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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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