www.elsevier.com/locate/ynimg
NeuroImage 34 (2007) 1270 – 1279
Semantic ambiguity processing in sentence context: Evidence from
event-related fMRI
Monika-Zita Zempleni, a,b,c,⁎ Remco Renken, b,c John C.J. Hoeks, a,b,c
Johannes M. Hoogduin, b,c and Laurie A. Stowe a,b,c
a
Department of General Linguistics, University of Groningen, The Netherlands
School of Behavioral and Cognitive Neurosciences, University of Groningen, The Netherlands
c
Neuroimaging Center, University of Groningen, The Netherlands
b
Received 6 May 2005; revised 14 September 2006; accepted 21 September 2006
Available online 4 December 2006
Lexical semantic ambiguity is the phenomenon when a word has
multiple meanings (e.g. ‘bank’). The aim of this event-related
functional MRI study was to identify those brain areas, which are
involved in contextually driven ambiguity resolution. Ambiguous
words were selected which have a most frequent, dominant, and less
frequent, subordinate meaning. These words were presented in two
types of sentences: (1) a sentence congruent with the dominant
interpretation and (2) a sentence congruent with the subordinate
interpretation. Sentences without ambiguous words served as a control
condition. The ambiguous words always occurred early in the
sentences and were biased towards one particular meaning by the
final word(s) of the sentence; the event at the end of the sentences was
modeled. The results indicate that a bilaterally distributed network
supports semantic ambiguity comprehension: left (BA 45/44) and right
(BA 47) inferior frontal gyri and left (BA 20/37) and right inferior/
middle temporal gyri (BA 20). The pattern of activation is most
consistent with a scenario in which initially a frequency-based
probabilistic choice is made between the alternative meanings, and
the meaning is updated when this interpretation does not fit into the
final disambiguating context. The neural pattern is consistent with the
results of other neuroimaging experiments which manipulated various
aspects of integrative and context processing task demands. The
presence of a bilateral network is also in line with the lesion and
divided visual field literature, but contrary to earlier claims, the two
hemispheres appear to play similar roles during semantic ambiguity
resolution.
© 2006 Elsevier Inc. All rights reserved.
Keywords: Functional MRI; Left hemisphere; Right hemisphere; Frontal
lobe; Temporal lobe; Language; Lexical semantic ambiguity; Homograph;
Disambiguation; Context; Integration
⁎ Corresponding author. Institute of Neuroradiology, University Hospital
of Zurich, Fraueklinikstrasse 10, 8091 Zurich, Switzerland.
E-mail address: monika.zempleni@usz.ch (M.-Z. Zempleni).
Available online on ScienceDirect (www.sciencedirect.com).
1053-8119/$ - see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.neuroimage.2006.09.048
Introduction
In natural language, many words have multiple meanings, for
instance, the English word ‘bank’ means a monetary institute as
well as ground next to a river. This phenomenon is referred to as
lexical semantic ambiguity. Words with alternative meanings which
are spelled identically are also called homographs. Although native
speakers encounter ambiguous words very frequently, they usually
do not perceive them as ambiguous, unless ambiguity is
emphasized as in puns for example. The lack of detection is
presumably due to fast and automatic disambiguation processes,
i.e. ambiguity resolution, under which one particular meaning of
the ambiguous word is chosen. The literature suggests that there
are two sources of information on which this choice is based. The
first is the context into which the interpretation has to fit; the
second is frequency-based meaning dominance, i.e. the relative
frequency of the alternative meanings. Depending on whether one
meaning is much more frequent than the alternatives or the
alternative meanings are equally frequent, an ambiguous word can
be unbalanced or balanced in frequency.
Various models of ambiguity resolution differ in the importance
they assign to context and frequency-based dominance during
ambiguity resolution. (For reviews on models and theories, see e.g.
Gorfein, 2001 or Coney and Evans, 2000.) Hereby we will focus
on the exhaustive access model proposed by Onifer and Swinney
(1981), which has received the most empirical support and which
provided the theoretical framework for the paradigm of the current
study. According to the exhaustive access model, all meanings
become active initially, regardless of frequency-based dominance.
This view is supported by numerous semantic priming studies (e.g.
Swaab et al., 2003; Swinney and Love, 2001). Semantic priming
refers to the phenomenon where a word meaning is more easily
accessed if the meaning of a semantically related word, i.e. prime,
has already been activated; this can be reflected in e.g. shorter
response times. Exhaustive meaning access is evinced by the fact
that reading an ambiguous word (e.g. ‘bank’) can facilitate
comprehension of words related to both alternative meanings
M.-Z. Zempleni et al. / NeuroImage 34 (2007) 1270–1279
(e.g. ‘money’ and ‘river’) irrespective of context. That is, both
context incongruent and context congruent meanings can cause
priming effects (at least for a couple of hundreds of milliseconds;
see studies below). Following exhaustive access of several
meanings, frequency-based dominance and contextual congruity
lead to a resolution to the appropriate meaning.
