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www.elsevier.com/locate/cogbrainres
Cognitive Brain Research
Research report
Spatio-temporal dynamics of top-down control: directing attention to
location and/or color as revealed by ERPs and source modeling
Heleen A. Slagtera,*, Albert Koka, Nisan Molb, J. Leon Kenemansb
aDepartment of Psychonomics, University of Amsterdam, Roeterstraat 15, 1018 W.B. Amsterdam, The NetherlandsbDepartments of Psychonomics and Psychopharmacology, Utrecht University, The Netherlands
Accepted 8 September 2004
Available online 18 October 2004
Abstract
This study investigated the nature and dynamics of the top-down control mechanisms that afford attentional selection using event-related
potentials (ERPs) and dipole-source modeling. Subjects performed a task in which they were cued to direct attention to color, location, a
conjunction of color and location or no specific feature on a trial-by-trial basis. Overall, similar ERP patterns were observed for directing
attention to color and location, suggesting that spatial and non-spatial attention rely to a great extent on similar control mechanisms. The
earliest attention-directing effect, at 340 ms, was localized to ventral posterior cortex and may reflect processes by which the cue is linked to its
associated feature. Only late in the cue-target interval, differences in ERP were observed between directing attention to color and location.
These originated from anterior and ventral posterior areas and may represent differences in, respectively, maintenance and perceptual biasing
processes. The ventral posterior sources estimated for these late effects of directing attention to location and color were located posterior to
those estimated for the modulatory effects of, respectively, spatial and non-spatial attention. This suggests that the precise neural populations
involved in perceptual biasing and attentional modulation may differ. Conjunction cues initially elicited less posterior positivity than color and
location cues, but evoked greater central positivity from 540 ms on. This central effect may reflect feature integration or ongoing processes
related to cue-symbol translation. These results extend our understanding of the spatio-temporal dynamics of top-down attentional control.
D 2004 Elsevier B.V. All rights reserved.
Theme: Neural basis of behavior
Topic: Cognition
Keywords: Spatial; Non-spatial; Attentional control; Attentional selection; Event-related potentials; Dipole modeling
1. Introduction
Functional neuroimaging studies have shown that stimuli
presented at attended positions in space (e.g., Ref. [18]) or
with an attended non-spatial stimulus feature, such as color
(e.g., Ref. [5]), elicit enhanced activation in sensory brain
areas corresponding to the attended stimulus dimension.
This attention-related sensory facilitation of target process-
ing enables us to respond faster and more accurately to
important external events. Advance knowledge of both
spatial and non-spatial stimulus characteristics has been
0926-6410/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cogbrainres.2004.09.005
* Corresponding author. Fax: +31 20 6391656.
E-mail address: H.A.Slagter@uva.nl (H.A. Slagter).
shown to improve behavior [29,30]. Nevertheless, results
from event-related potential (ERP) studies indicate that the
temporal dynamics of the neural mechanisms underlying
attentional modulation of target processing differ between
spatial and non-spatial attention. Whereas visuospatial
attention results in enhanced amplitudes of the exogenous
components P1 and N1 evident in the ERP to stimuli at both
attended and unattended locations as early as 80–90 ms
post-stimulus (e.g., Refs. [8,41]), selection based on non-
spatial visual stimulus features, such as color or form, is
reflected by effects starting at around 150 ms post-stimulus,
which are super imposed on the evoked components and
have a very different morphology (e.g., Refs. [16,20]).
Thus, results from ERP studies indicate that modulation
22 (2005) 333–348
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348334
effects are not only of longer latency when attention is
directed to a non-spatial stimulus feature, but they are also
qualitatively different for spatial and non-spatial attention.
Given these dissociations observed with ERP, one may
ask whether the control processes that direct the focus of
attention and may produce attentional modulation of
sensory responses differ between spatial and non-spatial
attention. Only fairly recently, research has turned to address
this question (for review, see Refs. [46,64] ). A straightfor-
ward way to investigate attentional control processes is to
examine brain activity in the period before the test stimulus
is presented, that is, when subjects direct their attention to a
relevant stimulus feature in response to an attention-
directing cue. Recent studies using functional magnetic
resonance imaging (fMRI) have revealed a network of
activated brain areas in the period between attention-
directing cue and test stimulus, encompassing both frontal
and parietal regions for spatial [6,22,25,26,60] as well as
non-spatial [37,48,49,58] attention. However, some domain-
specificity appears to be present within this network, with
dorsal frontal and parietal areas and ventral occipito-
temporal regions being more strongly activated by, respec-
tively, spatial and non-spatial attention-directing cues
[13,50]. In addition, several studies have observed increased
activation in visual areas not only in response to target
stimuli, but also in the period preceding the presentation of
the target stimulus (e.g., Refs. [13,22,26]). The common
interpretation of these findings is that higher order areas in
frontal and parietal cortex send biasing signals to function-
ally specialized sensory areas, so that they in turn can
selectively process target information [7].
Although fMRI provides detailed information about the
localization of neural processes, its temporal resolution is
still in the order of hundreds of milliseconds to a few
seconds at best [44]. This is much longer than the time it
takes to fully direct attention [39]. The high temporal
resolution of the event-related potential technique makes it
an excellent tool for the study of control processes and
preparatory states, as it can relate specific differences in
brain activation to changes in specific stages of information
processing. This temporal information is essential for a full
understanding of the attentional control mechanisms
reflected in fMRI activations. Even though fMRI studies
have shown involvement of roughly the same network of
brain regions in the directing of attention to spatial and non-
spatial stimulus attributes [13,50], the temporal sequence of
activation within these regions may be dependent on the
nature of the to-be-attended stimulus material.
Several ERP studies have previously investigated the
directing of attention to a location in space [9–11,15,
17,40,47,61,62] and non-spatial features [28,63]. However,
a comparison between results from these studies is at present
restrained by the fact that most studies of spatial attentional
control subtracted ERP responses to cues directing attention
to the left from ERP responses to cues directing attention to
the right hemifield [9–11,17,21,47,61,62] (but see Refs.
[15,40]). This comparison has revealed a sequence of effects
related to directing attention to a specific location in space,
consisting of an early directing attention negativity (EDAN)
at posterior parieto-occipital electrodes between 200 and 400
ms post-cue, an anterior directing attention negativity
(ADAN) at frontal electrodes between 300 and 500 ms
post-cue, and a late directing attention positivity (LDAP)
over lateral ventral occipito-temporal scalp regions starting at
around 500 ms post-cue. Yet, these effects cannot easily be
compared to results from studies of non-spatial attentional
control in which such an attend-left versus attend-right
comparison is obviously not possible. In addition, one may
ask whether these cue-direction-related effects reflect the full
temporal pattern of spatial attentional control (see also Ref.
