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Since the discovery of place cells, the hippocampus is thought to be the neural substrate of a cognitive map. The later discovery of head direction cells, grid cells and border cells, as well as of cells with more complex spatial signals, has led to the idea that there is a brain system devoted to providing the animal with the information required to achieve efficient navigation. Current questioning is focused on how these signals are integrated in the brain. In this review, we focus on the issue of how self-localization is performed in the hippocampal place cell map. To do so, we first shortly review the sensory information used by place cells and then explain how this sensory information can lead to two coding modes, respectively based on external landmarks (allothetic information) and self-motion cues (idiothetic information). We hypothesize that these two modes can be used concomitantly with the rat shifting from one mode to the other during its spatial displacements. We then speculate that sequential reactivation of place cells could participate in the resetting of self-localization under specific circumstances and in learning a new environment. Finally, we provide some predictions aimed at testing specific aspects of the proposed ideas. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Encoding of a goal with a specific value while performing a place navigation task involves the medial prefrontal cortex (mPFC) and the dorsal hippocampus (dHPC), and depends on the coordination between mPFC and the ventro-intermediate hippocampus (vHPC).The present work investigates the contribution of mPFC, dHPC, and vHPC when the rat has to update the value of a goal. Rats were trained to navigate to an uncued goal in order to release a food pellet in a continuous place navigation task. When they had reached criterion performance level in the task, they were subjected to a single 'flash session' in which they were exposed to an aversive strobe light during goal visits instead of receiving a food reward. Just before the flash session, the GABAA agonist muscimol was injected to temporarily inactivate mPFC, dHPC, or vHPC. The ability to recall the changed value of the goal was tested on the next day. We first demonstrate the aversive effect of the strobe light by showing that rats learn to avoid the goal much more rapidly in the flash session than during a simple extinction session in which goal visits are not rewarded. Furthermore, while dHPC inactivation had no effect on learning and recalling the new goal value, vHPC muscimol injections considerably delayed goal value updating during the flash session, which resulted in a slight deficit during recall. In contrast, mPFC muscimol injections induced faster goal value updating but the rats were markedly impaired on recalling the new goal value on the next day. These results suggest that, contrary to mPFC and dHPC, vHPC is required for updating the value of a goal. In contrast, mPFC is necessary for long-term retention of this updating. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Primary objective: Occupational exposure to static magnetic fields (SMF) increases, in particular due to the widespread use of Magnetic Resonance Imaging (MRI) for medical diagnosis, thus raising health concerns. This study investigated the behavioural effects of 128 mT SMF in rats and examined the hypothesis that iron supplementation (3 mg kg−1 for 5 days) potentiate the effects of SMF. Methods: Spatial learning abilities in the water maze, motor co-ordination in the rotarod and motor skills in the stationary beam and suspending string tests were assessed in iron-treated, SMF-exposed and co-exposed SMF-iron rats. Results: Acquisition of the water maze navigation task was unaffected in all groups. SMF-exposed and iron-treated rats showed a deficit in the 7-day retention test. No deficit was found in the rotarod and suspended string tests in all groups. Only iron-treated rats were impaired in the stationary beam test. A combination of iron and SMF treatments did not produce additional degradation of performance in all tests. Conclusion: SMF exposure had no massive effect but affected long-term spatial memory. Iron supplementation and 128 mT SMF had no synergistic effects. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: Evidence has accumulated that the entorhinal cortex (EC) is involved in memory operations underlying formation of a long-term memory. Because entorhinal-lesioned rats are impaired for long delays in delayed matching and non-matching to sample tasks, it has been proposed that EC contributes to the maintenance of information in short-term memory. In the present study, we asked whether such a time-limited role applies also when learning complex spatial information in a novel environment. We therefore examined the effects of EC lesions on habituation in an object exploration task in which a delay of either 4min or 10min is imposed between successive sessions. EC-lesioned rats exhibited a deficit in habituation at 10min but not 4min delays. Following habituation, reactions to spatial change (object configuration) and non-spatial change (novel object) were also examined. EC-lesioned rats were impaired in detecting the spatial change but were able to detect a non-spatial change, irrespective of the delay. Overall, the results suggest that EC is involved in maintaining a large amount of novel, multidimensional information in short-term memory therefore enabling formation of long-term memory. Switching to a novelty detection mode would then allow the animal to rapidly adapt to environmental changes. In this mode, EC would preferentially process spatial information rather than non-spatial information. [Copyright &y& Elsevier]

