Effects of Working Memory Span on Processing of Lexical Associations and Congruence in Spoken Discourse

Abstract

The goal of this study was to determine whether variability in working memory (WM) capacity and cognitive control affects the processing of global discourse congruence and local associations among words when participants listened to short discourse passages. The final, critical word of each passage was either associated or unassociated with a preceding prime word (e.g., "He was not prepared for the fame and fortune/praise"). These critical words were also either congruent or incongruent with respect to the preceding discourse context [e.g., a context in which a prestigious prize was won (congruent) or in which the protagonist had been arrested (incongruent)]. We used multiple regression to assess the unique contribution of suppression ability (our measure of cognitive control) and WM capacity on the amplitude of individual N400 effects of congruence and association. Our measure of suppression ability did not predict the size of the N400 effects of association or congruence. However, as expected, the results showed that high WM capacity individuals were less sensitive to the presence of lexical associations (showed smaller N400 association effects). Furthermore, differences in WM capacity were related to differences in the topographic distribution of the N400 effects of discourse congruence. The topographic differences in the global congruence effects indicate differences in the underlying neural generators of the N400 effects, as a function of WM. This suggests additional, or at a minimum, distinct, processing on the part of higher capacity individuals when tasked with integrating incoming words into the developing discourse representation.

Full text

ORIGINAL RESEARCH ARTICLE
published: 13 February 2013
doi: 10.3389/fpsyg.2013.00060
Effects of working memory span on processing of lexical
associations and congruence in spoken discourse
Megan A. Boudewyn*, Debra L. Long and TamaraY. Swaab
Department of Psychology and Center for Mind and Brain, University of California, Davis, CA, USA
Edited by:
Gina Kuperberg, Tufts University, USA
Reviewed by:
Cristiano Chesi, IUSS Pavia, Italy
Ellen F. Lau, University of Maryland,
USA
Ross Metusalem, University of
California, San Diego, USA
*Correspondence:
Megan A. Boudewyn, Department of
Psychology, University of California,
One Shields Avenue, Davis, CA
95616, USA.
e-mail: maboudewyn@ucdavis.edu
The goal of this study was to determine whether variability in working memory (WM)
capacity and cognitive control affects the processing of global discourse congruence and
local associations among words when participants listened to short discourse passages.
The final, critical word of each passage was either associated or unassociated with a pre-
ceding prime word (e.g., “He was not prepared for the fame and fortune/praise”). These
critical words were also either congruent or incongruent with respect to the preceding dis-
course context [e.g., a context in which a prestigious prize was won (congruent) or in which
the protagonist had been arrested (incongruent)]. We used multiple regression to assess
the unique contribution of suppression ability (our measure of cognitive control) and WM
capacity on the amplitude of individual N400 effects of congruence and association. Our
measure of suppression ability did not predict the size of the N400 effects of association or
congruence. However, as expected, the results showed that high WM capacity individuals
were less sensitive to the presence of lexical associations (showed smaller N400 asso-
ciation effects). Furthermore, differences in WM capacity were related to differences in
the topographic distribution of the N400 effects of discourse congruence. The topographic
differences in the global congruence effects indicate differences in the underlying neural
generators of the N400 effects, as a function of WM. This suggests additional, or at a
minimum, distinct, processing on the part of higher capacity individuals when tasked with
integrating incoming words into the developing discourse representation.
Keywords: N400, lexical association, discourse congruence, individual differences, working memory capacity
INTRODUCTION
Discourse comprehension requires language users to integrate
information from multiple levels of meaning, and individuals vary
considerably in their use of semantic and syntactic information
during sentence and discourse processing. Behavioral studies of
individual differences in discourse comprehension have impli-
cated a variety of factors, such as word-decoding ability, sup-
pression ability,working memory (WM) capacity, print exposure,
and background knowledge, as contributors to reading skill differ-
ences (see Long et al., 2006, for a review). Event-related potential
(ERP) studies of individual differences in comprehension are less
common, with most focusing on the comprehension of written
sentences. Few ERP studies have investigated individual variabil-
ity during online listening comprehension, but recent work has
implicated WM capacity as contributing to variability in process-
ing thematic relations in spoken sentences (Nakano et al., 2010),
and controlled suppression ability as contributing to individual
differences in processing word-level meaning relations in spoken
sentences (Boudewyn et al., 2012b).
Previous studies suggest that WM and cognitive control capac-
ity contribute to individual differences in the ability to construct a
globally coherent discourse representation, and also in the degree
to which comprehenders are influenced by meaning relations
among words (e.g., Gernsbacher, 1990;Gernsbacher and Faust,
1991;Just and Carpenter, 1992;Cantor and Engle, 1993;Gerns-
bacher, 1997;Van Petten et al., 1997;Vos et al., 2002;Vos and
Friederici, 2003;Bornkessel et al., 2004;Nakano et al., 2010;
Boudewyn et al., 2012b). The current study, for the first time,
combines electrophysiological measures of language processing
with measures of cognitive control and WM capacity in order to
examine if and how individual variability in these measures affects
processing of global discourse-level congruence and local word-
level meaning relations. In the following sections, we first discuss
studies that have examined the role of lexical associations and/or
message-level congruence during the comprehension of incoming
words in context. We then review evidence suggesting that indi-
vidual variation in WM capacity and cognitive control ability may
influence the processing of discourse-level meaning and meaning
relations among words in discourse context. Finally we turn to
specific predictions for this experiment.
DISCOURSE CONGRUENCE AND LEXICAL ASSOCIATIONS AS LEVELS OF
MEANING IN CONTEXT
Spoken discourse comprehension is a complex dynamic process
that requires the rapid translation of acoustic events into meaning-
ful speech and the integration of multiple sources of linguistic and
contextual information with the incoming speech signal. Many
studies have shown that information from previous context and
from background knowledge can be brought to bear quickly in
order to interpret and integrate incoming words (see van Berkum,
2009;Swaab et al., 2011 for reviews). Specifically,the processing of
incoming words can be facilitated by multiple factors, including
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Boudewyn et al. Working-memory and levels of context
(but not limited to) the presence of associatively related prime
words in the preceding context (e.g., Van Petten, 1993;Hoeks et al.,
2004;Coulson et al., 2005;Camblin et al., 2007;Boudewyn et al.,
2012a,b), the previous sentence or discourse context when it is
coherent, or predictive of upcoming words (e.g., Federmeier and
Kutas, 1999;van Berkum et al., 1999, 2003;Camblin et al., 2007;
Diaz and Swaab, 2007;Boudewyn et al., 2012a), and general world
knowledge (e.g., Hagoort et al., 2004;van Berkum et al., 2008). As
argued by Van Berkum in his“multiple-cause resource-intensified
memory retrieval” hypothesis, many types of information con-
tribute to the “interpretive background” of each incoming word in
context, all of which can facilitate the retrieval of the meaning(s) of
that word (van Berkum, 2009). In ERP studies, facilitation of the
processing of incoming words by supportive or consistent inter-
pretive background results in a reduction of the N400 waveform,
a negative-deflecting ERP that is sensitive to semantic aspects of
input (see Swaab et al., 2011 for a review).
Although many contextual variables impact the processing of
incoming input, these factors can vary in terms of their relative
influence on processing. Although word-level associations have
a significant impact on processing when presented in pairs or
lists (e.g., Williams, 1988;Marslen-Wilson and Zwitserlood, 1989),
they have a relatively weak influence on processing when embed-
ded in a sentence or discourse context, and are not consistently
observed. As noted above, several studies have found lexical asso-
ciation effects for words in context (Van Petten, 1993;Hoeks et al.,
2004;Coulson et al., 2005;Camblin et al., 2007;Boudewyn et al.,
2012a,b), but other studies have found association effects only
under certain conditions, such as when primes and targets are
embedded in the same clause (Carroll and Slowiaczek, 1986), when
primes and targets are congruent with the sentence-level meaning
(Morris, 1994), when primes are in linguistic focus (Morris and
Folk, 1998), or when targets appear in incongruent (Coulson et al.,
2005) or low-constraint (Hoeks et al., 2004) sentences.