[57]). Several studies of spatial top-down control have
reported behavioral cueing effects and attentional modulation
effects (i.e., P1, N1) in the absence of these cue-direction-
related ERP effects (i.e., EDAN [10,11], ADAN [15] and
LDAP [40,47]). This suggests that some attentional control
processes that may be mandatory for the establishment of an
attentional bias are not lateralized and, thus, do not show up in
the left–right subtraction. This further complicates an
integrative interpretation of the results from ERP studies of
spatial and non-spatial top-down control. Thus, in order to
adequately isolate the complete pattern of spatial or non-
spatial attentional control, one needs to compare the
attention-directing condition with a reference condition that
controls for processes that are not specific to the actual
initiation and directing of attention, such as cue-identification
and motor preparation processes, but that does not call upon
attentional control mechanisms.
In the present study, we examined the extent to which top-
down control processes are stimulus material-unspecific (i.e.,
general) or depend on the nature of the to-be-attended
stimulus feature (i.e., domain-specific) using a within-subject
design and a reference cue condition. ERPs elicited by
location and color attention-directing cues were compared to
ERPs elicited by reference cues to isolate processes related to
directing attention to location and color. The underlying
neural source configurations of the observed spatial and non-
spatial attention-directing effects were compared against each
other to reveal possible differences in the configuration and/
or timing of activated brain areas. Based upon position-
special models of attention [31,33,34,53,55,56], it can be
hypothesized that pre-target biasing effects of spatial
attention result from an initial activation in dorsal posterior
areas, which maintain location representations, which is
followed in time by activation in ventral posterior areas,
which maintain spatially corresponding feature representa-
tions. Biasing effects of non-spatial attention, on the other
hand, would be reflected by a reversed pattern of activation,
with ventral posterior areas being activated first and then
dorsal posterior areas, or activation of only ventral posterior
areas that hold representations of the non-spatial feature [19].
In LaBerge’s model of attention, for example, when the
location of the stimulus is predictable, parietal areas involved
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348 335
in coding spatial information can modulate featural informa-
tion of an object in the occipito-temporal lobe by constricting
the effective receptive fields of cells within this area, thereby
aiding in the selection of the object with the attended feature
[32,33].
In addition, the present study examined the relation
between perceptual biasing and attentional modulation
effects for spatial and non-spatial attention separately by
comparing the neural source configurations underlying these
effects. Based upon results of event-related fMRI studies,
which have shown increased baseline activity in the same
visual areas that were modulated by spatial attention (e.g.,
Refs. [22,26]), we expected to obtain similar source
solutions for pre-target perceptual biasing and post-target
attentional modulation effects.
Lastly, the present study investigated the direction of
attention in a condition where attention was to be directed
simultaneously both to a location in space and to a color. If
the two types of attention rely on completely different
control structures, no interaction, but pure additive effects of
directing attention to location and directing attention to
color are expected. If, on the other hand, the two types of
attentional control rely on similar mechanisms, simulta-
neously directing attention to location and color should
place greater demands on these general control mechanisms
as reflected by enhanced or prolonged attention directing-
related ERP effects. Another possibility would be that
directing attention to a conjunction of location and color
calls upon entirely new processes specific to the conjoining
of the two stimulus attributes [54].
Fig. 1. Examples of cues used in the location (most left panel), color
(second panel), conjunction (third panel) and no-feature (most right panel)
conditions. Horizontal lines denote the letter-symbol(s) to be used to direct
attention (L=attend left, R=attend right, G=attend to yellow, B=attend to
blue). When presented next to the fixation cross (i.e., no-feature cue), no
specific color or location had to be attended.
2. Method
2.1. Subjects
Sixteen healthy volunteers participated in the study. Two
subjects were discarded from the analyses because of poor
eye fixation in the interval between cue and test stimulus or
excessive blink activity during EEG recordings. Thus, 14
subjects (7 men, mean age of 23.2 years) remained in the
sample. All subjects were students at the University of
Amsterdam, were right-handed, had no history of mental or
sustained physical illness, and had normal or corrected-to-
normal vision by self-report. Subjects received credits as
part of an introductory course requirement at the University
of Amsterdam.
2.2. Stimuli and procedure
Each trial began with a 100-ms presentation of a cue
(0.928 in width and 2.88 in height) that was located at
fixation. After a random interval between 800 and 1500
ms (rectangular distribution), during which only the
fixation cross (0.318 in width and 0.208 in height) was
shown at the center of the screen, the cue was followed by
a test stimulus (38 in height, 38 in width). This test
stimulus was a blue or yellow square and appeared 7.138to center from fixation in either the left or the right visual
field and 1.738 to center above the horizontal meridian.
The interval between test stimulus offset and onset of the
next trial was varied randomly between 1400 and 2100 ms
(rectangular distribution). During this interval, the fixation
cross remained on the screen. All stimuli were presented
on a black background. Within a run, subjects were
randomly cued to attend to (a) a color (blue or yellow;
color condition (COL)), (b) a location (left or right;
location condition (LOC)), (c) a color and a location
(e.g., blue and left; conjunction condition (CONJ)) or (d)
to dnothingT (no-feature condition (N); see below).
Each cue consisted of four white uppercase letters (all
equal in width (0.368) and height (0.518)) presented around
the fixation cross in a vertical array: dBT, dGT, dLT and dRT(see Fig. 1). Each letter corresponded to a stimulus feature:
dBT to blue, dGT to yellow (dgeelT in Dutch), dLT to left and
dRT to right. Letter order was counterbalanced across
subjects with the restriction that the two blocationQ letters(dLT and dRT) and the two bcolorQ letters (dBT and dGT) werealways grouped together, resulting in eight possible combi-
nations of letters: BGLR, BGRL, GBLR, GBRL, LRBG,
RLBG, LRGB and RLGB. In the color and location
conditions, the color or location to which attention was to
be directed, was indicated by two short, horizontal lines
(0.208 in width, 0.088 in height), one on each side of a given
letter (e.g., when presented next to dLT, attention had to be
directed to the left (see Fig. 1)). In the conjunction
condition, two letters, one representing a color, the other a
location, were flanked by horizontal lines (0.108 in width,
0.088 in height) indicating that those both had to be used to
direct attention. In the no-feature condition, the two
horizontal lines (0.208 in width, 0.088 in height) were
presented next to the fixation cross. Conjunction cues were
presented on 40%, and color, location and no-feature cues
each on 20% of the trials.
In each task condition, the cue was followed by a test
stimulus, which was presented for either 50 ms (standard
duration; 75% of all trials in the attention-directing
conditions, 87.5% in the no-feature condition) or 150 ms
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348336
(deviant duration; 25% of all trials in the attention-directing
conditions, 12.5% in the no-feature condition). In case of an
attention-directing cue, subjects were instructed to respond
as fast and accurately as possible to test stimuli with the
attended feature(s) that were presented slightly longer (i.e.,
150 ms). On 50% of all trials, the test stimulus possessed the
attended attribute. On 12.5% of all trials, therefore, target
test stimuli were presented (with the attended attribute(s)
and of longer duration). In case of a no-feature cue, subjects
were asked to respond as fast and accurately as possible to
test stimuli that were presented slightly longer (i.e., 150 ms),
regardless of their color or location. Subjects used their right
index finger to respond to targets.
