Entorhinal grid cells are thought to provide a 2D spatial metric of the environment. In this study we demonstrate that in a familiar 1D circular track (i.e., a continuous space) grid cells display a ...novel 1D equidistant firing pattern based on integrated distance rather than travelled distance or time. In addition, field spacing is increased compared to a 2D open field, probably due to a reduced access to the visual cue in the track. This metrical modification is accompanied by a change in LFP theta oscillations, but no change in intrinsic grid cell rhythmicity, or firing activity of entorhinal speed and head-direction cells. These results suggest that in a 1D circular space grid cell spatial selectivity is shaped by path integration processes, while grid scale relies on external information.
Hippocampal place cells show position-specific activity thought to reflect a self-localization signal. Several reports also point to some form of goal encoding by place cells. We investigated this by ...asking whether they also encode the value of spatial goals, which is crucial information for optimizing goal-directed navigation. We used a continuous place navigation task in which male rats navigate to one of two (freely chosen) unmarked locations and wait, triggering the release of reward, which is then located and consumed elsewhere. This allows sampling of place fields and dissociates spatial goal from reward consumption. The two goals varied in the amount of reward provided, allowing assessment of whether the rats factored goal value into their navigational choice and of possible neural correlates of this value. Rats successfully learned the task, indicating goal localization, and they preferred higher-value goals, indicating processing of goal value. Replicating previous findings, there was goal-related activity in the out-of-field firing of CA1 place cells, with a ramping-up of firing rate during the waiting period, but no general overrepresentation of goals by place fields, an observation that we extended to CA3 place cells. Importantly, place cells were not modulated by goal value. This suggests that dorsal hippocampal place cells encode space independently of its associated value despite the effect of that value on spatial behavior. Our findings are consistent with a model of place cells in which they provide a spontaneously constructed value-free spatial representation rather than encoding other navigationally relevant but nonspatial information.
We investigated whether hippocampal place cells, which compute a self-localization signal, also encode the relative value of places, which is essential information for optimal navigation. When choosing between two spatial goals of different value, rats preferred the higher-value goal. We saw out-of-field goal firing in place cells, replicating previous observations that the cells are influenced by the goal, but their activity was not modulated by the value of these goals. Our results suggest that place cells do not encode all of the navigationally relevant aspects of a place, but instead form a value-free "map" that links to such aspects in other parts of the brain.
The existence of place cells, whose discharge is strongly related to a rat’s location in its environment, has led to the proposal that they form part of an integrated neural system dedicated to ...spatial navigation. It has been suggested that this system could represent space as a cognitive map, which is flexibly used by animals to plan new shortcuts or efficient detours. To further understand the relationships between hippocampal place cell firing and cognitive maps, we examined the discharge of place cells as rats were exposed to a Tolman‐type detour problem. In specific sessions, a transparent barrier was placed onto the maze so as to block the shortest central path between the two rewarded end locations of a familiar three‐way maze. We found that rats rapidly and consistently chose the shortest alternative detour. Furthermore, both CA1 and CA3 place cells that had a field in the vicinity of the barrier displayed local remapping. In contrast, neither CA1 nor CA3 cells that had a field away from the barrier were affected. This finding, at odds with our previous report of altered CA3 discharge for distant fields in a shortcut task, suggests that the availability of a novel path and the blocking of a familiar path are not equivalent and could lead to different responses of the CA3 place cell population. Together, the two studies point to a specific role of CA3 in the representation of spatial connectivity and sequences.
The processing of spatial information in the brain requires a network of structures within which the hippocampus plays a prominent role by elaborating an allocentric representation of space. The ...parietal cortex has long been suggested to have a complementary function. An overview of lesion and unit recording data in the rat indicates that the parietal cortex is involved in different aspects of spatial information processing including allocentric and egocentric processing. More specifically, the data suggest that the parietal cortex plays a fundamental role in combining visual and motion information, a process that would be important for an egocentric-to-allocentric transformation process. Furthermore, the parietal cortex may also have a role in the long-term storage of representation although this possibility needs further evidence. The data overall show that the parietal cortex occupies a unique position in the brain at the interface of perception and representation.
Place cells are hippocampal neurons whose discharge is strongly related to a rat's location in its environment. The existence of place cells has led to the proposal that they are part of an ...integrated neural system dedicated to spatial navigation, an idea supported by the discovery of strong relationships between place cell activity and spatial problem solving. To further understand such relationships, we examined the discharge of place cells recorded while rats solved a place navigation task. We report that, in addition to having widely distributed firing fields, place cells also discharge selectively while the hungry rat waits in an unmarked goal location to release a food pellet. Such firing is not duplicated in other locations outside the main firing field even when the rat's behavior is constrained to be extremely similar to the behavior at the goal. We therefore propose that place cells provide both a geometric representation of the current environment and a reflection of the rat's expectancy that it is located correctly at the goal. This on-line feedback about a critical aspect of navigational performance is proposed to be signaled by the synchronous activity of the large fraction of place cells active at the goal. In combination with other (prefrontal) cells that provide coarse encoding of goal location, hippocampal place cells may therefore participate in a neural network allowing the rat to plan accurate trajectories in space.
