Intact But Less Accessible Phonetic Representations in Adults with Dyslexia

Abstract

Dyslexia is a severe and persistent reading and spelling disorder caused by impairment to manipulate speech sounds. Here, we combine functional magnetic resonance brain imaging with multi-voxel pattern analysis and functional and structural connectivity analysis to disentangle whether dyslexics' phonological deficits are caused by poor quality of the phonetic representations or by difficulties in accessing intact phonetic representations. We show that phonetic representations are hosted bilaterally in primary and secondary auditory cortices, and that their neural quality is intact in adults with dyslexia. However, the functional and structural connectivity between bilateral auditory cortices and left inferior frontal gyrus (a region involved in higher-level phonological processing) is significantly hampered in dyslexics, suggesting deficient access to otherwise intact phonetic representations.Speech perception involves the mapping of spectrally complex and rapidly changing acoustic signals onto discrete and abstract phonetic sound categories or phonemes (1). Developmental dyslexia is a hereditary neurological disorder characterized by severe and persistent reading and/or spelling impairments (2). Individuals with dyslexia perform poorly on tasks that require phonological awareness, verbal short-term memory and lexical access. Performance on these phonological tasks predicts reading acquisition in both normal and dyslexic readers (3). One view is that success on these tasks reflects the quality of underlying phonological (phonetic) representations (4) and that these representations of speech sounds are distorted or less well specified in individuals with dyslexia (5). An alternative view holds that representations are intact, but access to the representations is problematic in people with dyslexia (6, 7). Here, we combine functional magnetic resonance imaging (fMRI) with multi-voxel pattern analysis (MVPA) (8-10) and functional and structural connectivity analysis to disentangle whether dyslexia is caused by poor quality of the phonetic representation or by difficulties in accessing an intact phonetic representation.We collected whole-brain functional images in 23 adults with a diagnosis of dyslexia and 22 matched normal readers (Table S1, 11-13), while they listened to different versions of four sublexical speech sounds (Fig. S1) and performed an easy phoneme discrimination task. The selection of stimuli allowed us to investigate both vowel and stop consonant discrimination, which relies on spectral versus spectrotemporal acoustic feature processing, respectively. If dyslexia is related to a deficit in the quality of phonetic representations, then we expect that the neural representations would be less robust and distinct in individuals with dyslexia than in normal readers. Given dyslexics' particular problems processing temporal cues, such as those involved in consonant discrimination (11), we expected the most prominent group differences for neural patterns distinguishing between consonants.We analyzed...

Individual Differences in Adult Reading Are Associated with Left Temporo-parietal to Dorsal Striatal Functional Connectivity

“…In addition to caudate findings, we also replicated findings of reduced TPJ to IFG functional connectivity in RD (Boets et al 2013). This disconnection at rest may imply differences in properties of connecting white matter (Boets et al 2013) that require compensatory overengagement of TPJ and IFG to achieve the same level of information transmission. Thus, together with the caudate findings, we observed both weaker positive and negative correlation between the TPJ and regions previously reported to show hyperactivation in task-based studies.…”

“…It has been suggested that the phonological deficit may be best understood as a difficulty with phonemic retrieval rather than representation (Boets et al 2013;Boets 2014), with retrieval difficulties caused by underconnectivity of left hemisphere language and auditory regions. In the present study, we also found a correlation between left TPJ to IFG connectivity and PDE scores, at a statistical threshold appropriate for an a priori ROI.…”

“…As previous work has shown reduced functional connectivity between left inferior frontal gyurs (IFG) and parietal language regions in individuals with RD or weaker reading skill (Koyama et al 2011;Boets et al 2013;Schurz et al 2014), we examined positive associations between PDE scores and connectivity from all left hemisphere seed regions to the left IFG at a reduced threshold of P < 0.001 uncorrected.…”

“…The left TPJ is thought to play a role in the encoding and retrieval of speech sounds (Ravizza et al 2004), and abnormal connectivity of this region has been hypothesized to specifically relate to difficulties in retrieval of speech sounds during reading (Boets et al 2013). Another theory of RD posits dysfunction in the magnocellular system as part of the dorsal visual processing stream (Demb et al 1998).…”

Enhancement of brain event-related potentials to speech sounds is associated with compensated reading skills in dyslexic children with familial risk for dyslexia

“…It has been proposed that perception of slow modulations in speech, reflected in rise time discrimination, would be impaired in dyslexics ). Furthermore, studies using functional magnetic resonance imaging (fMRI) to investigate speech processing have found deficits in access to phonetic representations without deficits at the cortical processing of speech sounds (Boets et al, 2013). However, there is no consensus yet on the mechanism how the deficit in phonological awareness is associated with the reading deficit or dyslexia.…”