Neurophysiological correlates of word recognition in dyslexia

Summary. The neurobiological basis of learning word spellings and recognition of recently learned words was assessed in a learning experiment in 9 dyslexics and 9 controls male adolescents. In a recognition paradigm previously learned pseudowords and graphic symbols were presented 50 times each interspersed pseudo-randomly between 3 unlearned items which were repeated 50 times and 150 filler pseudowords. The electrophysiological correlate of recognition of learned pseudowords and graphic symbols was a positivity around 600ms. For pseudowords the amplitude of this ERP component was significantly attenuated in the dyslexic group, no differences between the groups were found for recognition of graphic material. These data suggest that dyslexic children are able to learn the spelling of simple words, however, the neurophysiological correlate of recognition of these learned words is significantly attenuated. This result strengthens the view that dyslexic children are not generally impaired in recognition memory but specific for linguistic material like words.     

Stimulus-centered neglect in reading and object recognition

Abstract

We examine whether words and objects share a common stage of processing in which an item's shape is represented in a reference frame centered on the item, that is, a ‘stimulus-centered’ representation. The neurological patient MR neglected the left parts of words, letter-strings, and objects in a variety of reading and naming tests, regardless of where they were presented in the visual field. She identified only the right sides of chimeras, and failed to notice that they were not real objects. Her pattern of performance provides evidence consistent with a common pathway for word and object processing beyond the early, retinocentric levels of the visual system.     

Cerebellar function in children with and without dyslexia during single word processing

The cerebellar deficit hypothesis of dyslexia posits that dysfunction of the cerebellum is the underlying cause for reading difficulties observed in this common learning disability. The present study used functional magnetic resonance imaging (fMRI) and a single word processing task to test for differences in activity and connectivity in children with (n = 23) and without (n = 23) dyslexia. We found cerebellar activity in the control group when word processing was compared to fixation, but not when it was compared to the active baseline task designed to reveal activity specific to reading. In the group with dyslexia there was no cerebellar activity for either contrasts and there were no differences when they were compared to children without dyslexia. Turning to functional connectivity (FC) in the controls, background FC (i.e., not specific to reading) was predominately found between the cerebellum and the occipitaltemporal cortex. In the group with dyslexia, there was background FC between the cerebellum and several cortical regions. When comparing the two groups, they differed in background FC in connections between the seed region right crus I and three left‐hemisphere perisylvian target regions. However, there was no task‐specific FC for word processing in either group and no between‐group differences. Together the results do not support the theory that the cerebellum is affected functionally during reading in children with dyslexia.     

Neglect dyslexia: Frequency, association with other hemispatial neglects, and lesion localization

Patients with right hemisphere injury often omit or misread words on the left side of a page or the beginning letters of single words (neglect dyslexia). Our study involving a large sample of acute right hemisphere stroke investigated (1) the frequency of neglect dyslexia (ND), (2) the association between ND and other types of contralesional hemispatial neglect (CN), (3) the effect of visual field defect (VFD) on ND, and (4) the anatomical substrates for ND. Participants were 138 consecutive patients with right hemisphere stroke who underwent a neglect test battery including a test for ND. ND was considered present if the patient misread or omitted the left portion of the word in three or more of the 25 target words. CN was noted in 80/138 (58.0%) patients while ND was found in 31/138 (22.5%) patients. Of the 80 patients with CN, the frequency of neglect based on ND test was only 37.5% while the frequency of neglect based on other neglect tasks ranged from 51.3% to 86.3%. The severity of neglect was a significant predictor for ND. VFD was also a significant predictor for the occurrence of ND but this effect disappeared when the severity of neglect was controlled. Patients with CN had lesions in the superior and middle temporal gyri, inferior parietal lobule, and posterior insular cortex; patients with ND had additional lesions in the lingual and fusiform gyri. In summary, ND was dissociated from other types of neglect and was most often associated with severe neglect. VFD contributed to the occurrence of ND. ND resulted from lesions of temporoparietal junction areas (inferior parietal/superior temporal gyri) combined with those of lingual/fusiform gyri.     

Left neglect dyslexia and the effect of stimulus duration

The present study investigated the effects of the duration of the stimulus on the reading performance of right-brain-damaged patients with left neglect dyslexia. Three Italian patients read aloud words and nonwords, under conditions of unlimited time of stimulus exposure and of timed presentation. In the untimed condition, the majority of the patients' errors involved the left side of the letter string (i.e., neglect dyslexia errors). Conversely, in the timed condition, although the overall level of performance decreased, errors were more evenly distributed across the whole letter string (i.e., visual - nonlateralized - errors). This reduction of neglect errors with a reduced time of presentation of the stimulus may reflect the read out of elements of the letter string from a preserved visual storage component, such as iconic memory. Conversely, a time-unlimited presentation of the stimulus may bring about the rightward bias that characterizes the performance of neglect patients, possibly by a capture of the patients' attention by the final (rightward) letters of the string.     

