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Questions Left Unanswered: How the Brain Responds to
Missing Information
John C. J. Hoeks
1,2
*, Laurie A. Stowe
1,2
, Petra Hendriks
1,2
, Harm Brouwer
1,2
1Center for Language and Cognition Groningen, University of Groningen, Groningen, The Netherlands, 2BCN Neuroimaging Center, University of Groningen, Groningen,
The Netherlands
Abstract
It sometimes happens that when someone asks a question, the addressee does not give an adequate answer, for instance
by leaving out part of the required information. The person who posed the question may wonder why the information was
omitted, and engage in extensive processing to find out what the partial answer actually means. The present study looks at
the neural correlates of the pragmatic processes invoked by partial answers to questions. Two experiments are presented in
which participants read mini-dialogues while their Event-Related brain Potentials (ERPs) are being measured. In both
experiments, violating the dependency between questions and answers was found to lead to an increase in the amplitude
of the P600 component. We interpret these P600-effects as reflecting the increased effort in creating a coherent
representation of what is communicated. This effortful processing might include the computation of what the dialogue
participant meant to communicate by withholding information. Our study is one of few investigating language processing
in conversation, be it that our participants were ‘eavesdroppers’ instead of real interactants. Our results contribute to the as
of yet small range of pragmatic phenomena that modulate the processes underlying the P600 component, and suggest that
people immediately attempt to regain cohesion if a question-answer dependency is violated in an ongoing conversation.
Citation: Hoeks JCJ, Stowe LA, Hendriks P, Brouwer H (2013) Questions Left Unanswered: How the Brain Responds to Missing Information. PLoS ONE 8(10):
e73594. doi:10.1371/journal.pone.0073594
Editor: Claude Alain, Baycrest Hospital, Canada
Received February 7, 2013; Accepted July 23, 2013; Published October 2, 2013
Copyright: ß2013 Hoeks et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was funded by NWO (The Netherlands Organization for Scientific Research) PGW grant 10–26 awarded to John Hoeks and Harm Brouwer,
and NWO Open Access Incentive Fund (036.002.407). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of
the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: j.c.j.hoeks@rug.nl
Introduction
During conversation, speakers and listeners act upon certain
basic assumptions which enable them to communicate swiftly, and
seemingly effortlessly [1–5]. If, for instance, someone asks a
question, both speaker and hearer have knowledge of what would
constitute a valid answer. To be more specific, a question can be
said to impose constraints and create expectations regarding both
the information structure (i.e., specifying what is given and what is
new, and thus how the information contained in an utterance
should be linked to the existing discourse representation) and the
content of the answer. Consider for instance someone inquiring
about the activities of two protagonists, ‘John’ and ‘Peter’:
1. What did John and Peter do?
On the level of information structure, this question introduces
two entities that make them likely topics in the answer, where a
topic can be loosely described as the entity about which the
sentence imparts information [6]. On the content level, in turn,
the question requires the answer to impart on the activities of these
specific people (‘John’ and ‘Peter’), and not, for instance, about
their respective spouses. Answer (2) satisfies both of these
constraints.
2. John cleaned the house and Peter fixed the window.
In contrast, by leaving out information about the second
protagonist, answer (3) violates expectations regarding both
information structure and content. Utterance (3) is thus pragmat-
ically infelicitous as an answer to question (1).
3. John cleaned the house.
If there is no additional information, and the answer consists of
only this sentence, the person who posed the question is faced with
the task of determining what the speaker meant to communicate
by being incomplete. The speaker might, for instance, be taken to
convey that Peter did nothing, that what he did was of no
importance, or just that Peter is terribly lazy [1,7]. The
computation of such beliefs, and thus of a coherent mental
representation of intended meaning, may require extensive
pragmatic processing [Regel, Gunter, & Friederici [8] provide a
similar argument on the computation of ironic meaning]. How the
human language processor deals with this kind of processing is still
poorly understood, and neurocognitive investigations of such
phenomena are scarce.
This study presents two Event-Related brain Potential (ERP)
experiments that examine the neural correlates of the pragmatic
processes invoked by partial answers to questions. ERPs provide a
means of disentangling different processes involved in online
language comprehension, on the basis of the qualitatively different
signatures they leave behind. There are many ERP studies on
word- and sentence-level processing [Kutas, van Petten, &
Kluender [9] provide an overview], but researchers have only
recently started to use ERPs to investigate pragmatic processing
[8,10–13]. These latter studies provide evidence that pragmatic
processes such as the computation of bridging inferences or of
ironic meaning modulate the amplitude of the P600 component, a
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positive deflection of the ERP signal that usually peaks around
600 ms post stimulus onset.
