37_Using principal component analysis to capture individual differences

c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9

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Special issue: Research report

Using principal component analysis to capture
individual differences within a unified
neuropsychological model of chronic post-stroke
aphasia: Revealing the unique neural correlates of
speech fluency, phonology and semantics

Ajay D. Halai*, Anna M. Woollams and Matthew A. Lambon Ralph*

Neuroscience and Aphasia Research Unit, University of Manchester, UK

article info abstract

Article history: Individual differences in the performance profiles of neuropsychologically-impaired
Received 14 January 2016 patients are pervasive yet there is still no resolution on the best way to model and ac-
Reviewed 10 March 2016 count for the variation in their behavioural impairments and the associated neural
Revised 15 April 2016 correlates. To date, researchers have generally taken one of three different approaches: a
Accepted 18 April 2016 single-case study methodology in which each case is considered separately; a case-series
Published online 29 April 2016 design in which all individual patients from a small coherent group are examined and
directly compared; or, group studies, in which a sample of cases are investigated as one
Keywords: group with the assumption that they are drawn from a homogenous category and that
Individual differences performance differences are of no interest. In recent research, we have developed a
Principal component analysis complementary alternative through the use of principal component analysis (PCA) of
Post-stroke aphasia individual data from large patient cohorts. This data-driven approach not only generates
Symptomelesion mapping a single unified model for the group as a whole (expressed in terms of the emergent
principal components) but is also able to capture the individual differences between
patients (in terms of their relative positions along the principal behavioural axes). We
demonstrate the use of this approach by considering speech fluency, phonology and
semantics in aphasia diagnosis and classification, as well as their unique neural corre-
lates. PCA of the behavioural data from 31 patients with chronic post-stroke aphasia
resulted in four statistically-independent behavioural components reflecting phonolog-
ical, semantic, executiveecognitive and fluency abilities. Even after accounting for lesion
volume, entering the four behavioural components simultaneously into a voxel-based
correlational methodology (VBCM) analysis revealed that speech fluency (speech
quanta) was uniquely correlated with left motor cortex and underlying white matter
(including the anterior section of the arcuate fasciculus and the frontal aslant tract),

* Corresponding authors. Neuroscience and Aphasia Research Unit, School of Psychological Sciences, Zochonis Building, University of
Manchester, Brunswick Street, Manchester M13 9PL, England, UK.

E-mail addresses: [email protected] (A.D. Halai), [email protected] (M.A. Lambon Ralph).
http://dx.doi.org/10.1016/j.cortex.2016.04.016
0010-9452/© 2016 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/4.0/).

276 c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9

phonological skills with regions in the superior temporal gyrus and pars opercularis, and
semantics with the anterior temporal stem.
© 2016 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC

BY license (http://creativecommons.org/licenses/by/4.0/).

1. Introduction severity changes task performance (which otherwise is falsely
considered to reflect a behavioural dissociation). An alterna-
For both clinical practice and basic research, it is important to tive approach, single-case study, possesses the opposite set of
understand the patterns and neural bases of impaired per- advantages and disadvantages. Of course, whilst single-case
formance observed in both individual and groups of neuro- study avoids averaging away important behavioural dissoci-
psychological patients. Since the beginning of behavioural ations, it means that it is hard/impossible to place each pa-
neurology and neuropsychology, contrastive impairments tient within a broader context or to generate a coherent,
across patients have been used to make important inferences overarching cognitive and neuroanatomical model of the
about the underlying cognitive computations and the neural disorder. From a basic science perspective, this approach
structures that support them. In addition, stable models of could lead to the logical absurdity of there being as many
variable patient performance are critical in accurate neuro- theories as there are patients; whilst, from a clinical
logical differential diagnosis, clinical management and perspective, it reduces our ability to generalise clinical
treatment. knowledge from one patient to another e ultimately making it
impossible to successfully diagnose, manage and treat the
The generation of stable, reliable models of normal and patient's impairments. The single-case study makes it
impaired function rely on the ability to understand the nature impossible to map the relationship between severity of dam-
and sources of individual differences across patients. This has age and level of performance (which, like any mathematical
always been a key challenge for the field (Shallice, 1988), function, requires more than one datum). Over the past
leading to numerous debates and discussions (Caramazza & decade or so, multiple researchers have adopted a hybrid of
McCloskey, 1988; Lambon Ralph, Patterson, & Plaut, 2011; these two approaches in the form of case-series studies
Schwartz & Dell, 2011; Shallice, 1988), and arguably still is. (Hodges, Patterson, Oxbury, & Funnell, 1992; Lambon Ralph,
The kernel of the problem relates to the fact that there are McClelland, Patterson, Galton, & Hodges, 2001; Schwartz,
multiple sources that underlie variable neuropsychological Dell, Martin, Gahl, & Sobel, 2006; Woollams, Lambon Ralph,
results. Previous formal considerations of the issue have set Plaut, & Patterson, 2007). These investigations recruit a sam-
out a series of potential factors (Lambon Ralph, Moriarty, & ple of patients whom are all studied in detail at the individual
Sage, 2002; Shallice, 1988) but, for brevity, we will note three level (i.e., often to the same precision as single-case studies),
types here: (A) the type of difference that neuropsychological which can be used: to assess the consistency of performance,
dissociations and differential diagnosis are based on, namely i.e., the coherence, in the sampled cases (Lambon Ralph et al.,
stable performance differences that arise when a task is 2001; Schwartz et al., 2006; Woollams et al., 2007); to highlight
supported by one or more computations underpinned by patterns/mechanisms/lesion correlates which generalise
neuroanatomically-separated regions; (B) differences that reliably across cases; to identify meaningful, consistent pat-
relate, linearly or nonlinearly, to the severity of damage to the terns of individual differences (rather than random noise:
underlying neurocomputational component(s) e which can Woollams et al., 2007); as well as to relate impairment severity
provide critical information about the nature and character- to task performance (leading to mathematical/computational
istics of the core neurocognitive computations, as well as the models of the severity-based functions: Lambon Ralph et al.,
degree of recovery/deterioration in a patient's disease or dis- 2001; Schwartz et al., 2006; Woollams et al., 2007). Further-
order; and (C) sources of individual differences of no interest more, ‘comparative’ case-series can be used to strengthen
including random variations, measurement noise, and so important behavioural dissociations by demonstrating that,
forth. despite variations in severity, (i) the two case-series are sys-
tematically different in the expected way, yet (ii) are internally
Over the years, researchers have adopted different ap- coherent (Jefferies & Lambon Ralph, 2006; Lambon Ralph,
proaches to this issue. The most classical and common Lowe, & Rogers, 2007).
approach (also utilised by experimental psychology and
functional neuroimaging) is to recruit a coherent sample of Whilst the case-series methodology clearly has a number
patients and, through the power of central tendency, average of strengths which benefit basic science and clinical practice,
the behavioural results or lesion distributions in order to like group studies, the approach still relies heavily both on the
remove noise and other individual variations of no interest. As ability to recruit reasonably coherent patient groups and also
noted previously (Shallice, 1988), this method relies on having to know, a priori, what the relevant groupings of patient are.
or defining a ‘coherent’ group and, at worst, can suffer from Indeed, we note here that one of the most common uses and
two types of statistical error: (a) failing to uncover meaningful development platforms of the case-series methodology were
differences within the sample of patients which is lost made through investigations of semantic dementia (Hodges
through the averaging process; and (b) failing to detect or et al., 1992; Lambon Ralph et al., 2001; Woollams et al., 2007),
understand how systematic variation in patient/lesion which is a highly consistent group of patients and thus makes

