Special issue: Original article
The arcuate fasciculus and the disconnection theme in
language and aphasia: History and current state
Marco Catania,
* and Marsel Mesulamb
a
Natbrainlab, Section of Brain Maturation, King’s College London, Institute of Psychiatry, London, UK
b
Cognitive Neurology and Alzheimer’s Disease Center, Northwestern University, Chicago, IL, USA
a r t i c l e i n f o
Article history:
Received 27 March 2008
Reviewed 11 April 2008
Revised 14 April 2008
Accepted 15 April 2008
Keywords:
Arcuate fasciculus
Aphasia
Diffusion tensor imaging (DTI)
Language
Tractography
a b s t r a c t
Few themes have been more central to neurological models of aphasia than the disconnection
paradigm and the role of the arcuate fasciculus. Introduced by luminaries of 19th
Century neurology and resurrected by the charismatic work of Norman Geschwind, the
disconnection theme has triggered spectacular advances of modern understanding of
language and aphasia. But the disconnection paradigm had alternate fortunes, ranging
from irrational exuberance to benign neglect, and its followers have not always shared
the same view on its functional consequences and anatomical correlates. Our goal in
this paper is, first, to survey the 19th Century roots of the connectionist approach to
aphasia and, second, to describe emerging imaging technologies based on diffusion tensor
imaging (DTI) that promise to consolidate and expand the disconnection approach to
language and its disorders.
ª 2008 Elsevier Masson Srl. All rights reserved.
1. Introduction
Language is an exceedingly complex faculty that allows us to
encode, elaborate and communicate thoughts and experiences
through the mediation of arbitrary symbols known as
words. The coherent function of the language network and
its interactions with other neurocognitive networks depend
on an orderly set of interconnections. Much of current understanding
of language-related pathways is based on the pioneering
work of 19th Century neuroanatomists, such as Reil,
Burdach, Meynert, Wernicke, Dejerine. In the 1960s, in a series
of influential papers, Geschwind crystallized those early
anatomical findings adding new insights into brain connectivity
as derived from anatomical, physiological and
neuronographic studies both in animals and humans (Geschwind,
1965, 1970; Geschwind and Levitsky, 1968).
The neuroanatomy of the human brain that Geschwind
relied on was based principally on hand dissection of fixed
specimens and the tracing of degeneration in sections stained
for myelin. Recent developments in magnetic resonance
imaging have introduced new methods, based on diffusion
tensor imaging (DTI) tractography (see also Jones, 2008, this
issue; Catani and Thiebaut de Schotten, 2008, this issue) that
can reconstruct white matter pathways in the living human
brain. The resultant influx of information on human connectional
anatomy is likely to modernize the disconnection approach
to behavioural neurology and to reinvigorate models
of cognition based on distributed large-scale networks (Catani
* Corresponding author. Natbrainlab, Section of Brain Maturation PO50, Institute of Psychiatry, De Crespigny Park, SE5 8AF London, UK.
E-mail address: m.catani@iop.kcl.ac.uk (M. Catani).
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cortex
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and Mesulam, 2008, this issue). An overview of these trends,
and of their historical contexts, with a special focus on the arcuate
fasciculus and language, constitutes the subject matter
of this paper.
2. Disconnection accounts of language
disorders
The term disconnection is generally used to indicate classical
syndromes where lesions to white matter connections lead to
dysfunction of higher cognitive abilities (Catani and ffytche,
2005; Mesulam, 2005). The term became popular in the second
half of the 19th Century following Wernicke’s (1874) description
of the disconnection syndrome that was to become the
prototype for all others – conduction aphasia. Wernicke, like
his predecessor Theodore Meynert, conceived the brain as
a mosaic of areas containing ‘memory images’ related to
motor acts (localized in primary motor areas) and sensory experiences
(localized in primary visual, somesthestic, auditory,
olfactory and gustatory areas). He also assumed that higher
cognitive functions, in contrast to movements and perceptions,
are not localized in specific regions but emerge from
associative connections linking areas where images of motor
and sensory memories reside. On the basis of this ‘general
principle’, Wernicke (1874) elaborated the first network model
of language (Fig. 1): ‘.the first frontal gyrus [third frontal circonvolution
according to modern nomenclature], which has motor
function, acts as center for motor imagery; the first temporal gyrus,
which is sensory in nature, may be regarded as the centre of acoustic
images; the fibrae propriae, converging into the insular cortex, form
the mediating arc reflex.’ He argued that ‘aphasia may be caused by
any disruption of this pathway, the clinical picture, however, may
vary considerably and is related to the specific segment of the pathway
involved.’ According to Wernicke, the ‘production of spontaneous
movement, that is, the consciously formulated word, would be
brought about by the rearousal of the motor image through the associated
memory image of the sound.’ Spontaneous speech, in his
opinion, resulted from the interaction of distant cortical areas.
