Chapter
NEUROIMAGING AND APHASIOLOGY
IN THE XXI CENTURY:
EEG AND MEG STUDIES ON LANGUAGE
PROCESSING IN APHASIA SINCE
THE YEAR 2000
Marie Pourquié1, and Phaedra Royle2,†
∗
1
Basque Center on Cognition, Brain and Language;
Université de Montréal, École d’Orthophonie et d’Audiologie;
Centre for Research on Brain, Language and Music
2
Université de Montréal, École d’Orthophonie et d’Audiologie;
Centre for Research on Brain, Language and Music
ABSTRACT
This paper provides an update on a decade of neuroimaging studies
(using electroencephalography, EEG, and magnetoencephalography,
MEG) that have focused on linguistic processing in patients with aphasia.
The goal of this paper is to shed light on the challenges and usefulness in
using such techniques for the study of aphasia. We review study
objectives, techniques and results, and highlight the linguistic structures
that have been studied. The first part defines the challenges and
∗
†
E-mail: marie.pourquie@umontreal.ca.
E-mail:phaedra.royle@umontreal.ca.
2
Marie Pourquié and Phaedra Royle
usefulness of EEG and MEG techniques for aphasiology; the second
reports the procedures, targets and main results of reviewed studies; the
last section addresses the following issues: What does neuroimagery add
to classical behavioral studies on aphasia? Are they both necessary and
why?
INTRODUCTION
The term “aphasia” stands for language impairments that arise following a
brain lesion. Aphasiology can be defined as a science that infers
neurocognitive organization of the human faculty of language from the study
of deficits observed in aphasic patients. It also aims at developing therapeutic
treatments for language recovery.
Although the first generation of neuropsychologists (e.g. Bouillaud,
Broca, Wernicke) succeeded in identifying important brain areas involved in
language processing from an empirico-inductive perspective, this classical
approach has since been declared insufficient for the purpose of inferring
precise language organization in the brain (Démonet & Thierry, 2001).
Differences in lesion site and extent, and the particular background
characteristics of individual subjects (e.g. age, gender, bilingualism, treatment,
etc.) give rise to high levels of heterogeneity in aphasic manifestations. This
makes it difficult to correlate a given specific cognitive function with any
specific brain area, based only upon the study of aphasic symptoms. However,
advances in the development of neuroimaging tools have allowed researchers
to assess healthy individuals in order to explore the neural underpinnings of
language. They have shown that the language network in the brain is much
more extended that was thought before: it involves both hemispheres and the
coordinated activity of multiple structures (Hagoort & Poeppel, 2013). Thus,
neuroimaging techniques should be considered as complementary tools to
traditional approaches in aphasiology for the study of neurocognitive
underpinnings of human language. The present chapter is concerned with
neuroimaging studies that assess language processing in aphasia. We present
their goals, challenges and perspectives in the context of aphasiological
research. We specifically focus on studies developed over the last decade
using Electroencephalography (EEG) or Magnetoencephalography (MEG) 1.
1
For a review of fMRI research on language processing in aphasia, see Crosson, McGregor,
Gopinath, Conway, Benjamin, Chang, Bacon Moore, Raymer, Briggs, Sherod, Wierenga &
White, 2007.
Neuroimaging and Aphasiology in the XXI Century
3
1. CHALLENGES AND USEFULNESS
OF ELECTROENCEPHALOGRAPHY
AND MAGNETOENCEPHALOGRAPHY
TECHNIQUES FOR THE STUDY OF APHASIA
In the arena of clinical aphasiology, functional Magnetic Resonance
Imaging (fMRI) studies are ubiquitous. Despite the growing number of
Electroencephalography (EEG) and Magnetoencephalography (MEG) studies
on normal language processing that have the potential to provide useful data
for the further study of aphasia, only a few studies using such techniques to
assess language processing in aphasia have so far been run. Before discussing
the reasons why the use of these techniques appears to still be rare in
aphasiological research, we define some basic concepts in neuroimaging
research.
1.1. Neuroimaging Techniques
fMRI, EEG and MEG are non-invasive imaging techniques that aim to
characterize different aspects of neuronal activity in the brain, both in time and
in space. Whereas EEG and MEG track underlying electrical activity of the
brain, fMRI maps local changes in brain blood flow. EEG and MEG each
provide data with high temporal resolution (measured in milliseconds). While
data from EEG are limited in spatial resolution information, both MEG and
fMRI allow for high-level anatomical detail. Localized neural activity from
MEG data is called electromagnetic source imaging (EMSI). In turn, fMRI
detects local increases in relative blood oxygenation and provides good spatial
but relatively poor temporal resolution (Halchenko, Hanson & Pearlmutter,
2005).
Traditionally, much of the research on brain imaging in patients had used
magnetic resonance imaging (MRI, i.e. brain images based on magnetic
resonance fields) combined with fMRI methods to study language processing
in aphasia. MRI is used to visualize the brain structures and any architectural
pathology which may be present, but not the cognitive functions of the brain.
During functional magnetic resonance imaging (fMRI), participants perform a
particular task during the imaging process, causing increased metabolic
activity in the area of the brain believed to be linked to cognitive processing
demands caused by the task. In this case, changes in blood flow are measured
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Marie Pourquié and Phaedra Royle
and superposed on the MRI. The usefulness of fMRI is clearly linked to this
method’s high imaging resolution for brain structures and lesions, which have
obvious advantages when studying patient populations. However, this
approach has a major disadvantage in the time domain, as recording time
resolution is quite low (in the order of seconds, as it takes between 1 and 2
seconds for Blood-oxygen-level dependent (BOLD) levels, which are what are
measured using fMRI, to change following a stimulus). As we know that
language processing operates within hundreds of milliseconds after stimulus
presentation, and unfolds rapidly and quite automatically over time during the
processing of a stimulus or a sentence, much information on the time-course of
language processing is not available to us using this method.
More recently, the use of event-related potentials (ERPs), which are
electrical waves extracted from the EEG signal, have allowed for more finegrained interpretations of ongoing cognitive processes during language
processing, and have become an important source of evidence on the
functional organization and temporal dynamics of language processing. ERPs
are characteristic patterns of voltage change extracted from scalp-recorded
electrical brain activity, time-locked to the presentation of critical linguistic (or
other) stimuli (Luck, 2005). They can provide information at the level of
millisecond-by-millisecond changes in brain activity. Systematic differences
between ERP waves (or "components") in different experimental conditions
can be used to study and understand neurocognitive processes involved in
different aspects of language processing. ERP components differ in terms of
polarity (whether deflections are negative or positive going), onset latency
(when the curves depart from each other), duration (how long deflections last),
topography (what their distribution is on the scalp), latency shifts (differences
in onset or offset latency depending on experimental conditions or
populations), and amplitude (the size of the difference between ERPs in two or
more conditions).
