Intact verbal description of letters with diminished awareness of their forms.
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Visual processing and its conscious awareness can be dissociated. To examine the extent of dissociation between ability to read characters or words and to be consciously aware of their forms, reading ability and conscious awareness for characters were examined using a tachistoscope in an alexic patient. A right handed woman with 14 years of education presented with incomplete right hemianopia, alexia with kanji (ideogram) agraphia, anomia, and amnesia. Brain MRI disclosed cerebral infarction limited to the left lower bank of the calcarine fissure, lingual and parahippocampal gyri, and an old infarction in the right medial frontal lobe. Tachistoscopic examination disclosed that she could read characters aloud in the right lower hemifield when she was not clearly aware of their forms and only noted their presence vaguely. Although her performance in reading kanji was better in the left than the right field, she could read kana (phonogram) characters and Arabic numerals equally well in both fields. By contrast, she claimed that she saw only a flash of light in 61% of trials and noticed vague forms of stimuli in 36% of trials. She never recognised a form of a letter in the right lower field precisely. She performed judgment tasks better in the left than right lower hemifield where she had to judge whether two kana characters were the same or different. Although dissociation between performance of visual recognition tasks and conscious awareness of the visual experience was found in patients with blindsight or residual vision, reading (verbal identification) of characters without clear awareness of their forms has not been reported in clinical cases. Diminished awareness of forms in our patient may reflect incomplete input to the extrastriate cortex. (+info)
Visual word recognition in the left and right hemispheres: anatomical and functional correlates of peripheral alexias.
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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. (+info)
Reading aloud in jargonaphasia: an unusual dissociation in speech output.
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A patient is described who showed several dissociations between oral and written language processing after bilateral retrorolandic vascular lesion. Dissociation was firstly between abolished auditory comprehension and preserved written comprehension and then involved confrontation naming, clearly superior in the written modality. The third striking dissociation involved oral output; spontaneous speech, although fluent and well articulated, consisted of neologistic jargon, while reading aloud was clearly superior though not perfect. Data are discussed with reference to a cognitive model of word processing. The pattern of dissociation in word production may be due to a failure in retrieving the phonological word form from the semantic system. (+info)
Spatial frequency sensitivity differences between adults of good and poor reading ability.
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PURPOSE: To determine whether normal adults of good and poor reading ability exhibit different patterns of sensitivity to spatial frequency, as previously found between dyslexic and nondyslexic control subjects. METHODS: The visual acuity, spatial frequency sensitivity, and reading ability of 96 normal, nondyslexic adults was assessed. Participants were ranked according to reading ability. The top 50% were classified as good readers and the bottom 50% as poor readers. RESULTS: Despite no differences in visual acuity, good and poor readers showed different patterns of spatial frequency sensitivity. In particular, compared with good readers, poor readers showed reduced sensitivity to spatial frequencies between 2 and 6 cyc/deg, and no differences in sensitivity were found at lower or higher spatial frequencies. CONCLUSIONS: The findings indicate that spatial frequency sensitivity differences found previously between dyslexic and nondyslexic controls can extend to the normal (nondyslexic) adult population. (+info)
Pure alexia and right hemiachromatopsia in posterior dementia.
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A 66 year old, right handed woman presented with pure alexia and right hemiachromatopsia (PARH) in the context of a posterior dementia. PARH was accompanied by prosopagnosia, 2-D object agnosia, and environmental agnosia. Visual fields were normal to confrontation testing. The pathological anatomy of PARH involves circumscribed damage to the lingual and fusiform gyri and paraventricular white matter of the left occipital cortex, two contiguous cortical regions functionally specialised for processing colour and word form information, respectively. (+info)
Direct intracranial, FMRI, and lesion evidence for the causal role of left inferotemporal cortex in reading.
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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. (+info)
The role of left perisylvian cortical regions in spelling.
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In order to examine the role of left perisylvian cortex in spelling, 13 individuals with lesions in this area were administered a comprehensive spelling battery. Their spelling of regular words, irregular words, and nonwords was compared with that of individuals with extrasylvian damage involving left inferior temporo-occipital cortex and normal controls. Perisylvian patients demonstrated a lexicality effect, with nonwords spelled worse than real words. This pattern contrasts with the deficit in irregular word spelling, or regularity effect, observed in extrasylvian patients. These findings confirm that damage to left perisylvian cortex results in impaired phonological processing required for sublexical spelling. Further, degraded phonological input to orthographic selection typically results in additional deficits in real word spelling. (+info)
Sequential processing deficits of reading disabled persons is independent of inter-stimulus interval.
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Developmental dyslexia is a language-based learning disability with frequently associated non-linguistic sensory deficits that have been the basis of various perception-based theories. It remains an open question whether the underlying deficit in dyslexia is a low level impairment that causes speech and orthographic perception deficits that in turn impedes higher phonological and reading processes, or a high level impairment that affects both perceptual and reading related skills. We investigated by means of contrast detection thresholds two low-level theories of developmental dyslexia, the magnocellular and the fast temporal processing hypotheses, as well as a more recent suggestion that dyslexics have difficulties in sequential comparison tasks that can be attributed to a higher-order deficit. It was found that dyslexics had significantly higher thresholds only on a sequential, but not a spatial, detection task, and that this impairment was found to be independent of the inter-stimulus interval. We also found that the poor performance of dyslexics on the temporal task was dependent on the size of the required memory trace of the image rather than on the number of images. Our findings do not support the magnocellular theory and challenge the fast temporal deficit hypothesis. We suggest that dyslexics may have a higher order, dual mechanism impairment. We also discuss the clinical implications of our findings. (+info)