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Transcript of Attentional modulation of the human somatosensory evoked potential in a trial-by-trial spatial...
www.elsevier.com/locate/cogbrainres
Cognitive Brain Research 20 (2004) 491–509
Research report
Attentional modulation of the human somatosensory evoked potential in a
trial-by-trial spatial cueing and sustained spatial attention task measured
with high density 128 channels EEG
Regine Zopf, Claire Marie Giabbiconi, Thomas Gruber, Matthias M. Muller*
Institut fur Allgemeine Psychologie, Universitat Leipzig, Seeburgstraße 14-20, D-04103 Leipzig, Germany
Accepted 19 February 2004
Available online
Abstract
We investigated the modulation of the somatosensory evoked potential (SEP) elicited by mechanical stimuli in a spatial sustained
attention and a spatial trial-by-trial cueing design by means of high density electrode array EEG recordings. Subjects were instructed to detect
rare tactile target stimuli at the to-be-attended hand while ignoring stimuli at the other hand. Analysis of the SEP revealed a highly complex
pattern of results. The P50 component was significantly increased for attended stimuli in the sustained attention as opposed to the trial-by-
trial cueing condition. However, no difference in amplitude was found for attended as opposed to unattended stimuli. High density electrode
array recordings revealed a centero-frontal N140 component (N140c), which preceded the parietal N140 (N140p) by about 20 ms. The N140c
exhibited an attention effect in particular in the trial-by-trial spatial cueing condition. The N140p was significantly enlarged with attention
across both experimental conditions, but a closer inspection demonstrated that this was mainly due to the great attention effect in the trial-by-
trial spatial cueing condition. The late positive component (190–380 ms after stimulus onset) exhibited a significant attention effect in both
experimental conditions. The present experiment provides evidence that the attentional modulation of the SEP is different when tactile as
opposed to electrical stimuli were used and when only somatosensory stimuli are presented with no further sensory stimulation in other
modalities. Furthermore, transient as opposed to sustained spatial attention affected various components of the SEP in a different way.
D 2004 Elsevier B.V. All rights reserved.
Theme: Sensory systems
Topic: Somatosensory cortex
Keywords: Attention; Somatosensory system; Tactile stimuli; Human EEG
1. Introduction instructed them to detect rare tactile target stimuli at the
The present study was designed to study changes of the
somatosensory evoked potential (SEP) associated with se-
lective spatial attention to the left or the right hand by means
of high density electrode array EEG recordings. Further-
more, we were interested in the effect of trial-by-trial spatial
cueing and sustained spatial attention, i.e. focused spatial
attention to one hand for a number of trials, upon the SEP.
We presented our subjects’ mechanical tactile stimuli and
0926-6410/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cogbrainres.2004.02.014
* Corresponding author. Tel.: +49-341-97-35-962; fax: +49-341-97-35-
969.
E-mail address: [email protected] (M.M. Muller).
index finger at the to-be-attended hand, while ignoring all
stimuli at the unattended hand (finger). Previous studies,
using electrical stimuli reported of a significant increase in
amplitude for a number of components of the SEP when
subjects attended to a certain stimulus at one body location
(mainly left or right hand) as opposed to when they were not
attending to it [14,16,23,24,32,40–42].
However, from these studies a rather inconclusive picture
emerged. This might be due to the fact that a number of
different tasks and a huge variety of stimulus strengths were
involved in the respective studies. For example, Michie [40]
reported of no significant attention effect for the P1 and N1
component of the SEP when subjects had to detect stronger
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509492
shocks (1.6 mA) as opposed to weaker standard shocks (1
mA) presented with variable inter-stimulus intervals. In a
subsequent study, Michie et al. [41] presented again differ-
ent shocks but this time much stronger as opposed to the
first study. In this study, the weak shocks were 2.8 mA and
the strong shocks 3.5 mA on average. When subjects had to
detect strong targets among weak standards, the authors
reported of a significant attention effect of the N80. In the
opposite case, i.e. weak targets among strong standards a
significant effect for the P105 (P100) was found. The N150
exhibited an attention effect only for weak stimuli and
showed a bilateral postcentral maximum. Josiassen et al.
[32] on the other hand found a complex pattern of attention
effects, when stimulating one of four fingers (4.8 mA on
average) and subjects had to selectively attend to one finger.
Attention effects were found for the positive peaks P45,
P100, P190 and P400 plus the N230. Interestingly, the P400
was greater ipsilaterally. The N140 showed no target vs.
standard stimulus effect but was associated with a decrease
in amplitude for the unattended finger. These findings were
different from the ones by Desmedt et al. [14], in which also
multiple fingers were stimulated but only an increase in the
P400 component for the attended finger was found. The
P400 exhibited a bilaterally symmetrical scalp distribution.
And finally, to conclude these examples, Mima et al. [42] in
a MEG study found early and late attention effects with
latencies of 38, 68, 125 and 138 ms, respectively. Source
analysis of theses MEG fields localized the sources of the
early fields (38 and 68 ms) in SI. Sources of the two later
fields were located in SII.
Besides the differences in stimulation strengths and tasks
used in the studies mentioned above, it might be the case
that electrical stimuli applied to the median nerve or to
fingers are suboptimal to investigate neural mechanisms of
touch, since electrical stimuli are unable to mimic the
complex interactions between different mechanoreceptors
of the glabrous skin [34,38,50]. Although the differences in
stimulus quality do not necessarily influence the morphol-
ogy of the cortical SEP, it might well be the case that
mechanical as opposed to electrical stimuli have a different
impact on the attentional modulation of the SEP. Electrical
stimuli are sharp and last only for a fraction of a millisec-
ond. These characteristics are closer to pain stimuli although
the applied currents were well under the pain threshold.
Mechanical stimuli have not such a sharp onset (mostly they
are delivered in form of a sinusoid, see below) and made
contact with the skin for much longer. Therefore, mechan-
ical stimuli seem to mimic everyday experience of touch
more closely as opposed to electrical stimuli.
Previous work with mechanical stimuli has shown that
tactile pulses evoked a typical SEP with readily to identify
components P50, N70, P100, N140 and a positive late
component [26,29,48]. However, the early components with
latencies shorter than 50 ms were almost not visible with
tactile stimuli. Using mechanical tactile stimuli, Eimer et al.
[19–21] investigated crossmodal links in endogenous spatial
attention between somatosensory, auditory and visual pro-
cessing. They found a significant increase of the N140 and
N2 component when somatosensory as opposed to stimuli
from the other modality had to be attended. From these
studies it appears as if mechanical tactile stimuli produce a
more coherent picture of attention effects upon the SEP in
multi-modal spatial sensory integration studies. However, it
is hard to judge whether these effects are ‘‘pure’’ attentional
effects in the somatosensory modality, or whether they are a
consequence of the multi-modal nature of the stimulation.
Therefore, it seems appropriate to conduct an EEG study to
investigate the attentional modulation of the SEP evoked by
mechanical tactile stimuli in a uni-modal experiment. More-
over, previous studies have only used sparse electrode arrays
with between 8 and 21 electrodes not allowing to investigat-
ing the topographical scalp distribution of SEP components
and the respective attention effects. For that reason, we used a
128-channel montage to provide topographical scalp distri-
butions based on dense spatial sampling of roughly 2 cm. A
further goal was to directly compare the effects of spatial trial-
by-trial cueing and sustained spatial attention (focused atten-
tion to one finger for a number of trials) upon the SEP. To us
this seemed very important, given that both designs had been
used in previous studies, but very little is known on the
possible effects upon the SEP. Since trial-by-trial cueing
requires that the allocation of attention to the one or the other
body side has to be achieved in a rapid manner shortly after
the onset of the cue, it might well be the case that this time
consuming shift has a possible impact on early components of
the SEP, as- this was discussed for the visual evoked potential
[17,18].
2. Material and methods
2.1. Subjects
Fifteen healthy university students (8 female, 7 male)
received class credits or a small financial bonus for partic-
ipation. Their age ranged from 19 to 25 (mean 21.5 years;
S.D. 1.63 years). All subjects were right handed. Informed
consent was obtained from each subject after the nature of
the study was fully explained.