The experiment consisted of two sessions: a practice
session and an EEG session. The aim of the practice session
was to make subjects familiar with the specific task
requirements and to make sure that they did not show
excessive eye blink activity. It consisted of 8 runs of 80
trials (approximately 3.5 min each). During the EEG
recording session, subjects sat in a comfortable chair with
a computer monitor placed 80 cm in front of their eyes and
positioned so that the vertical and horizontal straight-ahead
lines of sight were the same for all subjects. After the
electrode cap was placed, subjects practiced the task once
and subsequently performed 24 task runs of 80 trials each
while their EEG was recorded. Subjects were asked to
minimize eye and body movements and allowed to pause
between the runs if they wished to do so.
2.3. ERP recordings
Recordings were made with 60 Ag-AgCl-electrodes
mounted in an elastic cap: FP1, FP2, AF7, AF8, AF3,
AF4, F7, F8, F5, F6, F3, F4, F1, F2, Fz, FT7, FT8, FC5,
FC6, FC3, FC4, FC1, FC2, FCz, T7, T8, C5, C6, C3, C4,
C1, C2, Cz, TP7, TP8, CP5, CP6, CP3, CP4, CP1, CP2,
CPz, P7, P8, P5, P6, P3, P4, P1, P2, Pz, PO7, PO8, PO5,
PO6, PO3, PO4, Poz, O1, O2, Oz and M1. All scalp
channels were referenced to the right mastoid. Horizontal
eye movements were monitored with two bipolar silver
chloride electrodes placed on the left and right of the
external canthi. Vertical eye movements and blinks were
measured bipolarly with two silver chloride electrodes
placed above and below the left eye. The EEG from each
electrode site was DC recorded with a low-pass filter of 60
Hz and digitized (16 bits) at 250 Hz. Impedances were kept
below 5 kV.
The raw data files were filtered off-line with a 40-Hz
low-pass filter (24 dB/oct, zero phase shift). Epochs were
created starting 100 ms before and ending 800 ms after
each cue of interest, and re-referenced to the mean of both
mastoids. Epochs were automatically eliminated if the
voltage exceeded F60 AV at the VEOG channel, F30 AVat the HEOG channel, or F60 AV at any of the other scalp
electrodes. Four types of cue-locked ERPs were con-
structed next: COL (average across B and Y cues), LOC
(average across L and R cues), CONJ (average across BL,
BR, YL and YR cues) and N (N cues). The EEG obtained
in response to test stimuli was averaged for standard (i.e.,
test stimuli of short duration) trails only in the color and
location single feature conditions. Four types of test
stimulus-locked ERPs were constructed: color attended,
color unattended, location attended and location unat-
tended. Trials with incorrect responses (i.e., button press to
non-target test stimulus) were not considered for averag-
ing. The resulting average VEOG and HEOG cue-and test
stimulus-locked waveforms were inspected for systematic
deviations of eye position. If residual horizontal (N2 AV)or vertical eye movement-related activity (greater voltage
at VEOG than FP1 or FP2) was present in the individual
average ERP waveforms, the epoched segments were
visually inspected and manually eliminated when contami-
nated with EOG activity. Two subjects, who showed
systematic EOG activity on too many trials (i.e., N33%
of the cue-locked epochs), had to be excluded from the
analysis.
2.4. Behavioral analyses
Repeated measures ANOVAs with the within-subject
factor condition (COL, LOC, CONJ, N) were performed
on response latencies of accurate responses to attended test
stimuli of longer duration and arc sin-transformed omitted
response rates. Furthermore, repeated measures ANOVAs
with the within-subject factor attention-cue-condition
(COL, LOC, CONJ) were performed on arc sin-trans-
formed false alarm rates to (a) attended test stimuli, which
were presented briefly, (b) unattended test stimuli, which
were presented slightly longer, and (c) unattended test
stimuli, which were presented briefly. These analyses were
performed to test for differences in behavioral performance
between cue conditions.
2.5. ERP analyses
2.5.1. Test stimulus-locked ERP analyses
The ERPs elicited by attended versus unattended stimuli
in spatial (P1 effect) and non-spatial (frontal selection
positivity (FP), occipital selection negativity (ON)) attention
tasks appear to be robust phenomena. Their presence was
examined in the present report to confirm that subjects had
indeed directed their attention to the cued stimulus
feature(s). The P1-effect was investigated at electrodes P7
and P8 between 80 and 140 ms post-stimulus for the
location condition. Voltage values, sampled every 4 ms
within these intervals, were submitted to repeated measures
ANOVAs, which tested for the effects of attention (attended,
unattended; LOCATT), hemisphere (left, right; HEMI) and
stimulus feature (left, right; LOC). The presence of FP and
ON effects was examined, respectively, at electrodes F3 and
F4 between 100 and 248 ms post-stimulus, and at electrode
Oz, between 148 and 300 ms for the color condition.
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348 337
Voltage values, sampled every 4 ms within these intervals,
were submitted to repeated measures ANOVAs, which
tested for the effects of attention (attended, unattended;
COLATT) and stimulus feature (blue, yellow; COL). In the
FP analyses, the additional factor hemisphere (left, right;
HEMI) was tested. Because of multiple interrelated com-
parisons, and hence the likelihood of false-positive spurious
significant effects, for all analyses performed, effects were
only considered reliable if they persisted for at least eight
successive time bins (4 ms each, p-valueb0.05).
2.5.2. Cue-locked ERP analyses
The 800-ms cue-target interval was divided into 40 time
bins of 20 ms (5 sample points) and, for each time bin, the
average voltage was computed for each electrode and task
condition of interest. The average voltage values thus
calculated were used as dependent variables in repeated
measurements ANOVA analyses that were performed to
isolate attentional control and/or domain-specific processes.
First, in order to detect attention-related differences
between the different cue conditions (COL, LOC, CONJ,
N), mean voltage values were subjected as dependent
variables to separate regional repeated measures ANOVAs
(anterior analysis (F7/F8, F3/F4, FC5/FC6), central analysis
(T7/T8, C3/C4, CP5/CP6) and posterior analysis (P7/P8,
P3/P4, PO5/PO6)) for each time bin in the cue-target
interval (0–800 ms post-cue). In these analyses, three factors
were tested within subjects: cue condition (COL, LOC,
CONJ, N; COND), electrode position within hemisphere
(e.g., P7/8, P3/4, PO5/6; SITE) and hemisphere (left, right;
HEMI). A main effect of condition or interaction effect of
condition with any of the other factors would be indicative
of a difference in attention-related processes between cue
conditions. In case of a significant effect, post-hoc contrasts
were used to determine which cue conditions specifically
differed from one another. The following three orthogonal
contrasts were specified for the factor COND: no-feature
versus all three attention-directing cue conditions (attention-
directing-related effect), conjunction versus single feature
(i.e., average of color and location) cue conditions
(interaction between spatial and non-spatial attentional
control) and color versus location cue condition (attention
domain-specific effect). Given our relatively small sample
size, only results from dmixed-modelT tests were examined
for all the repeated measurements analyses performed. The
Huynh-Feldt or Greenhouse-Geisser epsilon correction
factor (whenever the Huynh-Feldt epsilon was smaller than
0.75) was applied where appropriate, to compensate for
possible effects of non-sphericity in the measurements
compared. Only the corrected F- and probability values
and the uncorrected degrees of freedom are reported.
