| People spend a considerable amount of time for mental time travel,namely,remembering the past and imagining the future---the ability of great significance and value for people's survival and development.Academic study of mental time travel has already nearly 40 years of history,with a large number of reviews and research articles published.Tulving said that the ability for mental time travel was unique advanced cognitive abilities to humans.Studies have shown that mental time travel can be influenced by many factors,including age,time distance,event emotional valence,the significance and specificity of events,visual-spatial imaginary ability and emotional regulation strategies and so on.The published studies about remembering the past and imagining the future have reached a consistent finding—the activated brain regions during remembering the past and imagining the future showed a great degree of overlap----remembering the past and imagining the future share a common core brain system,including the medial temporal lobe(MTL),the medial prefrontal cortex(mPFC),the posterior cingulate(PCC),the precuneus and retrosplenial cortex,etc.Compared to the researches about this shared brain system,the studies focused on the age-related neural mechanisms in mental time travel are quiet few,and the studies researched on the time distance and age factors at the same time are very rare.This present study hoped to fill in this blank.Previous studies showed that compared with remembering the past,imagining the future can exhibit more activation only in right hippocampus,and relative to the young,the activation in the right hippocampus in the elderly people showed the lateralization effect of right hippocampus activation.The purpose of this study was to explore the age-related differences in the activation of brain regions and the functional connectivity based on the seed regions of the left and right hippocampus during remembering the past and imagining the future.In order to achieve the research purpose,this study recruited 20 Southwest University college students and 20 older adults.Let the participants to retrieve and imagine 9 pieces of recent(withinone month of the past or the future)and remote(between 5 ~ 10 years of the past or future)past/future event clues three days before the formal fMRI scan.Then they rate the event details,vivid degree,emotion valence,etc.,and the researchers chosed five clues under each of the four experimental conditions to use in later fMRI scanning according to these indicators.During the scanning,participants were asked to recall or imagine the specific events according to the event clues,and the brain imaging data during the remembering and imagining were collected to do further data analysis.Through the behavior data analysis,the two age groups were not significantly different on imagination ability scores.Then we did the age(old,young)× task(remembering,imagining)ANOVA analysis on the vivid degree,and only found the main effect of age,without the interaction of age and task.The older adults showed significantly higher vivid degree than younger adults on the two tasks.Then the brain imaging data,Firstly,we did brain activation analysis,and found that under both the conditions of remembering and imagining,only the main effect of age.Therefore,the further analysis focused on the age-related differences under the conditions of remembering the past and imagining the future.Based on the previous studies,we selected six ROIs(Region of Interest),the inferior frontal gyrus,superior temporal gyrus,the precuneus,the hippocampus,the superior parietal gyrus,the temporal_pole_sup,respectively.Then we extracted the beta value in these ROIs,and found that under the condition of remembering the past,the inferior frontal gyrus,precuneus,hippocampus,superior parietal gyrus and temporal_pole_sup showed significant age-related differences and under the condition of imagining the future,the superior temporal gyrus,precuneus,superior parietal gyrus and temporal_pole_sup exhibited significant age-related differences.The presence of age-related differences in these regions may indicate the decline of brain function in the older adults.Two sample t test showed that the older adults increased activation in none brain regions under remembering or imagining condition.Under both the conditions of remembering the past and imagining the future,brain regions showed the significant age-related differences(younger adults activated more than the older adults)included the left fusiform gyrus,superior parietal gyrus,the anterior cingulate cortex,thalamus,bilateral lingual gyrus,bilateral superior temporal gyrus,the right middle temporal gyrus,cuneus.And brain regions showed the significant age-related differences only under the condition of imagining the future included the right subcallosal gyrus,superior parietal gyrus,putamen,lateral globus pallidus,Paracentral Lobule,superior frontal gyrus,temporal_pole_sup,bilateral middle occipital gyrus,postcentral gyrus,precentral gyrus,medial globus pallidus,the left supplementary motor area,medial orbitalfrontal gyrus,inferior occipital gyrus,the brain regions showed the significant age-related differences only under the condition of remembering the past included the right precuneus,hippocampus and parahippocampal gyrus,temporal_pole_mid,lentiform nucleus,the left culmen,subcallosal gyrus,olfactory,middle frontal gyrus,frontal_inf_tri,middle temporal gyrus,bilateral medial frontal gyrus.The decreased activation occurred in the extensive brain areas in the older adults under both remembering and imagining conditions,may be the very result of a shared core brain system by mental time travel.Under the condition of remembering the past and imagining the future,the functional connectivity analysis of the whole brain based on the two seed regions,the left and right hippocampus were performed respectively.We found that under the above two conditions,the brain regions with significant functional connectivity with the left/right hippocampus in the older and the younger adults,showed great overlap with the forementioned common brain system regions during remembering the past and imagining the future,which may prompt that the left/right hippocampus played a great role in mental time travel by serving as the core nodes in this shared core brain system.Previous studies found that relative to remembering the past,when imagining the future the older adults activated the hippocampus significantly mainly on the right side,therefore the present study,tried to explore from the perspective of the functional connectivity whether there is a right tendency in the functional connectivity of hippocampus,in the younger and the older adults,respectively,when imagining relative to remembering.As a result,only the older adults,under the imagining relative to remembering condition,brain regions with significant functional connectivity with the right hippocampus were found: the left Frontal_Inf_Oper and the left Temporal_Pole_Sup,which proved the existence of the advantage of the right side of the hippocampus in the older adults,under the imagining condition.To sum up,we mainly found that: 1)there is only the main effect of age under the condition of remembering the past and imagining the future,with no interaction between age and time;2)Compared to the younger adults,under both the condition of remembering and imagining,the older adults showed no brain regions with increased activation,on the contrary,the younger adults showed increased activation in relatively extensive brain areas,suggesting the decline of brain function in the older adults during mental time travel;3)in the six ROIs selected,under both the conditions of remembering and imagining,the younger showed increased activation than the older adults in the right precuneus,the superior parietal gyrus and the temporal_pole_sup;4)under the condition of remembering and imagining respectively,the brain regions of significant functional connectivity with the left and right hippocampus in the two age groups showed a greater degree of overlap;5)compared to remembering the past,when imagining the future,only the functional connectivity of right hippocampus in the older adults presented two significant brain regions,Frontal_Inf_Oper and the Temporal_pole_sup,which proved that under the imagining compared to remembering condition,the functional connectivity bias based on the right hippocampus in the older adults,although the present study did not find the increased activation in the right hippocampus in the older adults under the imagining compared to the remembering condition.The present study,from the perspectives of behavior,brain activation and whole brain functional connectivity based on the left/right hippocampus,studied on the effect of aging on remembering the past and imagining the future.This work enriched the research findings of the age-related differences in the mental time travel,at the same time,provided the certain theoretical basis for the aging neural mechanism study about the mental time travel and provided methods and theoretical guidance for further studies. |
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