The goal of the current experiment is to examine the neural
bases of ambiguity resolution. Earlier studies suggest that the
neural substrate of ambiguity resolution might be of particular
interest for a neuroimaging experiment as both hemispheres are
suggested to be involved in this cognitive process. Burgess and
Simpson (1988) evaluated the effects of frequency-based meaning
dominance and the time course of disambiguation in a divided
visual field experiment. They presented homographs centrally
without context as primes followed by a target word presented
laterally; the targets thus were initially available only to one
hemisphere. The results indicated that both meanings were able to
facilitate the recognition of targets immediately (35 ms) in the left
hemisphere, but only the dominant meaning continued to elicit
priming at a longer delay (750 ms). In the right hemisphere, only
the dominant meaning led to an immediate priming effect, but both
meanings facilitated the targets at the long delay. The authors
concluded that the left hemisphere quickly accesses both meanings
and soon focuses on the dominant one, while in the right
hemisphere, access might be slower, particularly for the subordinate meaning, and that the right hemisphere either does not
choose between the alternatives or does so more slowly. Faust and
Chiarello (1998) also used the divided visual field paradigm to
investigate hemispheric differences in the use of context during
ambiguity resolution. They found that the left hemisphere is able to
select the contextually appropriate word meaning within a short
period (900 ms), as opposed to the right hemisphere. The authors
suggested that the right hemisphere may maintain both meanings,
which might be useful when revision of the initially selected
interpretation is necessary.
The divided visual field technique supports hemispheric
specialization, but gives no information about the brain regions
which are involved. The few lesion studies available suggest that
Broca’s area might be necessary for the quick access of multiple
meanings and that damage to this area at least slows ambiguity
resolution (Swaab et al., 1998; Swinney et al., 1989). Left nonthalamic subcortical and cortical (predominantly fronto-parietal
and fronto-temporal) damage apparently disrupts context-driven
processes during semantic ambiguity resolution (Copland et al.,
2000, 2002). Damage to either anterior temporal lobe (Zaidel et al.,
1995) or to various areas in the right hemisphere (e.g. basal
ganglia, fronto–temporo–parietal region) (Grindrod and Baum,
2005) has also been shown to cause deficits in lexical semantic
disambiguation.
Relatively few neuroimaging experiments have yet addressed
the neural substrates of ambiguity resolution. Copland et al. (2003)
studied the neural substrate of semantic priming using f MRI with a
test material in which the primes were ambiguous words. The
target words were either related to the dominant or the subordinate
meaning of the homograph. They compared the related conditions
to an unrelated target condition. They found less activation, due to
priming, in both related target conditions in the left middle
temporal gyrus and in the left inferior prefrontal cortex, which
suggests priming effects from both meanings, which provides
neuroimaging support for the exhaustive access view. However,
Copland et al. (2003) did not include an unambiguous condition to
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explicitly investigate the effect of ambiguity per se. Chan et al.
(2004) added unambiguous words to their study and compared
them to ambiguous words in a semantic relatedness generation
task. Ambiguous words elicited increased activation predominantly
in bilateral middle and superior frontal gyri and in the anterior
cingulate, whereas unambiguous words elicited predominantly
bilateral inferior frontal and temporal increases in activation.
Ambiguity resolution in sentential context was studied by Rodd et
al. (2005) using f MRI and by Stowe et al. (2005) using PET. Rodd
et al. (2005) found left posterior inferior temporal cortex and
bilateral inferior frontal gyri activation, whereas Stowe et al. (2005)
found only right inferior frontal activation extending into right
temporal pole during ambiguity resolution. The results of these
neuroimaging studies only partially overlap; due to the different
designs, however, it is likely that they addressed different aspects
of ambiguity resolution. We will come back to these results in more
detail in the Discussion section in comparison to the results of the
current study.
The main goal of the current study was to further clarify the
neural substrates of homograph comprehension using event-related
functional MRI. In the current paradigm, unbalanced homographs
were presented in a sentential context. The ambiguous words
always occurred at the beginning of sentences, and care was taken
that the words preceding the homograph did not provide context
biasing toward either meaning (i.e. neutral). The homograph was
followed by several additional words of neutral context. The final
word(s) of the sentence provided the biasing context, supporting
either the most frequent, dominant, or a less frequent, subordinate
interpretation of the homograph. Thus, there were sentences with a
dominant meaning congruent final context, hence dominant
sentences (D), and sentences with a subordinate meaning
congruent context, hence subordinate sentences (S). Matched
sentences without ambiguous words served as a control condition
(C). See Table 1, D, S, C conditions. Therefore, the novelty of the
current design compared to the earlier studies is that meaning
dominance is explicitly manipulated.
As said above, the theoretical framework in which the paradigm
was created is the exhaustive access model (Onifer and Swinney,
1981), which proposes that multiple word meanings are accessed
initially and kept active until a choice can be made between the
alternatives. Although this model proposes that multiple meanings
can be accessed even in the presence of preceding biasing context,
we additionally facilitated exhaustive access in the current
paradigm by the sentential structure: the homographs occurred
Table 1
Examples of the three experimental conditions with literal English
translations
Dominant sentence (D)
Subordinate sentence (S)
Control sentence (C)
De advocaat werd ∣ door het oudje ∣ keurig
benaderd.
The lawyer was/by the old-person/nicely
approached.
De advocaat werd ∣ door het oudje ∣ keurig
ingeschonken.
The egg liqueur was/by the old-person/nicely
poured-out.
Het gras werd ∣ door de tuinman ∣ nooit
gemaaid.
The grass was/by the gardener/never mowed.
Note. The homograph is underlined; the biasing context is typeset in bold;
vertical lines indicate presentational phrase borders.