Recent studies emphasize the importance of the entorhinal cortex in spatial representation and navigation. Furthermore, evidence is accumulating to show that spatial processing depends on interactions between the entorhinal cortex and the hippocampus. To investigate these interactions, we examined the effects of entorhinal cortex lesions on the activity of hippocampal CA1 place cells. Rats received bilateral radiofrequency lesions of the entorhinal cortex or sham lesions before place cell recording. Place cells were recorded as the rats performed a pellet-chasing task in a cylinder containing three cue-objects. Entorhinal cortex lesions did not abolish place cell spatial firing but reduced noticeably discharge rate and field size. Most importantly, the lesions affected firing field stability when cells were recorded both in constant conditions and following cue manipulations (object rotation, object removal). These findings indicate that the entorhinal cortex is necessary for the stability of hippocampal representations across exposures to a familiar environment. Consistent with the recent discovery of grid cells in the medial entorhinal cortex, our results suggest that the entorhinal cortex contributes to providing a spatial framework that would enable the hippocampus to maintain stable environment-specific representations. [ABSTRACT FROM AUTHOR]

Place cells are hippocampal pyramidal neurons that discharge strongly in relation to the rat's location in the environment. We recently reported that many place cells recorded from rats performing place or cue navigation tasks also discharged when they were at the goal location rather than in the primary firing field. Furthermore, subtle differences in discharge timing were found in the two navigation tasks, with activity occurring later in the place task compared to the cue task. Here we tested the possibility that such delayed firing in the place task may reflect the differential involvement of time estimation, which would allow the rat to predict forthcoming reward delivery. More specifically, we reasoned that failure to obtain a reward after a fixed 2s delay in the place task reliably reflected the rat's misplacement relative to the correct location, thus making time a valuable cue to help the rat perform the task. To test this hypothesis, well-trained rats were run on a partial extinction procedure in place and cue navigation tasks so that no feed-back signal was provided about their actual accuracy during extinction periods. Although the time estimation hypothesis predicts that only in the place task will the rat make correction movements at the end of goal periods during extinction, we found that such movements occurred in all rats, indicating correct time estimation in both place and cue tasks. We briefly discuss the results in the light of current computational theories of hippocampal function. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: It has been recently shown that lesions of parahippocampal areas including the entorhinal cortex do not disrupt place learning in the water maze, suggesting that the hippocampo-cortical circuitry is not important for spatial memory [Burwell RD, Saddoris MP, Bucci DJ, Wiig KA. Corticohippocampal contributions to spatial and contextual learning. J Neurosci 2004;24:3826–36]. The aim of the present study was to tax more directly the cooperation between the hippocampus and entorhinal cortex in two different spatial tasks, a place navigation task and a spontaneous object exploration task, using a disconnection procedure. Damaging the entorhinal–hippocampal system induced impairments in the two tasks but only in the spatial object exploration task rats with contralateral lesions displayed a greater deficit than rats with ipsilateral lesions. The results suggest that the cooperation between the hippocampus and the entorhinal cortex is modulated by the nature of the task and the cognitive processes involved in formation of spatial memory. [Copyright &y& Elsevier]

The hippocampus and associative parietal cortex (APC) both contribute to spatial memory but the nature of their functional interaction remains unknown. To address this issue, we investigated the effects of APC lesions on hippocampal place cell firing in freely moving rats. Place cells were recorded from APC-lesioned and control rats as they performed a pellet-chasing task in a circular arena containing three object cues. During successive recording sessions, cue manipulations including object rotation in the absence of the rat and object removal in the presence of the rat were made to examine the control exerted by the objects or by non-visual intramaze cues on place field location, respectively. Object rotations resulted in equivalent field rotation for all cells in control rats. In contrast, a fraction of place fields in APC-lesioned rats did not rotate but remained stable relative to the room. Object removal produced different effects in APC-lesioned and control rats. In control rats, most place fields remained stable relative to the previous object rotation session, indicating that they were anchored to olfactory and/or idiothetic cues. In APC-lesioned rats, a majority of place fields shifted back to their initial, standard location, thus suggesting that they relied on uncontrolled background cues to maintain place field stability. These results provide strong evidence that the hippocampus and the APC cooperate in the formation of spatial memory and suggest that the APC is involved in elaboration of a hippocampal map based on proximal landmarks. [ABSTRACT FROM AUTHOR]

Abstract Hippocampal place cells were recorded while rats with lesions of the striate visual cortex foraged for food pellets in a cylindrical arena. Compared to control rats, rats with striate damage had place cells whose firing was less well organized in space, according to a measurement of spatial coherence. More importantly, the spatial location of firing fields in rats with striate lesions was poorly controlled by three-dimensional objects, unlike the fields of either normal sighted rats or early blind rats. These findings suggest a possible contribution of the striate visual cortex to the selection of cues used for anchoring place cell firing fields in space. [ABSTRACT FROM AUTHOR]

