Summary
A bioRxiv preprint comparing rats with Chinese tree shrews reports that tree-shrew CA1 uses a hybrid spatial code, combining weaker position tuning with more multiplexed neural activity. The findings suggest that spatial representations can change within a conserved hippocampal circuit while retaining core network properties.
A comparison of rats and Chinese tree shrews suggests that the hippocampus can change how it represents space without replacing the underlying circuit architecture. In a bioRxiv preprint, researchers recorded activity from the CA1 region of the hippocampus while the two species performed the same tasks and were analysed using the same methods.
The rat hippocampus is commonly described as using a position-dominant code: populations of neurons are strongly associated with particular locations. The researchers report that tree-shrew CA1 instead used a hybrid representation. Position selectivity was weaker, while non-positional tuning and multiplexing were stronger. The authors describe this pattern as closer to the spatial representations reported in primates.
The study used rats and Tupaia belangeri chinensis, a tree shrew that the researchers describe as occupying an evolutionary and ecological position between rodents and primates. This makes the species useful for examining whether differences in spatial coding follow a simple evolutionary separation between rodents and primates or can emerge through changes in how a shared circuit is used.
A different code within a familiar circuit
Multiplexing refers to neural populations carrying information about more than one aspect of an experience rather than assigning activity primarily to a single variable. In this study, the tree-shrew population combined spatial information with stronger tuning that was not dominated by position. The supplied abstract does not specify the individual non-positional variables represented by those signals.
Despite the difference in coding format, the researchers report that several organisational features of hippocampal activity were preserved in tree shrews. These included a proximo-distal gradient, pattern completion and global remapping.
A proximo-distal gradient is a systematic change in neural properties along the length of a hippocampal subregion. Pattern completion describes the ability of a network to recover a familiar representation from partial or altered input. Global remapping is a broader reorganisation of spatial activity when an animal moves between different environments or contextual conditions. Together, the findings indicate that the tree-shrew hippocampus retained important network-level dynamics while using a different distribution of information across its neurons.
Fewer neurons, comparable location decoding
The researchers also report that multiplexed tree-shrew populations decoded location as accurately as rat populations dominated by position, while using fewer neurons. The result points to a possible computational advantage: information distributed across several dimensions may support accurate spatial decoding without requiring as many specialised position-selective cells.
This is a result about neural representation and decoding in animals, not a demonstration of improved navigation in humans or a medical finding. The study is a bioRxiv preprint, so the work has not yet undergone conventional peer review. The supplied abstract also does not report the numbers of rats, tree shrews or recorded neurons, nor does it provide the quantitative decoding metrics needed to assess the size of the reported advantage.
The authors interpret the results as evidence for gradual evolution of spatial coding within a conserved hippocampal scaffold. In that view, evolution can repurpose existing network dynamics to support a more multiplexed representation rather than requiring a wholly different circuit. The comparison provides an intermediate point between rodent-like position coding and the more multiplexed spatial representations associated by the authors with primates.