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this methodology particularly successful and powerful. It is in Before considering the PCA findings in this study, we
this context, therefore, that we consider a related approach e first briefly consider some of the key clinical characteristics
the use of principal component analysis (PCA) as a data-driven of chronic post-stroke aphasia. Speech production, phono-
method which uses the patterns of individual differences in logical and semantic impairments are common following
order both to reveal the statistically-reliable distinctions left hemisphere stroke and the nature of these deficits
within a patient dataset and also to place individual cases, forms the principal behavioural divisions in most aphasia
relative to each other, in the resultant multi-dimensional classification systems. Typically, patients are first divided
model (Butler, Lambon Ralph, & Woollams, 2014; Lambon into fluent (relatively effortless speech output) or non-
Ralph et al., 2002; Lambon Ralph, Patterson, Graham, fluent types (effortful, slow or reduced complexity and
Dawson, & Hodges, 2003). In effect, the PCA method spans length) and then additionally by the status of their
group and individual levels of analysis because the emergent phonological and semantic skills (Goodglass, 1993;
set of principal components provide a coherent, generalisable Goodglass & Kaplan, 1983; Kertesz, 1982). Despite their
set of factors or model of the underlying systems whilst, un- prominence in aphasia diagnostics and importance for
like group studies, variations in individual patient perfor- understanding intact language function, the unique neural
mance can be placed systematically along one or more of the correlates of speech dys-/fluency, semantic and phonolog-
principal axes. If a group is relatively homogenous then PCA ical deficits remain unclear. Furthermore, when these have
only generates one principal component along which all pa- been considered, they have been investigated separately
tients can be mapped. If, however, there are systematic dif- and thus the inter-relationship between them is unclear
ferences within the cohort, then one or more statistically- (e.g., is speech fluency, in part, a reflection of patients'
independent (orthogonal) factors emerge. In contrast unsys- phonological or semantic skills?).
tematic sources of variation, such as measurement error and
other random fluctuations are left as unexplained variance (or Classically, damage to left frontal areas was associated
loaded onto many additional components with little explan- with poor fluency (Broca, 1861); however, studies using high-
atory power, i.e., with eigenvalues less than 1). In addition, in resolution Magnetic Resonance Imaging (MRI) techniques
more recent work, we have demonstrated that the PCA factors have found that left frontal lesions do not always result in
can also be used to explore behaviourebrain correlates in a Broca's aphasia (Basso, Lecours, Moraschini, & Vanier, 1985;
new fashion; rather than traditional lesion-overlapping Willmes & Poeck, 1993) and production deficits can occur
methods which rely on coherent group studies (Caramazza following lesions outside of the frontal lobe, including white
& McCloskey, 1988; Shallice, 1988): or correlating raw test matter tracts and the anterior insula (aINS) (Bates et al., 2003;
scores with brain voxel status (as per lesionesymptom map- Blank, Scott, Murphy, Warburton, & Wise, 2002; Damasio,
ping: Bates et al., 2003), we have found that clear results 1992; Dronkers, 1996; Mohr et al., 1978; Wise, Greene, Bu¨ chel,
emerge when the behavioural principal components are & Scott, 1999). Functional neuroimaging studies have identi-
related to the patients' lesion distributions (Butler et al., 2014; fied a broader neural network during connected-speech pro-
Chase, 2014). duction. For example, by comparing propositional with
automatic speech (e.g., counting), a positron emission to-
In this study, therefore, we demonstrate and use the PCA mography (PET) imaging study found extensive left hemi-
approach in order to explore the nature of post-stroke sphere activation including the frontal lobe, pre-
aphasia. Aphasia is an interesting test case/problem for supplementary motor area (pre-SMA), angular gyrus (AG),
multiple reasons. Whilst it is possible to distinguish be- fusiform gyrus (FG), and perisylvian areas (Blank et al., 2002). A
tween patients with and without acquired language im- large-scale left hemisphere speech production network was
pairments, within the aphasic group there are multiple also identified in a study that measured fluency, complexity
sources of individual differences. Traditionally, category- and variety of post-stroke aphasic speech (Borovsky, Saygin,
based classification schemes have been developed in order Bates, & Dronkers, 2007). These critical sub-components
to assist with different diagnosis of aphasic subgroups, could not be separated when they were related to the pa-
clinical management and interventions, as well as for basic tients' neural damage, which hints at the multi-faceted nature
science studies that have tried to relate aphasic profiles to of speech fluency. Specifically, considerable anatomical
underlying lesion distributions. Whilst these category-based overlap was found across the measures: the number of speech
systems may provide a useful ‘broad brushstroke’ summary tokens (T) correlated with aINS, pre-motor cortex (PMC); mean
for individual patients, there are multiple limitations which length of utterance (MLU) with aINS, PMC and anterior supe-
follow from the fact that the underlying distribution of pa- rior temporal gyrus (aSTG); and typeetoken ratio (TTR) with
tient data reflects continuous variations along multiple di- AG, posterior middle temporal gyrus (pMTG) and aSTG. This
mensions rather than coherent, mutually-exclusive outcome probably reflects the inter-correlations between both
categories. Instead, there are both very fuzzy boundaries the behavioural measures and lesion distributions (middle
between ‘categories’ and considerable variation of profile cerebral artery e MCA lesions tend to sample the same subset
within each ‘category’. If this conceptualisation of post- of brain regions) (Phan, Donnan, Wright, & Reutens, 2005), and
stroke aphasia is correct then we might be able to use PCA because severity-related variance is shared across individual
in order to define the core underlying dimensions and the measurements. In addition, it is likely that fluency itself is a
relative positions of each individual patient along these multi-faceted aphasic feature (Basilakos et al., 2014), which
axes. Secondly, in turn, this multi-dimensional behavioural may benefit from decomposition into unique elements before
description can then be compared directly to the variation in the relationship with lesions can be satisfactorily explored.
the patients' lesion distributions. Plus, to identify the neural correlates of the full range of