Consequently, he interpreted the characteristic paraphasic
speech of patients with conduction aphasia as the expression
of the inability of temporal regions to monitor Broca’s area
speech output through subinsular connections. Wernicke’s
model was the forerunner of current network models of
cognition. His greatest merit was to anchor his ideas into the
clinical–anatomical correlation method, where he coupled
a careful description of the behavioural disturbances of his
patients to the anatomical findings from post-mortem dissections.
With him aphasiology became a discipline intimately
concerned with the connectional anatomy of the human
brain.
In France, the associationist theories were popularized by
Charcot who brought Wernicke’s ideas to his medical trainees
during his ‘lec¸ons du Mardi’ at the Salpetriere (Gelfand, 1999).
However, it was Jules Dejerine who formulated the most elegant
contribution of French neurology to the disconnection
paradigm. He beautifully explained the occurrence of reading
difficulties (i.e., pure alexia) in a patient with otherwise normal
writing ability using a pure disconnection mechanism,
which he was able to demonstrate with post-mortem dissections
(Epelbaum et al., 2008, this issue).
Shortly after Wernicke’s description of conduction aphasia,
Lichtheim (1885) extended the disconnection paradigm
to give a comprehensive account of different aphasic syndromes.
He hypothesized that Broca’s and Wernicke’s areas
are interconnected to an hypothetical ‘‘concept center’’ (not
anatomically localized) and added to Wernicke’s nomenclature
two other forms of aphasia, i.e., transcortical sensory
and transcortical motor aphasia, that he interpreted as resulting
from the disconnection of the concept center from the
motor and auditory language centers, respectively (Fig. 2). In
transcortical sensory aphasia, heard words cannot reach the
thought center leading to impairment in understanding
words, in transcortical motor aphasia thoughts cannot be
verbalised due to impaired transfer of inputs from the thought
center to Broca’s area.
Lichtheim translated Wernicke’s ideas into simple and
intuitive diagrams that became standard references for clinicians.
However, Lichtheim also introduced hypothetical centers
and connections backed by little supportive evidence.
His diagrams served the purpose of fitting a theoretical
framework that best explained clinical empirical observations
without a necessary anatomical correspondence. These diagrams
promoted a mechanical view of brain function where
connections represented ‘transferring devices’ between stores
of specialized information localized in individual cortical
Fig. 1 – Carl Wernicke (1848–1905) and his representation of the language network from his 1874 MD thesis.
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areas. This approach to brain function generated a wave of
criticisms and the clinico-anatomical correlation method
came under attack by many prominent investigators including
Hughling Jackson, Von Monakow, Henry Head, Karl
Lashley and Kurt Goldstein (for a review see Finger, 1994).
In many respects these authors brought forward important
criticisms that are still valid in modern neuroscience. First,
they warned that localization of symptoms and localization
of function were not identical. For example, for John Hughlings
Jackson, there was no doubt that verbal fluency is more
likely to be affected by damage to the left hemisphere than
the right hemisphere. Jackson had difficulty, however, with
the belief that observable symptoms specified the locations
of special centers for the affected functions. He argued that
it was entirely possible that some symptoms could be due to
secondary effects of the damage on other regions of the brain,
a distant ‘hodological effect’ according to more recent
terminology (Catani and ffytche, 2005; Catani, 2007). He also
believed that lesions were more useful for finding out what
the remaining unaffected parts of the brain did without the
benefit of the damaged area than what the damaged area
did when it was part of the intact brain (Finger, 1994).