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early in the sentences, without any preceding biasing context. What
we intended to test as the secondary aim of the current study was
whether the meaning choice is exclusively driven by biasing
context, which in this experiment occurs downstream toward the
end of the sentence, or whether in the case of the unbalanced
homographs used in this experiment an earlier choice can be
carried out, based on frequency-based meaning dominance, in
which case the context must ‘overwrite’ this initially favored
meaning if necessary.
The first scenario we considered is that, at the point of
disambiguation, both meanings are equally available. If this is the
case, the ambiguous conditions are expected to be more difficult
than the unambiguous sentences, but the two ambiguous conditions
are not expected to differ from each other because a choice has to
be made between alternatives in both cases. In brain areas which
support this process, we expect to see a pattern of activation in
which S = D > C. Hence, this pattern will be interpreted as reflecting
selection between equally available alternatives (selection).
The second hypothesis considered here is that, even though the
initial meaning access is exhaustive, still the alternative word
meanings are not kept equally active for an unlimited time. Instead,
an initial choice is made between the different interpretations on
the basis of frequency-based meaning dominance. If this favored
interpretation does not fit into the final disambiguating context, an
alternative meaning must be reactivated and integrated to the
context. If the homographs were 100% unbalanced, we might
expect a pattern of activation in which S > D = C because the
dominant meaning would always be chosen initially and the
meaning update would only be necessary for the subordinate
condition. However, the homographs used were not unbalanced to
this degree (see Materials section below), so it is likely that some
subjects initially favor the subordinate meaning in some sentences
at least. The sum of these two processes would therefore lead to a
pattern in which S > D > C. Thus, the S > D = C or S > D > C patterns
are both consistent with a scenario of an initial frequency-based
choice followed by updating the word meaning and reintegrating
the alternative with the disambiguating context. These patterns will
hence be referred to as the meaning update (update) patterns. Note
that areas showing these relative patterns of conditions would only
indicate where the meaning update takes place but cannot be
interpreted as sites where the initial meaning choice was carried
out.
Based on the results of the divided visual field literature (see
above), we were particularly interested in whether there is any
evidence that context- and frequency-based meaning dominance
are utilized differently in the two hemispheres; e.g. is there any
evidence that alternative word meanings are more available in the
right than in the left hemisphere?
Materials and methods
Materials
The materials consisted of three types of Dutch sentences:
sentences with a semantically ambiguous word (homograph)
biased either toward its dominant or toward its subordinate
meaning, and structurally identical sentences without ambiguous
words serving as a control. Eighty homographs (fifty nouns and
thirty verbs) were used in the ambiguous sentences. The
dominance of the alternative meanings was identified using the
results of an unpublished word meaning association study by
Bruinsma et al. (manuscript).1 In this offline study, forty-six naive
native speakers were asked to write down the first word that came
to mind on reading each homograph. Then, the number of
associates related to each alternative homograph meaning was
determined. For example, if the Dutch word ‘advocaat’ (eggliqueur or lawyer) was most often associated with words referring
to ‘courts’ or ‘law’ and less frequently referring to ‘alcohol’ and
‘drink’, then the meaning ‘lawyer’ was considered to be dominant
and the meaning ‘egg-liqueur’ was considered to be subordinate.
The difference in dominance was fairly strong; dominant-related
associations were given in 80.3% of the cases.
Two sentences were constructed containing each homograph as
illustrated in Table 1. In both versions, the homograph was
presented early in the sentence (underlined in the table), followed
by a neutral, non-biasing context that lasted at least three words,
and the sentence finished with a context, usually one word long
(bold in table), which strongly biased the homograph either
towards its dominant or subordinate interpretation.
As can be seen in Table 1, the difference between the dominant
and subordinate sentence versions, the biasing context, was usually
only one word. To check that the non-biasing context is indeed
neutral, we first consulted a panel of ten expert native speakers
(linguists and linguistics students) who confirmed that disambiguation to both meanings remained possible after the intervening
words. Then, a Cloze procedure test was carried out by twenty
naive native speakers on the sentence contexts without the final
disambiguating word(s). The results indicated that frequency-based
meaning dominance was not substantially altered by the intervening words. On average, 72% of the sentence fragments were
completed employing the dominant meaning (median percentage
82.5%), in 21.6% of the cases, the completions were consistent
with the subordinate meaning (median percentage 12.5%), which is
approximately the ratio indicated by the associate data. The
remaining responses were either ambiguous between the two
readings or were unclassifiable for other reasons. Last, the
plausibility ratings for the two conditions were similar (see below),
indicating that both versions of the final biasing context were
considered appropriate at the end of the sentence fragments. If the
intervening context created a bias toward one meaning, the other
completion should come across as implausible.
Unambiguous control sentences were constructed, each with the
same syntactic structure as two of the homograph sentence pairs.
Each sentence was broken into several phrases for presentation;
matched sentences were divided into phrases identically (generally
three phrases). Over all items, the dominant, subordinate, and
control sentences were matched for sentence structure, sentence
plausibility, sentence length, and average word frequency. The
word frequencies were taken from the CELEX data base (Baayen
et al., 1993). Plausibility was determined using an off-line rating
list with a five-point scale (1 = totally implausible and 5 = totally
plausible). The characteristics of the experimental conditions are
summarized in Table 2.
