Measuring the Allocation of Attention in the Stroop Task

8/10/2019 Measuring the Allocation of Attention in the Stroop Task

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Psychological Research

DOI 10.1007/s00426-011-0405-9

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ORIGINAL ARTICLE

Measuring the allocation of attention in the Stroop task: evidence

from eye movement patterns

Bettina Olk

Received: 6 September 2011 / Accepted: 6 December 2011

Springer-Verlag 2011

Abstract Attention plays a crucial role in the Stroop task,

which requires attending to less automatically processed

task-relevant attributes of stimuli and the suppression of

involuntary processing of task-irrelevant attributes. The

experiment assessed the allocation of attention by monitor-

ing eye movements throughout congruent and incongruent

trials. Participants viewed two stimulus arrays that diVered

regarding the amount of items and their numerical value

and judged by manual response which of the arrays con-

tained more items, while disregarding their value. DiVerent

viewing patterns were observed between congruent (e.g.,

larger array of numbers with higher value) and incongruent

(e.g., larger array of numbers with lower value) trials. The

direction of Wrst saccades was guided by task-relevant

information but in the incongruent condition directed more

frequently towards task-irrelevant information. The data

further suggest that the diVerence in the deployment of

attention between conditions changes throughout a trial,

likely reXecting the impact and resolution of the conXict.

For instance, stimulus arrays in line with the correct

response were attended for longer and Wxations were longer

for incongruent trials, with the second Wxation and consid-

ering all Wxations. By the time of the correct response, this

latter diVerence between conditions was absent. Possible

mechanisms underlying eye movement patterns are dis-

cussed.

Introduction

Research on attentional control shows that we are able to

ignore some features and stimuli in our environment, but not

others. One of the most prominent tasks to study control is

the Stroop interference task (Stroop, 1935), which requires

the suppression of involuntary processing of task-irrelevant

attributes of a stimulus and preferred attending to less auto-

matically processed task-relevant attributes (Macleod, 1991).

These demands make the Stroop task very informative for

the examination of attentional control (Banich et al., 2000).

The wordcolour task is probably the best known Stroop

task. A variant of the task is that participants are required to

report the colours in which words are presented. The words

can be congruent or incongruent with the colour. When the

colour and the word are congruent, i.e., the word GREEN is

shown in green, naming the colour is faster than when the

colour and the word are incongruent, i.e., the word GREEN

is shown in red. In the latter situation, the conXict slows

responding, which is reXected in the Stroop eVect. Other

variants of the Stroop task use numbers as items. In a numer-

ical version participants judge on which side of a display

more items are shown, irrespective of the identity of the

items (Washburn, 1994). In the congruent condition the

larger array of numbers consists of numbers with higher

value than the numbers of the smaller array. In the incongru-

ent condition the larger array of numbers is composed of

numbers with lower value than the numbers of the smaller

array (see Fig. 1for examples). Participants manual reaction

times (RT) are slower in the incongruent than in the congru-

ent condition, indicating that the task-irrelevant attribute

the value of the numbersis processed involuntarily and

slows down responding (Wolach, McHale, & Tarlea, 2004).

The allocation of attention is proposed to play a large

role in mediating the Stroop eVect. According to theoretical

B. Olk (&)

School of Humanities and Social Sciences,

Jacobs University Bremen, Campus Ring 1,

28759 Bremen, Germany

e-mail: [email protected]


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frameworks of attention, e.g., the biased competition

framework (Desimone & Duncan, 1995; Duncan, 2006),

attention is limited in capacity and selective, i.e., unwanted

information can be Wltered out to a certain extent. It is

understood that stimuli are competing for processing and

that this competition is biased, for instance by task rele-

vance and by stimulus characteristics. It is suggested that a

short-term description (attentional template) of currently

required information is used to bias competition (Desimone

& Duncan, 1995). Competitive processing as proposed by

the framework is seen as a stronger response to any one

object [] bought at the cost of weaker response to others

and this competition is shown in relative enhancement ofthe response to task-relevant objects, or relative suppres-

sion of objects to be ignored (Duncan, 2006, p. 5). These

mechanisms are also reXected in models that grant attention

an important role in the Stroop task. For instance, the paral-

lel distributed processing account (Cohen, Dunbar, & McC-

lelland, 1990; see also Botvinick, Braver, Barch, Carter, &

Cohen, 2001; Cohen, Aston-Jones, & Gilzenrat, 2004;

