Leo Blomert

Integration of letters and speech sounds in the human brain

Most people acquire literacy skills with remarkable ease, even though the human brain is not evolutionarily adapted to this relatively new cultural phenomenon. Associations between letters and speech sounds form the basis of reading in alphabetic scripts. We investigated the functional neuroanatomy of the integration of letters and speech sounds using functional magnetic resonance imaging (fMRI). Letters and speech sounds were presented unimodally and bimodally in congruent or incongruent combinations. Analysis of single-subject data and group data aligned on the basis of individual cortical anatomy revealed that letters and speech sounds are integrated in heteromodal superior temporal cortex. Interestingly, responses to speech sounds in a modality-specific region of the early auditory cortex were modified by simultaneously presented letters. These results suggest that efficient processing of culturally defined associations between letters and speech sounds relies on neural mechanisms similar to those naturally evolved for integrating audiovisual speech.

Orthographic Depth and Its Impact on Universal Predictors of Reading A Cross-Language Investigation

Alphabetic orthographies differ in the transparency of their letter-sound mappings, with English orthography being less transparent than other alphabetic scripts. The outlier status of English has led scientists to question the generality of findings based on English-language studies. We investigated the role of phonological awareness, memory, vocabulary, rapid naming, and nonverbal intelligence in reading performance across five languages lying at differing positions along a transparency continuum (Finnish, Hungarian, Dutch, Portuguese, and French). Results from a sample of 1,265 children in Grade 2 showed that phonological awareness was the main factor associated with reading performance in each language. However, its impact was modulated by the transparency of the orthography, being stronger in less transparent orthographies. The influence of rapid naming was rather weak and limited to reading and decoding speed. Most predictors of reading performance were relatively universal across these alphabetic languages, although their precise weight varied systematically as a function of script transparency.

Reduced neural integration of letters and speech sounds links phonological and reading deficits in adult dyslexia

Developmental dyslexia is a specific reading and spelling deficit affecting 4% to 10% of the population. Advances in understanding its origin support a core deficit in phonological processing characterized by difficulties in segmenting spoken words into their minimally discernable speech segments (speech sounds, or phonemes) and underactivation of left superior temporal cortex. A suggested but unproven hypothesis is that this phonological deficit impairs the ability to map speech sounds onto their homologous visual letters, which in turn prevents the attainment of fluent reading levels. The present functional magnetic resonance imaging (fMRI) study investigated the neural processing of letters and speech sounds in unisensory (visual, auditory) and multisensory (audiovisual congruent, audiovisual incongruent) conditions as a function of reading ability. Our data reveal that adult dyslexic readers underactivate superior temporal cortex for the integration of letters and speech sounds. This reduced audiovisual integration is directly associated with a more fundamental deficit in auditory processing of speech sounds, which in turn predicts performance on phonological tasks. The data provide a neurofunctional account of developmental dyslexia, in which phonological processing deficits are linked to reading failure through a deficit in neural integration of letters and speech sounds.

Deviant processing of letters and speech sounds as proximate cause of reading failure: a functional magnetic resonance imaging study of dyslexic children

Learning to associate auditory information of speech sounds with visual information of letters is a first and critical step for becoming a skilled reader in alphabetic languages. Nevertheless, it remains largely unknown which brain areas subserve the learning and automation of such associations. Here, we employ functional magnetic resonance imaging to study letter-speech sound integration in children with and without developmental dyslexia. The results demonstrate that dyslexic children show reduced neural integration of letters and speech sounds in the planum temporale/Heschl sulcus and the superior temporal sulcus. While cortical responses to speech sounds in fluent readers were modulated by letter-speech sound congruency with strong suppression effects for incongruent letters, no such modulation was observed in the dyslexic readers. Whole-brain analyses of unisensory visual and auditory group differences additionally revealed reduced unisensory responses to letters in the fusiform gyrus in dyslexic children, as well as reduced activity for processing speech sounds in the anterior superior temporal gyrus, planum temporale/Heschl sulcus and superior temporal sulcus. Importantly, the neural integration of letters and speech sounds in the planum temporale/Heschl sulcus and the neural response to letters in the fusiform gyrus explained almost 40% of the variance in individual reading performance. These findings indicate that an interrelated network of visual, auditory and heteromodal brain areas contributes to the skilled use of letter-speech sound associations necessary for learning to read. By extending similar findings in adults, the data furthermore argue against the notion that reduced neural integration of letters and speech sounds in dyslexia reflect the consequence of a lifetime of reading struggle. Instead, they support the view that letter-speech sound integration is an emergent property of learning to read that develops inadequately in dyslexic readers, presumably as a result of a deviant interactive specialization of neural systems for processing auditory and visual linguistic inputs.

