Halgren Eric, Dhond Rupali P, Christensen Natalie, Van Petten Cyma, Marinkovic Ksenija, Lewine Jeffrey D, Dale Anders M
MGH-NMR Center, Harvard Medical School, Boston, Massachusetts, USA.
Neuroimage. 2002 Nov;17(3):1101-16. doi: 10.1006/nimg.2002.1268.
Words have been found to elicit a negative potential at the scalp peaking at approximately 400 ms that is strongly modulated by semantic context. The current study used whole-head magnetoencephalography (MEG) as male subjects read sentences ending with semantically congruous or incongruous words. Compared with congruous words, sentence-terminal incongruous words consistently evoked a large magnetic field over the left hemisphere, peaking at approximately 450 ms. Source modeling at this latency with conventional equivalent current dipoles (ECDs) placed the N400 m generator in or near the left superior temporal sulcus. A distributed solution constrained to the cortical surface suggested a sequence of differential activation, beginning in Wernicke's area at approximately 250 ms, spreading to anterior temporal sites at approximately 270 ms, to Broca's area by approximately 300 ms, to dorsolateral prefrontal cortices by approximately 320 ms, and to anterior orbital and frontopolar cortices by approximately 370 ms. Differential activity was exclusively left-sided until >370 ms, and then involved right anterior temporal and orbital cortices. At the peak of the N400 m, activation in the left hemisphere was estimated to be widespread in the anterior temporal, perisylvian, orbital, frontopolar, and dorsolateral prefrontal cortices. In the right hemisphere, the orbital, as well as, weakly, the right anterior temporal cortices were activated. Similar but weaker field patterns were evoked by intermediate words in the sentences, especially to low-frequency words occurring in early sentence positions where there is little preceding context. The locations of the N400 m sources identified with the distributed solution correspond well with those previously demonstrated with direct intracranial recordings, and suggested by functional magnetic resonance imaging (fMRI). These results help identify a distributed cortical network that supports online semantic processing.
人们发现，单词会在头皮上引发一种负电位，该负电位在大约400毫秒时达到峰值，并受到语义语境的强烈调节。在男性受试者阅读以语义相符或不符的单词结尾的句子时，本研究使用了全脑磁脑图（MEG）。与相符的单词相比，句末语义不符的单词在左半球持续诱发一个大磁场，在大约450毫秒时达到峰值。在这个潜伏期，使用传统等效电流偶极子（ECD）进行源建模，将N400m发生器定位在左颞上沟或其附近。一种受皮质表面约束的分布式解决方案表明了一系列差异激活，大约在250毫秒时从韦尼克区开始，大约在270毫秒时扩散到颞前部位，大约在300毫秒时扩散到布洛卡区，大约在320毫秒时扩散到背外侧前额叶皮质，大约在370毫秒时扩散到眶额和额极皮质。差异活动在370毫秒之前完全集中在左侧，之后则涉及右侧颞前和眶皮质。在N400m的峰值时，左半球的激活估计在前颞叶、颞周、眶、额极和背外侧前额叶皮质广泛存在。在右半球，眶皮质以及右侧颞前皮质有微弱激活。句子中间的单词也会诱发类似但较弱的场模式，尤其是句子开头位置出现的低频单词，因为前面几乎没有语境。通过分布式解决方案确定的N400m源的位置与之前直接颅内记录所显示的位置以及功能磁共振成像（fMRI）所提示的位置非常吻合。这些结果有助于识别一个支持在线语义处理的分布式皮质网络。
