Research On The Maintenance And Retrieval Of Object Information In Visual Working Memory

Working memory(WM)was proposed as a broadly defined limited-capacity cognitive system for temporarily maintaining and processing representations of external information to support human cognition and behavior.WM can help us to understand things we see,hear and feel,and it is the basis for people to know and communicate with the world.Therefore,research on WM has been a hot spot in the field of cognitive neuroscience.Over the past few decades,there has been a growing researcher dedicate to reveal the underlying neural mechanism of WM.They designed several classical WM paradigms,e.g.delayed match to sample task,change detaction task and N-back task,which concludes the WM capacity is limited to 3-4 items.Researchers put forward three processing phases of WM,namely encoding,maintenance and retrieval.By using electroencephalogram(EEG)/ event-related potential(ERP)with high time resolution and functional magnetic resonance imaging(fMRI)with high space resolution,researchers observed the WM-related P300 activity in middle parietal area,and the slow wave component and alpha activity in fronto-occipital cortex,as well as WM-related fronto-parietal network.In the end,this thesis aims to explore the maintenance and retrieval of object information in WM,by using ERP,ERP-fMRI simultaneous recordings and transcranial magnetic stimulation(TMS)technique.1.Exploring the maintenance of object information in WM based on ERP.In the WM maintenance phase,task-irrelevant information would affect the maintenance of task-relevant information in WM.The existing load theory of attention suggests that the WM load affects selective attention at early and late stages.Thus,this study manipulated the WM load and used the high time resolution ERP measure to explore the interaction between WM load and task-irrelevant emotional distractors,and to found out whether this interaction occurred in the early or late stages.We used a delayed-match-to-sample(DMS)task and varied the WM load from 1 to 6,and the positive,negative and neutral faces were utilized as distractors.Behavioral results showed decreased accuracy and delayed reaction times(RTs)for high vs.low WM load.ERPs showed enhanced frontal N1 and occipital P1 amplitudes for negative faces vs.neutral or positive faces,implying rapid attentional alerting effects and early perceptual processing of negative distractors.However,high WM load appeared to inhibit face processing in general,showing decreased N1 amplitudes and delayed P1 latencies.An inverse correlation between the N1 activation difference(high-load minus low-load)and RT costs(high-load minus low-load)was found at left frontal areas when viewing negative distractors,suggesting that the greater the inhibition the lower the RT cost for negative faces.Emotional interference effect was not found in the late WM-related parietal P300,frontal positive slow wave(PSW)and occipital negative slow wave(NSW)components.In general,our findings suggest that the WM load modulates the early attention and perception of emotional distractors.2.Exploring the retrieval of object information in WM based on simultaneous ERPfMRI recordings and TMS.The subject’s performance in WM task is not only determined by the accurate maintenance of object representations,but also by the accurate retrieval of them.Previous theory demonstrated that object identity and location are bound together to form a unique integration that is maintained and processed in visual WM.Thus,this study manipulated the object location and WM load to investigate their effects on the object color change detection.Subjects responded faster and with greater accuracy in the repeated compared to the changed object location condition,when a higher WM load was utilized.These results support the spatial congruency advantage theory and suggest that it is more pronounced with higher WM load.Furthermore,the spatial congruency effect was associated with larger bilateral posterior N1 activity,greater activation of the right inferior frontal gyrus(rIFG)and less suppression of the right supramarginal gyrus(rSMG),when object location was repeated compared to when it was changed.The ERP-fMRI integrative analysis demonstrated that the object location discriminationrelated N1 component is generated in the rSMG.Furthermore,TMS stimulation of rSMG and rIFG comfirmed their special roles in the contribution of spatial congruency effect.3.Comparing the effects of EEG references on the ERP results in the simultaneous ERP-fMRI study.In current EEG-fMRI studies,average reference(AR),and digitally linked mastoid(LM)are popular re-referencing techniques in ERP analyses,although they may introduce their own physiological signals and alter the EEG/ERP outcome.We created the realistic cortex surface of individual subject based on the structural MRI and created the realistic electrode positions on the scalp surface based on the EEG gel artifact,which in turn to evaluate the “zero potential” accurately by using a reference electrode standardization technique(REST).As we hypothesized,we found that the adopted reference did not change the topography map of ERP components(N1 and P300 in the present study),but it did alter the task-related effect on ERP components.LM decreased or eliminated the WM load effect on P300,and the AR distorted the distribution of WM location-related effect at left posterior electrodes as shown in the statistical parametric scalp mapping(SPSM)of N1.Referring to the ERP cortical source estimate,fMRI result and a corresponding EEG-only study,we concluded that REST reference provided a more integrated and reasonable ERP result.Thus,we recommend the REST,especially with a realistic head model,as the optimal reference method for ERP data analysis in simultaneous ERP-fMRI studies.In summary,this thesis used ERP,simultaneous ERP-fMRI and TMS technique to investigate the neural mechanisms of which the maintenance and retrieval of object information in visual WM.This thesis revealed the WM system suppresses the interference of task-irrelevant distractors on the maintenance of task-relevant information in early stage,and revealed the object identity-location binding in WM retrieval phase.Furthermore,this thesis adopted a new reference electrode standardization technique and compared it with other commonly used reference methods,and we suggested REST as the reference method for ERP analyses in simultaneous ERP-fMRI studies.