Because of multiple interrelated comparisons, and hence
the likelihood of false-positive spurious significant effects,
effects were only considered reliable if they persisted for
at least two successive time bins (20 ms each, (corrected)
p-valueb0.05).
2.6. Source localization
To investigate the spatio-temporal dynamics and the
existence of domain specificity in attentional control, a
subtraction logic and source modeling were applied (cf. Ref.
[27]). For each electrode, six cue-locked grand average
difference waves were calculated: (1) location-no-feature
cue condition (single feature location (SFLOC)), (2) color-
no-feature cue condition (single feature color (SFCOL)), (3)
conjunction-color cue condition (conjunction location
(CJLOC)), (4) conjunction-location cue condition (conjunc-
tion color (CJCOL)), (5) left-no-feature cues (single feature
left (SFLEFT)) and (6) right-no-feature cues (single feature
right (SFRIGHT)). The latter two contrasts allowed for the
investigation of lateralization in the strength of effects with
respect to the cued location. In addition, stimulus-locked
attentional difference waveforms were created to investigate
the relationship between attention directing-related effects
and modulatory effect of attention on stimulus processing
for the contrasts attended-unattended location stimuli and
attended-unattended color stimuli. The following steps were
then performed for each computed grand average difference
waveform. The signal at each channel was first re-
referenced to the average signal across all channels. Then,
for each sample point, the global field power (GFP) was
calculated as the square root of the sum of squares of the
average-referenced activity over all channels. Peaks in the
global field power function are indicative of high variance
between channels and reflect a maximum of the total
underlying brain activity that contributes to the surface
potential field [36]. As a final step, one (or two, when the
residual variance (RV) was still higher than 5%) bilateral
dipole pair(s) with mirror-symmetric locations across hemi-
spheres was fitted at GFP peak latencies (plus and minus
two samples points (20 ms time bins)) of interest. Source
models were determined using the BESA program (V.4.2).
The default four shell ellipsoidal (i.e., head, scalp, bone, csf)
was used. Each dipole was characterized by six parameters
(three for location, three for orientation). The symmetry
constraint with respect to location reduced the number of
parameters to be fitted. An additional benergyQ constraint
(weighted 20% in the compound cost function, as opposed
to 80% for the RV criterion; see Ref. [2]) was used to reduce
the probability of interacting dipoles (i.e., nearby dipoles
producing high-amplitude potential fields of opposite
direction). This criterion was maintained so as to favor
solutions with relatively low dipole moments. Any differ-
ence in location and/or orientation parameters of the fitted
dipole pair(s) between the color-no-feature and location-no-
feature contrasts can be taken as evidence for a difference in
neural mechanisms between the two types of attention. To
evaluate apparent similarities/differences in equivalent
dipole locations across the different conditions (e.g.,
SFLOC and SFCOL) at grand average GFP peak latencies,
individual source parameters (dipole location, orientation
and strength) were estimated and entered into ANOVA’s or
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348338
paired t-tests. For a more detailed description of this
procedure, see Kenemans et al. [27].
3. Results
3.1. Behavioral results
There were no significant differences between the diffe-
rent task conditions in response latency (COL: 579, LOC:
576, CONJ: 567, N: 576 ms, relative to target onset) or the
number of omitted responses to target stimuli (COL: 12.1%,
LOC: 14.0%, CONJ: 13.8%, N: 15.5%). Neither were any
difference observed between the different cue conditions in
the number of false alarms to attended (COL: 1.4%, LOC:
1.7%, CONJ: 2.9%, N: 1.5%) or unattended (COL: 0.1%,
LOC: 0.1%, CONJ: 0.1%) test stimuli of short duration.
However, subjects made more false alarms to unattended
test stimuli of long duration in the CONJ condition (4.2%)
Fig. 2. (A) Grand average ERP waveforms to attended (Att) and unattended (Unat
(B, C) Grand average, average reference spline interpolated isopotential maps. Not
attended-unattended left test stimuli (left panel) and attended-unattended right test
(left panel) and right—no-feature cues (right panel) at 752 ms after cue onset.
than in the COL (0.9%) and LOC (1.8%) conditions
[F(2,26)=6.154, p=0.006].
3.2. ERPs
3.2.1. ERPs to test stimuli
As expected, P1 amplitudes were larger for stimuli
presented at attended compared to unattended locations in
the location condition between 112 and 140 ms post-
stimulus [6.2bF(1,12)b29.9, pb0.05] (see Fig. 2A). This
difference in attention-related activity was larger over
contralateral scalp regions as indicated by an interaction
between attention (attended, unattended), hemisphere (left,
right) and test stimulus feature (left, right) between 104 and
120 ms post-stimulus at electrodes P7 and P8
[5.6bF(1,12)b6.8, pb0.05]. Furthermore, compared to
stimuli of the unattended color, stimuli of the attended
color elicited a larger positive response at electrodes F3 and
F4 between 128 and 228 ms in the color condition
t) test stimuli presented left or right from fixation for electrodes P7 and P8
shaded: areas of positive amplitude. Shaded: areas of negative amplitude. B
stimuli (right panel) at 120 ms post-test stimulus. (C) Left—no-feature cues
.
:
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348 339
[5.7bF(1,12)b19.4, pb0.05] (see Fig. 3). In addition, a
significant main effect of attention was observed at Oz
between 152 and 188 ms [4.9bF(1,12)b39.7, pb0.05]
reflecting a larger positive response to stimuli of the
attended versus the unattended color. This positive response
was followed by greater attention-related negativity, which,
however, never reached significance.
3.2.2. ERPs to cues
Fig. 4 shows the representative waveforms elicited by
each type of cue (COL, LOC, CONJ, N) and the grand
average difference waveforms for the color, location and
conjunction effects (i.e., SFLOC, SFCOL, CJLOC and
CJCOL). Potential distributions corresponding to the
attention-directing-related effects are shown in Fig. 5. Table
1 lists time intervals and F-value ranges for main effects of
COND and interactions between this factor and the factors
HEMI and/or SITE within 0–800 ms post-cue, for each
regional analysis. Each of the regional effects will be
discussed next.
The posterior analyses (electrode sites: P7/P8, P3/P4,
PO5/PO6) revealed the earliest significant effect of condition.
