Journal of Behavioral and Brain Science, 2013, 3, 150-155 Published Online February 2013 (
How the Brain Process Stimulus-Response Conflict? New
Insights from Lateralized Readiness Potentials Scalp
Topography and Reaction Times
Marc E. Lavoie1,2,3, Johannes E. A. Stauder4
1Laboratoire de Psychophysiologie Cognitive et Sociale
2Centre de Recherche Fernand-Seguin, de l’Hôpital Louis-H. Lafontaine, Montréal, Canada
3Département de Psychiatrie, Université de Montréal, Montréal, Canada
4Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
Received December 14, 2012; revised January 15, 2013; accepted January 22, 2013
Stimulus-Response Compatibility (SRC) refers to the fact that some tasks are performed easier and better than others
because of the way stimuli and responses are paired with each other. To assess the brain responses to stimulus-response
conflicts, we investigated the behavioral (accuracy and Reaction Times: RTs) as well as the physiological response
(Lateralized Readiness Potentials: LRP) modulations in a positional blocked and a conditional mixed design in twelve
university students. Results revealed that the performance was less accurate and the RTs, as well as the LRP onset, were
delayed under the mixed conditional design. A greater compatibility effect was also noted on accuracy, RTs and LRP
onset latency in the mixed design. Consistent with these findings, smaller peak activation at fronto-central areas sug-
gests that more selective inhibition is needed in a mixed design context. Despite a smaller activation, the topographical
distribution is similar in both designs. These results indicate that the translation time between stimulus- and response
codes are greater under the mixed instruction, while the similar LRP topography suggests that common neural structures
underlie LRPs in response to both type of designs.
Keywords: Lateralized Readiness Potentials; Mixed-Blocked Designs; Stimulus Response-Compatibility; Reaction
1. Introduction
Stimulus-Response Compatibility (SRC) refers to the fact
that some tasks are performed easier and better than oth-
ers because of the way stimuli and responses are paired
with each other [1]. SRC are not restricted to particular
stimuli, modality or task, indicating the fundamental na-
ture of their underlying cognitive processes. In general
terms, two main theoretical views, the serial and the dual-
task automatic processing approach, attempted to explain
the mechanisms underlying stimulus response conflict.
Early SRC research papers [2,3], as well as recent ones
[4], proposed that incompatible mapping is slower than
compatible mapping because of increased translation time
between the stimulus and response code. Accordingly,
that coding process arises in series after stimulus identi-
fication, but before motor initiation. An alternative ex-
planation is provided by the dual-processing accounts,
which attribute SRC effects to the automatic activation of
responses [5]. At difference with the classical serial mo-
del, automatic activation is not dependent on the effi-
ciency of coding processes, but is induced by a separate
parallel route in which the response are activated directly
and have to be inhibited in case of an incompatible map-
ping [6]. Both SRC models were elaborated on the basis
of overt responses without direct inferences on brain pro-
cesses underlying stimulus-response conflict.
A considerable number of metabolic brain studies ad-
dressed movement related metabolic activity, although
none specifically addressed SRC. Increased regional
blood flow to hand and finger movements were observed
in the contralateral sensorimotor cortex [7,8], premotor
cortex, primary motor cortex, anterior cingulate cortex,
parietal cortex, the supplementary motor area (SMA) and
the prefrontal cortex [9,10]. Unfortunately, is the exact
functional role of most of these areas is still unclear. In-
creased blood flow in the Supplementary Motor Area
was observed to movements triggered by a stimulus as
compared to self paced movements [8] suggesting the
SMA to be task related to the contingency of the cue
triggering the movement. The anterior cingulate cortex
opyright © 2013 SciRes. JBBS
was associated response selection [9] and the modulation
of commands coming from different regions (e.g. pre-
frontal) to the motor system [11]. The anterior cingulate
cortex was mainly activated when the subjects are forced
to choose from a set of competing responses rather than
relying on well-established S-R associations. Although
these findings might suggest potential mechanisms im-
plicated in SRC, a satisfying understanding of the struc-
tures underlying SRC awaits a metabolic study with tasks
that specifically modulate stimulus response compatibil-
ity. However, even in case a metabolic study isolates the
brain structures, involved in the compatibility effect [12],
we still ignore the time course of activation, since none
of the metabolic techniques offers a high enough time
resolution to tear these processes apart. A solution to this
dilemma is a topographical Lateralized Readiness Poten-
tial (LRP) investigation, which offers a similar time re-
solution than a single lead LRP but also shows the dis-
tribution of the LRP across the scalp at each point in time
[13,14]. The latter may not only help in determining
whether there are qualitative differences in activation be-
tween different task manipulations but may also inform
us about the underlying brain sources implied in gener-
ating the LRP.