Cohen, Servan-Schreiber, & McClelland, 1992) illustrates

how attention may contribute to the Stroop eVect and the

resolution of conXict.1The model suggests two processing

pathways, one for the processing of the task-relevant attri-

butes of the stimulus, i.e., in the numerical version thiswould be the amount of numbers in an array, and one for

the processing of the task-irrelevant attributes, i.e., the

value of the numbers, which both converge on a common

response mechanism. Each pathway consists of input units,

intermediate units and output units, along which activation

is spread. When suYcient activation has accumulated to

exceed a response threshold, a response is given. Impor-

tantly, the model assumes two task demand/attention units

that are linked to the intermediate units in the two pathways

and their function is to allocate attention to one of the two

pathways. The instructions given to a participant, for exam-

ple, to assess on which side of the display more items are

shown, will activate the task demand/attention units. The

activation of a given task demand/attention unit will thensensitize the processing in the task-relevant pathway, sup-

porting eVective information Xow along it. In contrast,

accumulation of information in the other pathway is attenu-

ated. Thus, information, on which attention is allocated, is

processed to a greater degree than information on which no

or less attention is allocated. Attention is understood to act

as a modulator which selects one of the two competing pro-

cesses based on the task instructions. Later versions of the

model (e.g., Cohen et al., 2004) also recognize that features

of the stimuli and their representation may have an impact

on attentional allocation as well and that competition in

attention may occur. Thus, the presentation of a stimulus

will divert attention away from another stimulus (Cohen

et al., 1992) and the presentation of a distracting stimulus

can activate the task demand/attention units for the location

containing the distracting stimulus. Taken together, in this

model attention is deWned as the inXuence that activity in

the task-demand units has on processing in the two path-

ways and the representations of task instructions and of a

stimulus inXuence which dimensions of a stimulus are pro-

cessed (Cohen et al., 2004).

Considering the prominent role that is attributed to atten-

tion in the Stroop task, the present study set out to investi-

gate where attention is actually allocated in the course of a

trial in this task and whether the distribution of attention

diVers between congruent and incongruent trials, as would

be expected based on the theoretical considerations and

model outlined above. A powerful way of measuring where

attention is allocated is to monitor saccadic eye movements

and Wxations. It has been established that attention and eye

movements are ultimately linked (e.g., Deubel & Schnei-

der, 1996) and that the direction of saccades and the loca-

tion and duration of Wxations are informative with respect

to the allocation of attention during the solution process of

tasks and cognitive processing (Charness, Reingold, Pom-

plun, & Stampe, 2001; Grant & Spivey, 2003; Thomas &

Lleras, 2007; Olk, Harvey, & Gilchrist, 2002). Eye move-

ment patterns provide information that is not obtained by

measuring manual reaction times and error rates alone. As

but one example, a study by Grant and Spivey (2003) was

successful in revealing that attention and the thought pro-

cess during problem solving are related by measuring eye

movement patterns. In a similar manner, the present study

sought to investigate whether the conXict participants expe-

rience in the Stroop task is reXected in the allocation of

1 The parallel distributed processing account has originally been

applied to the wordcolour task.

Fig. 1 Examples of displays of trials in the (a) congruent condition,

and (b) incongruent condition in Experiment 1. Displays are not drawn

to scale


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attention during a trial, as assessed with eye movements. To

this end, saccades and Wxations in the course of a trial were

analysed.

In order to be able to analyse the direction of a saccade

to and the location of a Wxation on task-relevant and task-

irrelevant attributes, a numerical version of the Stroop task

was chosen because it provides the possibility to spatially

separate attributes of the stimuli that are critical for the con-Xict in the incongruent condition. The latter is not possible

in the wordcolour version as here word and colour occupy

the same location. The experiment contained two condi-

tions. In the congruent condition, an array containing more

numbers was composed of numbers with higher value than

the numbers of an array with fewer numbers, e.g., Wve 3s

versus four 2s. In the incongruent condition, an array con-

taining more items was composed of numbers of smaller

value than an array with fewer items, e.g., four 3s versus

Wve 2s (see Fig. 1). Participants judged by button press

which stimulus array was more numerous, irrespective of

the identity of the numbers. Importantly, in the incongruent

condition the two conXicting pieces of information related

to the concept of more are presented in separate visual

Welds. In one Weld task-relevant numerosity is larger and in

the other task-irrelevant value is higher. This allows assess-

ing which feature (task-relevant or task-irrelevant) may

guide attention and the eyes. By comparing eye movement

patterns between the congruent and incongruent condition

information with respect to the allocation of attention dur-

ing task completion can be gained as well as its relation to

the Stroop eVect. Participants were not instructed where to

look and for how long in order to assess their natural view-

ing behaviour.