Brouwer, Fitz, & Hoeks [14] have recently argued, on the basis
of a thorough review of the ERP literature, that the P600
component is best defined as a family of late positivities that reflect
the processing involved in the word-by-word construction,
reorganization, or updating of a mental representation of what is being
communicated (MRC)–see also [15,16]. Different varieties of the
P600-effect (in terms of electrophysiological properties like onset,
amplitude, duration, and scalp distribution) are assumed to reflect
different sub-processes of MRC construction. These sub-processes
may include, among other things, the accommodation of new
discourse referents, the establishment of relations between entities,
thematic role assignment and revision, and for instance, the
resolution of conflicts between different information sources (e.g.,
with respect to world knowledge). For instance, in the computation
of bridging inferences, as in a sentence pair like ‘‘We went for a
picnic. The beer was warm’’ [17], some of the sub-processes
involved will concern the accommodation of the new discourse
referent ‘‘The beer’’. The computation of ironic meaning, on the
other hand, may involve more sub-processes aimed at overcoming
the conflict between the unfolding discourse and the ‘literal
meaning’ of the ironic utterance—cf. ‘‘These artists are gifted!’’ in
the context of a bad musical performance, see [8].
The present study investigates whether the processes invoked by
partial answers to questions also produce an increase in P600
amplitude, which would provide strong support for the MRC
hypothesis discussed above (i.e., P600 amplitude reflects ease of
‘making sense’).
Results
Experiment 1
In the first experiment, participants read short question-answer
pairs that appeared word-by-word in the middle of a computer
screen, and were occasionally asked to answer a comprehension
question (see Procedure section below). During reading, brain
activity of the participants was monitored through ERP recording.
The question-answer pairs differed in the pragmatic felicity of the
answer given the preceding question. We used two types of
questions: ‘neutral’ questions like (4), which do not impose any
strong constraints on the information structure of the answer, and
questions such as (5) that require the answer to contain two topics
in a so-called ‘contrastive topic’ information structure—cf. [18].
For the answers we used Dutch sentences containing NP-
coordinations with a one-topic information structure, based on
materials taken from [19]. In these sentences, the NP following the
coordinator is temporarily ambiguous between being the subject of
a new clause, or the object of the present clause. In Dutch and also
in other languages, the object reading is preferred [20]. If such a
one-topic answer follows a contrastive-topic question, as in (5), this
constitutes a pragmatic violation: The question requires the
answer to impart on the activities of two topics (‘‘the mayor’’ and
‘‘the alderman’’); in the answer these entities are mentioned, but
only one of them (‘‘the mayor’’) turns out to be a topic.
It is important to note that in Dutch (unlike in English), the
presence of the adverb at the end of the sentence unambiguously
indicates that the ambiguous NP (‘‘the alderman’’) cannot be a
topic, and that the sentence only has one topic. Thus at the
adverb, the reader is confronted with a clear pragmatic violation.
It should be noted, however, that whereas in the experiment there
is no sentence following the partial answer, the missing informa-
tion could in principle be given in a next sentence (e.g., question:
‘‘What did the mayor and the alderman do?’’—answer: ‘‘The
mayor praised the councilor and the alderman exuberantly. The
alderman therefore thanked the mayor’’). It would be interesting
for a future experiment to manipulate the presence or absence of
such an additional sentence.
4. Neutral
Q: Wat gebeurde er?
‘What happened?’
A: De burgemeester prees het raadslid en de wethouder
uitbundig.
‘The mayor praised the councilor and the alderman exuber-
antly.’
5. Violation
Q: Wat deden de burgemeester en de wethouder?
‘What did the mayor and the alderman do?’
A: De burgemeester prees het raadslid en de wethouder
uitbundig.
‘The mayor praised the councilor and the alderman exuber-
antly.’
Data analysis. Participants were reading attentively, answer-
ing on average 85% (SD = 5.6) of the 35 content questions
correctly. ERP waveforms were time-locked to the presentation of
the critical adverb (‘‘exuberantly’’), see Figure 1.