278 c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9

aphasic features, it is critical to consider fluency simulta- scanning, were pre-morbidly left handed, had more than one
neously alongside phonological and semantic skills. stroke or had any other significant neurological conditions.
Informed consent was obtained from all participants prior to
To achieve this aim, the current study utilised the PCA participation under approval from the local ethics committee.
methodology (Butler et al., 2014; Chase, 2014) to reveal, for the Structural imaging data from a healthy age and education
first time, the unique neural correlates of speech fluency, se- matched control group (eight female, 11 male) were used to
mantic and phonological abilities in chronic post-stroke determine the lesion outline in the patients using the Seghier,
aphasia. This extends previous findings by taking into ac- Ramlackhansingh, Crinion, Leff, and Price (2008) lesion iden-
count speech fluency measures as part of the wider behav- tification toolbox.
ioural profile (phonological and semantic deficits), which is a
key dimension on which aphasia patients are typically clas- 2.2. Neuropsychological assessments and analysis
sified. Specifically, two approaches for tackling the analytical
challenges associated with brainebehaviour mapping in In order to assess speech production deficits, participants
chronic aphasia are to derive statistically-independent were asked to undertake a picture description task (‘Cookie
behavioural factors/dimension from a rotated PCA, and theft’ from the Boston Diagnostic Aphasia Examination)
lesion volume as a covariate for anatomical severity. Previous (Goodglass & Kaplan, 1983), which was audio recorded. Par-
studies using PCA on neuropsychological data have also ticipants were instructed to “tell me everything you see going
shown that it offers various benefits. First, there is additional on in this picture”. In addition to the fluency measures, we
statistical reliability by combining results from multiple tests utilised an extensive battery of neuropsychological tests to
(Lambon Ralph et al., 2002, 2003). Secondly, by maintaining assess participants' language and cognitive abilities (described
orthogonal components, it is possible to avoid problems of in Butler et al., 2014), enabling us to understand how speech
collinearity when undertaking lesionesymptom correlations. production measures relate to the patients' input and output
Thirdly, the use of varimax rotation promotes a clear cognitive phonological, semantic and general executive abilities. These
interpretation of each of the principal components. Finally, included subtests from the Psycholinguistic Assessments of
when the rotated components are combined with Language Processing in Aphasia (PALPA) battery (Kay, Lesser,
lesionesymptom mapping, it is possible to establish the & Coltheart, 1992): auditory discrimination using non-word
neural correlates of unique components of post-stroke (PALPA 1) and word minimal pairs (PALPA 2); and immediate
aphasic behaviour (Butler et al., 2014; Mirman, Chen, et al., and delayed repetition of non-words (PALPA 8) and words
2015; Mirman, Zhang, Wang, Coslett, & Schwartz, 2015). (PALPA 9). Tests from the 64-item Cambridge Semantic Battery
(Bozeat, Lambon Ralph, Patterson, Garrard, & Hodges, 2000)
In summary, this study investigated the multi- were included: spoken and written versions of the word-to-
dimensionality of post-stroke aphasic deficits and fluency picture matching task; Camel and Cactus Test (CCT e pic-
classification using this novel PCA-lesion mapping approach ture); and the picture naming test. To increase the sensitivity
(Butler et al., 2014). The investigation was conducted in three to mild naming and semantic deficits we used the Boston
stages: (i) confirmation of the basic results from a previous Naming Test (BNT) (Kaplan, Goodglass, & Weintraub, 1983)
focussed-investigation of post-stroke fluency (Borovsky et al., and a written 96-trial synonym judgement test (Jefferies,
2007) e which represents a more “standard” approach to Patterson, Jones, & Lambon Ralph, 2009). The spoken sen-
brainebehaviour correlation; (ii) decomposition of speech tence comprehension task from the Comprehensive Aphasia
production measures using rotated PCA and identification of Test (CAT) (Swinburn, Baker, & Howard, 2005) was used to
the neural correlates; and then, most importantly, (iii) simul- assess sentential receptive skills. The additional cognitive
taneous decomposition of a large behavioural dataset that tests included forward and backward digit span (Wechsler,
included measures of speech production, phonology, seman- 1987), the Brixton Spatial Rule Anticipation Task (Burgess &
tics and executive abilities using an omnibus PCA, and sub- Shallice, 1997), and Raven's Coloured Progressive Matrices
sequent identification of their unique neural correlates. (Raven, 1962). All scores were converted into percentage. As-
sessments were conducted with participants over several
2. Methods testing sessions, with the pace and number determined by the
participant.
2.1. Participants
The ‘Cookie theft’ description was recorded and then
Thirty one chronic stroke patients (either ischaemic or hae- transcribed. The coding procedure followed the method used
morrhagic) were recruited (the same patients as reported by by Borovsky et al. (2007). Each utterance was marked and
Butler et al., 2014), who had impairment in producing and/or bound morphemes, repetitions, false starts, retraces, unin-
understanding spoken language and their aphasia was clas- telligible material and interruptions were coded separately.
sified using the Boston Diagnostic Aphasia Examination Repeated and retraced utterances were excluded from anal-
(BDAE). No restrictions were placed according to aphasia type ysis and only correct full utterances were coded. When the
or severity (spanning global to minimal aphasia). All patients boundary of an utterance was unclear, or quite lengthy,
were at least 12 months post-stroke at the time of scanning transcribers applied the rule from Lee (1974) that only one
and assessment, were native English speakers with normal or “and” conjunction per sentence was allowed when the “and”
corrected-to-normal hearing and vision (see Table 1 for a connected two independent clauses. From these transcrip-
summary of demographic details). Our selection criteria tions, we extracted the number of word T and types, TTR,
excluded participants if they had any contraindications for number of morphemes and MLU in morphemes. In order to

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Table 1 e Participant background information. Cases are ordered according to BNT score.

Initials Age (years) Gender Years of Time post-stroke Lesion volume BDAE classification
education (months) (voxels)
Wernicke
1 DBb 66 M 12 59 42,687 Global
2 ES 69 M 11 39 28,146 Global
3 ESb 68 M 11 142 33,193 Broca
4 KW 81 M 10 24 11,393 Broca
5 BS 59 M 11 103 27,242 Mixed non-fluent
6 KL 55 M 13 31 14,625 Global
7 LM 63 M 11 13 14,990 Wernicke
8 DB 60 M 12 44 31,644 Wernicke/conduction
9 PE 73 F 16 22 Transcortical sensory aphasia
10 KS 59 M 12 12 6959 Broca
11 KK 48 M 12 33 5822 Mixed non-fluent
12 WM 77 M 11 66 20,043 Broca
13 GL 47 M 12 18 33,282 Broca
14 DCS 45 F 12 12 26,319 Mixed non-fluent
15 JSa 73 M 11 190 5273 Broca
16 JSc 78 M 12 76 45,875 Mixed non-fluent
17 JA 65 M 11 128 18,459 Anomia
18 JJ 84 M 12 25 26,097 Anomia
19 JM 62 M 11 110 8951 Anomia
20 JSb 72 M 11 23 15,492 Mixed non-fluent
21 ER 64 F 14 181 1481 Anomia
22 HN 81 M 10 56 26,480 Mixed non-fluent
23 BH 64 M 11 26 25,963 Anomia
24 EB 61 M 17 12 8795 Broca
25 DM 49 M 17 42 4806 Transcortical motor aphasia
26 DS 72 M 11 106 11,915 Broca
27 AG 55 M 11 131 11,446 Anomia
28 LH 65 M 11 81 21,270 Anomia
29 JMf 70 F 11 84 10,073 Anomia
30 AL 49 F 12 69 8921 Anomia
31 TJ 60 M 12 23 9767
19,975

