类别归纳的时间过程和源定位——事件相关电位研究提供的电生理证据

The Time Course and Source Location of Category Induction: Electrophysiological Evidence from ERP

采用事件相关电位(ERP)技术,以图形比较为实验作业探索类别归纳的神经机制。任务要求被试归纳两个图形刺激之间的共享特征,根据共享特征数量的不同构成三个任务:共享一个特征(S1F)、共享两个特征(S2F)和共享三个特征(S3F)。S1F任务和S2F任务被看作“归纳”任务,S3F任务被看作“非归纳”任务。对三个任务诱发的脑电分别进行叠加和平均,通过相减技术得到差异波(S1F-S3F,S2F-S3F)。在400到650m s时间窗口三个条件均诱发了LPC成分,地形图显示三个条件下LPC的头皮分布均出现在中-顶部;两个“归纳”任务与“非归纳”任务在LPC上存在显著差异,而两个“归纳”任务在LPC上没有显著差异。对两个差异波在400到650m s时段的数据进行偶极子源分析,结果显示每个差异波均只有一个偶极子源,且二者定位相似(均在海马附近)。由于已知海马与新异联系的形成有关,因此LPC可能反映了类别归纳的关键加工———新异联结的形成。

Category induction is a process of forming classification criterion. It involves abstraction of common features from limited samples of a specific category and turning the common features into a criterion characterizing this category. The common features provide a basis to judge whether a new stimulus can be classified into the category. Therefore, category induction is a prerequisite of classification processing, and a process of obtaining concepts and rules as well, occupying a primary position in human learning and knowledge acquirement. The event-related potential （ERP） was used to investigate the neural correlation of category induction, with geometric stimuli＇s comparison as experimental task. The present study developed Bigman and Pratt＇s （2004） study in the following ways： first, the successive presentation of S1 and S2 was changed to simultaneous presentation in order to eliminate the impact of the working memory load so that the meanings of related ERP components be clearer; second, with the limitation of its variation being considered, the size dimension was replaced with stripe orientation dimension, and the number of levels was changed from 3 to 4 to increase the range of feature variation; and third, a ＂ non-induction＂ condition was designed, where SI and S2 were identical, and the participants were also required to search common features. The EEG was recorded from 64 scalp sites using tin electrodes mounted in an elastic cap （ BrainProduct Inc. ）, with the averaged reference on the left and right mastoids, and a ground electrode （GND） was placed on the medial aspect of the frontal （on the middle line, between Fz and Fez ）. The ERP waves under three conditions were overlapped and averaged respectively. Brain Electrical Source Analysis program （ BESA, Version, 5.0, Software） was used to perform dipole source analysis. For dipole source analysis, the four-shell ellipsoidal head model was used. In order to focus on the scalp electrical activity related to the processing of pure induction processes effect, the averaged ERPs evoked by the ＂ non-induction＂ condition were subtracted from the ERPs evoked by the two ＂ induction＂ conditions respectively, and two difference waves were obtained accordingly （S1 F-S3 F and $2F-$3 F）. Principal component analysis （PCA） was employed in the interval from 400 to 650 ms in order to estimate the minimum number of dipoles. When the dipole points are determined, software will automatically determine the dipoles location. The relevant residual variance criterion was used. Two difference waves were obtained through subtracting S3F from S1F and subtracting S3F from S2F. In the time window of 400 and 650 ms, three conditions elicited obvious LPC component, and the topographies indicated that the components activated mainly over central-parietal scalp region. There was no significant difference between two ＂ induction＂ conditions, but there were significant differences both between two ＂ induction＂ conditions and ＂ non-induction＂ condition. Dipole source analysis （BESA 5 ） of two difference waves （S1F-S3F and S2F-S3F） indicated that there was only one dipole source under each difference wave, and the two dipole sources were located similarly （both were located approximately in hippocampus）. It is known that hippocampus is related to the formation of novel connection, therefore the LPC may reflect the key process of category induction： formation of novel connection.