Three time-windows for statistical analysis were chosen a priori:
a window in which early effects might be observed (150–350 ms
post-onset), a time-window in which possible N400 effects might
be observed (350–550 ms post-onset), and a later time-window for
a possible P600 (600–900 ms post-onset). For each of those
intervals, average ERPs were computed for participant, condition
and electrode separately. Prior to averaging, trials with ocular or
amplifier-related artifacts were excluded from the analysis. For
analysis purposes, three sets of electrodes were created: the three
prefrontal electrodes FP1, FZA, and FP2; the two occipital electrodes
O1 and O2; and the main set of the 15 remaining electrodes. For
each of those sets, Repeated Measures ANOVAs were conducted
with Violation (violation vs. neutral), Laterality and Anteriority as
within-participant factors. In the prefrontal analysis, Laterality had
3 levels (i.e., left, midline, and right side of the scalp); in the
occipital analysis, Laterality had 2 levels (i.e., left and right); for the
main analysis, Laterality had 5 levels (far left, left, middle, right, far
right), and Anteriority had 3 levels (anterior, central, and
posterior). Where appropriate, the Huynh-Feldt correction was
applied; corrected p-values will be reported with the original
degrees of freedom. Only effects involving the factor Violation will
be discussed.
Non-standard baseline. The pre-critical word (the ambiguous NP
‘‘the alderman’’) in the target sentence is introduced in the context
question of the violation condition, but not in the neutral
condition. This gives rise to 1) a ‘repetition’ N400-effect, where
the N400 in the violation condition is attenuated (as compared to
the neutral condition) through word repetition; 2) a P600 effect,
due to the fact that in the neutral condition ‘‘the alderman’’ is a
new discourse entity, whereas in the violation condition it is
already given [10,14,16]. As we wanted to avoid including these
effects in our baseline, we chose a baseline on the coordinator
‘‘en’’ (‘‘and’’) that precedes the ambiguous NP (i.e., ‘‘… and the
alderman exuberantly.’’). Importantly, the presence of the
positivity for the neutral condition may still affect the size of
subsequent effects (if we assume that ERP waves are additive), as
the violation condition starts out more negative than the neutral
condition at some of the electrodes. Hence, our ‘early-baseline’
procedure may overestimate the size of negativities following the
target word in the violation condition. Conversely, the fact that the
violation condition is more negative to begin with may have
decreased the amplitude of subsequent positivities associated with
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the violation condition. Thus, the early-baseline procedure may
underestimate the size of any positivity following the target word in
the violation condition.
Early Time Window (150–350 ms post-onset). In the
analysis of the main set of electrodes, there was a marginally
significant interaction of Violation6Anteriority (F(2,30) = 3.1;
p = .08). Follow-up analyses showed that this trend towards an
interaction was most probably caused by a negativity for the
violation condition (as compared to the neutral condition) that was
largest at the frontal electrodes (violation: 2.1 mV(SE = 0.6);
neutral: 4.3 mV(SE = 1.3)), smaller at central sites (violation:
2.7 mV(SE = 0.7); neutral: 3.7 mV(SE = 1.1)) and smallest at
posterior electrodes (violation: 1.5 mV(SE = 0.9)); neutral: 1.6 mV
(SE = 1.0)). At the prefrontal electrodes there was a marginally
significant main effect of condition, again with violation being
more negative than neutral (violation: 2.9 mV(SE = 0.7); neutral:
5.3 mV(SE = 1.1); F(1,15) = 3.9; p = .066). No effects were found in
the analysis of the occipital electrodes.
N400 Time-Window (350–550 ms post-onset). We did not
find significant effects for the main set or for the prefrontal electrodes
(all p-values..27). At the occipital electrodes there was a marginally
significant interaction of Violation6Laterality (F(1,15) = 3.5;
p = .08), most probably because the positivity elicited in the
violation condition was bigger at the left than at the right of the
scalp (Left: violation: 20.28 mV(SE = 0.9); neutral: 21.25 mV
(SE = 0.7); Right: violation: 20.65 mV(SE = 0.8); neutral:
20.88 mV(SE = 0.7)).