control for the length of response given by each participant, production factors related to the three previously-extracted
we also computed words-per-minute (WPM). We considered (Butler et al., 2014) factors relating to phonology, semantic
these measures as indices of speech fluency as they are and executive abilities.
among the simplest to quantify and mirror those commonly
used in other studies and clinical practice. There are a number 2.4. Acquisition of neuroimaging data
of additional measures that could be used to elaborate the
model of fluency, including syntactical features (i.e., see High-resolution structural T1-weighted MRI scans were ac-
Thompson et al., 2012). These, however, require a more open- quired on a 3.0 T Philips Achieva scanner (Philips Healthcare,
ended speech sample and are relatively time-consuming to Best, The Netherlands) using an eight-element SENSE head
code. We believe that between the four measures used here, coil. A T1-weighted inversion recovery sequence with 3D
we are capturing the amount of speech output within a acquisition was employed, with the following parameters: TR
confined context. All scores were converted into percentage, (repetition time) ¼ 9.0 msec, TE (echo time) ¼ 3.93 msec, flip
where the max score across participants was used. angle ¼ 8, 150 contiguous slices, slice thickness ¼ 1 mm, ac-
quired voxel size 1.0 Â 1.0 Â 1.0 mm3, matrix size 256 Â 256,
2.3. Principal component analysis field of view (FOV) ¼ 256 mm, TI (inversion time) ¼ 1150 msec,
SENSE acceleration factor 2.5, total scan acquisition
This analysis was split into two parts. First, only the speech time ¼ 575 sec.
production measures were entered into a PCA with varimax
rotation (SPSS 16.0). Factors with an eigenvalue exceeding 1.0 2.5. Analysis of neuroimaging data
were extracted and then rotated. Following orthogonal rota-
tion, the loadings of each test allowed a clear behavioural Structural MRI scans were pre-processed with Statistical
interpretation of each factor. Individual participants' scores Parametric Mapping software (SPM8: Wellcome Trust Centre
on each extracted factor were then used as behavioural for Neuroimaging, http://www.fil.ion.ucl.ac.uk/spm/). The
covariates in the neuroimaging analysis. A second PCA was images were normalised into standard Montreal Neurological
performed on a larger dataset which combined the back- Institute (MNI) space using a modified unified segmenta-
ground neuropsychological data with speech production tionenormalisation procedure optimised for focal lesioned
measures. This allowed us to determine how the speech brains (Seghier et al., 2008). Data from all participants with

280 c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9

stroke aphasia and all healthy controls were entered into the Wilson, & Stamatakis, 2005), a variant of voxel-lesion symp-
segmentationenormalisation. This procedure combines tom mapping (VSLM: Bates et al., 2003). VBCM does not require
segmentation, bias correction and spatial normalisation a binary classification of the intact/lesioned brain to be
through the inversion of a single unified model (see marked, as in the case of VSLM, as both the behaviour and
Ashburner & Friston, 2005 for more details). In brief, the tissue concentration measures are treated as continuous
unified model combines tissue class (with an additional tis- variables (conducted in SPM8). Three analyses were conduct-
sue class for abnormal voxels), intensity bias and non-linear ed and all reported anatomical regions were located on the left
warping into the same probabilistic models that are assumed hemisphere. First, each fluency measure was analysed sepa-
to generate subject-specific images. Images were then rately, in an attempt to confirm the basic results from
smoothed with an 8 mm full-width-half-maximum (FWHM) Borovsky et al. (2007). Secondly, the factor scores from the first
Gaussian kernel and used in the lesion analyses described speech production PCA were simultaneously entered to
below. The lesion of each patient was automatically identi- investigate how variation in tissue concentration corre-
fied using an outlier detection algorithm, compared to sponded to the unique components of speech dys-/fluency.
healthy controls, based on fuzzy clustering. The default pa- Finally, the participants' factor scores from the omnibus PCA
rameters were used apart from the lesion definition ‘U- of the entire neuropsychological test battery and speech
threshold’, which was set to .5 to create a binary lesion measures were entered into a VBCM analysis. In order to
image. We modified the U-threshold from .3 to .5 after ensure that the results were not merely attributable to lesion
comparing the results obtained from a sample of patients to size, each participants' lesion volume was calculated from the
what would be nominated as lesioned tissue by an expert lesion identified by the automated lesion identification
neurologist. The images generated for each patient were method (Seghier et al., 2008) and this was entered as a co-
individually checked and visually inspected with respect to variate in each VBCM. Hence, all analyses were performed
the original scan, and were used to create the lesion overlap with and without a correction for lesion volume. All
map in Fig. 1 and the individual lesion outlines in Fig. 3. We anatomical labels were based on the HarvardeOxford atlas in
selected the Seghier et al. (2008) method as it is objective and MNI space.
efficient for a large sample of patients (Wilke, de Haan,
Juenger, & Karnath, 2011), in comparison to a labour inten- 3. Results
sive hand-traced lesion mask. The method has been shown
to have a Dice overlap >.64 with manual segmentation of the 3.1. Neuropsychological and lesion profiles
lesion and >.7 with a simulated ‘real’ lesion (where real le-
sions are superimposed onto healthy brains; Seghier et al., Table 2 summarises the participants' scores on the speech
2008). We should note here, explicitly, that although production measures and a large neuropsychological battery
commonly referred to as an automated ‘lesion’ segmentation (Butler et al., 2014) and is ordered according to patients' scores
method, the technique detects areas of unexpected tissue on the BNT. A lesion overlap map for stroke aphasic partici-
class e and, thus, identifies missing grey and white matter pants is provided in Fig. 1A, and primarily covers the left
but also areas of augmented cerebral spinal fluid (CSF) space. hemisphere area supplied by the MCA (Phan et al., 2005). The
maximum number of participants who had a lesion in any one
We used the T1-weighted images with continuous signal voxel was 26 (À45, À23, 27; ventral portion of the postcentral
intensity values across the whole brain and correlated these gyrus).
with individual measures or PCA factor scores using a voxel-
based correlational methodology (VBCM: Tyler, Marslen-

Fig. 1 e (A) Lesion overlap map across the 31 patients (threshold 1e26). (B) VBCM between regional integrity and lesion
volume (t-scale 3e6).

Table 2 e Participant scores on the behavioural assessment battery and speech production me

ID NW NW W W rep Picture Boston NW min W min SWPM WWPM
rep I rep d rep I d naming naming pairs
pairs es
0 0 22.22
DBb 0 0 37.5 0 4.69 0 48.61 52.78 57.81 31.25
ES 0 0 00 0 0 54.17 54.17 78.13 90.63
ESb 0 0 00 1.56 0 75 50 87.5 60.94
KW 0 0 3.75 0 3.13 1.67 65.28 65.28 95.31 92.19
BS 3.33 0 5 1.25 4.69 1.67 75 75 92.19 100
KL 0 0 6.25 0 1.56 1.67 43.06 77.78 92.19 98.44
LM 13.33 3.33 7.81 8.33 87.5 54.17 67.19 53.13
DB 70 30 27.5 0 20.31 11.67 77.78 58.33 64.06 76.56
PE 13.33 3.33 85 83.75 31.25 13.33 94.44 86.11 96.88 100
KS 73.33 80 45 41.25 42.19 15 72.22 95.83 71.88 67.19
KK 33.33 3.33 93.75 95 39.06 25 47.22 95.83 93.75 95.31
WM 36.67 30 56.25 26.25 68.75 31.67 98.61 63.89 92.19 75
GL 93.33 63.33 55 41.25 67.19 43.33 97.22 97.22 96.88 95.31
DCS 40 56.67 100 81.25 62.5 46.67 75 97.22 100 98.44
JSa 30 3.33 72.5 68.75 71.88 53.33 75 77.78 92.19 98.44
JSc 36.67 63.33 75 65 79.69 63.33 90.28 86.11 98.44 98.44
JA 36.67 40 90 91.25 85.94 63.33 51.39 95.83 100 98.44
JJ 36.67 23.33 85 78.75 81.25 63.33 93.06 80.56 98.44 98.44
JM 83.33 83.33 82.5 73.75 75 63.33 76.39 95.83 100 100
JSb 63.33 36.67 100 98.75 71.88 65 81.94 88.89 93.75 90.63
ER 53.33 36.67 86.25 81.25 65.63 65 77.78 88.89 95.31 93.75
HN 36.67 23.33 70 81.25 95.31 66.67 93.06 76.39 93.75 93.75
BH 86.67 80 83.75 80 81.25 66.67 94.44 94.44 98.44 93.75
EB 83.33 53.33 100 96.25 75 71.67 80.56 98.61 98.44 100
DM 60 10 100 100 84.38 73.33 79.17 93.06 98.44 98.44
DS 56.67 33.33 73.75 68.75 87.5 78.33 100 77.78 100 100
AG 73.33 83.33 88.75 91.25 81.25 78.33 95.83 98.61 100 100
LH 56.67 50 77.5 87.5 96.88 80 90.28 97.22 96.88 100
JMf 93.33 66.67 82.5 88.75 93.75 88.33 91.67 95.83 100 100
AL 90 90 96.25 98.75 95.31 95 87.5 100 100 100
TJ 93.33 83.33 100 98.75 98.61 98.44 100
98.75 92.5