The allocation of attention in the congruent and incon-

gruent condition may vary throughout the trial. Therefore,

diVerent stages of each trial were evaluated. First, it was

assessed whether already at the beginning of a trial the allo-

cation of attention would diVer between congruent and

incongruent trials. It was analysed how fast participants

would leave the starting positionthe centre of the

screento explore the arrays, as reXected by the saccadic

reaction time (SRT) of the Wrst saccade after display onset.

It was hypothesised that a diVerence between the congruent

and incongruent conditionthe Stroop conXictcould

already be mirrored in the SRT, i.e., participants may take

longer to leave the start position in incongruent than in con-

gruent trials, in line with longer manual reaction times that

are typically found in the incongruent compared to congru-

ent condition. Second, it was analysed whether a diVerence

between conditions would be apparent when considering

the location to which attention is directed immediately after

leaving the central starting position. If so, a diVerence with

respect to the direction of the Wrst saccade after stimulus

onset should be found between conditions. Participants are

supposed to judge which array contains more items; hence,

considering the impact of task instructions, more Wrst sac-

cades should be directed to the stimulus array with more

items (task-relevant). However, this may mainly be the case

in the congruent condition and the presence of a conXict in

the incongruent compared to congruent condition may be

mirrored by fewer Wrst saccades to the array with more

items in the incongruent than the congruent condition. Infact, considering that the processing of the task-irrelevant

value of the numbers should not be suppressed completely

in this task (Washburn, 1994), leading to conXict in the

incongruent condition and the Stroop eVect, it is likely that

attention is attracted by numbers with a higher value and

hence within the incongruent condition more Wrst saccades

should be directed towards the stimulus array with numbers

with higher value (task-irrelevant). Third, the duration of

the Wrst and second Wxation after the Wrst saccade,2 sepa-

rately for array and condition, and fourth, the average dwell

time (Wxation duration) on a stimulus array considering all

Wxations during an entire trial were measured to assess

whether those Wxation durations would reXect the Stroop

eVect. In accordance with the task instructions, participants

should spend more time on the location that contains the

stimuli corresponding to the correct response, i.e., the

stimulus array with more items. However, such a pattern

may again be modulated by condition. In the incongruent

condition, attention may be attracted by the higher value of

numbers in an array and therefore Wxation duration may be

longer on the stimulus array with fewer items but numbers

of higher value in the incongruent condition. Finally, the

Wxation location and its duration at response were assessed

to elucidate the locus of attention when the response is

given. It is expected that by that time participants have

decided for the correct response and are therefore attending

more often/longer to the stimulus array with more items,

irrespective of condition.

Methods

Participants

Fifteen participants, ranging from 18 to 27 years of age

(M= 21 years), with normal or corrected vision gave

informed consent and received course credits or were paid

for their participation. The experiment was conducted in

accordance with the ethical standards laid down in the 1964

Declaration of Helsinki.

2 As the number of saccades and Wxations diVered between trials not

every saccade/Wxation made in a trial was analysed separately.


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Stimuli and procedure

At the beginning of a trial a Wxation dot (diameter 0.5) was

presented until Wxation was stable and the trial was initi-

ated. A new, identical Wxation dot was then presented for a

variable, random interval of another 8001,200 ms, and

then two arrays of stimuli were shown (Fig. 1). The stimu-

lus arrays appeared on the left and right on 50% of the trialsand above and below the centre in 50% of the trials. The

order of locations was randomized within an experimental

block. The arrays appeared simultaneously and until a man-

ual response was given; maximally for 3,000 ms. Each

array consisted of three to six items.3 The two arrays

diVered in numerosity by one or two items. The distance

between the central Wxation dot and the inner edge of a

number array was 2 of visual angle. Stimuli were black

and presented on a white background in the font Arial.

Each stimulus extended 1.51.9 in height and 0.91.1

in width. A stimulus array extended between 34.5 in

height and 34 in width. Participants heads rested on a

chin rest at a distance of 57 cm. Displays were presented on

a 21CRT monitor.