We propose to investigate SRC interference through
electrophysiological measures of motor activation, the
so-called LRPs, that reflects only the lateralized part of
the readiness potential, leaving aside electrical potentials
associated with non-motor activity [13]. LRPs reveal
initial activation of the incorrect response (negative de-
flection) followed by a delayed activation of the correct
response (positive deflection) in case of incompatible re-
sponse [15]. The concept of automatic activation found
considerable support by psychophysiological studies us-
ing LRP as an index of selective response preparation.
In order to further examine these hypotheses within a
psychophysiological setting, we compared one blocked
design, where arrows were presented in two separate
blocks of compatible and incompatible sequences and
one mixed design where stimulus-response configura-
tions are presented in mixed order and are dependant on
the color of the arrow. By means of comparing a mixed
with a blocked design, our primary aim is to assess the
occurrence of automaticity in SRC interference. Thus, if
SRC is best explained by a dual-task automatic process-
ing model, we predict that interference will be equally
present in both the blocked and mixed design (compati-
bility main effect). On the other hand, if SRC is best ex-
plained by a serial processing model, the interference
will be more pronounced in the mixed than in the
blocked design (design x compatibility interaction). Al-
though these findings might suggest potential mecha-
nisms implicated in SRC, an understanding of cerebral
structures underlying SRC awaits a topographical ap-
proach that offers a similar time resolution than a single
lead LRP, but with a distribution across the scalp. Hence,
our second aim is to investigate the topographical distri-
bution of the LRP to determine whether there are qualita-
tive differences of activation, which will inform us on
how the underlying neural structures are involved in sti-
mulus-response conflict, across designs and compatibil-
ity conditions (design by electrodes or a design by com-
patibility by electrodes).
2. Methodology
2.1. Participants
The participants were 8 female and 4 male students from
the University of Montreal. They were right handed, and
had normal color vision and normal or corrected to dis-
tant vision. Their average chronological age was 23.8
years (sd = 3.4) and the intelligence level was determined
by the Raven’s Advanced Progressive Matrices. The
scores on this test were 10.8 (sd = 1.5) for Set I (12/12)
and 25.5, (sd = 5.9) for Set II (36/36). The total average
raw scores (48/48) was 38.3, sd = 7.0. Handedness was
determined by the Edinburgh Inventory (Old-field, 1971)
on which the participants showed an average score of
85.6, sd = 27.8 (range 100 to 100). The color vision was
determined by means of the Dvorine Pseudo-Isochroma-
tic Plates (2ed, 1953) on which the participants correctly
identified 97 % of the 15 plates correct (sd = 0.05). None
of the participants had a score below the normal color
vision threshold of 89.5 % (2 unidentified plates). Letter
plates (Home vision care, Anaheim California), deter-
mined distance vision on which all subjects showed
100% correct letter identification above the 20/40 level.
The Table 1 shows the absence of sex differences on all
these variables.
Table 1. Demographic and psychological variables.