Abbreviations: W e word; NW e non-word; rep I e immediate repetition; rep D e delayed repetition; S/WW
backward digit span.
CAT column refers to spoken sentence comprehension subtest.

tion measures.

WPM CAT Synonym CCT Brixton Ravens Digit Digit WPM TTR MLU T

esentence FB

.25 12.5 48.96 53.13 38.18 30.56 25 0 63.33 53.91 60.74 23.89 c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9
.63 25 72.92 73.44 40 66.67 0 0 15.12 94.09 35.19 24.63
.94 34.38 52.08 43.75 23.64 38.89 00 0 0 0 0
.19 84.38 82.29 89.06 50.91 80.56
78.13 84.38 38.18 91.67 50 42.86 2 86.25 12.35 2.99
31.25 68.75 78.13 61.82 88.89 0 0 52.61 63.45 48.15 21.65
.44 28.13 57.29 68.75 32.73 61.11 0 0 11.87 44.72 21.16 13.44
.13 28.13 59.38 82.81 40 86.11 0 0 14.51 46 18.52 7.47
.56 31.25 79.17 84.38 41.82 80.56
84.38 68.75 52.73 86.11 37.5 14.29 17.42 38.33 65.36 89.58
50 81.25 84.38 76.36 100 25 28.57 49.1 60.62 93.7 100
.19 84.38 61.46 51.56 43.64 61.11 100 57.14 70.53 86.6 90.53 60.47
.31 46.88 75 73.44 58.18 91.67
91.67 95.31 81.82 100 0 0 14.75 62.21 50 45.54
50 81.25 76.56 67.27 83.33 37.5 28.57 5.44 65.71 22.22 10.45
.31 65.63 76.04 82.81 43.64 77.78 37.5 28.57 9.24 55.86 29.97 52.26
.44 93.75 63.54 87.5 61.82 80.56 62.5 57.14 14.83 92.74 70.37 23.14
.44 59.38 93.75 53.13 43.64 41.67 50 28.57 30.84 74.41 49.38 25.38
.44 75 82.29 81.25 76.36 94.44 62.5 42.86 23.9 90.36 61.11 20.9
.44 78.13 75 78.13 60 86.11 37.5 0 46.26 74.41 60.74 25.38
.44 56.25 84.38 90.63 41.82 38.89 62.5 42.86 41.98 85.19 82.72 40.31
85.42 85.94 25.45 75 50 57.14 79.16 77.63 84.77 59.72
100 83.33 73.44 67.27 66.67 62.5 28.57 41.18 92 47.62 26.13
.63 84.38 94.79 90.63 80 100 25 0 46.49 82.14 95.06 47.03
.75 56.25 95.83 98.44 50.91 91.67 50 42.86 25.4 91.27 91.36 47.03
.75 37.5 93.75 89.06 72.73 72.22 62.5 57.14 36.77 78.68 60.49 28.37
.75 78.13 89.58 75 56.36 75 75 57.14 78.87 67.16 98.52 93.31
92.71 87.5 76.36 88.89 37.5 0 23.63 84.74 50.79 28.37
71.88 91.67 93.75 50.91 83.33 50 28.57 34.53 100 100 17.17
.44 56.25 93.75 79.69 60 91.67 100 100 13.06 80.5 54.81 22.4
88.54 70.31 52.73 50 87.5 57.14 66.11 74.33 52.91 61.21
87.5 62.5 57.14 50.53 69 73.15 48.52
87.5 87.5 85.71 100 86.25 87.65 44.79
90.63 75 28.57 38.28 76.67 81.48 38.07
71.88
84.38
68.75

n; S/WWPM e spoken/written wordepicture matching; Digit F e forward digit span; Digit B e