Of the 320 trials per participant 50% of trials were con-

gruent, 25% were incongruent. Stimulus arrays were com-

posed of the numbers 2, 3, 4 or 5. In congruent trials the

larger array was composed of the numbers with higher

value. In incongruent trials the larger array was composed

of the numbers of smaller value. The diVerent types of trials

with respect to amount of items and value were counterbal-

anced between visual Welds. The remaining trials were Wller

trials, in which letter stimuli (A, B, C or D) were shown to

prevent participants from generating hypotheses with

respect to a relationship between the number of items in an

array and the value. All types of trials (congruent, incon-

gruent, Wller) were randomized within a block. The task

was to judge by button press, which array contained more

items (pressing the " key with the right index Wnger, #

with the right thumb, ! with the right middle Wnger and

with the left index Wnger on a keyboard), while ignor-

ing the value of the items. The spatial eccentricity of the

displays allowed solving the task without looking at the

arrays. No instructions were given regarding eye move-

ments. Accuracy and manual RT were recorded in addition

to eye movements. Participants obtained detailed written

instructions and examples before the experiment began.

Eye movement recording and data analysis

Eye movements were recorded with an EyeLink II eye

tracker (SR Research, Canada). Testing was preceded by a

9-point grid calibration and validation for which partici-

pants saccaded to a black circle (0.5) on white back-

ground, which appeared sequentially at 9 points in a square

array. Eye movements were recorded at 500 Hz sample rateat a spatial resolution of about 0.3. Participants viewed the

displays with both eyes, but only the data of the right eye

were recorded.

In all subsequent manual and saccadic reaction time as

well as saccade direction analyses, trials were rejected if

(a) a blink (3%) or recording error occurred (0.5%), (b) the

initial Wxation location at the start of a trial was not within

1 around the centre of the Wxation dot (9.1%), (c) a sac-

cade began before the stimuli appeared (2.9%) or (d) a but-

ton other than one of the assigned response buttons was

pressed (0.1%). For the eye movement analyses only trials

with correct responses were considered.

Results

Manual responses

One additional trial had to be excluded due to anticipation

(RT < 100 ms). No responses were slower than 3,000 ms.

The analysis of manual reaction times for correct responses

showed that participants responded signiWcantly faster in

the congruent (M= 656 ms) than in the incongruent

(M= 705 ms) condition, t(14) = 7.6,p < .001. The analysis

of the accuracy data showed that participants were also sig-

niWcantly more accurate in the congruent (M= 99.3% cor-

rect) than incongruent (M= 96% correct) condition,

t(14) = 3, p < .01, reXecting the Stroop eVect and showing

that task-irrelevant value information interfered with their

responses.

Trials without saccade

On 7% of the trials no saccade was made. Participants were

still able to provide a correct judgement. On congruent tri-

als they were 100% correct (out of 146 trials), on incongru-

ent trials in 90.6% of the trials (out of 64 trials). The

responses were also given quite fast (congruent:

M= 511 ms; incongruent: M= 550 ms), indicating that

making no saccade was not detrimental for solving the task.

It was thus possible to solve the task without making an eye

movement or, in other words, with deploying attention

covertly. However, trials without saccades were rare, sug-

gesting that participants preferred to allocate attention

overtly and to move their eyes. Because trials without

3 No Wller items were used (see Pansky & Algom, 2002) to allow easy

identiWcation of the amount of items without the need for detailed

visual search of a stimulus array as this would implicitly encourage

strategic eye movements, which would not be triggered by the amount

or the value of the numbers but simply the need to search.


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saccades were rare and not present for every participant,

those data were not analysed further.

Latency of theWrst saccade

The distribution of saccadic reaction times (SRT) was uni-

modal (M= 217 ms, SD = 72 ms). For the analysis the

mean SRT of the Wrst saccade was calculated separately forcongruent and for incongruent trials. On average SRT was

M= 221 ms (SD = 55 ms) on congruent trials and

M= 228 ms (SD = 71 ms) on incongruent trials. The diVer-

ence in SRT between conditions was not signiWcant,

t(14) = 1.3, p = .227. Because SRT was generally rather

short and hence some of the included saccades could be

anticipatory in nature, the data were also analysed including

only the Wrst saccades with an SRT > 200 ms, leaving

1,754 trials for the analysis. Including only those saccades,

mean SRT on congruent trials was M= 254 ms (SD =

41 ms) and on incongruent trialsM= 261 ms (SD = 60 ms).

Again, the diVerence between conditions was not signiW-

cant, t(14) = 1.1,p = .313.

Direction of theWrst saccade

Next, it was analysed in which percentage of trials in the

congruent and in the incongruent condition the Wrst saccade

was directed towards the array with more items and to the

array with fewer items. Trials in which a saccade was not

directed towards any of the arrays, i.e., when the stimuli

were shown on the left and right but a saccade was made

up- or downward or when the stimuli were shown above

and below the centre and a saccade was made to the left or

right, were rare and excluded (0.9%).