In contrast, evidence from recent ERP studies has shown that
discourse-level meaning has a powerful and early influence on
the processing of upcoming words, even though some traditional
accounts of discourse comprehension have suggested that while
the activation of individuals concepts is fast and automatic, inte-
grated representations of discourse context must be retrieved
from memory in order to be brought to bear on information
currently being processed (Kintsch, 1988;Ericsson and Kintsch,
1995). However, N400 amplitude differences between words that
fit well with the larger message compared to words that are less
congruent, or anomalous, are found without delay (Federmeier
and Kutas, 1999;van Berkum et al., 1999, 2003;Camblin et al.,
2007;Boudewyn et al., 2012b). In our studies, we have directly
manipulated discourse congruence and lexical association within
the same paradigm, and have found congruence effects to be more
robust and long-lasting compared to association effects,both dur-
ing reading and auditory comprehension (Camblin et al., 2007;
Boudewyn et al., 2012b).
Although the results of these studies suggest that word-level
association has a weaker influence than discourse congruence
on processing of incoming words, the relative influence of these
factors can vary depending on the particular context. For exam-
ple, readers sometimes fail to detect semantic anomalies within
discourse contexts such as“victim” in the following passage:“Child
abuse cases are being reported much more frequently these days.
In a recent trial, a 10-year sentence was given to the victim, but this
was subsequently appealed” (Sanford et al., 2010). In this example,
the passage is fully congruent up until the critical word “victim,”
and the failure of many readers to detect this break with the mes-
sage of the discourse context may be interpreted as an instance in
which readers over-rely on the associative relations among words
in the context (e.g., “victim” is strongly associated with, and pos-
sibly activated by, words in the context). These results suggest that
the relative influence of discourse-level meaning and word-level
meaning on processing is determined by the particular context
being processed.
As previously discussed, language processing is also influenced
by individual differences characteristics, and in the current study,
we test the hypothesis that the relative influence of discourse-level
meaning and word-level meaning on the processing of incoming
words is influenced by individual differences in WM capacity and
cognitive control ability. These individual differences characteris-
tics were selected for two reasons. First,WM capacity and cognitive
control are theoretically important during language comprehen-
sion: the maintenance of discourse context placesdemands on WM
resources, and control mechanisms (e.g., suppression ability) are
likely involved in the inhibition of context-irrelevant information
activated by language context. Second, individual differences in
both WM capacity and cognitive control ability have been impli-
cated by previous research as being related to differences in spoken
language comprehension (e.g., Nakano et al., 2010;Boudewyn
et al., 2012b). Thus, WM capacity and cognitive control abil-
ity may be particularly influential in determining sensitivity to
message-level meaning and lexical association during discourse
processing, as larger, multi-sentence discourse passages place great
demands on WM maintenance and on controlled integration of
context-relevant information.
WORKING MEMORY CAPACITY AND LANGUAGE PROCESSING
Working memory capacity has long been implicated in theories
of individual differences in language processing, and particularly
in studies investigating variability in the reading of syntactically
complex sentences (Just and Carpenter, 1992;Vos et al., 2002;Vos
and Friederici, 2003;Bornkessel et al., 2004). There are three major
accounts of the role of WM during language comprehension. The
first two accounts focus on the limited-capacity of WM resource(s)
(e.g., Just and Carpenter, 1992;Waters and Caplan,1996). Accord-
ing to the Capacity Theory (Just and Carpenter, 1992), language
processing and context maintenance draw upon a single limited-
capacity WM resource pool (of activation). Variability in the
amount of activation available, or capacity, produces individual
differences in sentence processing. This is especially apparent for
complex syntactic structures that are resource-demanding, and
leave few available resources for the maintenance of non-syntactic
information such as the meaning of individual words or the sen-
tence context as whole. Similarly, Mason and Just (2004) suggest
that the processing of discourse context requiresWM resources to
maintain contextually relevant information, so that comprehen-
ders can create anaphoric references and inferences. Therefore,
low WM capacity individuals are expected to be limited in their
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Boudewyn et al. Working-memory and levels of context
ability to maintain contextual information during the process-
ing of complex sentences and discourse context. In contrast, the
Separate Sentence Interpretation Resource (SSIR) theory (Waters
and Caplan, 1996) proposes that there are two separate WM
resource pools that may be tapped during sentence processing.
One resource pool underlies initial syntactic structure-building,
and does not vary across individuals. However, individuals vary
in the capacity of the second resource, which is more general and
comes into play during controlled, verbally mediated tasks such
as a conscious search of memory or logical reasoning (Waters and
Caplan, 1996). Even though Waters and Caplan (1996) made no
explicit predictions with respect to this second resource relative
to discourse processing, it is reasonable to assume that discourse
processing would tap into this resource as well, and that low capac-
ity comprehenders would have greater difficulty in maintaining
context-relevant information. Thus, even though the first two
accounts of the influence of WM capacity on language process-
ing make different predictions with respect to the processing of
syntactic information, they would likely make similar predictions
regarding the influence of WM capacity on discourse processing.
The third account regarding the role of WM in language com-
prehension attributes variability to individual differences in pro-
cessing efficiency, arising as a function of language experience.
This alternative proposal to limited-capacity models has been put
forward by MacDonald and Christiansen (2002), who argue that
“capacity” is a property of the efficiency and experience of the
processing system as a whole, rather than a limited resource pool.
According to this account, variability in sentence processing is
a result of individual differences in reading skill and experience
(MacDonald and Christiansen, 2002). Complex syntactic struc-
tures cause processing delays or difficulties, not because they are
resource-demanding, but because they are less frequently encoun-
tered than simpler structures, which are more common. Skilled
readers have more experience than less skilled readers with com-
plex syntactic structures. Therefore, facilitated comprehension of
difficult structures can be explained as a result of experience, rather
than of “high-capacity.” Indeed, there is evidence indicating that
increased exposure to complex syntactic structures can modify
processing of these structures in “low-span” subjects (MacDonald
et al., 1994;Long and Prat, 2002).
Whether individual variability on WM tasks and during sen-
tence comprehension is best attributed to differences in a limited-
capacity resource pool(s) or to differences in language experience,
the fact that individuals vary in their ability to construct and
maintain language context has important implications for our
understanding of language comprehension. As noted above, most
studies of the relation between WM capacity and language pro-
cessing have focused on the reading of syntactically ambiguous
or complex sentences (Just and Carpenter, 1992;Vos et al., 2002;
Vos and Friederici, 2003;Bornkessel et al., 2004). Here we will
review studies that are particularly relevant to the current study
because they have investigated the relation between WM capacity
and the use of meaning information during sentence or discourse
comprehension.
First, there is some evidence to suggest that high WM capac-
ity individuals integrate meaning across sentences in discourse
context more quickly and effectively than low WM capacity
individuals (Cantor and Engle, 1993, Experiment 2). In Cantor
and Engle, WM span was measured in the following way: Partici-
pants were presented with mathematical operation-word pairs, in
groups ranging from two to seven [e.g., (6/2)– 2 =1; DOG]. Fol-
lowing presentation and completion of all operation-word pairs,
participants were asked to recall as many words from the pre-
vious group as possible. The sum of correctly recalled words
(from groups where recall was perfect) was taken as the WM
span measure. Participants then read a series of passages that were
either three or six sentences long, and featured different subject
nouns (e.g., the teacher). Participants were asked to recall the
sentences associated with each subject (e.g., all sentences about
the teacher), and repeated the study-recall cycles until they were
able to accurately recall all items in a particular passage on three
consecutive trials. Following this learning phase, a verification
memory test was given to the participants, in which they were
asked to identify whether or not experimental and foil sentences
had been present in the study phase of the experiment. In contrast
to low WM capacity individuals, high WM capacity participants
required fewer study cycles in order to accurately recall the pas-
sages, and were faster to recognize sentences that had appeared in
longer passages compared to the shorter three-sentence passages
(Cantor and Engle, 1993). This suggests that high-capacity indi-
viduals utilized the additional information in the longer passages
to construct richer, more durable discourse representations. In
contrast, low WM capacity individuals exhibited the opposite pat-
tern, and were faster to recognize sentences that had appeared in
the shorter three-sentence passages. This pattern of results sug-
gests that the high WM span group integrated the individual
sentences of the longer passages into a coherent mental model
more quickly and successfully than the lowWM span group (Can-
tor and Engle, 1993). Therefore, higher capacity individuals may
be better able than lower-capacity individuals to maintain and
integrate across multiple sentences in order to construct a unified
discourse representation during comprehension.