P600 Time-Window (600–900 ms post-onset). The analy-
sis on the main set of electrodes produced a significant interaction
of Violation6nteriority6Laterality (F(8,120) = 2.5; p,.05). Fol-
low-up analyses per level of Laterality suggested that this
interaction was due to a specific pattern of results for electrodes
situated at the far left (Violation6Anteriority: F(2,30) = 3.3;
p = .059), indicating a positivity for the violation condition that
was present at T7 (violation: 3.2 mV(SE = 0.8); neutral: 1.6 mV
(SE = 0.7); F(1,15) = 4.5; p = .05) and P7 (violation: 1.1 mV
(SE = 1.1); neutral: 21.1 mV(SE = 1.0); F(1,15) = 6.0; p,.05), but
not at F7 (violation: 1.9 mV(SE = 0.8); neutral: 1.9 mV(SE = 1.3);
F,1). At the other levels of Laterality, the violation condition was
always more positive than the neutral condition, but none of these
differences were significant (e.g., left: violation: 3.5 mV(SE = 0.7);
neutral: 1.7 mV(SE = 1.0); middle: violation: 4.2 mV(SE = .7);
neutral: 3.1 mV(SE = 1.3); right: violation: 4.3 mV(SE = 0.7);
neutral: 2.9 mV(SE = 1.1); far right: violation: 3.1 mV(SE = 0.5);
neutral: 1.8 mV(SE = 1.0); all p-values..10). Analysis of the
occipital electrodes showed a significant interaction of Violation6
Laterality (F(1,15) = 2.8; p,.01), due to a larger positivity for the
violation condition at the left side (O1: violation: 0.9 mV(SE = 1.2);
neutral: 20.7 mV(SE = 1.2)) than at the right side (O2: violation:
0.6 mV(SE = 1.1); neutral: 0.3 mV(SE = 1.1)). At prefrontal
electrodes, the violation condition (4.6 mV(SE = 0.9)) was numer-
ically more positive than the neutral condition (3.2 mV(SE = 1.3))
but this difference did not reach significance (p..12).
Discussion. Leaving a question partially unanswered gave
rise to a significant, left-lateralized positive shift (600–900 ms after
the onset of the target) which we interpret as a P600. The
marginally significant effect at occipital electrodes in the ‘‘N400
time-window’’ suggests that this positivity already started earlier
(350–550 ms post-onset), though with a different scalp distribu-
tion. These findings are consistent with the MRC hypothesis [14],
where difficulties in creating a mental representation of language
input are assumed to be reflected in (late) positivities. In addition to
these positive effects, we found evidence for an early negativity
(150–350 ms post-onset) with a frontal focus.
To start with this early negativity, Lau, Stroud, Plesch, and
Phillips [21] reported a very similar finding in sentences containing
a word category violation. They interpreted this effect as an Early
Left Anterior Negativity or ELAN [22,23]—see [24] for a critical
review. ELAN effects are typically observed when the syntactic
category of the presented word does not match reader expectation.
In the present study, the question in the violation condition sets up
the expectation that the two protagonists in the answer act as
AGENTS, each involved in a separate event (e.g., an event
depicting what ‘‘the mayor’’ did, and another event depicting what
‘‘the alderman’’ did). However, instead of with the expected verb,
readers were presented with an adverb. This mismatch in category
may have produced the ELAN-effect.
After reading the disambiguating adverb, the reader must deal
with the fact that the mental representation of the sentence, based
on the assigned information structure and on the assigned
thematic roles, is partially incorrect and in need of revision: ‘‘the
alderman’’ is (i) not a topic, but should become part of the
comment, and (ii) not an AGENT but a PATIENT. However, this
‘local’ revision of the mental representation created thus far will
not solve the larger, more ‘global’ problem of the missing
information, which may require extensive pragmatic processing.
That is, after revising the interpretation to reflect that ‘‘the
alderman’’ is a PATIENT and part of a comment, rather than an
AGENT and a topic, one is still faced with the problem of what is
meant by leaving out information on what ‘‘the alderman’’ did.
Hence, to regain a coherent interpretation of the unfolding
dialogue, people have to update their mental representation to
reflect, for instance, that the speaker has left out the information
on purpose, for instance, to communicate that ‘‘the alderman’’ was
passive, and did nothing at all.
In the present experiment, it is not possible to separate processes
of local revision and global pragmatic processes, although one
might be tempted to speculate that the local revision is reflected by
the early positivity in the N400 window (the size of this effect was
rather small, but possibly underestimated through the early
baseline procedure, see Data Analysis section above), and the
global, more pragmatic processing by the later positivity. In order
to disentangle these processes, we conducted a second experiment,
using target sentences which did not contain the ambiguous NP
(‘‘the alderman’’), thereby eliminating the need for local revision.
Experiment 2
In Experiment 1, the target sentence contained all discourse
entities from the context question, only the information structure
was manipulated. In Experiment 2, we entirely removed the
ambiguous part of the target sentence, as shown in examples (6)
and (7) below (the critical word is underlined).
