Dysfunctional visual word form processing in progressive alexia

It remains under debate whether the fusiform visual word form area (VWFA) is specific to visual word form and whether visual expertise increases its sensitivity (Xue et al., 2006; Cohen et al., 2002). The present study examined three related issues: (1) whether the VWFA is also involved in processing foreign writing that significantly differs from the native one, (2) the effect of visual word form training on VWFA activation after controlling the task difficulty, and (3) the transfer of visual word form learning. Eleven native English speakers were trained, during five sessions, to judge whether two subsequently flashed (100-msec duration with 200-msec interval) foreign characters (i.e., Korean Hangul) were identical or not. Visual noise was added to the stimuli to manipulate task difficulty. In functional magnetic resonance imaging scans before and after training, subjects performed the task once with the same noise level (i.e., parameter-matched scan) and once with noise level changed to match performance from pretraining to posttraining (i.e., performance-matched scan). Results indicated that training increased the accuracy in parameter-matched condition but remained constant in performance-matched condition (because of increasing task difficulty). Pretraining scans revealed stronger activation for English words than for Korean characters in the left inferior temporal gyrus and the left inferior frontal cortex, but not in the VWFA. Visual word form training significantly decreased the activation in the bilateral middle and left posterior fusiform when either parameters or performance were matched and for both trained and new items. These results confirm our conjecture that the VWFA is not dedicated to words, and visual expertise acquired with training reduces rather than increases its activity.

In human occipitotemporal cortex, category-specific processing for visual objects seems to involve pairs of cortical regions, often with one located in the occipital cortex and another more anteriorly. We investigated whether such an arrangement might be the case for visual word processing. In addition to the Visual Word Form Area (VWFA) located in the occipitotemporal sulcus, we observed that another region in occipital lobe with robust responses to written words (Chinese characters). The current fMRI study investigated this area’s precise location and its functional selectivity using Chinese characters and other categories of visual images (cars, chairs and insects). In all the 13 subjects we could identify a cluster of voxels near the inferior occipital gyrus or middle occipital gyrus with stronger responses to Chinese characters than scrambled objects. We tentatively label this area as the Occipital Word Form Sensitive Area (OWA). The OWA’s response amplitudes showed similar preference to written words as the VWFA, with the VWFA showing a higher degree of word selectivity, which was confirmed by the result from spatial patterns of response. These results indicate that the OWA, together with the VWFA, are critical parts of the network for processing and representing the category information for word.

Reading normal and degraded words: Contribution of the dorsal and ventral visual pathways

Neural Representation in Visual Word Form Area during Word Reading

Ventral Occipital Temporal Cortex (VOTC) is home to a mosaic of categorically-selective functional regions that respond to visual stimuli. Within left VOTC lies the Visual Word Form Area (VWFA) - a text-selective region that develops as an individual learns to read. While there is consistency in the general location of text-selective responses - within the posterior portion of the occipitotemporal sulcus (OTS) - there is also substantial variability across individuals in the size and precise anatomical location. Moreover, there is an ongoing debate over the extent to which the VWFA a) encodes the visual features of text, versus b) is driven by the task of reading. Using functional magnetic resonance imaging, we scanned adults and children as they completed two tasks while viewing text, pseudo fonts, faces, objects, limbs. Experimental tasks were: (1) a fixation task where they were instructed to ignore the stimuli while making psychophysical judgements on the fixation dot; (2) a one-back task where they were instructed to attend to the stimuli and detect repeats. We found that a consistent VWFA location could be identified on each individual’s cortical surface using either task. At the same time, the response to text during the one-back task evoked a larger territory of text-selective response (leading to a larger ROI) than the fixation task. However, when averaged in template space, text-selective cortex could not be identified due to variability in the relative locations of text-, face-, object-, and limb-selective cortex. Thus, for both children and adults, a text-selective region (i.e., VWFA) can be localized on the individual cortical surface. This resulting region consistently responds to text irrespective of cognitive task, but text-selective responses are masked when averaged in template space due to variability in the exact configuration of category-selective regions.

Shortly after reading instruction, a region in the ventral occipital temporal cortex (vOTC) of the left hemisphere, the Visual Word Form Area (VWFA), becomes specialized for written words. Its reproducible location across scripts suggests important anatomical constraints, such as specific patterns of connectivity, notably to spoken language areas. Here, we explored the structural connectivity of the emerging VWFA in terms of its specificity relative to other ventral visual regions and its stability throughout the process of reading instruction in ten children studied longitudinally over 2years. Category-specific regions for words, houses, faces, and tools were identified in the left vOTC of each subject with functional MRI. With diffusion MRI and tractography, we reconstructed the connections of these regions at two time points (mean age ± standard deviation: 6.2 ± 0.3, 7.2 ± 0.4years). We first showed that the regions for each visual category harbor their own specific connectivity, all of which precede reading instruction and remain stable throughout development. The most specific connections of the VWFA were to the dorsal posterior parietal cortex. We then showed that microstructural changes in these connections correlated with improvements in reading scores over the first year of instruction but not 1 year later in a subsample of eight children (age: 8.4 ± 0.3years). These results suggest that the VWFA location depends on its connectivity to distant regions, in particular, the left inferior parietal region which may play a crucial role in visual field maps and eye movement dynamics in addition to attentional control in letter-by-letter reading and disambiguation of mirror-letters during the first stages of learning to read.