This effect was observed between 181 and 220ms post-cue at
Fig. 3. Grand average ERP difference waveforms (attended–unattended
color test stimuli) displaying the frontal selection positivity (FP) effect at
electrode Fz. The grand average, spline-interpolated isopotential map (two-
dimensional projection) shows the topographical distribution of this effect
at 144 ms post-test stimulus. The spacing between isopotentials in this map
is 0.2 AV. White areas denote areas of positive amplitude and dotted areas
denote areas of negative amplitude.
posterior sites. Post-hoc comparisons and inspection of the
data revealed that within this interval location cues elicited
less negativity than color, conjunction or neutral cues (main
effect of COND [2.9bF(3,39)b4.5, pb0.05]). This effect was
followed by a more pronounced effect of condition between
261 and 500 ms post-cue at posterior sites (main effect of
COND [4.6bF(3,39)b61.7, pb0.05]), which extended to
more central scalp locations [321–400 ms: 5.8bF(3,39)b7.9,
pb0.05]. Post-hoc comparisons and inspection of the grand
average ERP waveforms showed that, in this interval, single
feature cues elicited greater biphasic positivity than both no-
feature and conjunction cues and, furthermore, that no-
feature cues elicited greater positivity than conjunction cues
at posterior sites especially during the early part of this
biphasic positivity (261–400 ms post-cue). Scalp topogra-
phies of this early effect of cue condition show that it was
maximal over lateral parietal and occipital sites (CON-
D*SITE interaction [3.1bF(6,78)b7.8, pb0.05]) and that,
during the first phase, the effect was more prominent at right
compared to left posterior scalp locations, whereas during the
second phase, it was more pronounced at left compared to
right posterior scalp locations (COND*HEMI interaction
[3.8bF(3,39)b6.9, pb0.05]). In the later part of the cue-target
interval, starting at 661 ms, a third main effect of condition
was observed for posterior scalp sites [3.4bF(3,39)b12.4,
pb0.05]. This effect lasted until the end of the cue-target
interval and reflected larger positive voltage over dorsal
posterior sites for conjunction compared to single feature
cues and greater positivity to single feature cues than no-
feature cues over parieto-occipital sites (COND*SITE
interaction [3.1bF(6,78)b7.8, pb0.05]). This late posterior
effect spread to central scalp locations.
A further effect of condition was observed over fronto-
central scalp locations. Greater positive response was
revealed to no-feature cues compared to attention-directing
cues over central and anterior sites between, respectively,
401 and 640 ms [3.6bF(3,39)b15.7, pb0.05] and 441 and
600 ms [4.1bF(3,39)b17.3, pb0.05] after cue onset. This
effect reflects a more anterior distribution of the posterior
positivity in the no-feature compared to the other conditions.
Lastly, the anterior analyses revealed a further difference
between cue conditions: greater negativity to color cues
compared to location, conjunction and no-feature cues
[4.7bF(3,39)b10.5, pb0.05]. Between 641 and 760 ms,
color cues elicited a larger negative response than location
cues at frontal scalp locations. This effect was maximal over
midline frontal electrodes.
3.3. Source localization
A close correspondence in GFP peaks and the above-
described statistically significant ERP effects was observed.
At 184 ms post-cue, a small peak in GFP was only observed
for SFLOC, followed by a bigger peak at 340 ms post-cue.
The neural generators of these two effects (at 184 and 340
ms post-cue) were estimated first for the grand average
Fig. 4. Top part: Grand average, cue-locked ERP waveforms for the different cue conditions for a selected number of electrodes. Bottom part: Grand average
ERP difference waveform for the contrasts: conjunction–location cues (Conj-Loc; CJCOL), color–no-feature cues (Col-NF; SFCOL), conjunction–color cues
(Conj-Col; CJLOC) and location–no-feature cues (Loc-NF; SFLOC).
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348340
difference waveform (SFLOC), and then for the individual
subject difference waveforms, where the grand average
solution parameters were used as a starting point (cf. Ref.
[27]). Fitting of one symmetric dipole pair localized both
effects to the ventral-lateral compartment of posterior cortex
[RV=4.0% (184 ms) and 1.6% (340 ms)]. No differences in
location or orientation parameters were observed between
the source models obtained at 184 and 340 ms for SFLOC.
The main results of the grand average and per-subject
estimation procedures for the 340 ms SFLOC effect are
Fig. 5. Grand average spline-interpolated isopotential maps (two-dimensional projections) for the different contrasts at 180, 340, 520, 700 and 800 ms post-cue.
Col=color cues, NF=no-feature cues, Loc=location cues and Conj=conjunction cues. The spacing between isopotentials is 0.3 AV. White: areas of positive
amplitude. Shaded: areas of negative amplitude.
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348 341
summarized in Fig. 6A and B (second panel). Furthermore,
for both time points, no interaction was observed between
the attention-direction of the location cue (SFLEFT,
SFRIGHT) and hemisphere when dipole moments were
compared, suggesting that the two effects were not
lateralized with respect to the cued location.
Inspection of the grand average GFP functions revealed
that the second peak observed for SFLOC at 340 ms after
cue presentation was also present for the other contrasts
(SFCOL: 344 ms, CJLOC: 336 ms and CJCOL: 336 ms).
One bilateral dipole pair with mirror-symmetric locations
across hemispheres resulted in a model distribution explain-
ing more than 95% of the variance in each of the recorded
potential distributions [RV(SFCOL)=2.46%, RV(CJLOC)=
1.01% and RV(CJCOL)=0.70%]. The grand average and
individual subject instantaneous source models for the
different contrasts, which were derived at the d340T ms
GFP peak latencies, are summarized in Fig. 6. Statistical
analyses revealed that these dipoles did not differ
significantly with respect to location across conditions.
Table 1
Results from repeated measurement analyses at posterior, central and frontal elec
Posterior Central
COND 181–220 2.9bF(3,39)b4.5 321–800
261–500 4.6bF(3,39)b61.7
661–800 3.4bF(3,39)b12.4
COND*HEMI 241–380 3.8bF(3,39)b6.9 481–580
COND*SITE 281–780 3.1bF(6,78)b7.8 261–800
COND*HEMI 261–320 3bF(6,78)b4.1
*SITE 381–520 2.7bF(6,78)b4.6
561–800 2.6bF(6,78)b4.5
Time windows are given for each significant effect ( pb0.05 for two successive tim
condition with hemisphere (HEMI; left, right) and/or electrode site (SITE), along
This indicates that SFLOC, SFCOL, CJLOC and CJCOL
have equivalent dipole locations in the lateral ventral
posterior compartment of the cortex at around 340 ms post-
cue. The orientation of the dipoles, however, differed
across conditions [in the left hemisphere: x: F(3,39)=7.1,
y: F(3,39)=159.8, z: F(3,39)=20.0; in the right hemisphere:
x: F(3,39)=9.8, y: F(3,39)=72.0, z: F(3,39)=8.1]. The
dipole orientations for CJLOC and CJCOL were reversed
(flipped around the x-, y- and z-axes) relative to the
SFLOC and SFCOL dipole orientations. This effect reflects
the fact that color and location cues elicited greater
positivity over posterior scalp regions compared to both
no-feature and conjunction cues. The exact reversal in
orientation between the SFLOC and SFCOL, on the one
hand, and CJLOC and CJCOL, on the other hand,
illustrates the sensitivity of the modeling approach used
in the present study (cf. Ref. [27]).