6. Neutral
Q: Wat gebeurde er?
‘What happened?’
A: De burgemeester prees het raadslid.
‘The mayor praised the councilor.’
7. Violation
Figure 1. ERP waveforms for the two conditions in Experiment 1: Neutral (black line) and Violation (red line); topographic maps represent
Violation minus Neutral; there is an extended pre-stimulus time-window in which the onset of the coordinator (CRD), determiner (DET), and noun (N)
is indicated by arrows.
doi:10.1371/journal.pone.0073594.g001
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Q: Wat deden de burgemeester en de wethouder?
‘What did the mayor and the alderman do?’
A: De burgemeester prees het raadslid.
‘The mayor praised the councilor.’
Experiment 2 will thus provide an uncluttered view on how the
brain deals with missing information. Comparing the results from
this experiment with the results from Experiment 1 will enable us
to estimate to what extent the local revision processes contribute to
the positivity. And as the present manipulation does not involve
any kind of category violation, we expect the early negativity found
in Experiment 1 to disappear, which will give further credibility to
the notion that the negativity observed was in fact an ELAN.
Data analysis. Analysis methods were the same as for
Experiment 1, except that we now used a more ‘‘standard’’ pre-
stimulus baseline on the article (‘‘the’’) preceding the final noun
(‘‘councilor’’). See Figure 2 for a graphical display of the resulting
waveforms.
As in Experiment 1, we found a positivity (neutral condition
more positive than violation condition) starting around the onset of
the critical word. The cause of this effect is not completely clear,
but it may reflect the ease with which new information is
integrated into an existing mental representation. It is likely that
this integration is easier when there is already some kind of
representation, as in the violation condition, then when the mental
representation has to be created from the start [10]. As in the
previous experiment, this effect may overestimate any negativities
and underestimate any positivities that follow the presentation of the
target word in the violation condition. As we will see, this does not
affect the interpretation of the results: there are no negativities
(that may have been there spuriously), and the positive effects are
rather large (and hence not eliminated by the baseline positivity).
Early Time Window (150–350 ms post-onset). No signif-
icant effects were found in either of the analyses (all p-values..14).
N400 Time-Window (350–550 ms post-onset). We found
a significant effect of Violation for the main set of electrodes
(F(1,16) = 13.6; p,.005), with the violation condition being more
positive (2.2 mV(SE = 0.7)) than the neutral condition (2.05 mV
(SE = 0.4)). Additional significant main effects of Violation were
found for the prefrontal electrodes (violation: 3.2 mV(SE = 1.2);
neutral: 0.3 mV(SE = 1.0); F(1,16) = 9.0; p,.01) and the occipital
electrodes (violation: 0.4 mV(SE = 0.8); neutral: 21.7 mV
(SE = 0.6); F(1,16) = 6.0; p,.05).
P600 Time-Window (600–900 ms post-onset). The main
effect of Violation was significant for the prefrontal, electrodes
(F(1,16) = 7.3; p,.05), again with the violation condition (2.2 mV
(SE = 0.7)) being more positive than the neutral condition
(20.05 mV(SE = 0.4)). Main effects of Violation almost reached
significance at the main set of electrodes (violation: 3.0 mV
(SE = 0.9); neutral: 1.5 mV(SE = 0.5); F(1,16) = 4.1; p = .060) and
at the occipital electrodes (violation: 1.8 mV(SE = 0.6); neutral:
20.2 mV(SE = 1.0); F(1,16) = 3.6; p = .076).
Discussion
The results of both experiments show that the processes invoked
by a partial answer to a question modulate the amplitude of the
P600 component. In Experiment 1, all elements from the question
were present in the answer, but with the wrong information
structure. Disambiguation by the critical adverb required revision
of the mental representation built so far, and instigated a change in
information structure and thematic role assignment, as well as
pragmatic processing regarding the ‘global’ issue of missing
information. Therefore, the P600-effect that was found in
Experiment 1 was argued to reflect both local revision and global
pragmatic processing. In Experiment 2, by contrast, the entire
ambiguous phrase was eliminated, making the resulting P600-
effect a pure reflection of extensive (pragmatic) processing needed
to make sense of the dialogue—see also [8] for a similar P600-
effect for ironic versus non-ironic meaning.
The conclusion that both experiments produced a P600-effect
fits very well with a recent neurocognitive framework proposed by
Brouwer, Fitz, & Hoeks [14]—see also [15,16]. Traditionally, the
P600 component has been linked to syntactic processing [9,25].