EEG measures coordinated electrical brain activity. To record an EEG,
electrodes are placed over multiple areas of the scalp to detect patterns of
electrical activity. Scalp-recorded activity generated by the co-activation of
tens of thousands of neurons in a region of the neocortex engaged in cognitive
operations results in the ERP components observed. A number of ERP
components linked to linguistic sub-domains such as phoneme discrimination,
word segmentation, prosodic and intonational phrasing, morphology,
conceptual semantics, syntax, discourse processing, and more have been
identified in the last forty years (Hagoort & Poeppel, 2013). It should be noted
that there is no one-to-one mapping between specific linguistic sub-domains
Neuroimaging and Aphasiology in the XXI Century
5
and ERP components. That is, multiple domains of language processing can
elicit similar ERP components, while processes involving what are considered
to be unitary processes in linguistics (e.g., agreement checking) can elicit
multiple components in the ERP wave. However, the waves found in specific
linguistic contexts (e.g., morphosyntax, syntax, and word processing) are quite
stable across experiments and languages, and therefore allow us to study the
neurocognitive underpinnings of normal and impaired processing of language.
For example, the N400, a negative-going wave peaking around 400 ms after
stimulus presentation has been used since Kutas and Hillyard (1980) in
tracking lexical access and semantic integration.
Magnetic resonance imaging (MEG), which measures magnetic fields
linked to electrical activity in the brain, allows for similar fine-grained
analyses of ongoing neurocognitive processes. In addition, this technique can
provide better spatial resolution than ERPs because of the nature of the
recording (see Rodden & Stemmer, 2008). The MEG equivalent of ERP is the
event related field (ERF). ERFs measure changes in magnetic fields, again
linked to the processing of a critical stimulus. ERFs show similar timing and
amplitude changes as those found in ERPs but are always positive going,
never negative. As with ERPs, reliable ERF components are observed in
language-based experiments (see, e.g., Salmelin, 2007). However, they are not
as widely used as ERPs for the study of language processing, in part due to
higher operational costs for MEG.
1.2. Challenges in Neuroimaging Research on Aphasia
Research into the neural basis of language processing is complicated by
many factors linked to the specificities of aphasic populations (Démonet &
Thierry, 2001). The considerable variability of lesion size, shape, and location
and individual backgrounds may give rise to different spatial and temporal
brain activity patterns. In some cases, it might be more suitable to analyze
results at the individual subject level, rather than at group levels, in order to
draw conclusions on the neurofunctional organization of impaired language
processing.
On the other hand, the use of neuroimaging techniques also faces
methodological challenges when used to assess impaired linguistic systems
after brain damage. EEG and ERFs are characterized by their great temporal
resolution. This means that they are highly sensitive in detecting ongoing
cognitive events being processed in the brain, which are expressed through
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Marie Pourquié and Phaedra Royle
voltage or magnetic changes. But being sensitive to the most discrete implies
being affected by the less discrete, which can be a disadvantage for data
analysis and interpretation. Indeed, strong artifact signals generated by eye,
mouth and tongue movements can mask cortical activity measurements related
to cognitive processing.
Even though progress has been made to remove disruptive field patterns
from the MEG data (Salmelin, 2007), imaging language production in aphasia
remains challenging. For instance, in neurotypical participants, trial onset
timing is defined according to the tight temporal relationship between stimulus
presentation and response. However, patients’ average response latencies are
much longer and variable, making it tedious and less useful to set up an
experiment that integrates measures of speech output speed and accuracy
(Crosson et al., 2007). Moreover, ERP and MEG experiments need to present
numerous stimuli in order to overcome high signal-to-noise ratios. This,
coupled with the inherent inter-subject variability, creates larger signal to
noise ratios in patients than with neurotypical participants. This might explain
why, despite their apparent usefulness for the study of neurocognitive
processing, we find very few ERP and MEG studies that have been carried out
on aphasia and language production.
Rare neuroimaging studies have dealt with language production in aphasia
(Sörös, Cornelissen, Laine, and Salmelin, 2003). To our knowledge, no study
has yet assessed either phrase or sentence production in aphasics. The
assessment of overt speech in patients with aphasia is useful, as it allows
investigators to track response accuracy with the goal of determining if
specific brain areas are necessary for the production of correct linguistic
outputs. However, language production studies are constrained by the
necessity to remain still during data collection and recording, and are
additionally problematic with ERP as muscle movements create artifacts in the
EEG wave.
This is probably the main reason why most neuroimaging studies of
aphasia assess language comprehension. However, problems can emerge while
assessing comprehension in patients with aphasia, as it can also be delayed and
variable. Moreover, covert tasks prevent investigators from checking whether
participants apply instructions accurately or have understood the stimuli.
Finally, since aphasic participants may have difficulties either in retrieving
words, or in producing responses once words are retrieved, it remains unclear
whether observed activation patterns reflect word retrieval or post-retrieval
response production difficulties.
Neuroimaging and Aphasiology in the XXI Century
7
In addition, some studies reviewed here (e.g. Angrilli, Elbert, Cusumano,
Stegagno & Rockstroh, 2003) mainly focus on brain lateralization patterns
rather than the time course of cognitive processes, which is unusual in ERP
studies where timing effects are more relevant than their distribution over the
scalp, due to the inverse problem2. Because of poor spatial resolution in ERPs,
this methodological approach is usually not appropriate for research questions
bearing on localization. The technical challenges related to the use of
neuroimaging in aphasia (experiment modality, setting up time) in addition to
problems linked to behavioral approaches (high variability of lesion sites and
extent, distinct background, variable syndromes, different reorganization etc.)
raise the issue about the point of doing research on aphasia using such
techniques. The following section presents some benefits of neuroimaging
studies for aphasiological research.
1.3. Usefulness of Neuroimaging Techniques for the Study
of Language Processing in Aphasia
Currently, one of the main issues addressed by neuroimaging (especially
fMRI) studies of aphasia is the role of the non-dominant, usually right
hemisphere, versus perilesional cortex in language recovery. Meltzer, Wagage,
Ryder, Solomon, and Braun (2013) argue that the use of neuroimaging helps to
determine whether increased hemisphere activity during language tasks
performed by individuals with aphasia represents takeover of function by
regions homologous to the left-hemisphere language networks, maladaptive
interference, or adaptation of alternate compensatory strategies. In addition,
recent findings highlight the usefulness of temporally sensitive measures when
studying aphasia by demonstrating that the latency of electrophysiological
markers is of interest in patient populations (Zipse, Kearns, Nicholas &
Marantz, 2011).
Both EEG and MEG approaches have obvious advantages in the study of
language impaired populations, as they can be used with auditory stimuli,
avoiding reading processes that might be impaired in patients, in addition to
providing fine-grained information on the time-course of linguistic-cognitive
processing. The information that they provide is thus complimentary to
2
The fact that the neural correlates of the recorded activity on the scalp cannot be inferred from
the electrode(s) where the activity was recorded, due to distortion from brain and scalp
tissue. This is one reason why some researchers prefer MEG or fMRI to ERPs, as they do
not suffer from this problem.
