Jensen O, Lisman J E
Volen Center for Complex Systems, Brandeis University, Waltham, MA 02254, USA.
Learn Mem. 1996 Sep-Oct;3(2-3):279-87. doi: 10.1101/lm.3.2-3.279.
Hippocampal recordings show that different place cells fire at different phases during the same theta oscillation, probably at the peak of different gamma cycles. As the rat moves through the place field of a given cell, the phase of firing during the theta cycle advances progressively. In this paper we have sought to determine whether a recently developed model of hippocampal and cortical memory function can explain this phase advance and other properties of place cells. According to this physiologically based model, the CA3 network stores information about the sequence of places traversed during learning. Here we show that the phase advance can be understood if it is assumed that the hippocampus is in a recall mode that operates when the animal is already familiar with a path. In this mode, sensory information about the current position triggers recall of the upcoming 5-6 places (memories) in the path at a rate of one memory per gamma cycle. The model predicts that the average phase advance will be one gamma cycle per theta cycle, a value in reasonable agreement with the data. The model also correctly accounts for (1) the fact that the firing of a place cell occurs during approximately 7 theta cycles (on average) as the animal crosses the place field; (2) the observation that the phase of place cell firing depends more systematically on position than on time; and (3) the fact that traversal of an already familiar path produces further modifications (shifts the firing of a cell to an earlier position in the path). This later finding suggests that recall of previously stored information strengthens the memory of that information. In the model, this occurs because of a novel role of N-methyl-D-aspartate channels in recall. The general success of the model provides support for the idea that the hippocampus stores sequence information and makes predictions of expected positions during gamma-frequency recall.
海马体记录显示，在同一θ振荡期间，不同的位置细胞在不同阶段放电，可能是在不同γ周期的峰值。当大鼠穿过给定细胞的位置野时，θ周期内的放电相位会逐渐推进。在本文中，我们试图确定一种最近开发的海马体和皮质记忆功能模型是否能够解释这种相位推进以及位置细胞的其他特性。根据这个基于生理学的模型，CA3网络存储有关学习过程中所经过位置序列的信息。在这里我们表明，如果假设海马体处于回忆模式，即在动物已经熟悉一条路径时起作用，那么相位推进就可以得到解释。在这种模式下，关于当前位置的感觉信息会触发对路径中接下来5 - 6个位置（记忆）的回忆，每个γ周期回忆一个记忆。该模型预测，平均相位推进将是每个θ周期一个γ周期，这一数值与数据合理相符。该模型还正确地解释了以下几点：（1）当动物穿过位置野时，位置细胞的放电大约会在7个θ周期内发生（平均）；（2）观察到位置细胞放电的相位更系统地依赖于位置而非时间；（3）已经熟悉的路径的遍历会产生进一步的修改（将细胞的放电转移到路径中更早的位置）。这一后来的发现表明，对先前存储信息的回忆会加强对该信息的记忆。在模型中，这是由于N - 甲基 - D - 天冬氨酸通道在回忆中的新作用而发生的。该模型的总体成功为海马体存储序列信息并在γ频率回忆期间预测预期位置这一观点提供了支持。