However, in the last decade, an increasing number of studies have
found non-syntactic P600-effects [see [14,26,27] for overviews],
and a few of these studies have even found P600-effects for
specifically pragmatic phenomena such as bridging inferences [10–
12] and the processing of irony [8]. On the basis of a thorough
review of this literature, Brouwer and colleagues hypothesized that
the P600 component is best defined as a family of late positivitities
reflecting the different sub-processes involved in the word-by-word
construction, reorganization, or updating of a mental representa-
tion of what is being communicated. Examples of these sub-
processes are accommodating new discourse entities, establishing a
relation between the entities and assigning them a thematic role,
adding information to entities, revising already established
relations, revising already assigned thematic roles, and resolving
conflicts between information sources (e.g., with respect to world
knowledge). In the case of incomplete answers, some of these
processes might take the form of computing the meaning that is
implied by the answer. For instance, if the question ‘‘What did the
mayor and the alderman do?’’ is answered with ‘‘The mayor
praised the councilor’’, the observed P600-effect may reflect
processes involved in constructing for instance the belief that the
‘‘alderman’’ has been idle, which is then added to the developing
mental representation of the unfolding dialogue. To find out
whether language users do indeed create such meanings, attempts
should be made to investigate the mental representations that
participants actually construct, for instance by having them report
verbally on what they think is the intended meaning of an
utterance, or, more covertly, to probe this representation in a
priming paradigm.
An important difference in the results of the two experiments is
that in Experiment 2, the P600-effect seems much more
prominent from an early moment on (i.e., in the 350–550 ms
time window) than in Experiment 1, though Experiment 1 does
show a marginally significant positivity at the occipital electrodes.
This difference in prominence of the P600 effect is not quite what
would be expected. In Experiment 1, a local revision is required
before more global (pragmatic) processing can commence. In
Experiment 2, by contrast, there is no need for local revision.
Hence, we would have expected Experiment to engender more
extensive processing than Experiment 2. Close inspection of the data
from Experiment 1, however, suggests that there is an early
positivity that is likely to be underestimated as a result of the pre-
existing effect at baseline. As we discussed above, in Experiment 1
we chose a non-standard baseline because of an attenuated N400
as well as a P600-effect on the pre-critical word in the neutral
condition (see Data Analysis section of Experiment 1). The
presence of a positivity on the pre-critical word in the neutral
condition might have as a consequence that an actual P600-effect
on the critical word in the violation condition is underestimated.
Analysis of the three time-windows using a 100 ms within-stimulus
baseline confirms that this might indeed be the case: in addition to
the negativity in the 150–350 ms time window, and the positivity
in the 600–900 ms time window, this analysis also reveals a
significant positivity in the 350–550 ms time window (with a broad
scalp distribution). If we accept this tentative evidence, it suggests
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that both experiments show a positivity starting as early as 350 ms
after the onset of the critical word, signifying the effortful updating
of a mental representation of what is said.
Another difference between the experiments is the difference in
scalp distribution in the late 600–900 ms time window. In both
experiments, the P600-effect showed a standard broad centro-
parietal scalp distribution in the early 350–550 ms time window,
but in the later 600–900 ms time-window, the P600-effect
observed in Experiment 1 was strongly left-lateralized, whereas
the P600-effect observed in Experiment 2 was more pronounced at
prefrontal electrodes. Brouwer, Fitz, & Hoeks [14] argue that the
sub-processes invoked in the creation of a coherent mental
representation may be different in different phases and under
different circumstances, and that these differences may be reflected
in the P600 in terms of variations in ERP parameters such as
onset, amplitude, duration, and scalp distribution. This suggests
that the experiments may have invoked partly similar and partly
different processes.
Exactly what processes are involved still needs to be determined.
