A flexible hippocampal population code for experience relative to reward

2-photon imaging and behavioral data from hippocampal area CA1 during virtual reality navigation in mice. Included in Sosa, Plitt, & Giocomo, "A flexible hippocampal population code for experience relative to reward," Nature Neuroscience.

To reinforce rewarding behaviors, events leading up to and following rewards must be remembered. Hippocampal place cell activity spans spatial and non-spatial episodes, but whether hippocampal activity encodes entire sequences of events relative to reward is unknown. To test this, we performed two-photon imaging of hippocampal CA1 as mice navigated virtual environments with changing hidden reward locations. When the reward moved, a subpopulation of neurons updated their firing fields to the same relative position with respect to reward, constructing behavioral timescale sequences spanning the entire task. Over learning, this reward-relative representation became more robust as additional neurons were recruited, and changes in reward-relative firing often preceded behavioral adaptations following reward relocation. Concurrently, the spatial environment code was maintained through a parallel, dynamic subpopulation rather than through dedicated cell classes. These findings reveal how hippocampal ensembles flexibly encode multiple aspects of experience while amplifying behaviorally relevant information.

小鼠虚拟现实导航过程中海马CA1区的双光子成像（2-photon imaging）及行为学数据。该数据包含于Sosa、Plitt与Giocomo发表在《自然神经科学》（Nature Neuroscience）的论文《一种针对奖励相关经验的灵活海马群体编码》（"A flexible hippocampal population code for experience relative to reward"）中。

为强化奖赏行为，奖赏发生前后的事件需被记忆。海马位置细胞（hippocampal place cell）的活动涵盖空间与非空间事件，但海马活动是否编码奖赏相关的完整事件序列尚不清楚。为验证这一点，我们在小鼠导航至隐藏奖赏位置变化的虚拟环境时，对其海马CA1区进行双光子成像。当奖赏位置移动时，一部分神经元亚群会将其发放场（firing fields）更新至相对于奖赏的相同位置，构建覆盖整个任务的行为时间尺度序列。随着学习进程，更多神经元被募集，这种奖赏相关表征变得更加稳健；且奖赏相关发放的变化往往先于奖赏位置改变后的行为适应。同时，空间环境编码通过平行的动态神经元亚群而非专用细胞类群得以维持。这些发现揭示了海马神经元集群如何在放大行为相关信息的同时，灵活编码经验的多个方面。