Flexible encoding of objects and space in single cells of the dentate gyrus.

The hippocampus is involved in the formation of memories that require associations among stimuli to construct representations of space and the items and events within that space. Neurons in the dentate gyrus (DG), an initial input region of the hippocampus, have robust spatial tuning, but it is unclear how nonspatial information may be integrated with spatial activity in this region. We recorded from the DG of 21 adult mice as they foraged for food in an environment that contained discrete objects. We found DG cells with multiple firing fields at a fixed distance and direction from objects (landmark vector cells) and cells that exhibited localized changes in spatial firing when objects in the environment were manipulated. By classifying recorded DG cells into putative dentate granule cells and mossy cells, we examined how the addition or displacement of objects affected the spatial firing of these DG cell types. Object-related activity was detected in a significant proportion of mossy cells. Although few granule cells with responses to object manipulations were recorded, likely because of the sparse nature of granule cell firing, there was generally no significant difference in the proportion of granule cells and mossy cells with object responses. When mice explored a second environment with the same objects, DG spatial maps completely reorganized, and a different subset of cells responded to object manipulations. Together, these data reveal the capacity of DG cells to detect small changes in the environment while preserving a stable spatial representation of the overall context.

A theory of geometry representations for spatial navigation.

The geometric information of space, such as environment boundaries, is represented heterogeneously across brain regions. The computational mechanisms of encoding the spatial layout of environments remain to be determined. Here, we postulate a conjunctive encoding theory to illustrate the construct of cognitive maps from geometric perception. The theory naturally describes a spectrum of cell types including experimentally observed boundary vector cells, border cells, "annulus" and "bulls-eye" cells as special examples. In a similar way, inspired by the integration of egocentric and allocentric information as found in the postrhinal cortex, the theory also predicts a new cell type, named geometry cell. Geometry cells encode the geometric layout of the local space relative to the environment center, independent of the animal's positions and headings within the local space. The predicted geometry cell provides pure allocentric high-level representations of local scenes to support the quick formation of cognitive map representations capturing the spatial layout of complex environments. The theory sheds new light on the neural mechanisms of spatial cognition and brain-inspired autonomous intelligent systems.

Reaction to novelty as a behavioral assay of recognition memory in homing pigeons and Japanese quail.

Spontaneous novelty preference is apparent in a wide array of animals, including mammals, birds, reptiles, and fish. This provides a powerful behavioral assay to assess whether an animal can recognize a diverse array of stimuli in a common paradigm. Surprisingly, no research has been conducted in birds using novelty approach under conditions comparable to the spontaneous object recognition (SOR) protocols that have become standard across other animals. To correct this, the current study adapts a number of SOR protocols commonly used in mammals to characterize novelty approach in Silver King pigeons and Japanese quail. We show that, in general, both quail and pigeons readily approach novel objects or locations when tested using SOR protocols, although pigeons show a neophilic response under some conditions in which quail do not. Neither quail nor pigeons readily approach objects in novel contexts or novel locations. These data show that SOR can be successfully adapted to birds, allowing for more direct comparison between mammals and birds in tasks of shared ecological relevance.

The role of the dorsal dentate gyrus in object and object-context recognition.

The aim of this study was to determine the role of the dorsal dentate gyrus (dDG) in object recognition memory using a black box and object-context recognition memory using a clear box with available cues that define a spatial context. Based on a 10 min retention interval between the study phase and the test phase, the results indicated that dDG lesioned rats are impaired when compared to controls in the object-context recognition test in the clear box. However, there were no reliable differences between the dDG lesioned rats and the control group for the object recognition test in the black box. Even though the dDG lesioned rats were more active in object exploration, the habituation gradients did not differ. These results suggest that the dentate gyrus lesioned rats are clearly impaired when there is an important contribution of context. Furthermore, based on a 24 h retention interval in the black box the dDG lesioned rats were impaired compared to controls.

An analysis of the dentate gyrus function.

In this review article the emphasis will be on the role of the DG (dorsal and ventral) in supporting memory based on the operation of specific processes. Based on the development of computational models of dorsal dentate gyrus (dDG) and behavioral evidence based on dysfunction of dDG, this review will show that the dDG mediates mnemonic processing of spatial information. The processes subserved by dDG include (a) the operation of conjunctive encoding of multiple sensory inputs, implying an integration of sensory inputs to determine a spatial representation, and (b) pattern separation of spatial (especially metric) information, involving the reduction of interference between similar spatial locations (c) pattern separation of context (d) importance of context in object recognition, and (e) temporal integration and remote memory and spatial pattern separation based in part on neurogenesis. In addition the ventral dentate gyrus (vDG) mediates mnemonic processing of odor information as indicated by odor pattern separation.