2.2. Stimuli and apparatus
Subjects were seated in a comfortable chair in an
electrically shielded and sound attenuated cabin. Two ex-
perimental tables were placed left and right to the chair,
respectively, on which subjects’ hands rested in a comfort-
able position. Tactile stimulation was delivered by two
solenoid vibrators (V101, Ling Dynamic Systems, UK)
mounted under the table-tops, which drove a metal rod 6
mm in diameter with a plane surface. The rods made contact
with subjects’ second metacarpal of their index fingers
through an 8-mm-diameter hole in the table-top. Stimulation
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 493
was delivered to the second metacarpal of the left and right
index finger, respectively. Neither the vibrators nor the rods
were visible for the subjects. The vibrators were powered by
a dual-channel power amplifier, which was connected to a
digital-to-analogue conversion computer card. The ampli-
tude for all stimuli was set to 3.9 N at peak indentation. This
was achieved by previously determining the force/voltage
characteristic of each vibrator at DC by means of a force
transducer. The voltage amplitude of the sinusoidal signal
was set equal to the DC voltage required to generate a
constant force of 3.9 N, it being known that the force/
voltage characteristic of these vibrators is essentially linear
from DC to approximately 50 Hz. For each subject, the rods
were adjusted that no contact to the skin was established at
the rods resting state (zero crossing of the sinusoid) by a
laboratory jack.
The whole experiment consisted of six sustained atten-
tion and six trial-by-trial cueing blocks with 96 trials each,
which were presented in randomised order. In both, the trial-
by-trial cueing and the sustained attention task, a single 10
ms sinusoidal pulse served as standard stimulus (non-tar-
get), and a double pulse, which was built of two 10 ms
sinusoidal pulses with a gap of 50 ms, served as a target. In
30% of the trials, a target event occurred unpredictably at
the to-be-attended or the to-be-ignored finger. Stimuli were
presented in randomised order with a variable inter-stimulus
interval of 1500 to 2000 ms. In trial-by-trial cueing blocks, a
sinusoidal pulse of 100 ms in duration served as the
attention-directing cue, which preceded the target and non-
target stimuli by a variable interval between 1000 and 1200
ms. The interval between a stimulus and the next cue was
between 1500 and 2500 ms.
For the sustained attention condition, subjects had to
attend to the right or left finger in three blocks, respectively.
Attend to left and right hand blocks were presented in
randomised order. For all conditions (trial-by-trial cueing
and sustained attention) subjects were instructed (a) to
maintain visual fixation on a fixation point located on the
wall in front of them, (b) to avoid blinking, and (c) to detect
and respond to targets at the to-be-attended side by pressing a
button with their foot. The responding foot was changed to
the opposite one halfway through the experiment, and the
order in which feet were used was counterbalanced across
subjects. Throughout the experiment white noise was deliv-
ered through a centrally located loudspeaker to mask poten-
tial noise from the stimulation device.
2.3. Electrophysiological recordings
EEG was recorded continuously with an EGI (Electrical
Geodesics, 1998) 128-electrode array. A schematic represen-
tation of the electrode array and the corresponding extended
international 10–20 electrode sites, which were used for the
statistical analyses (see below) is given in Fig. 1.
The vertex (recording site Cz) was chosen as reference. As
suggested for the EGI high input impedance amplifier,
impedances were kept below 50 kV. Sampling rate was
500 Hz and all channels were pre-processed on-line by means
of 0.01–200-Hz band-pass filter. In addition, vertical and
horizontal eye movements were monitored with a subset of
the 128 electrodes. Further data processing was performed
off-line.
2.4. Data reduction and analysis
Only non-target stimuli were included in the present
analysis for both the sustained attention and trial-by-trial
spatial cueing condition. This is (a) in order to exclude
possible interference with the motor response, (b) because
targets and non-targets differed in their stimulus properties,
and (c) targets occurred only in 30% of the trials, resulting
in a substantially lower signal-to-noise ratio as compared to
non-targets. EEG was segmented to obtain epochs contain-
ing 500 ms prior to and 1500 ms following stimulus onset.
These longer epochs were chosen in order to allow the
analysis of the data in the frequency domain to explore the
role of induced high frequency EEG responses in somato-
sensory spatial attention. These data are currently analysed
and will be reported elsewhere.
These epochs were submitted for artifact rejection and
correction using a procedure developed by Junghofer et al.
[33] (statistical correction of artifacts in dense array studies,
SCADS). This procedure uses a combination of trial exclu-
sion and channel approximation based on statistical param-
eters of the data. In a first step, artifacts are detected using the
recording reference (Cz), and, subsequently, the average
reference. In a next interactive step, distinct sensors from
particular trials are removed on the basis of the distribution of
their amplitude, standard deviation and gradient. The infor-
mation of eliminated electrodes is replaced with a statistically
weighted spherical interpolation from the full channel set. In
a last step, the variance of the signal across trials is computed
to document the stability of the average waveform. The limit
for the number of approximated channels was set to 20
channels. With respect to the spatial arrangement of the
approximated sensors, it was ensured that the rejected sensors
were not located within one region of the scalp, because this
would make interpolation for this area invalid. Single epochs
with excessive eye-movements and blinks or more than 20
channels containing artifacts were discarded. Furthermore,
since it is known that looking to the to-be-attended body side
or part modulates tactile perception [49] and somatosensory
cortical processing [48], the horizontal EOG in all experi-
mental conditions was inspected for systematic horizontal
eye movements and trials with horizontal eye movements to
the to-be-attended side exceeding 2j were excluded
(corresponding to 20 AV in the horizontal EOG). A further
exclusion criteria which was linked to the analysis in the
frequency domain, was muscle activity, which cannot be
filtered out when interested in high frequency responses in
the EEG. As a consequence of these stringent criteria the
amount of rejected trials was somewhat higher as this would
Fig. 1. (A) Schematic representation of the 128 channels montage. Extended 10–20 sites used for statistical analysis are indicated. Note: 10–20 sites were
approximated to the closest electrode position on the net. (B) Schematic representation of electrodes, which were used in the statistics.
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509494
have been the case if one would concentrate on the SEP only.
Three subjects were totally excluded from further analysis
due to excessive eye blinks, horizontal eyemovements and/or
muscle artifacts in the EEG. For the remaining 12 subjects the
average rejection rate for both conditions was 30%, leaving
on average 70 trials per subject for each experimental
condition for analysis. For further analysis the average
reference was used.
2.5. Data analysis
2.5.1. Behavioural data
Mean reaction time and percentage of correctly detected
targets were subject to repeated-measures ANOVAs com-
prising the factors of Experimental condition (sustained
attention vs. trial-by-trial cueing), and Stimulus Side (right
vs. left). Only reaction times between 200 and 1000 ms after
target onset were considered to be correct responses. Reac-
tion times shorter or longer than that period were counted as
false alarms or missed responses.
2.5.2. Electrophysiological data
The remaining trials were averaged for the two exper-
imental conditions (sustained attention vs. trial-by-trial
cueing) for attended left and right as well as unattended
left and right standard stimuli. This resulted in 8 averaged
waveforms for each subject and electrode site. To obtain
the SEPs, each waveform was digitally filtered with a
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 495
Butterworth low-pass filter with a 25 Hz cut-off. The
amplitude of each SEP component was calculated as the
mean amplitude within a specified time window centred
around the peak latency in the root-mean square across all
electrodes, relative to a mean pre-stimulus amplitude of
100 ms. The following SEP components were extracted
(see Figs. 2 and 3, latencies are related to time after
stimulus onset): P50 (40–60 ms), N80 (64–84 ms),
P100 (100–120 ms), N144 (134–154 ms) and a late
component LC (190–380 ms). A closer inspection of the
latency of the negative component in the N144 time
Fig. 2. (A) Grand mean baseline corrected somatosensory evoked potentials fo
delivered to the right index finger at 10–20 electrode sites for the sustained spa
evoked potentials for attended (bold line) and unattended (thin line) tactile stimul
finger (right panel).
window revealed significant latency shifts between central
and posterior electrodes (see Results section). Based on
these findings we extracted a delayed N140 in a time
window between 160 and 180 ms at posterior electrodes.