For SFLOC and SFCOL, another GFP peak was
observed at 532 and 544 ms, respectively. In this time
window, a difference in positivity was observed over frontal
trode locations
Anterior
3.6bF(3,39)b15.7 421–800 2.9bF(3,39)b17.3
3.8bF(3,39)b8.1 501–580 3.1bF(3,39)b7
2.8bF(6,78)b20.6 281–640 2.8bF(6,78)b7.1
661–800 2.6bF(6,78)b5.1
e bins (i.e., 40 ms)) of condition (COND; SFLOC, SFCOL, CONJ, N), or of
with the minimum and maximum F-values for each effect.
Fig. 6. (A) Grand average source solutions at d340T ms post-cue for the contrasts: location–no-feature cues (Loc-NF; SFLOC), color–no-feature cues (Col-NF;
SFCOL), conjunction–color cues (Conj-Col; CJLOC) and conjunction–location cues (Conj-Loc; CJCOL). (B) Grand average (dark grey) and individual (black)
dipole solutions at GFP peak latency d340T ms displayed for each contrast (i.e., Loc-NF, Col-NF, Conj-Col and Conj-Loc) separately.
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348342
and central scalp locations between single feature and no-
feature cues. One bilateral dipole pair with mirror symmetric
locations in posterior cortex gave a good fit for both
contrasts [RV(SFLOC)=2.4% and RV(SFCOL)=1.4%] (see
Fig. 7). Furthermore, their estimated source parameters did
Fig. 7. Grand average source solutions for color–no-feature cues (Col-NF;
SFCOL) and location–no-feature cues (Loc-NF; SFLOC) for the early
posterior (344 and 340 ms, respectively) and intermediate (544 and 532 ms,
respectively) effects.
not differ, indicating that, at around 540 ms post-cue, similar
areas of cortex were differentially activated by the two
single feature cues versus the no-feature cue. These sources
were located more medially ( p=0.015) and anteriorly
( p=0.004), and somewhat more dorsally than the sources
that were estimated for the early posterior effects at around
340 ms post-cue.
Fig. 8. Grand average source solutions for color–no-feature cues (Col-NF
(SFCOL); black dipoles) and location–no-feature cues (Loc-NF (SFLOC);
dark grey dipoles) at 740 and 752 ms post-cue, respectively.
Fig. 9. Grand average ventral posterior sources of the late-latency attention-
directing effects (left panel) and the first attentional modulation effects
(right panel) for both spatial (A) and non-spatial (B) attention. Abbrevia-
tions: Col-NF=color–no-feature cues, Loc-NF=location–no-feature cues,
Att=attended and Unatt=unattended.
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348 343
Source modeling of SFLOC and SFCOL at GFP peaks at
752 and 740 ms post-cue, respectively, with one dipole pair
with symmetric location parameters, localized both of these
late effects of attentional control to the ventral posterior part
of cortex [RV(SFLOC)=12.4%, RV(SFCOL)=5.0%]. As the
RV for SFLOC in particular was relatively high, a second
dipole pair was added to the source models. For both SFLOC
and SFCOL, this second dipole pair moved to dorsal anterior
cortex, whereas the ventral posterior dipole pair did not, or
only slightly, change position (see Fig. 8). This time good fits
were obtained for both SFLOC (4.5%) and SFCOL (2.3%).
No differences in location parameters between SFLOC and
SFCOL were found for the anterior or posterior sources.
However, small differences in orientation of the dipoles were
observed [anterior sources: z-orientation (left hemisphere):
p=0.01, y-orientation (right hemisphere): p=0.01, posterior
sources: z-orientation (left hemisphere): p=0.01], indicating
that slightly different or more extended patches of anterior
and posterior cortex may have been activated by location
versus color cues in this latency range. Interestingly,
comparison of dipole moments revealed a significant
interaction between the attention-direction of the location
cue (SFLEFT, SFRIGHT) and hemisphere at 752 ms post-
cue onset [F(1,13)=5.4, p=0.018]. This effect reflects the
fact that at this latency, right cues elicited significantly
greater positivity in left compared to right ventral posterior
cortex ( p=0.004), whereas left cues activated both ventral
posterior regions to a similar extent (see Fig. 2C for the
modeled scalp topographies). Fig. 8 summarizes the results
from the grand average estimation procedures.
3.3.1. Relationship between late-latency cue effects and
attentional modulation effects
The ventral posterior sources of the 752-ms SFLOC and
740-ms SFCOL source models seemed very similar to
sources estimated previously for, respectively, the P1
attentional modulation effect [3,14,42] and the color
attentional modulation effects [1,35]. Also, the scalp
topographies of the late posterior spatial attention-directing
effect and the P1 selection effect were very comparable
(see Fig. 2B and C). It was therefore examined whether the
same ventral posterior areas that showed enhanced
activation to attention-directing cues at the end of the
cue-target interval were also modulated by attention. At
120 ms post, the difference in ERP between test stimuli
presented at attended and unattended locations (i.e., the P1
selection effect) was best explained with a symmetric
dipole pair in ventral-lateral occipital cortex and a single
dipole in medial anterior cortex (RV=2.6%). Modeling of
the P1 selection effect with just one symmetric dipole pair
resulted in an implausible solution.1 The grand average
1 This is conceivably related to the fact that the attentional selection
difference waveform was relatively noisy, as the individual difference
waveforms consisted of an average of 58 trials only. The third dipole was
added to model this noise.
ventral posterior source parameters of this and the late (752
ms) spatial attention-directing effect were very comparable
(see Fig. 9A). However, paired t-tests on the individual
subject source parameters indicated that the ventral posterior
sources were located slightly more anteriorly for the P1 effect
than the late-latency effect related to spatial attentional
control ( p=0.025).
At 144 ms, the difference in frontal positivity between
test stimuli of the attended versus unattended color was best
modeled with a symmetric dipole pair in the ventral central
part of the brain (RV=6.7%) (see Fig. 9B). This dipole pair
was located more anteriorly ( p=0.001) than the ventral
posterior dipole pair estimated for the late-latency (i.e., 740
ms) effect of directing attention to color. The first effects of
spatial (i.e., P1 effect) and non-spatial (i.e., FP effect)
attention were thus located more anteriorly than the late-
latency effects of directing attention to, respectively,
location and color.
4. Discussion
In this study, we investigated the nature and temporal
dynamics of top-down attentional control. The extent to
which the processes that direct the focus of attention depend
on the to-be-attended stimulus dimension was assessed by
comparing ERPs elicited by location and color attention-
directing cues to ERPs elicited by no-feature reference cues.