Other studies have investigated the relation between WM
capacity and the processing of meaning information during com-
prehension using ERPs. One study has linked WM capacity with
sensitivity to word-level animacy information in spoken sentences
(Nakano et al., 2010). Specifically, Nakano et al. (2010) found that
ERP signatures in sentences that violate the thematic constraints of
verbs (e.g., biting in “The box was biting the mailman”) compared
to word-knowledge-violated or non-anomalous verbs (e.g., biting
in “The poet/dog is biting the mailman”) varied as a function of
WM span: Thematic violations elicited a P600 effect in high-span
individuals, but an N400 effect in low-span individuals (Nakano
et al., 2010). N400 effects are typically produced by varying seman-
tic fit across conditions, whereas P600 effects arelinked to syntactic
manipulations or conflict at the semantics-syntax interface (see
Swaab et al., 2011 for a review). Therefore, the pattern of results in
this study suggests that high-span, but not low-span, participants
were able to quickly make use of animacy information from the
first noun in the sentence (i.e., that an inanimate noun such as box
is not a suitable subject for a verb like biting ) in order to begin the-
matic role assignment, leading to a P600 effect at the verb when the
interpretation requires revision [i.e., that the syntax dictates box
is actually the (anomalous) subject of the sentence]. In another
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Boudewyn et al. Working-memory and levels of context
ERP study, participants read sentences that contained associated
or unassociated word pairs, and that were either fully congruent
or anomalous (Van Petten et al., 1997). Only high WM partici-
pants distinguished between congruent and anomalous sentences
when no word-level associations were present,whereas all partici-
pants showed N400 effects of word-level association, regardless of
WM capacity, and of sentence congruence, when sentences con-
tained associations (Van Petten et al.,1997). This pattern of results
suggests that low WM capacity individuals are less sensitive than
high-capacity participants to sentence-level meaning.
COGNITIVE CONTROL ABILITY
Cognitive control ability may also play a crucial role in discourse
processing (Gernsbacher, 1990, 1997). It is important to note that
cognitive control is somewhat of a umbrella term, often used
to refer to any kind of controlled process, from the suppression
of irrelevant input through controlled maintenance of informa-
tion. In the current study, we focused on one specific aspect of
controlled processing: the ability to suppress or inhibit irrelevant
information. This is particularly important given that suppression
ability has been proposed to be related to individual variabil-
ity in discourse comprehension according to a prominent theory
of discourse comprehension, the Structure-Building Framework
(Gernsbacher, 1990, 1997;Gernsbacher and Faust, 1991). This
framework emphasizes the cognitive processes of mapping, which
involves incorporating incoming input into the current mental
representation, and shifting, which involves constructing a new
mental representation with the incoming input. Mapping occurs
when incoming input fits well with the current mental represen-
tation, whereas shifting occurs when the input is less coherent
(Gernsbacher, 1990). Successful construction of meaning repre-
sentations requires the enhancement of context-relevant informa-
tion, and the suppression of context-irrelevant information that
may be activated by incoming words and phrases. For example,
the “playing card” meaning of the word spade would be activated,
even though it is context-irrelevant, in a sentence involving the
“garden tool” meaning of spade. Ineffective suppression of irrel-
evant information may lead to excessive shifting, and therefore
to the creation of unnecessary sub-structures and disorganized
discourse representations (Gernsbacher, 1990, 1997;Gernsbacher
and Faust, 1991).
As noted in the “playing card” vs.“garden tool” example, words
that are related in meaning to the context-inappropriate sense
of ambiguous words (e.g., ace to spade in “He dug with the
spade”) represent one source of irrelevant information that is
activated during comprehension. Gernsbacher and colleagues pre-
sented sentences containing ambiguous words such as “spade” in
the example above, followed by probe words that were, in some
cases, related to the context-inappropriate meaning of the word
(ace), and asked to respond as to whether or not the probe word
was related to the sentence. Less skilled readers were found to be
slower than skilled readers at rejecting test words when they were
context-inappropriate (Gernsbacher and Faust, 1991; Experiment
4). This result provides evidence that skilled readers more effec-
tively suppress irrelevant information during comprehension,and
that low skilled readers are influenced to a greater extent by local
associations among word meanings.
Further evidence for the role of cognitive control in lan-
guage comprehension comes from several neuroimaging studies
that have found regions associated with domain-general cogni-
tive control functions, such as the dorso-lateral prefrontal cortex
(DLPFC) to be active under difficult sentence processing condi-
tions, such as complex or ambiguous sentence structures (January
et al., 2009; for a review, see Novick et al., 2005, 2010). The
DLPFC has also been implicated during the maintenance of simple
sentence contexts, with increased activation during maintenance
linked to context-appropriate responding in a word-completion
task (Kerns et al., 2004). Therefore, domain-general control, medi-
ated by the DLFPC, may be recruited even during simple sentence
comprehension in order maintain context information and guide
processing.
PREDICTIONS FOR THE CURRENT STUDY
The goal of this study was to use ERPs to investigate individual
variability in sensitivity to discourse-level congruence and lexical
association during spoken discourse comprehension. Discourse
congruence was manipulated by constructing three-sentence story
contexts in which the final, critical word was consistent with the
meaning of the preceding context (discourse congruent) or not
(discourse incongruent). For example, the critical word “fortune”
in the sentence “He was not prepared for the fame and fortune”
would be discourse congruent following a context in which a pres-
tigious prize is won, but discourse incongruent following a context
in which the protagonist had been arrested. In addition, the final
critical words were either associated to a preceding prime word
(“. . .fame and fortune) or unassociated (“. . .fame and praise). We
tested whether differences in WM capacity and cognitive control
contribute to differences in the use of global discourse context and
local meaning relations, using multiple regression to assess the
unique contribution of these measures on the amplitude of the
N400 congruence and association effects. This study is one of only
a handful of ERP studies that have applied a multiple regression
approach to investigate individual variability in ERP effects (see
Dambacher et al., 2006;Laszlo and Federmeier, 2011;Boudewyn
et al., 2012b, for other examples), and is the first to directly assess
relations among measures of WM and cognitive control, and the
real-time comprehension of spoken discourse. This is an impor-
tant goal, as previous studies have focused primarily on the reading
of single sentences. Although sentence processing studies are infor-
mative, and have suggested a link between WM and the ability
to maintain language context (e.g., Van Petten et al., 1997), as
well as between control (suppression ability), and the suppression
of irrelevant information during comprehension (Gernsbacher,
1990, 1997;Gernsbacher and Faust, 1991), there is good reason
to think that control and WM processes play a larger role dur-
ing the processing of discourse context. Discourse comprehension
places greater demands on language users than sentence compre-
hension, in that language users must integrate across sentences
in order to construct a message-level representation of the mean-
ing of a passage. In addition, the current representation must be
maintained in WM in order for incoming input to be integrated.
Finally, discourse context contains information at multiple levels,
as mentioned above, which may activate context-irrelevant words
or concepts, placing demands on suppression ability.