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Marie Pourquié and Phaedra Royle
traditional approaches for the assessment of language difficulties in special
populations, in particular because of their added value of measuring ongoing
processes in language comprehension and production, rather than providing
end-state data (reaction time and judgment values) or average brain activations
patterns over rather long time periods, as is found in fMRI.
Finally, most studies of aphasia have reported dissociations that emerge
behaviorally but not at the level of the brain. These indicate the possibility of
compensatory strategies in patients' behavior. For instance, Meltzer et al.
(2013) observed that right hemisphere activation can be observed with both
accurate and inaccurate comprehension performance, which they interpreted as
strategic recruitment of alternative networks rather than homologous takeover.
Zipse et al. (2011) report that whereas participants with aphasia did not show
semantic priming of the M350 (an electrophysiological marker of lexical
processing in MEG, roughly equivalent to the N400 in ERPs), they did exhibit
significant behavioral semantic priming. Semantic priming studies with
neurotypical subjects can also show dissociations between behavioral and ERP
data, especially with semantic priming (see Royle, Drury, Bourguignon &
Steinhauer, 2012 for a review). Sörös et al. (2003) note a dissociation between
noun and verb retrieval that only emerges after the disruption of the normal
language network, and which is not visible with electrophysiological tools.
Wassenaar and Hagoort (2007) also report that while control individuals show
on-line sensitivity to thematic role assignment3 between pictures and the
sentences, aphasic participants did not display typical on-line ERP patterns,
but showed off-line sensitivity to the sentence-picture mismatches.
Such data are of particular interest for aphasiological research as they
seem to indicate that aphasic performance is the result of specific deficits
bypassed by cognitive strategies. Thus, neuroimaging techniques offer the
opportunity to illustrate and measure an assumption that has already been
suggested in the literature on aphasia based on behavioral studies, namely that
aphasia is not the pure manifestation of underlying linguistic deficits, but the
sum of linguistic deficits compensated by cognitive strategies (Kolk & Van
Grunsven, 1985; Kolk, 1995; Nespoulous, 1996; Sahraoui & Nespoulous,
2012). It appears fruitful to pursue both on-line and off-line approaches in
order to obtain complimentary perspectives in our understanding of language
processing in aphasia.
3
That is, knowing which noun is the subject (agent) or the object (patient) of the verb, as in the
girl is pushed by the boy, where the first noun in the patient of the verb and the second one
the agent.
Neuroimaging and Aphasiology in the XXI Century
9
2. EEG AND MEG STUDIES ON LINGUISTIC PROCESSING
IN APHASIA: PROCEDURES, RESULTS AND ANALYSES
A general impression that we can take from ERP and MEG studies on
language processing is that only a few have used this approach to assess
language processing in aphasia. Moreover, most concern language
comprehension, and if they deal with language production then it usually is by
way of picture naming tasks, that is the production of isolated words. Finally,
they generally aim to determine which brain areas or hemispheres are
activated – either the non-dominant hemisphere or perilesional areas – in
individuals with aphasia performing specific linguistic tasks, in order to
identify brain areas involved in language recovery. The present section is
divided into three subsections: the first two present recent studies on lexicalsemantic and syntactic processing run with aphasic patients; the third contains
some remarks regarding our general assessment of these studies.
2.1. Lexical Processing
An ERP study by ter Keurs, Brown and Hagoort in 2002, investigated
written word recognition in Dutch-speaking patients with Broca's aphasia
(BA), and patients with right hemisphere damage without aphasia (RH), and
controls. They asked participants to categorize items according to their word
category (open and closed-class words). Of primary interest were ERP
negativities linked to early word detection (the early anterior N210-325) and
later lexical-semantic integration (the N400, for a review of studies on lexical
semantic-processing involving this component, see e.g., Holcomb & Grainger,
2006 and Kutas & Federmeier, 2011). Controls and RH patients showed
typical early and late negativities, in addition to a third later anterior
negativity, to closed-class words, while patients with BA did not show either
anterior component, in addition to presenting a delayed N400 that was longer
lasting that those found in the other two groups. It should be noted that the
items in this task were not controlled for frequency or length. Thus the closedclass items should have been perceptually salient, as they are typically shorter
and more frequent than open-class words4. These data seem to signal
4
Additional analyses integrating these factors did not show specific effects of frequency of word
length on the data, however.
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Marie Pourquié and Phaedra Royle
difficulties in early word class detection in patients with BA and may account
for observed syntactic difficulties in these patients.
Another ERP study on lexical access using semantic and associative
priming again investigated the N400, here modulating it with prime
presentation using associative relations (i.e., a word preceded by a
semantically related word, e.g., salt-pepper)5. In their Japanese auditory study
of lexical access in 10 participants with aphasia with auditory processing
impairments, and 10 controls, Kojima and Kaga (2003) presented related and
unrelated word pairs to participants. They observed longer onset and offset
latencies, as well and smaller amplitudes for the N400 to unrelated pairs in
patients. A correlation between the size of the N400 and an auditory
comprehension score that was gathered before the experiment was also found.
Behaviorally, the patients' recognition scores for the experimental items were
normal but their reaction times were slower that controls. Unfortunately, the
groups' ERPs and response patterns were not directly compared in the global
ANOVAs, but only in post-hoc t-tests. According to the authors, the
correlation between the offline measures and the N400 points to the usefulness
of the N400 in establishing or confirming a diagnosis of lexical access
impairment.
An ERP study led by Angrilli, Elbert, Cusumano, Stegagno and Rockstroh
(2003) presents data on rhyming and semantic judgment with word pairs in 10
Italian participants with non fluent aphasia and 10 controls. Using primed
rhyming pairs (i.e., a word preceded by another with which it rimed, e.g. cathat) or semantically related pairs (e.g., wood-tree) and fillers, they asked
patients to make rhyming and semantic judgments on the stimuli. Data show
different lateralization patterns in controls and patients, as well as longer
reaction times and higher error rates in patients. Unfortunately, data were not
analyzed on the target but rather the prime. Although stimuli were presented in
different blocks, where participants were specifically asked to perform a
phonological (rhyming) or a semantic task, thus promoting phonological
versus semantic strategies to word recognition, the fact that analyses were
made on the first rather than the second word makes it difficult to interpret this
data in terms of lexical access, and makes the interpretation more about
cognitive strategies used tasks rather than lexical access itself. We have no
way of telling whether rhyming or semantic priming had any effect on lexical
access and retrieval. In addition, the authors present no functional
interpretation of the ERP patterns.
5
Priming typically reduces the N400 amplitude.