The neural source configurations underlying the thus
observed spatial and non-spatial attention directing effects
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348344
were investigated and directly compared to reveal possible
differences in the configuration and/or timing of activated
brain areas between spatial and non-spatial attentional
control. Moreover, dipole modeling was used to explore
the relation between perceptual biasing and attentional
modulation effects. In addition, we examined attentional
control in a condition where attention was to be directed to a
conjunction of a color and a location.
4.1. The generality of attentional control
Overall, very similar activation patterns were observed
when attention was directed to location and color. The
finding of closely corresponding ERP patterns to color and
location cues is in line with results from recent event-related
fMRI studies [13,50], which observed great overlap in the
fronto-parietal networks involved in spatial and non-spatial
attentional control within the same subjects. The present
data supplement this functional anatomical knowledge by
showing that the temporal sequence of activation within
brain regions involved in attentional control is very similar
for spatial and non-spatial attention. Multiple processes
were linked to directing attention to location or color. Each
of these effects is described below with regard to current
neurophysiological models of attention.
4.1.1. Shortest-latency (184 and 340 ms) effects related to
attentional control
The shortest-latency differences in ERP between the
single feature (color and location) conditions and the no-
feature condition originated from ventral-lateral occipital
cortex (see Fig. 6). These effects, greater positivity over
parieto-occipital scalp regions, were already observed at 180
ms after the cue onset for the location condition, and at 260
ms for the color condition, and were largest at 340ms for both
conditions (see Figs. 4 and 5). Previous studies of spatial top-
down control also found increased activation over posterior
scalp regions in conditions where attention was directed to
the left or right compared to a reference condition between
200 [15] or 250 [40] and 500 ms after cue presentation. The
present findings show that this early posterior effect is
generated in occipital areas, is not specific for the directing of
attention to a location in space, but has a longer latency when
attention is directed to color. They also are in line with results
from a recent combined ERP and fMRI study, which located
the earliest observed effect of directing attention to color (at
240 ms post-cue) to ventral-posterior cortex [28].
It could be argued on the basis of its source location in
ventral-lateral occipital cortex that the enhanced posterior
positivity reflects the biasing of the areas in which perceptual
processing is modulated, rather than an attentional control
process. However, this is contradicted by the fact that the
early occipital activity was not lateralized with respect to the
cued location. Results from single cell recording [38] and
neuroimaging [22,26,60] studies generally support the notion
that, just like the effects of spatial attention on target
processing [25,52,59], the increase in baseline activity is
retinotopically organized. Therefore, the occipital effect at
340 ms after the cue onset likely does not reflect enhanced
activity of feature-specific visual areas.
Another, more probable, explanation for the differences
in posterior positivity between single feature and no-feature
cues is that it represents a dmeta-attentionT effect rather thanenhanced preparatory activity of feature-specific visual
areas. On both no-feature and attention-directing trials,
upon sensory processing of the cue-symbol, the cue-symbol
had to be mapped onto its corresponding task instruction.
Yet, only in case of an attention-directing cue, this task
instruction contained reference to a specific to-be-attended
feature. So, only on attention directing trials, the link
between the sensory information provided by the cue and its
functional properties (i.e., the specific to-be-attended
feature) had to be reinforced [12,23]. The enhanced
activation of ventral posterior areas to single feature versus
no-features cues may hence reflect increased activation of
visual association areas that hold representations of the cue-
symbol related to the invigoration of its functional
significance in the single feature tasks. This explanation is
in accordance both with the observed domain-independency
of this early effect and the fact that it was not lateralized
with respect to the cued location. It should be noted that this
early effect does not simply reflect cue-symbol interpreta-
tion processes [60], as the no-feature cue had to be
semantically interpreted as well. Attention thus seems to
be set up by generic processes that are additional to cue-
symbol interpretation processes and likely link the attention-
directing cue to its associated test stimulus feature.
4.1.2. Intermediate-latency (540 ms) effects
Between 400 and 640 ms post-cue, a difference in fronto-
central positivity was observed between the single feature and
no-feature cue conditions (see Fig. 5, third panel, first two
rows). In this interval, attention-directing cues elicited greater
negativity than no-feature cues. Mangun [40] also reported
greater negativity to spatial attention-directing cues com-
pared to neutral cues at central scalp sites between 500 and
700 ms after the cue onset. Here, this intermediate effect of
directing attention was located to similar parts of posterior
cortex for the location versus no-feature and color versus no-
feature cue contrasts, indicating that directing attention to
location and color relative to no specific feature resulted in
increased activity in the same areas at this latency (see Fig. 7).
It can accordingly be concluded that, at around 540 ms post-
cue, the same domain-independent processes were active in
the color and location attention-directing cue conditions.
The posterior sources estimated for the intermediate
effects were located more anteriorly and medially than the
sources found for the early posterior effects at 340 ms post-
cue (see Fig. 7). This may possibly reflect additional
contributions from parietal or frontal areas to these effects.
Recent event-related fMRI studies (e.g., Refs. [6,22,25])
revealed attention-directing cue-related activity in superior
2 As one reviewer pointed out, it should be noted that since color and
location cues were presented intermixed within the same run, attention had
to be reset before the start of each trial at both the feature and dimension
level. Differences in preparatory activity might have been more pronounced
had the two types of cues been presented in separate runs and attention
could have been tonically maintained at the dimension level.
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348 345
frontal cortex and superior and inferior parietal cortex. The
present data suggests that these areas may be active
relatively late after cue presentation, after processes related
to sensory identification of the cue-symbol and linkage of
the cue-symbol to the corresponding to-be-attended stimulus
feature have completed. This would be in line with the
common interpretation of these frontal and parietal activa-
tions as representing the actual execution of the task
instruction, i.e. the directing of attention (e.g., Ref. [4]).
4.1.3. Late (~750 ms) posterior and anterior effects of
directing attention to color or location and their
relationship to the early attentional modulation effects
FP and P1, respectively
Dipole-source modeling revealed that slightly different or
more extended patches of the same parts of dorsal anterior
and ventral posterior cortex were activated by spatial and
non-spatial top-down control at the end of the cue-target
interval, as the location of the estimated anterior and
posterior dipole pairs did not differ between the location
and color attention-directing contrasts, but their orientations
did (see Fig. 8). Hence, generic processes indifferent as to
what feature was task-relevant were followed in time by
processes that were in fact specific to the type of to-be-
attended feature. The difference in posterior dipole solutions
may represent differences between spatial and non-spatial
attentional control with respect to the precise neural
populations showing pre-target preparatory activity, as, in
case of spatial attention, dipole strength of the posterior
dipole pair was lateralized with respect to the cued location,
especially when attention was directed to the right hemi-
field. The close correspondence in scalp topography and
laterality between this late posterior spatial attention-
directing-related effect and the P1 selection effect and the
similarity in their estimated source parameters provides
further support for an interpretation of the late posterior
effect in terms of preparatory activity of visual areas
involved in the processing of the attended feature (see Figs.
