DRIFT AND STABILITY IN MEMORY-RELATED ACTIVITY OF HIPPOCAMPAL NEURONS IN A SPATIAL ALTERNATION TASK

The hippocampus plays a pivotal role in spatial navigation and memory encoding, yet the stability of its representations over time remains incompletely understood. This work investigates spatial representations in the CA1 region of the mouse hippocampus using longitudinal in vivo calcium imaging during five consecutive days in a spatial alternation task (modified M-maze). Using head-mounted miniscopes, neuronal activity from freely behaving mice was recorded and analysed to assess position encoding, representational drift, and task-relevant information processing. The fraction of neurons with place-specific tuning fluctuated between 30% and 75% over days. Notably, ~14% of recorded neurons encoded trial outcomes (rewards) within their calcium activity, while around half of the encoder cells lacked specific place selectivity.

Support vector machine classifiers decoded spatial position from CA1 activity with high accuracy within sessions, but failed to generalise across days, indicating substantial representational drift. Analysis of individual cells revealed heterogeneous temporal trends with stable and unstable behavior during the observation periods, respectively. These findings display a dynamic yet functionally resilient hippocampal code, reconciling neural plasticity with the preservation of behavioural performance and supporting models of memory encoding in a “stable-core/dynamic-periphery” architecture. The presence of neurons encoding successful trials independent of location suggests that the CA1 region might be involved in abstract task-variable encoding beyond spatial representation.