Neuroimaging and Aphasiology in the XXI Century
11
Another lexical-decision task using ERPs was developed by Justus,
Larsen, Yang, de Mornay Davies, Dronkers and Swick (2011). They studied a
group of patients with aphasia resulting from damage to the pars opercularis
(BA44) and the pars triangularis (BA45), i.e. roughly Broca’s area, and a
group of controls. The authors adopted lesion-based rather than symptombased inclusion criteria, because they wanted to test the hypothesis that
damage to frontal areas -- including Broca’s area -- results in a behavioral
dissociation between regular verb and irregular normal verb processing, linked
respectively to procedural and declarative memory systems (Ullman 1997,
2005). Several studies in healthy people have reported priming stronger effects
(reductions of the N400) for regular versus irregular verb forms using ERPs
(Münte, Say, Clahsen, Schiltz & Kutas, 1999)6 or behavioral methods (see
Forster, 1999, for a review). In previous studies, Justus and colleagues (Justus,
Larsen, de Mornay Davies & Swick, 2008; Justus, Yang, Larsen, de Mornay
Davies & Swick, 2009) observed that in neurologically healthy individuals,
hearing the past tense form of both regular and irregular verbs facilitates an
auditory lexical decision to the corresponding present-tense target (e.g.,
looked–look, spoke–speak) and is accompanied by a reduction in the N400
component to the target word. On the other hand, other studies reported
behavioral dissociations between regular and irregular morphological priming
in a group with aphasia (Marslen-Wilson & Tyler, 1997; Münte et al, 1999;
Tyler et al, 2002). Therefore, in their study, Justus et al (2011) aimed to test
whether this dissociation extend to ERPs in a group with Broca’s aphasia.
Again, their study highlights a dissociation between behavioral and ERP data:
whereas behavioral data show that irregular forms prime more than regular
ones, ERP data show similar priming effects for both regular and irregular
forms, in patients as well as controls. According to Justus et al. (2011), the
ERP data demonstrate preserved lexical integration of all verb types. Thus the
absence of behavioral regular-verb priming may not be attributable to prelexical or lexical access deficits. The authors reason that the absence of
significant regular-verb priming in the response time data rather resulted from
post-lexical events (covert articulation, segmentation strategies, and/or
cognitive control)7.
6
7
Although, see Stockall and Marantz (2006) and Morris and Stockall (2012) for data on
equivalent MRF and ERP priming for regular and irregular verbs, but not for semantically
and orthographically related words (e.g., boil-broil).
Note that the ERP data cannot be interpreted as reflecting only pre-lexical or lemma level
access to the lexicon since the primes are not masked. Post-lexical effects can be found in
ERP priming studies (see Kiefer & Brendel 2006; Kiefer, 2006; and Royle et al., 2012 for a
discussion of issues related to task designs and semantic priming effects).
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Marie Pourquié and Phaedra Royle
This study addressed current topics in the domain of neurolinguistics and
psycholinguistics, which are still being debated, such as the existence of a dual
division of English verb forms (regular/irregular) that links up to distinct
neurocognitive processes (procedural/declarative systems) in the brain;
whether English verb forms are better classified in a categorical (regular/
irregular) or rather a continuous way depending on morphophonological
properties of verb forms (Justus and colleagues defend the view of a gradual,
continuous, classification of verbs, distinguishing “weak” and “strong”
irregulars, e.g. send/sent, and drive/drove, as being more and less "regular"
respectively); and whether regular/irregular dissociations extend to ERPs.
However, we note that the discussion is developed around the relationship
between past tense and corresponding present tense forms, but the items used
in the present tense in English are morphophonologically identical to infinitive
verbs (e.g. speak). This may impact on the conclusions drawn by Justus et al.
"that the lexical entries for the present-tense targets were successfully preactivated by the past-tense primes" (p. 13). Indeed, the "cognitive path" (i.e.
computational operations) between past tense forms and corresponding
present-tense verb forms might not be the same as the one between past-tense
forms and infinitives.8 In order to test present tense forms in English that
cannot be confused with infinitives, 3rd singular forms (e.g. speaks) would be
more suitable.
Zipse et al. (2011) assessed lexical access in patients with aphasia, agematched and younger controls, all native speakers of English, using MEG.
Their goal was to explore whether individuals with aphasia exhibit differences
in the M350, an ERF linked to lexical access and integration (roughly the
equivalent of the ERP N400), compared with healthy controls. They also
attempted to explore whether M350 differences were associated with either
phonological or semantic processing deficits. They used a primed lexical
decision task including two stimulus types: identity primed (i.e., a word
preceded by itself, e.g., cake-cake) and semantically primed (e.g., bread-cake),
along with control pairs, with targets corresponding to the identity and
semantically related pairs but with different primes (e.g., leg-cake).
Participants were instructed to decide as quickly and as accurately as possible
whether the second word in each stimulus pair (i.e., the target) was a real or a
nonsense word. The MEG data were analyzed to examine M350 latency
amplitude in terms of (a) overall between-groups differences, (b) semantic
8
In addition to the fact that infinitives (citation forms) are typically the most frequent forms in
English.
Neuroimaging and Aphasiology in the XXI Century
13
priming within each group, and (c) identity priming within each group.
Consistent with the age-matched control group, the group with aphasia showed
both identity and semantic priming behaviorally. In contrast to the control
group, the patient group did not show either semantic or identity priming of
the M350 response. They also demonstrated longer M350 latencies than either
control group. Furthermore, within this group, M350 latencies were positively
correlated with a measure of semantic impairment. According to Zipse et al.
(2011), increased M350 latency appears to be indicative of a semantic
processing impairment9. Moreover, they suggest that the dissociation between
the electrophysiological and the behavioral results observed in the group of
patients — i.e. no semantic priming of the M350 but behavioral semantic
priming — is likely due to the fact that whereas the M350 directly tracks prelexical and lemma level activation, reaction times reflect the sum of many
cognitive processes, including post-lexical checking (Lorch, Balota, & Stamm,
1986). Therefore, they assume that some patients with aphasia might have a
lexical activation deficit at the semantic level, coupled with preserved abilities
to use strategic post-hoc lexical processes for word recognition. However,
since the groups were not directly compared on priming effects, these data are
not supported by appropriate statistical methods.
Sörös et al. (2003) present one of the few studies on language production
using MEG. However, production was limited to object and action naming
from picture stimuli, that is the production of isolated words (nouns and verbs
respectively). The purpose of their study was to assess whether the
dissociation largely described in the aphasic literature between noun and verb
processing can be observed at the neurophysiological level. The study
included 10 healthy Finnish speaking university students and one patient with
anomia (JP). The drawings illustrated a simple scene including an object and
an action. The same stimuli were used for action and object naming. JP
showed superior naming of verbs compared to nouns, due to a left posterior
parietal lesion. We note that the nature of the verb form elicited in action
naming task is not clear. Sörös et al. (2003) only mention that “[b]oth words
[objects and actions] used for the instructions are familiar Finnish expressions
and have an identical word length.” (p. 1789). However, the prompt “tekee”
used to elicit a verb is either translated ‘does’ or ‘is doing’. From a theoretical
perspective, it seems relevant to describe the linguistic nature of the stimuli,
because priming the production of, and producing, either infinitive,
9
However, note that they do not directly test phonological priming, as their "phonological" pairs
involved repetition priming.
