1 Between-subject prediction reveals a shared representational geometry in the rodent 2 hippocampus 3

2 hippocampus 3 Hung-Tu Chen, Jeremy R. Manning, Matthijs A. A. van der Meer* 4 Department of Psychological & Brain Sciences, Dartmouth College, Hanover, NH 03755 5 *Correspondence: mvdm@dartmouth.edu 6 Summary 7 The rodent hippocampus constructs statistically independent representations across 8 environments (“global remapping”) and assigns individual neuron firing fields to locations within 9 an environment in an apparently random fashion, processes thought to contribute to the role of 10 the hippocampus in episodic memory. This random mapping implies that it should be 11 challenging to predict hippocampal encoding of a given experience in one subject based on the 12 encoding of that same experience in another subject. Contrary to this prediction, we find that by 13 constructing a common representational space across rats (“hyperalignment”), we can predict 14 data of “right” trials (R) on a T-maze in a target rat based on 1) the “left” trials (L) of the target 15 rat, and 2) the relationship between L and R trials from a different source rat. These 16 cross-subject predictions relied on ensemble activity patterns including both firing rate and field 17 location, and outperformed a number of control mappings, such as those based on permuted 18 data that broke the relationship between L and R activity for individual neurons, and those 19 based solely on within-subject prediction. This work constitutes proof-of-principle for successful 20 cross-subject prediction of ensemble activity patterns in the hippocampus, and provides new 21 insights in understanding how different experiences are structured, enabling further work 22 identifying what aspects of experience encoding are shared vs. unique to an individual. 23