While predominant models of visual word form area (VWFA) function argue for its specific role in decoding written language, other accounts propose a more general role of VWFA in complex visual processing. However, a comprehensive examination of structural and functional VWFA circuits and their relationship to behavior has been missing. Here, using high-resolution multimodal imaging data from a large Human Connectome Project cohort (N = 313), we demonstrate robust patterns of VWFA connectivity with both canonical language and attentional networks. Brain-behavior relationships revealed a striking pattern of double dissociation: structural connectivity of VWFA with lateral temporal language network predicted language, but not visuo-spatial attention abilities, while VWFA connectivity with dorsal fronto-parietal attention network predicted visuo-spatial attention, but not language abilities. Our findings support a multiplex model of VWFA function characterized by distinct circuits for integrating language and attention, and point to connectivity-constrained cognition as a key principle of human brain organization.

Reading plays a key role in education and communication in modern society. Learning to read establishes the connections between the visual word form area (VWFA) and language areas responsible for speech processing. Using resting-state functional connectivity (RSFC) and Granger Causality Analysis (GCA) methods, the current developmental study aimed to identify the difference in the relationship between the connections of VWFA-language areas and reading performance in both adults and children. The results showed that: (1) the spontaneous connectivity between VWFA and the spoken language areas, i.e., the left inferior frontal gyrus/supramarginal gyrus (LIFG/LSMG), was stronger in adults compared with children; (2) the spontaneous functional patterns of connectivity between VWFA and language network were negatively correlated with reading ability in adults but not in children; (3) the causal influence from LIFG to VWFA was negatively correlated with reading ability only in adults but not in children; (4) the RSFCs between left posterior middle frontal gyrus (LpMFG) and VWFA/LIFG were positively correlated with reading ability in both adults and children; and (5) the causal influence from LIFG to LSMG was positively correlated with reading ability in both groups. These findings provide insights into the relationship between VWFA and the language network for reading, and the role of the unique features of Chinese in the neural circuits of reading.

A Real-World Size Organization of Object Responses in Occipitotemporal Cortex

The roles of the “visual word form area” in reading

Reading is of enormous importance in our modern information-based society and an essential activity in everyday life. While most children successfully learn to read within a few years, reading acquisition is severely impaired in about 5-10% of children, resulting in a specific reading disorder called “developmental dyslexia”. One of the core deficits of individuals with dyslexia is impaired automatic word recognition, which leads to slow, dysfluent reading. Neuroimaging studies in adult readers have linked this orthographic processing deficit to deficits in a specific section of the left occipitotemporal cortex (the visual word form area; VWFA) involved in visual word processing. However, the exact function of the VWFA and the larger left occipitotemporal VWF-system in healthy children, as well as its dysfunctioning in dyslexic children is still unclear. In addition, since the VWF-system and its core region (the VWFA) are part of the typical language network that encompasses several other regions crucially involved in reading, there is considerable interest in examining the connectivity between those brain areas. This Ph.D. thesis aimed to investigate both the function and functional connectivity comparing control children and children with dyslexia. In Study A we examined (i) two levels of functional specialization of visual word processing − (a) coarse specialization for print and (b) sensitivity to orthographic familiarity of letter strings − in the left occipitotemporal VWFsystem in control children, and (ii) whether these types of specialization were impaired in children with dyslexia. In Study B we investigated (i) the functional connectivity of the VWF-system and other major components of the language network in control children, and (ii) whether these connections were different in dyslexic children. We tested 18 dyslexic and 24 age-matched control children (age 9.7- 12.5 years) while they indicated if visual stimuli (real words, pseudohomophones, pseudowords and false-fonts) sounded like a real word. Five adjacent regions of interest (ROIs) in the bilateral occipitotemporal cortex were selected to cover the full anterior-posterior extent of the VWFsystem. In Study A, we used functional MRI to assess the brain activity in response to four types of visual letter strings varying in orthographic familiarity. In Study B, we used functional connectivity MRI to examine interregional cooperation within the language network, focussing on the VWFA. The results from Study A revealed that control children show a dissociation of two functional levels of specialization within the VWF-system: (1) coarse specialization for print, i.e. different processing of letter strings than of visual control stimuli and (2) sensitivity to orthographic familiarity of letter strings. Furthermore, we showed that both of these functional levels of VWF-system specialization could not be detected in children with dyslexia in the entire VWF-system. To answer the question whether this functional impairment of the VWF-system is associated with impaired cooperation of this system with other language areas, we looked at the functional connectivity of these regions. The results from Study B revealed that, in control children, the functional connections with typical language areas (left inferior parietal lobule and left inferior frontal gyrus) were specific for the VWFA and did not generalize to left occipitotemporal regions neighbouring the VWFA. In addition, we demonstrated that these functional connections between the VWFA and other major language areas are disrupted in children with dyslexia, whereas those of neighbouring areas were not affected. The results of both studies reveal an impairment of both function (of the VWF-system) and functional connectivity (of the VWFA with left inferior parietal and inferior frontal cortex) in dyslexic children. In conclusion, the findings presented in this work demonstrate that (1) word processing deficits in dyslexic children are associated with a pervasive developmental dysfunction of the entire VWF-system, and that (2) brain regions necessary for fluent, skilled reading may not work together properly during reading.

