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Starrfelt, R. & Behrmann, M. (2011). Number reading in pure alexia-A review.
Neuropsychologia, 49, 2283-2298.
Notes: It is commonly assumed that number reading can be intact in patients
with pure alexia, and that this dissociation between letter/word recognition
and number reading strongly constrains theories of visual word processing. A
truly selective deficit in letter/word processing would strongly support the
hypothesis that there is a specialized system or area dedicated to the
processing of written words. To date, however, there has not been a
systematic review of studies investigating number reading in pure alexia and
so the status of this assumed dissociation is unclear. We review the
literature on pure alexia from 1892 to 2010, and find no well-documented
classical dissociation between intact number reading and impaired letter
identification in a patient with pure alexia. A few studies report strong
dissociations, with number reading less impaired than letter reading, but
when we apply rigorous statistical criteria to evaluate these dissociations,
the difference in performance across domains is not statistically
significant. There is a trend in many cases of pure alexia, however, for
number reading to be less affected than letter identification and word
reading. We shed new light on this asymmetry by showing that, under
conditions of brief exposure, normal participants are also better at
identifying digits than letters. We suggest that the difference observed in
some pure alexic patients may possibly reflect an amplification of this
normal difference in the processing of letters and digits, and we relate
this asymmetry to intrinsic differences between the two types of symbols
Center for Visual Cognition, Department of Psychology, Copenhagen
University, O. Farimagsgade 2A, DK-1353 Copenhagen K, Denmark
Starrfelt, R.,
Habekost, T., & Gerlach, C. (2010).
Visual processing in pure alexia: a case study. Cortex, 46, 242-255.
Notes: Whether pure alexia is a selective disorder that affects reading
only, or if it reflects a more general visual disturbance, is highly
debated. We have investigated the selectivity of visual deficits in a pure
alexic patient (NN) using a combination of psychophysical measures,
mathematical modelling and more standard experimental paradigms. NN's naming
and categorization of line drawings were normal with regards to both errors
and reaction times (RTs). Psychophysical experiments revealed that NN's
recognition of single letters at fixation was clearly impaired, and
recognition of single digits was also affected. His visual apprehension span
was markedly reduced for letters and digits. His reduced visual processing
capacity was also evident when reporting letters from words. In an object
decision task with fragmented pictures, NN's performance was abnormal. Thus,
even in a pure alexic patient with intact recognition of line drawings, we
find evidence of a general visual deficit not selective to letters or words.
This finding is important because it raises the possibility that other pure
alexics might have similar non-selective impairments when tested thoroughly.
We argue that the general visual deficit in NN can be accounted for in terms
of inefficient build-up of sensory representations, and that this low level
deficit can explain the pattern of spared and impaired abilities in this
patient
Department of Psychology, Center for Visual Cognition, University of
Copenhagen, Denmark. randi.starrfelt@psy.ku.dk
Starrfelt, R.
(2009). Aleksi. In A.Gade, C. Gerlach, R. Starrfelt, & P. M. Pedersen
(Eds.), Klinisk neuropsykologi ( pp. 45-53). Copenhagen: Frydenlund.
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
Starrfelt, R. (2007). Selective alexia and agraphia sparing numbers-a case
study. Brain and Language, 102, 52-63.
Notes: Department of Psychology, University of Copenhagen, Linnesgade 22,
DK-1361, Copenhagen, Denmark. randi.starrfelt@psy.ku.dk
We report a patient (MT) with a highly specific alexia affecting the
identification of letters and words but not numbers. He shows a
corresponding deficit in writing: his letter writing is impaired while
number writing and written calculation is spared. He has no aphasia, no
visuo-perceptual or -constructional difficulties, or other cognitive
deficits. A similar pattern of performance has to our knowledge only been
reported once before [Anderson, S. W., Damasio, A. R., & Damasio, H. (1990).
Troubled letters but not numbers. Domain specific cognitive impairments
following focal damage in frontal cortex. Brain, 113, 749-766]. This study
shows that letter and number reading are dependent on dissociable processes.
More interestingly, it points to a common mechanism subserving the
perception and production of letters. We suggest that a deficit in a visuo-motor
network containing knowledge of the physical shape of letters might explain
the pattern of performance displayed by MT
Starrfelt, R. & Gerlach, C. (2007). The visual what for area: words and
pictures in the left fusiform gyrus. NeuroImage, 35, 334-342.
Notes: Center for Visual Cognition, Department of Psychology, University of
Copenhagen, Linnesgade 22, DK-1361 Copenhagen K, Denmark. randi.starrfelt@psy.ku.dk
<randi.starrfelt@psy.ku.dk>
An area in the left fusiform gyrus labelled the Visual Word Form Area (VWFA)
is claimed to be especially, or even selectively, responsive to words. We
explored how stimulus type and task demands affect activity in this area by
conducting a PET experiment where words and pictures were presented in two
conditions that differed in demands on shape processing: colour decision and
categorization. The subjects also performed an object decision task with
pictures only. The imaging data revealed a main effect of stimulus type:
rCBF was higher during word compared with picture processing. When compared
individually for colour decision and categorization, the difference between
words and pictures was only significant during colour decision, although a
trend was present during categorization also. rCBF in the VWFA was highest
during the object decision task, where only pictures were presented. Our
findings indicate that the putative VWFA is activated more by written words
than pictures, but only under certain circumstances. As demands on shape
processing increase, the difference in activation between words and pictures
decreases and can even be abolished. We suggest that activation in the VWFA
could reflect shape configuration-the integration of shape elements into
elaborate shape descriptions corresponding to whole objects or words. This
process may be required to different degrees for pictures and words
depending on task demands
Habekost, T. & Starrfelt, R. (2006). Alexia and quadrant-amblyopia: reading
disability after a minor visual field deficit. Neuropsychologia, 44,
2465-2476.
Notes: Center for Visual Cognition, Department of Psychology, University of
Copenhagen, Linnesgade 22, 1361 Copenhagen K, Denmark. Thomas.Habekost@psy.ku.dk
Reading difficulties caused by hemianopia are well described. We present a
study of alexia in a patient (NT) with a milder visual field deficit. The
patient had suffered a cerebral haemorrhage causing damage to the left
occipital cortex and underlying white matter. NT's text reading was slow and
prone to error, but recognition of single letters was preserved. Single word
reading was accurate, but slower than normal. On perimetric testing NT
initially showed an upper right quadrantanopia, but by attending covertly to
this quadrant he could achieve luminance detection except in a small scotoma
above the reading line. A whole report experiment showed that letter
perception was severely compromised in the quadrant, consistent with
cerebral amblyopia. On follow-up testing one and a half year post stroke, a
clear spontaneous recovery had occurred, reflected in improved text reading
with close to normal eye movements. Still, subtle reading difficulties and
oculo-motor abnormalities remained. Overall, the study shows how amblyopia
in one quadrant can lead to a characteristic form of alexia |
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