Dehaene,
S. & Cohen, L. (2011). The unique role of the visual word form area in
reading. Trends in Cognitive Sciences, 15, 254-262.
Notes: Reading systematically activates the left lateral occipitotemporal
sulcus, at a site known as the visual word form area (VWFA). This site is
reproducible across individuals/scripts, attuned to reading-specific
processes, and partially selective for written strings relative to other
categories such as line drawings. Lesions affecting the VWFA cause pure
alexia, a selective deficit in word recognition. These findings must be
reconciled with the fact that human genome evolution cannot have been
influenced by such a recent and culturally variable activity as reading.
Capitalizing on recent functional magnetic resonance imaging experiments, we
provide strong corroborating evidence for the hypothesis that reading
acquisition partially recycles a cortical territory evolved for object and
face recognition, the prior properties of which influenced the form of
writing systems
Institut National de la Sante et de la Recherche Medicale, Cognitive
Neuroimaging Unit, Gif sur Yvette, 91191 France; Commissariat a l'Energie
Atomique, Division of Life Sciences, Institute of Bioimaging, Neurospin, Gif
sur Yvette, 91191 France; Universite Paris 11, Orsay, France; College de
France, 11 Place Marcelin Berthelot, 75005 Paris, France
Pegado,
F., Nakamura, K., Cohen, L., & Dehaene, S. (2011).
Breaking the symmetry: mirror discrimination for single letters but not for
pictures in the Visual Word Form Area. NeuroImage, 55, 742-749.
Notes: Humans and primates can quickly recognize mirror images of previously
exposed pictures. This spontaneous mirror invariance, though advantageous
for visual recognition, makes it difficult to distinguish the orientation of
letters (e.g. to differentiate a "b" from a "d"), and may result in
classical mirror reading and writing errors in preschoolers. Mirror
invariance must therefore be overcome during reading acquisition. The Visual
Word Form Area (VWFA), a region in the ventral stream that develops with
reading expertise, was previously shown to discriminate words from their
mirror images in literate adults. Here we investigate whether this region
underlies mirror-image discrimination at the most elementary level of the
orthographic code, the single-letter level. Using an fMRI priming paradigm,
we demonstrate that the VWFA distinguishes the left-right orientation of
single letters in skilled readers, and yet exhibits mirror invariance for
simple pictures of matched complexity. These results clarify how letter
shapes, after reading acquisition, escape the process of mirror invariance
which is a basic property of the ventral visual shape recognition pathway
INSERM, U992, Cognitive Neuroimaging Unit, Gif/Yvette, France. felipepegado@yahoo.com
Szwed,
M., Dehaene, S., Kleinschmidt, A., Eger, E., Valabregue, R., Amadon, A. et
al.
(2011). Specialization for written words over objects in the visual cortex.
NeuroImage, 56, 330-344.
Notes: The Visual Word Form Area (VWFA) is part of the left ventral visual
stream that underlies the invariant identification of visual words. It
remains debated whether this region is truly selective for words relative to
common objects; why this particular part of the visual system is
reproducibly engaged in reading; and whether reading expertise also relies
on perceptual learning within earlier visual areas. In this fMRI study we
matched written words and line-drawings of objects in luminance, contour
length and number of features. We then compared them to control images made
by scrambling procedures that kept local features intact. Greater responses
to written words than to objects were found not only in the VWFA, but also
in areas V1/V2 and V3v/V4. Furthermore, by contrasting stimuli reduced
either to line junctions (vertices) or to line midsegments, we showed that
the VWFA partially overlaps with regions of ventral visual cortex
particularly sensitive to the presence of line junctions that are useful for
object recognition. Our results indicate that preferential processing of
written words can be observed at multiple levels of the visual system. It is
possible that responses in early visual areas might be due to some remaining
differences between words and controls not eliminated in the present
stimuli. However, our results concur with recent comparisons of literates
and illiterates and suggest that these early visual activations reflect the
effects of perceptual learning under pressure for fast, parallel processing
that is more prominent in reading than other visual cognitive processes
INSERM U992, Cognitive Neuroimaging Unit, Gif sur Yvette, France. mfszwed@gmail.com
Dehaene, S., Nakamura, K., Jobert, A., Kuroki, C., Ogawa, S., & Cohen, L.
