Bilateral NMDA lesions centered on the postrhinal cortex have minimal effects on hippocampal place cell firing

Spatial Information Encoding across Multiple Neocortical Regions Depends on an Intact Hippocampus

There has been considerable research showing populations of neurons encoding for different aspects of space in the brain. Recently, several studies using two-photon calcium imaging and virtual navigation have identified "spatially" modulated neurons in the posterior cortex. We enquire here whether the presence of such spatial representations may be a cortex-wide phenomenon and, if so, whether these representations can be organized in the absence of the hippocampus. To this end, we imaged the dorsal cortex of mice running on a treadmill populated with tactile cues. A high percentage (40-80%) of the detected neurons exhibited sparse, spatially localized activity, with activity fields uniformly localized over the track. The development of this location specificity was impaired by hippocampal damage. Thus, there is a substantial population of neurons distributed widely over the cortex that collectively form a continuous representation of the explored environment, and hippocampal outflow is necessary to organize this phenomenon.SIGNIFICANCE STATEMENT Increasing evidence points to the role of the neocortex in encoding spatial information. Whether this feature is linked to hippocampal functions is largely unknown. Here, we systematically surveyed multiple regions in the dorsal cortex of the same animal for the presence of signals encoding for spatial position. We described populations of cortical neurons expressing sequential patterns of activity localized in space in primary, secondary, and associational areas. Furthermore, we showed that the formation of these spatial representations was impacted by hippocampal lesion. Our results indicate that hippocampal inputs are necessary to maintain a precise cortical representation of space.

The Neurocognitive Basis of Spatial Reorientation

The ability to recover one's bearings when lost is a skill that is fundamental for spatial navigation. We review the cognitive and neural mechanisms that underlie this ability, with the aim of linking together previously disparate findings from animal behavior, human psychology, electrophysiology, and cognitive neuroscience. Behavioral work suggests that reorientation involves two key abilities: first, the recovery of a spatial reference frame (a cognitive map) that is appropriate to the current environment; and second, the determination of one's heading and location relative to that reference frame. Electrophysiological recording studies, primarily in rodents, have revealed potential correlates of these operations in place, grid, border/boundary, and head-direction cells in the hippocampal formation. Cognitive neuroscience studies, primarily in humans, suggest that the perceptual inputs necessary for these operations are processed by neocortical regions such as the retrosplenial complex, occipital place area and parahippocampal place area, with the retrosplenial complex mediating spatial transformations between the local environment and the recovered spatial reference frame, the occipital place area supporting perception of local boundaries, and the parahippocampal place area processing visual information that is essential for identification of the local spatial context. By combining results across these various literatures, we converge on a unified account of reorientation that bridges the cognitive and neural domains.

The postsubiculum and spatial learning: the role of postsubicular synaptic activity and synaptic plasticity in hippocampal place cell, object, and object-location memory

Visual landmarks exert stimulus control over spatial behavior and the spatially tuned firing of place, head-direction, and grid cells in the rodent. However, the neural site of convergence for representations of landmarks and representations of space has yet to be identified. A potential site of plasticity underlying associations with landmarks is the postsubiculum. To test this, we blocked glutamatergic transmission in the rat postsubiculum with CNQX, or NMDA receptor-dependent plasticity with d-AP5. These infusions were sufficient to block evoked potentials from the lateral dorsal thalamus and long-term depression following tetanization of this input to the postsubiculum, respectively. In a second experiment, CNQX disrupted the stability of rat hippocampal place cell fields in a familiar environment. In a novel environment, blockade of plasticity with d-AP5 in the postsubiculum did not block the formation of a stable place field map following a 6 h delay. In a final behavioral experiment, postsubicular infusions of both compounds blocked object-location memory in the rat, but did not affect object recognition memory. These results suggest that the postsubiculum is necessary for the recognition of familiar environments, and that NMDA receptor-dependent plasticity in the postsubiculum is required for the formation of new object-place associations that support recognition memory. However, plasticity in the postsubiculum is not necessary for the formation of new spatial maps.

Differential contribution of hippocampus, perirhinal cortex and postrhinal cortex to allocentric spatial memory in the radial maze

Rats with hippocampal, perirhinal cortex and postrhinal cortex lesions were trained in a reference spatial memory task to determine whether these structures contribute differentially to the acquisition and retention of spatial information. The results of Experiment 1 indicated that hippocampal lesions profoundly impaired the acquisition of the task. However, postrhinal lesions produced only a mild deficit and perirhinal lesions produced no deficit whatsoever in the learning of the task. During acquisition, hippocampus-damaged rats committed more perseverative errors than postrhinal rats, suggesting that the nature of the operations performed by each of these structures is different. The results of Experiment 2 showed a profound deficit in retention in hippocampal and postrhinal-lesioned animals tested 24 days after training. Perirhinal-lesioned animals, however, executed the task just as well as the control subjects did. These functional data, in consonance with existing connectivity data, suggest that each of these medial temporal lobe regions makes a different contribution to allocentric spatial learning and memory.