We labelled the earlier centero-frontal N140 as N140c, and
the parietal N140 as N140p. For statistical analysis we
applied two analysis approaches. In the first approach we
analysed all components except the N140p at electrodes
C3 and C4 only, because these electrodes had been chosen
for analysis in a number of previous studies, cited in the
Introduction. This analysis was mainly done to allow a
r attended (bold line) and unattended (thin line) tactile non-target stimuli
tial attention condition. (B) Grand mean baseline corrected somatosensory
i at electrode locations C3/4 delivered to the right (left panel) or left index
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509496
better comparison of the present data with previous experi-
ments from other groups. In the second approach, a subset
of 36 electrodes was selected, which are related to extend-
ed 10–20 electrode locations [2], depicted in Fig. 1. These
electrodes covered frontal (FP1/FP2, AF7/AF8, AF3/AF4,
F7/F8, F3/F4, F1/F2), central (FC5/FC6, FC3/FC4, FC1/
FC2, C5/C6, C3/C4, C1/C2) and parietal (CP5/CP6, CP3/
CP4, CP1/CP2, P7/P8, P3/P4, PO3/PO4) areas on the
scalp surface of the left and right hemisphere, respectively.
The selection of electrode locations was guided to cover
frontal, central and parietal scalp areas equally well and
keeping the region comparable to an extended 10–20
electrode montage.
Repeated-measures ANOVAs were conducted for the
mean amplitudes for each SEP component comprising the
factors of Experimental condition (sustained attention vs.
trial-by-trial cueing), Stimulus side (right vs. left hand),
Attention (attended vs. unattended), Hemisphere (contralat-
eral vs. ipsilateral to the stimulus side) and Scalp region
(frontal vs. central vs. parietal), and Scalp site within Scalp
region (FP1/FP2, AF7/AF8, AF3/AF4, F7/F8, F3/F4, F1/
F2 for frontal), (FC5/FC6, FC3/FC4, FC1/FC2, C5/C6, C3/
C4, C1/C2, for central), and (CP5/CP6, CP3/CP4, CP1/
CP2, P7/P8, P3/P4, PO3/PO4, for parietal)). For the C3/C4
comparison the ANOVA model comprised of the factors of
Experimental condition, Stimulus side, Attention, and
Hemisphere (represented by electrodes C3/4). For a com-
parison of the scalp distributions of SEP components
indicated by significant interactions with the factors of
Experimental condition and/or Attention, the amplitude
values were z-transformed. In contrast to the normalization
suggested by McCarthy and Wood [37], a z-score trans-
formation is less affected by noise as compared to taking
the minimum and maximum [35]. Normalized amplitude
values were subject to ANOVAs with the factors listed
above.
Huyn-Feldt adjustments for non-sphericity were applied
whenever appropriate. Post-hoc tests were carried out
using paired Student’s t-tests corrected for multiple com-
parisons by the Bonferroni–Dunn criterion. Means and
standard errors are presented throughout the paper. Iso-
contour voltage maps were plotted for the SEP compo-
nents defined above, or the difference amplitude (attended
minus unattended) of a certain SEP component on the
basis of all 128 electrodes using the spherical spline
algorithm of Perrin et al. [44].
Table 1
Average reaction time and detection rates and standard errors for target stimuli for s
the left and right hand
Sustained attention
Reaction times in ms Detection rates in %
Left 626.17, S.E.: 28.57 98.09, S.E.: 0.93
Right 622.45, S.E.: 27.83 97.56, S.E.: 0.65
3. Results
3.1. Behavioural data
No significant differences were found for target detection
rates or reaction times. Overall, subjects detected nearly
100% of the targets. Table 1 shows the mean target detection
rates and reaction times across all 12 subjects for both
experimental conditions for the left and right hand, respec-
tively. False alarms and misses together were only between
0.8% and 2.7%.
3.2. Electrophysiological data
In Figs. 2 and 3, the grand mean SEPs across 12
subjects are depicted for attended and unattended stimuli
at the right index finger for sustained attention and trial-
by-trial cueing, respectively. In the upper panel the electro-
des of the international 10–20 system are depicted in the
lower panel electrodes C3 and C4 are zoomed out for
attended and unattended stimuli at the left or right finger,
respectively.
Tactile stimuli elicited a typical SEP with easily to
identify components P50, N80, P100, N144, and a late
positive component (LC) in both experimental conditions.
3.3. P50
3.3.1. Electrodes C3/4
In the sustained spatial attention condition the P50 ampli-
tude was significantly greater than in the trial-by-trial spatial
cueing condition (F(1,11) = 5.35, p < 0.05). As expected, the
overall amplitudes were significantly greater at the contra-
lateral hemisphere (F(1,11) = 25.13, p < 0.0005). When sub-
jects attended constantly for an entire block to one hand, the
P50 amplitude at the contralateral site was greater as opposed
to trial-by-trial cueing. Almost no difference in amplitude
between these two conditions for the unattended stimulus was
present (Experimental condition�Attention�Hemisphere,
F(1,11) = 4.84, p= 0.05).
3.3.2. Thirty-six electrodes
Fig. 4 depicts the grand average scalp distributions of the
P50 amplitude for all experimental conditions. The top-
ographies show a clear positive peak at posterior electrode
locations contralateral to the stimulated finger and a polarity
ustained attention (left) and trial-by-trial cueing (right) across 12 subjects for
Trial-by-trial cueing
Reaction times in ms Detection rates in %
637.65, S.E.: 35.06 98.79, S.E.: 0.65
642.47, S.E.: 29.01 98.97, S.E.: 0.53
Fig. 3. (A and B) Same as in Fig. 2A and B, but for the trial-by-trial spatial cueing condition.
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 497
reversal at fronto-central electrodes, indicating the activity
of one dipolar source.
The topographical features shown in Fig. 4 were con-
firmed by the statistical tests. The greatest amplitudes were
found at parietal and central electrodes contralateral to the
stimulated finger, whereas at frontal sites an amplitude
reversal to negative values was present with no difference
in amplitudes at contralateral and ipsilateral electrodes
(Hemisphere� Scalp region, F(2,22) = 34.9, p < 0.0001).
This interaction is depicted in Fig. 5. As shown in Fig. 4,
the greatest P50 amplitudes were present at parietal elec-
trodes and electrode locations C3/4 and C5/6 contralateral
to the stimulated finger (Hemisphere� Scalp region�Electrode, F(10,110) = 17.87, p< 0.0001). Furthermore, the
significant interaction Stimulus side�Hemisphere� Scalp
region (F(2,22) = 4.53, p < 0.05) indicated different ampli-
tudes for the P50 component for stimuli presented to the left
or right finger. Stimulation of the left finger resulted in
greater negative amplitudes at ipsilateral central electrodes
(t(1,11) = 2.9, p < 0.05). With regard to right finger stimula-
tion, we found a trend towards greater negative amplitudes at
contralateral frontal (t(1,11) =� 2.19, p <0.06) and central
(t(1,11) =� 2.12, p < 0.06) scalp regions. A closer inspection
of Figs. 2 and 3, and the results based on the analysis of
electrode locations C3/4, reported above, suggested greater
P50 amplitudes for attended stimuli during sustained atten-
tion as opposed to attended stimuli during trial-by-trial
cueing at central and parietal electrodes. This impression
was supported by looking at the interaction comprising the
factors of Experimental condition, Attention, Hemisphere,
Scalp region, and Electrode (F(10,110) = 2.70, p < 0.05; for
normalized amplitude values, F(10,110) = 3.02, pV 0.01),
Fig. 4. Grand mean isocontour voltage maps of the P50 elicited by attended (left) and unattended (right) non-target stimuli during sustained spatial attention to
the right vs. left hand index finger (upper four maps) and during trial-by-trial spatial cueing to the right vs. left hand index finger (lower four maps).