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Boudewyn et al. Working-memory and levels of context
It is important to note that cognitive control and WM may
not be separable constructs, particularly given that performance
on WM tasks has been linked to performance on cognitive control
tasks (see Engle, 2002 for a review). Therefore,in this study, we have
adopted the approach of (1) specifying which aspects of control
(suppression ability) and WM (maintenance) we wish to investi-
gate, and (2) adopting a multiple regression approach in order to
account for the unique contribution of each measure in predicting
individual ERP effects related to language processing. Importantly,
the tasks employed in the current study were designed to tap into
different aspects of controlled processing. Our control measure,
the Stroop task, is primarily a measure of suppression ability, as
participants are asked to respond to the font color of printed color
words (e.g., red for the words blue/red when printed in red). Trials
involving a mismatch between the color term and the font color
(e.g., blue in red) require that participants overcome conflict and
suppress the meaning of the word in order to respond appropri-
ately. In contrast, our WM task requires that participants listen to
sets of sentences, evaluate whether, or not each one is a true state-
ment, and keep the last word of each sentence in a set in mind to
be recalled after the set ends. In short, it is a dual-task paradigm,
involving memory but also the controlled allocation of attentional
resources and the maintenance of information over time. There-
fore, although there is likely to be some overlap between control
and WM, the control and WM tasks used in this study tap into
different aspects of controlled processing.
Given that previous work has linked WM capacity with sensi-
tivity to sentence- (Van Petten et al., 1997;Nakano et al., 2010)
and discourse context (Cantor and Engle,1993), we predicted that
WM span would significantly predict ERP effects of congruence.
Specifically, high WM span was expected to predict larger N400
effects of congruence (larger N400 deflection to incongruent tar-
get words than congruent target words). Likewise, if individuals
with lower WM spans are less able to maintain and use language
context, and are therefore less sensitive to discourse congruence,
then lower WM span individuals may have a smaller portion of
the preceding context active and available in WM than higher WM
span individuals. Critically, the associated prime words appear in
the local context immediately preceding the target words, leading
the local primes to occupy a larger, more prominent position in
the portion of context that is active for lower WM span individuals
than for higher WM span individuals. If this is the case, then lower
WM span individuals may prove to be more sensitive to the local
word-level context (associations; larger N400 deflection to unasso-
ciated target words than associated target words). In other words,
local word-level context may increase in salience as a function of
WM span, in that word-level context may be more influential for
lower-span individuals who are less able to maintain the larger,
discourse-level context. In short, N400 effects of discourse con-
gruence are expected to increase with higher WM span, whereas
N400 effects of lexical association are expected to decrease as a
function of higher WM span.
In contrast, we predicted that cognitive control (suppression)
ability, as measured by the Stroop task, would predict sensitiv-
ity to the presence of local lexical associations. As noted above,
suppression of irrelevant information is an important aspect of
language comprehension, and skilled readers have been shown to
be more effective at suppressing context-irrelevant lexical associa-
tions during sentence comprehension (Gernsbacher et al., 1990).
Given the task of listening to connected discourse for comprehen-
sion, the presence of lexically associated words has been shown
to be a weak source of information compared to discourse-level
context, resulting in smaller N400 effects (Camblin et al., 2007;
Boudewyn et al., 2012a). In this case, it may be important to sup-
press lexical associates during discourse comprehension. Indeed,
in a previous study,we found that high control participants showed
smaller N400 effects of lexical association for words in sentences
(Boudewyn et al., 2012a). Following from this,in the cur rent study,
we predicted that increased suppression ability would be associated
with smaller effects of association (larger N400 deflection to unas-
sociated target words than associated target words), and poten-
tially with larger effects of congruence (larger N400 deflection to
incongruent target words than congruent target words).
MATERIALS AND METHODS
PARTICIPANTS
Twenty-six undergraduates (17 female) from the University of
California, Davis gave informed consent before participating in
the study. All were right-handed, native speakers of English,
with no reported problems with hearing or reading or neuro-
logical/psychiatric disorders. They were compensated with either
course credit or at a rate of 10 dollars per hour. The mean age of
participants was 20.38 (range: 18–29; SD: 2.45).
METHODS: ERP SESSION
Stimuli
The stimuli included 72 experimental story sets, in which discourse
congruence (congruent or incongruent) and lexical association
(associated or unassociated) was orthogonally varied to produce
four conditions (see Table 1): congruent-associated; congruent-
unassociated; incongruent associated; incongruent unassociated.
These stimuli were adapted from those used in Camblin et al.
(2007) and are the same as those used in Boudewyn et al. (2012a).
The first two sentences of each passage established a discourse
context and the third (final) sentence contained the associated or
unassociated target word (see Table 2). Target words were either
congruent or incongruent with the discourse context. All except
22 of the prime and target pairs were separated by one interven-
ing word; the remaining stimuli had either two intervening words
(8), or three (14). Target words were always congruent with the
meaning of the final sentence. That is, target words made sense in
context if the sentence were to be heard in isolation.
For the association manipulation, associated prime-target word
pairs were selected using the Edinburgh Associative Thesaurus
(Kiss et al., 1973) as well as association pre-tests conducted as
part of a previous study (Camblin et al., 2007). Associated target
words had an average association strength of 39.8% (range: 20–
90%), while unassociated words pairs had an average association
strength of 0.2% (range: 0–4%). For the congruence manipula-
tion, previous pre-testing of the congruence of the stimuli showed
that incongruent stories were significantly rated as less congruent
than the stories in the congruent conditions on a five-point scale
(1 =completely congruent; 5 =completely incongruent). Aver-
age congruence ratings were: 1.44 (Congruent/Associated); 1.5
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Boudewyn et al. Working-memory and levels of context
Table 1 | Example stimulus sets showing each of the four conditions described in the text.
Condition Context Target sentence
Associated
congruent
Rick was unaware that his sister had submitted his poem in the prestigious contest.
He was shocked when he won the award and the hefty cash prize
He was not prepared for the fame
and FORTUNE
Unassociated
congruent
Rick was unaware that his sister had submitted his poem in the prestigious contest.
He was shocked when he won the award and the hefty cash prize
He was not prepared for the fame
and PRAISE
Associated
incongruent
Rick was mortified when the videotape of his arrest was shown on the news. After
the news show aired, he was ridiculed by the entire neighborhood
He was not prepared for the fame
and FORTUNE
Unassociated
incongruent
Rick was mortified when the videotape of his arrest was shown on the news. After
the news show aired, he was ridiculed by the entire neighborhood
He was not prepared for the fame
and PRAISE
Associated
congruent
Although he tried very hard, Ben’s cooking skills were pathetic at best. His latest
attempt at making marinara sauce was particularly bland and unappetizing
Luckily Ben had picked up some salt
and PEPPER
Unassociated
congruent
Although he tried very hard, Ben’s cooking skills were pathetic at best. His latest
attempt at making marinara sauce was particularly bland and unappetizing
Luckily Ben had picked up some salt
and BASIL
Associated
incongruent
Todd slipped on a large patch of ice near his front step. He wanted to be sure the ice
melted before anyone else took a fall
LuckilyTodd had picked up some
salt and PEPPER
Unassociated
incongruent
Todd slipped on a large patch of ice near his front step. He wanted to be sure the ice
melted before anyone else took a fall
LuckilyTodd had picked up some
salt and BASIL
For clarification, the primes are shown in italics and target words are capitalized; during the experiment these words were not specifically emphasized.
Table 2 | Results of discourse congruence and lexical association from
repeated measuresANOVA, including topographic factors of
Anteriority and Hemisphere.
300–500 ms F p
Congruence 7.56 *
Association 32.99 ***
Congruence ×association <1 ns
Congruence ×hemisphere <1 ns
Association ×hemisphere <1 ns
Congruence ×association ×hemisphere <1 ns
Congruence ×anteriority <1 ns
Association ×anteriority <1 ns
Congruence ×association ×anteriority 7.62 *
Congruence ×hemisphere ×anteriority 2.67 ns
Association ×hemisphere ×anteriority <1 ns
Congruence ×association ×hemisphere ×anteriority 3.72 ∧
Degrees of freedom for all F values are 1, 25. ***p <0.001, **p <0.01, *p <0.05,
∧p<0.07.
