An fMRI Study of Working Memory for Phonological and orthographic Information in Normal and Dyslexic Chinese Children

The relationship between working memory and reading disability was investigated in this fMRI study. According to Baddeley's multiple-component theory, verbal working memory is characterized by temporary manipulation and maintenance of verbal information for further processing. It consists of three components which are served by distinct neural circuitries: central executive, which is mediated by mid-superior frontal regions; subvocal rehearsal recruiting inferior frontal gyri, cerebellum, premotor and supplementary motor areas; and phonological store served by left parietal cortex. Working memory for various kinds of verbal information maybe associated with distinct neural substrates. The present study explored the possible dissociation between phonological and orthographic working memories in Chinese dyslexic and normal children.

Thirty-three children from a primary school in Beijing participated in the fMRI experiments. These children were divided into a normal group and a dyslexic one based on their performance in a reading test and Chinese language course. All of them were tested on both phonological and orthographic verbal working memory tasks as well as corresponding control tasks.

Results showed that dyslexic children had weaker activity in brain areas for subvocal rehearsal and central executive processes than normal controls during both phonological and orthographic working memory tasks. Specifically, dyslexic children had less active prefrontal cortex, premotor and supplementary motor areas and cerebellum, when responding to the two working memory tasks. Moreover, dyslexic children??s bilateral inferior frontal cortex, along with fusiform gyri, middle temporal cortex were not as active as they were in normal readers in the phonological working memory condition. In contrast, dyslexics, unlike normal readers, did not take advantage of the rehearsal strategy and might rely on the left precuneus for recalling visual spatial information (e.g., Cavanna & Trimble, 2006). Brain activation patterns of the normals and dyslexics were similar in performing orthographic tasks, except for activation in left premotor and left cerebellum. Normal children might use subvocal rehearsal to help short-term maintenance of visually presented verbal information, while poor readers could not use explicit vocal repetition to assist and therefore might have to use motor rehearsal for help, i.e. copying the character in mind (e.g., Lewis et al., 2003). The findings suggested that impaired phonological and orthographic working memory systems could partly account for reading disabilities in Chinese dyslexics who show less utilization of phonological rehearsal strategy, less superior executive control ability and a heavy dependence on visual strategy and motor action in reading and working memory tasks. General comparison of performances on different working memory tasks reveals, apart from a common neural network for short-term memory of verbal information, phonological and orthographic working memory systems are different in their specialized emphasis on phonological and orthographic processing.