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Marie Pourquié and Phaedra Royle
progressive, or inflected 3rd singular present tense verbs may have different
implications for neurocognitive models of language processing, and might also
simply promote different responses in different patients.
No evident differences were observed in brain activation related to noun
and verb retrieval in any participant, despite the fact that a clear behavioral
dissociation between noun and verb retrieval was observed in JP, who showed
more difficulties in naming nouns than verbs. MEG measurements revealed a
specific activation pattern for JP’s object naming in the left inferior frontal
cortex, roughly Boca’s area, that was distinct from his action naming as well
as from the naming-related responses in controls. Broca’s area seemed to be
involved both in action and in object naming, but was activated significantly
earlier and more strongly when naming objects. On the other hand, since JP
showed increased naming latencies for nouns compared with verbs, the
authors argue that this result suggests a temporally deviant and impaired
activation of Broca’s area in JP, and conclude that differences in brain
activation related to noun and verb retrieval are not evident in healthy
individuals but only emerge after the disruption of normal language networks.
They also claim that this study is the first to demonstrate a neurofunctional
basis for a dissociation of verb and noun production in an aphasic patient.
In sum, several studies used EEG or MEG to assess lexical access in
aphasia. They concern various languages: English, Dutch, Italian, Japanese,
and Finnish have been investigated. All but one used a priming task. The study
by Söros and al. (2003) is the only one that used a language production task
that involved picture naming. All mentioned studies assessed lexical access
using isolated words; none used a sentential context. Lexical priming involved
open and closed-class words, words of the same semantic fields, rhyming
pairs, non words, regular and irregular verbs, past verb forms, and nouns and
verbs. Generally, they tracked the N400 ERP component (or the M350 ERF
one), to investigate lexical processing and deficits in this domain in
participants with aphasia. Results from these studies show a varied picture of
impairment, where ERP components in patients do not always reflect, or
correlate with, behavioral data.
2.2. Syntactic Processing
Using ERPs to identify strengths and weaknesses in Dutch patients with
Broca’s aphasia, Wassenaar and colleagues developed three auditory
Neuroimaging and Aphasiology in the XXI Century
15
experiments involving sentence comprehension (Wassenaar, Brown &
Hagoort, 2004; Wassenaar & Hagoort, 2005, 2007).
Wassenaar and colleagues (2004) assessed morphosyntactic agreement
processing in comprehension of simple and complex sentences that required
Subject-Verb agreement within conjoined versus embedded clauses. They
compared processing of sentences such as De vrouwen betalen de bakker en
nemen/*neemt het brood mee naar huis, ‘The women pay the baker and
take/*takes the bread home’ (conjoined sentences with less complex syntax)
and De vrouwen die de bakker betalen, nemen/*neemt het brood mee naar
huis. ‘The women who pay the baker take/*takes the bread home' (embedded
clauses with more complex syntax). The study included three groups of
participants: patients with Broca’s aphasia (BA), patients with a right
hemisphere lesion without aphasia (RH) and controls. While controls showed
a typical Syntactic Positive Shift (SPS, often also called the P600, a positive
component emerging around 600 ms after critical stimulus) to agreement
errors, this component was modulated by deficit severity in the BA group.
That is, the stronger the deficit in the patient, the smaller the P600 was. The
RH group evidenced normal ERP components and behavioral results.
However, no between-group analyses were performed to support differences
between patients and controls, which is less than ideal (Nieuwenhuis,
Forstmann & Wagenmakers, 2011).
In a follow up study, Wassenaar and Hagoort (2005) compared processing
of syntactic and semantic violations in order to track syntactic processing
difficulties in a similar group of patients with Broca’s aphasia, again in
comparison to patients with right hemisphere lesions without aphasia and
controls. In the syntactic violation condition, a word-category violation was
created by adding a verbal suffix (e.g., -t) on a noun stem, in a position that
was ungrammatical given the syntactic context (e.g., De houthakker ontweek
de ijdele schroef/*schroeft op dinsdag. ‘The lumberjack dodged the vain
propeller/*propelled on Tuesday'. In the semantic violation, sentences ended
with a word that violated sentential-semantic constraints (e.g. De timmerman
kreege en compliment van zijn baas/!bloem. ‘The carpenter got a compliment
from his boss/!flower.’ ERPs in the syntactic condition show similar results to
the previous study, that is a SPS (or P600) for both the Control and RH
groups, and a late and very reduced SPS for the BA group. In the semantic
condition, results show expected N400 and SPS components for both the
control and RH groups, and a reduced and more focal N400, as well as a
reduced SPS, in the BA group. The BA group also shows reduced P300s in an
oddball task used to track automatic detection of stimulus (tone) changes.
16
Marie Pourquié and Phaedra Royle
However, their results on the P300 did not correlate with the size of the P600,
pointing to distinct cognitive mechanisms. Between-group analyses support
differences on syntactic but not on semantic processing. The authors argue that
ERPs can identify syntactic structure processing difficulties with BA, and that
these processing difficulties can be independent of lexical access, which
appears to be normal in the BA patients.
Finally, Wassenaar and Hagoort (2007) assessed thematic processing in
similar groups (BA, RH and controls), by way of a picture-sentence matching
task, contrasting semantically irreversible active sentences (e.g., The young
woman reads the exciting book), semantically reversible active sentences (e.g.,
The tall man pushes the young woman), and semantically reversible passive
sentences (e.g., The woman is pushed by the tall man). Previous studies on
neurotypical German participants report a negativity followed by a positive
shift (N400-P600) in argument structure violations where thematic roles
(agents and patients) do not match up with the verb structure (Friederici &
Frisch, 2000; Frisch, Hahne & Friederici, 2004). Results on irreversible active
sentences with picture mismatches (i.e., the participants hear The exciting book
reads the young woman and see [WOMAN READING BOOK]) show a
typical biphasic N400-P600 in controls and RH groups, but only a late in P600
in the BA group. Processing reversible active sentences with incongruent
images (i.e., the participants hear The tall man pushes the young woman and
see [WOMAN PUSHING MAN]) induced a similar N400-P600 complex in
controls and the RH groups (the P600 being non significant in controls), while
no effect was observed in the BA group at the verb, and a slight negativity was
observed sentence finally. Finally, reversible passive sentences in incongruous
conditions (i.e., the participants hear The woman is pushed by the tall man and
see [WOMAN PUSHING MAN]) elicited an N400-P600 in controls, a small
P600 (with statistically marginal effects) in the RH group and no ERP
components in the BA group. The ERPs of the BA group were clearly
different from RH and control groups: whereas typical ERPs were observed in
all sentence types in RH and control groups, the BA group showed a positive
shift time-locked to the onset of the verb only for semantically irreversible
sentences. Interestingly, in this study, the authors highlight a dissociation
between BA participants’ online electrophysiological and offline behavioral
measures, suggesting that strategies might be used to compensate their on-line
syntactic processing difficulties. Results demonstrate that BA participants
seem to have strong difficulties matching up thematic roles in sentences and
images even though off-line behavioral sensitivity to the sentence–picture
mismatches were observed.
