Biomedical subjects

E Save

Publications and source records attributed to E Save.

At least 19 recordsLinked to original sources

PubMed2006

Spatial discrimination of visually similar environments by hippocampal place cells in the presence of remote recalibrating landmarks.

Place cells in the rat hippocampus commonly show place-related firing activity in the animal's current environment. Here, we evaluated the capability of the place cell system to discriminate visually identical environments. Place cell activity was first recorded while rats moved freely in a cylinder divided into three connected sectors. Two sectors were visually identical whereas the third sector was made distinctive by the addition of visual and tactile cues. When in a given sector, the rats could not perceive the cues present in the other two sectors. Most cells had distinctive place fields in each sector, including the two identical sectors. To rule out the influence of non-controlled cues, rotations of the cylinder (+/- 120 degrees) were conducted. When successful, cylinder rotations resulted in equivalent field rotation for all cells. These results suggest that the place cell system is able to form a specific spatial representation for all sectors, so that the rat knows, at any time, in which sector it is currently located. Presumably, such discrimination relies on angular path integration in which the computational errors stemming from self-motion cues would be corrected by environmental landmarks provided by the distinctive sector.

Analysis of Variance↗

PubMed2005

Coding for spatial goals in the prelimbic/infralimbic area of the rat frontal cortex.

Finding one's way in space requires a distributed neural network to support accurate spatial navigation. In the rat, this network likely includes the hippocampus and its place cells. Although such cells allow the organism to locate itself in the environment, an additional mechanism is required to specify the animal's goal. Here, we show that firing activity of neurons in medial prefrontal cortex (mPFC) reflects the motivational salience of places. We recorded mPFC neurons from rats performing a place navigation task, and found that a substantial proportion of cells in the prelimbic/infralimbic area had place fields. A much smaller proportion of cells with such properties was found in the dorsal anterior cingulate area. Furthermore, the distribution of place fields in prelimbic/infralimbic cells was not homogeneous: goal locations were overrepresented. Because such locations were spatially dissociated from rewards, we suggest that mPFC neurons might be responsible for encoding the rat's goals, a process necessary for path planning.

Action Potentials↗

PubMed2004

Comparison of the effects of entorhinal and retrosplenial cortical lesions on habituation, reaction to spatial and non-spatial changes during object exploration in the rat.

This study was aimed at comparing the effects of damage to the entorhinal cortex and retrosplenial cortex on exploration and reaction-to-change in rats. Following habituation to a configuration of objects, a familiar object was displaced therefore producing a spatial change. A non-spatial change was made by substituting a familiar object by a novel object. Both entorhinal cortex lesions and retrosplenial cortex lesions elicited a deficit in the rat's reaction to a spatial change. In addition, rats with entorhinal lesions showed hypoactive object exploration and exhibited a mild deficit in reaction to a non-spatial change. This may reflect the importance of temporal cortical input in object recognition. Overall, these results suggest that the entorhinal cortex and the retrosplenial cortex contribute to the processing of the geometric properties of the environment and have thus close functional links.

Animals↗

PubMed2004

Independent coding of connected environments by place cells.

Place cells are hippocampal neurons that have a strong location-specific firing activity in the rat's current environment. Collectively, place cells also provide a signature of the rat's environment as their ensemble activity is markedly different when recorded in distinct apparatuses. This phenomenon, referred to as 'remapping', suggests that each environment activates a different hippocampal map. In this study, we sought to determine the independence of such maps. In Experiment 1, we used a cylinder apparatus that was divided into two equal halves by a central barrier with an aperture allowing the rat to freely commute between the two sides. A local change in one side failed to induce field remapping in the changed side, thus precluding any significant conclusion to be drawn. We therefore designed Experiment 2 in which place cells were first recorded while rats explored three distinct high-walled boxes. Most cells had distinctive firing fields in each box. A runway was then added to connect two initially unrelated boxes. This manipulation altered the firing of some cells but the fields in each box were still clearly distinguishable. The final manipulation consisted of changing one box and allowing the rat to commute freely between the changed and unchanged boxes. While the firing fields remapped in the changed box, they were most usually unaltered in the unchanged box. These results suggest that the hippocampus holds a set of independent maps for each box, and that each specific map is activated mainly according to the rat's current sensory environment.

Animals↗

PubMed2004

Spatial navigation and hippocampal place cell firing: the problem of goal encoding.