Eye movement correlates of acquired central dyslexia

Based on recent progress in theory and measurement techniques, the analysis of eye movements has become one of the major methodological tools in experimental reading research. Our work uses this approach to advance the understanding of impaired information processing in acquired central dyslexia of stroke patients with aphasia. Up to now there has been no research attempting to analyze both word-based viewing time measures and local fixation patterns in dyslexic readers. The goal of the study was to find out whether specific eye movement parameters reflect pathologically preferred segmental reading in contrast to lexical reading.We compared oral reading of single words of normal controls (n = 11) with six aphasic participants (two cases of deep, surface and residual dyslexia each). Participants were asked to read aloud lines of target words differing in length and frequency. Segmental reading was characterized by deviant spatial distribution of saccadic landing positions with initial fixations located mainly at the beginning of the word, while lexical readers showed the normative ‘preferred landing positions’ left to the center of the words. Contrary to expectation, word length did not distinguish between segmental and lexical readers, while word frequency showed the expected effect for lexical readers only. Their mean fixation duration was already prolonged during first pass reading reflecting their attempts of immediate access to lexical information. After first pass reading, re-reading time was significantly increased in all participants with acquired central dyslexia due to their exceedingly higher monitoring demands for oral reading.     

Letter and letter-string processing in developmental dyslexia

This study evaluated letter recognition processing in Italian developmental dyslexics and its potential contribution to word reading. Letter/bigram recognition (naming and matching) and reading of words and non-words were examined. A group of developmental dyslexics and a chronologically age-matched group of skilled readers were examined. Dyslexics were significantly slower than skilled readers in all tasks. The rate and amount model (RAM, Faust et al., 1999) was used to detect global and specific factors in the performance differences controlling for the presence of over-additivity effects. Two global factors emerged. One (“letter-string” factor) accounted for the performance in all (and only) word and non-word reading conditions, indicating a large impairment in dyslexics (more than 100% reaction time - RT increase as compared to skilled readers). All the letter/bigram tasks clustered on a separate factor (“letter” factor) indicating a mild impairment (ca. 20% RT increase as compared to skilled readers). After controlling for global factor influences by the use of the z-score transformation, specific effects were detected for the “letter-string” (but not the “letter”) factor. Stimulus length exerted a specific effect on dyslexics' performance, with dyslexics being more affected by longer stimuli; furthermore, dyslexics showed a stronger impairment for reading words than non-words. Individual differences in the “letter” and “letter-string” factors were uncorrelated, pointing to the independence of the impairments. The putative mechanisms underlying the two global factors and their possible relationship to developmental dyslexia are discussed.     

Top‐down and bottom‐up influences on the left ventral occipito‐temporal cortex during visual word recognition: An analysis of effective connectivity

The functional role of the left ventral occipito‐temporal cortex (vOT) in visual word processing has been studied extensively. A prominent observation is higher activation for unfamiliar but pronounceable letter strings compared to regular words in this region. Some functional accounts have interpreted this finding as driven by top‐down influences (e.g., Dehaene and Cohen [ 2011 ]: Trends Cogn Sci 15:254–262; Price and Devlin [ 2011 ]: Trends Cogn Sci 15:246–253), while others have suggested a difference in bottom‐up processing (e.g., Glezer et al. [ 2009 ]: Neuron 62:199–204; Kronbichler et al. [ 2007 ]: J Cogn Neurosci 19:1584–1594). We used dynamic causal modeling for fMRI data to test bottom‐up and top‐down influences on the left vOT during visual processing of regular words and unfamiliar letter strings. Regular words (e.g., taxi) and unfamiliar letter strings of pseudohomophones (e.g., taksi) were presented in the context of a phonological lexical decision task (i.e., “Does the item sound like a word?”). We found no differences in top‐down signaling, but a strong increase in bottom‐up signaling from the occipital cortex to the left vOT for pseudohomophones compared to words. This finding can be linked to functional accounts which assume that the left vOT contains neurons tuned to complex orthographic features such as morphemes or words [e.g., Dehaene and Cohen [ 2011 ]: Trends Cogn Sci 15:254‐262; Kronbichler et al. [ 2007 ]: J Cogn Neurosci 19:1584–1594]: For words, bottom‐up signals converge onto a matching orthographic representation in the left vOT. For pseudohomophones, the propagated signals do not converge, but (partially) activate multiple orthographic word representations, reflected in increased effective connectivity. Hum Brain Mapp 35:1668–1680, 2014. © 2013 Wiley Periodicals, Inc.     

