Neural systems and effective connectivity for Chinese reading : mapping between orthography and phonology

Abstract

The mapping between orthography and phonology plays an essential role in reading and how this process is implemented in the human brain is of great interest. Previous investigations into the neural mechanisms underlying orthography-phonology mapping have overwhelmingly focused on the functional localization of specialized brain areas, but little is known as to how these areas interact with each other. In the present research, therefore, we investigated the neural systems and cortical interactions for orthography-phonology mapping of Chinese characters. Three functional magnetic resonance imaging studies were conducted with adult subjects to investigate the orthography-phonology mapping for Chinese characters by emphasizing different aspects of the process. Dynamic causal modeling (DCM) was used to compute effective connectivity. In Study 1, we manipulated visual complexity of Chinese characters and examined brain systems and connectivity that were modulated by the visual-orthographic load during a tone judgment task with visually presented characters. Results showed that high-stroke characters produced greater activation than low-stroke characters mainly in the bilateral middle frontal gyri (MFG), left inferior frontal gyrus (IFG), bilateral superior parietal lobules (SPL), and bilateral occipitotemporal cortices (vOT). DCM analyses among the left-hemisphere regions showed that increased visual-orthographic load strengthened the connections from vOT to SPL and from SPL to MFG, suggesting increased involvement of the pathway from vOT to MFG via SPL for fine-grained analysis of the characters. Moreover, MFG exerted inhibitory top-down modulation on vOT, which may enable more efficient information processing in vOT. In Study 2, we manipulated the mapping load at the phonological processing stage by comparing brain activity provoked by multi-pronunciation characters to single-pronunciation characters. Using a tone judgment task and a homophone judgment task, we identified task-independent neural systems that were sensitive to the phonological mapping load, including bilateral MFG, bilateral ventral IFG and left posterior parietal lobule (PPC). DCM analyses indicated that increased phonological mapping load enhanced connection from left PPC to left MFG, suggesting an important role of this connection for maintaining and manipulating phonological information. In Study 3, by varying the degrees of conflict between orthographic and phonological information in a homophone judgment task, we identified neurocircuitry for the interaction between orthography and phonology, including anterior cingulate cortex (ACC), bilateral inferior and middle frontal gyri, bilateral PPC and the left vOT. Connectivity analyses showed that conflict processing reduced connectivity from left vOT to ACC and increased connectivity from left MFG to ACC, which might reflect less reliance on the visual form information but more reliance on the accurate orthography-phonology mapping to resolve the conflict. In short, the findings provided novel insights into the neural systems and effective connectivity for orthography-phonology mapping of Chinese characters and highlighted universal and language-specific components for the brain mechanisms underlying reading.