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509498
which showed greater P50 amplitudes for attended stimuli in
the sustained attention condition as opposed to attended
stimuli in the trial-by-trial cueing condition at all posterior
electrodes and the central electrode positions C3/4 and C5/6
contralateral to the stimulated finger.
3.4. N80
3.4.1. Electrodes C3/4
We found a trend towards more negative N80 amplitudes
for the sustained attention condition at the contralateral
hemisphere and more negative amplitudes for the cueing
condition at the ipsilateral hemisphere (F(1,11) = 3.97, p <
0.08). Other than that, no further statistically significant
effects were found for the N80 component at electrodes
C3/4.
3.4.2. Thirty-six electrodes
Considering 36 electrodes in the statistical analysis
resulted in no attention related significant main effect or
interaction. As depicted in Fig. 6, the N80 exhibited the
greatest negative amplitudes at frontal and central scalp
regions (Scalp region, F(2,22) = 13.02, p < 0.005) contralat-
eral to the stimulated finger (Hemisphere� Scalp� region,
F(2,22) = 8.47, p < 0.01), and this negativity was greater
when the right finger was stimulated (Stimulus side�Hemi-
Hemisphere� Scalp region�Electrode, F(10,110) = 5.12,
p < 0.001; for normalized amplitude values, F((10,110) =
4.48, p < 0.001).
A significant interaction comprising the factors Exper-
imental condition and Scalp region (F(2,22) = 4.62,
p < 0.05; for normalized amplitude values, F(2,22) =7.02,
p < 0.05) showed that the amplitude of the N80 was
Fig. 5. Mean P50 amplitudes for non-target stimuli across 12 subjects (plus
standard errors) averaged across frontal, central and parietal 10–20
electrode locations, respectively, at the contralateral (black bars) and
ipsilateral (grey bars) hemisphere.
Fig. 6. Mean N80 amplitudes across 12 subjects (plus standard errors) for
non-target stimuli averaged across frontal, central and parietal 10–20
electrode locations, respectively, at the contralateral (black bars) and
ipsilateral (grey bars) hemisphere.
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 499
greater for the trial-by-trial cueing as opposed to the
sustained attention condition at frontal (t(1,11) = 2.35,
p < 0.05) and central (t(1,11) = 2.1, p < 0.06) scalp regions
across both hemispheres.
3.5. P100
3.5.1. Electrodes C3/4
The P100 amplitude at electrode locations C3/4 exhibited
a greater amplitude for unattended stimuli as opposed to
attended stimuli at the contralateral hemisphere, which was
opposite at the ipsilateral hemisphere (Attention�Hemi-
Hemisphere, F(1,11) = 11.59, p < 0.01). However, as can be
seen in Figs. 2 and 3, this pattern was most pronounced for
the right hand in both experimental conditions. For the left
hand, trial-by-trial cueing resulted in a pronounced attention
effect with a greater P100 amplitude for the attended stimuli
at the contralateral hemisphere, whereas sustained attention
produced this attention effect at the ipislateral hemisphere
(Experimental condition� Stimulus side�Attention�Hemisphere, F(1,11) = 6.12, p < 0.05).
3.5.2. Thirty-six electrodes
In Fig. 7, the grand mean P100 isocontour voltage maps
are depicted for all experimental conditions. The P100
exhibited a centero-parietal positive maximum, which was
slightly shifted towards the hemisphere contralateral to the
stimulated hand.
Statistical analyses found a significant interaction Hemi-
sphere� Scalp region (F(2,22) = 4.14, p < 0.05), depicted in
Fig. 8. This interaction confirms that the P100 was greatest
at parietal regions with a shift towards greater amplitudes at
contralateral parietal electrodes and a polarity reversal to
negative values at frontal scalp electrodes (Scalp region,
F(2,22) = 10.4, p < 0.01).
Stimulating the left finger resulted in greater overall
P100 amplitudes as opposed to stimulating the right finger
(Stimulus side, F(1,11) = 5.15, p< 0.05). A closer inspection
showed that the P100 amplitude for left as opposed to right
finger stimulation was greater at contralateral central and
parietal electrodes with smaller negative values at frontal
electrodes. At ipsilateral electrodes there was basically no
difference in amplitude between stimulating the left or right
finger (Stimulus side�Hemisphere� Scalp region�Elec-
trode, F(10,110) = 2.41, p < 0.05; for normalized amplitude
values, F(10,110) = 2.55, p < 0.05).
For the left hand finger there was no difference in P100
amplitude for attended and unattended stimuli. For the right
hand finger on the other hand, the overall P100 for attended
stimuli was more positive as compared to unattended stimuli
(Stimulus side�Attention, F(1,11) = 4.80, pV 0.05). How-
ever, for normalized values this interaction became not
significant. Other interactions with attention, which were
significant for the uncorrected amplitude values resulted in
p-values well above 0.1 when the normalized values were put
into the ANOVA model. A valid interpretation of topograph-
ical differences between experimental conditions was there-
fore questionable, and hence, the interactions will not be
reported here.
3.6. N140
A closer inspection of the SEP for the attended stimuli
showed that the peak latency of the negative component in
the N140 range differed considerably between central and
parietal electrode locations. Fig. 9 depicts the SEP for
attended stimuli at the right finger for one frontal, central
and parietal electrode for the trial-by-trial cueing and
sustained attention condition, respectively. The latency shift
of the negative component following the P100 between C3
and P3 is clearly visible.
Fig. 7. Grand mean isocontour voltage maps of the P100 elicited by attended (left) and unattended (right) non-target stimuli during sustained spatial attention to
the right vs. left hand index finger (upper four maps) and during trial-by-trial spatial cueing to the right vs. left hand index finger (lower four maps).
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509500
In order to test this latency shift, the latencies of the peak
following the P100 were extracted in a time window
between 110 and 200 ms after stimulus onset and averaged
across central and parietal electrodes, respectively. The
latency was significantly longer for parietal electrodes
(M = 165.46 ms, S.E. = 3.32 ms) compared to central
(M = 145.57 ms, S.E. = 4.42 ms, t(1,11) =� 2.96, p < 0.05).
Given these latencies, it seems highly plausible to see the
N140 as a component consisting of at least two separate
subcomponents with a peak latency of about 146 ms at
central and about 165 ms at parietal electrode sites. The
assumable dipolar structure of the generator of the posterior
N140 resulted in a polarity reversal at frontal electrode leads
(see Fig. 9). We labelled the two subcomponents of the
N140 complex as N140c (central) and N140p (parietal).
3.7. N140c
3.7.1. Electrodes C3/4
At electrode sites C3/4 the main effect attention slightly
failed to reach the 5% level (F(1,11) = 3.97, p = 0.07). The
biggest attention effect was found for right hand stimula-
tion at the contralateral hemisphere and for left hand
stimulation at the ipsilateral hemisphere (Stimulus side�Attention�Hemisphere, F(1,11) = 6.67, p < 0.05). This pic-
ture was in particular true for the trial-by-trial cueing
condition, whereas the sustained attention condition
exhibited an attention effect at the contralateral hemisphere
for the left hand stimulation as well (Experimental con-
dition� Stimulus side�Attention�Hemisphere, F(1,11)=
14.77, p < 0.005).
Fig. 8. Mean P100 amplitudes across 12 subjects (plus standard errors) for
non-target stimuli averaged across frontal, central and parietal 10–20
electrode locations, respectively, at the contralateral (black bars) and
ipsilateral (grey bars) hemisphere.
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 501
3.7.2. Thirty-six electrodes
In Fig. 10, the grand mean N144 isocontour voltage
maps for all experimental conditions including the
Fig. 9. Grand mean baseline corrected SEPs for attended (upper panel) and unatt
(dashed line), and P3 (bold line) for sustained spatial attention (right) and trial-by
difference maps (attended minus unattended) are
depicted.