A repeated measures ANOVA showed that partici-

pants looked more frequently to the array with more

items, F(1, 14) = 25.6, p < .001. This eVect was modu-

lated by an interaction between congruency amount of

items (more, fewer), F(1, 14) = 23.6,p < .001. To inves-

tigate the interaction further, paired-samples ttest were

conducted and showed that in the congruent and in the

incongruent condition more Wrst saccades were directed

to the visual Weld that contained the stimulus in line with

the correct response, i.e., with more items (congruent:

68.1 vs. 31.9%, t(14) = 6.7,p < .001; incongruent: 59.7

vs. 40.3%, t(14) = 3.2,p < .01, see Fig. 2). The analysis

shows that the task (where is more?) guided the Wrst sac-

cades in both conditions. Importantly though, the inter-

action and following comparisons between both

conditions indicate that the value of the numbers had

also an eVect. The percentage of the Wrst saccades to the

array with more items was signiWcantly higher in the

congruent condition (more items/higher value: 68.1%)

than in the incongruent condition (more items/lower

value: 59.7%), t(14) = 4.9, p < .001. In the incongruent

condition, signiWcantly more Wrst saccades went to the

array with fewer items (fewer items/higher value:

40.3%) than in the congruent condition (fewer items/

lower value: 31.9%), t(14) = 4.9, p < .001. Thus, the

task-irrelevant higher value of the numbers did attract

the Wrst saccades in the incongruent condition. The anal-

ysis only including saccades with SRT > 200 ms showed

the same results, a main eVect of amount of items,

F(1, 14) = 35.5, p < .001, modiWed by an interaction,

F(1, 14) = 13, p < .005. In the congruent and in the

incongruent condition more saccades were directed to

the array with more than with fewer items (congruent:

71 vs. 29%, t(14) = 7.4, p < .001; incongruent: 62 vs.

38%, t(14) = 3.9, p < .005). Comparing the conditions,

more saccades went to the array with more items in the

congruent condition (higher value) than in the incongru-

ent condition (lower value), t(14) = 3.6, p < .005. The

opposite was true for saccades to fewer items with more

saccades to those arrays in the incongruent condition,

t(14) = 3.6,p < .005.

Duration of theWrst Wxation

Duration of the Wrst Wxation (following the Wrst saccade)

showed that participants Wxated longer on the array that

contained more items (congruent: M= 235 ms; incongru-

ent:M= 229 ms) than fewer items (congruentM= 164 ms;

incongruent M= 172 ms), F(1, 14) = 64.2, p < .001. This

eVect was not modulated by an interaction with congru-

ency,p > .05, showing that this pattern was true for congru-

ent as well as incongruent trials. There was also no

diVerence between congruent and incongruent trials with

respect to longer Wxation durations on the arrays with more

or with fewer items, allp > .05. The results are illustrated in

Fig. 3a.

Fig. 2 Percentage of Wrst saccades directed to the stimulus arrays,

considering the amount of items (more/fewer) and value of the num-

bers (higher/lower), shown separately for the congruent and incongru-

ent condition.Error barsindicate standard errors

68 %60 %

32 %40 %

0

20

40

60

80

100

congruent incongruentPercentage

offirstsaccades

Condition

Percentage of first saccades to arrays

more items fewer items

highervalue

highervalue

lowervalue

lowervalue


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Duration of the second Wxation

With respect to the second Wxation, participants Wxated

longer on incongruent than congruent trials, F(1, 14) = 6,

p < .05, and Wxated longer on the array that contained more

than fewer items, F(1, 14) = 26.4, p < .001. This pattern

was true in the congruent and incongruent condition (con-

gruent more: M= 243 ms vs. fewer: 203 ms; incongruent

more: M= 272 ms vs. fewer: 212 ms), as the interaction

with congruency was not signiWcant, p > .05, illustrated in

Fig. 3b.

Fixation duration on stimulus arrays during the trial

In order to assess for how long during a trial participants

dwelled on the stimuli, it was calculated how much time

they spent, on average, in each condition (congruent, incon-

gruent), on each of the two stimulus arrays averaged over

all Wxations until the correct response button was pressed.

