Hui, Y.-Q. | bioRxiv Science

bioRxiv2026

Gradual evolution of multiplexed spatial coding within a conserved hippocampal scaffold

The hippocampus constructs spatial maps for navigation, yet their format differs markedly across species. Whether divergences reflect ecological adaptation, phylogenetic architecture, or flexible coding strategy within a conserved circuit has been difficult to disentangle. We recorded CA1 in rats and tree shrews (Tupaia belangeri chinensis), a mammal phylogenetically and ecologically between rodents and primates, using identical tasks and analyses. Unlike the position-dominant code of rats, tree shrew CA1 exhibited a hybrid representation: weaker position selectivity, stronger non-positional tuning, and prevalent multiplexing approaching primate characteristics. Multiplexed populations decoded location as accurately as rat position populations while using fewer neurons, suggesting computational advantages of high-dimensional coding. Despite this representational shift, proximo-distal gradient, pattern completion, and global remapping persisted. Representational format can be tuned without altering the underlying circuit. Tree shrews occupy an evolutionary intermediate in which ancestral network dynamics are repurposed for efficient multiplexed coding, tracing a transition toward primate-like spatial representations.

neuroscience↗

bioRxiv2023

A comparison of behavior paradigms assessing spatial memory in tree shrew

Spatial cognition enables animals to navigate the environment. Impairments in spatial navigation are often preclinical signs of Alzheimers disease (AD) in human. Therefore, evaluating spatial memory deficits is valuable when assessing incipient AD in animal models. The Chinese tree shrew, a close relative of primates, possesses many features that make it suitable for AD research. However, there is a scarcity of reliable behavior paradigms to monitor changes in spatial cognition in this species. To address this, we established reward-based paradigms in the radial-arm maze and the cheeseboard maze for tree shrew, and tested spatial memory of a group of twelve male animals in both tasks, along with a control water maze test, before and after bilateral lesions to the hippocampus, the brain region essential for spatial navigation. Tree shrews memorized target positions during training, and their task performance improved gradually until reaching a plateau in all three mazes. After the lesion, spatial learning was compromised in both newly-developed tasks, whereas memory retrieval was impaired in the water maze. Furthermore, individual task performance in both dry-land paradigms depended heavily on the size of remaining hippocampal tissue. Notably, all lesioned animals displayed spatial memory deficits in the cheeseboard task, but not in the other two paradigms. Our results suggest that the cheeseboard task currently represents the most sensitive paradigm for assessing spatial memory in tree shrew, with the potential to monitor progressive cognitive declines in aged or genetically modified animals developing AD-like symptoms. Significance StatementCognitive tests that monitor impairments in spatial memory play a crucial role in evaluating animal models with early-stage Alzheimers disease (AD). The Chinese tree shrew possesses many features suitable for an AD model, yet behavior tests assessing spatial cognition in this species are lacking. Here we developed novel behavior paradigms tailored to measure spatial memory in tree shrews and evaluated their sensitivity to changes in spatial learning by examining a group of hippocampus-lesioned animals. Our results indicate that the cheeseboard task effectively detects impairments in spatial memory and holds potential for monitoring the progressive cognitive decline in aged or genetically modified tree shrews that develop AD-like symptoms. This research may facilitate the use of tree shrew model in AD research.

animal behavior and cognition↗