Statistical analysis of the N144 with more electrodes
revealed a more complex picture as opposed to analysing
C3/4 only. As depicted in Figs. 10 and 11, the amplitude of
the N144 exhibited a shift from frontal negative to parietal
positive values (Scalp region, F(1,11) = 5.17, p < 0.05). How-
ever, for the trial-by-trial cueing condition the N144 exhibited
a more negative amplitude at frontal and a more positive
amplitude at parietal scalp areas as opposed to the sustained
attention condition (Experimental condition� Scalp region,
F(1,11) = 3.88, p < 0.05). This was in particular the case, as
depicted in Fig. 11, for the unattended stimuli (Experimental
condition�Attention� Scalp region, F(1,11) = 8.52,
p < 0.01; for normalized values, F(1,11) = 3.68, pV 0.06).
Subsequent post-hoc t-tests revealed significant attention
differences only for trial-by-trial cueing at frontal (t(1,11)
= 2.97, p < 0.05) and parietal (t(1,11) =� 5.46, p < 0.0005)
scalp regions. Similarly to the above reported C3/4 findings,
an overall N144 attention effect at the contralateral hemi-
sphere for right hand stimulation and an ipsilateral effect for
left hand stimulation was found (Stimulus side�Atten-
tion�Hemisphere, F(1,11) = 5.72, p < 0.05; for normalized
values, F(1,11) = 3.95, p < 0.075; see Fig. 11).
ended (lower panel) right tactile stimuli at electrodes AF3 (solid line), C3
-trial spatial cueing (left).
Fig. 10. Grand mean isocontour voltage maps of the N144c in the sustained spatial attention condition for attended right and left (first row), unattended right
and left (second row) non-target stimuli. The difference in isocontour voltage maps (attended minus unattended) for the sustained spatial attention condition are
depicted in the third row. Rows three to six: the same as above but for the trial-by-trial spatial cueing condition. Note: Different scales.
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509502
3.8. N140p
Given the topography of the N140p, we tested this
component for parietal electrodes only. This resulted in a
highly significant attention effect (F(1,11) = 28.99, p <
0.0005). The attention effect was mainly due to the big
difference in N140p amplitude between attended and unat-
tended stimuli in the trial-by-trial cueing condition (Exper-
imental condition�Attention, F(1,11) = 17.57, p < 0.002;
for the normalized values, F(1,11) = 5.96, p < 0.05), which
was confirmed by a post-hoc t-test (t(11) = 5.93, p < 0.0001,
see Fig. 12). Basically no difference in N140p amplitude
was found for attended stimuli between the two experimen-
tal conditions. The pattern depicted in Fig. 12 was also
responsible for a significant main effect Experimental con-
dition (F(1,11) = 8.7, p < 0.05) with a greater overall posi-
tive amplitude for the trial-by-trial cueing condition.
3.9. LC
3.9.1. Electrodes C3/4
At electrode locations C3/4 the amplitude of the LC was
significantly greater for the trial-by-trial cueing compared
with sustained attention (F(1,11) = 39.34, p < 0.0001) and
unattended as opposed to attended stimuli (F(1,11) = 24.22,
pV 0.0005), which found its expression in the significant
Attention� Experimental condition interaction (F(1,11) =
26.89, p < 0.0005), depicted in Fig. 13.
Fig. 11. Mean N144c amplitudes across 12 subjects (plus standard errors) for non-target stimuli averaged across frontal, central and parietal 10–20 electrode
locations, respectively, for attended (black bars) and unattended non-target stimuli (grey bars) for the sustained spatial attention (left panel) and trial-by-trial
spatial cueing condition (right panel).
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 503
Post-hoc t-tests revealed a significant increase in ampli-
tude for the unattended as opposed to the attended stimuli
for the sustained attention (t(11) = 2.20, p < 0.05) and trial-
by-trial cueing condition (t(11) = 6.35, p < 0.0001). Further-
more, the LC amplitude was significantly greater for the
trial-by-trial cueing compared with sustained attention for
attended (t(11) = 3.68, p < 0.005) and unattended stimuli
Fig. 12. Mean N140p amplitudes across 12 subjects (plus standard errors)
for non-target stimuli averaged across parietal 10–20 electrode locations
for attended (left) and unattended stimuli (right) for sustained spatial
attention (black bars) and trial-by-trial spatial cueing (grey bars).
(t(11) = 7.80, p < 0.0001). In addition, we found a trend
towards greater amplitudes at the electrode contralateral to
the stimulated hand (F(1,11) = 3.80, p < 0.08).
3.9.2. Thirty-six electrodes
Fig. 14 depicts the isocontour voltage maps for the LC
component for all experimental conditions. In this figure,
Fig. 13. Mean LC amplitudes across 12 subjects (plus standard errors) for
non-target stimuli averaged across electrodes C3/C4 for attended (left) and
unattended stimuli (right) for sustained spatial attention (black bars) and
trial-by-trial spatial cueing (grey bars).
Fig. 14. Grand mean isocontour voltage maps of the LC elicited by attended (left) and unattended (right) non-target stimuli during sustained spatial attention to
the right vs. left hand index finger (upper four maps) and during trial-by-trial spatial cueing to the right vs. left hand index finger (lower four maps). Note:
Different scales.
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509504
the positive peak around electrode Cz is clearly visible.
Identical to the analysis restricted to C3/4, the peak was
greater for unattended (M = 1.63, S.E. = 0.23) compared
with attended stimuli (M = 3.67, S.E. = 0.26; F(1,11) =
38.91, p < 0.0001).
This focused positive activity at central electrode loca-
tions extended to parietal scalp regions and reversed in
polarity at frontal areas (Scalp region, F(1,11) = 18.41,
p < 0.0001). Furthermore, the trial-by-trial cueing condition
was linked to greater amplitudes (Experimental condi-
tion� Scalp region, F(1,11) = 43.09, p < 0.0001; for the
normalized values, F(1,11) = 12.04, p < 0.005) at central
(t(1,11) =� 2.84, p < 0.05), parietal (t(1,11) =� 6.11,
p < 0.0001), and frontal (t(1,11) = 6.94, p < 0.0001) scalp
regions. As can be seen in Fig. 14, there was a shift towards
greater amplitudes at central and parietal scalp regions
contralateral to the stimulated hand (Hemisphere� Scalp
region, F(1,11) = 20.6, p < 0.0001), but this became signif-
icant at parietal regions only (t(1,11) = 3.93, p < 0.005).
The significant interaction Experimental condition�At-
tention (F(1,11) = 20.33, p < 0.001; for the normalized val-
ues, F(1,11) = 4.51, p < 0.06), depicted in Fig. 15, shows that
the overall attention effect was present in the sustained
attention (t(1,11) =� 2.69, p < 0.05) and trial-by-trial cueing
condition (t(1,11) =� 7.29, p < 0.0001). However, the over-
all amplitude for unattended stimuli during sustained atten-
tion was significantly smaller as opposed to trial-by-trial
cueing (t(1,11) =� 1.46, p < 0.05).
Fig. 15. Mean LC amplitudes across 12 subjects (plus standard errors) for
non-target stimuli averaged across all 10–20 electrode locations for
attended (black bars) and unattended non-target stimuli (grey bars) for the
sustained spatial attention (left panel) and trial-by-trial spatial cueing
condition (right panel).
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 505
We further found a significant interaction Attention�Hemisphere� Scalp regions (F(1,11) = 13.48, p< 0.001; for
the normalized values, F(1,11) = 14.75, p < 0.0001) shown
in Fig. 16. Significant attention effects were found at
contralateral frontal (t(11) = 3.97, p< 0.01), central (t(11) =
Fig. 16. Mean LC amplitudes across 12 subjects (plus standard errors) for non-t
locations, respectively, for attended (black bars) and unattended non-target stimul
electrodes.
3.08, pV 0.01) and ipsilateral frontal (t(11) = 4.97, p <0.001),
central (t(11) = 4.41, p < 0.005) and parietal (t(11) =2.25,
p < 0.05) scalp regions.