(2010).
Why do children make mirror errors in reading? Neural correlates of mirror
invariance in the visual word form area. NeuroImage, 49, 1837-1848.
Notes: Young children often make mirror errors when learning to read and
write, for instance writing their first name from right to left in English.
This competence vanishes in most adult readers, who typically cannot read
mirror words but retain a strong competence for mirror recognition of
images. We used fast behavioral and fMRI repetition priming to probe the
brain mechanisms underlying mirror generalization and its absence for words
in adult readers. In two groups of French and Japanese readers, we show that
the left fusiform visual word form area, a major site of learning during
reading acquisition, simultaneously shows a maximal effect of mirror priming
for pictures and an absence of mirror priming for words. Thus, learning to
read recruits an area which possesses a property of mirror invariance,
seemingly present in all primates, which is deleterious for letter
recognition and may explain children's transient mirror errors
INSERM, Cognitive Neuroimaging Unit, Gif sur Yvette 91191, France.
stanislas.dehaene@cea.fr
Qiao, E., Vinckier, F., Szwed, M., Naccache, L., Valabregue, R., Dehaene, S.
et al. (2010). Unconsciously deciphering handwriting: subliminal invariance
for handwritten words in the visual word form area. NeuroImage, 49,
1786-1799.
Notes: Expert readers exhibit a remarkable ability to recognize handwriting,
in spite of enormous variability in character shape-a competence whose
cerebral underpinnings are unknown. Subliminal priming, combined with
neuroimaging, can reveal which brain areas automatically compute an
invariant representation of visual stimuli. Here, we used behavioral and
fMRI priming to study the areas involved in invariant handwritten word
recognition. Compared to printed words, easily readable handwritten words
caused additional activity in ventral occipitotemporal cortex, particularly
in the right hemisphere, while difficult handwriting also mobilized an
attentional parietofrontal network. Remarkably, however, subliminal
repetition effects were observed across printed and handwritten styles,
whether easy or difficult to read, both behaviorally and in the activation
of the left visual word form area (VWFA). These results indicate that the
left inferotemporal VWFA possesses an unsuspected degree of fast and
automatic visual invariance for handwritten words, although surprisingly
this invariance can be reflected both as repetition suppression and as
repetition enhancement
INSERM, Cognitive Neuro-imaging Unit, IFR 49, Gif sur Yvette, France
Cohen, L., Dehaene, S., Vinckier, F., Jobert, A., & Montavont, A. (2008).
Reading normal and degraded words: contribution of the dorsal and ventral
visual pathways. NeuroImage, 40, 353-366.
Notes: Department of Neurology I, AP-HP, Hopital de la Salpetriere, 47/83
Boulevard de l'Hopital, Paris CEDEX 13, France. laurent.cohen@psl.aphp.fr
Fast, parallel word recognition, in expert readers, relies on sectors of the
left ventral occipito-temporal pathway collectively known as the visual word
form area. This expertise is thought to arise from perceptual learning
mechanisms that extract informative features from the input strings. The
perceptual expertise hypothesis leads to two predictions: (1) parallel word
recognition, based on the ventral visual system, should be limited to words
displayed in a familiar format (foveal horizontal words with normally spaced
letters); (2) words displayed in formats outside this field of expertise
should be read serially, under supervision of dorsal parietal attention
systems. We presented adult readers with words that were progressively
degraded in three different ways (word rotation, letter spacing, and
displacement to the visual periphery). Behaviorally, we identified
degradation thresholds above which reading difficulty increased
non-linearly, with the concomitant emergence of a word length effect on
reading latencies reflecting serial reading strategies. fMRI activations
were correlated with reading difficulty in bilateral occipito-temporal and
parietal regions, reflecting the strategies required to identify degraded
words. A core region of the intraparietal cortex was engaged in all modes of
degradation. Furthermore, in the ventral pathway, word degradation led to an
amplification of activation in the posterior visual word form area, at a
level thought to encode single letters. We also found an effect of word
length restricted to highly degraded words in bilateral occipitoparietal
regions. Those results clarify when and how the ventral parallel visual word
form system needs to be supplemented by the deployment of dorsal serial
reading strategies
Epelbaum, S., Pinel, P., Gaillard, R., Delmaire, C., Perrin, M., Dupont, S.
