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The subjective sense of space may result in part from the combined activity of place cells in the hippocampus and grid cells in posterior cortical regions such as the entorhinal cortex and pre- and parasubiculum. In horizontal planar environments, place cells provide focal positional information, whereas grid cells supply odometric (distance measuring) information. How these cells operate in three dimensions is unknown, even though the real world is three-dimensional. We investigated this issue in rats exploring two different kinds of apparatus: a climbing wall (the 'pegboard') and a helix. Place and grid cell firing fields had normal horizontal characteristics but were elongated vertically, with grid fields forming stripes. It seems that grid cell odometry (and by implication path integration) is impaired or absent in the vertical domain, at least when the rat itself remains horizontal. These findings suggest that the mammalian encoding of three-dimensional space is anisotropic. [ABSTRACT FROM AUTHOR]

Self-localization requires that information from several sensory modalities and knowledge domains be integrated in order to identify an environment and determine current location and heading. This integration occurs by the convergence of highly processed sensory information onto neural systems in entorhinal cortex and hippocampus. Entorhinal neurons combine angular and linear self-motion information to generate an oriented metric signal that is then ‘attached’ to each environment using information about landmarks and context. Neurons in hippocampus use this signal to determine the animal''s unique position within a particular environment. Elucidating this process illuminates not only spatial processing but also, more generally, how the brain builds knowledge representations from inputs carrying heterogeneous sensory and semantic content. [Copyright &y& Elsevier]

The three-dimensional topography of the environment is a potentially important source of orienting information for animals, but little is known about how such features affect either navigational behaviour or the neural representation of place. One component of the neural place representation comprises the hippocampal place cells, which show location-specific firing that can be oriented by directional cues in the environment. The present study investigated whether a simple topographical feature, terrain slope, could provide such orienting information to place cells. Place cells were recorded as rats explored a tilted (30°) square box located in the centre of a dark, curtained and visually symmetrical circular enclosure. The orientation of the tilted surface was varied, first in conjunction with that of a visible cue card (to stabilise the system) and then in the absence of the cue card, when the slope of the box was the only remaining stable polarising cue in the environment. In the latter condition, place fields continued to be reliably oriented by the slope. Thus, terrain slope provides sufficient orienting information to set and probably maintain the orientation of the hippocampal place system. This may explain previous behavioural observations that spatial orientation is improved when slope information is available. [ABSTRACT FROM AUTHOR]

In a symmetrical environment possessing no other polarizing visual cues, the spatially localized firing of hippocampal place cells can be primarily orientated by a reliable distal visual stimulus, such as a white cue card. However, if such a directional cue is made unreliable by being frequently moved in full view of the rat, the rat's internal sense of direction comes, over the course of a few days, to control the orientation of place fields instead. We investigated whether this simple form of ‘cue-instability’ learning would transfer to a new context, in which the firing patterns of the place cells become reorganized and in which a new spatial representation is thus active. We found that after cue-instability learning, the ‘remapped’ place field representation in the new environment was also orientated by the internal sense of direction of the rat rather than by the cue card, showing that the cue learning generalized from one context (and hence spatial representation) to another. This contrasts with another kind of place cell learning, in which the cells can acquire the ability to discriminate two spatial locations in one context but do not transfer this discrimination to a new context. We discuss the different effects of context changes on learned place cell activity in terms of the possible architecture of the inputs to place cells. [ABSTRACT FROM AUTHOR]

The spatially localized firing of rodent hippocampal place cells is strongly determined by the local geometry of the environment. Over time, however, the cells can acquire additional inputs, including inputs from more distal cues. This is manifest as a change in firing pattern (‘remapping’) when the new inputs are manipulated. Place cells also reorganize their firing in response to non-geometric changes in ‘context’, such as a change in the colour or odour of the environment. The present study investigated whether the new inputs acquired by place cells in one context were still available to the cells when they expressed their altered firing patterns in a new context. We found that the acquired information did not transfer to the new context, suggesting that the context inputs and the acquired inputs must interact somewhere upstream of the place cells themselves. We present a model of one possible such interaction, and of how such an interaction could be modified by experience in a Hebbian manner, thus explaining the context specificity of the new learning. [ABSTRACT FROM AUTHOR]

Hippocampal place cells show spatially localized activity that can be modulated by both geometric information (e.g., the distances and directions of features in the environment) and nongeometric information (e.g., colors, odors, and possibly behaviors). Nongeometric information may allow the discrimination of different spatial contexts. Understanding how nongeometric (or contextual) information affects hippocampal activity is important in light of proposals that the hippocampus may play a role in constructing a representation of spatial context. We investigated the contextual modulation of place cell activity by recording hippocampal place cells while rats foraged in compound contexts comprising black or white color paired with lemon or vanilla odor. Some cells responded to the color or odor changes alone, but most responded to varying combinations of both. Thus, we demonstrate, for the first time, that there is a heterogeneous input by contextual inputs into the hippocampus. We propose a model of contextual remapping of place cells in which the geometric inputs are selectively activated by subsets of contextual stimuli. Because it appears that different place cells are affected by different subsets of contextual stimuli, the representation of the entire context would require activity at the population level, supporting a role for the hippocampus in constructing a representation of spatial context. [ABSTRACT FROM AUTHOR]

The firing pattern of entorhinal 'grid cells' is thought to provide an intrinsic metric for space. We report a strong experience-dependent environmental influence: the spatial scales of the grids (which are aligned and have fixed relative sizes within each animal) vary parametrically with changes to a familiar environment's size and shape. Thus grid scale reflects an interaction between intrinsic, path-integrative calculation of location and learned associations to the external environment. [ABSTRACT FROM AUTHOR]

Much behavioural and physiological evidence suggests that the hippocampus encodes space. Puzzlingly, however, hippocampal damage also disrupts episodic memory. A recent study shows how these two faculties might be related, finding that the spatial firing of hippocampal ‘place cells’ is sometimes modulated by what the animal has recently done or what it will do next. Thus, the cells encode something resembling a context, or episode, collectively forming a potential substrate for episodic memory. [Copyright &y& Elsevier]

In this article the authors comment on a study related to formation of spatial memory by David Dupret and colleagues, published in the 2010 issue. The study showed that spatial learning involves reorganization of place fields of hippocampal CA1 neurons and this is reactivated in NMDA receptors for memory consolidation. The authors state that the findings are consistent with the previous reports and provide convincing evidence that the cognitive map is plastic which resembles spatial memory.