In order to avoid repetition effects, the sentences were assigned
to two experimental lists, which contained either the dominant or
the subordinate version of a homograph sentence pair; this is why
twice as many homograph sentences were created as control
sentences. The two lists contained the same set of control sentences
(40); the dominant and subordinate sentence pairs (80) were
1
For further information on the Bruinsma et al. study, please contact the
corresponding author.
M.-Z. Zempleni et al. / NeuroImage 34 (2007) 1270–1279
distributed so that subjects saw only one version of a sentence pair
(40 dominant and 40 subordinate sentences). The sentence
conditions on the two lists were also matched for sentence
structure, plausibility, phrase number, length in letters, and for
average word frequency.
Thirty-six filler sentences with various structures were added to
both lists. Ambiguous words also occurred in some filler sentences.
After each filler sentence, a target word was presented, as
illustrated in Table 3, and subjects decided whether it was related
to the sentence in meaning. Half of the targets were related to the
fillers, whereas the other half were unrelated to them. The semantic
relationship between the fillers and targets was intentionally rather
easy, so a poor performance on this task would indicate lack of
cooperation or attention. This task was also intended to distract
subjects from the homograph sentences. As the task was imposed
on the filler sentences only, the hemodynamic response function to
the target stimuli should not be affected by motor responses or
decision strategies.
The order of the stimuli in each list was pseudo-random, with
items in each condition spread evenly across the entire list and no
more than two items of the same condition presented consecutively. In order to minimize effects of fatigue and lack of
concentration, the experiment was divided into four equally long
runs. The order of the four runs was counterbalanced over subjects
resulting in 2 lists by 2 presentation orders.
Apparatus and procedure
The sentences were presented visually, phrase-by-phrase (see
Tables 1 and 3), and centered on the computer screen. For
programming and presentation, we used E-Prime (Psychology
Software Tools Inc., 2001). The text was printed in black on a
white background, with a font size which allowed for comfortable
reading. A projector transmitted the stimuli from the computer to a
screen which was visible to the subjects via a coil-mounted mirror.
Subjects were instructed to read and comprehend the sentences
silently. In order to monitor attention and cooperation during the
scanning session, participants were also asked to carry out the
relatedness decision described above (see filler sentences in Table
3). Responses were made by pressing buttons on a response box
with the index and middle fingers of the right hand.
Each item was presented as follows. First, a fixation cross was
presented in the center of the screen, simultaneously with the
beginning of a scan of a brain volume. This remained until
replaced by the first phrase of the sentence; sentence onsets were
jittered between 0 and 3 s, which was the volume acquisition (TR)
time, to decouple the sequence of slices from the onset of stimuli.
Each phrase remained centered on the screen for approximately
100 ms per character and then was replaced by the following
phrase. After the final phrase, a fixation cross was displayed until
the beginning of the following item, which was initiated by a
trigger from the MR scanner. The filler sentences were followed by
Table 2
Condition match of the test materials
Condition
Number of
sentences
Dominant (D) 80
Subordinate (S) 80
Control (C)
40
Plausibility
Length in
letters
Average word
frequency
4.14 (0.45)
3.83 (0.59)
4.39 (0.42)
55 (10)
55 (10)
55 (10)
2.27 (1.07)
2.30 (1.05)
2.07 (1.27)
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Table 3
Examples of the filler sentence—target word pairs with literal English
translations
Filler sentencerelated target
Filler sentenceunrelated target
Toen het donker werd ∣ besloten de soldaten ∣
te stoppen.—oorlog
When it dark became, ∣ decided the soldiers ∣
to stop.—war
Tijdens de bevalling ∣ moest de vrouw ∣
al haar energie gebruiken.—gordijn
During the childbirth ∣ must the woman ∣
all her energy use.—curtain
Note. Vertical lines indicate presentational borders.
a 600 ms fixation cross then the target appeared for 3000 ms and
then another fixation that lasted until the next item. The total
presentation time for the sentences was 5.5 s on average
(SD = 1.2 s, minimum = 3 s, maximum = 10 s) for each condition.
The total time for each fixation–sentence–fixation combination
was 21 s, the time necessary to acquire seven complete brain
volumes. The long interstimulus interval was chosen to allow the
hemodynamic response to return to baseline before initiating the
following trial.
The measurements were carried out using a Philips Intera 3 T
MRI in the Neuroimaging Center of the University of Groningen.
Echo planar images (T2* weighted) were acquired (TR = 3000 ms,
TE = 35 ms, flip angle = 90°). Each volume consisted of 46 slices
covering the whole brain (slice thickness = 3.5 mm, slice
gap = 0 mm, field of view 224 × 224 × 161 mm, in plane resolution:
64 × 64).
Homograph sentence comprehension was tested after the
scanning session using a relatedness decision procedure, similar
to that used in the scanner, but in this case imposed on the
ambiguous sentences. A subset of ambiguous sentence pairs from
the scanner materials were selected, and three target words were
chosen for each of them, based on the associated words collected
by Bruinsma et al. (manuscript).1 One target was related to the
dominant but not to the subordinate meaning, another was related
to the subordinate but not to the dominant meaning, and one target
was unrelated to either meaning. This produced six conditions: the
dominant sentence followed by the congruent dominant-related
target (D-con); the dominant sentence followed by the incongruent
subordinate-related target (D-inc); and the dominant sentence
followed by the unrelated target (D-unr); and the same for the
subordinate sentences, i.e. S-con, S-inc, and S-unr. For example,
the sentence ‘De advocaat werd door het oudje keurig benaderd/
ingeschonken’ (‘The lawyer/egg liqueur was by the old person
nicely approached/poured out’) was followed by the word
‘meubel’ (furniture) as unrelated target or the words ‘rechtbank’
(‘court’) versus ‘drankje’ (‘drink’) as congruent or incongruent
targets depending on the dominant versus subordinate contexts.2
The behavioral material was programmed and presented the same
way as the materials in the scanner (see above), and the task was
implemented using a pc. Subjects were instructed to respond as
accurately as possible by pressing the control and left arrow keys
of the pc using the index and middle fingers of the right hand. The
average error rate was calculated for each subject in order to
establish whether the subject was capable of comprehending the
test sentences.
