Harvard Medical School, Martinos Center for Biomedical Imaging, Brigham and Women's Hospital, Boston, MA, USA.
Neuropsychologia. 2012 Dec;50(14):3764-74. doi: 10.1016/j.neuropsychologia.2012.08.021. Epub 2012 Sep 13.
The posteromedial cortex (PMC) is strongly linked to episodic memory and age-related memory deficits. The PMC shows deactivations during a variety of demanding cognitive tasks as compared to passive baseline conditions and has been associated with the default-mode of the brain. Interestingly, the PMC exhibits opposite levels of functional MRI activity during encoding (learning) and retrieval (remembering), a pattern dubbed the encoding/retrieval flip (E/R-flip). Yet, the exact role of the PMC in memory function has remained unclear. This review discusses the possible neurofunctional and clinical significance of the E/R-flip pattern. Regarding neurofunctional relevance, we will review four hypotheses on PMC function: (1) the internal orienting account, (2) the self-referential processing account, (3) the reallocation account, and (4) the bottom-up attention account. None of these accounts seem to provide a complete explanation for the E/R-flip pattern in PMC. Regarding clinical relevance, we review work on aging and Alzheimer's disease, indicating that amyloid deposits within PMC, years before clinical memory deficits become apparent. High amyloid burden within PMC is associated with detrimental influences on memory encoding, in particular, the attenuation of beneficial PMC deactivations. Finally, we discuss functional subdivisions within PMC that help to provide a more precise picture of the variety of signals observed within PMC. Collective data from anatomical, task-related fMRI and resting-state studies all indicate that the PMC is composed of three main regions, the precuneus, retrosplenial, and posterior cingulate cortex, each with a distinct function. We will conclude with a summary of the findings and provide directions for future research.
后内侧皮质（PMC）与情景记忆和与年龄相关的记忆缺陷密切相关。与被动基线条件相比，PMC 在各种高要求的认知任务中表现出去激活，并且与大脑的默认模式相关。有趣的是，PMC 在编码（学习）和检索（记忆）期间表现出相反水平的功能磁共振成像（fMRI）活动，这种模式被称为编码/检索翻转（E/R-翻转）。然而，PMC 在记忆功能中的确切作用仍不清楚。这篇综述讨论了 E/R-翻转模式的可能神经功能和临床意义。关于神经功能相关性，我们将回顾 PMC 功能的四个假设：（1）内部定向假说，（2）自我参照加工假说，（3）重新分配假说，以及（4）自上而下注意假说。这些假设似乎都不能完全解释 PMC 中的 E/R-翻转模式。关于临床相关性，我们回顾了衰老和阿尔茨海默病的研究工作，表明在临床记忆缺陷出现之前数年，PMC 内的淀粉样蛋白沉积。PMC 内的高淀粉样蛋白负荷与记忆编码的有害影响有关，特别是对有益的 PMC 去激活的衰减。最后，我们讨论了 PMC 内的功能细分，以帮助提供 PMC 内观察到的各种信号的更精确图像。来自解剖学、任务相关功能磁共振成像（fMRI）和静息状态研究的集体数据均表明，PMC 由三个主要区域组成，即楔前叶、后扣带回和后顶叶皮质，每个区域都具有独特的功能。我们将总结研究结果，并为未来的研究提供方向。
