fNIRS evidence for left middle frontal gyrus involved in visual-spatial analysis of Chinese characters

The left middle frontal gyrus (MFG) is a typical region identified in the brain mechanism of Chinese character reading. A common view suggests that the MFG is responsible for visual-spatial processing in reading Chinese characters, since the Chinese writing system has extremely complex structures. However, this explanation has not received direct evidence. Moreover, which kind of visual-spatial analysis underlying the MFG's activation is not clear. The current functional Near-Infrared Spectroscopy (fNIRS) study examined the functional role of the MFG in Chinese character reading by directly manipulating the spatial frequency characteristics of the Chinese character materials. The experiment adopted a 3 (characters type: Real, Pseudo, and Artificial character) by 3 (spatial frequency: Full, Low, and High spatial frequency) within-participant factorial design. All Real characters were phonograms containing two components: phonetic and semantic radicals. The phonetic radical was not pronounceable and meaningless. The semantic radical was not pronounceable and meaningful that can provide the meaning category. The Pseudo characters were created by switching the position of radicals from Real characters. The Artificial characters were scrambled strokes from Pseudo characters. Each type of character was presented in three ways with different spatial frequencies: Full, Low, and High spatial frequency. In the Full spatial frequency, items were typical stimuli. In the Low and High spatial frequency, items were spatially filtered by Gaussian filter to remain the low/high spatial frequency features. Each type of character has 60 stimuli. All 540 stimuli for nine conditions were presented in 6 runs. Each run contained 1 similar to 2 blocks for each condition and lasted for about 8 minutes. Thirty-one participants took part in the experiment to perform a one-back task in each block by detecting whether the current stimulus was the same as the previous one. Meanwhile, a multi-channel fNIRS system was used to record brain activity at the left MFG. The results found a significant main effect of character type at left MFG. The activation of MFG was stronger for Pseudo characters than for Real and Artificial characters. The left MFG also showed a significant interaction between character type and spatial frequency. The difference among the three types of characters was significant for Low spatial frequency but not for Full and High spatial frequency. For materials presented in Low spatial frequency, Pseudo characters elicited more robust activation than Real and Artificial characters. These results suggested that the left MFG was sensitive to the spatial information of Chinese characters, especially for materials presented in low spatial frequency and for materials that required more graphemic/orthographic processing. In sum, the finding provided direct evidence that the left MFG engaged in visual-spatial processing in Chinese character reading.