Female Male
Mean (SD) Mean (SD)t-test
Age (years) 24.1 (4) 23 (1) ns
Laterality (% right) 82.5 (29) 82.7 (27) ns
Schooling (years) 17.1 (2) 16.5 (1) ns
Intelligence (raw score)
Raven set I 10.75 (2) 11 (1) ns
Raven set II 26.63 (7) 29.25 (4) ns
Raven—total 37.38 (8) 40.25 (6) ns
Color vision (%) 98 (0.5) 96 (0.7) ns
Letter plates (%)
Left eye 100 97 ns
Right eye 100 94 ns
Both eyes 100 100 ns
Copyright © 2013 SciRes. JBBS
2.2. Procedures
Participants were administered three tasks: 1) In the posi-
tional blocked compatible tasks (100 trials), participants
were asked to respond with the hand corresponding to the
direction of the arrow, while ignoring its color. 2) The
positional blocked incompatible (100 trials) required re-
sponding with the hand opposite to the direction of the
arrow, while ignoring color. 3) The conditional mixed
task (200 trials) required a hand response corresponding
to the direction of the arrow to one color and with the
hand opposite to the direction of the arrow to the other
color. The arrows were presented for 350 ms (Inter-Sti-
mulus-Interval = 2000 - 2500 ms).
The EEG signals were recorded concomitantly and
sampled at 250 Hz, with a high- and low-pass filter of
0.01 and 30 Hz respectively, recorded from 20 tin elec-
trodes mounted in a nylon cap, placed at F7, F8, FC3,
FC4, T7, T8, C3, C4, C1, C2, CP3, CP4, TP7, TP8, P7,
P8, P3, P4, O1, O2 referenced to linked earlobes (impe-
dance < 5 kOhms). The EEG was corrected for artifacts
with a dynamic regression in the frequency domain [16]
and correct trials averaged off-line from stimulus onset.
2.3. Statistics and Processing
The stimulus-locked LRP, obtained from subtraction [13],
were reduced to 10 locations (F7’, F3’, FC3’, C1’, C3’,
P3’, P7’, T7’, TP7’, O1’). The proportional onset was
determined as 20% of the maximum positive peak of the
LRP within 50 - 600 ms post-stimulus. Two factors in-
cluding DESIGN (mixed vs blocked) and COMPATIBI-
LITY (compatible vs incompatible) were analyzed for
Reaction Times (RTs) and accuracy separately. Repeated
measures factor (ELECTRODES) was incorporated in
the analysis with LRP onset and peak amplitude.
3. Results
3.1. Performances
Accuracy was significantly better to the blocked (2% error
rate) than to the mixed (8% error rate) design (F1,11) =
18.33, p < 0.001). Consistently, RTs were significantly
faster in the blocked (372 ms) than in the mixed (561 ms)
design (F(1,11) = 255.83, p < 0.001). In both blocked and
mixed design (Figure 1), the compatible yielded faster
RTs than the incompatible condition (F(1,11) = 15.44, p
< 0.005), but that incompatibility effect was only sig-
nificant in the blocked design (t(11) = 5.12, p < 0.001).
3.2. LRP Onset Latency
The LRP onset latency revealed that there were main
effects of design (F(1,11) = 48.02, p < 0.001) and com-
patibility (F(1,11) = 5.88, p < 0.05). The LRP onset was
longer in the mixed (290 ms) than in the blocked (196 ms)
design and the compatible (218 ms) yielded faster la-
tency than the incompatible (268 ms). In addition, there
was a design by compatibility interaction (F(1,11) = 8.78,
p < 0.05) revealing that the LRP latency difference be-
tween compatible and incompatible conditions was larger
in the mixed (90 ms) than in the blocked (4 ms) design
(Figure 2). The LRP waveforms (Figure 3) revealed an
incorrect activation (negative deflection) peaking at 250
ms in the mixed design only. This was followed by a cor-
rect activation (positive deflection) peaking at approxi-
mately 300 ms in response to the blocked design and at
500 ms post-stimulus in the mixed design.
3.3. LRP Peak Amplitude
The results of the LRP peak amplitude activation re-
vealed that there was main effects of design (F(1,11) =
22.87, p < 0.001) and compatibility (F(1,11) = 4.90, p <
0.05). The LRP amplitude was smaller in the mixed (3.1
uV) than in the blocked (4.3 uV) design and the incom-
patible (3.5 uV) yielded smaller amplitude than the com-
Figure 1. Reaction times in function of compatibility, in
comparing blocked and mixed designs.
Figure 2. Comparison of the SRC compatibility effect from
the LRP onset latency, related to the correct activation in
response to the mixed and blocked design.
