Abstract:

The ability to intentionally forget emotional irrelevant or unwanted materials is vital for memory function. The item method, which is the one of two main variants of directed forgetting paradigm, is the typical approach to exploring the mechanisms of intentional forgetting in the laboratory. When the item method was used, the predominant explanation of directed forgetting effects emphasizes selective encoding, while evidence supporting inhibition control during encoding and retrieval is also observed. ERPs studies on directed forgetting during encoding attempt to seek inhibition evidence and separate encoding effort from encoding success. Many studies have found that the to-be-forgotten cues elicited early enhanced positive waves in frontal and prefrontal areas, indicating strong activation of inhibitory processes. Meanwhile, intentional forgetting involves different brain areas compared with unintentional forgetting and intentional remember. Most of the researches on directed forgetting of emotional materials showed that emotional stimuli are exempt from forgetting or have a diminished directed forgetting effect when compared with neutral stimuli. Unlike previous studies on directed forgetting of emotional events with neutral symbols as cues (eg, “RRRR” as the to-be-remembered cue and “FFFF” as the to-be-forgotten cue), the current study used emotional pictures as cues to create two experiment manipulations. The first manipulation used positive pictures as the to-be-remembered cues and negative pictures as the to-be-forgotten cues; the second used negative pictures as the to-be-remembered cues and positive pictures as the to-be-forgotten cues. The purpose of the two manipulations is to examine the electrophysiological correlates of the intentional forgetting of neutral nouns using the item-method directed forgetting paradigm. Twenty female undergraduate students participated in the experiment. During study phase, participants were instructed to remember half of a series of presented words (TBR) and to forget the other half (TBF) according the emotional picture cues. During test phase, they pressed one of the mouse buttons to indicate whether the word was presented at study phase or not. We used the behavioral recognition data to sort ERPs time-locked to the picture-cues into 4 conditions based on the combination of memory instruction and behavioral outcome: RR (TBR_hit), RF (TBR_miss), FR (TBF_hit) and FF (TBF_miss), to investigate the emotional effects on the 4 experimental conditions and the processes underlying successful implementations of intentions to forget (i.e., RF vs. FF ) and intentions to remember (i.e., RR vs. FR). Directed forgetting, that is reduced recognition of to-be-forgotten words, occurred for both positive picture as cues and negative conditions. At 350~600ms, it was found that positive picture as to-be-remembered cues elicited a more positive wave than negative ones (under the ‘RR’ and the ‘RF’ condition) in the middle frontal and central areas which are involved in the evaluation of the emotional valence of stimuli, and this result could reflect a preference toward pleasant pictures. But ERP amplitudes were larger for negative pictures compared to positive ones under the ‘FF’ condition in the left parietal region, which suggests weaker inhibition for the item under the condition of negative pictures as ‘FF’ cues. There were no significant differences between the positive and negative pictures under the ‘FR’ condition. Moreover, when the positive pictures are as cues, there were significant differences between the intentional and unintentional remember conditions (‘RR’ vs. ‘FR’) and between the intentional and unintentional forget conditions (‘RF’ vs. ‘FF), whereas when the negative pictures are as cues, no such comparisons were observed to be significant. Our results show that flexible control of memory may be effective even in conditions in which negative pictures were used as cues, although it requires more effort than that in conditions using positive pictures as cues. Moreover, positive emotion is more effective as to-be-forgotten cues.

Key words: directed forgetting, emotion effects of cues, encoding intention, event-related potentials