Complementary deficits in perceptual classification in pure alexia and acquired prosopagnosia - New insights from two classic cases.

Pure alexia and prosopagnosia traditionally have been seen as prime examples of dissociated, category-specific agnosias affecting reading and face recognition, respectively. More recent accounts have moved towards domain-independent explanations that postulate potential cross-links between different types of visual agnosia. According to one proposal, abnormal crowding, i.e. the impairment of recognition when features of adjacent objects are positioned too closely to each other, might provide a unified account for the perceptual deficits experienced by an agnosic patient. An alternative approach is based on the notion of complementary visual subsystems favouring the processing of abstract categories and specific exemplars, respectively. To test predictions of these two approaches with regard to pure alexia and prosopagnosia, we present previously unpublished data on digit recognition and visual crowding from two in the neuropsychological literature extensively studied patients, KD and MT (e.g., Campbell et al., 1986; Landis and Regard, 1988; Rentschler et al., 1994). Patient MT, diagnosed with pure alexia, showed pronounced abnormal foveal crowding, whereas KD, diagnosed with prosopagnosia, did not. These results form a distinct double dissociation with the performance of the two patients in other perceptual classification tasks involving Gabor micropatterns and textures, as well as Glass patterns, which revealed a significantly greater impairment in KD relative to MT. Based on an analysis of the specific task demands we argue that prosopagnosia and pure alexia may involve complementary deficits in instantiation and abstraction, respectively, during perceptual classification, beyond any category specificity. Such an explanation appears in line with previous distinctions between a predominantly left-hemispheric, abstract-category and a predominantly right-hemispheric, specific-exemplar subsystem underlying object recognition.

Can the right hemisphere read? A behavioral and disconnectome study on implicit reading in a patient with pure alexia.

Patients with pure alexia have major difficulties in reading aloud. However, they often perform above chance level in reading tasks that do not require overt articulation of the target word - like lexical decision or semantic judgment - a phenomenon usually known as "implicit reading." There is no agreement in the literature on whether implicit reading should be attributed to relative sparing of some left hemisphere (LH) reading centers or rather to signs of compensatory endeavors by the right hemisphere (RH). We report the case of an 81-year-old patient (AA) with pure alexia due to a lesion involving the left occipital lobe and the temporal infero-mesial areas, as well as the posterior callosal pathways. Although AA's reading was severely impaired and proceeded letter by letter, she showed an above-chance-level performance for frequent concrete words in a tachistoscopic lexical decision task. A structural disconnectome analysis revealed that AA's lesion not only affected the left occipital cortex and the splenium: it also disconnected white-matter tracts meant to connect the visual word-form system to decision-related frontal areas within the LH. We suggest that the RH, rather than the LH, may be responsible for patient AA's implicit reading.

Correlación neurorradiológica de las afasias. Mapa cortico-subcortical del lenguaje / Neuroradiologic correlation with aphasias. Cortico-subcortical map of language

La afasia es una alteración adquirida del lenguaje debida a una lesión cerebral, que se caracteriza por errores en la producción, la denominación o la comprensión del lenguaje. Aunque la mayoría de las afasias suelen ser mixtas, desde un punto de vista práctico se clasifican en diferentes tipos según sus rasgos clínicos principales: afasia de Broca, afasia de Wernicke, afasia de conducción, afasia transcortical y alexia con o sin agrafia. Presentamos los hallazgos clínicos de los principales subtipos representándolos con casos radiológicos, y proporcionamos una revisión actualizada de la red del lenguaje con imágenes de resonancia funcional y de tractografía

Aphasia is an acquired language disorder due to a cerebral lesion; it is characterized by errors in production, denomination, or comprehension of language. Although most aphasias are mixed, from a practical point of view they are classified into different types according to their main clinical features: Broca's aphasia, Wernicke's aphasia, conduction aphasia, transcortical aphasia, and alexia with or without agraphia. We present the clinical findings for the main subtypes of aphasia, illustrating them with imaging cases, and we provide an up-to-date review of the language network with images from functional magnetic resonance imaging and tractography

Pure alexia: two cases and a new neuroanatomical classification.