(Congruent/Unassociated); 4.31 (Incongruent/Associated); 4.32
(Incongruent/Unassociated). In order to match across conditions
for story coherence and semantic overlap, story sets were matched
across conditions using LSA values, which area measure of similar-
ity of meaning among words, sentences, or multi-sentence stories
(Landauer et al., 1998). LSA scores were computed using the “sen-
tence comparison”function (available at http://lsa.colorado.edu/),
and the “general reading up to first year college” semantic space
(300 factors). LSA values were obtained between sentence 1 and
sentence 2 of each passage, and between sentence 2 and sentence
3; the average was computed and served as the LSA score for that
item. Values ranged from 0.21 to 0.22 across conditions.
Overall story cloze probability was low (all items <33%);
and was below 10% in three of the conditions (Congru-
ent/Unassociated: 7%; Incongruent/Associated: 6%; Incongru-
ent/Unassociated: 3%). In the Congruent/Associated condition
the average was 29%. With respect to criteria used in previous
ERP and behavioral studies (e.g., Federmeier and Kutas), all of
these values constitute low cloze probability. However, in order
to confirm that cloze probability did not influence the results,
a cloze-matched subset of materials were tested in two previ-
ous studies, which showed that cloze probability did not change
or contribute to the pattern of results (Camblin et al., 2007;
Boudewyn et al., 2012a). The critical target words were matched
for frequency across conditions using Francis and Kucera (1982)
(Associated =83.71; Unassociated =83.9). Target word duration
did not differ between conditions (see below).
Story sets were divided into four lists and counterbalanced
such that the critical target words and the two sentences preced-
ing the final sentence were not repeated within-subjects. Each list
contained 144 experimental stories, 36 in each condition.An addi-
tional 40 filler stories were included; 20 were congruent and 20
ended with a word that was anomalous at both the discourse and
sentence-level.
Story sets and fillers were spoken by a female speaker, with
natural inflection and at a natural speaking rate. The words were
digitally recorded using a Schoeps MK2 microphone and Sound
Devices USBPre A/D (44,100 Hz, 16 bit). Speech onset and offset
of each word was determined by visual inspection of the speech
waveform and by listening to the words using speech editing
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Boudewyn et al. Working-memory and levels of context
software (Audacity, by Soundforge). Discourse context (first two
sentences of the passage) and the critical sentences were recorded
separately. The onset and offset of each critical word in all con-
ditions was determined by visual inspection of the speech wave-
form and by listening to the words using speech editing software
(Audacity, by Soundforge). The average duration of the stories
was 8951 ms (ranging from 7917 to 9359 ms), and the average
duration of the critical words was 568 ms (ranging from 293 to
861 ms). The duration of the critical words did not differ between
conditions (t<1). The duration between prime and target was
the same for the congruent/incongruent associated and for the
congruent/incongruent unassociated conditions because the same
final sentence was used for the two associated conditions and the
two unassociated conditions. This was accomplished by insert-
ing a 1-s silence between the speech files that contained the first
two context sentences and the speech files that contained the final
sentences using Presentation software1. There were no statistically
significant differences in duration of the onset of the prime to
the onset of the target between the associated and unassociated
conditions (p=0.43). The target sentences were the same in all
conditions up to the sentence-final target word.
Comprehension questions did not focus on the prime or the tar-
get but rather on the context of the discourse. The same true/false
question was asked in the congruent associated/unassociated
and incongruent associated/unassociated conditions. Half of the
questions required a true response, and half required a false
response.
Procedure
The ERP session always preceded the behavioral testing session.
ERP and behavioral testing sessions were always conducted on
consecutive days,w ith the exception of two participants, who com-
pleted the behavioral testing session as part of an unrelated study
4 months prior to their ERP session. Participants were informed
at the beginning of the ERP session that the experiment was
focused on language comprehension. During the ERP session,
participants were seated in a comfortable chair in an electrically
shielded, sound-attenuating booth. The stimuli were presented
through Beyer dynamic headphones using Presentation software.
The discourse trials began with a white fixation cross at the center
of the screen, approximately 100cm in front of the participants.
The fixation cross was present from 900ms before onset of the
stimuli and during presentation of the entire passage until the off-
set of the final word. The fixation cross was then replaced by a
visually presented comprehension question about the preceding
discourse, after 1000ms. Subjects were asked to make a true/false
response by pressing a yes or no button with the index fingers of
each hand. The comprehension question remained on the screen
until participants made a response.
Each experimental session began with a practice block consist-
ing of filler passages, after which two of the four counterbalanced
lists were presented in random order, each containing both exper-
imental trials and fillers in a pseudorandom order. Each list was
divided into four blocks for presentation purposes such that each
1The duration of this pause was set to 1 s in order to match the average of the
naturally produced pauses between sentences one and two.
participant listened to eight blocks of stimuli (two lists). The order
of blocks was counterbalanced.
Participants were asked to keep their eyes fixated on the white
fixation cross and to refrain from blinking or eye-movements as
long as it remained on the screen. This was done in order to min-
imize movement-related artifacts in the EEG signal. When the
fixation cross was replaced by the true/false question, participants
were instructed that they could blink and move their eyes freely
until they made a response. Condition-specific stimulus codes
were sent out at the onset of the critical words and these codes
were used for later off-line averaging of the EEG signal.
ERP recording and data reduction
EEG was recorded from 29 tin electrodes, mounted in an elastic
cap (ElectroCap International). The right mastoid electrode was
used as the recording reference (except for electrodes used to mea-
sure blinks and eye-movements: one electrode beneath the left
eye was referenced to FP1 and two placed at the outer canthi of
each eye were referenced to each other). The left mastoid was also
recorded for later off-line algebraic re-referencing. The EEG signal
was amplified with band pass cutoffs at 0.01 and 30 Hz, and digi-
tized online at a sampling rate of 250 Hz (Neuroscan Synamp I).
EEG was digitized continuously along with accompanying stim-
ulus codes used for subsequent averaging. Impedances were kept
below 5 kΩ.
Prior to off-line averaging, all single-trial waveforms were
screened for amplifier blocking, muscle artifacts, horizontal eye-
movements, and blinks over epochs of 900 ms, starting 100 ms
before the onset of the critical target words. Average ERPs were
computed over artifact-free trials in the related and unrelated con-
ditions. All ERPs were filtered off-line with a Gaussian low-pass
filter with a 25 Hz half-amplitude cutoff. Statistical analyses were
conducted on the filtered data.
METHODS: BEHAVIORAL BATTERY
Before beginning the behavioral battery session, participants were
told that the battery was composed of a series of psychological
tests designed to assess constructs such as memory, and that some
mistakes were to be expected. The instructions for each task were
given just prior to task presentation.
Listening span
The Listening Span task was adapted from Daneman and Carpen-
ter (1980), and consisted of 25 sets of sentences ranging from two
sentences per set to six; there were five sets of each set length. Par-
ticipants were instructed to listen to all sentences within each set
for comprehension, and then to indicate whether each sentence
was true or false immediately after hearing the whole sentence. In
addition, participants were instructed to remember the final words
of each sentence in the set, and were asked to recall them in any
order after the whole set was presented. There was a 1500 ms pause
in between each sentence during which participants made their
true/false response. Presentation of sets was random. Each cor-
rect response (correct recall of the final words) was scored as one
point, for a maximum of 100 points. This task, which was adapted
from its visual counterpart (Reading Span), predicts reading com-
prehension, and syntactic parsing abilities, particularly when used
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Boudewyn et al. Working-memory and levels of context
in conjunction with other tasks (Waters and Caplan, 2003). Fur-
ther, Nakano et al. (2010) showed that visual and auditory span
measures are highly correlated, and are also sensitive predictors of
individual differences in spoken language comprehension.
