The distinction between neural mechanisms of explicit and implicit expressions of memory has been well studied at the retrieval stage, but less at encoding. Several studies employed a novel paradigm to measure explicit memory and priming-without-explicit memory in one test, and contrasted the neural signals of these two processes at the encoding stage via a Dm analysis. However, dissociations obtained in these studies are complicated because of the contamination from familiarity, a fast automatic process in which memory judgments can be driven by the increased fluency of reprocessing studied information. Familiarity is also a form of explicit memory but different from recollection. Growing evidence has indicated that familiarity-based recognition judgments might rely on the same process that supports implicit memory or priming. Therefore, it is necessary to concurrently acquire and compare the encoding processes yielding later recollection, familiarity and priming in a single test.

In this study, a two-stage forced-choice recognition test was adopted within the subsequent memory (Dm) paradigm, so as to simultaneously acquire neural correlates of recollection, familiarity and priming in a single test. During the study phase, participants were instructed to judge the color of each word. There were two stages during the forced-choice recognition test phase. In the first stage, two words (one from the study list, and one new) appeared concurrently, and participants were instructed to indicate the studied word. If they could not recognize the studied word, a guess was permitted to make a choice. In the second stage, a cue to make a confidence judgment appeared directly following the recognition response, and participants indicated whether the foregoing studied-selection was based on remembering, knowing or guessing. Here, “remembering” refers to the retrieval of specific details from the study phase supporting the recognition decision, “knowing” refers to the recognition supported by a weak feeling of familiarity with few details retrieved from the study phase, and “guessing” refers to “absolutely no feeling of memory” such that the stimulus in no way felt “old”. The study ERP data were then classified into four categories as “subsequent remembered” (later retrieved with detailed information), “subsequent known” (later retrieved with a feeling of familiarity)”, “subsequent primed” (later retrieved without conscious awareness) and “subsequent forgotten” (not retrieved). Differences in subsequent memory effects (Dm effects) were measured by comparing ERP waveform associated with later memory based on recollection, familiarity or priming with that associated with later forgotten items. In addition, interference during encoding was introduced in Experiment 2 to determine whether three Dm effects were different from Experiment 1. The interference task was to judge the orientation of arrow which appeared with word at the same time.

The results showed that, in Experiment 1, the recollection Dm effect involved a robustly sustained (onset at 400 ms) prefrontal positive-going Dm effect which was right-lateralized, and a later (onset at 800 ms) occipital negative-going Dm effect. Familiarity involved an earlier (300~400 ms) prefrontal positive-going Dm effect and a later (500~600 ms) parietal positive-going Dm effect. Priming involved a negative-going Dm effect which onset at 700 ms, mainly distributed over anterior brain sites. In Experiment 2 with an interference task during encoding, a similar priming Dm effect that was negative-going during 600~800 ms at central site, and a similar familiarity Dm effect that was positive-going during 600~800 ms at frontal site were still observed. However, there was no evidence of Dm effect associated with recollection.