Van Petten & Luka [28] speculate that more frontally pronounced
positivities reflect prediction errors, whereas more parietally
pronounced positivities reflect reprocessing costs. It is difficult to
see how this fits with our data, as one could argue that in both
experiments there is some kind of prediction error (of the expected
information structure), and there is a requirement for reprocessing
(or at least more extensive processing in order to recover the
intended meaning). Hence, it is unclear whether our data should
evoke a frontal P600-effect, a parietal one, or some kind of
combined positivity, reflecting both prediction error and repro-
cessing cost. Brouwer and Hoeks [15] have recently suggested an
alternative explanation for the origin of characteristically different
P600-effects. They hypothesize that the different sub-processes
involved in MRC construction, elicit (late) positivities because
these processes are implemented by neural generators residing in
different parts of the left Inferior Frontal Gyrus (lIFG) (a
conglomerate of areas around and including Broca’s Area). This
is consistent with the data of the present study. The experiments
may have invoked similar sub-parts of the lIFG in the early 350–
550 ms time-window, yielding very similar ‘early’ P600-effects,
and different, but potentially overlapping, sub-parts of the lIFG in
the later 600–900 ms time-window, producing characteristically
distinct P600-effects. In addition, the right hemisphere homologue
of lIFG, the rIFG, has been shown to be active when processing
complex stimuli [29], which may also affect the scalp distribution
of the P600. A categorization of different instances of the P600 and
the different circumstances under which they are elicited, may
help us to further unravel what kinds of processing, including
pragmatic processing, constitute the creation of meaning.
Finally, the results of the experiments combined are consistent
with the interpretation that the early negativity found in
Experiment 1 is actually an ELAN reflecting a category violation.
In Experiment 1, the question in the violation condition requires
the answer to talk about two AGENTs. This makes it very likely
that participants expected to get a verb instead of an adverb (e.g.,
‘‘exuberantly’’). This category violation may have lead to an
ELAN. In Experiment 2, there is no such category violation, and
we did not find an ELAN there. Steinhauer & Drury [24] have
recently argued that the functional significance of the ELAN
component, which they associate strongly with Friederici’s syntax-
first model of language processing, is still rather unclear. Our
present results, however, suggest that there is some sort of early
effect of a mismatch between linguistic input and expectation. We
do not believe, however, that the existence of category violation
effects per se makes it necessary to adopt syntax-first models, as
also other models assume that the language comprehension system
engages in some form of prediction [14,26,27].
Materials and Methods
Ethics statement
The treatment of the participants conformed to APA and BPS
ethical standards. Participants gave written consent for participa-
tion. The protocol was approved by the Medical Ethical Committee of
the University Medical Center Groningen (METc UMCG).
Participants
Experiment 1. Eighteen undergraduate students from the
University of Groningen (6 male, age-range 18–29, average 20)
participated, receiving payment or course credits for taking part in
the experiment. All were right-handed native speakers of Dutch
with normal, uncorrected vision.
Experiment 2. Twenty participants (8 male, age-range 18–
34, average 22) that were either university students or recent
university graduates, volunteered to take part in the experiment.
All were right-handed native speakers of Dutch with normal or
corrected-to-normal vision.
Materials and Design
Experiment 1. Besides the two kinds of experimental
question-answer pairs (20 per condition) where the answer
sentence contained an NP-coordination, there were 40 filler
dialogues (20 with a neutral question and 20 with a two-topic
question) in which the answer consisted of an S-coordinated
sentence, such as in the answer to (8):
8. Q: Wat gebeurde er?/Wat deden de ridder en de hertog?
‘What happened?’/‘What did the knight and the duke do?’
A: De ridder bevocht de prins en de hertog vluchtte.
‘The knight fought the prince and the duke fled’.
The use of S-coordinated sentences as fillers should minimize
the chance of participants developing processing strategies for the
NP-coordinated sentences in the critical dialogues. In addition,
there were 100 filler items from an unrelated experiment on the
processing of relative clauses.
Experiment 2. In experiment 2, we adapted the critical
sentences of experiment 1 so that they ended after the first NP of
the NP-coordination (see example dialogues in (9)):
9. Q: Wat gebeurde er?/Wat deden de burgemeester en de
wethouder?
‘What happened?’/‘What did the mayor and the alderman do?’
A: De burgemeester prees het raadslid.
‘The mayor praised the councilor.’
The S-coordination fillers were the same as in experiment 1.
There were also 116 fillers from an unrelated experiment. In both
experiments, lists were created using a Latin Square, with equal
numbers of items occurring in each condition on each list, and no
list containing more than one version of a given item. The order in
which experimental and filler items appeared was determined
semi-randomly (i.e., allowing maximally three experimental items
in consecutive order, but never two consecutive items in the same
condition) and was the same for all lists. Each lists was presented to
Figure 2. ERP waveforms for the two conditions in Experiment 2: Neutral (black line) and Violation (red line); topographic maps represent
Violation minus Neutral.
doi:10.1371/journal.pone.0073594.g002
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PLOS ONE | www.plosone.org 7 October 2013 | Volume 8 | Issue 10 | e73594
an equal number of participants and each participant saw only one
list.
