Neural Basis Of Temporal Order Memory:Effects Of Source Context And Temporal Distance

Much of our experience is perceived and interpreted through the sequences of events that occur.Temporal order memory allows us to re-experience the past by recovering the sequences of events that characterize those prior experiences.However,how the relative order of two events is extracted from our memory is still controversial.Association-based theory proposes that people reconstruct the relative order information through the degree of association between two events.The closer the association is,the better the temporal order memory is.However,the context-based theory argues that people retrieve the relative order information through the comparision of difference of source context between the two events.The larger the difference is,the better the temporal order memory is.Temporal distance between two events is another important factor affecting temporal order memory.The temporal distance effect,probably the most ubiquitous findings in recency discrimination in which order judgments are facilitated by increasing temporal distance between two events,have been shown in many studies.However,one recent study has shown that the change of source context affects our subjective representation of the temporal distance between the two events.For instance,assuming the objective temporal distance is constant,the individual will subjectively overestimate the temporal distance between the two events if their beloging source contexts are different.The result implies that the source context and the temporal distance of two events may interact within retrieving temporal information.The dissertation aims to investigate the neural basis of temporal order memory and the effect of source context and temporal distance.The dissertation was divided into three parts,with a total of five experiments.Participants watched naturalistic materials first and made discriminative choices of two still-frames from the materials.Participants were required to choose the still-frame that happened earlier in the materials.Specifically,Experiment 1 and Experiment 2 combined naturalistic materials with Magnetic Resonance Imaging(MRI)to explore the neural basis of source context affecting temporal order memory.Experiment 3 sought to identify brain areas that covariated with objective temporal distance between two events.Experiment 4 and Experiment 5 studied the interaction effect of source context and subjective temporal distance on temporal order memory,and the interaction effect of source context and objective temporal distance on temporal order memory respectively.Specifically,Experiment 4 presented a behavioral study that explored the interaction effect of source context and subjective temporal distance on temporal order memory.Experiment 5 further explored the interaction effect of source context and objective temporal distance on temporal order memory by combining functional MRI,representational similarity analysis,noninvasive neuromodulation techniques(Transcranial Magnetic Stimulation,TMS)with an interactive-video memory paradigm.The overall results are summarized as follows.Firstly,regardless of whether the video clips were played consecutively or intermittently,compared to the sourcedifferent condition,the recency judgment was faster and more accurate in the sourcesame condition(experiment 1 and experiment 2).Secondly,the fMRI univariate activation analysis indicated that the source-different condition induced significant activation of the right angular gyrus and the bilateral precuneus,whereas the sourcesame condition induced significant activation of the prefrontal cortices,the visual cortices and the temporal lobes.Multivoxel decoding analysis confirmed that the visual cortices,angular gyrus,and precuneus were involved in distinguishing the two source contexts of events(source-same vs source-different,experiment 2).Thirdly,as the objective temporal distances between events increase,the accuracy in recency discrimination increases.The fMRI parametric modulation analysis revealed significant modualtions of the left inferior temporal lobe,the occipital gyrus,the left inferior frontal gyrus and the precuenus associated with trials as a function of temporal distance.Specifically,the retrieval-related activation of the left inferior temporal lobe increased with increasing temporal distances between two events,while the retrievalrelated activation of the occipital gyrus,the left inferior frontal gyrus and the precuenus increased with decreasing temporal distances between two events(Experiment 3).Fourthly,when the objective temporal distance was constant,the proportion of trials labeled close in the source-same condition was higher than the proportion of pairs labeled close in the source-different condition.Further,the proportion of pairs labeled far was greater in the source-different condition compared to the source-same condition.Importantly,compared to those trials labeled close,the accuracy was significantly higher in those trials labeled far condition,but there was no difference between the two response-related conditions in recency discrimination(Experiment 4).Fifthly,we revealed a multivoxel representation of temporal distances separating pairs of episodic event-moments in the posterior medial memory system,especially when the events were experienced within the source-same condition.We further showed such multivoxel representations to be vulnerable to disruption through targeted repetitive transcranial magnetic stimulation and that perturbation to the mnemonic abstraction altered the neural-behavior relationship across the wider parietal memory network(Experiment 5).The above results show that the effect of temporal distance on recency discrimination is modulated by source context.When the source contexts of the two events are the same,the recency discrimination becomes faster and more accurate with the increase of temporal distance.The parietal cortices,especially the precuneus,can distinguish two kinds of source contexts and characterize the representation of temporal distance,which supports the cognitive mechanism of temporal memory retrieval based on the comparison of situational contexts,indicating a key role of the posterior medial parietal memory system in temporal context memory.Future research needs to further disentangle a variety of cognitive neural mechanisms involved in the process of temporal memory,and to clarify the complex relationships between different functional sub-regions of the parietal memory system at different stages of memory encoding and retrieval,and at different time scales and different temporal contexts,before we can establish an integrated cognitive model of our memory of time and context.