Three stages of emotional word processing: an ERP study with rapid serial visual presentation

Rapid responses to emotional words play a crucial role in social communication. This study employed event-related potentials to examine the time course of neural dynamics involved in emotional word processing. Participants performed a dual-target task in which positive, negative and neutral adjectives were rapidly presented. The early occipital P1 was found larger when elicited by negative words, indicating that the first stage of emotional word processing mainly differentiates between non-threatening and potentially threatening information. The N170 and the early posterior negativity were larger for positive and negative words, reflecting the emotional/non-emotional discrimination stage of word processing. The late positive component not only distinguished emotional words from neutral words, but also differentiated between positive and negative words. This represents the third stage of emotional word processing, the emotion separation. Present results indicated that, similar with the three-stage model of facial expression processing; the neural processing of emotional words can also be divided into three stages. These findings prompt us to believe that the nature of emotion can be analyzed by the brain independent of stimulus type, and that the three-stage scheme may be a common model for emotional information processing in the context of limited attentional resources.     

The transition from vision to language: Distinct patterns of functional connectivity for subregions of the visual word form area

Reading entails transforming visual symbols to sound and meaning. This process depends on specialized circuitry in the visual cortex, the visual word form area (VWFA). Recent findings suggest that this text-selective cortex comprises at least two distinct subregions: the more posterior VWFA-1 is sensitive to visual features, while the more anterior VWFA-2 processes higher level language information. Here, we explore whether these two subregions also exhibit different patterns of functional connectivity. To this end, we capitalize on two complementary datasets: Using the Natural Scenes Dataset (NSD), we identify text-selective responses in high-quality 7T adult data (N = 8), and investigate functional connectivity patterns of VWFA-1 and VWFA-2 at the individual level. We then turn to the Healthy Brain Network (HBN) database to assess whether these patterns replicate in a large developmental sample (N = 224; age 6–20 years), and whether they relate to reading development. In both datasets, we find that VWFA-1 is primarily correlated with bilateral visual regions. In contrast, VWFA-2 is more strongly correlated with language regions in the frontal and lateral parietal lobes, particularly the bilateral inferior frontal gyrus. Critically, these patterns do not generalize to adjacent face-selective regions, suggesting a specific relationship between VWFA-2 and the frontal language network. No correlations were observed between functional connectivity and reading ability. Together, our findings support the distinction between subregions of the VWFA, and suggest that functional connectivity patterns in the ventral temporal cortex are consistent over a wide range of reading skills.     

The relationship between visual perception and visual mental imagery: a reappraisal of the neuropsychological evidence

Visual perception and visual mental imagery, the faculty whereby we can revisualise a visual item from memory, have often been regarded as cognitive functions subserved by common mechanisms. Thus, the leading cognitive model of visual mental imagery holds that visual perception and visual imagery share a number of mental operations, and rely upon common neural structures, including early visual cortices. In particular, a single visual buffer would be used "bottom-up" to display visual percepts and "top-down" to display internally generated images. The proposed neural substrate for this buffer consists of some cortical visual areas organised retinotopically, that is, the striate and extrastriate occipital areas. Empirical support for this model came from the report of brain-damaged patients showing an imagery deficit which parallels a perceptual impairment in the same cognitive domain. However, recent reports of patients showing double dissociations between perception and imagery abilities challenged the perception-imagery equivalence hypothesis from the functional point of view. From the anatomical point of view, the available evidence suggests that occipital damage is neither necessary nor sufficient to produce imagery deficits. On the other hand, extensive left temporal damage often accompanies imagery deficits for object form or colour. Thus, visual mental imagery abilities might require the integrity of brain areas related to vision, but at an higher level of integration than previously proposed.     

Different influence of backward masking on visual pattern recognition in the early stages of presenile dementia and normal ageing

The effect of backward masking with illumination of all fields or with a checkerboard pattern, respectively, on the recognition of pattern + or T was measured on four groups of subjects: 21 presenile onset DAT patients, 16 patients with a questionable vascular dementia of the same age (CVD group), 15 age-matched control subjects (EC group) and 16 younger control subjects (YC group, mean age 22 years). The pattern stimulus had a duration of 20 ms and was followed by the masking stimulus pattern (20 ms duration) with increasing intervals in steps of 5 ms. The mean recognition threshold (RT) for each of the four possible pattern–mask combinations was measured in five trials. The RTs decreased within the five trials in all groups. DAT patients and CVD patients adapted with different speeds during the five trials. We found a small significant RT difference between elder and younger control subjects, whereas patients had increased RTs in the first trial and differences in the four variants in the further trials compared with age-matched controls. The test contributes to a set of data for diagnosis and differential diagnosis in the early stages of presenile dementia.     