Modified Stroop
The Modified Stroop Task was adapted from that used in Van
Veen and Carter (2005), and consisted of visual presentation of
color words (blue, green, yellow, and red). Words were printed in
each of these colors, and participants were instructed to respond
to the color of the font and not the word itself; responses were
mapped such that a left button press indicated the word was in
red or yellow font color, and a right button press indicated that
the word was in blue or green font. Trials were either congruent
(e.g., “red” in red; 50%), semantically incongruent (SI) in that the
font color, and word were not the same but were mapped onto
the same finger (e.g., “red” in yellow; 25%), or response incongru-
ent (RI), in that the font color and word were not the same and
additionally were not mapped onto the same finger (e.g., “red”
in green; 25%). There were 124 total words presented in ran-
dom order per block, and three experimental blocks. Each word
appeared on the screen for 300 ms, separated by a 2700 ms inter-
stimulus interval. This version of the Stroop task was designed
to separate conflict at the representational level and conflict at
the response level by including SI trials in addition to RI and
congruent trials. RI trials are assumed to reflect both semantic
and response conflict, because they involve a mismatch between
(1) the font color and the color being named, and (2) the but-
ton response required for the font color and the button response
that is required for the color being named. An example of an
RI trial is the word “red” printed in green font, when red font
has been mapped to one button/finger and green font has been
mapped to another. In contrast, SI trials are assumed to reflect
only semantic conflict because the button response for both the
font color and the color being named is the same. An exam-
ple of an SI trial is “red” printed in yellow font, when both red
and yellow fonts are mapped to the same response button/finger
(De Houwer, 2003;Van Veen and Carter, 2005). In a previous
study, we showed that this task significantly predicts the size of
N400 association effects in spoken sentences (Boudewyn et al.,
2012a).
RESULTS: BEHAVIORAL DATA
Comprehension questions
On average, participants responded with 92.7% accuracy (SD:
4.4; range: 81.2–99.3%) on the true/false comprehension ques-
tions that followed each story. Accuracy did not differ among
experimental conditions, with participants answering 93.5,
93, 92, and 92.5% correctly for the congruent/associated,
congruent/unassociated, incongruent/associated, and incongru-
ent/unassociated conditions, respectively.
Listening span
Listening Span was calculated as the total number of correctly
recalled items (out of a total of 100). The average score was 63.65
(range: 42–79; SD =10.18).
Modified Stroop
Mean RTs were 644.21, 651.82, and 697.75 ms for congruent (C),
SI, and RI words, respectively. Planned contrasts revealed no
significant difference between the C and SI conditions (p=0.1),
but there was a significant difference between the C and RI condi-
tions (p=0.0001). Mean accuracy was 95.7% (C), 95% (SI), and
92.7% (RI); the difference in accuracy between the C and RI con-
ditions was significant (p=0.001). Planned contrasts revealed no
significant difference in accuracy between the C and SI conditions
(p=0.14), but there was a significant difference between the C and
RI conditions (p=0.01). For the purposes of the regression and
correlation analyses reported below, a single measure of Stroop
Interference was created from the Stroop RT scores. Specifically,
for each subject, the average difference between RI and C trials
was calculated, following from previous work using this task (Van
Veen and Carter, 2005;Boudewyn et al., 2012a).
Correlations between behavioral measures
Listening Span performance and Stroop Interference scores were
not significantly correlated (p=0.654).
RESULTS: ERP DATA
The grand averages across all participants showed effects of dis-
course congruence and association, such that the N400 waveform
to congruent words was reduced compared to incongruent words,
and the N400 waveform to associated words was reduced com-
pared to unassociated words (see Figure 1). A repeated measures
ANOVA was conducted in the 300–500ms N400 epoch with the
factors Congruence (Congruent, Incongruent), and Association
(Associated, Unassociated), and the topographic factors of Anteri-
ority (Anterior, Posterior) and Hemisphere (Left, Right), thereby
creating four quadrants of electrodes: Left/Frontal (F3, F7, FC1,
FC5). Right/Frontal (F4, F8, FC2, FC6), Left/Posterior (CP1, CP5,
P3, T5), and Right/Posterior (CP2, CP6, P4, T6) as within-subjects
factors. This configuration was chosen because we were inter-
ested in potential differences in the topographic distribution of
effects of WM and Control on our manipulations of congruence
and association. If participants recruit more WM and/or con-
trol processes to maintain contextually relevant information, than
these effects could have a more frontal distribution, as frontal
ERP effects have often been linked to controlled maintenance
(e.g., Kiss et al., 2007). In contrast, if they are heavily influenced
by word-level meaning relations among single words, then these
effects could have a more typical, posterior N400 distribution (see
Swaab et al., 2011, for a review). The Greenhouse–Geisser cor-
rection was applied to all Ftests with more than one degree of
freedom in the numerator. These analyses showed a main effect
of Congruence [F(1,25) =7.56; p<0.05], a main effect of Asso-
ciation [F(1,25) =32.99; p<0.001]. In addition, there was a sig-
nificant interaction of Congruence by Association by Anteriority
[F(1,25) =7.62; p<0.05], and a marginal interaction of Congru-
ence by Association by Hemisphere by Anteriority [F(1,25) =3.72;
p=0.0653]. These results are reported in Table 2.
In order to follow-up on the interactions of Congruence
and Association with topographic factors, additional repeated
measures ANOVAs were conducted for each electrode quad-
rant, separately (Factors: Congruence (Congruent, Incongruent),
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Boudewyn et al. Working-memory and levels of context
FIGURE 1 | Main effects of congruence (top) and association (bottom) shown for electrodes in all four quadrants: Left/Frontal (F3, F7, FC1, FC5),
Right/Frontal (F4, F8, FC2, FC6), Left/Posterior (CP1, CP5, P3,T5), and Right/Posterior (CP2, CP6, P4,T6).
Association (Associated, Unassociated), and Electrode Site (four
sites) as within-subjects factors. Significant main effects of congru-
ence were found for the Right/Frontal and Right/Posterior quad-
rants; the effect of congruence was marginal in the Left/Frontal
(p=0.0591) quadrant, and did not reach significance in the
Left/Posterior quadrant [but did significantly interact with Elec-
trode (p<0.05) in this quadrant]. In contrast, main effects of
Association and interactions of Association by Electrode were
found across all four quadrants (p<0.001). In addition, there
was a trend toward a Congruence by Association interaction
(p=0.088) in the Right/Frontal quadrant. Results for these
analyses are reported in Table 3.
RESULTS: REGRESSION ANALYSIS
Dependent measures
We estimated the size of individual N400 effects by cal-
culating the mean amplitude of the effects of Congruence
(Incongruent – Congruent) and Association (Unassociated –
Associated) for each individual in the typical N400 time window
(300–500 ms).
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Boudewyn et al. Working-memory and levels of context
Table 3 | Results of discourse congruence and lexical association from
repeated measuresANOVA, for each quadrant separately.
300–500 ms Df F p
LEFT/FRONTAL
Congruence 1.25 3.91 ∧
Association 1.25 27.1 ***
Congruence ×association 1.25 <1 ns
Congruence ×electrode 3.75 <1 ns
Association ×electrode 3.75 13.43 ***
Congruence ×association ×electrode 3.75 <1 ns
RIGHT/FRONTAL
Congruence 1.25 4.79 *
Association 1.25 30.71 ***
Congruence ×association 1.25 3.15 ns
Congruence ×electrode 3.75 <1 ns
Association ×electrode 3.75 10.09 ***
Congruence ×association ×electrode 3.75 <1 ns
LEFT/POSTERIOR
Congruence 1.25 4.04 ∧
Association 1.25 25.58 ***
Congruence ×association 1.25 <1 ns
Congruence ×electrode 3.75 5.12 *
Association ×electrode 3.75 26.9 ***
Congruence ×association ×electrode 3.75 <1 ns
RIGHT/POSTERIOR
Congruence 1.25 8.35 **
Association 1.25 26.9 ***
Congruence ×association 1.25 <1 ns
Congruence ×electrode 3.75 4.46 *
Association ×electrode 3.75 15.48 ***
Congruence ×association ×electrode 3.75 <1 ns
***p <0.001 **p <0.01 *p <0.05 ∧p<0.07.
Correlation analysis. N400 effect estimates were computed for
each individual as described above for each quadrant of electrodes
tested in the previous ANOVAs (Right/Frontal, Left/Frontal,
Right/Posterior, Left/Posterior). These quadrants were chosen
because the ANOVA above revealed differences in the topographic
distribution of the effects of congruence and association. The two
posterior quadrants include electrode sites for which the N400 is
typically maximal (see Swaab et al., 2011 for a review). The results
of correlation tests revealed significant correlations between (1)
Listening Span and the effect of congruence in the Right/Frontal
quadrant, and (2) Listening Span and the effects of congruence
and association in the Left/Posterior quadrant. Additionally, the
effect of congruence was correlated with the effect of association
in the Left/Posterior quadrant, such that larger effects of congru-
ence were linked to larger effects of association. These results are
reported in Table 4.
