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ORIGINAL PAPER
Stress reduces attention to irrelevant information: Evidence
from the Stroop task
Rob Booth ÆDinkar Sharma
ÓSpringer Science+Business Media, LLC 2009
Abstract Stroop interference can be reduced by stress,
and this has been taken as evidence that stress decreases the
attention paid to irrelevant information, a theory known as
‘Easterbrook’s hypothesis’. This contradicts more recent
theories, which state that attentional control deteriorates in
stress. Fifty-five participants undertook a Stroop task under
high stress (loud white noise) and low stress conditions.
Attention to the irrelevant word information was assessed
by manipulating the proportion of congruent trials (e.g. the
word RED in the colour red); it is known that Stroop
interference increases when many such trials are presented.
This effect was reduced when participants were stressed,
which is evidence that stress does indeed reduce attention
to irrelevant information. This pattern of results was not
present in participants with low working memory spans,
presumably because these participants had less attentional
control. These findings highlight an important weakness in
contemporary theories of cognition in stress.
Keywords Stroop Selective attention Stress
Working memory Easterbrook’s hypothesis
Introduction
Easterbrook’s hypothesis (Easterbrook 1959) states that
emotional individuals pay less attention to ‘peripheral’ (i.e.,
less relevant) information. Easterbrook did not himself
specify the mechanism behind this effect; presumably stress
overloads the attention system to some extent, and so reduces
the attention resources available for allocation to less rele-
vant information (c.f. perceptual load, Lavie et al. 2004).
Easterbrook’s hypothesis has been supported by results
indicating that stressed participants show reduced Stroop
interference. In the Stroop task (Stroop 1935), participants
ignore the meaning of words and respond to the colour in
which they are presented; Stroop interference is the differ-
ence in response time or accuracy between incongruent trials
(e.g., the word RED in green print) and control trials (e.g.
??? in green print). The word information is irrelevant, so
its influence should be reduced under stress because less
attentional resources are allocated to it (Easterbrook did not
specifically posit a spatial narrowing of attention, as is
sometimes suggested).
This prediction has been repeatedly supported. For
example, O’Malley and Poplawsky (1971) used a traditional
Stroop task, with multiple stimuli presented on cards. Par-
ticipants who were stressed by hearing loud bursts of white
noise were generally faster at this task, and showed less
interference, compared to participants who performed the
task in silence. O’Malley and Gallas (1977) replicated this
finding with an improved control condition: participants
exposed to bursts of white noise at 85 dB showed reduced
interference compared to those exposed to noise at 75 dB.
Chajut and Algom (2003, Experiment 2) also replicated this
in a similar experiment using a more modern computer-
controlled single-trial Stroop task, and also showed that
Stroop interference could be reduced by a very difficult fake
This research was carried out while Rob Booth was a doctoral student
at the University of Kent, supervised by Dinkar Sharma.
R. Booth (&)D. Sharma
Department of Psychology, Centre for Cognitive Neuroscience
and Cognitive Systems, Keynes College, University of Kent,
Canterbury CT2 7NP, UK
e-mail: rob.booth.psych@googlemail.com
D. Sharma
e-mail: D.Sharma@kent.ac.uk
123
Motiv Emot
DOI 10.1007/s11031-009-9141-5
IQ test. Substances that increase physiological arousal have
also been found to reduce interference (Callaway 1959;
Kenemans et al. 1999).
Easterbrook’s (1959) hypothesis has been neglected in
more recent theories of cognition in emotion, which tend to
claim that attentional selection deteriorates with stress or
anxiety (e.g. Eysenck et al. 2007; Mathews and Mackintosh
1998): Eysenck et al. predict that Stroop interference should
increase in anxious states (see Pallak et al. 1975). Therefore,
the fact that interference is usually found to decrease seems
to present a problem for recent theories of cognition in
anxiety. If this problem is to be accepted, it must be shown
that stress’s effects on Stroop interference do indeed result
from improved selectivity, and not from some strategy
change made by the participants. For example, it is possible
that stress simply increases the participants’ motivation
levels (c.f. McFall et al. 2009), so that they wish to complete
the task (and escape the stress manipulation) as quickly as
possible. This will lead to an overall decrease in RTs.
