面孔分类中空间高低频表征的神经机制:一个颅内脑电研究

Neural Mechanism of Spatial Frequency Representation in Face Categorization: an ECoG Study

来自多方面的研究表明,面孔的分类和识别位于特定脑区.同时,已有行为实验研究表明,图像的空间高低频特征在面孔分类的不同范畴中起不同的贡献,例如身份更多被低频信号传递,性别被高低频共同传递,而表情更多被高频传递.然而,空间频率在面孔分类中的贡献,其表征和神经机制目前相关研究很少.利用特定癫痫患者植入颅内电极的监控期,呈现不同类型面孔图像,同时记录其颅内脑电,用事件相关电位方法考察了据认为是面孔特定成分的相关电位的潜伏期在170ms的波形(N170波形)的变化;用电极反应显著性分析考察了空间频率在不同分类特征上的贡献.结果表明,空间高频(HSF)图像的N170潜伏期显著延迟.只呈现空间低频(LSF)图像,N170的潜伏期对普通人面孔会延迟,而对熟悉的名人则没有这个差异.女性面孔诱发的N170在HSF条件下潜伏期明显晚于LSF条件,而男性面孔诱发的波形则不存在这个差异.表情在N170上没有体现出任何差异.但是基于电极的显著性分析表明,有更多的额叶电极参与了表情的加工;身份特征加工有更多电极在空间低频上表现出差异,而性别加工则空间高低频比较平衡.与以往行为结果不同的是,表情加工也有更多低频贡献,而且表情的差异可以在早达114ms的时候就发生.这符合表情信息在颞枕区域有一个快速基本加工,再传递到其他脑区的认知模型.因此,空间高低频信息在身份和性别上的贡献,可能发生在经典的面孔加工脑区,由N170表达,表情信息不由N170表达,而是在颞枕较广泛的范围内快速加工再传递到别的脑区,如额叶.这是首次利用颅内脑电就空间频率在面孔分类中的贡献的神经机制进行研究,为深入理解脑内面孔各种特征加工的动态过程提供了一个新的切入点.

Converging evidences show that face categorization and recognition are represented in specific brain areas. It is also known from behavioral studies that spatial frequency of pictures takes different roles in different face categorizations, e.g. identity information is carried more by low frequency features, and gender is represented by both high and low frequencies while high frequency is more important for face expression. However, there is no solid experimental data about the neural mechanism of the different contributions of spatial frequencies on face categorization up to now. ECoG was collected from epilepsy patients while they underwent a monitoring session to locate their epilepsy foci with implanted subdural surface electrodes. Pictures with different gender, facial expression and identities were presented to the patients, while they were required to perform a simple detection task to maintain their focus on the pictures. The changes of traditional face related components, N170, were analyzed by event related potentials and the significant changes was also verified on each electrode by permutation test. The N170 latency is found to be delayed for high spatial frequency （HSF） pictures. For faces of unknown person, low spatial frequency （LSF） presentation results in a longer N170 latency, but not for faces of well-known movie stars. Female faces show longer N170 latency in HSF conditions compared to LSF, but male faces have no such effect. No significant effect was found on N170 component for expression. However, analysis based on individual electrodes showed that more frontal electrodes are involved in expression representation; the identity-specific sites are more likely to respond to LSF stimuli, and the gender-specific sites have equal responses to both LSF and HSF stimuli, while the expression-specific sites are also more likely to respond to the LSF stimuli, which is inconsistent with existing behavioral studies, and show significant differences as early as 114 milliseconds. It fits with the cognitive model that expression related information has been analyzed briefly in occipital-temporal areas in the early stage before other brain areas are involved for further processing. In summary, spatial frequency＇s contributions to identity and gender could be represented by N170 in traditional face related brain areas while expression information, instead of carrying by N170, is distributed in a wider area in occipital temporal region for fast analysis and relayed to other areas, e.g. frontal cortex. This is the first study to study the neural mechanism of spatial frequency＇s contributions in face categorization by ECoG, and provides a new perspective to understand the brain dynamics of feature processes for face perception.