Dissociation in the recovery from neglect dyslexia and visuospatial unilateral neglect: a case report

The recovery of the ability to read of a patient affected by persistent visuospatial neglect suggests the functional independence of the two phenomena. Neglect dyslexia seems to be an example of a dissociation between an implicit and explicit knowledge of the characteristics of the stimulus.

---

Sommario Il recupero della capacità di lettura in un paziente con persistente neglect visuo-spaziale suggerisce l'indipendenza funzionale dei due fenomeni. La neglect dyslexia sembra costituire un esempio di dissociazione tra conoscenza implicita ed esplicita delle caratteristiche dello stimolo.

     

Effect of word and syllable frequency on activation during lexical decision and reading aloud

This functional MRI (fMRI) study investigated the effect of lexical and syllable frequency on visual word processing during lexical decision and reading aloud. Previous research has shown a dissociation of syllable and word frequency effects in Spanish using behavioral and electrophysiological measures, suggesting that sublexical (syllabic) representations are computed and mediate the firing of lexical candidates. Here, we characterize the neuroanatomical basis of these lexical and sublexical manipulations and their dependence on task. During lexical decision, words with low vs. high lexical frequency increased activation in left frontal, anterior cingulate, supplemental motor area (SMA), and pre-SMA regions; while words with high vs. low syllable frequency increased activation in a left anterior inferior temporal region. In contrast, when the words were read aloud those with low vs. high syllable frequency increased activation in the left anterior insula, with no other significant effects. On the basis of the neuroanatomy, we propose that the contrasting effects of syllable frequency during lexical decision and reading aloud reflect two different cognitive processes in visual word processing. Specifically, words with high-frequency syllables may increase lexical competition in the inferior temporal lobe while facilitating articulatory planning in the left anterior insula.     

Cortical dynamics of word recognition

While functional neuroimaging studies have helped elucidate major regions implicated in word recognition, much less is known about the dynamics of the associated activations or the actual neural processes of their functional network. We used intracerebral electroencephalography recordings in 10 patients with epilepsy to directly measure neural activity in the temporal and frontal lobes during written words' recognition, predominantly in the left hemisphere. The patients were presented visually with consonant strings, pseudo-words, and words and performed a hierarchical paradigm contrasting semantic processes (living vs. nonliving word categorization task), phonological processes (rhyme decision task on pseudo-words), and visual processes (visual analysis of consonant strings). Stimuli triggered a cascade of modulations in the gamma-band (>40 Hz) with reproducible timing and task-sensitivity throughout the functional reading network: the earliest gamma-band activations were observed for all stimuli in the mesial basal temporal lobe at 150 ms, reaching the word form area in the mid fusiform gyrus at 200 ms, evidencing a superiority effect for word-like stimuli. Peaks of gamma-band activations were then observed for word-like stimuli after 400 ms in the anterior and middle portion of the superior temporal gyrus (BA 38 and BA 22 respectively), in the pars triangularis of Broca's area for the semantic task (BAs 45 and 47), and in the pars opercularis for the phonological task (BA 44). Concurrently, we observed a two-pronged effect in the prefrontal cortex (BAs 9 and 46), with nonspecific sustained dorsal activation related to sustained attention and, more ventrally, a strong reflex deactivation around 500 ms, possibly due to semantic working memory reset.     

The role of visual attention in dyslexia: Behavioral and neurobiological evidence

Poor phonological processing has typically been considered the main cause of dyslexia. However, visuo‐attentional processing abnormalities have been described as well. The goal of the present study was to determine the involvement of visual attention during fluent reading in children with dyslexia and typical readers. Here, 75 children (8–12 years old; 36 typical readers, 39 children with dyslexia) completed cognitive and reading assessments. Neuroimaging data were acquired while children performed a fluent reading task with (a) a condition where the text remained on the screen (Still) versus (b) a condition in which the letters were being deleted (Deleted). Cognitive assessment data analysis revealed that visual attention, executive functions, and phonological awareness significantly contributed to reading comprehension in both groups. A seed‐to‐voxel functional connectivity analysis was performed on the fluency functional magnetic resonance imaging task. Typical readers showed greater functional connectivity between the dorsal attention network and the left angular gyrus while performing the Still and Deleted reading tasks versus children with dyslexia. Higher connectivity values were associated with higher reading comprehension. The control group showed increased functional connectivity between the ventral attention network and the fronto‐parietal network during the Deleted text condition (compared with the Still condition). Children with dyslexia did not display this pattern. The results suggest that the synchronized activity of executive, visual attention, and reading‐related networks is a pattern of functional integration which children with dyslexia fail to achieve. The present evidence points toward a critical role of visual attention in dyslexia.     