et al.
(2008). Pure alexia as a disconnection syndrome: new diffusion imaging
evidence for an old concept. Cortex, 44, 962-974.
Notes: Functional neuroimaging and studies of brain-damaged patients made it
possible to delineate the main components of the cerebral system for word
reading. However, the anatomical connections subtending the flow of
information within this network are still poorly defined. Here we study the
connectivity of the Visual Word Form Area (VWFA), a pivotal component of the
reading network achieving the invariant identification of letter strings,
and reproducibly located in the left lateral occipitotemporal sulcus.
Diffusion images and functional imaging data were gathered in a patient who
developed pure alexia following a small surgical lesion in the vicinity of
his VWFA. We had a unique opportunity to compare images obtained before,
early after, and late after surgery. Analysis of diffusion images with white
matter tractography and voxel-based morphometry showed that the VWFA was
mainly linked to the occipital cortex through the inferior longitudinal
fasciculus (ILF), and to perisylvian language areas (supramarginal gyrus)
through the arcuate fasciculus. After surgery, we observed the progressive
and selective degeneration of the ILF, while the VWFA was anatomically
intact. This allowed us to establish the critical causal role of this fiber
tract in normal reading, and to show that its disruption is one
pathophysiological mechanism of pure alexia, thus clarifying a long-standing
debate on the role of disconnection in neurocognitive disorders
Hopital de la Salpetriere, Department of Neurology, Paris, France
Gaillard, R., Naccache, L., Pinel, P., Clemenceau, S., Volle, E., Hasboun,
D. et al. (2006). Direct intracranial, FMRI, and lesion evidence for the
causal role of left inferotemporal cortex in reading. Neuron, 50,
191-204.
Notes: Models of the "visual word form system" postulate that a left
occipitotemporal region implements the automatic visual word recognition
required for efficient reading. This theory was assessed in a patient in
whom reading was explored with behavioral measures, fMRI, and intracranial
local field potentials. Prior to surgery, when reading was normal, fMRI
revealed a normal mosaic of ventral visual selectivity for words, faces,
houses, and tools. Intracranial recordings demonstrated that the left
occipitotemporal cortex responded with a short latency to conscious but also
to subliminal words. Surgery removed a small portion of word-responsive
occipitotemporal cortex overlapping with the word-specific fMRI activation.
The patient developed a marked reading deficit, while recognition of other
visual categories remained intact. Furthermore, in the post-surgery fMRI map
of visual cortex, only word-specific activations disappeared. Altogether,
these results provide direct evidence for the causal role of the left
occipitotemporal cortex in the recognition of visual words
INSERM, U562, CEA/DSV, IFR 49, Orsay, France
Dehaene, S., Cohen, L., Sigman, M., & Vinckier, F. (2005). The neural code
for written words: a proposal. Trends in Cognitive Sciences, 9,
335-341.
Notes: INSERM unit 562, Cognitive Neuroimaging, Service Hospitalier Frederic
Joliot, CEA/DRM/DSV 4 Place du General Leclerc, 91401 Orsay cedex, France.
dehaene@shfj.cea.fr
How is reading, a cultural invention, coded by neural populations in the
human brain? The neural code for written words must be abstract, because we
can recognize words regardless of their location, font and size. Yet it must
also be exquisitely sensitive to letter identity and letter order. Most
existing coding schemes are insufficiently invariant or incompatible with
the constraints of the visual system. We propose a tentative neuronal model
according to which part of the occipito-temporal 'what' pathway is tuned to
writing and forms a hierarchy of local combination detectors sensitive to
increasingly larger fragments of words. Our proposal can explain why the
detection of 'open bigrams' (ordered pairs of letters) constitutes an
important stage in visual word recognition.