Place cells are hippocampal neurons whose discharge is strongly related to a rat's location in the environment. The existence of such cells, combined with the reliable impairments seen in spatial tasks after hippocampal damage, has led to the proposal that place cells form part of an integrated neural system dedicated to spatial navigation. This hypothesis is supported by the strong relationships between place cell activity and spatial problem solving, which indicate that the place cell representation must be both functional and in register with the surroundings for the animal to perform correctly in spatial tasks. The place cell system nevertheless requires other essential elements to be competent, such as a component that specifies the overall goal of the animal and computes the path required to take the rat from its current location to the goal. Here, we propose a model of the neural network responsible for spatial navigation that includes goal coding and path selection. In this model, the hippocampal formation allows for place recognition, and stores the set of places that can be accessed from each position in the environment. The prefrontal cortex is responsible for encoding goal location and for route planning. The nucleus accumbens translates paths in neural space into appropriate locomotor activity that moves the animal towards the goal in real space. The complete model assumes that the hippocampal output to nucleus accumbens and prefrontal cortex provides information for generating solutions to spatial problems. In support of this model, we finally present preliminary evidence that the goal representation necessary for path planning might be encoded in the prelimbic/infralimbic region of the medial prefrontal cortex.

Action Potentials↗

PubMed2001

Re-evaluation of the spatial memory deficits induced by hippocampal short lasting inactivation reveals the need for cortical co-operation.

Evidence has accumulated that the rat hippocampus plays a central role in spatial memory. In complement to lesion studies, reversible lidocaïne-induced inactivations have been used to investigate the time-course of the memory processes mediated by the hippocampus. A number of studies suggest that, in some conditions, the hippocampus is not necessary for online acquisition of spatial information. To test this hypothesis, we examined the effects of bilateral lidocaïne-induced inactivations of the dorsal hippocampus in the acquisition of new spatial information. After initial learning of a place navigation task in the water maze, rats were tested for acquisition of a new platform location and received injections of lidocaïne in the hippocampus prior to each daily four-trial block. The training blocks were separated by a 24-h period allowing the hippocampus to recover from inactivation. The results show that lidocaïne-injected rats were able to learn the new platform location like controls. Inactivations, however, was found to induce a within-block learning impairment. This suggests that the hippocampus can perform off-line processing and that another structure is able to handle spatial information during hippocampal inactivations. Parietal-lesioned rats that received an injection of lidocaïne were still able to learn the new platform location suggesting that the parietal cortex does not sustain this role. Overall, our results suggest that the hippocampus is not necessary for all stages of memory formation and co-operates with other brain, possibly cortical, structures which remain to be determined.

Anesthetics, Local↗

PubMed2001

Evidence for a relationship between place-cell spatial firing and spatial memory performance.

The rat hippocampus contains place cells whose firing is location-specific. Although many properties of place cells have been uncovered, little is known about their actual contribution to the animal's spatial performance. In this study, we addressed this issue by recording place cells while rats solved a continuous spatial alternation task in which they had to alternate between the two arms of a Y-maze to get a food reward in the third (goal) arm. By manipulating the information available to the animals, we induced the cells to establish their fields in locations that were out of register relative to their standard position, thus making them inconsistent with the learned spatial task. When this happened, the rats' performance in the alternation task was markedly decreased. In addition, the nature of the behavioral errors during inconsistent field placements also changed dramatically in a way that was highly indicative of the rats' spatial disorientation. These results suggest that there is a functional relationship between the spatial firing patterns of place cells and the spatial behavior of the rat, thus strengthening the idea that these cells are part of a navigational system.

Animals↗

PubMed2001

Dissociation of the effects of bilateral lesions of the dorsal hippocampus and parietal cortex on path integration in the rat.

Rodents are able to rely on self-motion (idiothetic) cues and navigate toward a reference place by path integration. The authors tested the effects of dorsal hippocampal and parietal lesions in a homing task to dissociate the respective roles of the hippocampus and the parietal cortex in path integration. Hippocampal rats exhibited a strong deficit in learning the basic task. Parietal rats displayed a performance impairment as a function of the complexity of their outward paths when the food was placed at varying locations. These results suggest that the parietal cortex plays a specific role in path integration and in the processing of idiothetic information, whereas the hippocampus is involved in the calibration of space used by the path integration system.

Animals↗

PubMed2001

Place-cell firing does not depend on the direction of turn in a Y-maze alternation task.

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.

Action Potentials↗

PubMed2001

[Neural basis for spatial memory in animals: what do hippocampal neurons tell us?].

