Prompt Structure Reorganizes In-Context Sequential Memory in LLMs

Large language models (LLMs) are sensitive to where information appears in their context. Research on human sequential memory shows that position is not a single quantity: beyond its absolute rank in the whole sequence, an item in a grouped sequence carries a global rank, the position of its group, and a local rank, its position within the group, which are represented along separable neural dimensions. Existing evaluations of LLMs ask what appears at a position, not where an item is, so whether LLMs hold several positional descriptions of an item, and whether the prompt determines which one is accessible, remains unknown. Here we show, across five LLMs probed with controlled English and Chinese sequences of familiar words, that structural cues reorganize positional retrieval. Stating that the sequence consists of familiar words mainly improves reports of a target's group and within-group position. Making group boundaries visible improves these reports further, but often lowers exact absolute-position accuracy and redirects absolute-position errors toward the displayed grouping. These boundary effects persist after the familiar-word structure is destroyed and when group lengths vary, where exact absolute reports can collapse while reported positions remain strongly rank-correlated with true positions. Retrieval thus depends on the match between the organization a cue emphasizes and the coordinate a query demands, paralleling hierarchical working memory in humans, although in LLMs this organization is largely cue-dependent. Prompts therefore act as organizational interventions on in-context sequential memory, and memory systems should be evaluated across the positional readouts they support and for interference between them.