内隐与外显记忆编码阶段脑机制的重叠与分离

ERP Dissociation and Connection between Implicit and Explicit Memory at Encoding

记忆包括编码和提取两个重要阶段,以往研究多关注内隐和外显记忆在提取阶段的脑机制分离,那二者在编码阶段的脑机制是否不同?为探讨这个问题,该实验采用两阶段的迫选再认方式,把学习项目区分成三类:随后被选择且告知为见过的项目(记住),随后被选择但告知为猜测的项目(启动)以及随后未被选择的项目(忘记)。结果表明,从200ms开始的颞区负走向Dm效应为内隐记忆(启动vs忘记)所特有,反映了对刺激的知觉加工过程,400～500ms前额区正走向的Dm效应为外显记忆(记住vs忘记)所特有,反映了对刺激的精细加工过程,而200～300ms中央区及600ms开始的顶区负走向的Dm效应为两种记忆类型所共有,它们分别反映了对刺激的注意状态以及把编码后的刺激信息登记进相应记忆系统的过程。因此,内隐记忆与外显记忆在编码阶段的脑机制既存在着分离,也存在着重叠的现象。

The distinction between explicit and implicit memory is fundamental to current memory research. Explicit memory involves conscious remembering of prior episodes, often by means of intentional retrieval of those episodes, whereas implicit memory involves influences of prior episodes on current behaviour without intentional retrieval, and sometimes without conscious remembering of those prior episodes. Much evidence confirms that explicit memory and implicit memory have different neural bases at the retrieval stage, but what about the encoding stage？ Little evidence is provided owing to methodological ambiguities in prior studies which often compared incidental tests with intentional tests. In fact, brain activity in one test can reflect not only implicit （memory） but also explicit memory. Addressing these ambiguities has awaited a theoretical approach that distinguishes implicit （memory） and explicit memory for specific episodes in one test. To explore this question, a forced-choice recognition was conducted to produce priming without awareness of memory retrieval. We suggest that recognition mechanisms allied with explicit memory are different from recognition mechanisms allied with implicit memory. An ERP experiment was conducted with a study-to-test paradigm, in which participants performed a color study task, followed by a forced-choice recognition. There are two stages during recognition. Two words （one old and one new） were presented in a forced-choice recognition, and subjects were asked to choose the old one. If subjects could not choose a studied word, they were encouraged to guess. After choosing, subjects would report whether the word was from the study stage or not. Neural activities during the study phase were recorded. The Dm for explicit memory was identified by contrasting ERPs to words＇ for which the studied word was selected and endorsed it as an old word versus ERPs to words for which the studied word was unselected; The Dm for implicit memory was identified by contrasting ERPs to words for which the studied word was selected but failed to endorse it as an old word versus ERPs to words for which the studied word was unselected. The results showed that implicit and explicit memory share a 200~300ms frontal-central negative-going Dm effect, which maybe reflect attention at encoding, so that these words can be retrieved implicitly or explicitly. Implicit memory involved a temporal negative-going Dm effect from 200ms after stimulus onset, which maybe reflect encoding into the perceptual representation system. Explicit memory involved an earlier （400-600ms） right prefrontal, positive-going Dm effect, as well as a late （600-1200ms） parietal negative-going Dm effect. These effects maybe reflect elaborated processing and encoding into the episodic memory system. The results suggested that implicit and explicit memory are not completely independent of each other. The truth is that they have both independent and shared components at encoding.