Visuographemic alexia: a new form of a peripheral acquired dyslexia

Acquired dyslexias following temporal lesions

The acquisition of reading by children is supported by deep changes in the brain systems devoted to vision and language. The left temporal lobe contributes critically to both systems, and lesions affecting it may therefore cause both peripheral vision-related and central language-related reading impairments. The diversity of peripheral dyslexias reflects the anatomical and functional division of the visual cortex into early visual regions, whose lesions have a limited impact on reading; ventral regions, whose lesions are mostly associated to Pure Alexia; and dorsal regions, whose lesions may yield spatial, neglect-related, and attentional dyslexias. Similarly, central alexias reflect the broad distinction, within language processes, between phonological and lexico-semantic components. Phonological and surface dyslexias roughly result from impairment of the former and the latter processes, respectively, while deep dyslexia may be seen as the association of both. In this chapter, we review such types of acquired dyslexias, their clinical features, pathophysiology, and anatomical correlates.

The role of allograph representations in font-invariant letter identification

The literate brain must contend with countless font variants for any given letter. How does the visual system handle such variability? One proposed solution posits stored structural descriptions of basic letter shapes that are abstract enough to deal with the many possible font variations of each letter. These font-invariant representations, referred to as allographs in this paper, while frequently posited, have seldom been empirically evaluated. The research reported here helps to address this gap with 2 experiments that examine the possible influence of allograph representations on visual letter processing. In these experiments, participants respond to pairs of letters presented in an atypical font in 2 tasks-visual similarity judgments (Experiment 1) and same/different decisions (Experiment 2). By using representational similarity analysis (RSA) in conjunction with linear mixed effect models (LMEM; RSA-LMEM) we show that the similarity structure of the responses to the atypical font is influenced by the predicted similarity structure of allograph representations even after accounting for font-specific visual shape similarity. Similarity due to symbolic (abstract) identity, name, and motor representations of letters are also taken into account providing compelling evidence for the unique influence of allograph representations in these tasks. These results provide support for the role of allograph representations in achieving font-invariant letter identification. (PsycINFO Database Record

Alexia With and Without Agraphia: An Assessment of Two Classical Syndromes

Background:

Current cognitive models propose that multiple processes are involved in reading and writing.

Objective:

Our goal was to use linguistic analyses to clarify the cognitive dysfunction behind two classic alexic syndromes.

Methods:

We report four experiments on two patients, one with alexia without agraphia following occipitotemporal lesions, and one with alexia with agraphia from a left angular gyral lesion.

Results:

The patient with occipital lesions had trouble discriminating real letters from foils and his reading varied with word-length but not with linguistic variables such as part of speech, word frequency or imageability. He read pseudo-words and words with regular spelling better, indicating preserved use of grapheme-to-phoneme pronunciation rules. His writing showed errors that reflected reliance on ‘phoneme-to-grapheme’ spelling rules. In contrast, the patient with a left angular gyral lesion showed better recognition of letters, words and their meanings. His reading was better for words with high imageability but displayed semantic errors and an inability to use ‘grapheme-to-phoneme’ rules, features consistent with deep dyslexia. His agraphia showed impaired access to both an internal lexicon and ‘phoneme-to-grapheme’ rules.

Conclusion:

Some cases of pure alexia may be a perceptual word-form agnosia, with loss of internal representations of letters and words, while the angular gyral syndrome of alexia with agraphia is a linguistic deep dyslexia. The presence or absence of agraphia does not always distinguish between the two; rather, writing can mirror the reading deficits, being more obvious and profound in the case of an angular gyral syndrome.

Prelexical representations and processes in reading: evidence from acquired dyslexia

We report a detailed and extensive single-case study of an acquired dyslexic patient, L.H.D., who suffered a left-hemisphere lesion as a result of a ruptured aneurysm. We present evidence that L.H.D.'s reading errors stem from a deficit in visual letter identification, and we use her deficit as a basis for exploring a variety of issues concerning prelexical representations and processes in reading. First, building on the work of other researchers, we present evidence that the prelexical reading system includes an allograph level of representation that represents each distinct visual shape of a letter (e.g., a, A, etc., for the letter A). We extend a theory proposed by Caramazza and Hillis [Caramazza, A., & Hillis, A. (1990a). Spatial representation of words in the brain implied by studies of a unilateral neglect patient. Nature, 346, 267-269] to include an allograph level, and we probe the nature of the allograph representations in some detail. Next, we explore the implications of visual similarity effects and letter perseverations in L.H.D.'s reading performance, arguing that these effects shed light on activation dynamics in the prelexical reading system and on the genesis of L.H.D.'s errors. We also probe the processing of letter case in the visual letter identification process, proposing that separate abstract letter identity and case representations are computed. Finally, we present evidence that the allograph level as well as the abstract letter identity level implement a word-based frame of reference.

Number reading in pure alexia--a review

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.

Selective response to letter categorization within the left fusiform gyrus

Neuroimaging studies that look at reading processes using words, pseudowords, nonwords and letters frequently report specific left fusiform gyrus (BA37) activations. In the present study, we examined fMRI signal variations within the left and right BA37 for paired Latin letters, Korean letters and geometrical figures in discrimination and categorization tasks. Data of Pernet et al. (Pernet, C., Franceries, X., Basan, S., Cassol, E., Démonet, J.F., Celsis, P., 2004. Anatomy and time course of discrimination and categorization processes in vision: an fMRI study. NeuroImage 22, 1563-1577) were re-analyzed using a ROI methodology that highlights the selective response of the left BA37 to Latin letter categorization. First, differences according to stimulus type were observed for the categorization task only. Second, we found weaker activation for Latin letter categorization than for both geometrical figure and Korean letter categorization. Third, only Latin letter categorization elicited as left-sided activation, although the direct comparison between regions did not demonstrate a significant difference. These data suggest that the left fusiform gyrus sustains access to letter representations in memory; and results are discussed with reference to the relationship between letter categorization and word recognition and to selective vs. specific (i.e. task-independent) neural response.

Automatic grapheme processing in the left occipitotemporal cortex

In a previous event-related fMRI study, we showed that the left occipitotemporal cortex was specifically involved in the abstract categorization of visually presented alphabetic symbols. Here, we duplicate the fMRI study by measuring visual event-related potentials to verify whether the left posterior cortex supports the processing of graphemic representations at a perceptual, prelexical level. N170 amplitudes elicited by the categorically ambiguous stimulus 'O' were about twice as small in the left as in the right occipitotemporal region, and comparable to that of other letters on the left, and to that of other geometric figures on the right. The side asymmetry suggests that a graphemic module is unilaterally implemented in the left hemisphere, where it automatically processes heteromorphic representations that do not depend on the physical characteristics of the signal, whereas the difference in amplitude suggests that this left graphemic module is in series with, precedes and does not transmit information to, or inhibits a general shape-analysis module.