2
For more information on the behavioral dataset, please contact the
corresponding author.
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M.-Z. Zempleni et al. / NeuroImage 34 (2007) 1270–1279
The experiment was approved by the Medical Ethical
Committee of the University Medical Center Groningen. Participants gave written informed consent prior to participating in
accordance with the Helsinki Declaration. Before inclusion, the
participants were screened for MRI incompatibility, and during
scanning, the standard MRI safety regulations were followed.
Subjects
Data from sixteen subjects were included in the study, after
excluding one subject from the seventeen scanned due to
movement artifacts. All participants were healthy, native Dutch
speakers (8 females, 8 males; average age 32.5 years, SD = 10.1;
average educational level 15.8 years, SD = 4.1, handedness score
0.99, SD = 0.1, indicating almost exclusive right handedness (Van
Strien, 1992)). All subjects had normal or corrected to normal
vision, normal hearing, and no history of neurological or
psychiatric disorder.
Data analysis and model specification
The raw f MRI data were converted into analyze format using
the MRIcro software package (Rorden and Brett, 2000). SPM2
(Wellcome Department of Cognitive Neurology, London, UK) was
used for spatial preprocessing, such as realignment, normalization
(2 × 2 × 2 mm voxel size), and smoothing with a 10 mm Gaussian
kernel (Friston, 1994). The realigned data were checked for
movement artifacts; translation movements bigger than 5 mm and
rotation movements bigger than 3° were rejected causing exclusion
of one subject out of seventeen scanned.
Since the aim of the study was to evaluate the neural substrate
of contextual disambiguation, we modeled the point of disambiguation as an event. The biasing context becomes available at the
end of the sentences (in 90% of the sentences, the disambiguating
word is the final word). Therefore, the end of the presentation of
the last phrase was used as the time point of this event. In 10% of
the sentences, additional words followed the disambiguating one in
order to make the sentences more plausible; the presentation time
of these extra words varied between 300 ms and 1300 ms (760 ms
on average). Considering the 3000 ms volume acquisition time,
this short additional presentation time in a low percent of sentences
is very unlikely to influence the results, particularly given
individual variability in reading speed and reading strategies.
SPM2 was used for statistical analysis, during which beta
estimates were calculated for each subject for each condition (D, S
and C) modeled as events at the sentence end-point (first level
analysis). These beta estimates were entered into a random effects
one-way ANOVA on the second level (Henson and Penny, 2005;
Penny et al., 2003), during which a non-sphericity correction was
applied. After identifying significant areas of hemodynamic
response, a further analysis was carried out in order to identify
which conditions contributed to the significant difference.
Basically, we were interested whether the subordinate and dominant conditions differed significantly in any of the areas which were
significant in the F-test since this would exclude the possibility of
the selection hypothesis, which would suggest equal availability of
the alternative meanings at the sentence end-points. For this reason,
we created an inclusive mask of the F-test and the S > D t-test.
Additionally, we checked if any of the areas showed significant
difference between both the S > D and the D > C contrasts. For this
reason, we made a conjunction analysis of these two contrasts.
Results
Behavioral performance in the scanner and on the homograph
comprehension task
f MRI participants performed well on the relatedness decision
task in the scanner (imposed on filler sentences; see Apparatus and
procedure section). The good performance indicates that participants were attentive and cooperative during scanning (average
error rate: 0.06, SD: 0.06, minimum = 0, maximum = 0.017).
Therefore, no dataset had to be left out due to inappropriate inscanner behavioral performance.
Subjects also performed well on the post scanner homograph
comprehension task (see Apparatus and procedure section) as
indicated by their low error rates across all conditions (average = 0.12, SD = 0.06, minimum = 0.03, maximum = 0.23). This
suggests that participants were able to comprehend the homograph
sentences and to choose the context congruent meaning of the
homographs. Therefore, no dataset had to be left out due to
inappropriate behavioral performance.
f MRI results
Table 4 lists those brain areas which showed significant effects
at p uncorrected ≤ 0.0002 (F ≥ 11.47) and consisted of at least 5
voxels. The areas are organized according to anatomical regions.
kE refers to cluster size expressed as number of voxels. F stands
for the maximum F value within the cluster. Lat stands for
hemispheric lateralization; LH: left hemisphere; RH: right hemisphere. Montreal Neurological Institute coordinates (x, y, z) and the
Brodmann areas (BA) are provided in separate columns. As can be
seen, five clusters showed significant hemodynamic change at this
threshold: two clusters in the left inferior frontal gyrus, in fact two
maxima in a single larger area as indicated in Fig. 1 (see below);
one in the left inferior/middle temporal gyrus, in the right inferior/
middle temporal gyrus, and in the right inferior frontal gyrus.