For trials on which the stimulus arrays were shown to the

left and right of the centre, participants looked only rarely

above or below the centre, some participants not at all. For

trials on which the stimulus arrays were shown above and

below the centre, participants looked only rarely to the left

or right, with some participants not doing so at all. Thoserare Wxations were excluded (0.2% of trials). Fixation dura-

tion was signiWcantly longer on incongruent than congruent

trials, F(1, 14) = 5.9, p < .05, and on the stimulus array

with more items and corresponding to the correct response,

F(1, 14) = 32.4,p .001. As the interaction was not signiWcant,

this pattern was true for congruent as well as incongruent

trials. On congruent trials participants spent on average

235 ms on the array with more items and 192 ms on the

array with fewer items. On incongruent trials they spent on

average 244 ms on the array with more items and 194 ms

on the array with fewer items. The results are illustrated in

Fig. 3c.

Fixation location and duration at response

The percentage of trials participants Wxated the array with

more or fewer trials while they gave their manual response

was analysed. Here, no diVerence in percentage of Wxations

at response between the arrays with more or fewer items or

between congruency of conditions was found, all p > .05.

In congruent trials, the Wnal Wxation was on the array with

Fig. 3 Average Wxation duration on stimulus arrays for (a) the Wrst

Wxation, (b) the second Wxation, (c) all Wxations, and (d) at response,

considering amount of items (more/fewer) and value of the numbers

(higher/lower), depicted separately for the congruent and incongruent

condition.Error barsindicate standard errors

235 ms 229 ms

164 ms172 ms

0

50

100

150

200

250

300

350

congruent incongruent

Fixationduration(inms)

Condition


Condition


Condition


Condition

Average duration on arrays (first fixation)


highervalue

highervalue

lowervalue

lowervalue

243 ms272 ms

203 ms

212 ms

0

50

100

150

200

250

300

350

Fixationduration

(inms)

Average duration on arrays (second fixation)


highervalue

highervalue

lowervalue

lowervalue

235 ms 244 ms192 ms

194 ms

0

50

100

150

200

250

300

350

Fixationduration(inms)

Average duration over all fixations on arrays

during a trial


highervalue

highervalue

lowervalue

lowervalue

282 ms 289 ms

210 ms 213 ms

0

50

100

150

200

250

300

350

Fixationduratio

n(inms)

Average fixation duration on arrays at response

more i tems fewer items

highervalue

highervalue

lowervalue

lowervalue

(a)

(b) (d)

(c)


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more items on 49% of the trials and on the array with fewer

items on 51% of the trials. For incongruent trials, the array

with more items was Wxated in 53% and with fewer items

on 47%. With respect to the duration of the last Wxations

(see Fig. 3d), participants Wxated not longer on incongruent

than congruent trials, p > .05, but longer on the array with

more than with fewer items, F(1, 14) = 65.2,p < .001. The

interaction was not signiWcant, p > .05, showing that thiseVect was true in the congruent (M= 282 ms,M= 210 ms,

respectively), and incongruent condition (M= 289 ms,

M= 213 ms, respectively).

Discussion

The goal of the present study was to characterize the alloca-

tion of attention in the course of a trial in the Stroop task by

analysing and comparing eye movement patterns in the

congruent and incongruent condition. Participants indicated

by button press which of two stimulus arrays contained

more stimuli and their eye movements were recorded while

they did so. The manual reaction time and accuracy mea-

surements showed that a conXict occurred, and that task-

irrelevant information was processed and interfering and

hence RT was longer and accuracy lower in the incongruent

condition. This Wnding is in line with a vast amount of liter-

ature (Macleod, 1991) and proving the eVectiveness of the

materials used in the present study.

To demonstrate where attention was deployed during

this task eye movement patterns between the congruent and

incongruent condition were compared. The Wrst analysis

focused on the beginning of a trial, i.e. how fast participants

are leaving the central starting position and are initiating

their Wrst saccade to explore the arrays. No diVerence

between congruent and incongruent trials was found, show-

ing that participants are not slower to initiate a move of

attention away from the centre at the beginning of the trial

in the incongruent condition when the correct response is

given. These data suggest that the Stroop eVect observed in

the manual response times with slower manual RT in the

incongruent condition is not related to the speed with which

the Wrst eye movement is initiated.