Henry,
C., Gaillard, R., Volle, E., Chiras, J., Ferrieux, S., Dehaene, S. et al.
(2005). Brain activations during letter-by-letter reading: a follow-up
study. Neuropsychologia, 43, 1983-1989.
Notes: Service de Neurologie 1, IFR 70, Hopital de la Salpetriere, AP-HP,
47/83 Bd de l'Hopital, 75651 Paris Cedex 13, France
Lesions affecting the ventral cortex of the left temporal lobe commonly
yield a selective reading impairment known as pure alexia. It is thought to
result from the disruption or deafferentation of the Visual Word Form Area (VWFA),
a region in the left lateral occipitotemporal sulcus activated whenever
normal subjects are viewing alphabetic strings. Most pure alexic patients
retain the ability to identify single letters, and develop a strategy of
letter-by-letter (LBL) reading. We recently studied fMRI activations in LBL
readers and clarified the underlying mechanisms. However, LBL reading is a
dynamic process which may improve over months or years of practice, although
the cerebral bases of this continuing improvement are currently unknown. We
had the opportunity to run the same behavioural testing and fMRI experiment
a second time in an alexic patient, 8 months after collecting the data
reported by Cohen et al. [Cohen, L., Henry, C., Dehaene, S., Molko, N.,
Lehericy, S., Martinaud, O., Lemer, C., and Ferrieux, S. (2004). The
pathophysiology of letter-by-letter reading. Neuropsychologia, 42,
1768-1780]. We analyze the changes that occurred over this period in the
pattern of reading-related activations, while the patient's LBL reading
improved. The activation level decreased in most of the overall network
between the two sessions. This general trend contrasted with a focal
increase restricted to specific left frontal and parietal areas. When
studying the contrast between words and consonant strings, which may be
taken as a correlate of LBL reading, we also found a general decrease,
except for similar left frontal and parietal regions, which showed a
significant increase. We suggest that the pattern of evolution fits with the
minimal hypothesis of normal strategic abilities and skill learning,
associated with perceptual tuning in right-hemispheric structures able to
substitute the disrupted VWFA
Cohen,
L., Henry, C., Dehaene, S., Martinaud, O., Lehericy, S., Lemer, C. et al.
(2004). The pathophysiology of letter-by-letter reading. Neuropsychologia,
42, 1768-1780.
Notes: Institut de Neurologie, Hopital de la Salpetriere, 47/83 Bd de
l'Hopital, 75651 Paris CEDEX 13, France. laurent.cohen@psl.ap-hop-paris.fr
Pure alexia is a frequent and incapacitating consequence of left
occipitotemporal lesions. It is thought to result from the disruption or the
disconnection of the visual word form area (VWFA), a region reproducibly
located within the left occipito-temporal sulcus, and encoding the abstract
identity of strings of visual letters. Alexic patients often retain
effective single letter recognition abilities, and develop an effortful
letter-by-letter reading strategy which is the basis of most rehabilitation
techniques. We study a patient who developed letter-by-letter reading
following the surgical removal of left occipito-temporal regions. Using
anatomical and functional MRI in the patient and in normal controls, we show
that alexia resulted from the deafferentation of left fusiform cortex, and
we analyze the network of brain regions subtending letter-by-letter reading.
We propose that during letter-by-letter reading (1) letters are identified
in the intact right-hemispheric visual system, with a central role for the
region symetrical to the VWFA; (2) letters are serially transferred to the
left hemisphere through the intact segment of the corpus callosum; (3) word
identity is eventually recovered in the left hemisphere through verbal
working memory processes involving inferior frontal and supramarginal
cortex.