4. Discussion
In the present study, we investigated the attentional
modulation of the somatosensory evoked potential evoked
by using mechanical stimuli in a spatial sustained attention
and trial-by-trial spatial cueing design by means of high
density electrode array EEG recordings. Subjects were
instructed to detect rare tactile target stimuli at the to-be-
attended hand. Behavioural data provided evidence that
target detection was reasonably easy for both conditions at
both hands with an average detection rate of 98%. Further-
more, reaction times were very similar in both conditions for
left and right hand targets.
Electrophysiological data revealed a rather complex
pattern of results. Similar to previous studies with tactile
pulses [19–21,26,29,48], our tactile stimuli evoked a SEP
with readily to identify components such as P50, N80,
P100, N140, and a late positive component (LC). With
respect to the N140, our high-density electrode array record-
ings revealed that the N140 consisted of at least two
subcomponents with an earlier central (N140c) and a later
arget stimuli averaged across frontal, central and parietal 10–20 electrode
i (grey bars) at contralateral (left panel) ipsilateral (right panel) hemisphere
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509506
parietal peak (N140p). All these SEP components were
present in the trial-by-trial spatial cueing and sustained
spatial attention condition of the experiment. In the follow-
ing we will discuss our findings in the temporal order of the
respective SEP components.
The P50 showed a precentrally negative and postcen-
trally positive deflection. The positive deflection was great-
est at parietal electrodes contralateral to the stimulated hand
(finger). The precentral negative deflection exhibited a
centero-frontal maximum. Thus, the scalp distribution of
the P50 of the present study was similar to the one reported
previously with sparse electrode arrays [23,26,41]. Previous
studies have identified the sources of the P50 in SI [1,27,42]
and the isocontour voltage maps of the present study would
be in line with a source in the contralateral primary
somatosenory cortex. We found the P50 significantly en-
larged at central and posterior electrodes contralateral to the
stimulated finger in the sustained spatial attention condition
compared with trial-by-trial spatial cueing. Although the
interval between cue and stimulus was relatively long in the
present study, shifting attention to the left or the right hand
in the trial-by-trial cueing condition might have had a
consequence on the P50 amplitude. Contrary to the studies
by Josiassen et al. [32] and Mima et al. [42], but similar to
Desmedt et al. [14], we found no attention effect for the
P50. The reasons for these inconsistent findings are not
entirely clear at present, but it seems plausible that the
attentional modulation of the P50 might be linked to the
quality (electrical vs. mechanical) and strength of the
stimuli [42].
The peak of the N80 exhibited its maximum at frontal
and central scalp sites, contralateral to the stimulated hand,
with a polarity reversal at posterior electrodes. Similar to the
P50, the generators of the N80 had been found in SI
[27,30,42]. Contrary to Michi et al. [41], we found no
attention effect for the N80. Mima et al. [42] reported of
an attention effect for the magnetic field with a latency of 68
ms poststimulus. This latency is in between the P50 and
N80 of the present study. Interestingly, Michie and cow-
orkers found an attentional modulation of the N80 only in
their 1987 study, in which they used much stronger electri-
cal stimuli as opposed to their 1984 study. In addition, the
N80 modulation was only found in the condition in which
subjects had to detect strong targets among weak standards.
Thus, similar to the P50 it seems as if attentional modulation
of the N80 is to some extend dependent on the strength of
electrical stimuli. The fact that tactile stimuli are different
from electrical stimuli might be related to the fact that
neither the present nor other studies [20] that used tactile
stimuli have found an attentional modulation of the N80.
Previous studies found an attentional modulation of the
P100 [15,32,41,42]. Due to the bilateral scalp distribution in
previous studies, the generators of the P100 have been
found in the secondary somatosensory cortex of the left
and right hemisphere [25–27,42]. The present study found a
somewhat different scalp distribution, with a positive max-
imum over parietal scalp areas, contralateral to the stimu-
lated hand. Left finger stimulation resulted in greater
amplitudes at contralateral central and parietal electrodes,
whereas no overall difference between left and right finger
stimulation was found ipsilaterally. A significant attention
effect was only found for right hand stimulation, and no
overall difference (i.e. across all electrodes and experimental
conditions) between attended and unattended stimuli was
present for the left hand. When we restricted our analysis to
electrodes C3 and C4, the P100 exhibited greater amplitudes
for attended stimuli across all experimental conditions at the
ipislateral electrode. However, this was mainly determined
by the sustained spatial attention condition. Trial-by-trial
spatial cueing resulted in greater P100 amplitudes for
attended stimuli at the contralateral electrode. Thus, the
present findings are only partially in line with studies
recording the SEP with sparse electrode arrays, in which
an attentional modulation of the P100 was found at central
electrodes such as C3/4 (see above). Further, we have not
found a bilateral scalp distribution of the P100. In our study,
the P100 exhibited a trend towards greater amplitudes at
contralateral parietal electrodes with a polarity reversal at
frontal electrodes. The present findings seem to confirm that
the pattern of P100 modulation as a function of spatial
attention is highly complex. High density electrode array
recordings allowed us to draw isocontour voltage maps with
a much better spatial resolution, resulting in the shift in
topography towards a contralateral maximum of the P100.
The complex results with respect to the experimental
manipulation (trial-by-trial spatial cueing vs. sustained spa-
tial attention) and left vs. right hand stimulations need future
elaboration.
Somewhat similar to our study, Hamalainen et al. [25]
reported of a double peak of the N140 elicited by slow
pulses, but their double peak was at posterior electrodes
only. The second peak was roughly in the latency range of
the N140p of the present experiment. Other than the report
by Hamalainen, we are not aware of any other study
suggesting that the N140 seems to consist of a complex
with at least two subcomponents, and, therefore, possible
different generators for the N140c and N140p. Previous
studies were mainly analysing—what we labelled—the
N140c, which is most certainly due to sparse electrode
array recordings and the concentration on C3/4 for statis-
tical analysis. In theses studies, the N140 showed a
consistent attention effect only in multi-modal studies
[20]. As outlined in the Introduction, in studies investigat-
ing uni-modal effects of attention on the SEP the picture is
inconclusive [14,16,23,24,32,40–42]. In our study we
found a trend towards an attention effect, when we
restricted our analysis to electrode locations C3 and C4
for the N140c. This parallels the recent findings by Eimer
et al. [21], where in a cross-modal study attention to tactile
stimuli also tended to enlarge the N140 amplitude at
electrodes C3/4 only. However, earlier work showed a
significant attention effect [19], demonstrating inconsisten-
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 507
cy with regard to the N140, which needs further experi-
mental elaboration. In the present study, the N140c
exhibited a bilateral tempero-frontal scalp distribution with
a polarity reversal to positive amplitudes at parietal scalp
sites. This scalp distribution is consistent with bilateral SII
activation as suggested in previous studies [27,31,42].
Interestingly, we found the attention effect for right and
left hand stimuli over the left hemisphere, suggesting that
neural activity for attended stimuli is always greater in the
hemisphere contralateral to the dominant hand (all subjects
were right handed). To test this hypothesis, the study must
be conducted with left-handed subjects to see whether one
finds right hemisphere dominance in the N140c range.
A further possible explanation for the left cortical
hemisphere dominance of the N140c was suggested by
recent works by Rushworth et al. [45–47]. They reported
left hemisphere dominance in the anterior parietal cortex,
slightly posterior to the central sulcus (supramarginal
gyrus) in motor attention [45]. It is subject to future studies
to examine whether or not a similarity between motor and
somatosensory attention exists. Anatomical studies in the
macaque suggest such a link, given that the macaque
homologue of the supramarginal gyrus is connected with
the somatosensory and premotor cortices [7,8]. On the
other hand, this alternative explanation is in variance with
the studies cited above, localizing the N140 in SII and with
the topographical distribution of the present study. Inter-
estingly, trial-by-trial spatial cueing resulted in greater
N140c amplitudes at frontal and parietal electrodes, and,
as depicted in Fig. 11, in a significant attention effect at
these electrodes. Sustained spatial attention showed no
attention effect. Hence, it becomes clear that the overall
attention effect was mainly due to the effect in the trial-by-
trial spatial cueing condition and that trial-by-trial spatial
cueing seems to modulate the N140c in a more pronounced
fashion as compared to when subjects maintained attention
to one hand.