Navigation in rodents depends on both self-motion (idiothetic) and external (allothetic) information. Idiothetic information has a predominant role when allothetic information is absent or ...irrelevant. The vestibular system is a major source of idiothetic information in mammals. By integrating the signals generated by angular and linear accelerations during exploration, a rat is able to generate and update a vector pointing to its starting place and to perform accurate return. This navigation strategy, called path integration, has been shown to involve a network of brain structures. Among these structures, the entorhinal cortex (EC) may play a pivotal role as suggested by lesion and electrophysiological data. In particular, it has been recently discovered that some neurons in the medial EC display multiple firing fields producing a regular grid-like pattern across the environment. Such regular activity may arise from the integration of idiothetic information. This hypothesis would be strongly strengthened if it was shown that manipulation of vestibular information interferes with grid cell activity. In the present paper we review neuroanatomical and functional evidence indicating that the vestibular system influences the activity of the brain network involved in spatial navigation. We also provide new data on the effects of reversible inactivation of the peripheral vestibular system on the EC theta rhythm. The main result is that tetrodotoxin (TTX) administration abolishes velocity-controlled theta oscillations in the EC, indicating that vestibular information is necessary for EC activity. Since recent data demonstrate that disruption of theta rhythm in the medial EC induces a disorganization of grid cell firing, our findings indicate that the integration of idiothetic information in the EC is essential to form a spatial representation of the environment.
Excitotoxic lesions are frequently used to assess the role of cerebral structures in cognitive processes in rodents. However, the precise site and extent of these lesions remain unknown without ...histological verifications. Using a 7-Teslas MRI system and a T2-weighted turbo-RARE sequence, MR images were acquired at several time points following NMDA lesions (1h, 6h, 24h, 48h, 1 week and 2 weeks). NMDA infusions into the parenchyma induced a clear and delineable hyperintense signal from 1h up to 1-week post-surgery. Hyperintensity volumes were compared with NeuN and Cresyl violet histological quantifications of the lesion magnitude. NMDA-induced hypersignal is observed as soon as 1h post-injection and is a reliable estimate of the presence (or absence) of a lesion. Compared to NeuN, Cresyl violet staining underestimates the extent of the lesion in significant proportions. The MRI hyperintensity generated by NMDA instillation into the parenchyma can be used as a powerful tool to confirm the diffusion of the drug into the cerebral tissue, to ascertain the locus of injection and predict with a high success rate the fate of NMDA lesions as soon as 1h post-surgery. This approach could be very useful in a large variety of lesion studies in rodents.
Place cells are hippocampal neurons whose discharge is strongly related to a rat's location in its environment. The existence of place cells has led to the proposal that they are part of an ...integrated neural system dedicated to spatial navigation. To further understand the relationships between place cell firing and spatial problem solving, we examined the discharge of CA1 and CA3 place cells as rats were exposed to a shortcut in a runway maze. On specific sessions, a wall section of the maze was removed so as to open a shorter novel route within the otherwise familiar maze. We found that the discharge of both CA1 and CA3 cells was strongly affected in the vicinity of the shortcut region but was much less affected farther away. In addition, CA3 fields away from the shortcut were more altered than CA1 fields. Thus, place cell firing appears to reflect more than just the animal's spatial location and may provide additional information about possible motions, or routes, within the environment. This kinematic representation appears to be spatially more extended in CA3 than in CA1, suggesting interesting computational differences between the two subregions.
Hippocampus place cell discharge is an important model system for understanding cognition, but evidence is missing that the place code is under the kind of dynamic attentional control characterized ...in primates as selective activation of one neural representation and suppression of another, competing representation. We investigated the apparent noise ("overdispersion") in the CA1 place code, hypothesizing that overdispersion results from discharge fluctuations as spatial attention alternates between distal cues and local/self-motion cues. The hypothesis predicts that: (1) preferential use of distal cues will decrease overdispersion; (2) global, attention-like states can be decoded from ensemble discharge such that both the discharge rates and the spatial firing patterns of individual cells will be distinct in the two states; (3) identifying attention-like states improves reconstructions of the rat's path from ensemble discharge. These predictions were confirmed, implying that a covert, dynamic attention-like process modulates discharge on a approximately 1 s time scale. We conclude the hippocampus place code is a dynamic representation of the spatial information in the immediate focus of attention.
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