This dialectic between the localizationists and their opponents
lasted for several decades, until the work of Norman
Geschwind in the 1960s. Geschwind brought new credibility
to the localizationist approach by re-interpreting the functional
role of connections and specialized cortical areas
according to evidence arising from the new neuroscience of
the 20th Century. He also extended the disconnection paradigm
beyond white matter lesions to lesions of association
cortex. In Geschwind’s (1965) model, even a lesion confined
to association cortex could cause a disconnection syndrome,
little distinction being made between such lesions and those
restricted to white matter tracts (see also Glickstein and
Berlucchi, 2008, this issue). He argued that ‘lesions of association
cortex, if extensive enough, act to disconnect primary receptive or
motor areas from other regions of the cortex in the same or in the
opposite hemisphere.. Thus a ‘disconnexion lesion’ will be a large
lesion either of association cortex or of the white matter leading
from this association cortex’ (Geschwind, 1965).
Based on this broader view, Geschwind reappraised conduction
aphasia as a disconnection syndrome resulting
either from a lesion of the white matter connections or of
the perisylvian cortex, the latter acting as relay station between
Wernicke’s and Broca’s areas. In Geschwind’s view,
Wernicke’s aphasia could also be conceptualized as a disconnection
syndrome (Fig. 3). He argued for ‘the importance of the
angular gyrus in acting as a region involved in corss-modal associations,
particularly in cross-association between either vision, or
touch and hearing. If the angular gyrus is important in the process
of associating a heard name to a seen or felt object, it is probably
also important for associations in the reverse direction. A name
passes through Wernicke’s area, then via the angular gyrus
arouses associations in the other parts of the brain’ (Geschwind,
1965). Wernicke’s aphasia could then result either from a lesion
of Wernicke’s area or of its connections to the angular
gyrus. But Geschwind admitted that his intuitions, pending
experimental anatomical evidence, were to be regarded as
‘speculative’.
With the advent of new information arising from structural
and functional imaging, it appeared that parts of the
Geschwind–Wernicke model represented an over-simplification.
Kempler et al. (1988), for example, showed that lesions to the
arcuate fasciculus were associated with hypometabolism in
Wernicke’s and Broca’s areas only in 50% of the patients,
the remaining showing hypometabolism only in Wernicke’s
area. Furthermore, the Geschwind–Wernicke model predicted
that lesions at any point along the course of the arcuate
fasciculus result in an identical aphasia. Yet, clinically, this
emerged not to be the case with conduction aphasias forming
a heterogeneous group ranging from ‘‘Broca-like’’ to ‘‘Wernicke-like’’
deficits (Levine and Calvanio, 1982). These studies
began to raise questions concerning the validity of existing
neurocognitive formulations of language.
The dilemma that aphasiologists in specific, and behavioural
neurologists in general, had to face stemmed
Fig. 2 – Ludwig Lichtheim (1845–1928) and his representation of the language network from his 1885 Brain paper.
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principally from the lack of sufficient information on human
neuroanatomy (see also Catani and Mesulam, 2008, this
issue). In contrast to the giant strides made in unravelling
the connectivity of the monkey brain, the details of connection
pathways in the human brain remained stuck in the
methodology of the 19th Century. In a scientific commentary
in Nature Crick and Jones (1993) voiced these concerns to the
scientific community: ‘‘to interpret the activity of the living
human brains, their anatomy must be know in detail.’’ They
urged the ‘‘development of new techniques since most of
the methods used in the monkeys cannot be used on
humans.’’ A year later, in 1994, Basser et al. (1994) published
their seminal paper where they describe for the first time DTI.
DTI, coupled to tractography, offers a non-invasive technique
that reconstructs white matter trajectories in the living
human brain (see also Jones, 2008, this issue; Catani and
Thiebaut de Schotten, 2008, this issue). By measuring the
diffusivity of water along different directions and by tracing
a pathway of least hindrance to diffusion, DTI tractography
can visualise continuous pathways as inferred from the movement
of water molecules subjected to a magnetic gradient
(Basser et al., 2000; Le Bihan, 2003). Tractography findings
are not necessarily equivalent to data obtained from postmortem
dissections. Nevertheless, tractography results are
likely to reflect highly reproducible features of the human
brain anatomy (Catani et al., 2002; Wakana et al., 2004), and
tractography-based dissections currently represent the only
way to study the connectional anatomy of language pathways
in living subjects. As will be shown below, the anatomy of the
arcuate fasciculus is one question that has been addressed
very fruitfully by DTI tractography.
