Context versus content: contributions of the hippocampus and cortical organization to spatiotemporal memory

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Demchuk, Aubrey M.
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Lethbridge, Alta. : University of Lethbridge, Dept. of Neuroscience
During learning, the hippocampus is proposed to link the distributed components of neocortical memory representations to a spatiotemporal framework. However, the continued involvement of the hippocampus after memory consolidation is debated. Moreover, the relative encoding of spatial context (descending ‘where’ information) versus sensory content (ascending ‘what’ information), and how cortical architecture supports these interactions, remains poorly understood. Thus, in vivo two-photon microscopy of genetically encoded Thy1-GCaMP6s calcium indicator mice was used to longitudinally image cortical volumes from retrosplenial and motor cortices during behaviour both before and after bilateral lesions of the dorsal hippocampus. While the global spatial context remained constant throughout training and imaging, two familiar obstacles were added, interchanged, or removed from a cued treadmill to decouple sensorimotor activity from location-specific activity. The results support that (1) the online binding of unreliable sensory attributes to a coherent spatial framework remains mediated by the hippocampus in pre-operatively familiar environments, (2) the descending suppression of cortical responses by the hippocampus obeys hierarchical interareal and interlaminar gradients opposite to the flow of ascending sensory inputs, (3) motor cortices exhibit weak columnar spatial tuning, and (4) probabilistic cell dynamics drive the temporal drift of memory representations. Altogether, even in familiar environments, the hippocampus appears to actively integrate current sensory information with an internal model of the environment and propagate top-down predictions throughout the cortical hierarchy to improve the efficiency of ongoing information processing.
Hippocampus , Spatiotemporal memory , Cortical , Spatial context , Sensory content