Profound retrograde but absence of anterograde amnesia for cued place learning in rats with hippocampal lesions

Previous studies in our lab have shown that slight modifications in the spatial reference memory procedure can overcome the deficit in spatial learning typically observed in rats with hippocampal damage. However, it is unknown if memory acquired under such training circumstances is spared after hippocampal lesions. With this aim a four-arm plus-shaped maze and a spatial reference memory paradigm were used, in which the goal arm was doubly marked: by an intramaze cue (a piece of sandpaper positioned on the floor of the arm) and by the extramaze constellation of stimuli around the maze. Experiment 1 replicated previous findings showing that hippocampally damaged rats can learn a place response just as well as the controls when the intramaze cue is present during the training, but they are unable to do so in the absence of the intramaze signal. When the learning procedure was doubly signaled, a transfer test performed 24h after the end of acquisition demonstrated that lesioned rats showed perfect memory for the goal arm when the intramaze cue was removed. Experiment 2 investigated the effect of hippocampal damage 1 day after the learning. Results showed that regardless of the training procedure employed (with or without the intramaze cue), hippocampal lesions produced a profound retrograde amnesia. Thus, although the absence of anterograde amnesia suggests that structures other that the hippocampus can take charge of the acquisition, the presence of retrograde amnesia indicates the critical role of the normal hippocampus in the long-term formation of allocentric information.

Origins of landmark encoding in the brain

The ability to perceive one's position and directional heading relative to landmarks is necessary for successful navigation within an environment. Recent studies have shown that the visual system dominantly controls the neural representations of directional heading and location when familiar visual cues are available, and several neural circuits, or streams, have been proposed to be crucial for visual information processing. Here, we summarize the evidence that the dorsal presubiculum (also known as the postsubiculum) is critically important for the direct transfer of visual landmark information to spatial signals within the limbic system.

The velocity-related firing property of hippocampal place cells is dependent on self-movement

Hippocampal place cells have the interesting property of increasing their firing rate when a freely moving animal increases its running speed through the cell's place field. A previous study from this laboratory showed that this movement-related firing property is disrupted by lesions of the perirhinal cortex (PrhC). It is possible, therefore, that PrhC lesions disrupt speed-modulated sensory information such as optic flow or motor efferent or proprioceptive input that might be available to the hippocampus from the PrhC. To test this hypothesis, rats with single unit recording electrodes implanted in the CA1 region of the hippocampus received different levels of optic flow stimulation in both a freely moving and a passive movement condition. The effects of PrhC lesions were also tested. Although increasing the amount of optic flow information available decreased place field size, it had no discernable effect on the movement-firing rate relationship in the place cells of control animals run in the free-movement condition. In lesioned animals the relationship was disrupted, replicating our previous results. In the passive movement condition many place cells stopped firing. In those cells that did fire, however, the movement-firing rate relationship was no longer evident. These data indicate that the movement-firing rate relationship is not driven by vestibular or optic flow cues, but rather depends on either motor efferent or proprioceptive input, or that it results from some other form of input that may be modulated by self-motion, such as from the vibrissae.

Preserved learning about allocentric cues but impaired flexible memory expression in rats with hippocampal lesions

Several studies have shown that slight modifications in the standard reference spatial memory procedure normally used for allocentric learning in the Morris water maze and the radial maze, can overcome the classic deficit in allocentric navigation typically observed in rats with hippocampal damage. In these special paradigms, however, there is only intramaze manipulation of a salient stimulus. The present study was designed to investigate whether extramaze manipulations produce a similar outcome. With this aim a four-arm plus-shaped maze and a reference spatial memory paradigm were used, in which the goal arm was marked in two ways: by a prominent extramaze cue (intermittent light), which maintained a constant relation with the goal, and by the extramaze constellation of stimuli around the maze. Experiment 1 showed that, unlike the standard version of the task, using this special training procedure hippocampally-damaged rats could learn a place response as quickly as control animals; importantly, one day after reaching criterion, lesioned and control subjects performed the task perfectly during a transfer test in which the salient extramaze stimulus used during the acquisition was removed. However, although acquisition deficit was overcomed in these lesioned animals, a profound deficit in retention was detected 15 days later. Experiment 2 suggests that although under our special paradigm hippocampal rats can learn a place response, spatial memory only can be expressed when the requisites of behavioral flexibility are minimal. These findings suggest that, under certain circumstances, extrahippocampal structures are sufficient for building a coherent allocentric representation of space; however, flexible memory expression is dependent, fundamentally, on hippocampal functioning.