Cohen, L., Lehericy, S., Henry, C., Bourgeois, M., Larroque, C.,
Sainte-Rose, C. et al. (2004). Learning to read without a left occipital
lobe: right-hemispheric shift of visual word form area. Annals of
Neurology, 56, 890-894.
Notes: Institut de Neurologie, Hopital de la Salpetriere, AP-HP, Paris,
France. laurent.cohen@psl.ap-hop-paris.fr
Using anatomical and functional magnetic resonance imaging, we studied the
pattern of brain lateralization during spoken and written language tasks, in
an 11-year-old girl who underwent a left occipitotemporal resection for a
Sturge-Weber angioma at the age of 4 years, that is, after the development
of speech but before the acquisition of reading. We observed a selective and
successful shift to the right hemisphere of the visual component of reading,
particularly the Visual Word Form Area, whereas the verbal components
remained strongly left-lateralized. This emphasizes the potential utility of
a precise functional and developmental cartography of language for the
surgical treatment of focal brain lesions in children
Cohen,
L., Jobert, A., Le Bihan, D., & Dehaene, S. (2004).
Distinct unimodal and multimodal regions for word processing in the left
temporal cortex. NeuroImage, 23, 1256-1270.
Notes: Institut de Neurologie, Hopital de la Salpetriere, AP-HP, Paris,
France. laurent.cohen@psl.ap-hop-paris.fr
How are word recognition circuits organized in the left temporal lobe? We
used functional magnetic resonance imaging (fMRI) to dissect cortical
word-processing circuits using three diagnostic criteria: the capacity of an
area (1) to respond to words in a single modality (visual or auditory) or in
both modalities, (2) to modulate its response in a top-down manner as a
function of the graphemic or phonemic emphasis of the task, and (3) to show
repetition suppression in response to the conscious repetition of the target
word within the same sensory modality or across different modalities. The
results clarify the organization of visual and auditory word-processing
streams. In particular, the visual word form area (VWFA) in the left
occipitotemporal sulcus appears strictly as a visual unimodal area. It is,
however, bordered by a second lateral inferotemporal area which is
multimodal [lateral inferotemporal multimodal area (LIMA)]. Both areas might
have been confounded in past work. Our results also suggest a possible
homolog of the VWFA in the auditory stream, the auditory word form area,
located in the left anterior superior temporal sulcus
Cohen, L. & Dehaene, S. (2004). Specialization within the ventral stream:
the case for the visual word form area. NeuroImage, 22, 466-476.
Notes: Is there specialization for visual word recognition within the visual
ventral stream of literate human adults? We review the evidence for a
specialized "visual word form area" and critically examine some of the
arguments recently placed against this hypothesis. Three distinct forms of
specialization must be distinguished: functional specialization,
reproducible localization, and regional selectivity. Examination of the
literature with this theoretical division in mind indicates that reading
activates a precise subpart of the left ventral occipitotemporal sulcus, and
that patients with pure alexia consistently exhibit lesions of this region
(reproducible localization). Second, this region implements processes
adequate for reading in a specific script, such as invariance across upper-
and lower-case letters, and its lesion results in the selective loss of
reading-specific processes (functional specialization). Third, the issue of
regional selectivity, namely, the existence of putative cortical patches
dedicated to letter and word recognition, cannot be resolved by positron
emission tomography or lesion data, but requires high-resolution
neuroimaging techniques. The available evidence from single-subject fMRI and
intracranial recordings suggests that some cortical sites respond
preferentially to letter strings than to other categories of visual stimuli
such as faces or objects, though the preference is often relative rather
than absolute. We conclude that learning to read results in the progressive
development of an inferotemporal region increasingly responsive to visual
words, which is aptly named the visual word form area (VWFA)
Cohen,
L., Martinaud, O., Lemer, C., Lehericy, S., Samson, Y., Obadia, M. et al.
(2003). Visual word recognition in the left and right hemispheres:
anatomical and functional correlates of peripheral alexias. Cerebral
Cortex, 13, 1313-1333.
