Ng Chi-Wing, Elias Gabriel A, Asem Judith S A, Allen Timothy A, Fortin Norbert J
Center for the Neurobiology of Learning and Memory, Department of Neurobiology and Behavior, University of California Irvine, Irvine, CA 92697, USA.
Department of Psychology, Florida International University, Miami, FL 33199, USA.
Behav Brain Res. 2018 Nov 15;354:39-47. doi: 10.1016/j.bbr.2017.10.015. Epub 2017 Oct 28.
The hippocampus plays a critical role in the memory for sequences of events, a defining feature of episodic memory. To shed light on the fundamental mechanisms supporting this capacity, we recently recorded neural activity in CA1 as rats performed a nonspatial odor sequence memory task. Our main finding was that, while the animals' location and behavior remained constant, a proportion of CA1 neurons fired differentially to odors depending on whether they were presented in or out of sequence (sequence cells). Here, we further examined if such sequence coding varied along the distal-to-proximal axis of the dorsal CA1 region (distal: toward subiculum; proximal: toward CA3). Differences in information processing along this axis have been suggested by recent anatomical and electrophysiological evidence that odor information may be more strongly represented in the distal segment, whereas spatial information may be more strongly represented in the proximal segment. Recorded neurons were grouped into four arbitrary sections of dorsal CA1, ranging from distal to proximal. We found that, although sequence cell coding was observed across the distal-to-proximal extent of CA1 from which we recorded, it was significantly higher in intermediate CA1, a region with more balanced anatomical input from lateral and medial entorhinal regions. More specifically, in that particular segment of CA1, we observed a significant increase in the magnitude of sequence coding of all cells, as well as in the sequential information content of sequence cells. Importantly, a different pattern was observed when examining the distribution of spatial coding from the same electrodes. Consistent with previous reports, our results suggest that spatial information was more strongly represented in the proximal section of CA1 (higher proportion of cells with place fields). These findings indicate that nonspatial sequence memory coding is not uniformly distributed along the transverse axis of CA1, and that this distribution does not simply follow the expected gradient based on the stimulus modality or the degree of spatial selectivity. Instead, the observed distribution suggests this form of sequence coding may be associated with convergent input from lateral and medial entorhinal regions, which is present throughout the proximodistal axis but greater in intermediate CA1.
海马体在事件序列记忆中起着关键作用,这是情景记忆的一个决定性特征。为了阐明支持这种能力的基本机制,我们最近在大鼠执行非空间气味序列记忆任务时记录了CA1区的神经活动。我们的主要发现是,虽然动物的位置和行为保持不变,但一部分CA1神经元根据气味是按顺序呈现还是乱序呈现而产生不同的放电(序列细胞)。在此,我们进一步研究了这种序列编码是否沿背侧CA1区的远侧到近侧轴变化(远侧:朝向海马下脚;近侧:朝向CA3)。最近的解剖学和电生理学证据表明,沿该轴的信息处理存在差异,即气味信息可能在远侧段有更强的表征,而空间信息可能在近侧段有更强的表征。记录的神经元被分为背侧CA1区从远侧到近侧的四个任意部分。我们发现,虽然在我们记录的CA1区从远侧到近侧的范围内都观察到了序列细胞编码,但在中间CA1区显著更高,该区域从外侧和内侧内嗅区接受的解剖学输入更为平衡。更具体地说,在CA1区的那个特定部分,我们观察到所有细胞的序列编码幅度以及序列细胞的序列信息含量都有显著增加。重要的是,在检查来自同一电极的空间编码分布时观察到了不同的模式。与先前的报道一致,我们的结果表明空间信息在CA1区的近侧部分有更强的表征(具有位置野的细胞比例更高)。这些发现表明,非空间序列记忆编码并非沿CA1区的横轴均匀分布, 并且这种分布并不简单地遵循基于刺激模式或空间选择性程度的预期梯度。相反,观察到的分布表明这种序列编码形式可能与来自外侧和内侧内嗅区的汇聚输入有关,这种输入在整个近远轴上都存在,但在中间CA1区更大。