Do FN400 potentials index conceptual priming during old/new judgment?

Manson Cheuk-Man FONG, Ivan Yifan ZOU, Patrick Chun Kau CHU, and William Shi-Yuan WANG, Department of Chinese and Bilingual Studies, The Hong Kong Polytechnic University

Objective: The frontal N400 (FN400) and parietal late positive complex (LPC) are two event-related potential (ERP) components commonly elicited in an old/new judgment task, in which subjects are asked to determine whether a given item belongs to a studied list (old) or not (new). Consistent with a dual-processing notion of memory in which the neural substrates supporting familiarity and recollection are distinct, these two temporally separated ERP components have frequently been accepted as indices of familiarity and the confidence in the recollection of studied items, respectively.  However, another prominent proposal postulates that familiarity and recollection are both reflected in the LPC component, with a reduction in the FN400 magnitude indexing conceptual priming instead. The present study aims to examine the functional role of FN400 by including both pictures and visual words in the studied list.

Rationale and hypothesis:  Previous works have reported that FN400 effect is present only if the studied items are presented in the same visual form during the subsequent old/new judgment, e.g., both as pictures or visual words.  This result contradicts the conceptual priming hypothesis for FN400, which would predict that an FN400 effect could still be observed even if pictures and visual words were to be used, respectively, in the studied list and during the old/new judgment. One potential reason why such picture-to-word conceptual priming has not been observed is that previous studies have not included a subsequent recollection task to measure the confidence of the old/new judgment, a factor that is known to influence both the FN400 and LPC amplitudes. In this ongoing work, we therefore re-examine the generality of the conceptual priming hypothesis by determining whether a picture-to-word priming effect can be found, when the trials in which subjects fail to recollect episodic details of the studied items are excluded.

Method: A group of 20+ university students will be asked to name a list of 80 items, which alternate between pictures and disyllabic Chinese words presented visually in groups of 20 items. Given that increasing the number of times the studied list is presented is known to enhance both FN400 and LPC, the whole list will be presented twice to increase the “memory strength” of the studied items.  Subjects will then be asked to perform an old/new judgment for a word list that comprises 120 words of three memory categories: (1) old picture words (corresponding to studied pictures); (2) old repeated words (identical to the visual words in the studied list); and (3) new words. The three sets of words are matched in semantic category, familiarity, and number of strokes (separately for the first and second Chinese characters). For each word that is judged “old”, subjects will be further asked to recollect whether it has first been presented to them as picture or visual word. This recollection task provides the basis for isolating the trials in which the old/new judgment is made with high confidence for ERP analysis. The peak amplitudes and latencies of FN400 and LPC components will be measured at the frontal and parietal electrodes, respectively, and analyzed by planned pairwise comparisons among the three memory categories.

Expected findings: Both the old picture words and old repeated words are expected to elicit a less negative FN400 and more positive LPC than new words.  The presence of picture-to-word priming will support the conceptual priming hypothesis of the FN400. Whether a difference in FN400 magnitude can be observed between the two types of old words depends on a number of factors (e.g., the strength of form priming, and the differential efficiency of memory encoding for pictures and words). Thus, such comparison will be done for the sole purpose of evaluating the relative efficacy of pictures and words in eliciting the old/new effects.

Research significance: This work will shed light on the conceptual priming hypothesis of the FN400, which helps resolve the two prominent proposals regarding the association of FN400 to either conceptual priming or familiarity.  Also, the present work is an initial experiment for establishing that the FN400 and LPC effects can be consistently elicited in the present paradigm, which is intended to be subsequently used for evaluating decline in episodic memory in normal aging and pathological aging. The paradigm will find potential use in the diagnosis of various neurological diseases, particularly those in which episodic memory deficit is one of the first behavioral symptoms (e.g., Alzheimer’s disease).