Recent studies relying on the recording of neuronal unit activity in freely moving rats show the existence of two populations of neurons signalling the animal's location or head direction: place cells found primarily in the hippocampus and head direction cells found in brain areas anatomically and functionally related to the hippocampus. The properties of these two neuronal populations suggest that their activity strongly depends upon information cues stemming from the spatial environment, and also suggest their involvement in spatial memory. Place cells and head direction cells would jointly participate in a neural network allowing the animal to orient in space and to store spatial locations in memory. This network would also be operating in humans, in particular for encoding specific events in episodic memory.

Animals↗

PubMed2000

Contribution of multiple sensory information to place field stability in hippocampal place cells.

Hippocampal place cells in rats display spatially selective firing in relation to both external and internal cues. In the present study, we assessed the effects of removing visual and/or olfactory cues on place field stability. Place cell activity was recorded as rats searched for randomly scattered food in a cylinder. During an initial recording session, the lights were on and the only available cue was a single white cue card. Following this session, three sessions were run in a row with the cue card removed. In addition, the lights were either turned off or left on and the floor was either cleaned or left unchanged, thus creating four conditions: dark/cleaning, dark/no cleaning, light/cleaning, and light/no cleaning. A fifth session was run with the cue card back on the cylinder wall and the lights turned on. The rat remained in the cylinder during all sessions without being removed at any time. In the dark/cleaning and light/cleaning conditions, most place fields were not stable (i.e., abruptly shifted position). In addition, half of the cells stopped firing in the dark/cleaning condition. In contrast, in the dark/no cleaning and light/no cleaning conditions, most place fields remained stable across sessions. These results suggest that 1) rats are not able to rely on only movement-related information to maintain a stable place representation, 2) visual input is necessary for the firing of a large number of cells, and 3) olfactory information can be used to compensate for the lack of visuospatial information.

Action Potentials↗

PubMed2000

Hippocampal-parietal cortical interactions in spatial cognition.

Growing evidence suggests that the associative parietal cortex (APC) of the rat is involved in the processing of spatial information. This observation raises the issue of the respective functions of the APC and the hippocampus in spatial processing as well as of their possible interactions. In this paper, we review neuroanatomical, electrophysiological, and behavioral data that support the existence of such functional interactions. Our hypothesis is that the APC is involved in the initial combination of visuospatial information and self-motion information necessary for the integration of egocentrically acquired information into allocentrically coded information, the latter step being completed in the hippocampus. The dialogue between the hippocampus and the APC is therefore crucial, particularly when the elaboration and/or updating of an allocentric representation depends on complex combinations of visuospatial and self-motion information.

Animals↗

PubMed2000

Involvement of the hippocampus and associative parietal cortex in the use of proximal and distal landmarks for navigation.

Rats with dorsal hippocampus or associative parietal cortex (APC) lesions and sham-operated controls were trained on variants of the Morris water maze navigation task. In the 'proximal landmark condition', the rats had to localize the hidden platform solely on the basis of three salient object landmarks placed directly in the swimming pool. In the 'distal landmark condition', rats could rely only on distal landmarks (room cues) to locate the platform. In the 'beacon condition', the platform location was signaled by a salient cue directly attached to it. Rats with hippocampal lesions were impaired in the distal and to a less extent in the proximal landmark condition whereas rats with parietal lesions were impaired only in the proximal landmark condition. None of the lesioned groups was impaired in the beacon condition. These results suggest that the processing of information related to proximal, distal landmarks or associated beacon are mediated by different neural systems. The hippocampus would contribute to both proximal and distal landmark processing whereas the APC would be involved in the processing of proximal landmarks only. Navigation relying on a cued-platform would not require participation of the hippocampus nor the APC. Assuming that the processing of proximal landmarks heavily depends on the integration of visuospatial and idiothetic information, these results are consistent with the hypothesis that the APC plays a role in the combination of multiple sensory information and contributes to the formation of an allocentric spatial representation.

Animals↗

PubMed2000

Sensory and memory properties of hippocampal place cells.