Pure alexia without visual or language accompanying deficits (isolated pure alexia), represents an infrequent finding in clinical practice. It has been linked to lesions involving the splenium of the callosal corpus in classical descriptions; however, it has also been reported after occipito-temporal cortex damage in the absence of white matter implication. In this regard, a functional region called the visual word form area has been recently related to the posterior aspect of the occipitotemporal gyrus. We report two new cases of cortical hematomas leading to this rare condition and we discuss the neuroanatomical evolution of this syndrome. Finally, we propose a new classification of pure alexia based on the neuroanatomical location of the lesion, namely: (1) disconnection alexia, after posterior and dorsal lesions involving the splenium of the callosal corpus or the paraventricular white matter, often associated with visual deficits, and (2) cortical alexia, after more anterior and ventral lesions in the occipito-temporal cortex with damage of the visual word form area, that usually manifests as isolated pure alexia.

Reading on the right when there's nothing left? Probabilistic tractography reveals hemispheric asymmetry in pure alexia.

We present a patient with reading inexpertise and right hemianopia following left posterior cerebral artery (PCA) stroke. We examine the extent of disruption to reading performance and the extent of white matter tract damage relative to a patient with more limited PCA infarction and isolated right hemianopia. We show white matter disconnection of the temporal occipital fusiform cortex in our pure alexia patient. Connectivity-based laterality indices revealed right hemisphere laterality in the alexia patient; this was not associated with improved reading function. We speculate that the degree of premorbid laterality may be a critical factor affecting the extent of reading dysfunction in alexia.

Brain mechanisms of recovery from pure alexia: A single case study with multiple longitudinal scans.

Pure alexia is an acquired reading disorder, typically due to a left occipito-temporal lesion affecting the Visual Word Form Area (VWFA). It is unclear whether the VWFA acts as a unique bottleneck for reading, or whether alternative routes are available for recovery. Here, we address this issue through the single-case longitudinal study of a neuroscientist who experienced pure alexia and participated in 17 behavioral, 9 anatomical, and 9 fMRI assessment sessions over a period of two years. The origin of the impairment was assigned to a small left fusiform lesion, accompanied by a loss of VWFA responsivity and by the degeneracy of the associated white matter pathways. fMRI experiments allowed us to image longitudinally the visual perception of words, as compared to other classes of stimuli, as well as the mechanisms of letter-by-letter reading. The progressive improvement of reading was not associated with the re-emergence of a new area selective to words, but with increasing responses in spared occipital cortex posterior to the lesion and in contralateral right occipital cortex. Those regions showed a non-specific increase of activations over time and an increase in functional correlation with distant language areas. Those results confirm the existence of an alternative occipital route for reading, bypassing the VWFA, but they also point to its key limitation: the patient remained a slow letter-by-letter reader, thus supporting the critical importance of the VWFA for the efficient parallel recognition of written words.

Visual attention in posterior stroke and relations to alexia.

Impaired visual attention is common following strokes in the territory of the middle cerebral artery, particularly in the right hemisphere, while attentional effects of more posterior lesions are less clear. Commonly, such deficits are investigated in relation to specific syndromes like visual agnosia or pure alexia. The aim of this study was to characterize visual processing speed and apprehension span following posterior cerebral artery (PCA) stroke. In addition, the relationship between these attentional parameters and single word reading is investigated, as previous studies have suggested that reduced visual speed and span may explain pure alexia. Eight patients with unilateral PCA strokes (four left hemisphere, four right hemisphere) were selected on the basis of lesion location, rather than the presence of any visual symptoms. Visual attention was characterized by a whole report paradigm allowing for hemifield-specific measurements of processing speed and apprehension span. All patients showed reductions in visual span contralateral to the lesion site, and four patients showed bilateral reductions in visual span despite unilateral lesions (2L; 2R). Six patients showed selective deficits in visual span, though processing speed was unaffected in the same field (ipsi- or contralesionally). Only patients with right hemifield reductions in visual span were impaired in reading, and this could follow either right or left lateralized stroke and was irrespective of visual field impairments. In conclusion, visual span may be affected bilaterally by unilateral PCA-lesions. Reductions in visual span may also be confined to one hemifield, and may be affected in spite of preserved visual processing speed. Furthermore, reduced span in the right visual field seems to be related to reading impairment in this group, regardless of lesion lateralization.

Progressive Wallerian Degeneration of the Corpus Callosal Splenium in a Patient with Alexia Without Agraphia: Advanced MR Findings.