A somewhat similar pattern occurred at parietal electro-
des for the N140p. Similar to the N140c, we found a
significant attention effect for the trial-by-trial spatial cueing
condition. As depicted in Fig. 12, this was mainly due to the
fact that ignored stimuli in the trial-by-trial spatial cueing
condition exhibited significantly greater positive amplitudes
in the N140p time range compared with sustained spatial
attention. The N140p elicited by attended stimuli was
almost identical for both experimental conditions. The
latency difference of about 20 ms between the N140c and
N140p can only be explained by assuming two different
generators, with the N140p generator being more posterior
as opposed to the N140c generator. Based on their findings
of the N140 double peak, Hamalainen et al. [25] suggested
that several different mechanisms participated in the gener-
ation of the N140, without any further discussion of the
nature of these mechanisms. Different to the N140c, the
amplitude of the N140p for attended stimuli did not differ
between sustained and trial-by-trial spatial attention. It is
tempting to assume that the N140p might be related to
spatial information processing, i.e. to distinguish between
the left and right hand.
Support for such an interpretation comes from recent
studies in the visual and somatosensory modality. In the
visual modality the parietal cortex has been found to be
involved in spatial information processing [9–13]. Recently,
positron emission tomography (PET) studies in humans
demonstrated the activation of parietal cortex in somatosen-
sory spatial attention tasks [5,36,37]. Intracortical record-
ings in monkey cortex further support these findings [4] and
lesions of the right posterior parietal cortex were associated
with neglect of visual and somatosensory stimuli, contralat-
eral to the lesion [39,43]. Since lesions of the parietal cortex
resulted also in neglect of visual and auditory stimuli [22],
polysensory neural mechanisms seem to be linked to spatial
information processing in the parietal lobe [3,6,28,36,37].
Similar to Hamalainen et al. [26], the late positive
component of the present study exhibited its maximum at
the vertex and parietal scalp areas. This peak was also found
in other previous studies [14,16,41]. In the present study, a
clear attention effect across both experimental conditions
was only found for this component, but, contrary to the
above studies and studies by Eimer et al. [19,21], it
exhibited smaller positive amplitudes for attended stimuli.
However, the positive component in the studies by Eimer
and colleagues was found roughly between about 400 and
500 ms poststimulus. In the study by Michie et al. [41] the
late positive component had a latency of 415 ms, which is
significantly later compared to the latency of what we
labelled the late positive component. A closer inspection
of Figs. 2 and 3 of the present study shows that in the
latency range of about 400 ms, attended stimuli were related
to greater positive amplitudes as opposed to unattended
stimuli, and, thus, resembles the findings of these previous
studies. It might well be the case, that what we labelled the
late positive component was in fact the N2 with a longer
latency as opposed to the studies by Eimer [19,21]. In these
studies, the N2 was also found to exhibit positive values
with smaller positive values for attended as opposed to
unattended stimuli, which was exactly what we found in our
study. Since Hamalainen et al. [26] were not comparing
attended and unattended stimuli, it might be the case that no
pronounced N2 was elicited in their study.
In conclusion, the present study found a complex pattern
of SEP modulations as a function of attention and experi-
mental condition (trial-by-trial spatial cueing vs. sustained
spatial attention). Only when we restricted our analysis to
electrodes C3 and C4, the N140c tended to be increased
when attending to tactile stimuli. Overall, trial-by-trial
spatial cueing affected the N140c more pronounced as
opposed to sustained spatial attention. The N140p showed
the same pattern. The finding of similar amplitudes for the
N140p for attended stimuli at posterior electrodes tempted
us to link that component speculatively to spatial informa-
tion processing. A robust attention effect was found in the
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509508
time window between 190 and 380 ms poststimulus. Thus,
the present experiment provides evidence that attentional
modulation of the SEP is highly sensitive to (1) the nature of
the stimulation (mechanical vs. electrical stimuli), (2) the
modalities involved (unimodal vs. multi-modal), and (3) the
experimental designs used (trial-by-trial spatial cueing vs.
sustained spatial attention).
Acknowledgements
We thank Nicola Williams for her help in data recording.
This research was funded by the German Research
Foundation. Regine Zopf’s research stay was supported by
a stipend from the DAAD.
References
[1] T. Allison, G. McCarthy, C.C. Wood, S.J. Jones, Potentials evoked in
human and monkey cerebral cortex by stimulation of the median
nerve, Brain 114 (1991) 2465–2503.
[2] American, Electroencephalographic and Society, Guidelines for stan-
dard electrode position nomenclature, Journal of Clinical Neuro-
physiology 8 (1991) 200–202.
[3] H.A. Buchtel, C.M. Butter, Spatial attentional shifts: implications for
the role of polysensory mechanisms, Neuropsychologia 26 (1988)
499–509.
[4] H. Burton, R.J. Sinclair, S.-Y. Hong, J.R. Pruett, K.C. Whang, Tac-
tile–spatial and cross-modal attention effects in the second somato-
sensory and 7b cortical areas of rhesus monkeys, Somatosensory and
Motor Research 14 (1997) 237–267.
[5] H. Burton, N.S. Abend, A.M.K. MacLeod, R.J. Sinclair, A.Z. Snyder,
M.E. Raichle, Tactile attention tasks enhance activation in somato-
sensory regions of parietal cortex: a positron emission tomography
study, Cerebral Cortex 9 (1999) 662–674.
[6] C.M. Butter, H.A. Buchtel, R. Santucci, Spatial attentional shifts:
further evidence for the role of polysensory mechanisms using visual
and tactile stimuli, Neuropsychologia 27 (1989) 1231–1240.
[7] C. Cavada, P.S. Goldman-Rakic, Posterior parietal cortex in rhesus
monkey: I. Parcellation of areas based on distinctive limbic and sen-
sory corticocortical connections, Journal of Comparative Neurology
287 (1989) 393–421.
[8] C. Cavada, P.S. Goldman-Rakic, Posterior parietal cortex in rhesus
monkey: II. Evidence for segregated corticocortical networks linking
sensory and limbic areas with the frontal lobe, Journal of Comparative
Neurology 287 (1989) 422–445.
[9] M. Corbetta, Frontoparietal cortical networks for directing attention
and the eye to visual locations: identical, independent, or overlapping
neural systems, Proceedings of the National Academy of Sciences 95
(1998) 831–838.
[10] M. Corbetta, F.M. Miezin, G.L. Shulman, S.E. Petersen, A PET study
of visuospatial attention, The Journal of Neuroscience 13 (1993)
1202–1226.
[11] M. Corbetta, G.L. Shulman, F.M. Miezin, S.E. Petersen, Superior
parietal cortex activation during spatial attention shifts and visual
feature conjunction, Science 270 (1995) 802–805.
[12] M. Corbetta, J.M. Kincade, J.M. Ollinger, M.P. McAvoy, G.L.
Shulman, Voluntary orienting is dissociated from target detec-
tion in human posterior parietal cortex, Nature Neuroscience 3
(2000) 292–297.
[13] J.T. Coull, A.C. Nobre, Where and when to pay attention: the neural
systems for directing attention to spatial locations and to time inter-
vals as revealed by both PET and fMRI, The Journal of Neuroscience
18 (1998) 7426–7435.
[14] J.E. Desmedt, D. Robertson, Differential enhancement of early and
late components of the cerebral somatosensory evoked potentials dur-
ing forced-paced cognitive tasks in man, Journal of Physiology 271
(1977) 761–782.
[15] J.E. Desmedt, N.T. Huy, M. Bourguet, The cognitive P40, N60 and
P100 components of somatosensory evoked potentials and the earliest
signs of sensory processing in man, Electroencephalography and
Clinical Neurophysiology 56 (1983) 272–282.
