Can the right hemisphere read? A behavioral and disconnectome study on implicit reading in a patient with pure alexia (original) (raw)
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Cerebral Cortex, 2003
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.
Right hemisphere reading mechanisms in a global alexic patient
Neuropsychologia, 2004
We investigated the implicit, or covert, reading ability of a global alexic patient (EA) to help determine the contribution of the right hemisphere to reading. Previous studies of alexic patients with left hemisphere damage have suggested that the ability to derive meaning from printed words that cannot be read out loud may reflect right hemisphere reading mechanisms. Other investigators have argued that residual left hemisphere abilities are sufficient to account for implicit reading and moreover do not require the postulation of a right hemisphere system that has no role in normal reading processes. However, few studies have assessed covert reading in patients with lesions as extensive as the one in EA, which affected left medial, inferior temporal-occipital cortex, hippocampus, splenium, and dorsal white matter. EA was presented with lexical decision, semantic categorization, phonemic categorization, and letter matching tasks. Although EA was unable to access phonology and could not overtly name words or letters, she was nevertheless capable of making lexical and semantic decisions at above chance levels, with an advantage for concrete versus abstract words. Her oral and written spelling were relatively intact, suggesting that orthographic knowledge is retained, although inaccessible through the visual modality. Based on her ability to access lexical and semantic information without contacting phonological representations, we propose that EA's implicit reading emerges from, and is supported, by the right hemisphere. Finally, we conclude that her spelling and writing abilities are supported by left hemisphere mechanisms.
Dysfunctional visual word form processing in progressive alexia
Brain, 2013
Progressive alexia is an acquired reading deficit caused by degeneration of brain regions that are essential for written word processing. Functional imaging studies have shown that early processing of the visual word form depends on a hierarchical posterior-to-anterior processing stream in occipito-temporal cortex, whereby successive areas code increasingly larger and more complex perceptual attributes of the letter string. A region located in the left lateral occipito-temporal sulcus and adjacent fusiform gyrus shows maximal selectivity for words and has been dubbed the 'visual word form area'. We studied two patients with progressive alexia in order to determine whether their reading deficits were associated with structural and/or functional abnormalities in this visual word form system. Voxel-based morphometry showed left-lateralized occipito-temporal atrophy in both patients, very mild in one, but moderate to severe in the other. The two patients, along with 10 control subjects, were scanned with functional magnetic resonance imaging as they viewed rapidly presented words, false font strings, or a fixation crosshair. This paradigm was optimized to reliably map brain regions involved in orthographic processing in individual subjects. All 10 control subjects showed a posterior-to-anterior gradient of selectivity for words, and all 10 showed a functionally defined visual word form area in the left hemisphere that was activated for words relative to false font strings. In contrast, neither of the two patients with progressive alexia showed any evidence for a selectivity gradient or for word-specific activation of the visual word form area. The patient with mild atrophy showed normal responses to both words and false font strings in the posterior part of the visual word form system, but a failure to develop selectivity for words in the more anterior part of the system. In contrast, the patient with moderate to severe atrophy showed minimal activation of any part of the visual word form system for either words or false font strings. Our results suggest that progressive alexia is associated with a dysfunctional visual word form system, with or without substantial cortical atrophy. Furthermore, these findings demonstrate that functional MRI has the potential to reveal the neural bases of cognitive deficits in neurodegenerative patients at very early stages, in some cases before the development of extensive atrophy. Keywords: progressive alexia; letter-by-letter reading; posterior cortical atrophy; logopenic primary progressive aphasia; visual word form system Abbreviations: PCA = posterior cortical atrophy; VWFA = visual word form area
The pathophysiology of letter-by-letter reading
Neuropsychologia, 2004
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.
Reading therapy strengthens top-down connectivity in patients with pure alexia
Brain, 2013
This study tested the efficacy of audio-visual reading training in nine patients with pure alexia, an acquired reading disorder caused by damage to the left ventral occipitotemporal cortex. As well as testing the therapy's impact on reading speed, we investigated the functional reorganization underlying therapy-induced behavioural changes using magnetoencephalography. Reading ability was tested twice before training (t1 and t2) and twice after completion of the 6-week training period (t3 and t4). At t3 there was a significant improvement in word reading speed and reduction of the word length effect for trained words only. Magnetoencephalography at t3 demonstrated significant differences in reading network connectivity for trained and untrained words. The training effects were supported by increased bidirectional connectivity between the left occipital and ventral occipitotemporal perilesional cortex, and increased feedback connectivity from the left inferior frontal gyrus. Conversely, connection strengths between right hemisphere regions became weaker after training.
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 wordresponsive 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. Neuron 192
How Does iReadMore Therapy Change the Reading Network of Patients with Central Alexia
Central alexia (CA) is an acquired reading disorder co-occurring with a generalized language deficit (aphasia). The roles of perilesional and ipsilesional tissue in recovery from poststroke aphasia are unclear. We investigated the impact of reading training (using iReadMore, a therapy app)ontheconnectionswithinandbetweentherightandlefthemisphereofthereadingnetworkofpatientswithCA.Inpatientswithpurealexia, iReadMore increased feedback from left inferior frontal gyrus (IFG) region to the left occipital (OCC) region. We aimed to identify whether iReadMore therapy was effective through a similar mechanism in patients with CA. Participants with chronic poststroke CA (n ϭ 23) completed 35 h of iReadMore training over 4 weeks. Reading accuracy for trained and untrained words was assessed before and after therapy. The neural response to reading trained and untrained words in the left and right OCC, ventral occipitotemporal, and IFG regions was examined using event-related magnetoencephalography. The training-related modulation in effective connectivity between regions was modeled at the group level with dynamic causal modeling. iReadMore training improved participants' reading accuracy by an average of 8.4% (range, Ϫ2.77 to 31.66) while accuracy for untrained words was stable. Training increased regional sensitivity in bilateral frontal and occipital regions, and strengthened feedforward connections within the left hemisphere. Our data suggest that iReadMore training in these patients modulates lower-order visual representations, as opposed to higher-order, more abstract representations, to improve word-reading accuracy.
2008
Pure alexia is a well-documented syndrome characterized by impaired reading in the context of relatively intact spelling, resulting from lesions of the left temporo-occipital region (Coltheart, 1998). It is considered a disconnection syndrome in that visual information about written words does not reach the cortical areas critical for word recognition (i.e., the orthographic lexicon). The relative preservation of orthographic knowledge is confirmed in individuals with pure alexia by the fact that they can spell and they can also recognize orally spelled words with little difficulty. To compensate for their deficit, pure alexics often use a letter-by-letter (LBL) strategy, where reading is achieved by sequentially identifying the component letters of a word. In some cases, however, letter identification is impaired and a LBL reading approach is obviated. The inability to name letters may reflect a visual perceptual deficit, or a disconnection of visual-verbal information, where the letters are correctly perceived but fail to access letter names.