Multiple regression. Listening Span and Stroop Interference were
entered into a multiple regression analysis as predictors of N400
effects of congruence and association. This approach allowed us to
assess the unique contribution of these two measures in predicting
the size of the N400 effects. Stroop Interference did not emerge as
Table 4 | Results of the correlation analyses.
Variable 1 2 3 4
RIGHT/FRONTAL QUADRANT
1. Congruence effect
2. Association effect 0.055
3. Stroop interference 0.037 0.099
4. Listening span −0.486* 0.001 0.094
LEFT/FRONTAL QUADRANT
1. Congruence effect
2. Association effect 0.223
3. Stroop interference −0.125 0.05
4. Listening span −0.283 −0.044 0.094
RIGHT/POSTERIOR QUADRANT
1. Congruence effect
2. Association effect −0.225
3. Stroop interference 0.262 0.035
4. Listening span −0.032 0.159 0.094
LEFT/POSTERIOR QUADRANT
1. Congruence effect
2. Association effect 0.455*
3. Stroop interference −0.132 0.131
4. Listening span 0.516** 0.622** 0.094
Correlations among ERP effects of congruence and association and behavioral
measures of Stroop Interference and Listening Span were tested for each quad-
rant, separately.
*p < 0.05, **p < 0.01
a significant predictor of any ERP effects tested in any quadrant.
In contrast, Listening Span significantly predicted (1) the effect of
congruence in the Right/Frontal [r=0.493 (r2=0.243; p<0.05)]
and Left/Posterior quadrants [r=0.547 (r2=0.299; p<0.01)],
and (2) the effect of association in the Left/Posterior quadrants
[r=0.626 (r2=0.392; p<0.001; see Tables 5 and 6)]. Figures 2
and 3depict the relations between effects of Listening Span and
ERP effects of Discourse Congruence and Association. For illustra-
tive purposes only, participants were divided into High Listening
Span (n=12) and Low Listening Span (n=12) groups, exclud-
ing participants at the median in order to display differences in
topographic distribution as a function of Span.
DISCUSSION
The goal of this study was to investigate if and how individual
differences in cognitive control and WM span affect the pro-
cessing of discourse-level congruence and word-level meaning
relations. A multiple regression approach was used in order to
assess the unique influence of these measures on the amplitude of
individual N400 effects of congruence and association. Across all
participants, significant effects of lexical association and discourse
congruence were observed. However, the effect of congruence was
marginal in the Left/Frontal quadrant, and was only significant
at some sites in the Left/Posterior quadrant. This was surpris-
ing, given that discourse congruence effects have previously been
found to be robust in comparison to effects of lexical associ-
ation in this paradigm, as noted in the introduction (Camblin
et al., 2007;Boudewyn et al., 2012a). This difference in the relative
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Boudewyn et al. Working-memory and levels of context
Table 5 | Multiple regression analysis of discourse congruence effect
size (two predictors) showing unstandardized (b) and standardized
(β) partial coefficients, and probability levels (p).
Predictor bβp
LEFT/FRONTAL
Constant 1.718
Listening span −0.03 −0.274 0.194
Stroop interference −0.002 −0.099 0.632
RIGHT/FRONTAL
Constant 3.055
Listening span −0.053 −0.494 0.015*
Stroop interference 0.002 0.083 0.66
LEFT/POSTERIOR
Constant −3.236
Listening span 0.042 0.533 0.007**
Stroop interference −0.003 −0.183 0.319
RIGHT/POSTERIOR
Constant −0.776
Listening span −0.007 −0.057 0.785
Stroop interference 0.006 0.268 0.208
Table 6 | Multiple regression analysis of lexical association effect size
(two predictors) showing unstandardized (b) and standardized (β)
partial coefficients, and probability levels (p).
Predictor bβp
LEFT/FRONTAL
Constant −0.935
Listening span −0.006 −0.049 0.819
Stroop interference 0.001 0.055 0.8
RIGHT/FRONTAL
Constant −1.285
Listening span −0.001 −0.009 0.967
Stroop interference 0.002 0.1 0.645
LEFT/POSTERIOR
Constant −6.49
Listening span 0.077 0.615 0.001***
Stroop interference 0.002 0.073 0.666
RIGHT/POSTERIOR
Constant −2.899
Listening span 0.02 0.157 0.466
Stroop interference 0 0.02 0.925
size of the effects underscores the variability amongst individu-
als in sensitivity to different aspects of language context. Indeed,
as the regression analysis revealed, the different topographic dis-
tribution of the effects of congruence varied as a function of
Listening Span. These topographic differences could have con-
tributed to the reduced size of the congruence effect in this study
compared to in previous work. In addition, Listening Span, but
not Stroop Interference, significantly predicted the size of the
N400 effects of association in the Left/Posterior quadrant, with
higher-span participants showing smaller effects of association
than lower-span participants. Interestingly, Listening Span also
predicted the size of the N400 effect of discourse congruence, but
the direction of the effect varied by topographic distribution: in
the Left/Posterior quadrant, higher-span participants had smaller
effects of congruence than did lower-span participants, while in
the Right/Frontal quadrant, higher-span participants had larger
effects of congruence. This pattern is depicted in Figure 3, and sug-
gests that there is a difference in the neural substrates recruited by
high compared to low listening span individuals during discourse
processing.
The links between Listening Span and the ERP effects of con-
gruence and association were consistent with our predictions,
suggesting that higher WM span individuals are less sensitive to
the presence of word-level associations within discourse context
than lower WM span individuals. Local, word-level context may
be more influential, or salient, for individuals with lower WM
spans, potentially because lower WM span participants may have
less of the discourse context actively maintained and available in
WM. If so, then it would follow that the local context immediately
preceding the critical target words (i.e., the associated primes)
would feature prominently in the portion of context that is active.
The pattern of results for the discourse congruence manipula-
tion was complex: higher WM span was not invariably linked
with larger ERP effects of congruence, but rather was linked with
right frontal ERP effects of congruence. In contrast, lower WM
span was predictive of left posterior congruence effects. Previous
work has also suggested a link between WM span and the use of
discourse context (Cantor and Engle, 1993), as well as sentence
context (Van Petten et al., 1997;Nakano et al., 2010). Specifically,
as mentioned in the introduction, behavioral work has shown that
higher WM individuals may be better able than lower-capacity
individuals to construct coherent discourse representations (Can-
tor and Engle, 1993). In addition, ERP work has indicated that high
WM individuals are more sensitive to sentence-level congruence
than low WM individuals (Van Petten et al., 1997). The results of
the current study are novel in showing not only that N400 effects
of discourse congruence are linked to WM span, but that WM
span predicts topographic differences in the distribution of the
effects.
It is interesting that the topography of the N400 effects of
congruence, but not of association, varied with listening span.