Responding more quickly could mean a smaller Stroop effect
because less time has elapsed for interference to build
up to affect performance. It is therefore important to dis-
count a strategic explanation of stress’s effects on Stroop
interference.
One way to address whether stress-related decreases in
Stroop interference are strategic is to simultaneously assess
the attention paid to the irrelevant word information with
another method. For example, if many congruent trials (e.g.,
BLUE in blue print) appear in the Stroop task, interference
increases (Kane and Engle 2003; Lindsay and Jacoby 1994).
Although the cause of this ‘proportion-congruent’ effect is
still controversial (Blais et al. 2007), it is clear that such a
manipulation can only affect participants’ behaviour if they
attend to the irrelevant word information to some degree. If
stressed participants attend less to the irrelevant word
information, they should be less likely to notice the con-
gruent trials and their behaviour should not change. In other
words, the Stroop interference shown by stressed partici-
pants should be unaffected by the proportion of congruent
trials presented, and remain low.
This pattern would not be expected if stress’s reduction of
Stroop interference was due to some strategic factor. Pre-
senting congruent trials increases the salience of the dis-
tracter words, because a correlation is introduced between
the colour information and the word information (see
Melara and Algom 2003). This makes the irrelevant word
information harder for the participant to ignore, Melara and
Algom argue, because the attention system is predisposed to
seek out such correlations. If stress’s reduction in Stroop
interference is caused by some strategic change—i.e., if the
irrelevant word information is still processed in stress—then
the correlation between word information and colour
information will still decrease the participants’ ability to
inhibit the distracter words, and will still increase interfer-
ence. In other words, the proportion-congruent effect in
stress and non-stress conditions should be equal. On the
other hand, if stress does indeed decrease attention to the
irrelevant word information as Easterbrook’s (1959)
hypothesis implies, then this correlation is more likely to go
unnoticed, and the proportion-congruent effect should be
small in the stress condition.
To increase the power of the experiment, participants’
working memory (WM) spans were assessed. Although
WM span ostensibly relates to the storage capacity of
memory, it is also related to the control of attention, and
the maintenance of task goals (De Fockert et al. 2001;
Kane and Engle 2003; Lavie et al. 2004). This attention
control aspect of working memory has been present since
its inception (see Baddeley 1992), and some theorists have
suggested that individual differences in working memory
span may be largely or entirely attributable to attention
control, rather than storage capacity per se (Kane et al.
2007).
It therefore seems reasonable that WM span and stress,
if they both relate to attention control and/or selectivity,
will interact in their effects on Stroop interference. Spe-
cifically, at any given moment, participants with higher
WM spans (‘high spans’) are more likely to be treating as
irrelevant information which actually is irrelevant to their
task, and those with lower WM spans (‘low spans’) are
more likely to be suffering a ‘slip’ of attention and mis-
takenly treating target colour information as irrelevant.
Easterbrook’s hypothesis suggests that attention resources
are withdrawn from irrelevant stimuli, but if the aspect of a
Stroop stimulus that is treated as irrelevant is less consis-
tent for low spans, then they would be expected to show
less consistent stress effects. High spans on the other hand
are more likely to maintain attentional control to selec-
tively attend to the target colour so that stress decreases
Stroop interference. For this reason, high and low spans
were identified after the experiment so that their perfor-
mance could be compared.
To summarise, the present experiment investigated
whether Stroop interference was reduced in stress, partic-
ularly in groups with high WM spans. A constant white
noise at 84 dBC was used to induce stress; this was found
to be effective by Chajut and Algom (2003), and a similar
manipulation was used by O’Malley and colleagues
(O’Malley and Gallas 1977; O’Malley and Poplawsky
1971). The proportion of congruent trials was also
manipulated. If stress reduces attention to irrelevant
information as described by Easterbrook (1959), stressed
participants should not notice this manipulation, and their
behaviour should not change in response to it. WM span
was assessed, as it was predicted that the above pattern of
results should be more clear in high spans.