281

282 c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9

3.2. Confirmation of previous basic findings on speech 3.4. Identifying principal speech-production components
fluency alone
The speech production measures were subjected to a rotated
A bivariate correlation analysis on the speech production PCA and produced a two independent factor solution, which
measures showed a high degree of inter-correlation. The accounted for 84.65% of variance in participants' performance
token measure was correlated with MLU and WPM (r ¼ .633, (F1 ¼ 59.28%; F2 ¼ 25.35%). The factor loading of each pro-
p < .001 and r ¼ .513, p < .001 respectively). The MLU measure duction measure is given in Table 3. The measures that tapped
was correlated with TTR and WPM (r ¼ .505, p ¼ .004 and into overall quantity of speech (e.g., number of words and
r ¼ .674, p < .001 respectively). No other correlations reached WPM) loaded heavily on factor 1; hence we refer to this factor
significance. As noted by Borovsky et al. (2007), it is not un- as ‘speech quanta’. Factor 2 was interpreted as ‘semantic va-
expected to find inter-correlations between these measures. riety’, as the TTR measurement loaded heavily on it (e.g.,
Indeed, the shared variance could reflect common factors proportion of unique words spoken). The MLU loaded heavily
underlying speech, including semantics, phonology and gen- on factor 1, while loading to a lesser degree on factor 2. Lesion
eral aphasia severity. As highlighted in the Introduction, these analysis for each factor, entered simultaneously, is displayed
high inter-correlations provide a challenge when attempting in Fig. 2E. The speech quanta measure (F1) was uniquely
to relate the behavioural measures to unique aspects of the correlated with the left pre-central gyrus, superior insula and
patients' lesions (see below). putamen, and the underlying white matter (including the
superior longitudinal fasciculus, caudateepremotor tracts
The first VBCM analysis examined the neural correlates and the frontal ‘aslant’ tract (FAT) that connects the medial-
of fluency (T), complexity (MLU), semantic variety (TTR) and superior portion of the frontal lobe to the inferior-lateral
WPM each entered separately (Fig. 2AeD). The results for T frontal region) (Basilakos et al., 2014; Catani et al., 2012;
revealed involvement of the superior-posterior insula and Rojkova et al., 2015). The semantic variety factor (F2) was
portions of the frontal lobe including the pre-central gyrus uniquely correlated with the SMG and pMTG/STG plus the
and middle frontal gyrus. The status of underlying white anterior/orbito frontal gyrus. Weaker correlation is observed
matter was also correlated. The analysis for MLU revealed along the STG to anterior lateral portions. After adding a co-
an overlapping albeit broader region, extending throughout variate for lesion volume, no significant clusters remained for
the left inferior frontal gyrus (IFG), insula and putamen, F1 or F2.
along the SMA, motor cortex and the underlying white
matter. 3.5. Identifying principal speech, language and executive
behavioural factors
The results for TTR, contrasted with those for T and
MLU, in revealing a region that extended from super- In order to determine the relationship between speech pro-
marginal gyrus (SMG) and pMTG/STG through to aSTG as duction measures and a range of language and cognitive skills,
well as a portion of the anterior/orbito frontal gyrus. Peak the factor scores from the speech PCA were correlated with
voxels were in the IFG/frontal pole, SMG and posterior su- the factor scores for phonology, semantics and executive
perior temporal gyrus (pSTG). Finally, the analysis for WPM abilities, computed in our previous study (Butler et al., 2014).
revealed no significant correlations with lesion areas, The speech quanta factor only correlated with the first
although at an uncorrected voxel height of p ¼ .01 (cluster (phonology) factor (r ¼ .404, p ¼ .01). The semantic variety
extent 50 voxels) the results were very similar to those speech factor only correlated with the second (semantics)
obtained for T. factor (r ¼ .452, p < .001). To understand the relationship be-
tween speech fluency and various language and cognitive
3.3. Effect of lesion size measures more thoroughly, an omnibus PCA was conducted.
This resulted in a four factor solution including phonological
Given that some regions are more likely than others to be skills (54.8% variance), semantic ability (11.6% variance) and
damaged after MCA stroke (Phan et al., 2005), we explored cognitiveeexecutive function (8.3% variance). We note here
the intrinsic relationship between lesion location and size, that a variety of different tasks over-and-above the executive
and then controlled for this factor in the subsequent tests loaded on this third factor. These include measures
lesionesymptom analyses. Each participant's volume was associated with auditoryephonological processing (e.g., min-
calculated from the lesion identified by the modified seg- imal pairs) or semantic memory (e.g., CCT), although in both
mentationenormalisation procedure. Lesion volume itself cases there was additional high loadings on the component
was correlated with the integrity of various regions with the one would typically associate with these tests. This outcome
MCA territory (Fig. 1B), representing the outer belt of the lesion probably reflects the fact that the PCA process ‘decomposes’
overlap map (Fig. 1A). Crucially, including lesion size in the the components of each task (reflecting the fact that no task is
VBCM analysis for each speech measure reduced the signifi- a pure measure of a single underpinning cognitive function).
cance across all measures and the underlying neural corre- In this case, all the tasks that load onto the third factor require
lates (see Fig. 2AeD). The TTR and T measures did not a high degree of additional ‘problem-solving’ and executively-
correlate with any regions after accounting for lesion volume loaded working memory.
(even at a reduced image threshold of p < .01, FWE-cluster
corrected p < .05). MLU was the only measure to reveal sig- In this omnibus PCA, the ‘semantic variety’ characteristics
nificant relationships with the left superior insula, putamen of speech production were subsumed into the general se-
and underlying white matter after accounting for lesion mantics factor and a new additional fourth factor emerged
volume.

c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9 283

Fig. 2 e VBCM analysis using each speech production measure as the dependent measure [each t-map (scale 3) is
thresholded at p < .005, cluster corrected at FWE of p < .001; without lesion volume correction]. Note each result reflects the
raw (non-unique) correlation with each speech fluency measure (see text). (A) Lesion correlates for T. (B) Lesion correlates for
mean length per utterance, (C) TTR. (D) Lesion correlates for WPM (uncorrected threshold p ¼ .01, voxel-extent 50). (E) Lesion
correlates of the unique PCA factors: F1 reflects speech quanta (yellow) and F2 reflects semantic variety (cyan). Note e no
significant cluster remains for any factor after lesion volume correction (see text for further details & Fig. 1B).

Table 3 e Factor loadings for PCA of speech production based entirely on ‘speech quanta’ (6.3% variance) (see Table 4
measures. for factor loadings and Table 5 for the patients' factors scores).
The VBCM analysis for each independent factor, entered
Measure F1 F2 simultaneously, is shown in Fig. 3A and peak MNI co-
ordinates are shown in Table 6. Each cluster shows where
TTR .082 .973 tissue concentration covaries uniquely with a given factor
MLU .803 .488 score, after accounting for lesion volume (the results without
T .904 À.157 the lesion volume covariate were very similar). Results are
WPM .800 .258 thresholded at p < .005 voxel-level, p .001 family-wise error
(FWE)-corrected cluster-level.
Footnote: Factor loadings exceeding .5 are marked in bold.

284 c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9

Table 4 e Factor loadings from the omnibus PCA. Performance on the phonological factor was uniquely
correlated with voxels across a number of left hemisphere
1 234 regions: primary auditory cortex (Brodmann area 41 and 42);
mid to posterior middle and superior temporal gyri (MTG and
Non-word repetition immediate .822 .108 .220 .326 STG); superior temporal sulcus (STS); and posterior portions of
Non-word repetition delayed .912 .106 .126 .175 the insula, Heschl's gyrus and the planum temporale. The
Word repetition immediate .780 .230 .122 .430 phonological cluster also overlapped with white matter re-
Word repetition delayed .783 .315 .168 .411 gions, encompassing part of the arcuate fasciculus, a key
Forward digit span .841 .239 .058 .198 aspect of the dorsal language pathway (Catani & ffytche, 2005;
Backward digit span .774 .191 .166 .070 Catani, Jones, & ffytche, 2005; Duffau et al., 2005; Parker et al.,
Non-word minimal pairs .584 .203 .683 .167 2005; Saur et al., 2008).
Word minimal pairs .586 .421 .505 .237
CAT e sentence .726 .388 .374 À.050 Performance on the semantic factor was uniquely related to
Cambridge 64-item naming .712 .600 .096 .168 a cluster in the left hemisphere anterior temporal lobe (ATL;
BNT .659 .629 .013 .209 see Fig. 3A). The cluster overlapped with the anterior-MTG and
the temporal stem (including the dorsal edges of the inferior
96-Synonym judgement .353 .714 .347 .244 temporal gyrus e ITG and FG). Thus, with regards to white
matter, the cluster included part of the ventral language route,
Spoken word-to-picture matching .311 .722 .378 À.190 overlapping with the inferior longitudinal fasciculus, inferior
fronto-occipital fasciculus and uncinate fasciculus (Duffau,
Written word-to-picture matching .156 .699 .578 .022 Gatignol, Moritz-Gasser, & Mandonnet, 2009; Mummery, Ash-
burner, Scott, & Wise, 1999; Parker et al., 2005; Saur et al., 2008).
Camel and Cactus e pictures À.078 .461 .702 .276
Raven's Coloured Progressive .091 .034 .918 .079 Performance on the speech quanta factor was uniquely
Brixton spatial anticipation test .391 .232 .629 .018 related to a cluster in the left hemisphere pre-central gyrus,
superior insula and putamen, extending medially to the
TTR .235 .749 .055 .160 caudate nucleus. With regards to white matter, the cluster
WPM .310 .103 .013 .716
Mean length per utterance .294 .397 .000 .814
T .154 À.152 .350 .818

Footnote: Factor loadings exceeding .5 are marked in bold.