Why word length only matters in the left visual field

During visual word recognition, string length affects performance when stimuli are presented to the left visual field (LVF), but not when they are presented to the right visual field (RVF). Using a lexical-decision experiment, we investigated an account of this phenomenon based on the SERIOL model of letter-position encoding. Bottom-up activation patterns were adjusted via positional manipulations of letter contrast. This manipulation eliminated the LVF length effect by facilitating responses to longer words, thereby demonstrating that a length effect is not an inherent property of right-hemisphere processing. In contrast, the same manipulation slowed responses to longer words in the RVF, creating a length effect. These results show that hemisphere-specific activation patterns are the source of the asymmetry of the length effect.     

Judging the angles formed by visible and imaginary clock hands: a study of hemispatial effects in healthy volunteers

Healthy, right-handed volunteers (six male, six female) either saw or imagined the hands of a clock set a particular time. In both conditions, they then judged whether the angle between the clock hands was greater than or less than 90 degrees. Subjects pressed one of two response keys to indicate their decision, and hand of response (left/right) was counterbalanced within and between subjects. The subjects had significantly longer reaction times and made significantly more errors when the imaginary angles formed by the clock hands were located in left hemispace (e.g. 8:30) than right hemispace (e.g. 4:30). With visible hands, there was no reaction time difference between visual hemifields, although significantly more errors were made when the angle formed by the hands fell within the left visual field. In the perceptual task (visible hands), reaction times and error rates increased monotonically as the distance between the hands approximated more closely to 90 degrees. This psychophysical relationship was not found in the representational task (imaginary hands). Rather, there was a significant positive correlation between reaction times/error rates and the magnitude of the number indicative of the position of the minute hand. The latter finding is consistent with the hypothesis that the lateral asymmetry in the representational task (reaction times and error rates are higher in left hemispace) is due to the time taken to mentally rotate the imaginary minute hand in a clockwise direction. No such operation is required in the perceptual condition where the hands are clearly visible.     

Hippocampal underactivation in an fMRI study of word and face memory recognition in schizophrenia

Schizophrenia is a major mental disorder which is characterized by several cognitive deficits. Investigations of the neural basis of memory dysfunctions using neuroimaging techniques suggest that the hippocampus plays an important role in declarative memory impairment. The goal of this study was to investigate possible dysfunctions in cerebral activation in schizophrenic patients during both word and face recognition memory tasks. We tested 22 schizophrenics and 24 controls matched by gender, age, handedness and parental socioeconomic status. Compared to healthy volunteers, patients with schizophrenia showed decreased bilateral hippocampal activation during word and face recognition tasks. The whole brain analysis also showed a pattern of cortical and subcortical hypoactivation for both verbal and non-verbal recognition. This study provides further evidence of hippocampal involvement in declarative memory impairments of schizophrenia.     

A tale of two recognition systems: implications of the fusiform face area and the visual word form area for lateralized object recognition models

Two areas of current intense interest in the neuroimaging literature are that of the visual word form area (VWFA) and of the fusiform face area (FFA) and their roles in word and face perception, respectively. These two areas are of particular relevance to laterality research because visual word identification and face identification have long been shown to be especially lateralized to the left hemisphere and the right hemisphere, respectively. This review therefore seeks to evaluate their significance for the broader understanding of lateralization of object recognition. A multi-level model of lateralized object recognition is proposed based on a combination of behavioral and neuroimaging findings. Rather than seek to characterize hemispheric asymmetries according to a single principle (e.g., serial-parallel), it is suggested that current observations can be understood in terms of three asymmetric levels of processing, using the framework of the Janus model of hemispheric function. It is suggested that the left hemisphere represents features using an abstract-category code whereas the RH utilizes a specific-exemplar code. The relationships between these features are also coded asymmetrically, with the LH relying on associative co-occurrence values and the RH relying on spatial metrics. Finally, the LH controlled selection system focuses on isolating features and the RH focuses on conjoining features. It is suggested that each hemisphere utilizes efficient (apparently parallel) processing when stimuli are congruent with its preferred processing style and inefficient (apparently serial) processing when they are not, resulting in the typical left-lateralization for orthographic analysis and right-lateralization for face analysis.