Copyright © 2013 SciRes. JBBS
Copyright © 2013 SciRes. JBBS
Panel A. Block design Panel B. Mixed conditional design
Figure 3. Scalp distribution of the stimulus-locked LRP compatibility effect related to the blocked (panel A) and mixed design
(panel B). For both panels, the solid bold line shows the LRPs to the incompatible, while the dashed line shows the LRPs in
response to the compatible. The y-axis denotes the amplitude in microvolts, while the x-axis denotes the time scale in milli-
seconds. The vertical dashed lines refer to the stimulus onset, while the solid vertical line shows the mean reaction times (RTs)
for each task. The negative polarity deflections denote the incorrect activation of the response while the positive deflections
indicate the correct activation of the response.
patible (4 uV). Finally, a design by electrode interaction
was also found on the LRP peak amplitude (F(3.98,43.83)
= 3.74, p < 0.05) showing that the LRP amplitude was
larger in the blocked than in the mixed design at FC3’,
C3’and CP3’ respectively (see Figure 4).
4. Discussion
Our findings consistently showed that performance was
less accurate and the RTs, as well as LRP onset, were
delayed under the mixed conditional design as compared
with the blocked design. This indicates that the transla-
tion time between stimulus and response codes are greater
under the mixed condition. Consistent with these find-
ings, smaller peak activation at fronto-central regions sug-
gests that more selective inhibition is solicited, when the
program for the correct activation is retrieved in the
mixed design context [17]. Single neuron studies also
support response competition effects of preliminary vis-
ual information in the frontal cortex revealed by the
LRPs [18,19]. Despite the smaller activation observed in
the mixed design, the topographical distribution is simi-
lar than in the blocked design, suggesting that common
neural structures underlie LRPs in response to both de-
Additionally, a greater compatibility effect on accu-
racy, RTs and LRP onset latency in blocked than in
mixed design, supports a model of serial activation of the
response as it were found in similar SRC studies [20,21].
This suggests that, when stimuli are presented in a
blocked design, both compatible and incompatible condi-
tions directly (or automatically) activate the correct re-
sponse. However, when stimuli are presented in a mixed
design an additional step must be performed i.e. the acti-
vated response together with its program is first aborted
(negative waveform peaking at 300 ms post-stimulus) in
the incompatible condition, and the program for the cor-
rect response is retrieved later in time (positive wave-
form peaking at 500 ms post-stimulus), favoring a dual
route processing explanation of the mixed SRC interfer-
ence [22]. When the response spatially corresponds to
Figure 4. Stimulus-locked LRP scalp distribution from lat-
eralized locations showing the correct maximum activation
(with standard error of the mean) to blocked and mixed de-
signs (averaged across compatible-incompatible conditions).
The figure show a design by electrode interaction found on
the LRP peak amplitude. The LRP amplitude w as larger in
the blocked than in the mixed design at FC3’, C3’and CP3’
stimulus location a fast direct route is activated, whereas
a slower indirect (controlled) route activates the inten-
tionally selected response [23]. However, the discrepancy
in the compatibility effect between RT and LRP onset
measures in the mixed design could be attributed to the
larger LRP-RT difference to the mixed (271 ms) as com-
pared to the blocked design (176 ms). Thus, in the con-
text of a mixed design, LRP onset is more distant from
the RT and produces a delayed speed of response activa-
tion as compared to the blocked design because of a de-
layed translation time to decode the response program.
In conclusion, our data showed a slower LRP onset
and reaction times to the incompatible mapping, greater
in the mixed design and favor a serial processing expla-
nation of the stimulus-response conflict. Moreover, the
LRP amplitude activation during the mixed design con-
sistently showed smaller amplitude at fronto-central ar-
eas (design by electrode interaction), revealing that more
selective inhibition was necessary to perform the condi-
tional mixed task.
5. Acknowledgements
This work was supported by a Fonds pour la Recherche
en Santé du Québec (FRSQ) clinical research (5271) and
the FRSQ chercheur-boursier awarded to MEL. At last
but not the least, we thank all participants for their pre-
cious contribution in this study.
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