3. The anatomy of the arcuate fasciculus
Reil (1809, 1812) was the first to identify, almost two Centuries
ago, a group of fibres running deeply into the white matter
of the temporal, parietal and frontal regions located
around the Sylvian fissure of each hemisphere (Fig. 4). In
1822 Burdach (1819–1826) described in detail this system of
perisylivan fibres and named it the Fasciculus Arcuatus (Arcuate
fasciculus), for the arching shape of its longest fibres. Subsequently,
Dejerine (1895) confirmed the findings of the
German neuroanatomists but attributed the discovery to Burdach.
Dejerine also believed that the arcuate fasciculus was
mainly composed of short associative fibres connecting
Fig. 3 – Norman Geschwind (1926–1984) and his representation of the language network from his 1970 Science paper.
Fig. 4 – Johann Christian Reil (1759–1813) and his description of the arcuate fibres from his 1812 Archiv fu¨r die Physiologie
paper.
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neighbouring perisylvian cortex. As we have seen above,
Wernicke hypothesized that language relied on the integrity
of a ‘‘psychic reflex arc’’ between temporal and frontal regions.
But the arcuate fasciculus was not part of Wernicke’s
original anatomical model (Wernicke, 1874). He thought
that the temporal and frontal language areas were mutually
interconnected by fibres passing through the external capsule
and relaying in the cortex of the insula. It was Constantin
Von Monakow who first identified the arcuate fasciculus
as the tract connecting Broca’s and Wernicke’s areas,
a view later accepted by Wernicke in 1908 (Geschwind,
1967). Von Monakov’s statement soon became a dogma in
neurology and still today provides the backbone of anatomical
models of language.
4. Recent contribution from DTI tractography
Although the existence of the arcuate fasciculus has been
confirmed in several post-mortem studies in humans, these
methods (e.g., blunt dissections, axonal staining of degenerating
axons, etc.) have not shed much light on the detailed
anatomy of the relevant fibres. More powerful methods have
been used to trace homologous axonal pathways in the
monkey but the absence of language in non-human primates
raises doubts on the possibility of translating connectional
anatomy of putative language pathways from animal to man.
Tractography studies are showing that the anatomy of the
arcuate fasciculus is more complex than previously thought
(Fig. 5) (Catani et al., 2005). In addition to the long direct segment
connecting Wernicke’s area with Broca’s area, there is
an indirect pathway consisting of two segments, an anterior
segment linking Broca’s territory with the inferior parietal
lobule and a posterior segment linking the inferior parietal
lobule with Wernicke’s territory. This arrangement not only
supports the more flexible architecture of parallel processing
(Mesulam, 1990), but also is in keeping with some of the
classical neurological models of aphasia, contemporary
models of verbal working memory (Baddeley, 2003) and recent
functional neuroimaging findings (Jung-Beeman, 2005; Sakai,
2005; Stephan et al., 2003). Additional support for the existence
of the three perisylvian segments of the ‘‘arcuate fasciculus’’
comes from human intraoperative electrocorticography
(Matsumoto et al., 2004), functional connectivity (Schmithorst
and Holland, 2007), post-mortem dissections (Lawes et al.,
2008), and experiments in homologous parts of the monkey
brain (Deacon, 1992).
Another unexpected finding derived from the tractography
dissections of the arcuate fasciculus is the extension of its
putative cortical terminations beyond the classical limits of
Broca’s and Wernicke’s areas to include part of the middle
and precentral frontal gyrus and the posterior middle temporal
gyrus, respectively (Catani et al., 2005).
Fig. 5 – Tractography reconstruction of the arcuate
fasciculus. Numbers indicate the cortical projections of the
segments: 1, superior temporal lobe; 2, middle temporal
lobe; 3, inferior frontal and precentral gyrus; 4, middle
frontal and precentral gyrus; 5, supramarginal gyrus; 6,
angular gyrus (mod. from Catani et al., 2005).
Group2 mild left
lateralization (~20%)
Group3 bilateral,
symmetrical (~20%)
Group1 extreme left
lateralization (~60%)
A
Group1
(85%)
Group2 (10%)
Group3
(5%)
MalesFemales
Group1
(40%)
Group2 (30%)
Group3
(30%)
B C
Fig. 6 – Distribution of the pattern of lateralization of the long segment in the normal population and between genders (mod.
from Catani et al., 2007).
