The Effect Of Encoding Task And Reward Anticipation On The Recognition Of Associative Memory In College Students ——Evidence From ERP

BackgroundMemory is ubiquitous in our daily life.Item memory is the memory of individual items;associative memory is the memory of the relationship of item-item or item-context.There are many factors that affects memory retrieval,previous studies have explored that reward plays a greater role when the difficulty（medium）of encoding task is more appropriate,but it remains unclear whether and how reward and encoding task in associative memory.ObjectiveWe used event-related potential（ERPs）technique with high temporal resolution to further explore the temporal characteristics of the effects as well as the neural mechanisms of reward and processing depth on item and associative memory retrieval.MethodsIn the first study,we used neutral object pictures and neutral background pictures to record the behavioral results and EEG signals of 30（14 males）participants during task completion.The reward cue was introduced in the encoding phase,and the encoding tasks included congruity-judgment task and size-judgment task,and the serial judgment paradigm and associative recall paradigm were used to separately test item memory and inter-item associative memory.The behavioral and ERP data were analyzed by multi-factor repeated measure ANOVA,and ERPs were analyzed from five representative midline electrodes（Fz,FCz,Cz,CPz,and Pz）.In the second study,we recorded the behavioral results and EEG signals of 32（18males）participants during task completion.The experimental materials and paradigm were the same as those in the first study,and the timing of the introduction of reward cues was altered（retrieval phase）.The behavioral and ERP datas were analyzed by multi-factor repeated measure ANOVA,and ERPs were analyzed from 12 representative electrodes,incorporating frontal-frontocentral electrodes（F3,Fz,F4,FC3,FCz,and FC4）and centroparietal-parietal electrodes（CP3,CPz,CP4,P3,Pz,and P4）.In the third study,we used neutral object pictures and neutral Chinese two-character words to record the behavioral results and EEG signals of 30（15 males）participants during task completion.The reward cue was introduced in the encoding phase,and the encoding tasks included congruity-judgment task and color-judgment task,and the associative recognition paradigm were used to explore the between-domain inter-item associative memory.The behavioral and ERP datas were analyzed by multi-factor repeated measure ANOVA,and ERPs were analyzed from five representative midline electrodes（Fz,FCz,Cz,CPz,and Pz）.ResultsIn the first study,the results of item retrieval showed that the accuracy of rewarded items was higher than unrewarded items only in the congruity-judgment task（P = 0.001）,and the FN400（300–500 ms）and LPC reward effects（500–700 ms）were greater in the congruity-judgment task than those of the size-judgment task from electrode Fz to electrode Pz.The results of source retrieval showed that the average accuracy of rewarded items was higher than unrewarded items,and that the difference in the size-judgment task was significantly larger（P = 0.026）,and that the reward effect at FN400（300–500 ms）were greater in the size-judgment task than the congruity-judgment task.In the second study,in item retrieval,the reward differences at Prs in the congruity-judged object items were significantly greater than the size-judged items（P =0.005）,and the significant late LPN reward effects（700–1000 ms）were found in the congruity-judgment task in more areas.The source retrieval results showed that reward（relative to no reward）significantly improved recognition accuracy（P = 0.004）and the significant P300（300–500 ms）and late LPC（500–700 ms）,LPN（700–1000 ms）reward effects were found only in the congruity-judgment task.In the third study,the reward differences at Prs in the color-judgment task were greater than those in the congruity-judgment task（P = 0.014）.ERP results further showed that significant reward effects at FN400（300–500 ms）and LPC（500–800 ms）were found in the color-judged items both in intact and rearranged conditions,and the reward effects at LPC were distributed more widely than the reward effects at FN400.ConclusionBoth encoding tasks and reward anticipation have interactive impact on item memory and associative memory retrieval.Whether item retrieval or item retrieval,reward plays a greater role when the difficulty is more appropriate（medium difficulty level）,and the impact of this effect on the two associative memory types is different in time course: the impact in the early stage of within-domain inter-item associative memory retrieval,while later memory stage,which is related to between-domain inter-item associative memory,will be greater.