CHARACTERISTICS OF PLACE FIELD FORMATION IN THE HIPPOCAMPAL CA1 AND CA3 REGIONS

Hippocampal pyramidal cells (PCs) exhibit spatially tuned activity (i.e ’place fields’, PFs) producing environment-specific spatial codes. PFs can emerge in a single lap via behavioral timescale synaptic plasticity (BTSP) extensively characterised in CA1PCs (Bittner et al, 2015, 2017), where activity in the first lap is stronger and is shifted forward relative to later laps. BTSP has also been reported in CA3PCs, but with no shift (Li et al., 2024, Madar et al., 2025). BTSP is associated with strong reorganization of the synaptic weights of active inputs, but how these synaptic changes influence the spatial code in other, related environments is not known. Furthermore, PF formation (PFF) in CA1PCs can manifest through multiple mechanisms (Sümegi et al., 2025), justifying further inquiry. Here we used two-photon Ca²⁺ imaging in CA1PCs and CA3PCs of head-fixed Thy1-GCaMP6s mice navigating in two randomly alternating familiar virtual environments. We calculated two features of PFF: activity strength during formation relative to subsequent laps (‘gain’) and shift of PF position after formation. We show that new PFs exhibit larger backward shifts in CA1PCs than CA3PCs while gain distributions are similarly positively skewed, indicating that BTSP is prominent in PFF but its properties differ in the two regions. Furthermore, we found that PFF in one environment can lead to elevated activity in the other environment, especially at locations with similar sensory cues. Our results reveal variability in PFF across pyramidal cells and cross-context interactions between spatial codes as a consequence of synaptic plasticity mechanisms.