Reading difficulty (RD), also known as dyslexia, is a prevalent neurodevelopmental disorder affecting 5-17% of individuals across various languages. Characterized by difficulties in accurate and fluent reading despite adequate cognitive abilities and instruction, RD's challenges often persist into adulthood. Phonological deficits are a key characteristic of RD, manifesting in difficulties with speech processing from early childhood. Abnormal brain activity during speech processing is a potential biomarker for early diagnosis, as these responses are automatic and detectable even in newborns. Longitudinal studies have demonstrated a predictive relationship between early speech perception and later literacy skills. While phonological deficits influence both speech perception and production, research on speech production in RD is limited due to challenges in data collection from young children. This study addresses this gap by examining foreign language (Spanish) speech perception and production in older children and adults with RD to identify potential neurological markers of the disorder. Previous research has established speech perception deficits in individuals with RD, including reduced brain activation in prefrontal and superior temporal cortices, atypical phoneme boundaries, and altered mismatch negativity (MMN) responses. In Chinese-speaking individuals with RD, similar deficits have been reported, including reduced activation in the left inferior frontal gyrus during auditory rhyming. Speech perception and production are interconnected through shared phonological representations; deficits in phonological representation can lead to challenges in speech motor planning, resulting in slower speech rate and increased pauses. Longitudinal studies have observed these production difficulties in young children with RD, persisting into adulthood. Existing research largely focuses on speech production speed, pauses, and errors; more sophisticated acoustic analyses, such as voice onset time (VOT) and vowel formant frequencies, are needed for a detailed assessment of phonological deficits. This study investigates whether and how phonological deficits manifest in foreign speech perception and production, examining differences between children and adults with RD. Spanish, phonologically distinct from Chinese, was chosen to increase the chances of identifying RD-related deficits. The current study aims to investigate foreign speech production quality and brain activity during foreign speech perception and production in Chinese children and adults with RD using fNIRS, a method particularly suitable for studying speech production due to its tolerance of motion artifacts.

Extensive research documents speech perception deficits in individuals with RD. Studies using neuroimaging techniques like fMRI and EEG have shown reduced brain activation in various regions, including the left prefrontal cortex during auditory rhyming, the superior temporal cortex during auditory speech processing, and bilateral superior temporal regions during phoneme discrimination tasks. EEG studies have also revealed smaller amplitude and longer latency of MMN/MMR and LDN responses in children and adults with RD, indicating reduced phoneme discrimination. A meta-analysis confirmed persistent speech perception deficits in adults with RD, suggesting these difficulties don't diminish with age. In the context of Chinese RD, reduced brain activation in the dorsal left IFG during auditory rhyming and reduced MMN responses have been observed. However, research comparing adults and children with Chinese RD regarding speech perception deficits is lacking. The connection between speech perception and production is strong, as both processes utilize the same phonological representations. Poorly specified phonological representations in individuals with RD lead to difficulties in formulating speech-motor plans, resulting in slower speaking rates and increased pauses. Studies have reported these production issues in toddlers later diagnosed with RD, persisting into adulthood. However, research on the acoustic quality of speech in RD, using measures such as VOT and vowel formant frequencies, remains limited, as does research on brain activity patterns during speech production in individuals with RD. While studies have shown deficits in foreign speech perception and production in individuals with RD, they haven't thoroughly examined the developmental differences between children and adults. This study aims to fill these gaps by examining both behavioral and neurological aspects of foreign speech processing in RD.

This study employed a cross-sectional design, recruiting fifth-grade children and college students. Participants with RD were identified based on Raven's Standard Progressive Matrices scores (above 80) and z-scores below -1.5 on at least one of three Chinese reading tests (character naming, sentence reading fluency, and one-minute character naming). Controls had Raven's scores above 80 and z-scores above -1 on all three tests. The final sample included 20 children with RD, 24 age-matched controls, 20 adults with RD, and 23 age-matched controls. All participants were native Chinese speakers, right-handed, and had no neurological or psychiatric disorders, nor prior Spanish learning. Behavioral assessments included the three reading tests used for screening, plus tests of phonological, morphological, and orthographic awareness, working memory (forward and backward digit spans), and rapid automatized naming (RAN). fNIRS was used to measure brain activity during speech perception and production tasks. The speech perception task involved listening to pairs of Spanish CV syllables varying in similarity to Chinese sounds. The speech production task required participants to imitate 26 multi-syllabic Chinese pseudowords and 26 Spanish words. Phonetic analyses of the Spanish speech production data measured VOT for /b/ and /d/ and vowel formant frequencies for five vowels. fNIRS data were preprocessed using Homer2, including motion correction and bandpass filtering. General linear models (GLMs) were used to analyze task-evoked neural activation, focusing on HbO changes. Multivariate pattern analysis (MVPA) using Support Vector Machines (SVM) with LOOCV and recursive feature elimination (RFE) was employed to classify individuals with and without RD based on brain activation patterns.

Behavioral assessments revealed significant differences between RD and control groups across various reading and language tasks, with RD participants showing lower scores. Phonetic analysis of Spanish vowel pronunciation showed that children, in general, were more similar to native speakers than adults, but children with RD showed less accuracy than control children. There was no significant difference in VOT between groups. fNIRS GLM analysis of the speech perception task revealed a significant three-way interaction (age × group × similarity to Chinese) in the right IFG and right DLPFC. Adults with RD showed greater hemodynamic responses than adults without RD when perceiving Spanish syllables highly similar to Chinese. In the speech production task, a significant interaction between language and group was found in the left MTG. Individuals without RD showed greater deactivation for Spanish than Chinese, whereas individuals with RD showed greater deactivation for Chinese. Brain-behavioral correlations showed a negative correlation between right DLPFC activation during Spanish perception (high similarity to Chinese) and pseudoword rhyming judgment in children with RD but not in adults. MVPA using SVM achieved 60-90% accuracy in classifying RD participants, with consistent high discriminative power from the left MTG, left premotor cortex, left SMA, and left IFG across various classifications.

This study provides both behavioral and neurological evidence for impaired speech perception and production in individuals with RD. The reduced differences between adults and children with RD in right IFG and right DLPFC activation during Spanish speech perception suggests a slowed developmental trajectory in these regions, crucial for phoneme discrimination and working memory. The reduced language differentiation in left MTG activity during speech production in individuals with RD indicates difficulties in efficiently suppressing the default mode network during challenging foreign language tasks. This might be linked to atypical learning experiences and the DMN's role in learning. The high classification accuracy of MVPA underscores the potential of brain activation patterns as reliable markers of RD, especially involving the left MTG (involved in phonological representation and speech perception), left premotor cortex, left SMA (essential for speech motor planning), and left IFG (crucial for speech production and phonological processing). These findings align with previous research showing abnormalities in these regions in individuals with RD.

This study provides strong evidence for neurological differences associated with phonological deficits in RD, reflected in poorer foreign language speech perception and production. Brain activation patterns in specific regions, particularly the left MTG, left premotor cortex, left SMA, and left IFG, serve as robust classifiers of RD, regardless of age or task. These findings provide insights into the developmental aspects of foreign speech processing abnormalities in RD.

The relatively small sample size and cross-sectional design might limit the ability to fully capture developmental changes in RD. The findings might not be generalizable to other languages or populations. Further research with larger samples and longitudinal designs is needed to confirm these findings and investigate the developmental trajectories more comprehensively.