Selected references
Seghier, M. L., Neufeld, N. H., Zeidman, P., Leff, A. P., Mechelli, A.,
Nagendran, A. et al.
(2012). Reading without the left ventral occipito-temporal cortex.
Neuropsychologia, 50, 3621-3635.
Notes: The left ventral occipito-temporal cortex (LvOT) is thought to be
essential for the rapid parallel letter processing that is required for
skilled reading. Here we investigate whether rapid written word
identification in skilled readers can be supported by neural pathways that
do not involve LvOT. Hypotheses were derived from a stroke patient who
acquired dyslexia following extensive LvOT damage. The patient followed a
reading trajectory typical of that associated with pure alexia, re-gaining
the ability to read aloud many words with declining performance as the
length of words increased. Using functional MRI and dynamic causal modelling
(DCM), we found that, when short (three to five letter) familiar words were
read successfully, visual inputs to the patient's occipital cortex were
connected to left motor and premotor regions via activity in a central part
of the left superior temporal sulcus (STS). The patient analysis therefore
implied a left hemisphere "reading-without-LvOT" pathway that involved STS.
We then investigated whether the same reading-without-LvOT pathway could be
identified in 29 skilled readers and whether there was inter-subject
variability in the degree to which skilled reading engaged LvOT. We found
that functional connectivity in the reading-without-LvOT pathway was
strongest in individuals who had the weakest functional connectivity in the
LvOT pathway. This observation validates the findings of our patient's case
study. Our findings highlight the contribution of a left hemisphere reading
pathway that is activated during the rapid identification of short familiar
written words, particularly when LvOT is not involved. Preservation and use
of this pathway may explain how patients are still able to read short words
accurately when LvOT has been damaged
Wellcome Trust Centre for Neuroimaging, Institute of Neurology, UCL, London
WC1N 3BG, UK. Electronic address: m.seghier@fil.ion.ucl.ac.uk
Starrfelt, R., Habekost, T., & Leff, A. P. (2009). Too little, too late:
reduced visual span and speed characterize pure alexia. Cerebral Cortex,
19, 2880-2890.
Notes: Whether normal word reading includes a stage of visual processing
selectively dedicated to word or letter recognition is highly debated.
Characterizing pure alexia, a seemingly selective disorder of reading, has
been central to this debate. Two main theories claim either that 1) Pure
alexia is caused by damage to a reading specific brain region in the left
fusiform gyrus or 2) Pure alexia results from a general visual impairment
that may particularly affect simultaneous processing of multiple items. We
tested these competing theories in 4 patients with pure alexia using
sensitive psychophysical measures and mathematical modeling. Recognition of
single letters and digits in the central visual field was impaired in all
patients. Visual apprehension span was also reduced for both letters and
digits in all patients. The only cortical region lesioned across all 4
patients was the left fusiform gyrus, indicating that this region subserves
a function broader than letter or word identification. We suggest that a
seemingly pure disorder of reading can arise due to a general reduction of
visual speed and span, and explain why this has a disproportionate impact on
word reading while recognition of other visual stimuli are less obviously
affected
Department of Psychology, Center for Visual Cognition, Copenhagen
University, DK-1361 Copenhagen, Denmark. randi.starrfelt@psy.ku.dk
Leff, A. P. & Behrmann, M. (2008). Treatment of reading impairment after
stroke. Current Opinion in Neurology, 21, 644-648.