The rat hippocampus contains place cells whose firing is location-specific. These cells fire only when the rat enters a restricted region of the environment called the firing field. In this review, we examine the sensory information that is fundamental to the place cell system for producing spatial firing. While visual information takes precedence in the control of firing fields when it is available, local (olfactory and/or tactile) cues combined with motion-related cues can permit stable spatial firing. Motion-related cues are integrated by hippocampal place cells, but in the absence of external cues do not support stable firing over long periods. While firing fields are based on a variety of sensory cues, they do not strictly depend on such cues. Rather, sensory information is important for activating the representation appropriate to the current environment as reflected by the firing properties of place cell ensembles. Specific sensory channels as well as the memory properties of place cells can support ongoing firing under manipulations of the environment. These memory features raise the question of the role of the place cell system in the acquisition, storage and retrieval of spatial information. Based on the existing literature about the effects of hippocampal lesions and about the metabolic activations in spatial memory tasks, we suggest that a function of the place cell system is to automatically provide the organism with information about its current location so as to allow for the rapid acquisition of novel information.

Animals↗

PubMed1998

Spatial firing of hippocampal place cells in blind rats.

The rat hippocampus contains cells that are characterized by location-specific firing. Previous work has shown that the angular position of hippocampal place cell firing fields is accurately controlled by the position of visual cues, suggesting that vision plays a important role in triggering place cell activity. However, a role for other types of information has also been suggested because place cell activity can be recorded while animals are moving in the darkness. In this study, we asked whether place fields can get established in rats that have never seen their environment. We studied place cell activity in early blind rats and found that these rats had place cells very similar to those recorded from sighted rats. This result suggests that early vision is not necessary for normal firing of hippocampal place cells. Dynamic, motion-related information in conjunction with stimulus recognition seems to be sufficient.

Animals↗

PubMed1998

Posterior parietal cortex lesions severely disrupt spatial learning in DBA mice characterized by a genetic hippocampal dysfunction.

C57BL/6 (C57) and DBA/2 (DBA) inbred mice with posterior parietal cortex or sham lesions were tested in a radial eight-arm maze task with all the paths baited. In the high learner C57 strain, parietal lesions produced a limited impairment of performance without affecting maze-running strategies while the same lesions were found to affect more severely performance in the poor learner DBA strain. Because (1) the processing of spatial information has been found to depend on the conjunctive participation of the hippocampus and the posterior parietal cortex, and (2) DBA mice represent a genetic model of hippocampal dysfunction, the fact that parietal lesions impair spatial performance more severely in the DBA strain suggests that the contribution of the posterior parietal cortex to spatial learning depends on the degree of functionality of the hippocampus.

Animals↗

PubMed1996

Effortful information processing in a spontaneous spatial situation by rats with medial prefrontal lesions.

Previous research has suggested that the rat prefrontal cortex might play a role in spatial information processing and in divided attention. More recent work showed that the effect of prefrontal lesions is more important when the task involves response selection in complex situations. The first aim of the present study was to test the effect of lesions of the prelimbic area of the rat prefrontal cortex in spatial exploration, a situation involving the processing of spatial and non-spatial information, but requiring no response selection. The second aim was to manipulate the degree of cognitive effort required by the task. The latter effect was tested by manipulating the number of items to explore. Rats explored either a simple (3 objects) or a complex (6 objects) situation. We reasoned that acquiring spatial information so as to react adequately to spatial or non spatial changes involved more effortful processing in the complex situation than in the simpler one. The results suggest that the medial prefrontal cortex is not crucially involved in effortful processing when the task requires no response selection.

Animals↗

PubMed1996

The differences shown by C57BL/6 and DBA/2 inbred mice in detecting spatial novelty are subserved by a different hippocampal and parietal cortex interplay.

Inbred C57BL/6 (C57) and DBA/2 (DBA) mice with hippocampus, posterior parietal cortex or sham lesions were placed in an open-field containing five objects and their reactivity to the displacement (spatial novelty) or the substitution (object novelty) of some of these objects was examined. C57 mice reacted to spatial novelty by exploring more the displaced than the non-displaced objects while DBA mice did not show any consistent reaction. In the highly reactive C57 strain, the peak of exploratory responses directed towards the displaced objects was completely abolished by hippocampal and posterior parietal cortex lesions. In the non-reactive DBA strain, hippocampal lesions induced an aspecific decreased interest towards the two categories of objects while posterior parietal cortex lesions did not produce any behavioral modification. The high reactivity of C57 mice to spatial change appears to be subserved by the conjunctive participation of the hippocampus and the posterior parietal cortex. Conversely, the deficit shown by DBA mice in that situation seems to be related to: (i) a poorly functional hippocampus; and (ii) the non-involvement of the posterior parietal cortes. The present data suggest that the participation of the posterior parietal cortes to the detection of spatial novelty may depend on the degree of functionality of the hippocampus.

Animals↗