The corpus callosal splenium is an uncommon location for Wallerian degeneration (WD), which may be mistaken for new pathology on magnetic resonance imaging (MRI). We describe the case of a 69-year-old woman with a left posterior cerebral artery infarct in whom progressive WD of the splenium of the corpus callosum seen on MRI was misinterpreted as new infarction or neoplasm. We address how magnetic resonance spectroscopy, perfusion MRI, diffusion tensor MRI, and serial imaging were utilized in establishing the correct diagnosis. Interestingly, the patient also presented with alexia without agraphia, which has never been reported in association with splenial WD. It is conceivable that WD affected critical splenial association fibers resulting in this uncommon dissociation syndrome.

An eye movement study on the role of the visual field defect in pure alexia.

Pure alexia is a severe impairment of word reading which is usually accompanied by a right-sided visual field defect. Patients with pure alexia exhibit better preserved writing and a considerable word length effect, claimed to result from a serial letter processing strategy. Two experiments compared the eye movements of four patients with pure alexia to controls with simulated visual field defects (sVFD) when reading single words. Besides differences in response times and differential effects of word length on word reading in both groups, fixation durations and the occurrence of a serial, letter-by-letter fixation strategy were investigated. The analyses revealed quantitative and qualitative differences between pure alexic patients and unimpaired individuals reading with sVFD. The patients with pure alexia read words slower and exhibited more fixations. The serial, letter-by-letter fixation strategy was observed only in the patients but not in the controls with sVFD. It is argued that the VFD does not cause pure alexic reading.

Fluorodeoxyglucose F18 positron emission tomography in a case of slowly progressive pure alexia.

We describe a 71-year-old patient with slowly progressive pure alexia in which analysis of her fluorodeoxyglucose FDG-PET scan revealed an area of focal hypometabolism in the visual word form area. She presented with difficulty reading. Examination revealed pure alexia with preservation of other cognitive domains. Brain MRI revealed only slight atrophy. A Fluorodeoxyglucose F18 positron emission tomography scan revealed hypometabolism in the occipital cortex bilaterally, left greater than right, with normal metabolism elsewhere in the brain. This case highlights the utility of FDG-PET scan in evaluating focal neurodegenerative conditions before clear atrophy can be seen on MRI.

Alexia without agraphia in Creutzfeldt-Jakob disease.

Progressive dementia uncommonly presents with focal deficits referable to posterior cortical dysfunction. We describe a 62 year-old man who presented with progressive visual disturbance in whom detailed cognitive testing documented alexia without agraphia. The only finding from diagnostic investigations was hypoperfusion of the posterior left temporoparietal region on brain SPECT scan. He rapidly progressed and Creutzfeldt-Jakob disease (CJD) was confirmed at autopsy. Although reading disorders may develop in association with posterior cortical atrophy due to CJD, this is the first reported autopsy-confirmed case presenting as alexia without agraphia.

Improvement in Tc-99m HMPAO brain SPECT findings during donepezil therapy in a patient with pure akinesia.

A 58-year-old man presented with a history of disturbance in initiating gait. His history revealed meningoencephalitis five years prior to admission. Neurological examination included gait disturbance as difficulty in initiation and a hesitating speech with many freezing episodes and micrographia Magnetic resonance imaging (MRI) showed diffuse hyperintensity of frontal subcortical white matter on T2 weighted images. He was diagnosed with PA. L-Dopa up to the dosages of 1000 mg/ day and selegiline 10 mg/day were given. First brain SPECT using technetium-99m labeled D,L-hexamethylpropylene amine oxime (Tc-99m HMPAO) was performed when he was taking L-dopa and selegiline. In visual evaluation, hypoperfusion in bilateral frontoparietal cortex was seen (Fig. 2). Treatment with L-dopa and selegiline produced no benefit. Donepezil 10 mg/day was begun. This therapy regimen resulted in dramatic clinical improvement within several days that was confirmed by blinded raters who watched the patient's video recordings. During this response second brain perfusion SPECT study was repeated during donepezil therapy. Markedly increased perfusion in bilateral frontoparietal cortex was observed. This is the first case of PA to develop possibly after an episode of bacterial pneumococcal meningoencephalitis and who responded to donepezil as documented by changes in clinical findings and Tc-99m HMPAO brain SPECT studies.