HIPPOCAMPAL TIME CELLS ENCODE SUB-SECOND TRACE INTERVALS WITHOUT REMAPPING WHEN INTERVALS CHANGE

The hippocampus encodes temporal information across diverse timescales, but whether sub-second and multi-second timing relies on similar neural mechanisms remains unclear. We addressed this by examining time cell dynamics during trace eye-blink conditioning with varying increments of trace intervals (250ms to 550ms) using 2-photon calcium imaging from dorsal hippocampal CA1 pyramidal neurons in head-fixed mice (n=4 animals, >7000 cells across 120 sessions). Critically, we employed an interleaved protocol alternating 250ms and 550ms trials within a session to eliminate learning-dependent confounds in remapping analyses. We discovered that sub-second time cells employ different strategies than those reported in working memory tasks. The time cells maintained remarkably consistent peak timing across 250ms and 550ms trace intervals presented in interleaved blocks (r=0.883, p<0.00001). This stability occurred despite mice successfully adjusting eye-blink timing to match trace durations. Surprisingly, the activity of time cell sequences extended for more than 5 seconds post-stimulus, far beyond the behavioral relevance window and trace durations. These sequences subdivide into distinct epochs characterized by different time cell proportions and temporal tuning widths. Time cells were present before trace association learning but declined significantly during behavioral extinction (from 13.7±10.6% to 2.4±2.0%, p<0.0001), while at the same time the population's general activity remained the same. This is suggestive of an active suppression of time cells rather than reversion to pre-learning states. Overall, these findings reveal distinct features of sub-second temporal encoding in the hippocampal population.