Further, it was analysed whether the Stroop conXict

would be apparent when considering the location to which

attention is directed immediately after leaving the central

starting position. It was expected that participants may look

more often towards the array with more items, in line with

task demands, but that a diVerence between the congruent

and incongruent condition may be apparent. More speciW-

cally, the presence of a conXict in the incongruent com-

pared to congruent condition could be reXected by fewer

Wrst saccades to the array with more items in the incongru-

ent than the congruent condition. In line with the hypothe-

sis, in both conditions more Wrst saccades were directed to

the visual Weld with the stimulus with more items but the

percentage of Wrst saccades to this array was higher in the

congruent than in the incongruent condition. In the incon-

gruent condition, more Wrst saccades were directed to the

array with fewer items/higher value than in the congruent

condition, indicating that the task-irrelevant attribute

(higher value) attracted attention in the incongruent condi-tion. The direction of the Wrst saccade thus reXects the con-

Xict and may demonstrate that the task (where is more?)

activates a concept or sets up an attentional template

(Desimone & Duncan, 1995) that is initially still indistinct

enough to bias attention not only towards task-relevant

(more items) but also task-irrelevant (higher value) attri-

butes or, alternatively, initiallyat the beginning of a

trialhas less impact. In relation to the parallel distributed

processing model (e.g., Cohen et al., 2004), this Wnding

shows that also information in the task-irrelevant pathway

is attended to, albeit to a lesser degree than information in

the task-relevant pathway, because overall, more Wrst sac-

cades were directed to the array with more items. The latter

conclusion is supported by the Wxation durations that were

analysed, as described further down below.

The data of the Wrst saccade thus shows that participants

look more often towards the array with stimuli with higher

value in the incongruent compared to the congruent condi-

tion but are equally fast to initiate the Wrst saccade in both

conditions. It had been expected that they may be slower to

initiate an eye movement in the incongruent condition. It

thus seems that the conXict is not apparent in the latency

and direction data alike. Considering that saccadic

responses in both directions in the incongruent condition

(to more items as well as to higher value) are in line with

the concept or template that may be created, it is possible

that the speed with which a saccade is initiated does not dis-

tinguish between task-relevant (more) and task-irrelevant

(higher). Alternatively, bearing in mind that saccades were

initiated quite fast, it cannot be ruled out that a Xoor eVect

occurred and that an SRT diVerence was only present

numerically but not signiWcant.

The analyses of the durations of the Wrst and second Wxa-

tion after the Wrst saccade, the average Wxation duration

considering all Wxations of a trial and Wxation duration at

response, separately for array and condition, showed that

participants spent more time Wxating the array with more

items and this was the case in both conditions.4 Thus,

although attention is directed to both arrays, it dwells for

4 It is unlikely that the direction of saccades and Wxation durations were

simply directed to the array containing more items because those were

physically slightly larger. In an experiment with similar stimuli but

diVerent instructions participants did not look more often to the physi-

cally larger arrays (Olk, 2007).


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1 3

longer on the stimulus array that is in line with task

demands and on which the stimuli corresponding to the cor-

rect response were shown. It therefore appears that even

though the Wrst sweep of attention as indicated by the direc-

tion of Wrst saccades was aVected by number value and may

be directed towards the location that does not correspond

with the correct response in the incongruent condition, the

nature of a stimulus array is discerned quickly and thenattention is preferably allocated to the array corresponding

to the correct response and accordingly, dwells there for

longer. This Wnding is in line with the idea that attention is

allocated with respect to task demands and instructions and

that it supports the correct response (e.g., Desimone &

Duncan, 1995; Duncan, 2006; Cohen et al., 2004). Such a

behaviour is also in agreement with Wndings reported in the

eye movement literature, which show that high-level fac-

tors such as goals, knowledge and attentional and working

memory capacity (Henderson, 2003; Kane, Bleckly, Con-

way, & Engle, 2001; Kappas & Olk, 2008; Olk & King-

stone, 2003, 2009; Yarbus, 1967) aVect where we attend

and look. In the present case, it is further possible that the

impact of task demands increases throughout a trial. For

instance, the concept or attentional template may gain

more impact or it may be reWned more in the course of the

trial, i.e., referring more to larger array than higher

value.

Interestingly, with their second Wxation and when all

Wxations of a trial were considered participants spent more

time Wxating in the incongruent condition. What could be

the reason as to why participants looked longer in the

incongruent condition? A viable answer may be that after

some initial processing (after the Wrst saccade and the Wrst

Wxation) the actual conXict is mostly perceived in the

incongruent condition. The time that participants spent

Wxating in this condition may partly reXect the time

required to verify which array corresponds to the correct

response. In the congruent condition, both attributes of an

array, numerosity and number value, converge, i.e., for the

array with more items they are in line with the activated,

reWned concept or template more, but in the incongruent

condition this is not the case. A further mechanism in play

could be the inhibition of irrelevant information. For

instance, the Selective Attention Model (SLAM, Phaf, van

der Heijden, & Hudson, 1990) suggests that at Wrst all stim-

uli may be processed and in the following irrelevant infor-

mation may be actively inhibited (Phaf et al., 1990).