Notes: PURPOSE OF REVIEW: Reading impairments after left or right hemisphere
stroke are common yet receive little attention from clinicians and
therapists. In this review, we focus on the classification of acquired
alexia and the current theory and practice underlying the rehabilitation of
this diverse set of disorders. RECENT FINDINGS: The underlying behavioural
impairments that dictate reading ability in the acquired alexias are
becoming better understood; this, in turn, has led to targeted therapies
being undertaken, mainly on a single subject basis. In hemianopic alexia,
the most 'peripheral' of the acquired alexias, where text reading speed is
determined largely by damage to the visual field, therapies have been
directed at improving reading eye movements. In 'pure' alexia, techniques
are usually aimed at improving whole-word recognition. In central alexic
syndromes, where other language functions are also involved, the emphasis
has been on strengthening connections between lexical and semantic
representations, strengthening phonological representations, or both, and
their association with lexical/semantic knowledge. SUMMARY: Despite targeted
approaches to the rehabilitation of patients with alexia caused by stroke,
there is still a preponderance of largely descriptive, single-case studies
in the literature. In some syndromes, small trials have been attempted and
the hope is that, in the future, more systematic investigations will be
carried out so rehabilitation efforts can be built on a strong theoretical
and empirical foundation. Well designed, single-case studies continue to
play an important role in informing therapy, as these disorders are, by
nature, heterogeneous
Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University
College London, UK. a.leff@fil.ion.ucl.ac.uk
Leff, A. P., Spitsyna, G., Plant, G. T., & Wise, R. J. (2006). Structural
anatomy of pure and hemianopic alexia. Journal of Neurology, Neurosurgery
and Psychiatry, 77, 1004-1007.
Notes: BACKGROUND: The two most common types of acquired reading disorder
resulting from damage to the territory of the dominant posterior cerebral
artery are hemianopic and pure alexia. Patients with pronounced hemianopic
alexia have a right homonymous hemianopia that encroaches into central or
parafoveal vision; they read individual words well, but generate inefficient
reading saccades when reading along a line of text. Patients with pure
alexia also often have a hemianopia but are more disabled, making frequent
errors on individual words; they have sustained damage to a brain region
that supports efficient word identification. OBJECTIVE: To investigate the
differences in lesion site between hemianopic alexia and pure alexia groups,
as rehabilitative techniques differ between the two conditions. METHODS:
High-resolution magnetic resonance images were obtained from seven patients
with hemianopic alexia and from six patients with pure alexia caused by a
left occipital stroke. The boundary of each lesion was defined and lesion
volumes were then transformed into a standard stereotactic space so that
regional comparisons could be made. RESULTS: The two patient groups did not
differ in terms of damage to the medial left occipital lobe, but those with
pure alexia had additional lateral damage to the posterior fusiform gyrus
and adjacent tissue. CONCLUSIONS: Clinicians will be able to predict the
type of reading disorder patients with left occipital lesions have from
simple tests of reading speed and the distribution of damage to the left
occipital lobe on brain imaging. This information will aid management
decisions, including recommendations for reading rehabilitation.
Leff, A. P., Crewes, H., Plant, G. T., Scott, S. K., Kennard, C., & Wise, R.
J. (2001). The functional anatomy of single-word reading in patients with
hemianopic and pure alexia. Brain, 124, 510-521.
Notes: MRC Clinical Sciences Centre, Cyclotron Unit, Hammersmith Hospital,
London W12 0NN, UK. alexander.leff@ic.ac.uk
We investigated single-word reading in normal subjects and patients with
alexia following a left occipital infarct, using PET. The most posterior
brain region to show a lateralized response was at the left occipitotemporal
junction, in the inferior temporal gyrus. This region was activated when
normal subjects, patients with hemianopic alexia and patients with an
incomplete right homonymous hemianopia, but no reading deficit, viewed
single words presented at increasing rates. This same area was damaged in a
patient with pure alexia ("alexia without agraphia") and no hemianopia, who
read words slowly using a letter-by-letter strategy. Although the exact
level of the functional deficit is controversial, pure alexia is the result
of an inability to map a percept of all the letters in a familiar letter
string on to the mental representation of the whole word form. However, the
commonest deficit associated with "pure" alexia is a right homonymous field
defect; an impairment that may, by itself, interfere with single-word
reading because of inability to see the letters towards the end of a word.
The relative contributions of pure and hemianopic alexia in individual
patients needs to be assessed, as the latter has been shown to respond well
to specific rehabilitation programmes |
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