Motiv Emot
123
Method
Participants
Sixty-three participants (50 females), aged 17–59 years
(M=22) took part in the experiment. All were native
English speakers with normal or corrected hearing andvision.
They received £5 or course credit for their participation.
Design
The Stroop task employed a 2 9392 within-participants
design. The factors were Trial Type (incongruent or con-
trol), Proportion-Congruent (proportion of congruent trials
within the block: 0, 25 or 50%), and the Volume of the
noise (stressful, 84 dBC, or unstressful, 65 dBC).
Apparatus and stimuli
The experiment was run using a Dell Inspiron 510 M
computer running E-prime v1.0.1, a CRT monitor, and a
PST response box. A chin rest was positioned one metre
from the screen. Noise was created using Philips SBC
HC202 headphones with the transmitter switched off (this
produces a broad-range hiss). Volumes were set using a
TES 1351 sound level meter.
As a manipulation check for stress, skin conductance
data were collected using a BIOPAC MP35. This recorded
at 10 Hz via 10 mm-diameter Ag–AgCl disposable elec-
trodes treated with isotonic gel, attached to the distal
phalanges of the third and fourth fingers of the left hand.
Stimuli subtended 0.9°vertically and averaged 3.4°
horizontally, and were presented in red, blue, yellow or
green print on a black background. Incongruent stimuli
consisted of a colour word presented in a contrasting col-
our, e.g. BLUE in green print. Congruent stimuli consisted
of a word in a matching colour, e.g. BLUE in blue print.
Control stimuli consisted of strings of three to six ?’s in
one of the four colours above. Neutral stimuli consisted of
a neutral word presented in one of the four colours. Words
were randomly selected for each participant from lists of
mono- and bi-syllabic words of up to eight letters, which
were matched for letter length, syllable length and CELEX
frequency (measured using Lexicus, Frankish 2002). These
later appeared as targets in a recognition test: no significant
effects were recorded, so further details are omitted.
Procedure
Participants were seated in front of the screen, and the
procedure was explained. They were reassured about the
safety of the noise. The skin conductance electrodes were
applied and tested, and recording was initiated. During a
practice phase, participants familiarised themselves with
the task. Participants learnt the colour to which each button
corresponded, and were advised that speeded responding
was unnecessary. The experimenter left the room during all
phases of the Stroop task. The practice phase consisted of
the words ‘HOUSE’, ‘TREE’ and ‘CAT’, randomly pre-
sented 25 times in each of the four print colours. Each trial
began with a 1000 ms blank black screen before the
stimulus, which remained on-screen until a response was
made. The practice phase also served as a habituation
period for the skin conductance recording.
At the end of the practice phase the experimenter reit-
erated the nature of the task, stressing that the participant
should now respond as quickly and accurately as possible,
and explaining that three blocks of trials would be pre-
sented. The experimenter then set the headphones to the
required volume. The order of the two volume conditions
was counterbalanced across participants.
The first experimental phase consisted of three blocks,
one for each Proportion-Congruent condition. Each block
included 48 critical trials (24 control and 24 incongruent).
The 0% congruent block also included 48 neutral word
trials; the 25% congruent block included 24 neutral word
trials and 24 congruent trials; and the 50% congruent block
included 48 congruent trials. Note that the neutral trials
were present to replace the congruent trials without altering
the relative frequency of incongruent and control trials (see
Melara and Algom 2003); responses to these trials, and
indeed to congruent trials, were not analysed. The blocks
were presented in a random order, and trials were presented
in a random order within each block. Trials proceeded as in
the practice phase.