[16] J.E. Desmedt, D. Robertson, E. Bronko, J. Debecker, Somatosensory
decision tasks in man: early and late components of the cerbral poten-
tials evoked by stimulation of different fingers in random sequences,
Electroencephalography and Clinical Neurophysiology 43 (1977)
404–415.
[17] M. Eimer, Attending to quadrants and ring-shaped regions: ERP
effects of visual attention in different spatial selection tasks, Psycho-
physiology 36 (1999) 491–503.
[18] M. Eimer, An ERP study of sustained spatial attention to stimulus
eccentricity, Biological Psychology 52 (2000) 205–220.
[19] M. Eimer, J. Driver, An event-related brain potential study of cross-
modal links in spatial attention between vision and touch, Psycho-
physiology 37 (2000) 697–705.
[20] M. Eimer, J. Driver, Crossmodal links in endogenous and exogenous
spatial attention: evidence from event-related brain potentials, Neuro-
science and Biobehavioral Reviews 25 (2001) 497–511.
[21] M. Eimer, J. van Velzen, J. Driver, Cross-modal interactions between
audition, touch, and vision in endogenous spatial attention: ERP
evidence on preparatory states and sensory modulations, Journal of
Cognitive Neuroscience 14 (2002) 254–271.
[22] M.J. Farah, A.B. Wong, M.A. Monheit, L.A. Morrow, Parietal lobe
mechanisms of spatial attention: modality-specific or supramodal,
Neuropsychologia 27 (1989) 461–470.
[23] L. Garcia-Larrea, H. Bastuji, F. Mauguire, Mapping study of somato-
sensory evoked potentials during selective spatial attention, Electro-
encephalography and Clinical Neurophysiology 80 (1991) 201–214.
[24] L. Garcia-Larrea, A.C. Lukaszewicz, F. Mauguire, Somatosensory
responses during selective spatial attention: the N120-to-N140 tran-
sition, Psychophysiology 32 (1995) 526–537.
[25] H. Hamalainen, M. Sams, A. Pertovaara, S. Carlson, K. Reinikainen,
R. Naatanen, Different functional roles of SI and SII somatosensory
cortices as reflected by evoked potentials and multiple-unit responses
to mechanical stimulation in awake monkey, Neuroscience Research
Communications 2 (1988) 143–150.
[26] H. Hamalainen, J. Kekoni, M. Sams, K. Reinikainen, R. Naatanen,
Human somatosensory evoked potentials to mechanical pulses and
vibration: contributions of SI and SII somatosensory cortices to P50
and P100 components, Electroencephalography and Clinical Neuro-
physiology 75 (1990) 13–21.
[27] R. Hari, K. Reinikainen, E. Kaukorana, M. Hamalainen, R. Ilmo-
niemi, A. Penttinen, J. Salminen, D. Teszner, Somatosensory evoked
cerebral magnetic fields from SI and SII in man, Electroencephalog-
raphy and Clinical Neurophysiology 57 (1984) 254–263.
[28] S.H. Hendry, S.S. Hsiao, M.C. Bushnell, Somatic sensation, in: M.J.
Zigmond, F.E. Bloom, S.C. Landis, J.L. Roberts, L.R. Squire (Eds.),
Fundamental Neuroscience, Academic Publisher, San Diego, 1999,
pp. 761–789.
[29] N. Ishiko, T. Hanamori, N. Murayama, Spatial distribution of so-
matosensory responses evoked by tapping the tongue and finger in
man, Electroencephalography and Clinical Neurophysiology 50
(1980) 1–10.
[30] H. Johannsen-Berg, D. Llyoyd, The physiology and psychology
of selective attention to touch, Frontiers in Bioscience 5 (2000)
894–904.
[31] H. Johansen-Berg, V. Christensen, M. Woolrich, P.M. Matthews,
Attention to touch modulates activity in both primary and secondary
somatosensory areas, NeuroReport 11 (2000) 1237–1241.
R. Zopf et al. / Cognitive Brain Research 20 (2004) 491–509 509
[32] R.C. Josiassen, C. Shagass, R.A. Roemer, D.V. Ercegovac, J.J.
Straumanis, Somatosensory evoked potential changes with a selective
attention task, Psychophysiology 19 (1982) 146–159.
[33] M. Junghofer, T. Elbert, D.M. Tucker, B. Rockstroh, Statistical cor-
rection of artifacts in dense array EEG/MEG studies, Psychophysio-
logy 37 (2000) 523–532.
[34] E.R. Kandel, T.M. Jessel, Touch, in: E.R. Kandel, J.H. Schwartz, T.M.
Jessell (Eds.), Principles of Neural Science, Prentice Hall International,
London, 1991, pp. 367–384.
[35] S.J. Luck, S.A. Hillyard, Electrophysiological evidence for parallel
and serial processing during visual search, Perception and Psycho-
physics 48 (1990) 603–617.
[36] E. Macaluso, C. Frith, J. Driver, Selective spatial attention in vision
and touch: unimodal and multimodal mechanisms revealed by PET,
Journal of Neurophysiology 83 (2000) 3062–3075.
[37] E. Macaluso, C. Frith, J. Driver, Directing attention to locations and to
sensory modalities: multiple levels of selective processing revealed
with PET, Cerebral Cortex 12 (2002) 357–368.
[38] J.H. Martin, T.M. Jessel, Modality coding in the somatic sensory
system, in: E.R. Kandel, J.H. Schwartz, T.M. Jessell (Eds.), Prin-
ciples of Neural Science, Prentice Hall International, London, 1991,
pp. 341–352.
[39] M.M. Mesulam, Spatial attention and neglect: parietal, frontal
and cingulate contributions to the mental representation and at-
tentional targeting of salient extrapersonal events, Philosophical
Transactions of the Royal Society of London. B 354 (1999)
1325–1346.
[40] P.T. Michie, Selective attention effects on somatosensory event-related
potentials, Annals of the New York Academy of Sciences 425 (1984)
250–255.
[41] P.T. Michie, H.M. Bearpark, J.M. Crawford, L.C.T. Glue, The effects
of spatial selective attention on the somatosensory event-related po-
tential, Psychophysiology 24 (1987) 449–463.
[42] T. Mima, T. Nagamine, K. Nakamura, H. Shibasaki, Attention modu-
lates both primary and secondary somatosensory cortical activities in
humans: a magnetoencephalographic study, Journal of Neurophysio-
logy 80 (1998) 2215–2221.
[43] M. Moscovitch, M. Behrmann, Coding spatial information in the
somatosensory system: evidence from patients with neglect following
parietal lobe damage, Journal of Cognitive Neuroscience 6 (1994)
151–155.
[44] F. Perrin, J. Pernier, O. Bertrand, J.F. Echallier, Spherical splines for
scalp potential and current source density mapping, Electroencepha-
lography and Clinical Neurophysiology 72 (1989) 184–187.
[45] M.F.S. Rushworth, A. Ellison, V. Walsh, Complementary localization
and lateralization of orienting and motor attention, Nature Neuro-
science 4 (2001) 656–661.
[46] M.F.S. Rushworth, M. Krams, R.E. Passingham, The attentional role
of the left parietal cortex: the distinct lateralization and localization of
motor attention in the human brain, Journal of Cognitive Neuroscience
13 (2001) 698–710.
[47] M.F.S. Rushworth, T. Paus, P.K. Sipila, Attention systems and the
organization of the human parietal cortex, The Journal of Neuroscience
21 (2001) 5262–5271.
[48] M. Taylor-Clarke, S. Kennett, P. Haggard, Vision modulates somato-
sensory cortical processing, Current Biology 12 (2002) 233–236.
[49] S.P. Tipper, D. Lloyd, B. Shorland, C. Dancer, L.A. Howard, F.
McGlone, Vision influences tactile perception without proprioceptive
orienting, NeuroReport 9 (1998) 1741–1744.
[50] M. Zimmermann, Das somatoviscerale sensorische System, in: R.F.
Schmidt, G. Thews (Eds.), Physiologie des Menschen, Springer-
Verlag, Berlin, 1990, pp. 207–233.