Abstract Hippocampal place cells were recorded while rats solved a continuous spatial alternation task requiring short-term spatial memory. All cells that had a firing field on the stem of the Y-shaped maze were found to have a very similar pattern of discharge whether the rat was about to make a right or a left turn, and whether the preceding turn was a right or a left turn. Thus, the view that place cells encode a variety of events (including the direction of turns) useful for solving memory tasks is not well supported by the present data. We suggest several possible explanations to account for the discrepancy with other recent studies showing turn-related modulation of place-cell activity. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rats exposed to 900MHz electromagnetic field (EMF) combined or not with iron supplementation. [•] Rats trained in spatial tasks, biogenic monoamines and oxidative stress measured. [•] EMF: deficits in the exploration task only, monoamines altered and no global oxidative stress. [•] EMF+iron overload: no greater behavioral and neurochemical deficits than EMF alone. [•] EMF affects cognitive and neurochemical processes but no synergistic effects between EMF and iron. [Copyright &y& Elsevier]

Abstract: The rat medial prefrontal cortex has been suggested to be involved in executive functions and, more specifically, in working memory and response selection. Here, we looked for prefrontal neural correlates as rats performed a modified radial arm maze task that taxed such functions. Rats had to learn the position of four rewarded arms among eight, and visit each rewarded arm only once, thus avoiding repeated visits. In addition, rats were left on the maze after the four successful visits to baited arms until they had visited all the arms twice. Prefrontal neural activity was examined during choice periods, i.e. 2s before the rat entered the arms. We found that a substantial proportion of recorded medial prefrontal neurons were selectively activated before either the first or second visit to the arms irrespective of their reward status, thereby tagging already visited arms. These behavioral correlates show that, within the rodent medial prefrontal cortex, neuronal populations demonstrate behavioral correlates suggestive of its role in guiding prospective search behavior and thus executive functions. [Copyright &y& Elsevier]

Abstract: Low-frequency electromagnetic fields (EMF) have been suggested to affect the brain via alterations of blood–brain barrier permeability to iron. Because of an immature blood–brain barrier, the young brain may be particularly vulnerable to EMF exposure. It is therefore possible that behavioral and neurotoxic effects resulting from EMF-induced iron excess in the brain would be greater in young adults. The objective of the present study was to investigate the interaction between low-frequency EMF and iron overload in young rats. In Experiment 1, we tested the effects of iron overload on spatial learning and memory. Iron treatment did not affect performance in a reference (Morris water maze) and a working memory task (8-arm radial maze). In contrast, detection of a spatial change in an object exploration task was impaired. These effects correlated with modifications of the serotoninergic metabolism. In Experiment 2, the combination of EMF exposure and iron overload was tested. As in Experiment 1, rats were not impaired in reference and working memory tasks but were mildly impaired in the detection of the spatial change. Overall, the results showed an effect of iron overload on spontaneous spatial memory processes. However, low-frequency EMF exposure did not potentiate the effects of iron overload in young rats. [Copyright &y& Elsevier]

The medial (MEC) and the lateral (LEC) regions of the entorhinal cortex send a major input to the hippocampus and have been proposed to play a foremost role in combining spatial and non-spatial attributes of episodic memory. In addition, it has been recently suggested that the MEC is involved in the processing of information in a global reference frame and the LEC in the processing of information in a local reference frame. Whether these putative functions could be generalized to navigation contexts has not been established yet. To address this hypothesis, rats with MEC or LEC NMDA-induced lesions were trained in two versions of a navigation task in the water maze, a global cue condition in which they had to use distal room cues and a local cue condition in which they had to use 3 objects placed in the pool. In the global cue condition, MEC-lesioned rats exhibited slower acquisition and were not able to precisely locate the submerged platform during the probe trial. In contrast LEC-lesioned rats exhibited control-like performance. In the local cue condition, navigational abilities were spared in both lesion groups. In addition when the 3 different objects were replaced by 3 identical objects, all groups maintained their navigation accuracy suggesting that the identity of objects is not crucial for place navigation. Overall, the results indicate that the MEC is necessary for place navigation using a global reference frame. In contrast, navigation using a local reference frame does not require the LEC nor the MEC. [ABSTRACT FROM AUTHOR]

When facing a choice at a decision point in a maze, rats often display hesitations, pauses and reorientations. Such "vicarious trial and error" (VTE) behavior is thought to reflect decision making about which choice option is best, and thus a deliberation process. Although deliberation relies on a wide neural network, the dorsal hippocampus appears to play a prominent role through both its neural activity and its dynamic interplay with other brain areas. In contrast, the involvement of the ventral hippocampus in deliberation is unexplored. Here, we compared directly the effects of dorsal (dHPC) and ventral intermediate (vHPC) hippocampal inactivations induced by intracerebral muscimol injections on VTE behavior as a model of deliberation. To this aim, we analyzed VTE events as rats were required to switch strategy to a new unlearned reward rule. We used a protocol in which task performance in muscimol-injected animals was minimally altered so as to evidence specific effects on VTE behavior. Our results show subtle alterations in VTE behavior following dHPC, but not vHPC, inactivations, therefore suggesting a specific contribution of the dorsal hippocampus to deliberation through its role in prospective evaluation of future actions. [ABSTRACT FROM AUTHOR]