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The hippocampus forms unique neural representations for distinct experiences, supporting the formation of different memories. 1,2,3,4,5,6 Hippocampal representations gradually change over time as animals repeatedly visit the same familiar environment ("representational drift"). 7,8,9,10,11,12 Such drift has also been observed in other brain areas, such as the parietal, 13,14 visual, 15,16,17 auditory, 18,19 and olfactory 20 cortices. While the underlying mechanisms of representational drift remain unclear, a leading hypothesis suggests that it results from ongoing learning processes. 20,21,22 According to this hypothesis, because the brain uses the same neural substrates to support multiple distinct representations, learning of novel stimuli or environments leads to changes in the neuronal representation of a familiar one. If this is true, we would expect drift in a given environment to increase following new experiences in other, unrelated environments (i.e., off-context experiences). To test this hypothesis, we longitudinally recorded large populations of hippocampal neurons in mice while they repeatedly visited a familiar linear track over weeks. We introduced off-context experiences by placing mice in a novel environment for 1 h after each visit to the familiar track. Contrary to our expectations, these novel episodes decreased place cells' representational drift. Our findings are consistent with a model in which representations of distinct memories occupy different areas within the neuronal activity space, and the drift of each of them within that space is constrained by the area occupied by the others. [Display omitted] • Ca²⁺ imaging was used to quantify representational drift in a familiar environment • Mice were exposed to novel enriched environments to boost hippocampal learning • Off-context novel experiences constrain hippocampal drift rather than accelerate it • Off-context novel experiences specifically affect drift in spatial tuning A leading hypothesis suggests that representational drift in a given context stems from ongoing learning processes, such as those induced by novelty in other contexts. However, Elyasaf et al. find evidence to the contrary: novel experiences constrain hippocampal representational drift rather than accelerating it. [ABSTRACT FROM AUTHOR]

Navigation requires the knowledge of one's location ('map') (1-3) and direction in space ('compass') (4,5). The neural representation of spatial location in mammals includes place-cells which compute position (1,6-12), together [...], Navigation requires a sense of direction ('compass'), which in mammals is thought to be provided by head-direction cells, neurons that discharge when the animal's head points to a specific azimuth. However, it remains unclear whether a three-dimensional (3D) compass exists in the brain. Here we conducted neural recordings in bats, mammals well-adapted to 3D spatial behaviours, and found head-direction cells tuned to azimuth, pitch or roll, or to conjunctive combinations of 3D angles, in both crawling and flying bats. Head-direction cells were organized along a functional-anatomical gradient in the presubiculum, transitioning from 2D to 3D representations. In inverted bats, the azimuth-tuning of neurons shifted by 180°, suggesting that 3D head direction is represented in azimuth x pitch toroidal coordinates. Consistent with our toroidal model, pitch-cell tuning was unimodal, circular, and continuous within the available 360° of pitch. Taken together, these results demonstrate a 3D head-direction mechanism in mammals, which could support navigation in 3D space.

The integration of olfactory and spatial information is critical for guiding animal behavior. The lateral entorhinal cortex (LEC) is reciprocally interconnected with cortical areas for olfaction and the hippocampus and thus ideally positioned to encode odor-place associations. Here, we used mini-endoscopes to record neural activity in the mouse piriform cortex (PCx) and LEC. We show that in head-fixed mice, odor identity could be decoded from LEC ensembles, but less accurately than from PCx. In mice freely navigating a linear track, LEC ensemble activity at the odor ports was dominated by spatial information. Spatial position along the linear track could be decoded from LEC and PCx activity, however, PCx but not LEC exhibited strong behavior-driven modulation of positional information. Together, our data reveal that information about odor cues and spatial context is differentially encoded along the PCx-LEC axis., Competing Interests: Competing interests Authors declare that they have no competing interests.