Neuroimaging and Aphasiology in the XXI Century
17
Similarly, Kielar, Meltzer-Asscher and Thompson (2012) used ERPs to
investigate verb argument structure processing in agrammatic aphasia. A
grammatical judgment task was assigned to English-speaking participants with
agrammatic aphasia, as well as healthy young and older adults. Two
conditions were tested: argument structure violations and semantic violations.
The latter was included to determine whether participants evidenced the
classical N400 effect to semantic anomalies and to investigate possible
differences in sensitivity to argument structure and semantic information
during sentence processing. In the argument structure condition, a transitive
verb was replaced by an intransitive verb, creating an abnormal syntactic
context frame (e.g., *Anne sneezed the doctor and the nurse), since an
intransitive verb does not select direct objects. In the semantic violation
condition the final word of each sentence was semantically inappropriate (e.g.,
Anne visited the doctor and the !socks). Participants were instructed to press a
green or red button based on their correctness judgment of each sentence.
While controls showed a negativity followed by a positive shift (N400-P600),
agrammatic individuals only showed a late positivity in response to argument
structure mismatches. Additionally, participants with agrammatism showed a
relatively preserved but reduced N400 response to semantic violations.
According to the authors, these data show that individuals with agrammatism
do not demonstrate normal real-time sensitivity to verb argument structure
during sentence processing. They conclude that individuals with agrammatic
aphasia are impaired in the on-line use of verb information during the
integration of individual arguments into an overall sentence context.
More recently and using MEG, Meltzer et al. (2013) assessed sentence
comprehension in aphasia by manipulating syntactic complexity in English
sentences. Their study included 25 participants with aphasia of various types
(e.g. mild-moderate anomic aphasia; mild nonfluent¸ mild to moderate fluent
aphasia; mild anomic aphasia; mild conduction aphasia; moderate Broca's
aphasia; very mild anomic aphasia) who retained sufficient language
comprehension abilities to participate in the task. Their results were compared
to data previously collected from young healthy controls assessed using the
same paradigm (Meltzer & Braun, 2011), in addition to controls that were
matched in age with the patients. A sentence-picture matching task was used
to assess the comprehension of clauses with different levels of syntactic
complexity (simple active, subject-embedded, and object embedded), which
were either semantically irreversible (simple active: e.g. The man is washing
the glass in the kitchen sink; subject embedded: e.g. The man who is washing
the car is doing a sloppy job; object-embedded: e.g. The glass that the man is
18
Marie Pourquié and Phaedra Royle
washing has a small chip in it) or reversible (simple active: e.g. The man is
teaching the woman about a hard math problem; subject-embedded: e.g. The
man who is teaching the woman is discussing a hard problem; objectembedded: e.g. The man who the woman is teaching is discussing a hard
problem).
The main goal of this study was to compare behavioral and imaging
measures within the group of aphasic patients. Moreover, this study aimed to
determine whether increased right hemisphere activity during language tasks
reflects functional takeover by regions homologous to the left-hemisphere
language networks, maladaptive interference, or adaptation of alternate
compensatory strategies. According to Meltzer et al. (2013), their data provide
a measure of neural activity related to sentence comprehension, and of
sentence content maintenance in short-term memory, which in turn allows
them to deal with the controversy opposing two points of view on the nature of
syntactic comprehension deficits in patients with aphasia (and objectembedded relative clauses, in particular): Whereas some investigators assume
a loss of specialized syntactic processing mechanisms (e.g. Caramazza,
Capitani, Rey, & Berndt, 2001; Drai & Grodzinsky, 1999), others suggest a
depletion of more general-purpose cognitive resources that are required to
handle processing demands of such sentences in working memory (Caplan,
Waters, Dede, Michaud & Reddy, 2007). As expected by the investigators,
results showed that patients performed well above chance on irreversible
sentences, and at chance on reversible sentences of high complexity on off-line
tasks. Moreover, they did not observe more activation of the right temporal
lobe in aphasics relative to controls, nor that activation correlated with
performance. But accurate comprehension was correlated with neural activity
in the more dorsal fronto-parietal lobes of the right-hemisphere, particularly
during the memory delay period. Since these areas have been assumed to be
involved in working memory (see e.g., Postle, 2006, and Wager & Smith,
2003) and other tasks involving high demands on general cognitive resources
such as attention and executive function, Meltzer and colleagues (2013)
suggest that patients who successfully compensate for left temporal lobe
damage seem to recruit more general cognitive resources during the effortful
process of determining sentence meaning. The authors interpret successful
sentence comprehension in aphasia as being supported by reanalysis of
sentences in verbal short-term memory, which may draw more heavily on right
hemispheric networks in aphasic patients than in controls.
The patient group heterogeneity is a useful addition to the discussion
about the nature of syntactic comprehension deficits, as argued by the authors:
Neuroimaging and Aphasiology in the XXI Century
19
indeed, the fact that every single one of their patients exhibited comprehension
failures on the same clause type (i.e. object-relative clause), regardless of the
size, location, and extent of their lesion, supports a resource depletion account
rather than the loss of specific comprehension mechanisms that would be
“located” in one specific part of the brain.
However, in our opinion, in relation to the experimental protocol used in
this study, some parameters should be more carefully taken into account in
assessing patients with aphasia using neuroimaging techniques. In particular,
the total time of the experiment, which was “approximately one and a half
hours, including preparation” (Meltzer et al. 2013, p. 1251), seems too long
and might induce fatigue, especially given the nature of the task (syntactically
complex sentence comprehension) and environment (staying in a machine).
After such a long session in such conditions, and because the task consisted in
pressing the left or right button on a fiber optic response box to indicate which
picture correctly depicted the action described in the sentence, the
interpretation of randomness in response patterns is questionable.
In sum, ERP and MEG studies on aphasic morphosyntactic
comprehension mentioned in this chapter concern languages less diverse than
in studies that deal with lexical processing (see previous section): English,
Dutch and German, that is only Indo-European (Germanic) languages. They
study subject-verb agreement processing in sentences with different degrees of
syntactic
complexity
(conjoined/embedded,
active/passive
and
irreversible/reversible), verb argument structure (transitive/intransitive) and
thematic roles (agent/patient), as well as semantic processing within sentences.
Generally, they track both negative (LAN/N400) and positive (SPS/P600)
neural signatures linked to lexical and syntactic processing in aphasic
individuals, in comparison with neurologically impaired participants without
aphasia or controls. Most evidence seems to point to difficult online
processing of grammatical errors and complex syntactic structures in patients,
as compared to semantic processing.
