Tambini Arielle, Miller Jacob, Ehlert Luke, Kiyonaga Anastasia, D'Esposito Mark
Center for Biomedical Imaging and Neuromodulation, Nathan Kline Institute for Psychiatric Research, Orangeburg, NY.
Department of Psychiatry, New York University Grossman School of Medicine, New York, NY.
bioRxiv. 2023 Apr 7:2023.04.06.535935. doi: 10.1101/2023.04.06.535935.
Influential views of systems memory consolidation posit that the hippocampus rapidly forms representations of specific events, while neocortical networks extract regularities across events, forming the basis of schemas and semantic knowledge. Neocortical extraction of schematic memory representations is thought to occur on a protracted timescale of months, especially for information that is unrelated to prior knowledge. However, this theorized evolution of memory representations across extended timescales, and differences in the temporal dynamics of consolidation across brain regions, lack reliable empirical support. To examine the temporal dynamics of memory representations, we repeatedly exposed human participants to structured information via sequences of fractals, while undergoing longitudinal fMRI for three months. Sequence-specific activation patterns emerged in the hippocampus during the first 1-2 weeks of learning, followed one week later by high-level visual cortex, and subsequently the medial prefrontal and parietal cortices. Schematic, sequence-general representations emerged in the prefrontal cortex after 3 weeks of learning, followed by the medial temporal lobe and anterior temporal cortex. Moreover, hippocampal and most neocortical representations showed sustained rather than time-limited dynamics, suggesting that representations tend to persist across learning. These results show that specific hippocampal representations emerge early, followed by both specific and schematic representations at a gradient of timescales across hippocampal-cortical networks as learning unfolds. Thus, memory representations do not exist only in specific brain regions at a given point in time, but are simultaneously present at multiple levels of abstraction across hippocampal-cortical networks.
关于系统记忆巩固的主流观点认为，海马体快速形成特定事件的表征，而新皮质网络提取跨事件的规律，形成图式和语义知识的基础。新皮质对图式记忆表征的提取被认为发生在长达数月的时间尺度上，尤其是对于与先验知识无关的信息。然而，这种关于记忆表征在长时间尺度上的理论演变，以及不同脑区巩固的时间动态差异，缺乏可靠的实证支持。为了研究记忆表征的时间动态，我们通过分形序列让人类参与者反复接触结构化信息，同时进行为期三个月的纵向功能磁共振成像研究。在学习的前1 - 2周，海马体中出现了序列特异性激活模式，一周后高级视觉皮层出现激活，随后是内侧前额叶和顶叶皮层。学习3周后，前额叶皮层出现了图式化的、序列一般性的表征，随后是内侧颞叶和颞叶前部皮层。此外，海马体和大多数新皮质的表征显示出持续而非限时的动态变化，这表明表征在学习过程中倾向于持续存在。这些结果表明，特定的海马体表征早期出现，随后随着学习的展开，在海马体 - 皮质网络的不同时间尺度上出现特定和图式化的表征。因此，记忆表征并非仅在特定时间点存在于特定脑区，而是同时存在于海马体 - 皮质网络的多个抽象层次上。