As discussed above, recently encountered prime words may be
more salient to low-span individuals than to high-span individu-
als, resulting in the link between low-span and larger association
effects over more classic, central posterior electrode sites. In con-
trast, congruence effects place greater demands on maintenance
and integration over time, and the non-canonical, frontal dis-
tribution of this effect for the high listening span individuals is
particularly interesting given that frontal negativities in this time
window have often been associated with memory manipulations,
especially those associated with familiarity (Rugg and Curran,
2007; but see Voss and Federmeier, 2011) and controlled main-
tenance of verbal WM (Kiss et al., 2007). Although we cannot
infer the underlying neural generators from the scalp topogra-
phy of ERP effects, differences in scalp distribution do indicate
differences in the configuration and/or number of neural gener-
ators producing the effect. Studies using intracranial recordings
have implicated the left, and possibly the right, temporal lobes as
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Boudewyn et al. Working-memory and levels of context
FIGURE 2 | Correlations of listening span and congruence. At top are
topographic maps showing the scalp distribution of the N400 effect of
Discourse Congruence (Incongruent – Congruent) for High-Span participants
(left) and Low-Span participants (right). Correlations are shown at bottom. It is
important to note that larger N400 effects are reflected by more negative
values.
likely generators of the scalp-recorded N400 effect (Nobre et al.,
1994;Nobre and McCarthy, 1995). However, other studies have
suggested a possible generator in left inferior frontal cortex (i.e.,
DLPFC), using fMRI and MEG (Halgren et al., 2002;Hagoort
et al., 2004). Therefore, it is an interesting possibility that the
more frontal N400 effect of discourse congruence than was seen
in the higher listening span participants in the current study is
driven by the recruitment or increased engagement of prefrontal
cortex during controlled maintenance and use of discourse-level
context.
Frontiers in Psychology | Language Sciences February 2013 | Volume 4 | Article 60 | 12

Boudewyn et al. Working-memory and levels of context
FIGURE 3 | Correlations of listening span and association. At top are
topographic maps showing the scalp distribution of the N400 effect of
Lexical Association (Unassociated – Associated) for High-Span participants
(left) and Low-Span participants (right). Correlations are shown at bottom.
It is important to note that larger N400 effects are reflected by more
negative values.
It is also interesting that the higher-span participants showed
larger effects of discourse congruence over right frontal electrode
sites than did lower-span participants. Again, we cannot conclude
based on the ERP data in this study alone that this was caused
by a right hemisphere (RH) generator. However, there is a rich
literature that implicates the RH in discourse processing (e.g., St.
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Boudewyn et al. Working-memory and levels of context
George et al., 1999;Xu et al., 2005; see also Johns et al., 2008, for a
review). Specifically, there is evidence for increased engagement of
the RH during comprehension as complexity of the language input
increases, with RH activation being maximal at the discourse-level
of processing (e.g., Xu et al., 2005). It is possible that, in the current
study, increased sensitivity to the discourse-level meaning rep-
resentation for higher WM capacity individuals led to increased
RH involvement, which could have contributed to the relation
among WM span, discourse congruence effects, and topographic
distribution that we observed.
The lack of a link between our measure of control, Stroop
Interference, and the ERP effects of either discourse congruence
or lexical association was unexpected. We predicted that Stroop
Interference would be predictive of increased lexical association
effects, which we have previously found to be the case during
the comprehension of short sentences (Boudewyn et al., 2012a).
In Boudewyn et al., participants listened to simple spoken sen-
tences that averaged only 11 words per sentence, including the
prime and target pairs. Therefore, the context prior to the crit-
ical words was minimal and unlikely to tax WM; indeed, WM
did not predict the N400 effect of lexical association in that study.
Instead, Stroop Interference significantly predicted the N400 effect
of lexical association, with higher-control individuals showing
smaller association effects than lower-control individuals. This
was interpreted as evidence that higher-control (better suppres-
sion ability) individuals were more effective than lower-control
individuals at suppressing the word-level meaning relation,which
represents information that is somewhat irrelevant in a sentence
processing context (Boudewyn et al., 2012a). Previous studies of
word priming effects have also suggested that small or absent
priming effects may be attributed to suppression of the seman-
tic relation rather than to an absence of semantic activation
(Mari-Beffa et al., 2000), and that lexical associates of words in
sentence contexts may be activated but then inhibited, when the
task demands are to understand the sentence as a whole (Nor-
ris et al., 2006). In the current study, however, Stroop Interfer-
ence was not significantly related to the effects of local lexical
association.
In our view, there are at least two possible interpretations of
this result. The first is that, when associated prime-target pairs
that are embedded in sentences are additionally embedded within
discourse context, sensitivity to the local meaning association is
primarily determined by WM rather than by control/suppression
ability. There is some reason to think that this may be the case: as
discussed previously, cognitive control as measured by the Stroop
task in the current study primarily taps into suppression abil-
ity and conflict resolution, rather than controlled maintenance
of information over time. In contrast, our Listening Span task
does require the maintenance of context information over time.
Therefore, when a larger context is available to the language user,
it may be the case that Listening Span accounts for individual
differences in sensitivity to both discourse congruence and lex-
ical association better than Stroop Interference. In other words,
although sensitivity to the presence of word associations may be
largely determined by suppression ability when there is a minimal
maintenance load (simple sentence context), the added mainte-
nance demands of a larger, more complex discourse context may
result in context representations that vary in the prominence of
the prime word, as a function of WM span. Individuals with high
WM span are likely to maintain representations in which a greater
amount of context information is active and available; thus, the
local prime words would feature less prominently. Individuals with
low WM span may have less of the preceding context available in
memory, leading the local prime words to occupy a more promi-
nent and salient position in memory when the target word is then
encountered. Under these circumstances, WM span may trump
suppression ability as the dominating factor influencing sensitiv-
ity to local associations during the processing of the incoming
target words.
We cannot completely rule out the possibility that suppres-
sion ability does play a role in determining the size of individual
N400 effects of lexical association, even for words in discourse
context, but that it simply failed to emerge as significant in the
current study. This could be a result of insufficient power, but
we do not think this to be the case because there was sufficient
power to detect significant relations among Listening Span and
our ERP effects of interest. Another possible explanation is that
participant strategy influenced our measure of Stroop Interfer-
ence: on average, participants in the current study had slower
reaction times on all trial types in the Stroop Interference task than
those reported by Boudewyn et al., 2012a[644.21 and 697.75 ms
for congruent (C) and RI words in the current study, com-
pared to 604.95 and 677.68 ms in Boudewyn et al., 2012a]. It is
possible that participants in the current study adopted a more
conservative strategy on this task, taking longer to respond and
potentially making this measure less sensitive to raw suppres-
sion ability, as a result. It is important to keep in mind, however,
that the lack of significant relations among Stroop Interference
and ERP effects of lexical association and discourse congruence
does not imply that control processes, and specifically suppres-
sion ability,are unimportant in discourse comprehension. Instead,
it suggests that the maintenance of context information over
time may be the aspect of controlled processing that best deter-
mines sensitivity to multiple levels of meaning during discourse
comprehension.
CONCLUSION
The results of this study show that listeners vary in sensi-
tivity to local lexical associations embedded within discourse
context as a function of WM span. High-span individuals are
less sensitive to the presence of associatively related prime
words in context. WM span was also related to differences
in the topographic distribution of effects of discourse congru-
ence: N400 effects of discourse congruence were maximal for
high-span participants at right frontal electrode sites, whereas
congruence effects were maximal for low-span participants at
left posterior electrode sites. This topographic difference in
the distribution of the effects can only be explained by dif-
ferences in the configuration of the underlying neural gen-
erators of the N400 effects of congruence, as a function of
WM capacity. This pattern of results suggests additional, or at
least distinct, processing on the part of higher-span individu-
als when integrating incoming input with the current discourse
representation.
Frontiers in Psychology | Language Sciences February 2013 | Volume 4 | Article 60 | 14

Boudewyn et al. Working-memory and levels of context
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Conflict of Interest Statement: The
authors declare that the research was
conducted in the absence of any
commercial or financial relationships
that could be construed as a potential
conflict of interest.
Received: 17 September 2012; accepted:
28 January 2013; published online: 13
February 2013.
Citation: Boudewyn MA, Long DL and
Swaab TY (2013) Effects of working
memory span on processing of lexical
associations and congruence in spoken
discourse. Front. Psychology 4:60. doi:
10.3389/fpsyg.2013.00060
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