Table 5 e PCA factor scores from Butler et al. (2014), fluency-only PCA and the omnibus PCA.

Butler et al. (2014) Speech PCA All scores combined

F1-phon F2-sem F3-exe F1-flu F2-var F1 F2 F3 F4

DBb À.332 À2.321 À2.018 .290 À.499 À.775 À1.600 À2.606 1.043
ES À1.668 À.266 À.331 À1.024 .797 À1.842 .384 À.564 À.256
ESb À1.078 À.701 À1.360 À1.923
KW À1.211 À.202 À1.911 À1.853 À3.030 À.300 À1.773 À1.290 À1.929
BS À1.935 .267 .972 .540 À.832 .387 .700
KL À1.810 À.072 .678 À.136 À1.952 .249 .604 .120
LM .972 À1.046 À.217 À1.391 À1.175
DB À.873 À1.682 À1.171 À1.185 À.843 À.516 1.193
PE .315 À2.295 À.765 À1.072 À.149 À1.445 À.946 À.606
KS .594 1.151 À2.058 À1.426 À2.555 1.491
KK À1.067 .358 .459 1.961 1.274 .658 1.723
WM 1.726 À2.395 .718 1.209 À.980 À1.129 À.090 .706 1.274
GL À.164 .616 À1.524 À.015 À.149
DCS À1.207 .161 1.319 À1.364 .363 1.504 À1.458
JSa À.036 À.525 À1.344 À.284 À.642 .971 À.158 À1.198 À.715
JSc .618 À.361 À.642 À.297 .346 À.567 1.107 À.819
JA .208 .583 À.453 À1.181 À.240 À1.148 1.351 À.398
JJ À.321 .045 1.680 À.643 1.080 .461 .221 À.500
JM .416 .309 À.099 .009 .564 À.344 À.170
JSb À.124 .356 .233 .148 .131 .358 .417
ER .236 .593 À.256 .307 .947 .998 .657 À2.027 .348
HN .993 1.429 1.328 .279 .537 .377 .805 .683
BH .472 À.053 .308 À.456 .706 À.622 1.420 .003 À.409
EB À.241 .006 À2.197 .227 À.534 À.210 À.957 1.189
DM À.089 1.337 .677 .854 1.422 .269 À.754 .927
DS 1.204 .712 .863 .253 .612 .451 1.273 À.198 À.522
AG .760 .216 .120 À.214 .909 À.727 1.048 1.215 1.739
LH À.607 .174 À1.092 2.256 .392 À.051 .090 .551 À.013
JMf .128 1.269 À.692 À.566 À.526 1.810 À.204 À.282 .129
AL 1.419 1.045 À.296 À.199 .525 .765 1.239 À.018 À1.321
TJ .704 .210 1.150 À.759 1.760 .755 1.653 .885 .234
.811 .354 .420 .830 .451 1.353 .244 .294 .467
1.446 .737 .041 .624 À.240 1.165 .101 .000 .834
1.141 .262 À.064 1.285 À.131 .391 À1.013 .161
1.029 .807 .265 .842 .365
.110 .373 .718
.228
À1.289

c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9 285

Fig. 3 e (A) VBCM analysis showing significant and unique neural correlates to phonological (red), semantic (blue) and
speech quanta (green) performance, including lesion volume covariate. Image thresholded at p < .005, cluster corrected at
FWE of p < .01 [image scale (t) 3e5]. Illustrative exemplar cases are shown in panels B and C. Panel B (patients with good
executive scores) e patient EB (yellow) with good fluency versus KW (orange) with poor fluency. Panel C (patients with poor
executive skills) e patient KS (pink) with good fluency versus ESb (purple) with poor fluency.

Table 6 e Neural correlates for omnibus PCA factors after accounting for lesion volume.

Principal component Location Extent (voxels) Z MNI co-ordinates
xyz

Factor 1 (phonology) Left temporal lobe 4387 4.24 À48 À16 0
Factor 2 (semantic) Planum polare 1030 4.23
Factor 4 (speech quanta) Posterior supramarginal gyrus 2164 4.06 À48 À50 14
Posterior middle temporal gyrus
3.68 À62 À18 À14
Left temporal lobe 3.63
Anterior middle temporal gyrus 2.77 À60 À6 À26
Anterior temporal fusiform cortex À38 À6 À28
Posterior ITG 3.85 À70 À24 À30
3.80
Left prefrontal lobe 3.58 À24 À4 32
Corticospinal tract À30 À4 28
Pre-central gyrus À54 8 32
Pre-central gyrus

included an area corresponding to the anterior part of the utility, it is important to explore how individual results relate
dorsal language route (superior longitudinal fasciculus) as to the group-level maps for each languageecognitive factor.
well as the caudateepremotor tracts and the FAT (Rojkova Secondly, exemplar cases help interpretation of the behav-
et al., 2015). It is notable that the effect of lesion size on the ioural factors and their neural correlates (given that PCA, by
speech quanta factor when calculated using all neuropsy- design, generates scores that are one step removed from raw
chological scores is negligible which was not the case when clinical measures: see Lambon Ralph et al., 2003). To achieve
using the speech production measures alone (see above, Figs. this aim, the group was divided into two using a median split
2E and 3A). based on executive factor scores to cover a range of patient
types. Two participants were selected from each split to
In contrast to the phonological, semantic and fluency fac- provide contrasting pairs who scored high or low on the
tors, there were no clusters that correlated uniquely with the speech quanta factor (see Fig. 3B and C). In the high executive
executive factor score which survived correction for multiple group, patient KW (Broca's aphasia) had the lowest speech
comparisons. This factor did, however, correlate with lesion quanta score (orange outline) and EB (anomic) had the
volume in a simple correlation [r(31) ¼ À.373, p ¼ .039], sug- highest (yellow outline). In the low executive group, patient
gesting that larger lesions result in poorer executive ability. ESb (global) had the lowest speech quanta score (purple
outline) and KS (transcortical sensory aphasia) the highest
3.6. Individual cases (pink outline). It is clear and encouraging from these exam-
ples that the individual patient lesions and associated
The relationship of individual patients to the group-level behavioural profiles both align with the group-level VBCM
analyses was explored for two reasons. First, to test clinical

