Neural Basis Of Processing Bindings In Working Memory

The binding problem is one of the core issues in the field of working memory(WM).In the WM field,bindings are usually classified into two types: conjunctive binding and relational binding.The conjunctive binding refers to an integrated object representation of different features within an item,while the relational binding refers to an integrated episodic representation of different items or an item and its spatiotemporal context.Regarding the neural basis of retaining bindings in WM,previous studies have controversial conclusions,whose experimental design cannot resist the influence of confounding factors such as perception and attention.Their data analyses are not sensitive to detect the representation of bindings in the brain as well.In addition,previous studies mainly focus on a single type of binding,which cannot examine whether conjunctive binding and relational binding share a generic representation in the brain.Therefore,the present study aims to systematically investigate the neural basis of retaining bindings in WM with functional magnetic resonance imaging(fMRI)and transcranial magnetic stimulation(TMS),which employs rigorous criteria based on the characteristics of binding processing in WM and new advances in research techniques of cognitive neuroscience.In the current study,we investigated the neural basis of retaining conjunctive and relational bindings in WM through two parts(six experiments in total).We employed a change detection paradigm and used color-shape bindings and color-location bindings as stimuli.In the first part,we used fMRI and performed the general linear model and the multivariate pattern analysis(MVPA).We relied on the criteria of the “quality” and“quantity” of retaining bindings to identify the core brain regions representing conjunctive binding(Experiment 1)and relational binding(Experiment 2)in WM.These regions should reflect both the differences in the nature of WM representations for features versus bindings(“quality”)and the load effect of bindings(“quantity”).If there were common regions for retaining conjunctive and relational bindings in WM,we performed cross-category MVPA and functional connectivity analysis to further locate the crucial region for generic binding representations(the joint analysis of Experiment 1 and Experiment 2).The two criteria are as follows:(1)The neural patterns of the conjunctive and relational binding should be similar in the crucial region;(2)The crucial region should exhibit the reentrant processes when retaining two types of bindings in WM.In the second part,we used TMS to verify the causal link between the crucial region and WM for conjunctive binding(Experiments 3 & 5)and relational binding(Experiments 4 & 6).We applied single-pulse TMS on the crucial region in the early stage of WM after perceptual processing was completed.The main conclusions were obtained as follows:(1)The core brain networks for representing conjunctive and relational bindings in WM were distributed in the frontoparietal cortex and bilateral insula.For both two types of bindings,the superior parietal lobule,precentral gyrus,and supplementary motor area in the left hemisphere were involved.The angular gyrus,inferior frontal gyrus,and insula in the right hemisphere were only involved in representing conjunctive binding.The right inferior parietal lobule and left insula were only involved in representing relational binding.The activation intensity of the above regions was enhanced with increasing WM load of bindings.Their activation patterns could be used to discriminate whether the WM representation of binding was present,which reflected the differences in the nature of WM representations and suggested a specific neural representation of binding in the core regions.(2)The crucial region for representing generic binding in WM was located in the left superior parietal lobule.This area exhibited a reentrant process when retaining conjunctive and relational bindings in WM.Moreover,the neural representations for both types of bindings were similar in the crucial region,and the activity of the crucial region was causally related to the WM for both types of bindings.