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Hemispheric asymmetry is a key feature of the language
network. Differences in the arcuate fasciculus between left
and right hemispheres have been demonstrated by microscopic
examination of post-mortem specimens (Galuske
et al., 2000), by structural T1 MRI (Paus et al., 1999) and by
DTI (Buchel et al., 2004; Hagmann et al., 2006; Nucifora
et al., 2005; Powell et al., 2006; Catani et al., 2007). Tractography
analysis of the degree of lateralization of the three segments
(as measured by an indirect index of segment
volume) showed an extreme degree of leftward lateralization
in w60% of the normal population (Fig. 6A) (Catani
et al., 2007). The remaining w40% of the population shows
either a mild leftward lateralization (w20%) or bilateral,
symmetrical pattern (w20%). An extreme degree leftward
lateralization is observed in 40% of the female population
(Fig. 6B), whereas 85% of males appear to be extremely left lateralised
(Fig. 6C). The overall prevalence of leftward asymmetry
(Groups 1 and 2 in Fig. 6) in the direct segment of the
arcuate fasciculus in the right-handed population is higher
(w80%) than that reported for the planum temporale (w65%),
the region of the posterior superior temporal gyrus classically
associated with language lateralization (Geschwind and Levitsky,
1968). Considering that the prevalence of left functional
‘dominance’ for language is >90%, asymmetry of the direct
segment may represent a more critical anatomical substrate
for language lateralization than planum temporale asymmetry
(Catani et al., 2007).
Surprisingly, the extreme left lateralization of the direct
long segment is associated with worse performance on
a complex verbal memory task that relies on semantic clustering
for retrieval (i.e., California Verbal Learning Test,
CVLT). These findings suggest that lateralization of language
to the left hemisphere is a key aspect of human brain organization.
Paradoxically less than extreme lateralization
might ultimately be advantageous for certain cognitive
functions (Catani et al., 2007) (see also Doron and Gazzaniga,
2008, this issue).
5. Beyond the arcuate fasciculus: the ventral
pathways
The arcuate fasciculus belongs to the core perisylvian circuitry
underlying language. Functional imaging experiments and
clinicopathological observations of a language-based neurodegenerative
syndrome known as primary progressive aphasia
(PPA) have been expanding the boundaries of this core
circuitry (for a recent review see Mesulam, 2007). One of the
most interesting developments has been the demonstration
that areas in the medial, inferior and anterior temporal cortices,
traditionally considered outside the canonical language
network, may play crucial roles in semantic processing. The
interaction of these additional areas with the canonical
perisylvian language network may be mediated by a set of
ventral tracts such as the inferior longitudinal fasciculus,
the uncinate fasciculus, and the inferior fronto-occipital fasciculus
(Fig. 7) (for an anatomical description of these tracts
see Catani and Thiebaut de Schotten, 2008, this issue). The inferior
longitudinal fasciculus carries visual information from
occipital areas to the temporal lobe (Catani et al., 2003a) and
it is likely to play an important role in visual object recognition,
and in linking object representations to their lexical
labels (Mummery et al., 1999). The uncinate fasciculus interconnects
the anterior temporal lobe to the orbitofrontal
area, including the inferior frontal gyrus (Catani et al., 2002),
and may play an important role in lexical retrieval, semantic
associations, and aspects of naming that require connections
from temporal to frontal components of the language network
(e.g., the naming of actions) (Grossman et al., 2004; Lu et al.,
2002). The inferior fronto-occipital fasciculus is arguably the
only direct connection between occipital and frontal cortex
in the human brain (Catani, 2007). It is considered as part of
the mirror neuron system and there is preliminary evidence
suggesting that this tract is not present in monkey. The relevance
of this fasciculus to language is not fully understood
but may involve reading and writing (for other functional aspects
of these three segments see Gaffan and Wilson, 2008,
this issue; Fox et al., 2008, this issue; Ross, 2008, this issue;
Epelbaum et al., 2008, this issue; Doricchi et al., 2008, this issue;
Raudruff et al., 2008, this issue; Catani and Thiebaut de
Schotten, 2008, this issue). These ventral pathways are linked
to the perisylvian network at least in two different regions,
posteriorly, through short U-shaped fibres connecting Wernicke’s
area to lateral temporo-occipital cortex and anteriorly
through intralobar fibres connecting lateral orbitofrontal cortex
to Broca’s area.
