Attentional modulation of the human somatosensory evoked potential in a trial-by-trial spatial...

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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. Mu ¨ller * Institut fu ¨r Allgemeine Psychologie, Universita ¨t 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 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 instructed them to detect rare tactile target stimuli at the 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 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. Mu ¨ller). www.elsevier.com/locate/cogbrainres Cognitive Brain Research 20 (2004) 491 – 509

Transcript of Attentional modulation of the human somatosensory evoked potential in a trial-by-trial spatial...

Page 1: 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

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

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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

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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

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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

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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

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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

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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),

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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

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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.

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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).

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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).

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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.

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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).

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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).

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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

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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-

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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

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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.

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