The areas are also shown in Fig. 1, projected on a standard
anatomical template (MRIcro; Rorden and Brett, 2000). The
picture shows the areas at a slightly lower threshold (see color bar)
in order to show that the two activation maxima in the left inferior
Table 4
Anatomical regions showing significant effects during homograph
comprehension
kE F
Lat Anatomical region
Frontal lobe
14 12.38 LH Inferior frontal gyrus
9 12.38 LH Inferior frontal gyrus
6 12.88 RH Inferior frontal gyrus
x
− 48
− 52
34
y
z
26
20
16
26
20 −10
BA
45
44
47
Temporal lobe
31 15.50 LH Inferior/middle temporal gyri − 50 − 48 −12 20/37
18 13.21 RH Inferior/middle temporal gyri
56 − 34 −16
20
Note. The analysis was time-locked to the end of each sentence, to the point
of disambiguation. All areas were significant at p ≤ 0.0002 (F ≥ 11.47), the
spatial extent of activation (kE) was ≥5 voxels. Areas are presented with
Montreal Neurological Institute coordinates (x, y, z), the cytoarchitectural
designation according to Brodmann (BA), the maximum F value (F), of the
hemodynamic response in the area and its extent (kE). In order to emphasize
laterality effects, the hemispheric lateralization (Lat) is presented in a
separate column. LH: left hemisphere; RH: right hemisphere.
M.-Z. Zempleni et al. / NeuroImage 34 (2007) 1270–1279
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Fig. 1. Notes: The picture above shows the anatomical regions whose activation maximum showed significant hemodynamic response during disambiguation at a
threshold of p uncorrected ≤ 0.0002 (F ≥ 11.47) and kE ≥5. The picture shows the areas at a slightly lower threshold (see color bar) in order to show that the two
activation maxima in the left inferior frontal gyrus (area IV) are in fact part of one continuous region. The statistical map is projected on a standard anatomical
template (MRIcro; Rorden and Brett, 2000). BA: Brodmann area. The plots show the zero-mean corrected magnitude of the effect of conditions, averaged over the
whole cluster in each area. The error bars show the 90% confidence interval. D: dominant condition; S: subordinate condition; C: unambiguous control condition.
frontal gyrus (area IV on the figure) are in fact two maxima of a
continuous area. This figure also shows the contrast estimates in
the different regions. As already can be seen on the plots, the
significance is mainly due to the effect of the subordinate sentence
condition. In order to determine the different patterns, we created
an inclusive mask of the F-test and the S > D contrast. This
revealed that the subordinate condition elicited significantly higher
(p uncorrected ≤ 0.0002; F ≥ 11.47; kE ≥ 5) activation than the
dominant condition in each of the areas. Additionally, the areas in
the left inferior frontal gyrus showed significance when evaluated
in a conjunction analysis of the S > D and the D > C contrasts at a
lower threshold (p uncorrected ≤ 0.005; T ≥ 2.75; and kE ≥ 5). These
results suggest that the relative pattern of conditions is most
consistent with an S > D > C pattern in the left inferior frontal gyrus
and with the S > D = C pattern in the other areas. There is no
indication for the S = D > C pattern in any of the areas.
Discussion
In this f MRI experiment, the neural substrate of lexical semantic
ambiguity comprehension was studied, with a special focus on the
hemispheric contribution to context-driven disambiguation. The
results indicate that a bilaterally distributed network underlies
context-driven semantic ambiguity resolution. The majority of the
activated voxels are located in the left hemisphere, in the left inferior
frontal gyrus (BA 45/44) and in the left inferior/middle temporal gyri
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(BA 20/37). Significant activations were also seen, however, in the
right hemisphere, in the right inferior frontal gyrus (BA 47), and in
the right inferior/middle temporal (BA 20) gyri.
These findings are consistent with the results of earlier
neuroimaging experiments which evaluated ambiguity comprehension in sentential context, in particular, with the findings of Rodd
et al. (2005). In an f MRI study, they compared sentences
containing several ambiguous words (with disambiguating information in the sentence) versus sentences without ambiguous
words. The high ambiguity versus unambiguous sentence contrast
elicited activation in the left posterior inferior temporal gyrus and
in the left inferior frontal gyrus; the left hemisphere activations
were very similar to those we found in the current experiment.
They also reported right inferior frontal activation but in a more
lateral and superior region than in the current study. Stowe et al.
(2005) presented ambiguous words in the beginning of sentences
and disambiguated them at the end of the sentences to the
subordinate meaning. They compared this condition to an
unambiguous sentence condition in a blocked design PET
experiment and found right inferior frontal activation; this area
was also significant in the current study. The lack of a more
extensive activation in the Stowe et al. (2005) study might be due
to the slower time course of PET which makes it less sensitive to
relatively short responses within the block or to the fact that each
ambiguous sentence was disambiguated to the subordinate meaning making the task more transparent for the participants. To sum
up, three of the areas identified in the current study (left and right
inferior frontal gyri and the left inferior/middle temporal gyri)
were also activated in other studies, which studied semantic
ambiguity resolution in sentence context, although the experimental designs and methodology were not identical. The
occurrence of bilateral activation in both the frontal and the
temporal lobes is likely due to the novel element of our design; in
the current paradigm, frequency-based meaning dominance and
contextual disambiguation were systematically manipulated which
might have increased the processing demand recruiting additional
areas.