2.3. Remarks on Linguistic Manipulations in Experimental
Tasks
A common approach to semantic and word finding deficits in aphasic
patients is to study naming and other lexically-based processing, which is
clearly not optimal when we want to assess language processing in its full
complexity. The use of sentence structures to study semantics can give us a
20
Marie Pourquié and Phaedra Royle
clearer view of semantic processing in-context, and would also allow for a
better understanding of the facilitating and hindering factors in semantic
processing within sentences in aphasics. As we saw, patients' results on
semantically oriented questions in the sentence processing tasks were
occasionally similar to controls. Semantic processing within sentences, a
common technique in ERPs (Kutas & Hillyard, 1980; Kutas & Federmeier,
2000), should be developed to better understand what aspects of sentence
processing are impaired, or processed differently, in patients with aphasia,
especially those with semantic deficits.
In addition, we note that no study compared different groups of aphasic
speakers. Some, such as Wassenaar and colleagues compare neurologically
impaired patients with and without aphasia, but not between different aphasic
subgroups. Since similar symptoms can arise from different lesions -- for
example, word retrieval deficits technically called anomia, are found in many
aphasia subtypes -- it might be useful to assess neural activity of a group of
individuals with anomia in order to identify different deficits and strategies
used during lexical access, even though off-line data show the similar
superficial behavior across patients. Studies on single word lexical processing
in patients with word-finding problems could also be refined in multiple ways
using priming with phonological, orthographic, morphological or semantic
priming pairs in order to better understand the naming deficits observed
(Royle et al., 2012; Royle & Courteau, this book).
In 2008, Laganaro, Morand, Schwitter, Zimmermann and Schnider
published an ERP study on naming in four anomic patients and controls, using
delayed picture naming. Patients were tested pre and post therapy, in order to
check whether specific deficit patterns and subsequent recovery could be
mirrored by ERP patterns and changes in these over time. The delayed naming
paradigm they used avoided movement artifacts in the ERP, while allowing for
the evaluation of lexical activation in preparation for production. They asked
French-speaking participants to name a series of line drawings. Behaviorally,
patients showed improvement and maintained improved naming after
treatment. They showed intermediate results on their ERP patterns, with
normalization in the ERP signatures for the three patients with lexicalphonological difficulties but persistent deviance patterns in the patent with a
lexical-semantic impairment. Further testing after six months showed
additional pattern normalization in two patients with lexical-phonological
difficulties in early time windows (100-300 ms after stimulus presentation) but
more deviant patterns in later times windows (300-600 ms). Hemispherization
changes were also observed in the patients, which might indicate the use of
Neuroimaging and Aphasiology in the XXI Century
21
compensatory strategies in lexical access. This study seems to show that early
automatic word (or image) detection can normalize over time but later
(lexical-phonological or lexical-semantic) processes do not, despite better
behavioral responses. It also appears that specific response error types (or
strategies) can be reflected in the ERP profile. Unfortunately, little information
is presented on the interventions that were used for the patient's lexical access
difficulties.
In a follow-up study in 2009, Laganaro, Morand and Schnider tested 16
patients with anomia, half with lexical-semantic deficits and the other half
with lexical-phonological ones, and controls, again using delayed picture
naming. Here a more detailed analysis of differences between the two groups
of patients gives added depth to their data, namely showing that the N400
component linked to lexical access and integration was reduced in lexicalsemantic patients, while present in lexical-phonological ones. A later positivegoing deflection was observed in lexical-phonological patients only. This later
component is believed by the authors to reflect difficulties in word-form
encoding. This study shows that it is possible to identify different anomic
impairment types and response strategies in patients. However, one must note
that in this study, the patients were quite variable in their post onset times (1
month to 6 years). Because normalization in patients can take 6 months to one
year, and, as the previous study shows, patients pre- and post-treatment can
exhibit changing ERP patterns, the data cannot be used to conclusively support
neurocognitive distinctions between subtypes of anomia patients.
As we have seen throughout this chapter, many neuroimaging studies use
comprehension tasks. They have the added advantage of not eliciting verbal
responses in patients while targeting aspects of language that are difficult to
probe in elicitation or spontaneous speech paradigms. Future studies should
put more emphasis on morphosyntactic comprehension in aphasics and take
advantage of cross-linguistic richness in multiple languages in order to create
experiments that can assess morphosyntactic comprehension, a known domain
of difficulty in patients. For example, Pourquié (2013) demonstrates that prodrop languages are perfectly suitable for the assessment of verb inflection
comprehension in aphasia. Since the pronoun or noun phrase subject is
optionally phonologically realized in pro-drop languages, speakers are often
forced to extract grammatical information about subjects through verb
inflection only (e.g. the Spanish verb form: [PRO3s] come/ [PRO3p] comen,
eat.3s/p `s/he eats/they eat’). In comparable English contexts, the grammatical
properties of the language, i.e. obligatory overt subjects, make it unclear
whether speakers distinguish singular and plural agreement through verb
22
Marie Pourquié and Phaedra Royle
inflection (-s/Ø) or pronouns (he/they), and where patient's difficulties may lie.
Hence, pro-drop languages are prefect candidates to assess morphosyntactic
comprehension, as they allow the creation of tasks focused on verb inflection
comprehension. Conversely, since many published studies of aphasia concern
English, tasks assessing verb inflection usually investigate past tense
processing because its marking is morphologically overt: e.g. They play vs.
They played (e.g., Holland, Brindley, Shtyrov, Pulvermüller, Patterson, 2012).
There is extensive literature on tense deficits in aphasia – and past tense
processing in particular, (e.g., Bastiaanse, 2013) – reporting that individuals
with agrammatic aphasia have problems producing inflected verb forms. In
addition to assessing past tense processing using neuroimagery, it would also
be relevant to test verb comprehension in the present tense in order to identify
possible deficits not involving PAST. This would allow investigators to
examine whether or not aphasic individuals show difficulties comprehending
verb inflection, even in simple structures (e.g. active sentence in the present
tense).
Therefore, the present chapter reiterates the need for more cross-linguistic
studies and studies of multilingual speakers that would provide further insight
on the neurocognitive underpinnings of impaired linguistic systems after brain
damage, and rehabilitation. Indeed, as mentioned above, studies that assessed
morphosyntactic comprehension in patients with aphasia concern languages
that are structurally close. Given the varieties of syntactic structure
configurations found across languages (Cinque & Rizzi, 2008), more crosslinguistic comparisons would be useful to further examine these innovative but
preliminary findings.
Hence, a general remark we want to express through this chapter is that
neuroimaging studies of aphasia should not only be neuroanatomically
motivated but also linguistically and psycholinguistically driven. We have
noted that neuroimaging studies on aphasia are far more concerned with
neuroanatomic aspects of language recovery, i.e. neural plasticity, than by
cognitive flexibility. Crosson et al. (2007) note that “clearly more attention
should be focused on what are the structures that can contribute, the
circumstances under which they can contribute, how to engage those structures
in the service of language rehabilitation, and how to suppress activity in
structures that might interfere with optimizing language performance.” (p.