Procedure
Experiment 1. Participants were instructed to read each
sentence for comprehension, and to respond to an occasional
content question (by means of pressing yes/no buttons on a button
box; 35 for the entire experiment; example: ‘‘What did the
detective and the greengrocer do?’’–question: ‘‘Was there a
detective?’’). At the beginning of each trial, a fixation mark
appeared for 1 second. After that, the dialogues were presented
word-by-word in the center of the screen. Each word remained on
screen for 243 ms, followed by a blank screen of 243 ms. Between
question and answer there was a pause of 729 ms. The experiment
took about 100 min, including preparation.
Experiment 2. Participants were instructed to read each
sentence for comprehension, and to judge the semantic relatedness
of an occasional probe word to the preceding sentence (by means
of pressing yes/no buttons on a computer keyboard; example:
‘‘The lackey spied upon the baroness and the lady-in-waiting
screamed’’–probe: ‘‘nobility?’’). At the beginning of each trial, a
fixation mark appeared for 500 ms. This fixation mark was
followed by a blank screen of the same duration. After that, the
dialogues were presented word-by-word in the center of the
screen. Each word remained on the screen for 240 ms, followed by
a blank screen of 240 ms. Between question and answer there was
a pause of 960 ms. The experiment took about 100 min, including
preparation.
EEG recording parameters
Experiment 1. EEG activity was recorded by means of 20 tin
electrodes mounted in an elastic cap: FP1, FP2, FZA, F7, F3, FZ,
F4, F8, T7, C3, CZ, C4, T8, P7, P3, PZ, P4, P8, O1, and O2.
Bipolar horizontal EOG was recorded between electrodes at the
outer canthi; bipolar vertical EOG was recorded for both eyes.
Electrode impedances were kept below 5 kV. EOG and EEG
signals (referred to the average of the two mastoids) were sampled
at 1000 Hz, amplified (EEG: 0.2 mV/V; EOG 0.5 mV/V; time
constant: 10 sec.), and digitally low-pass filtered with a cut-off
frequency of 30 Hz.
Experiment 2. EEG activity was recorder by means of 62 tin
electrodes mounted in an elastic cap: FP1, FP2, F3, F4, C3, C4,
P3, P4, O1, O2, F7, F8, T7, T8, P7, P8, AFZ, CZ, FZ, PZ, AF3,
AF4, FC3, FC4, CP3, CP4, PO3, PO4, PO7, PO8, AF7, AF8,
FPZ, OZ, FC1, FC2, C1, C2, FCZ, FT9, FT10, F5, F6, FC5,
FC6, C5, C6, CP5, CP6, P1, P2, P5, P6, P9, P10, PO9, PO10,
CPZ, POZ, O9, O10, and IZ. Bipolar EOG was recorded
between electrodes at the outer canthi; bipolar vertical EOG was
recorded for the left eye only. EOG and EEG signals (referred to
the average of the two mastoids) were sampled at 250 Hz,
amplified (EEG: 0.2 mV/V; EOG 0.5 mV/V; time constant:
10 sec.), and digitally band-pass filtered with a low cut-off
frequency of 0.01 Hz and a high cut-off frequency of 50 Hz.
Conclusions
Violating the strong dependency between questions and answers
by omitting information that language users have asked for,
invoked pragmatic processes that modulate the amplitude of the
P600 component. We interpret this increased P600 amplitude as a
reflection of increased effort in constructing a coherent represen-
tation of what is being communicated. Possibly, this involves the
computation of what the addressee of the question wants to
communicate by leaving out part of the answer, and adding this
information to the unfolding representation of the linguistic input
in order to create coherence. Our results add to the as-of-yet small
range of pragmatic phenomena that modulate the amplitude of
the P600 component. This marks the importance of the P600 as an
index of making sense, both in discourse and in conversation.
Supporting Information
Materials S1 ‘‘Questions Left Unanswered’’: Stimulus Materi-
als.
(XLSX)
Acknowledgments
We are grateful to Ingeborg Prinsen and Charlotte Wunderink for
collecting the data for Experiment 1, and to Ryan Taylor for collecting the
data for Experiment 2. We would also like to thank Lotte Schoot for her
help in running and analyzing Experiment 2.
Author Contributions
Conceived and designed the experiments: JH HB PH LS. Performed the
experiments: JH LS. Analyzed the data: JH HB. Contributed reagents/
materials/analysis tools: JH. Wrote the paper: JH HB PH LS.
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