Unlocking the Secrets of Language Processing in the Brain
With the advent of the functional magnetic resonance imaging (fMRI) technology, studies on dyslexia has been able to search for relevant brain regions that are involved in reading. fMRI is a non-invasive technology that is widely utilised in studies on cognitive functions. Major brain regions that are responsible for reading has been identified:
Alphabetic languages
It has been found that the left temporo-parietal region is crucial in alphabetic language reading (Figure 1). The activation of this region among dyslexics is weaker during phonological or reading processing (Figure 2). It has been proposed that this region is responsible for the letter-sound mapping. Together with the results from behavioural studies, we can infer that dyslexia in alphabetic languages mainly stems from phonological deficits. As dyslexics show poorer phonological processing, they face difficulties in letter-sound mapping and this gives rise to the failure in word decoding. This suggests that teaching phonics is one of the effective ways in improving the reading ability of dyslexic children.
Figure 1. A lot of studies revealed a converging brain region of decreased activation among dyslexics of alphabetic languages.
Figure 2. The highlighted region indicates significant reduced activation among dyslexic adults when performing an English word naming task (Siok et al., in preparation).
Chinese language
While the behavioural and cognitive bases of Chinese reading and dyslexia are not conclusive yet, we have found an emerging pattern of brain activation over the past decade. It has been found that, during homophone judgement and lexical decision tasks, the left middle frontal region was activated among normal readers but the activation was weaker among dyslexics (Siok, Perfetti, Jin & Tan, 2004) (Figure 3 and 4).
Figure 3. The brain activation of normal and reading impaired participants when performing a Chinese homophone judgement task. The activation concentrated at the middle frontal region (green circle). Reading impaired participants showed weaker activation in that region.
Figure 4. The brain activation of normal and reading impaired participants when performing a Chinese lexical decision task. Reading impaired participants showed weaker activation in the middle frontal region (green circle).
A follow-up study showed both reduced activation and grey matter volume in the middle frontal region among dyslexic children when compared to normal readers (Siok, Niu, Jin, Perfetti, & Tan, 2008) (Figure 5).
Figure 5. The volume of grey matter in the middle frontal region (blue circle) among dyslexics is significantly less than that of the normal readers.
Due to the unique activation pattern of the left middle frontal region and its role in Chinese reading, we hope to identify the cognitive processes which it is responsible for. Currently, we propose that it is involved in multi-modal conversion (character-sound-meaning mapping), the processing of visual complexity and syllable-level phonological processing. We also try to compare Chinese reading with a similar language, Korean, which is known for its square written blocks and the mapping of each character into a syllable.
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