The experimenter then told the participant that they
would complete the experimental phase a second time, but
at the other volume. The second experimental phase con-
sisted of the same blocks as in the first phase, in a newly-
randomised order.
Ospan task
After the Stroop task participants completed the Ospan
WM test (Tuholski and Engle 2002), as described by
Turner and Engle (1989). The Ospan involves remember-
ing lists of words while solving arithmetic problems. The
task was controlled by the experimenter, and participants
responded on pre-printed sheets. Having completed the
Ospan, participants were given a full debriefing.
Results
Overall accuracy was very high (M=0.95). Eight partic-
ipants made more than 10% errors; these participants were
Motiv Emot
123
considered as outliers, and removed from further analyses.
Including these cases does not change the overall pattern of
results.
Ospan task
The Ospan is scored by summing the length of all completely
correct lists. Scores ranged between two and 32 (M=14.96,
SD =7.78), and fit the commonly-found distribution for the
task (Kane and Engle 2003). The sample was median split to
produce a high span group with scores of 15 and above
(N=26, M=21.85, SD =4.66) and a low span group with
scores of 14 and below (N=29, M=8.79, SD =3.76).
Skin conductance data
The skin conductance data were examined to gauge the
effectiveness of the stress manipulation. Data from two
participants were lost due to experimenter error. For the 53
remaining participants, data were averaged for each of the
six blocks of Stroop trials: these correspond to the chro-
nological order of the blocks. This was done to test whether
participants’ arousal levels varied with time.
These data were subjected to a 2 (Volume) 93
(Block) 92 (WM Group) ANOVA. There was a significant
interaction between Volume and Block (F(2,102) =7.41,
MSE =0.37, p\0.001, g
p
2
=0.13), such that the arousal
difference between Volume conditions decreased with time
(First block, 84 dBC: M=7.39 lS, SD =4.87 lS; 65 dBC:
M=7.02 lS, SD =4.79 lS. Second block, 84 dBC:
M=7.12 lS, SD =4.94 lS; 65 dBC: M=6.75 lS,
SD =5.09 lS. Third block, 84 dBC: M=6.94 lS,
SD =5.01 lS; 65 dBC: M=7.12 lS, SD =5.13 lS). This
shows that the stress manipulation was effective, although its
effectiveness disappeared in the final block. This does not
affect the Stroop results as these effects are distributedequally
across the three Proportion-Congruent conditions.
Stroop task
Correct response times were subjected to a participant- and
condition-specific outlier trimming procedure, using a non-
recursive moving criterion (Van Selst and Jolicoeur 1994).
Two interference scores were then calculated, one for
response time (incongruent–control, see Fig. 1) and one for
accuracy rate (control–incongruent). Note that interference
scores could not be calculated with congruent or neutral
trials, as these were not present in every Proportion-Con-
gruent condition: there were no congruent trials in 0% con-
gruent condition, and no neutral trials in 50% congruent
condition. RT interference was subjected to a 2 (Volume) 9
3 (Proportion-Congruent) 92 (WM Group) ANOVA.
There was a significant three-way interaction (F(2,106) =
4.17, MSE =5150, p\0.05, g
p
2
=0.07). Specifically,
among high spans the Proportion-Congruent effect was only
significant at 65 dBC (F(2,50) =6.01, MSE =5634,
p\0.01, g
p
2
=0.19) and not at 84 dBC (F(2,50) =2.00,
MSE =3111, p=0.15, g
p
2
=0.07); among low spans, the
Proportion-Congruent effect was significant at 84 dBC
(F(2,56) =9.53, MSE =5563, p\0.001, g
p
2
=0.25) but
not at 65 dBC (F(2,56) =0.85, MSE =6441, p=0.85,
g
p
2
=0.01).
1
There was also a Proportion-Congruent main effect
(F(2,106) =12.18, MSE =5316, p\0.001, g
p
2
=0.19) as
expected, but no other effects reached significance (all
Fs\1.5, all ps[0.2).