Notes: Institut de Neurologie, Hopital de la Salpetriere, AP-HP, Paris,
France. laurent.cohen@psl.ap-hop-paris.fr
According to a simple anatomical and functional model of word reading,
letters displayed in one hemifield are first analysed through a cascade of
contralateral retinotopic areas, which compute increasingly abstract
representations. Eventually, an invariant representation of letter
identities is created in the visual word form area (VWFA), reproducibly
located within the left occipito-temporal sulcus. The VWFA then projects to
structures involved in phonological or lexico-semantic processing. This
model yields detailed predictions on the reading impairments that may follow
left occipitotemporal lesions. Those predictions were confronted to
behavioural, anatomical and functional MRI data gathered in normals and in
patients suffering from left posterior cerebral artery infarcts. In normal
subjects, alphabetic stimuli activated both the VWFA and the
right-hemispheric symmetrical region (R-VWFA) relative to fixation, but only
the VWFA showed a preference for alphabetic strings over simple
chequerboards. The comparison of normalized brain lesions with
reading-induced activations showed that the critical lesion site for the
classical syndrome of pure alexia can be tightly localized to the VWFA.
Reading impairments resulting from deafferentation of an intact VWFA from
right- or left-hemispheric input were dissected using the same methods,
shedding light on the connectivity of the VWFA. Finally, the putative role
of right-hemispheric processing in the letter-by-letter reading strategy was
clarified. In a letter-by-letter reader, the R-VWFA assumed some of the
functional properties normally specific to the VWFA. These data corroborate
our initial model of normal word perception and underline that an
alternative right-hemispheric pathway can underlie functional recovery from
alexia
McCandliss, B. D., Cohen, L., & Dehaene, S. (2003). The visual word form
area: Expertise for reading in the fusiform gyrus. Trends in Cognitive
Sciences, 7, 293-299.
Notes: B.D. McCandliss, Sackler Inst. Devmtl. Psychobiology, Weill Med.
Coll. of Cornell Univ., Box 140, 1300 York Avenue, New York, NY 10021
Brain imaging studies reliably localize a region of visual cortex that is
especially responsive to visual words. This brain specialization is
essential to rapid reading ability because it enhances perception of words
by becoming specifically tuned to recurring properties of a writing system.
The origin of this specialization poses a challenge for evolutionary
accounts involving innate mechanisms for functional brain organization. We
propose an alternative account, based on studies of other forms of visual
expertise (i.e. bird and car experts) that lead to functional
reorganization. We argue that the interplay between the unique demands of
word reading and the structural constraints of the visual system lead to the
emergence of the Visual Word Form Area
Cohen, L., Lehericy, S., Chochon, F., Lemer, C., Rivaud, S., & Dehaene, S.
(2002). Language-specific tuning of visual cortex? Functional properties of
the Visual Word Form Area. Brain, 125, 1054-1069.
Notes: Institut de Neurologie, Hopital de la Salpetriere, AP-HP, Paris,
France. laurent.cohen@psl.ap-hop-paris.fr
The first steps in the process of reading a printed word belong to the
domain of visual object perception. They culminate in a representation of
letter strings as an ordered set of abstract letter identities, a
representation known as the Visual Word Form (VWF). Brain lesions in
patients with pure alexia and functional imaging data suggest that the VWF
is subtended by a restricted patch of left-hemispheric fusiform cortex,
which is reproducibly activated during reading. In order to determine
whether the operation of this Visual Word Form Area (VWFA) depends
exclusively on the visual features of stimuli, or is influenced by
language-dependent parameters, brain activations induced by words, consonant
strings and chequerboards were compared in normal subjects using functional
MRI (fMRI). Stimuli were presented in the left or right visual hemifield.