286 c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9

results and are stable regardless of overall executive func- uniquely correlated with damage to left perisylvian areas,
tioning (as should be expected given that the PCA approach including left mid to posterior STG, MTG, STS and Heschl's
extracts statistically-independent factors). gyrus (HG), as well as the underlying white matter (arcuate
fasciculus portion of the dorsal language pathway). The se-
4. Discussion mantic factor correlated uniquely with left anterior-MTG and
the underlying temporal stem. The speech quanta factor
Mirroring the principal features of natural language, speech correlated uniquely with left hemisphere pre-central gyrus,
fluency, phonological and semantic deficits are common superior insula and putamen, extending medially to the
following left MCA stroke. In most aphasia classification sys- caudate nucleus. With regards to white matter, the cluster
tems, patients are first divided by speech fluency followed by included an area corresponding to the anterior part of the
the relative balance of the patients' phonological and se- dorsal language route (superior longitudinal fasciculus) as
mantic abilities (Goodglass & Kaplan, 1983; Kertesz, 1982). By well as the caudateepremotor tracts and the FAT. The ex-
measuring and applying novel analyses to the overall rate, ecutive factor did not covary, uniquely, with any brain re-
complexity and variety of naturalistic speech simultaneously gions, as found previously (Butler et al., 2014).
with other measures of language performance, we were able
to provide insights into their neural substrates. The results The speech quanta-related region included key cortical
provide novel insights not only about the core features and and subcortical areas associated with deficits of articulatory
neural bases of naturalistic language, but also how these un- planning (Bates et al., 2003; Borovsky et al., 2007; Dronkers,
derpin the varieties and nature of post-stroke aphasia. 1996) and motor coordination of speech related movements
(Ackermann & Riecker, 2004). Likewise, direct stimulation of
In the first analysis, we largely confirmed the basic speech ventral PMC and pars opercularis leads to speech inhibition
fluency results from Borovsky et al. (2007), by showing that the (Kinoshita et al., 2014). The underlying white matter damage
rate of T and MLU in the patients' speech correlated with the might also be crucial and, indeed, the core of the identified
insula, SMA and underlying white matter. In contrast, speech quanta-related region overlaps with the FAT (Rojkova
TTR correlated with the SMG, pMTG, aSTG, and anterior/orbito et al., 2015; see Fig. 4). The integrity of this tract has been
frontal gyrus. Whilst these results are interesting and support shown to correlate with non-fluent speech in primary pro-
previous observations that speech fluency is a multi-faceted gressive aphasia (Catani et al., 2013; Mandelli et al., 2014) and,
aphasic feature (Ackermann & Riecker, 2004; Basilakos et al., when combined with the integrity of the anterior section of
2014), their interpretation is complicated by the fact that the arcuate fasciculus, to correlate with clinician-rated
these behaviourelesion correlations were not significant fluency in post-stroke aphasia (Basilakos et al., 2014). In
when lesion volume was controlled and no account is taken of keeping with our finding of a separate principal component
the patients' other aphasic features e thus the picture of the for speech quanta over and above phonology and semantic
lesionesymptom mappings is incomplete. skills, Catani et al. (2013) did not find a relationship between
FAT integrity and repetition or syntactic abilities in primary
We addressed these limitations by utilising the PCA-VBCM progressive aphasia. Additional convergent evidence has been
methodology in the two subsequent stages. We initially un- derived from neurosurgical investigations: direct electrical
dertook a PCA of the speech production measures alone. This stimulation of FAT inhibits speech and subsequent surgical
PCA suggested that the clinical concept of ‘fluency’ reflects resection leads to transient postoperative inhibition
two more general and independent factors: ‘speech quanta’ (Kinoshita et al., 2014).
(the quantity of speech produced) and ‘variety’ (the range of
different words/token irrespective of the quantity produced). The results for the phonological, semantic and executive
Scores from the speech quanta factor correlated uniquely factors reproduced those observed in our previous study and
with tissue damage in the left pre-central gyrus, superior related investigations (Butler et al., 2014; Mirman, Chen, et al.,
insula and putamen, and the neighbouring white matter 2015; Mirman, Zhang, et al., 2015). It is interesting to note that
(including the superior longitudinal fasciculus, caudateepre- speech ‘variety’ seems to be inherently linked to the patients'
motor tracts and the FAT that connects the medial-superior degree of remaining semantic abilities (factor 2) presumably
portion of the frontal lobe to the inferior-lateral frontal re- because the use of a wide vocabulary is underpinned by fine
gion) (Rojkova et al., 2015). In contrast, the speech variety semantic distinctions. More generally, the observed neural
factor was uniquely related to the SMG and pMTG/STG plus correlates of the phonological and semantic factors are highly
the anterior/orbito frontal gyrus. However, again the results consistent with data from other methods. Phonological pro-
for these speech production-only data were largely cessing was associated with left HG, mid to posterior MTG, STG
confounded with lesion volume. and STS, which have all been identified in various previous
reviews of phonological processing (Hickok & Poeppel, 2004,
These twin challenges of inter-correlation and severity 2007; Price, 2010, 2012; Vigneau et al., 2006; Wise et al., 2001)
were overcome in the final step in which we completed an and repetitive transcranial magnetic stimulation (rTMS) to
omnibus PCA containing all the detailed behavioural data pSTG has been shown to increase error rates in the speech
and the speech production measures, which also allowed us production of neurologically-intact participants (Acheson,
to explore the relationship between speech fluency and other Hamidi, Binder, & Postle, 2011). The neural substrate for se-
aphasic features (Basilakos et al., 2014). This analysis mantic performance (left anterior temporal regions) again
revealed four unique factors (phonology, executive process- converges with findings from large-scale functional magnetic
ing, semantic function which included the speech variety resonance imaging (fMRI) reviews (Binder, Desai, Graves, &
measure, and speech quanta). The phonological factor Conant, 2009; Price, 2010; Vigneau et al., 2006; Visser,

c o r t e x 8 6 ( 2 0 1 7 ) 2 7 5 e2 8 9 287

Fig. 4 e Overlap (yellow) between the speech quanta-related lesion correlate (green) and the FAT (red). The speech quanta
image is thresholded at p < .005, FWE of p < .01 [image scale (t) 3e5], and the FAT is a probabilistic map (image scale .5e1).

Jefferies, & Lambon Ralph, 2009), data from semantic dementia rather than the strong inter-correlations that are found be-
(Mion et al., 2010), rTMS (Pobric, Jefferies, & Lambon Ralph, tween the raw test results). Whilst we have applied this
2007), direct electrical stimulation of inferior fronto-occipital PCA-VBCM approach to post-stroke chronic aphasia in the
fasciculus (IFOF) (Duffau, Gatignol, Mandonnet, Capelle, & current investigation, this methodological approach should
Taillandier, 2008) and neuroanatomically-constrained models be applicable and beneficial across a range of acute and
(Ueno, Saito, Rogers, & Lambon Ralph, 2011). progressive neurological conditions.

One thing that must be kept in mind when using PCA is Acknowledgements
that the factor solution is always dependent on the tests
included in the decomposition. Thus, if an analysis fails to We are grateful to all the patients and carers for their
include behavioural measures that reflect a core feature of continued support of our research programme. We thank Dr.
the target clinical populations then, by definition, the Rebecca Butler for collection of the speech data analysed in
resultant PCA model will not derive a principal dimension this study and to Prof. Marco Catani and Dr. Michel Thiebaut
for this clinically-relevant feature. Our current PCA model de Schotten for providing the anatomical overlay for the FAT.
contains measures of phonological, semantic, speech This research was support by funding from the Rosetrees
fluency and executive skill. Whilst this model captures Trust, an NIHR (UK) senior investigator award and a Medical
some of the core aspects of post-stroke aphasia, future Research Council (UK) programme grant to MALR (MR/
studies will need to augment the range of assessments in J004146/1).
order to capture other key features of this clinical disorder
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