To test against a possible motivational account of stress’s
effects on Stroop interference, the RT analysis was repeated
with mean correct RTs to control trials (i.e., not interference
scores) as the DV. There was no effect of Volume
(F(1,53) =1.83, MSE =6217, p=0.18, g
p
2
=0.03, see
Table 1), and there were no interactions between Volume,
WM Group, and Proportion-Congruent (all Fs\0.8,
ps[0.4).
Fig. 1 Stroop response time interference scores (incongruent RT–
control RT) for high and low working memory span groups, under
stressful (84 dBC) and non-stressful (65 dBC) conditions (ms)
1
The Stroop data were re-analysed with the upper and lower tertiles
as the WM groups. The same pattern of results were found as reported
in Fig. 1. Furthermore, when Proportion-Congruent effects were
calculated by subtracting 0% condition interference scores from 50%
condition interference scores, it was found that Proportion-Congruent
effects in the 84 dBC condition tended to correlate negatively with
Ospan score (r(53) =-0.224, p=0.099); Proportion-Congruent
effects in the 65 dBC condition did not correlate with Ospan scores
(r(53) =0.123, p=0.26). These findings support our conclusions
from our median split analyses.
Motiv Emot
123
Accuracy interference scores were subjected to the same
analyses. There was a tendency for Volume to interact with
WM Group (F(1,53) =3.60, MSE =0.003, p=0.06,
g
p
2
=0.06), so that the louder noise tended to reduce inter-
ference for high spans (84 dBC: M=0.005, SD =0.032;
65 dBC: M=0.020, SD =0.040) and increase interference
for low spans (84 dBC: M=0.011, SD =0.044; 65 dBC:
M=0.002, SD =0.041). However, these simple effects did
not approach significance (both Fs\2.6, both ps[0.12).
There was also a Proportion-Congruent main effect
(F(2,106) =5.24, MSE =0.004, p\0.01, g
p
2
=0.09), so
that interference was highest in the 50% congruent condition
(M=0.025), and much lower in the 25% (M=0.005) and
0% (M=-0.002) conditions. No other effects reached sig-
nificance (all Fs\0.6, all ps[0.5).
Discussion
As predicted, stress effects on Stroop interference were
jointly moderated by Proportion-Congruent and WM
Group. This interaction (Fig. 1) supports the prediction
made above, that proportion-congruent effects will be less
apparent in stressed high spans. This is evidence that these
participants are indeed less aware of the irrelevant word
information, and therefore seems to support Easterbrook’s
(1959) hypothesis. This reduced proportion-congruent
effect also meant that high spans tended to show less
Stroop interference under stress, although this lower-order
interaction did not reach significance in its own right,
perhaps because it is subsumed within the significant
three-way interaction. The present work suggests the
intriguing possibility that the detectability of stress’s
reduction of Stroop interference depends upon the compo-
sition of the task itself. For example, high spans did not
show reduced interference under stress in the 0% congruent
condition, perhaps because they were already strongly
attending to the target colour (Kane and Engle 2003). Future
work will need to examine this possibility more closely.
Note that these effects cannot be easily attributed to an
increase in participants’ motivation (see, e.g., Huguet et al.
2004). Participants did not show a significant decrease in
RTs to control trials in the 84 dBC condition, which would
be expected if the loud noise motivated them to respond
quickly. Furthermore, if the noise motivated them to
respond as quickly as possible, they should have been
tempted to pay more attention to the irrelevant word
information in the 50% congruent condition, as this would
have enabled them to respond quickly and efficiently to the
large number of congruent trials presented in these blocks.
However, the 84 dBC noise reduced high spans’ interfer-
ence in this condition, suggesting that they were paying
less attention to the word information. Clearly, this is more
consistent with Easterbrook’s hypothesis than it is with a
strategic or motivational account.