In contrast to the studies above, another neuroimaging study on
semantic ambiguity comprehension by Chan et al. (2004) yielded a
completely different set of areas. They presented blocks of
semantically ambiguous and unambiguous words, without context,
to which participants had to covertly generate semantically related
words. Ambiguous words elicited increased activation predominantly in bilateral middle and superior frontal gyri and in the
anterior cingulate, whereas unambiguous words elicited predominantly bilateral inferior frontal and temporal activation. However,
in this study, the experimenters did not provide disambiguating
context, which would force participants to choose a particular
meaning that is congruent with this context. In this case, therefore,
subjects could only base their decision when generating a related
word on the relative frequency of the alternative meanings.
Taken together, these data suggest that the presence of any
disambiguating information may profoundly influence the neural
network involved in comprehension of semantically ambiguous
words. In other words, disambiguation on purely probabilistic
grounds with no need for contextual integration presents a different
cognitive scenario calling for a different neural network than the
situation when one particular meaning has to be chosen according
to the context. Therefore, the network which we see here appears to
be responsible for contextual updating/integration. This hypothesis
is clearly supported by the S > D = C and the S > D > C patterns seen
in the activated areas. As pointed out in the Introduction, these
patterns suggest that the network is primarily concerned with
updating the sentence meaning when context turns out to be
inconsistent with an initially favored meaning.
A support for this interpretation of our data is that the areas in
the current study have also been found to be significant in other
neuroimaging studies which have manipulated other aspects of
semantic/contextual integrative demand, for example, by introducing anomalies into the test sentences or using unexpected
sentence completions. Kuperberg et al. (2003) found that the left
inferior frontal gyrus (BA 44/45, 47, 9/46) is sensitive to
pragmatic violations whereas semantic violations elicited right
middle and superior temporal gyri activation (BA 21/22) in
another study (Kuperberg et al., 2000). These activations are
similar to those found in the current study: left BA 45/44 and a
slightly more ventral region in the right temporal lobe (right
inferior/middle temporal gyrus; BA 20). In another f MRI study,
Baumgartner et al. (2002) manipulated the degree to which
sentence final nouns were expected as sentence completions
(expected, unexpected, and violated sentence completions).The
unexpected and anomalous conditions elicited activations in the
left inferior frontal and left posterior middle temporal gyri, in areas
similar to those we found in the current study. It has to be noted
that the sentences in our study were all judged to be plausible in an
off-line rating, but the subordinate sentences might have
temporarily been detected as implausible and in any case they
had less expected sentence completions by definition. Additionally, the left middle temporal gyrus has also been reported to be
involved in the comprehension of complex and semantically less
transparent phrases and sentences (Homae et al., 2002). Last,
bilateral frontal and temporal areas, with left dominance, have
been implicated in generating the event-related potential occurring
in the 300–500 ms range (N400) which is extremely sensitive to
factors which affect semantic integration difficulty such as
expectancy violations or increased demand for semantic integration. See for example Halgren et al., 2002 or Van Petten and Luka,
2006 for recent reviews.
Turning to the functions of the individual areas, the involvement of the prefrontal cortex during context-based semantic
ambiguity resolution is not surprising given that this brain area is
generally implicated in context processing (Cohen and ServanSchreiber, 1992). Additionally, the left inferior frontal gyrus,
mostly BA 44 and 45 areas, has been suggested to subserve a
general mechanism for selecting among competing semantic
representations; its involvement in selection has been demonstrated
using different cognitive tasks, e.g. verbal fluency (Hirshorn and
Thompson-Schill, 2006), verb generation, object classification and
semantic comparison (Thompson-Schill, 2003). This area is also
involved in dealing with competing syntactic representations as
can be seen in activations elicited by syntactic garden-path
sentences (Mason et al., 2003; Stowe et al., 2004). Badre et al.
(2005) demonstrated that the left mid-ventrolateral prefrontal
cortex (BA 45) is crucial in mnemonic processing and supports a
general post-retrieval selection mechanism, i.e. selecting relevant
knowledge from competing information, which is a distinct cognitive process from the top–down retrieval of semantic knowledge
localized more to the left anterior ventrolateral prefrontal cortex
(BA 47).
From a wider perspective, selection between competing
representations may call on some aspect of working memory.
The involvement of the left inferior frontal gyrus in the current
M.-Z. Zempleni et al. / NeuroImage 34 (2007) 1270–1279
paradigm is in line with the (verbal) working memory literature
(D’Esposito et al., 2000). There is considerable empirical evidence
suggesting that the left ventrolateral prefrontal cortex, including
Broca’s area, is crucial for maintaining and manipulating
information in working memory (for maintenance, see Fiez et al.,
1996; Paulesu et al., 1993; for manipulation, see Gelfand and
Bookheimer, 2003). Left ventrolateral prefrontal cortex also
appears to support executive control processes in working memory,
such as overcoming proactive interference, i.e. inhibiting prepotent
response tendencies (D’Esposito et al., 2000; Jonides et al., 1998).
