编码任务对来源记忆的影响:一项事件相关电位研究

本研究采用ERPs技术，探讨编码任务对来源记忆的影响及其神经机制。在学习阶段，被试进行两种编码操作：匹配判断任务和大小判断任务；在测验阶段，让被试进行来源判断任务。行为结果发现：在编码阶段，匹配判断任务比大小判断任务需要更长的时间；在测验阶段，匹配判断任务在“项目旧/背景旧”和“部分旧”条件的正确率显著高于大小判断任务，匹配判断任务在“项目旧/背景旧”条件的反应时显著短于大小判断任务。测验阶段ERPs结果发现：两种编码任务的新旧效应在200~700ms均有较广泛的头皮分布，而在700~1100ms呈前额皮层最大化分布；匹配判断任务的新旧效应比大小判断任务更大，表现在600~700ms的FPz电极以及700~800ms的Pz、P4电极。由此可见，在来源记忆中，编码任务影响来源记忆，主要表现在后部顶区位置；晚期前额皮层新旧效应不受任务难度因素的影响。

Effects of Encoding task on Source Memory: An Event-Related Potentials Study

Item memory and source memory are two important aspects of episodic memory. Source memory is defined as the time, place and background information of the object that we should remember. In event-related potentials (ERPs) researches, when participants were asked to retrieve additional information about a studied stimulus in a source memory test, there are two kinds of old/new effects: early old/new effect and late old/new effect. The early old/new effect which appears across widespread regions of the scalp, but maximal at centroparietal scalp sites is observed from around 200 to 400 ms after stimulus onset. The later old/new effect over prefrontal cortex (PFC) begins around 700 to 800 ms after stimulus onset. It is well established that source memory retrieval engages later old/new effect more than simple item memory. Some researchers think that the early old/new effect which depend on the integrity of the medial temporal lobe and diencephalon indexes successful item recognition, and the late old/new effect reflects the executive process. Thus，this late-onset PFC old/new effect play an important role in source retrieval. So far, it is still not clear why source retrieval places greater demands on prefrontal cortex than old/new recognition judgements. In the current experiment, using event-related potentials technique, we explored the effects of encoding task on source memory and its neural correlates. Seventeen college students (7 male, 10 females) took part in our experiment. Stimuli were 640 digitized pictures in which an object was superimposed on a background. During the study phase, subjects were received two kinds of tasks. For one kind of task, they were asked to make judgments about the adaptability between items and backgrounds. For the other, they were received a size-judgment task, i.e., whether the items (object) were larger or smaller than the CRT monitor. In the subsequent test phase, there were four categories stimuli (“old/ old”, “old/ new”, “new/old”, “new/ new”). Subjects were told to determine whether the picture was “completely old” “part old” or “completely new”. The ERPs for “completely old” and “completely new” were averaged separately. We supposed that the early old/new effect were greater for congruity-judgment task than size-judgment task, and the prefrontal old/new effect was insensitive to encoding task and manipulation of difficulty. The behavioral results showed that, in the study phase, the reaction times of congruity-judgment task were slower than size-judgment task. In the test phase, the source accuracy of congruity-judgment task was reliably better than that of size-judgment task for the “completely old” and “part old” conditions while the reaction times of congruity-judgment task for “completely old” condition were significantly faster than that of size-judgment task. The ERPs results showed that early and late old/new effect were observed under both encoding conditions. It was found that during about 600~700ms after stimulus onset, the amplitude of ERPs of congruity-judgment task was more positive than that of size-judgment task at FPz, and the same pattern also appeared from 700~800ms, at Pz and P4. Based the above results, conclusions could be drawn that (1) early ERP old/new effect and late prefrontal ERP old/new effect may index two important processes in source memory retrieval; (2) the influence of encoding task on source memory was mainly manifested at the more posterior brain cortex; (3) the late prefrontal old/new effect did not co-vary with the retrieval difficulty. This research promoted our understanding the neural mechanism of retrieval process of episodic memory.