The VWFA was identified in both a blocked-design experiment and an
event-related experiment as a left-hemispheric inferotemporal area showing a
stronger activation to alphabetic strings than to chequerboards, and
invariant for the spatial location of stimuli. In both experiments, stronger
activations of the VWFA to words than to strings of consonants were
observed. Considering that the VWFA is equally activated by real words and
by readable pseudowords, this result demonstrates that the VWFA is initially
plastic and becomes attuned to the orthographic regularities that constrain
letter combination during the acquisition of literacy. Additionally, the use
of split-field stimulation shed some light on the cerebral bases of the
classical right visual field (RVF) advantage in reading. A left occipital
extrastriate area was found to be activated by RVF letter strings more than
by chequerboards, while no symmetrical region was observed in the right
hemisphere. Moreover, activations in the precuneus and the left thalamus
were observed when subjects were reading RVF versus left visual field (LVF)
words, and are likely to reflect the attentional component of the RVF
advantage
Dehaene, S., Le Clec'H, G., Poline, J. B., Le Bihan, D., & Cohen, L. (2002).
The visual word form area: a prelexical representation of visual words in
the fusiform gyrus. Neuroreport, 13, 321-325.
Notes: Unite INSERM 334, France
Event-related fMRI was used to test the hypothesis that the visual word form
area in the left fusiform gyrus holds a modality-specific and prelexical
representation of visual words. Subjects were engaged in a
repetition-detection task on pairs of words or pronounceable pseudo-words
that could be written or spoken. The visual word form area responded only to
written stimuli, not to spoken stimuli, independently of their semantic
content. We propose that the occasional activation of the fusiform gyrus
when listening to spoken words is due to the topdown recruitment of visual
orthographic or object representations
Molko, N., Cohen, L., Mangin, J. F., Chochon, F., Lehericy, S., Le Bihan, D.
et al. (2002). Visualizing the neural bases of a disconnection syndrome with
diffusion tensor imaging. Journal of Cognitive Neuroscience, 14,
629-636.
Notes: INSERM, Service Hospitalier Frederic Joliot, Orsay cedex, France
Disconnection syndromes are often conceptualized exclusively within
cognitive box-and-arrow diagrams unrelated to brain anatomy. In a patient
with alexia in his left visual field resulting from a posterior callosal
lesion, we illustrate how diffusion tensor imaging can reveal the anatomical
bases of a disconnection syndrome by tracking the degeneration of neural
pathways and relating it to impaired fMRI activations and behavior. Compared
to controls, an abnormal pattern of brain activity was observed in the
patient during word reading, with a lack of activation of the left visual
word form area (VWFA) by left hemifield words. Statistical analyses of
diffusion images revealed a damaged fiber tract linking the left ventral
occipito-temporal region to its right homolog across the lesioned area of
corpus callosum and stopping close to the areas found active in fMRI. The
behavioral disconnection syndrome could, thus, be related functionally to
abnormal fMRI activations and anatomically to the absence of a connection
between those activations. The present approach, based on the "negative
tracking" of degenerated bundles, provides new perspectives on the
understanding of human brain connections and disconnections
Dehaene, S., Naccache, L., Cohen, L., Bihan, D. L., Mangin, J. F., Poline,
J. B. et al.
(2001). Cerebral mechanisms of word masking and unconscious repetition
priming. Nature Neuroscience, 4, 752-758.
Notes: Unite INSERM 334, IFR 49, Service Hospitalier Frederic Joliot, CEA/DSV,
4 Place du General Leclerc, 91401 Orsay cedex, France. dehaene@shfj.cea.fr
We used functional magnetic resonance imaging (fMRI) and event-related
potentials (ERPs) to visualize the cerebral processing of unseen masked
words. Within the areas associated with conscious reading, masked words
activated left extrastriate, fusiform and precentral areas. Furthermore,
masked words reduced the amount of activation evoked by a subsequent
conscious presentation of the same word. In the left fusiform gyrus, this
repetition suppression phenomenon was independent of whether the prime and
target shared the same case, indicating that case-independent information
about letter strings was extracted unconsciously. In comparison to an
unmasked situation, however, the activation evoked by masked words was
drastically reduced and was undetectable in prefrontal and parietal areas,
correlating with participants' inability to report the masked words
Cohen, L. & Dehaene, S. (2000). Calculating without reading: Unsuspected
residual abilities in pure alexia. Cognitive Neuropsychology, 17,
563-583.