Reading is built upon transformations that map representations of written words to their pronunciations, names, and meanings. However, unlike many other visual skills, literacy is unique to humans and has only become widespread in the past few hundred years, therefore neural circuits that underly reading cannot have evolved for reading. Thus, scientists have long debated the nature of the neural re-tuning of visual networks that must occur to support these transformations in a highly skilled manner. A key node of the reading brain, the visual word form area (VWFA), lies where circuits that underpin key aspects of visuolinguistic transformations diverge from earlier visual processing in ventral occipitotemporal cortex. Furthermore, different reading deficits co-occur with parallel deficits in these visuolinguistic transformations. Evidence suggests that as literacy is acquired during development, preexisting visuolinguistic circuits that support representational transformations compatible with reading become tuned for understanding written words. This model implies that the VWFA is where it is because reading requires functional capacities that are parallel to those needed to recognize and name objects on one hand and those needed for multimodal integration during speech/language comprehension on the other. The VWFA is at a key location that gets input from earlier visual regions and has lateral connectivity into the auditory-visual speech and language network and ventral connectivity into visual naming circuits. We review neurophysiological, neuroanatomical, and neurodevelopmental evidence that support a model in which literacy emerges from the specialization of preexisting circuits that have the natural capacities for vision to linguistic transformations needed to acquire and facilitate fluent reading.

A functional region of left fusiform gyrus termed "the visual word form area" (VWFA) develops during reading acquisition to respond more strongly to printed words than to other visual stimuli. Here, we examined responses to letters among 5- and 6-year-old early kindergarten children (N = 48) with little or no school-based reading instruction who varied in their reading ability. We used functional magnetic resonance imaging (fMRI) to measure responses to individual letters, false fonts, and faces in left and right fusiform gyri. We then evaluated whether signal change and size (spatial extent) of letter-sensitive cortex (greater activation for letters versus faces) and letter-specific cortex (greater activation for letters versus false fonts) in these regions related to (a) standardized measures of word-reading ability and (b) signal change and size of face-sensitive cortex (fusiform face area or FFA; greater activation for faces versus letters). Greater letter specificity, but not letter sensitivity, in left fusiform gyrus correlated positively with word reading scores. Across children, in the left fusiform gyrus, greater size of letter-sensitive cortex correlated with lesser size of FFA. These findings are the first to suggest that in beginning readers, development of letter responsivity in left fusiform cortex is associated with both better reading ability and also a reduction of the size of left FFA that may result in right-hemisphere dominance for face perception.

a:2:{s:4:"lang";s:2:"en";s:7:"content";s:1392:"<h3>Background/Objectives</h3> Alexia without agraphia is a disorder of reading inability with intact writing, usually caused by infarction in the left occipital lobe involving the splenium. A significantly rarer form arises following lesions in the left inferior temporal lobe, affecting the visual word form area (VWFA), a specialised region responsible for word recognition. We describe a 51-year-old male with alexia without agraphia due to a strategic infarction affecting the VWFA, with complete recovery several weeks later. <h3>Methods</h3> The patient‘s history and examination findings are reported, with imaging findings presented, and a discussion of the literature. <h3>Results</h3> The patient demonstrated selective impairment in whole-word reading, with preserved writing and letter-by-letter reading, and no visual field defects. MR brain demonstrated VWFA infarction. The patient experienced complete recovery. <h3>Discussion</h3> This case illustrates a ‘cortical’ variant of alexia without agraphia due to strategic infarction in the VWFA, distinct from the better-recognised ‘disconnection syndrome’ involving the occipital lobe and splenium. This report further evidences the VWFA’s role in word recognition, providing valuable insights into the functional localisation of reading and writing abilities. andrewmacphie{at}gmail.com ";}

Combined ERP/fMRI evidence for early word recognition effects in the posterior inferior temporal gyrus