This novel finding seems to contradict many recent
theories on how stress and anxiety affect cognition. In the
future, it will be important to further study this problem by
manipulating the salience of the irrelevant word informa-
tion using other methods (e.g., the size of the irrelevant
word: see e.g. Melara and Algom 2003).
Oddly, high spans appear to show more interference
than low spans in the 50% condition, when not under
Table 1 Mean correct response
times (RTs) and accuracy rates
(Acc.) from the Stroop task, by
Working Memory Span Group
(WM) and Proportion-
Congruent (P–C). Standard
deviations are in brackets
WM P–C Volume
(dBC)
Incongruent Control
RT Acc. RT Acc.
Low 0% 65 720 (130) 0.960 (0.056) 657 (95) 0.958 (0.042)
84 688 (77) 0.957 (0.032) 638 (84) 0.958 (0.051)
25% 65 735 (148) 0.961 (0.046) 668 (114) 0.954 (0.048)
84 708 (150) 0.944 (0.045) 663 (115) 0.954 (0.048)
50% 65 752 (141) 0.945 (0.073) 678 (102) 0.960 (0.045)
84 775 (168) 0.943 (0.062) 654 (97) 0.964 (0.041)
Total 65 736 (139) 0.955 (0.059) 668 (103) 0.957 (0.044)
84 724 (141) 0.948 (0.048) 652 (99) 0.959 (0.046)
High 0% 65 695 (124) 0.976 (0.038) 656 (81) 0.981 (0.024)
84 691 (109) 0.984 (0.027) 640 (92) 0.971 (0.035)
25% 65 735 (160) 0.963 (0.066) 665 (120) 0.976 (0.039)
84 714 (142) 0.968 (0.032) 667 (106) 0.971 (0.035)
50% 65 777 (181) 0.938 (0.075) 666 (95) 0.979 (0.029)
84 734 (137) 0.950 (0.059) 658 (96) 0.974 (0.036)
Total 65 736 (158) 0.959 (0.063) 662 (99) 0.979 (0.031)
84 713 (130) 0.967 (0.043) 655 (98) 0.972 (0.035)
Motiv Emot
123
stress.
2
This would contradict the notion that WM corre-
lates with goal maintenance. Previous work has found that
WM span differences in Stroop interference only appear
when there are many congruent trials, though high spans
should show less interference than low spans (e.g. Kane
and Engle 2003). However, Kane and Engle warned their
participants about the congruent trials, and instructed them
to remain focussed on the colour information; in the
present experiment, such a warning was not possible. Non-
stressed high spans may therefore have switched to an
easier word-reading strategy in the 50% congruent condi-
tion; this would have impeded their responses to incon-
gruent trials, leading to increased interference (see Lindsay
and Jacoby 1994).
One weakness of the present experiment is that it did not
directly assess participants’ mood with, for example, an
anxiety questionnaire. We feel that this is an acceptable
omission given our physiological validation of our stress
manipulation, together with the fact that loud noise manip-
ulations have a long history in stress research. Noise has also
been previously shown to increase arousal (Geen and
McCown 1984).
To summarise, stress reduced proportion-congruent
effects on Stroop interference, among participants with high
working memory span. This provides further evidence that
stress reduces the attention paid to irrelevant information
(Easterbrook 1959), and suggests stress’s reduction of
Stroop interference is not likely to be strategic in nature.
These effects remain a problem for theories stating that
anxious states decrease attentional control (e.g. Eysenck
et al. 2007; Mathews and Mackintosh 1998). Eysenck et al.
argue that Easterbrook-like effects in other paradigms (e.g.
Weltman et al. 1971) can be explained in terms of decreases
in attentional control: stressed individuals’ attention is
drawn to the most salient stimulus, but in these experiments
the most salient stimulus is actually the target, not the dis-
tracter. The present experiment suggests that such accounts
must be expanded to cover the Stroop task, where the dis-
tracter word is usually regarded as much more salient than
the target (Melara and Algom 2003). Future work should
attempt to reconcile these two apparently contradictory
theoretical positions.
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