Brain Asymmetry in Syntactic Processing of Word Class in Chinese: An ERP study

While recent evidence suggests that the right hemisphere (RH) is not as insensitive to syntactic information as previously thought, the extent to which the RH can engage combinatorial syntactic processes like the left-hemisphere (LH) does across languages that are lexically coded to different degrees is still poorly understood. The present study targeted Chinese native speaker to investigate (1) whether similar qualitative difference between two hemispheres during syntactic processing of word class information previously found in English can also be found in Chinese and (2) what kind(s) of the underlying mechanisms or inter-hemispheric communication are implied for such functional asymmetry of language. We conducted four experiments collecting data from 28 right-handed young adults without familial sinistrality background (FS-) to research on these topics, the first two being ERP experiments seeking answer for question (1) and the last two behavioral tasks attempting to answer question (2). Experiment 1 investigated word class violation with central presentation, in which participants read either match or mismatch word pairs presented at the center of the computer screen. The results successfully replicated previous findings by showing grammaticality effects indexed by N400 (300-600 ms) and P600 (600-1100 ms). Experiment 2 then investigated word class violation with divided-visual field presentation, in which the targets were presented either in left-visual field or right-visual field, assessing the LH- and RH-biased responses respectively. The results showed a left-lateralized P600 effect, similar to what was reported in the English literature. As suggested by past studies that interhemispehric communication of coordination between two hemispheres or unilateral inhibition (from LH on RH) to be the most possible mechanisms underlying the functional asymmetry of language, we designed experiment 3 and 4, bilateral flanker task and word-matching task, to assess the ability of interhemispheric inhibition and coordination for each participant. In the bilateral flanker task participants saw pairs of arrows presented across left and right visual fields. The arrows were either congruent or incongruent in direction. They were asked to attend to one of the visual fields and judge for the direction of the target arrow while ignoring the distractor from the other visual field. In the word-matching task, participants were shown two Chinese one-syllable words either bilaterally shown to each visual field or unilaterally within a same visual field. Participants were asked to make semantic relatedness judgment. The results revealed a negative correlation between the degree of language lateralization and the inerhemispheric coordination ability (assessed using the word-matching task), but not interhemispheric inhibition ability (assessed using the bilateral flanker task), suggesting that larger degree of language lateralization reflects greater interference between hemispheres during tasks requiring bilateral integration and thus results in worse interhemispheric coordination. Combining the results from both ERP experiments and behavioral tasks, the present findings established the functional lateralization patterns of brain responses during word class processing in Chinese. In addition, the present findings demonstrated that among young adults with stronger lateralization pattern in language processing, the degree of lateralization is related to interhemispheric coordination, pointing out the close relationship between parallel processing and bilateral integration.