The memory for time and space differentially engages the proximal and distal parts of the hippocampal subfields CA1 and CA3

A well-accepted model of episodic memory involves the processing of spatial and non-spatial information by segregated pathways and their association within the hippocampus. However, these pathways project to distinct proximodistal levels of the hippocampus. Moreover, spatial and non-spatial subnetworks segregated along this axis have been recently described using memory tasks with either a spatial or a non-spatial salient dimension. Here, we tested whether the concept of segregated subnetworks and the traditional model are reconcilable by studying whether activity within CA1 and CA3 remains segregated when both dimensions are salient, as is the case for episodes. Simultaneously, we investigated whether temporal or spatial information bound to objects recruits similar subnetworks as items or locations per se, respectively. To do so, we studied the correlations between brain activity and spatial and/or temporal discrimination ratios in proximal and distal CA1 and CA3 by detecting Arc RNA in mice. We report a robust proximodistal segregation in CA1 for temporal information processing and in both CA1 and CA3 for spatial information processing. Our results suggest that the traditional model of episodic memory and the concept of segregated networks are reconcilable, to a large extent and put forward distal CA1 as a possible “home” location for time cells.

公认的情景记忆（episodic memory）模型认为，其通过分离的神经通路处理空间与非空间信息，并在海马体（hippocampus）内实现这些信息的关联。然而，这些通路投射至海马体的不同近远轴层级。近期，研究者借助带有空间或非空间显著性维度的记忆任务，已描述了沿该轴分离的空间与非空间子网络。本研究中，我们通过检测小鼠脑内的Arc核糖核酸（Arc RNA），探究当情景记忆所具备的双显著性维度同时存在时，CA1与CA3脑区的神经活动是否仍保持分离，以此验证分离子网络概念与传统模型是否可兼容。同时，我们还研究了绑定于物体的时序或空间信息，是否分别与单纯的项目或位置激活相似的子网络。为此，我们分析了小鼠近端与远端CA1、CA3脑区的脑活动与空间和/或时序辨别率之间的相关性。我们发现，时序信息处理在CA1脑区存在显著的近远轴分离现象，而空间信息处理在CA1与CA3脑区均存在该分离特征。本研究结果表明，情景记忆的传统模型与分离网络概念在很大程度上是可兼容的，并提出远端CA1可能是时间细胞（time cells）的潜在驻留区域。