160). Here the word "structure" refers to anatomical structure.
From a more cognitive and linguistically driven approach, and in the
interests of both theoretical and therapeutic perspectives, it would be of
particular relevance to determine for example, which linguistic structures (e.g.
Neuroimaging and Aphasiology in the XXI Century
23
active, passive, declarative, negative, question, embedded or not) or domains
of language (e.g. lexical, morphological, syntactic, phonological, prosodic) are
recoverable and which ones are not; how peripheral and core cognitive
functions linked to language are impaired; whether the same type of linguistic
structure is equally impaired in one language or another in multilingual
patients or across languages (Menn & Obler, 1990; Gitterman, Goral & Obler,
2012); whether or not a significant relationship is identifiable between the
functional nature of recovery and neuroanatomical reorganization of the brain.
This opens a large program of research, which should not forget that aphasia
is, in its essence, a “linguistic problem” (Jakobson, 1963) and that today,
aphasiology is an interdisciplinary research field that benefits not only from
cutting-edge technologies but from the combined expertise of linguists,
psycholinguists and neuroscientists (Nespoulous, 1994). Finally, although
most studies suggest the deployment of cognitive strategies in aphasia, there
has been little characterization of these strategies in terms of linguistic and
cognitive processes.
3. WHAT DOES NEUROIMAGERY ADD
TO CLASSICAL BEHAVIORAL STUDIES
ON LANGUAGE PROCESSING IN APHASIA?
Functional neuroimaging techniques, fMRI in particular, have been
primarily used in the context of aphasiology research to study reorganization
of brain-based language substrates in aphasia. More recently studies using
EEG and MEG techniques attempt to measure brain activation involved in
different linguistic tasks in aphasic individuals. In addition to studying
language processing in patients with aphasia and to comparing it to
neurotypical language processing, neuroimaging studies can be useful in
determining training benefits and their impact on neural reorganization, in the
context of aphasiology. For instance, Cornelissen, Laine, Tarkiainen,
Järvensivu, Martin and Salmelin (2003) show that behavioral improvements
were accompanied by changes in cortical dynamics in language training (relearning) in chronic anomic patients.
Other studies have developed linguistically driven treatments. The
Treatment of Underlying Forms (TUF) (Thompson & Shapiro, 2005)
considers both the lexical and syntactic properties of the sentences used during
treatment as well as those selected for generalization testing. An important
observation emerging from their study is that training on more complex but
24
Marie Pourquié and Phaedra Royle
related structures yields more wide-ranging treatment improvement than using
simpler structures as a starting point. Such results have important implication
for intervention. Using MEG, Faroqi-sha (2008) reports results from a study of
treatment effects on recovery in 6 individuals with chronic agrammatic
aphasia. The task focused on morphosyntactic comprehension. Sentences were
correct (e.g., Yesterday the teacher graded the exams), semantically
anomalous (e.g. Yesterday the !honeybee graded the exams) or
morphologically anomalous (e.g. Tomorrow the teacher *graded the exams).
Semantically anomalous sentences were included as a control condition since
individuals with agrammatic aphasia are known to process these sentences
with high accuracy (Hagoort, Wassenaar & Brown, 2003). All participants
demonstrated quantitative and qualitative changes in neural activity following
therapy, and the final spatiotemporal patterns were similar to those found in
neurotypical participants. According to Faroqi-sha (2008), these results
demonstrate that neurophysiological changes following language therapy were
specific to the linguistic process that was targeted, and not the result of general
cognitive or compensatory mechanisms.
Thus, in the context of therapeutic research, the use of neuroimaging can
help to measure the effect of treatment patient populations, as well as on
specific language processes. Another avenue of neuroimaging research would
be to study the effects of specific types of treatment on recovery (or changing)
behavioral patterns in patients, and their link to normalization or maintenance
of deviant ERP components. ERP studies on bilingualism have recently shown
convergence on first language learner profiles by second language learner's
ERPs (White, Genesee & Steinhauer, 2012), in addition to the effects of
language training approaches on this process (Morgan-Short, Steinhauer, Sanz
& Ullman, 2012). A research program of this type integrating therapeutic
approaches should be a fruitful avenue of investigation.
In sum, studies on intervention and neuroimaging in aphasia language
processing support the following claims. 1. Neuroimaging research on
sentence and word processing in healthy speakers can be used as a
complimentary approach to behavioral studies of aphasia, in particular for the
development on neurocognitive models of language processing following
brain lesions. 2. Neuroimaging studies of aphasia can help distinguish deficits
and strategies, and specify the role of right hemisphere and perilesional areas
(as long as MEG or fMRI are used for localization) in recovery, as well as
measure treatment effects on brain activation during language processing. 3.
Neuroimaging and behavioral approaches are both necessary and provide
Neuroimaging and Aphasiology in the XXI Century
25
complimentary data on language deficits and processes, as they focus on
different aspects of language production and comprehension: neuroimaging is
not always suitable for the assessment of language production, and it cannot
clearly assess the qualitative strategic aspects of recovery, while off-line
language comprehension tasks can result in ambiguous results in terms of their
interpretation. In fact, brain activation should not be the only matter of
interest, as structures produced by patients in off-line responses to specific
targets can provide us with rich information about processes and strategies
used to provide linguistic outputs. Thus, an integrated approach to language
processing deficits in aphasia should be pursued, not only using on-line and
off-line methods for evaluation, but also with the combined knowledge of
linguistics, clinical professionals such as speech language pathologists, and
neuroscientists.
CONCLUSION
The classical syndrome approach of aphasia is considered to be unstable
for the identification of neurocognitive underpinnings of normal language
processing. Correlations between lesion areas and language impairments can
be unreliable due to systematic post-lesional reorganization of the brain and
high inter-individual differences in aphasic populations. However,
neuroimaging allows us to examine patients as individuals or groups, and can
be regarded as a complementary tool to the behavioral approach, in the context
of aphasiology, especially in view of the information they can provide
regarding the times course of underlying cognitive processes. On the other
hand, many challenges exist within different neuroimaging techniques for the
study of normal language processing, and even more exist in aphasiology.
Despite the complexities of neuroimaging experimental design, some studies
are being carried out on aphasia and open a promising research pathway.
Aphasiological research enriched by the development of neuroimaging
addresses issues such as the differentiation between language deficits and
cognitive strategies used to overcome these. Brain activation and areas
involved in language recovery, and treatment effects on cognitive flexibility
and brain plasticity are some of the domains that can be directly assessed using
neuroimaging techniques. Since behavioral results show dissociations not
observable in imaging, the classical syndrome approach should however not
be abandoned or overlooked. In particular, this chapter outlines the necessity
26
Marie Pourquié and Phaedra Royle
to qualify the linguistic nature of deficits and strategies in addition to
illustrating them through brain imaging. As both approaches fulfill different
functions and bring to the fore qualitatively different data, aphasiological
research should be able to take advantage of their distinctive and
complimentary benefits in further studies.
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