Four phases and temporal threshold of population calcium response during locomotion in cortical astrocytes

Calcium activity is a major form of astrocyte excitability essential to their function and plasticity. Astrocytic calcium events arise both spontaneously and in response to behaviour 1. In this study, we examined how these unitary events shape population-level calcium activity during quiescence and locomotion. Using two-photon imaging of astrocytic calcium in the primary somatosensory cortex (S1) of awake mice running on a freely rotating disk, we found that spontaneous fluctuations during quiescence were driven mainly by the co-occurrence of new calcium events rather than by their enlargement. During locomotion, population responses progressed through four phases: the emergence of new events, their merging into a superevent, fragmentation of this superevent, and an afterburst. New events appeared immediately at locomotion onset, indicating a rapid astrocytic response. The response also exhibited a temporal threshold of [~]5 seconds: when locomotion was shorter than this, the superevent phase failed to develop, and the population response was markedly reduced. In suprathreshold responses, the superevent displayed a {Delta}F/F intensity pattern that reproduced reliably across locomotion episodes. Hotspots within this pattern had shorter latencies and were also more active during quiescence, suggesting that the response is at least partly deterministic. Consistent with this, we were able to reproduce both the response phases and the superevent pattern using a dynamic mode decomposition with control (DMDc) model driven by synthetic or real locomotion inputs. Together, these findings dissect the population calcium response into quantifiable properties of unitary calcium events and reveal the formation and reproducibility of the superevent pattern, offering a potential framework for the astrocytic component of the engram 2,3.