A Resting-state Functional MR Imaging Study Of Hippocampus Focusing On Memory

BackgroundMemory impairment is one of the first symptoms of diseases with cognitive dysfunctions.It is an inherent phenotype of many neuropsychiatric disorders.The decline of memory function causes a series of serious damage to individual physiological,psychological and social activities.Investigations into the neural bases of memory have targeted the hippocampus.And dynamic functional connectivity between hippocampus and certain cortex is critical to memory.However,despite the improvement of people's understanding of memory decline and related neuropsychiatric disorders in recent years,most patients are still in the dilemma of trying to improve cognitive memory function,but unable to effectively protect cognitive memory function in the early stage or prevent further decline of memory.On one hand,memory is very complex.In the field of cognitive psychology,memory is classified into separate systems based on time or content aspects.It should be noted that different categories and stages of memory often work in parallel and are therefore not easy to completely isolated in practice.This situation greatly increases the difficulty of behavioral cognitive assessment of certain aspects of memory function.On the other hand,Neuroimaging studies have shown that different memory systems involve different brain regions,but the mechanisms of neurological function related to different memory functions remain yet to be studied,and the differences of hippocampal function at individual level have not been evaluated.Therefore,to figure out the differences of hippocampal function in different types of memory,analyze the possible neural basis of functional network,and master the imaging characteristics of magnetic resonance hippocampus in different types of memory remain important problems that need to be solved urgently in the field of brain imaging of magnetic resonance of memory.For these reasons,this study aims to use resting-state functional MRI and standardized cognitive psychometric assessment to conduct a series of hippocampal neuroimaging studies about different kinds of memory.(1)To further investigate the patterns of hippocampal neural networks;(2)To analyze the possible intrinsic relationship between hippocampus and different kinds of memory;to provide new clues for us to study the cognitive dysfunction caused by different types of memory impairment and to understand the different progression patterns of neurodegenerative diseases,such as Alzheimer's disease,semantic dementia and Parkinson's disease.MethodsExperiment 1:We recruited 100 healthy participants in the age of 19-29.Resting state functional and structural MRI data were acquired and voxel-wise correlation analysis was performed to functionally divide the bilateral hippocampus.The average template of anterior and posterior hippocampus was obtained.We further used DICE to evaluate the repeatability of the template obtained by the functional segmentation method.Experiment 2:A total of 100 healthy young people(aged 19-29 years)were enrolled in the study.We investigated the inter-hemispheric hippocampal-cortical functional connectivity in the anterior and posterior hippocampal neural networks,and compared the differences of the hippocampal functional networks between the left and right hemispheres.Experiment 3:A total of 100 healthy young people(aged 19-29 years)were enrolled in the study.Every participant took the standard assessment of verbal and visual memory using verbal(California Verbal Learning Test ?,CVLT-?)and visual subtests(Rey-Osterrieth Complex Figure,ROCF).Using resting-state functional MRI data,we performed neural network and seed-based functional connectivity analysis to detect the functional connectivity between hippocampus and certain cortex related to different types of memory,and to identify different function of hippocampal neural networks in different memory processes.ResultsExperiment 1:In both left and right hemispheres,the hippocampal was functionally segmented based on voxel-wised correlation analysis,and the average volume of the anterior hippocampus was smaller than that of the posterior hippocampus.Moreover,according to DICE coefficients,this data-driven method of functional segmentation of anterior and posterior hippocampus showed good repeatability.Experiment 2:In both left and right hemispheres,the entorhinal cortex(EC)and posterior parahippocampal gyrus(PPHG)showed the strongest connection with anterior and posterior hippocampus,respectively.Hemispherical lateralization was shown in posterior hippocampal networks.We found that the resting functional connectivity between posterior hippocampus and lingual gyrus(LG),posterior cingulate(PCC)in the left hemisphere were stronger than those in the right hemisphere(paired t-test,p<0.001).In addition,the resting functional connectivity between posterior hippocampus and thalamus(THA)in the right hemisphere was stronger than that in the left hemisphere(paired t-test,p<0.001).There was no significant difference in hippocampal-cortical functional connectivity between the left and right hemispheres across the anterior hippocampal networks.Experiment 3:Based on the analysis of hippocampal neural network and the resting functional connectivity analysis of seed points,our results showed that the anterior and posterior hippocampal networks involved differently in verbal and visual memory.Only the posterior hippocampal neural network is involved in verbal memory,while both the anterior and posterior hippocampal neural networks are related to visual memory.Anterior and posterior hippocampal neural network analysis showed that:Intra-hemispheric resting functional connectivity between left posterior hippocampus and posterior parahippocampal gyrus(PPHG)was positively correlated with CVLT-?Trail 2 Immediate Free Recall(verbal memory)(r=0.223,p=0.029).Intra-hemispheric resting functional connectivity between left posterior hippocampus and posterior cingulate(PCC)was negatively correlated with ROCF Immediate Recall(visual memory)(r=-0.217 p=0.034).Intra-hemispheric resting functional connectivity between left anterior hippocampus and temporal pole(TP)negatively correlated with ROCF Delayed Recall(visual memory)(r=-0.228,p=0.025).Seed-based functional connectivity analysis showed that:Functional connectivity between left posterior hippocampus and internal temporal lobe was correlated with CVLT-short delayed free recall(r=-0.39309,t=-4.179,cluster size=29).Functional connectivity between left posterior hippocampus and superior frontal lobe was correlated with ROCF immediate recall(r=-0.41596,t=-4.4583,cluster size=17).Functional connectivity between left anterior hippocampus and middle frontal lobe was correlated with ROCF immediate recall(r=-0.38257,t=-4.0359,cluster size=17).ConclusionsOur findings suggest that(1)voxel-level correlation analysis can be used to functionally segment the left and right hippocampus into anterior and posterior parts.This segmentation method has high accuracy and stability;(2)the anterior and posterior hippocampi are functionally related to different cortical brain regions and belong to different neural functional networks.There is hemisphere asymmetry between the anterior and posterior hippocampal neural networks,which the laterality of the posterior hippocampal neural networks is more obvious;(3)the anterior and posterior hippocampi and hippocampal networks involved differently in verbal and visual memories,which may have implications for a functionally synergetic and dissociable role of the hippocampus in different kinds of memory.The different imaging characteristics of hippocampus in resting-state functional MR imaging provided imaging basis for the study of neuromechanism in early memory damage,memory network remodeling and compensation.