2B,C and 9A). In line with this, previous studies using the
high spatial resolution of fMRI have shown preparatory
activity in the same visual areas that were modulated by
attention (e.g., Refs. [22,26]).
Yet, it is puzzling in this respect that the posterior sources
estimated for the P1 spatial attentional modulation effect
were located more anteriorly than those estimated for the
late-latency effect of directing attention to location (see Fig.
9A). The same pattern of results was observed for non-
spatial attention; the posterior sources estimated for the FP
effect were located more anteriorly than those estimated for
the posterior non-spatial attention-directing effect at the end
of the cue-target interval (see Fig. 9B). One explanation for
these unanticipated findings may be that lowering the
threshold to task-relevant input in one brain area results in
modulation of activity in the next area. It is noteworthy in
this respect that while preparatory activity has been
observed in V1 (e.g., Ref. [24]), this area is not modulated
by selective attention (e.g., Refs. [3,43]). Albeit speculative,
a similar mechanism might be at work for higher-level
visual areas (i.e., preparatory activity leads to modulation at
the next processing level) and explain the more anterior
location of the posterior dipole pair for the test stimulus-
locked attentional difference waveforms.
The late-latency anterior dipoles were located close to
premotor areas (see Fig. 8). Differences in their orientation
between the location and color attention-directing contrasts
likely reflects the domain-dependent effect observed
between 640 and 760 ms after the cue onset at frontal
electrodes. Next to the difference in early posterior
positivity between the location and color cue conditions
around 184 ms post-cue, this was the only other difference
in cue-related ERP between the two types of top-down
control. In this late latency time window, color cues elicited
greater frontal negativity than location cues. Several fMRI
studies have observed domain-dependent segregation of
frontal cortex during the delay period (e.g., Refs. [13,51]).
The difference in anterior dipole orientations between
spatial and non-spatial attentional control is consistent with
these findings and may hence reflect differences in
maintenance processes between the spatial and non-spatial
attention-directing cue conditions.
To summarize, the present data indicate that the tem-
poral sequence of activation within brain regions involved
in directing the attentional focus is very similar for spatial
and non-spatial attention.2 No evidence was found for
differences in the timing of activation of dorsal and ventral
posterior areas between spatial and non-spatial attention-
directing cues as may be predicted based upon position-
special theories [31,33,34,53,55,56]. The postulated special
role of spatial attention in visual processing may arise from
post-test stimulus differences between spatial and non-
spatial attention in processes related to the detection of
behaviorally relevant stimuli, rather than differences in goal-
directed selection of stimuli. It may be argued that subjects
were required to orient attention spatially in all conditions,
even in the color cue condition, and that similarities in the
location and color cue-related responses may therefore be
due to the task design rather than to fundamental similarities
in attentional control mechanisms. Yet, as we statistically
compared ERPs elicited by color cues to ERPs elicited by
no-feature cues, all putative activity related to dividing
attention across the two peripheral locations where the test
stimulus could be presented, should have been cancelled
out. It is also important to note in this respect that results
from a recent event-related fMRI study by Giesbrecht et al.
[13] indicated that the brain regions involved in orienting
H.A. Slagter et al. / Cognitive Brain Research 22 (2005) 333–348346
attention to color do not critically depend on whether the
color stimulus is presented foveally or in the periphery.
Brain activation patterns to color cues were virtually
identical whether the task-relevant stimulus was presented
at the fovea or in the periphery. This supports our reasoning
that the observed overlap in location and color cue-related
responses in the present study truly reflects similarities in
attentional control mechanisms. All in all, the present
findings suggest that, generally, the parts of the brain that
extract the meaning of the cue and that are able to relate this
to current goals generalize over the type of feature to-be-
attended.
4.2. Directing attention to a conjunction of color and
location
Attentional control was further investigated in a
condition in which subjects were cued to direct attention
to a conjunction of a location and a color. Conjunction
cues initially elicited less posterior positivity than single
feature location and color cues, but evoked greater central
positivity from 540 ms on, as can be seen in Figs. 4 and 5.
The delayed enhanced posterior positivity in the conjunc-
tion condition suggests that it may have taken longer to
derive the information about the meaning of the cue-
symbol in the conjunction compared to the single feature
conditions. It is conceivable that it was more difficult to
link the cue symbol to its associative properties (i.e., color
and location) in the conjunction condition, as actually two
symbols (letters) had to be used to direct attention in this
condition. Starting at 540 ms and persisting throughout the
rest of the cue-target interval, greater activation was
observed over central scalp regions to conjunction com-
pared to single feature cues (see Figs. 4 and 5, rows 3 and
4). No such central positivity was found for color versus
no-feature cues or location versus no-feature cues. This
topographical difference shows that directing attention to a
conjunction of location and color is not simply the
summation of directing attention to location plus directing
attention to color, but may call upon extra brain
mechanisms. These may be specific to the conjoining of
two features, such as processes related to the integration of
the spatial and non-spatial feature information [54] or
processes representing the selective recruitment of frontal
areas that simultaneously maintain spatial and object
information on line [45]. Alternatively, the late central
positivity to conjunction relative to single feature cues may
reflect ongoing activity of brain regions involved in the
control operations by which the cue-symbol is translated
into a selective pattern of activation, related to the fact that
in the conjunction condition two symbols had to be used to
direct attention. More false alarms to test stimuli of longer
duration were observed in the conjunction than the single
feature conditions, suggesting that attentional control
processes may indeed have taken longer to complete when
two stimulus attributes were task-relevant.
5. Conclusions
Our results indicate that a feature non-specific process,
originating from ventral posterior cortex and possibly related
to reinforcement of the link between attention-directing cue
and its associated to-be-attended feature, initiates attentional
control. They, furthermore, showed that this process takes
about 340 ms to reach full strength when attention is to be
directed to one stimulus feature. The brain areas involved in
this cue association process are conceivably linked to
occipital areas involved in preparatory processes and higher
order areas in fronto-parietal cortex involved in maintenance.
Which areas these are specifically seems to partially depend
on the nature of the to-be-attended feature (i.e., color,
location). Our results, in addition, suggest that directing
attention to a spatial and non-spatial stimulus feature
simultaneously involves a process that can be dissociated
from the process of directing attention to a single feature. This
process may be specific to the conjoining of a spatial and non-
spatial stimulus feature or reflect ongoing activity of brain
areas involved in the translation of the cue-symbol into a
pattern of selective activation. Future studies combining the
high temporal resolution of ERPs and the high spatial
resolution of fMRI should further explore the sequence of
brain activity involved in top-down control of spatial and
non-spatial attention using a within-subject design and
appropriate reference task.
Acknowledgments
We would like to thank Marcus Spaan for his technical
assistance and Durk Talsma and Tineke Grent-’t Jong for
reading earlier versions of this manuscript. This research
was supported by Dutch NWO grant 42520206 to A.K. and
J.L.K.
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