Cognitive tasks affect the relationship between representational pattern similarity and subsequent item memory in the hippocampus

Episodic memories are records of personally experienced events, coded neurally via the hippocampus and surrounding medial temporal lobe cortex. Information about the neural signal corresponding to a memory representation can be measured in fMRI data when the pattern across voxels is examined. Prior studies have found that similarity in the voxel patterns across repetition of a to-be-remembered stimulus predicts later memory retrieval, but the results are inconsistent across studies. The current study investigates the possibility that cognitive goals (defined here via the task instructions given to participants) during encoding affect the voxel pattern that will later support memory retrieval, and therefore that neural representations cannot be interpreted based on the stimulus alone. The behavioral results showed that exposure to variable cognitive tasks across repetition of events benefited subsequent memory retrieval. Voxel patterns in the hippocampus indicated a significant interaction between cognitive tasks (variable vs. consistent) and memory (remembered vs. forgotten) such that reduced voxel pattern similarity for repeated events with variable cognitive tasks, but not consistent cognitive tasks, supported later memory success. There was no significant interaction in neural pattern similarity between cognitive tasks and memory success in medial temporal cortices or lateral occipital lobe. Instead, higher similarity in voxel patterns in right medial temporal cortices was associated with later memory retrieval, regardless of cognitive task. In conclusion, we found that the relationship between pattern similarity across repeated encoding and memory success in the hippocampus (but not medial temporal lobe cortex) changes when the cognitive task during encoding does or does not vary across repetitions of the event.

情景记忆（Episodic memories）是个体亲身经历事件的记录，由海马体（hippocampus）及周围内侧颞叶皮层（medial temporal lobe cortex）进行神经编码。当对体素（voxel）间的激活模式进行检视时，可通过功能磁共振成像（fMRI）数据测得与记忆表征对应的神经信号。既往研究发现，待记忆刺激重复呈现时体素模式的相似性，能够预测后续的记忆提取效果，但不同研究间的结果并不一致。本研究旨在探讨一种可能性：编码阶段的认知目标（本文通过给被试的任务指令进行定义）会影响后续支持记忆提取的体素模式，因此神经表征不能仅通过刺激本身进行解读。行为实验结果显示，在事件重复呈现过程中施加多变的认知任务，能够提升后续的记忆提取表现。海马体的体素模式分析显示，认知任务（多变vs. 固定）与记忆状态（记住vs. 遗忘）之间存在显著交互作用：即仅当事件重复伴随多变认知任务时，重复事件的体素模式相似性降低，才会支持后续的记忆成功；而固定认知任务组未出现该效应。在内侧颞叶皮层或外侧枕叶中，未观察到认知任务与记忆成功之间在神经模式相似性上的显著交互作用。与之相反，右侧内侧颞叶皮层的体素模式相似性越高，越与后续的记忆提取相关，且不受认知任务的影响。综上，本研究发现，当编码阶段的认知任务在事件重复过程中发生变化或保持固定时，海马体（而非内侧颞叶皮层）中重复编码阶段的模式相似性与记忆成功之间的关联会发生改变。