| 其他摘要 | Spatial navigation, an essential and integrated cognitive function in daily life, degenerates early in healthy aging and is highly susceptible to loss of deficits at advanced ages. Numerous studies have shown that the hippocampus provides critical neurological support for spatial navigation, and this region itself exhibits great aging sensitivity and heterogeneity. Therefore, elucidating the differential aging of hippocampal sub-regions and their different contributions to spatial navigation processes and their underlying mechanisms is key to a deeper understanding of the hippocampus and spatial navigation. In this paper, we propose a gradient topology law of the hippocampal axis to characterize the functional organization of the hippocampus and model hippocampus-related cognitive processes continuously along the hippocampal axis from head to tail. Specifically, one end of the axis (hippocampal tail) processes concrete and instantiated information, the other end of the axis (hippocampal head) processes abstract and generalized information, and the middle position function of the axis is determined by its topological distance from the head and tail endpoints. In this paper, we construct a paradigm for MRI functional imaging studies of natural situational spatial navigation, and use a combined cross一sectional and longitudinal experimental design to achieve verification of the above gradient law.
In Study 1, behavioral and MRI scan data were collected from 30 young people, and the spatial navigation performance of the young people kept improving with learning, indicating the validity of the experimental paradigm and the learning effect of the behavioral performance of spatial navigation. With the help of inter-voxel similarity analysis, representational similarity analysis, and multi-voxel pattern analysis, we measured functional gradients in three dimensions of the hippocampus during spatial navigation, including representational granularity gradient, representational organization gradient, and representational mapping gradient. On the representational granularity gradient, the hippocampus showed a decrease in representational granularity from head to tail during spatial navigation, reflecting a change in the scale of the hippocampal location domain, i.e., how large a spatial extent the voxels in this region respond to, which gradually decreases from the head to tail of the hippocampus; and on the representational organization gradient, the hippocampus showed a transition from conceptual to instantiated information organization from head to tail, which allows the hippocampus to take into account information on multiple abstract scales to aid navigation, such as specific routes versus groups of routes with the same rules; and in terms of representational mapping relationships, we found a one-to-one mapping of neural representations to stimuli only in the caudal region, implying that information was represented concretely and correspondingly to stimuli in the hippocampal caudal region, rather than in a many-to-one or one-to-many mapping relationship. In Study 2, based on the data from Study 1, another 30 elderly participants were recruited to compare the spatial navigation performance of young people with that of the elderly, and it was evident that spatial navigation was impaired in the elderly. And a significant flattening was demonstrated on all three gradients of the hippocampal axis, including an overall gradient level change and a reduction in the difference in hippocampal cephalocaudal function. This reduction in cephalocaudal function was mainly due to the impairment of hippocampal head function when comparing the results in young adults. Study 3 followed older adults at 2-year intervals, and by comparing the data from the two visits, this study found that the overall level of representational size gradient in older adults continued to age over the two years, while the representational tissue and representational mapping gradients remained stable.
In this study, we systematically explored the function of the hippocampus and proposed the law of gradient topology in the hippocampus, and we verified the three dimensional gradients, including representational granularity, representational organization, and representational mapping gradients, present in the hippocampal axis during spatial navigation. Future research is expected to extend this hippocampal gradient topological law to other cognitive domains and provide a unified and guiding research framework for conducting hippocampal research. In addition, using cross-sectional and longitudinal data, we clarified the influence of aging factors on the hippocampal functional gradient, which will continue to slow down during individual aging, and the dominant factor of this process may come from damage to the hippocampal head. |
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