In the current homograph comprehension paradigm, the dominant
word meaning can be considered as a prepotent response;
therefore, the involvement of the BA 45 area may reflect the
necessity for actively suppressing the dominant meaning and
allowing information update and the reintegration of the less potent
subordinate word meaning; reintegration and updating may also
make use of verbal maintenance processes.3
With regard to the right inferior frontal lobe, its role in linguistic
context updating is more tendentious, but recent neuroimaging
studies have shown that, when task demands increase during
language processing, the right inferior frontal gyrus tends to get
involved, e.g. degree of speech compression (Poldrack et al., 2001)
or when subjects were asked not only to comprehend but also to
‘repair’ syntactic violation (Meyer et al., 2000). Note that this latter
study involved a process which is in some sense similar to
‘repairing’ an initial ‘misinterpretation’ during semantic ambiguity
resolution.
Although context processing in general has been associated with
prefrontal regions, language related semantic integration which
requires context updating has also been associated with temporal
regions. For example, many studies, which applied sentential or
lexical semantic manipulations, have found left inferior/middle
temporal activations (see for example the Kuperberg and the
Baumgartner studies cited above; or for a review, see Stowe et al.,
2005). Rodd et al. (2005) also found a similar posterior area during
their semantic ambiguity comprehension task (see above).
Involvement of the right temporal lobe has not been reported
as frequently, but several studies suggest that, under certain
circumstances, it responds to increased context processing
demands, e.g. Kuperberg et al. (2000), see above. In another
f MRI study, Kircher et al. (2001) asked subjects to complete
sentence context without a missing final word; the sentence stems
were characterized by low Cloze probability. In the generation
condition, subjects had to generate a plausible sentence ending. In
the decision condition, subjects had to choose between two words,
presented by the experimenters. The control condition was reading
complete sentences. The generation condition versus the other two
conditions contrast elicited, among other areas, extensive right
lateral temporal activation. The decision versus reading contrast
revealed left inferior frontal and bilateral middle–superior
temporal gyri activations at a slightly more dorsal location than
in our study. The authors argue that multiple meanings have to be
accessed and evaluated during this task, especially in the
generation condition, because the sentence stems had low Cloze
3
Working memory has typically been investigated using delayed
response tasks. These tasks are hypothesized to elicit several cognitive
subcomponents, such as information encoding, maintenance, manipulation,
retrieval, response selection and inhibition. There is evidence that these
subprocesses are carried out by different parts of the lateral prefrontal cortex
(D’Esposito et al., 2000).
1277
probability. Since our material requires the access and evaluation
of alternative interpretations, our findings are in line with this
suggestion.
Finally, let us consider the bilateral nature of the neural
network involved. This finding is consistent with the lesion
literature and explains why both left hemisphere damage (Copland
et al., 2000, 2002; Swaab et al., 1998; Swinney et al., 1989) and
right hemisphere damage (Grindrod and Baum, 2005; Zaidel et al.,
1995) can disrupt lexical semantic ambiguity comprehension.
Similarly, this bilateral network involvement is in line with the
proposals based on divided visual field experiments (Faust and
Chiarello, 1998; Burgess and Simpson, 1988). However, the
relative condition patterns do not provide support for the claims
that the two hemispheres contribute in a substantially different
manner. Our results did not support the hypothesis that, at least in
the right hemisphere, the alternative meanings are equally
available for use during context updating because the S = D > C
pattern is not seen in any of the significant areas. It has to be
noted, however, that the studies mentioned above evaluated the
availability of the alternative meanings at shorter delays (900 ms in
the former and 750 ms in the latter study) whereas in the current
study the presentation time of the intervening phrase(s) which
contained a neutral context was 1835 ms on average. Taken
together, this evidence may indicate that the right hemisphere is
capable of carrying out contextual disambiguation, but it does so
more slowly than the left hemisphere. An alternative explanation
that is more consistent with the model proposed by Faust and
Chiarello (1998) is that the left hemisphere carries out the initial
choice and the right hemisphere only participates in the
reactivation of the alternative meanings. Further studies are
needed, applying for example combined f MRI and EEG, to more
precisely determine the time course of context-driven disambiguation in the two hemispheres.
The results reported here suggest that frequency-based meaning
dominance and contextual congruency invoke two totally different
processes in the brain. However, in order to substantiate this view,
the role of these two factors has to be investigated more
systematically in future studies. For example, by varying the
degree of imbalance between the availability of the different
meanings and the degree to which context supports these
meanings, we could get a clearer picture of the areas which
respond to these factors. Since these factors are graded rather than
discrete, treating them as variables within a regression analysis
conducted over items could identify areas which parametrically
vary in intensity of activation due to these variables.
Conclusions
The present study investigated the neural substrate of lexical
semantic ambiguity in sentence context. The results indicate the
involvement of a bilaterally distributed network: bilateral inferior
frontal gyri and bilateral inferior/middle temporal gyri. These
findings are consistent with the lesion literature as well as with
existing neuroimaging data. The pattern of activation suggests that,
even if multiple meanings are accessed during the comprehension
of ambiguous words, they are not maintained for an unlimited
time. Instead, a meaning frequency-based choice is most likely, at
least in case of unbalanced homographs. If a later biasing context
occurs which does not match the initially favored choice, the
alternative word meaning has to be updated and integrated to the
context.
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M.-Z. Zempleni et al. / NeuroImage 34 (2007) 1270–1279
Acknowledgments
We would like to thank Marco Haverkort for contributing to the
test materials; members of the Center for Language and Cognition
Groningen group for valuable comments on an earlier draft. Remco
Renken is a Hazewinkel-Beringer fellow; Monika-Zita Zempleni
has been supported by an Ubbo Emmius scholarship provided by
the Faculty of Arts, University of Groningen.
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