura et al., 1996) but at variance with other
reports which located the generator in area 1 (Allison et al.,
1991; Buchner and Scherg, 1991; Kakigi, 1994).
No relationship among handedness, N20 asymmetry, and
asymmetric SI morphometry was detected. However, with
only one extreme left-hander in our sampling, correlations
were not sensitive. But the fact that extreme right-handers
who are supposed to exhibit the most pronounced left hemi-
sphere dominance (LeMay, 1992; Habib et al., 1995) did not
show any differences in asymmetry of the N20 component
or SI morphology compared to non-right-handers indicated
no simple direct relationship of handedness to N20 asym-
metry or lateralized SI morphology.
Previous studies investigating the relationship between
handedness and other structural cerebral asymmetries (pla-
num temporale and parietale, frontal and occipital petalia,
sylvian fissure) led to diverse results: Both correlations
(Bear et al., 1986; Weinberger et al., 1982; Steinmetz et al.,
1991; Foundas et al., 1995; Ja¨ncke et al., 1994) and no
correlations (Witelson and Kigar, 1992; Good et al., 2001)
were found. If sensorimotor regions or functions were di-
rectly investigated as in the present study, no reflection of
handedness by structural or functional asymmetries was
detected (White et al., 1997; Dassonville et al., 1997; So¨ro¨s
et al., 1999; Rossini et al., 1994; but Amunts et al., 1996).
This is in accordance with our data.
In summary, we have documented that asymmetry of
median nerve SEP occurs at the cortical level only. This
asymmetry was due to different source strengths and not to
source locations or orientations. As a possible morpholog-
ical correlate of the leftward asymmetry of the N20 SEP
component, the total rostral surface area of the left postcen-
tral gyrus exceeded that of the right. However, the degree of
lateralization of the N20 response and SI morphology were
not related. Moreover, functional and morphological corti-
cal asymmetry of somatosensory representation appears to
vary independently of motor (and language) functions.
921P. Jung et al. / NeuroImage 19 (2003) 913–923

Acknowledgments
This study was supported by the Deutsche Forschungs-
gemeinschaft (Tr236/13). It contains essential parts of the
M.D. thesis of P. Jung which will be submitted to the faculty
of Medicine, Johannes Gutenberg-University Mainz, Ger-
many. Special thanks to Gertrud Schatt for technical assis-
tance and Harald Bornfleth for his advice concerning BESA.
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