6. Additional directions for DTI and
tractography
As illustrated in Fig. 8, information on the anatomy of connections
can potentially help to resolve dilemmas posed by cases
that superficially appear to defy established neurocognitive
Fig. 7 – Tractography reconstruction of the ventral
pathways of the left hemisphere.
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models. For example, the site of maximal lesion overlap for
a specific syndrome may extend into axonal pathways that
interconnect a different set of remote areas, raising the possibility
that the critical factor is not necessarily the destruction
in the cortical area of overlap but a disconnection of the two
remote areas (Fig. 8).
DTI tractography also has the potential of detecting pathway
changes at early stages of neurodegenerative processes
affecting language function so that the effects of such changes
upon the resultant aphasias can be studied (Catani, 2006). In
primary progressive aphasia, for example, the loss of cortical
neurons is accompanied by axonal degeneration along
specific white matter pathways (Fig. 9) (Catani et al., 2003b;
Borroni et al., 2007). Up to now, morphometric work on PPA
had focused on the relationship of cortical degeneration to
details of the language impairment. An equally interesting development
would be to use DTI to measure microstructural
changes in specific tracts and to correlate them with the
symptom profile (Catani, 2006).
Individual differences in the asymmetry of the arcuate
fasciculus detected by DTI could conceivably also help to
assess recovery potential in aphasias. It is not unreasonable
to assume that greater symmetry is likely to lead to better
recovery following stroke or neurosurgery. This is an assumption
that can be tested experimentally with currently available
methodology.
Fig. 8 – Topological and hodological approaches in clinico-anatomical correlation studies. In the upper row an example of
a lesion overlap study for clinico-anatomical correlation is represented where four patients present with similar neurological
deficits and their respective brain images are overlapped in order to identify a common anatomical substrate. Here we want to
highlight that the conclusion that one may draw from this type of studies depends on the hypothesis that is tested and the
general framework adopted. (A) A strict topological approach considers brain functions as localized in specific cortical regions.
Within this framework the critical area for the same neurological deficit manifested by a group of stroke patients (four in the
example, where each area, from 1 to 4, represents the extension of the lesion for each patient) is located at the cortical region
of maximum lesion overlap (region b in the example). (B) The hodological (network) approach to brain–behaviour correlation
includes a consideration of brain pathways that pass through the damaged area. Within this framework, the neurological
deficit could also be attributed to a disconnection between a and c because all lesions affect the same a to c pathway at
different levels (red circle). Note that A and B represent the same experiment (i.e. same patients and image analysis), however
the conclusions are opposite due to the different approach. (C) Image of the brain of Broca’s aphasic patient showing a lesion
to the inferior frontal cortex. Broca, who worked within a topological framework, considered that his patient’s speech deficit
was the consequence of the cortical lesion in the inferior frontal lobe. (D) Sagittal MRI image (mod. from Dronkers et al., 2007)
of the same brain shown in (C). Clearly the lesion extends into the white matter of the arcuate (red arrows) of the left
hemisphere. If Broca had worked within a hodological framework and performed dissections of his patient’s brain it is
probable that he would have attributed the speech deficit to a lesion of the arcuate fasciculus.
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7. Concluding remarks and future directions
In this review we have tried to highlight the merits of the
hodological (pathway-based) approach to behavioural neurology
and its modern pursuit with DTI tractography as applied
to language and the arcuate fasciculus. We realize, of course,
that mapping symptoms onto single tracts is subjected to the
same criticisms directed to narrow cortical localizationism,
that our knowledge of human white matter anatomy is still
very limited, and that giant strides are needed to reach the
level of pathway characterization that has been obtained in
the monkey. Nonetheless, DTI tractography applied to the
arcuate fasciculus and other pathways is likely to offer productive
insights into the connectivity of the human brain
and to reconfirm our belief that the disconnection paradigm
has still a lot to offer to neurology and psychiatry.
Acknowledgements
MC is funded by the Medical Research Council (UK), the AIMS
network MM is funded by the National Institute of Deafness
and Communication Disorders (DC008552), National Institute
on Aging (AG13854).
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