The neural signature of orthographic–phonological binding in successful and failing reading development

Learning to read in alphabetic orthographies starts with learning a script code consisting of letter-speech sound pairs. Although children know which letters belong to which speech sounds within months, it takes much longer to automatically integrate them into newly constructed audiovisual objects. This extended learning process corresponds with observations that reliable letter and word specific activations in the fusiform cortex also occur relatively late in reading development. The present review discusses electrophysiological and neuroimaging studies of the nature and mechanisms involved in letter-speech sound integration in normal and dyslexic readers. It is demonstrated that letter-speech sound associations do not develop in parallel with visual letter recognition but immediately work in concert to form orthographic-phonological bonds which remain active even in experienced reading. Effective letter-speech sound integration may be necessary for reliable letter recognition to develop. In contrast, it is this basic integration of letters and speech sounds which poses an immediate problem for beginning dyslexic readers, and remains problematic in adult dyslexic readers. It is hypothesized that a specific orthographic-phonological binding deficit may not only act as a proximal cause for reading deficits in dyslexia, but may also explain the notorious lack of reading fluency. Finally, it is suggested that similar integrated audiovisual representations may also exist for larger grain-sizes in the same posterior occipitotemporal/inferoparietal network as identified for orthographic-phonological integration of letters and speech sounds.

Predictors of developmental dyslexia in European orthographies with varying complexity

BACKGROUND  The relationship between phoneme awareness, rapid automatized naming (RAN), verbal short-term/working memory (ST/WM) and diagnostic category is investigated in control and dyslexic children, and the extent to which this depends on orthographic complexity. METHODS General cognitive, phonological and literacy skills were tested in 1,138 control and 1,114 dyslexic children speaking six different languages spanning a large range of orthographic complexity (Finnish, Hungarian, German, Dutch, French, English). RESULTS Phoneme deletion and RAN were strong concurrent predictors of developmental dyslexia, while verbal ST/WM and general verbal abilities played a comparatively minor role. In logistic regression models, more participants were classified correctly when orthography was more complex. The impact of phoneme deletion and RAN-digits was stronger in complex than in less complex orthographies. CONCLUSIONS Findings are largely consistent with the literature on predictors of dyslexia and literacy skills, while uniquely demonstrating how orthographic complexity exacerbates some symptoms of dyslexia.

Coping with phonological assimilation in speech perception: Evidence for early compensation

The pronunciation of the same word may vary considerably as a consequence of its context. The Dutch wordtuin (English,garden) may be pronouncedtuim if followed bybank (English,bench), but not if followed bystoel (English,chair). In a series of four experiments, we examined how Dutch listeners cope with this context sensitivity in their native language. A first word identification experiment showed that the perception of a word-final nasal depends on the subsequent context. Viable assimilations, but not unviable assimilations, were often confused perceptually with canonical word forms in a word identification task. Two control experiments ruled out the possibility that this effect was caused by perceptual masking or was influenced by lexical top-down effects. A passive-listening study in which electrophysiological measurements were used showed that only unviable, but not viable, phonological changes elicited a significant mismatch negativity. The results indicate that phonological assimilations are dealt with by an early prelexical mechanism.

Numerical predictors of arithmetic success in grades 1–6

Math relies on mastery and integration of a wide range of simpler numerical processes and concepts. Recent work has identified several numerical competencies that predict variation in math ability. We examined the unique relations between eight basic numerical skills and early arithmetic ability in a large sample (N = 1391) of children across grades 1-6. In grades 1-2, childrens ability to judge the relative magnitude of numerical symbols was most predictive of early arithmetic skills. The unique contribution of childrens ability to assess ordinality in numerical symbols steadily increased across grades, overtaking all other predictors by grade 6. We found no evidence that childrens ability to judge the relative magnitude of approximate, nonsymbolic numbers was uniquely predictive of arithmetic ability at any grade. Overall, symbolic number processing was more predictive of arithmetic ability than nonsymbolic number processing, though the relative importance of symbolic number ability appears to shift from cardinal to ordinal processing.

Long-term cognitive dynamics of fluent reading development

Most theories of reading development assume a shift from slow sequential subword decoding to automatic processing of orthographic word forms. We hypothesized that this shift should be reflected in a concomitant shift in reading-related cognitive functions. The current study investigated the cognitive dynamics underlying reading development in a large school sample ranging from beginning to experienced readers. The results showed that phonological awareness (PA) and rapid automatized naming (RAN) contributed substantially to reading fluency over all six primary school grades. However, the relationship between PA and word (but not pseudoword) reading fluency decreased as a function of reading experience, whereas the relationship between RAN and word reading fluency increased gradually. Moreover, this cognitive shift was most pronounced for high-frequency words. The results seem to point to the development of one (and only one) reading network for all types of words in which processing load or type of processing depends on word familiarity and amount of reading experience.

Naming problems do not reflect a second independent core deficit in dyslexia: Double deficits explored

The double deficit hypothesis states that naming speed problems represent a second core deficit in dyslexia independent from a phonological deficit. The current study investigated the main assumptions of this hypothesis in a large sample of well-diagnosed dyslexics. The three main findings were that (a) naming speed was consistently related only to reading speed; (b) phonological processing speed and naming speed loaded on the same factor, and this factor contributed strongly to reading speed; and (c) although general processing speed was involved in speeded naming of visual items, it did not explain the relationship between naming speed and reading speed. The results do not provide support for the existence of a second independent core naming deficit in dyslexia and indicate that speeded naming tasks are mainly phonological processing speed tasks with an important addition: fast cross-modal matching of visual symbols and phonological codes.