Robust and Task-Independent Spatial Profile of the Visual Word Form Activation in Fusiform Cortex

Written language represents a special category of visual information. There is strong evidence for the existence of a cortical region in ventral occipitotemporal cortex for processing the visual form of written words. However, due to inconsistent findings obtained with different tasks, the level of specialization and selectivity of this so called visual word form area (VWFA) remains debated. In this study, we examined category selectivity for Chinese characters, a non-alphabetic script, in native Chinese readers. In contrast to traditional approaches of examining response levels in a restricted predefined region of interest (ROI), a detailed distribution of the BOLD signal across the mid-fusiform cortical surface and the spatial patterns of responses to Chinese characters were obtained. Results show that a region tuned for Chinese characters could be consistently found in the lateral part of the left fusiform gyrus in Chinese readers, and this spatial pattern of selectivity for written words was not influenced by top-down tasks such as phonological or semantic modulations. These results provide strong support for the robust spatial coding of category selective response in the mid-fusiform cortex, and demonstrate the utility of the spatial distribution analysis as a more meaningful approach to examine functional magnetic resonance imaging (fMRI) data

Chinese and Korean Characters Engage the Same Visual Word Form Area in Proficient Early Chinese-Korean Bilinguals

A number of recent studies consistently show an area, known as the visual word form area (VWFA), in the left fusiform gyrus that is selectively responsive for visual words in alphabetic scripts as well as in logographic scripts, such as Chinese characters. However, given the large difference between Chinese characters and alphabetic scripts in terms of their orthographic rules, it is not clear at a fine spatial scale, whether Chinese characters engage the same VWFA in the occipito-temporal cortex as alphabetic scripts. We specifically compared Chinese with Korean script, with Korean script serving as a good example of alphabetic writing system, but matched to Chinese in the overall square shape. Sixteen proficient early Chinese-Korean bilinguals took part in the fMRI experiment. Four types of stimuli (Chinese characters, Korean characters, line drawings and unfamiliar Chinese faces) were presented in a block-design paradigm. By contrasting characters (Chinese or Korean) to faces, presumed VWFAs could be identified for both Chinese and Korean characters in the left occipito-temporal sulcus in each subject. The location of peak response point in these two VWFAs were essentially the same. Further analysis revealed a substantial overlap between the VWFA identified for Chinese and that for Korean. At the group level, there was no significant difference in amplitude of response to Chinese and Korean characters. Spatial patterns of response to Chinese and Korean are similar. In addition to confirming that there is an area in the left occipito-temporal cortex that selectively responds to scripts in both Korean and Chinese in early Chinese-Korean bilinguals, our results show that these two scripts engage essentially the same VWFA, even at the level of fine spatial patterns of activation across voxels. These results suggest that similar populations of neurons are engaged in processing the different scripts within the same VWFA in early bilinguals

Coordinate of VCFA and VKFA and the peak point distance (mm) between these two areas.

<p>Coordinate of VCFA and VKFA and the peak point distance (mm) between these two areas.</p

Selectively activated area for Chinese characters and Korean characters in the left occipito-temporal cortex.

<p>Brain activation maps in Talairach space of each subject show the activation of Chinese characters and Korean characters when contrasted with faces, respectively (q(FDR)<10⁻³). The right side of the image corresponds to the left hemisphere. The cross in each image is centered at the coordinate for the peak category selective response point. The color bar shows the t values.</p

Correlation coefficients for within-category stimuli (left bar, C-C, K-K, L-L, F-F) and correlation coefficients for between-category stimuli (right bar, C-L, K-L, C-F, K-F, F-L).

<p>The correlation coefficient C-K (middle bar) is not significantly different from the within-category correlations, but significantly different from the between category correlations (<i>p</i> = 0.000).</p

Voxel number of VCFA, VKFA, VCFA ∩ VKFA and VCFA ∪ VKFA and overlap indexes between VCFA and VKFA.

<p>Note: Overlap index1, VCFA ∩ VKFA / VCFA; overlap index2, VCFA ∩ VKFA / VKFA; overlap index3, VCFA ∩ VKFA / VCFA ∪ VKFA.</p

Schematic depiction of the experimental paradigm.

<p>In block-design runs, subjects viewed Chinese characters, Korean characters, unfamiliar faces and line drawings in separate blocks. In each block 20 stimuli were presented and each stimulus was presented for 250 ms with an inter-stimulus interval of 750 ms. Each run contained 12 blocks with 3 blocks for each category of stimuli.</p

Cross-correlation coefficients of beta values as a function of spatial position between the two tasks (passive vs. active).

<p>For each category of the test stimuli, the beta distributions were not shifted from the passive task to the active task (peak correlation at zero lag), and the average correlation coefficient of the zero lag was 0.92±0.06, indicating that neither the peak positions nor the shapes of the beta distributions were changed between these two tasks.</p

Schematic depiction of the experimental paradigm.

<p>Participants viewed Chinese characters (and native English speakers viewed English words in addition), faces, and line drawings in block-design runs with each block consisting of 20 stimuli.</p

Spatial profile of response selectivity across mid-fusiform cortex in the active (naming/reading) and passive (position judgment) tasks.

<p>Beta distributions were extracted from the grey matter surface along the mid-fusiform cortex and were plotted as a function of spatial position. The anatomical images were of a single subject (A) and the beta distributions were averaged from all subjects (B). The distributions for the three categories of stimuli (Chinese characters, faces, and line drawings) were similar in the passive task (left panels) and the active task (right panels), supporting a robust and task-independent response selectivity for Chinese characters in the left lateral mid-fusiform cortex.</p