Notes: L. Cohen, Service de Neurologie 1, Clinique Paul Castaigne, Hopital
de la Salpetriere, 47/83 Bd de l'Hopital, 75651 Paris Cedex 13, France ;
ABSTRACT: In pure alexic patients, clear-cut dissociations between impaired
naming and preserved comprehension abilities can be found in the domain of
number processing (Cohen and Dehaene, 1995). In the present study, we report
a novel case of pure alexia with striking preservation of some calculation
abilities. The patient was fully able to decide which of two numbers was the
larger, or whether a number was odd or even, even with 2-digit numerals for
which she made close to 90% reading errors. In arithmetic the patient,
though unable to read aloud correctly the operands of visually presented
problems, could still produce verbally the exact result of the very same
problems. For instance, when presented visually with the subtraction problem
8 - 6, the patient read the problem aloud as 'five minus four', but
nevertheless produced the correct result 'two.' Such capacities for
'calculating without reading' were observed in subtraction, addition, and
division tasks, but not in multiplication tasks. We discuss how both the
existence of residual abilities and the pattern of dissociations between
operation types can be explained by current theories of the cerebral
substrates of number processing (Dehaene and Cohen, 1995)
Cohen,
L., Dehaene, S., Naccache, L., Lehericy, S., Dehaene Lambertz, G., Henaff,
M. A. et al.
(2000). The visual word form area: spatial and temporal characterization of
an initial stage of reading in normal subjects and posterior split-brain
patients. Brain, 123, 291-307.
Notes: Service de Neurologie 1, INSERM U334, Service Hospitalier Frederic
Joliot, CEA/DSV, Orsay, France. laurent.cohen@psl.ap-hop- paris.fr ;
ABSTRACT: A standard model of word reading postulates that visual
information is initially processed by occipitotemporal areas contralateral
to the stimulated hemifield, from whence it is subsequently transferred to
the visual word form (VWF) system, a left inferior temporal region
specifically devoted to the processing of letter strings. For stimuli
displayed in the left visual field, this transfer proceeds from the right to
the left hemisphere through the posterior portion of the corpus callosum. In
order to characterize the spatial and temporal organization of these
processes, reading tasks with split-field presentation were performed by
five control subjects and by two patients suffering from left hemialexia
following posterior callosal lesions. The subjects' responses were studied
using behavioural measures and functional brain imaging techniques,
providing both high spatial resolution (functional MRI, fMRI) and high
temporal resolution (high-density event- related potentials, ERPs). Early
visual processing was revealed as activations contralateral to stimulation,
located by fMRI in the inferior occipitotemporal region and presumably
coincident with area V4. A negative wave occurring 150-160 ms post-stimulus,
also strictly contralateral to stimulation, was recorded over posterior
electrodes. In contrast with these hemifield-dependent effects, the VWF
system was revealed as a strictly left- hemispheric activation which, in
control subjects, was identical for stimuli presented in the left or in the
right hemifield and was located in the middle portion of the left fusiform
gyrus. The electrical signature of the VWF system consisted of a unilateral
sharp negativity, recorded 180- 200 ms post-stimulus over left inferior
temporal electrodes. In callosal patients, due to the inability of visual
information to pass across the posterior part of the corpus callosum, the
VWF system was activated only by stimuli presented in the right visual
field. Similarly, a significant influence of the word/non- word status on
ERPs recorded over the left hemisphere was discernible for either hemifield
in controls, while it affected only right-hemifield stimuli in callosal
patients. These findings provide direct support for the main components of
the classical model of reading and help specify their timing and cerebral
substrates
Cohen, L. & Dehaene, S. (1995). Number processing in pure alexia: The effect
of hemispheric asymmetries and task demands.
Neurocase, 1,
121-137. |
|
|