Kavli Institute for Systems Neuroscience and Centre for the Biology of Memory, Norwegian University of Science and Technology, Olav Kyrres gate 9, MTFS, 7489 Trondheim, Norway.
Nature. 2011 Sep 28;478(7368):246-9. doi: 10.1038/nature10439.
The ability to recall discrete memories is thought to depend on the formation of attractor states in recurrent neural networks. In such networks, representations can be reactivated reliably from subsets of the cues that were present when the memory was encoded, at the same time as interference from competing representations is minimized. Theoretical studies have pointed to the recurrent CA3 system of the hippocampus as a possible attractor network. Consistent with predictions from these studies, experiments have shown that place representations in CA3 and downstream CA1 tolerate small changes in the configuration of the environment but switch to uncorrelated representations when dissimilarities become larger. However, the kinetics supporting such network transitions, at the subsecond timescale, is poorly understood. Here we show in rats that instantaneous transformation of the spatial context does not change the hippocampal representation all at once but is followed by temporary bistability in the discharge activity of CA3 ensembles. Rather than sliding through a continuum of intermediate activity states, the CA3 network undergoes a short period of competitive flickering between preformed representations of the past and present environment before settling on the latter. Network flickers are extremely fast, often with complete replacement of the active ensemble from one theta cycle to the next. Within individual cycles, segregation is stronger towards the end, when firing starts to decline, pointing to the theta cycle as a temporal unit for expression of attractor states in the hippocampus. Repetition of pattern-completion processes across successive theta cycles may facilitate error correction and enhance discriminative power in the presence of weak and ambiguous input cues.
离散记忆的回忆能力被认为依赖于递归神经网络中吸引子状态的形成。在这样的网络中，当记忆被编码时存在的提示的子集可以可靠地重新激活表示，同时最小化来自竞争表示的干扰。理论研究指出海马体的 CA3 系统可能是一个吸引子网络。与这些研究的预测一致，实验表明 CA3 和下游 CA1 中的位置表示可以容忍环境配置的微小变化，但当差异变大时，会切换到不相关的表示。然而，在亚秒级的时间尺度上，支持这种网络转变的动力学还知之甚少。在这里，我们在大鼠中表明，空间上下文的瞬时转换不会立即改变海马体的表示，而是随后在 CA3 集合的放电活动中出现暂时的双稳定性。CA3 网络不是通过连续的中间活动状态滑动，而是在选择后者之前，在过去和现在环境的预先形成的表示之间经历短暂的竞争闪烁。网络闪烁非常快，通常在下一个 theta 周期内，活动集合完全被替换。在单个周期内，当发射开始下降时，分离性更强，这表明 theta 周期是在海马体中表达吸引子状态的时间单位。在连续的 theta 周期中重复模式完成过程可能有助于在存在弱和模糊输入提示时进行错误校正和提高辨别力。
