Buzsáki György
Center for Molecular and Behavioral Neuroscience, Rutgers, The State University of New Jersey, Newark, 07102, USA.
Hippocampus. 2005;15(7):827-40. doi: 10.1002/hipo.20113.
Five key topics have been reverberating in hippocampal-entorhinal cortex (EC) research over the past five decades: episodic and semantic memory, path integration ("dead reckoning") and landmark ("map") navigation, and theta oscillation. We suggest that the systematic relations between single cell discharge and the activity of neuronal ensembles reflected in local field theta oscillations provide a useful insight into the relationship among these terms. In rats trained to run in direction-guided (1-dimensional) tasks, hippocampal cell assemblies discharge sequentially, with different assemblies active on opposite runs, i.e., place cells are unidirectional. Such tasks do not require map representation and are formally identical with learning sequentially occurring items in an episode. Hebbian plasticity, acting within the temporal window of the theta cycle, converts the travel distances into synaptic strengths between the sequentially activated and unidirectionally connected assemblies. In contrast, place representations by hippocampal neurons in 2-dimensional environments are typically omnidirectional, characteristic of a map. Generation of a map requires exploration, essentially a dead reckoning behavior. We suggest that omnidirectional navigation through the same places (junctions) during exploration gives rise to omnidirectional place cells and, consequently, maps free of temporal context. Analogously, multiple crossings of common junction(s) of episodes convert the common junction(s) into context-free or semantic memory. Theta oscillation can hence be conceived as the navigation rhythm through both physical and mnemonic space, facilitating the formation of maps and episodic/semantic memories.
在过去的五十年里，五个关键主题一直回荡在海马体 - 内嗅皮层（EC）研究中：情景记忆和语义记忆、路径整合（“航位推算”）和地标（“地图”）导航，以及theta振荡。我们认为，单细胞放电与局部场theta振荡中反映的神经元集合活动之间的系统关系，为深入理解这些术语之间的关系提供了有益的见解。在训练大鼠进行方向引导（一维）任务的实验中，海马体细胞集合按顺序放电，不同的集合在相反方向的奔跑中活跃，即位置细胞是单向的。这类任务不需要地图表征，并且在形式上与学习一个事件中依次出现的项目相同。赫布可塑性在theta周期的时间窗口内起作用，将行进距离转化为依次激活且单向连接的集合之间的突触强度。相比之下，海马神经元在二维环境中的位置表征通常是全向的，这是地图的特征。生成地图需要探索，本质上是一种航位推算行为。我们认为，在探索过程中通过相同地点（交汇点）的全向导航会产生全向位置细胞，从而产生不受时间背景影响的地图。类似地，事件的共同交汇点的多次交叉会将共同交汇点转化为无上下文或语义记忆。因此，theta振荡可以被视为穿越物理空间和记忆空间的导航节律，促进地图以及情景/语义记忆的形成。