Applying this idea to the present task, as such processes,

may take time, it is conceivable that longer Wxations in

incongruent trials are the result. For instance, after initial

processing the impact of task demands may increase and

the array with more items is activated and inhibition may be

imposed on the numbers with higher value (of the array

with fewer items). Or, inhibition may be imposed within

the array with more items/lower value on the attribute

lower value or on number value as such because it is in

conXict with the task demands. The longer Wxation duration

in the incongruent condition may thus reXect such a resolu-

tion of the conXict.

Finally, the Wxation location and its duration at response

were assessed to monitor the locus of attention when the

response was given. Here no diVerence in percentage ofWxations on arrays with more or less items was found in

either condition but participants spent more time on the

array with more items in both conditions. Note that the time

spent Wxating in the incongruent compared to congruent

condition was not longer anymore. It thus seems that by the

time of the response participants have decided for the cor-

rect response, are attending more often/longer to the stimu-

lus array with more items, irrespective of condition. A lack

of a diVerence between conditions may indicate that at this

point the conXict in the incongruent condition was resolved

and inhibition may dissipate.

Taken together, the present experiment shows that atten-

tion is mostly deployed to the array in line with the correct

response. Initially though, it is also guided towards the

task-irrelevant attribute higher value in the incongruent

condition, as indicated by the direction of the Wrst saccade.

Considering Wxation duration, particularly in the course of a

trial until conXict resolution, more time is spent on the

array with more items and in the incongruent condition.

The present experiment is the Wrst that measured the

allocation of attention, as indicated by eye movement pat-

terns, throughout a trial in a Stroop task. The Wndings are

important because they not only demonstrate that monitor-

ing of eye movements is a very informative method for the

investigation of cognitive conXicts but also because the

obtained data suggest potential underlying mechanisms and

lay the foundation for much further work. For instance,

future studies may wish to incorporate eye movement mea-

surements to investigate eVects of varying discriminability

and salience of numerosity and numerical value (Pansky &

Algom, 2002) or the similarity between the representations

of relevant and irrelevant information (Pavese & Umilta,

1998). Further, the role of working memory (WM) could be

addressed. Recently, Kane and Engle (2003) have shown

that working memory span is related to maintaining a goal

in the wordcolour Stroop task. High-WM-span partici-

pants demonstrated less interference than did low-span par-

ticipants. It would thus be most instructive to assess

whether low-span participants allocation of attention

would diVer from attentional allocation of high-span partici-

pants. For example, low-span participants may look less

often towards the stimulus array in line with the task goal.

Additional research could also explore whether persons

with lapses of intention (West, 1999), with deWcient inhibi-

tory control or who may have diYculties to suppress the


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1 3

task-irrelevant information and may thus be prone to

enhanced interference (Kaufmann et al., 2008; Spieler,

Balota, & Faust, 1996) would show diVerent eye movement

patterns, i.e., would be attracted more by the value of the

numbers. Further, it could be explored how guiding atten-

tion in a Stroop task could aVect the conXict. In line with

Grant and Spivey (2003) who showed that guiding attention

towards the problem-relevant features promoted problemsolving, in the presently used task guiding attention

towards the array with more items at the beginning of a trial

could reduce the Stroop eVect. Interestingly, Perret and

Ducrot (2010) could show that the Stroop eVect in the

wordcolour version could be reduced by presenting the

word stimuli in a way that participants Wrst Wxation was on

a position in the word that is not optimal for reading, i.e.,

towards the end of the word, reducing the eYciency of

word processing. Attentional eVects could also be investi-

gated further by varying the percentage of congruent and

incongruent trials and thereby altering participants strate-

gies of dividing attention and allocating attentional weight

to the relevant and irrelevant dimension (Logan &

ZbrodoV, 1979). Future work may also wish to apply a very

diYcult Stroop task in which error rates would be high

enough to be able to explore the diVerences in attentional

allocation between trials in which correct and incorrect

responses were given. Last but not least, the proposed

mechanisms regarding the role of the impact of a concept

and attentional template throughout a trial and inhibition

of irrelevant information should be investigated further.

Acknowledgements I would like to thank Bernhard Hommel, Wer-

ner Schneider and Artem Belopolsky for very constructive commentson previous drafts of this manuscript.

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Measuring the Allocation of Attention in the Stroop Task