Entorhinal cortex lesions disrupt the transition between the use of intra- and extramaze cues for navigation in the water maze

A map of spatial navigation for neuroscience

Spatial navigation has received much attention from neuroscientists, leading to the identification of key brain areas and the discovery of numerous spatially selective cells. Despite this progress, our understanding of how the pieces fit together to drive behavior is generally lacking. We argue that this is partly caused by insufficient communication between behavioral and neuroscientific researchers. This has led the latter to under-appreciate the relevance and complexity of spatial behavior, and to focus too narrowly on characterizing neural representations of space-disconnected from the computations these representations are meant to enable. We therefore propose a taxonomy of navigation processes in mammals that can serve as a common framework for structuring and facilitating interdisciplinary research in the field. Using the taxonomy as a guide, we review behavioral and neural studies of spatial navigation. In doing so, we validate the taxonomy and showcase its usefulness in identifying potential issues with common experimental approaches, designing experiments that adequately target particular behaviors, correctly interpreting neural activity, and pointing to new avenues of research.

Phthalates and neurotoxic effects on hippocampal network plasticity

Phthalates are synthetically derived chemicals used as plasticizers in a variety of common household products. They are not chemically bound to plastic polymers and over time, easily migrate out of these products and into the environment. Experimental investigations evaluating the biological impact of phthalate exposure on developing organisms are critical given that estimates of phthalate exposure are considerably higher in infants and children compared to adults. Extensive growth and re-organization of neurocircuitry occurs during development leaving the brain highly susceptible to environmental insults. This review summarizes the effects of phthalate exposure on brain structure and function with particular emphasis on developmental aspects of hippocampal structural and functional plasticity. In general, it appears that widespread disruptions in hippocampal functional and structural plasticity occur following developmental (pre-, peri- and post-natal) exposure to phthalates. Whether these changes occur as a direct neurotoxic effect of phthalates or an indirect effect through disruption of endogenous endocrine functions is not fully understood. Comprehensive investigations that simultaneously assess the neurodevelopmental, neurotoxic, neuroendocrine and behavioral correlates of phthalate exposure are needed to provide an opportunity to thoroughly evaluate the neurotoxic potential of phthalates throughout the lifespan.

Dorsolateral striatal lesions impair navigation based on landmark-goal vectors but facilitate spatial learning based on a "cognitive map"

In three experiments, the nature of the interaction between multiple memory systems in rats solving a variation of a spatial task in the water maze was investigated. Throughout training rats were able to find a submerged platform at a fixed distance and direction from an intramaze landmark by learning a landmark-goal vector. Extramaze cues were also available for standard place learning, or "cognitive mapping," but these cues were valid only within each session, as the position of the platform moved around the pool between sessions together with the intramaze landmark. Animals could therefore learn the position of the platform by taking the consistent vector from the landmark across sessions or by rapidly encoding the new platform position on each session with reference to the extramaze cues. Excitotoxic lesions of the dorsolateral striatum impaired vector-based learning but facilitated cognitive map-based rapid place learning when the extramaze cues were relatively poor (Experiment 1) but not when they were more salient (Experiments 2 and 3). The way the lesion effects interacted with cue availability is consistent with the idea that the memory systems involved in the current navigation task are functionally cooperative yet associatively competitive in nature.

Relative contributions of CA3 and medial entorhinal cortex to memory in rats

The hippocampal CA1 field processes spatial information, but the relative importance of intra- vs. extra-hippocampal sources of input into CA1 for spatial behavior is unclear. To characterize the relative roles of these two sources of input, originating in the hippocampal field CA3 and in the medial entorhinal cortex (MEC), we studied effects of discrete neurotoxic lesions of CA3 or MEC on concurrent spatial and nonspatial navigation tasks, and on synaptic transmission in afferents to CA1. Lesions in CA3 or MEC regions that abolished CA3-CA1, or reduced MEC-CA1 synaptic transmission, respectively, impaired spatial navigation and unexpectedly interfered with cue response, suggesting that in certain conditions of training regimen, hippocampal activity may influence behavior otherwise supported by nonhippocampal neural networks. MEC lesions had milder and temporary behavioral effects, but also markedly amplified transmission in the CA3-CA1 pathway. Extensive behavioral training had a similar, but more modest effect on CA3-CA1 transmission. Thus, cortical input to the hippocampus modulates CA1 activity both directly and indirectly, through heterosynaptic interaction, to control information flow in the hippocampal loop. Following damage to hippocampal cortical input, the functional coupling of separate intra- and extra-hippocampal inputs to CA1 involved in normal learning may initiate processes that support recovery of behavioral function. Such a process may explain how CA3 lesions, which do not significantly modify the basic features of CA1 neural activity, nonetheless impair spatial recall, whereas lesions of EC input to CA1, which reduce the spatial selectivity of CA1 firing in foraging rats, have only mild effects on spatial navigation.

From objects to landmarks: the function of visual location information in spatial navigation

Landmarks play an important role in guiding navigational behavior. A host of studies in the last 15 years has demonstrated that environmental objects can act as landmarks for navigation in different ways. In this review, we propose a parsimonious four-part taxonomy for conceptualizing object location information during navigation. We begin by outlining object properties that appear to be important for a landmark to attain salience. We then systematically examine the different functions of objects as navigational landmarks based on previous behavioral and neuroanatomical findings in rodents and humans. Evidence is presented showing that single environmental objects can function as navigational beacons, or act as associative or orientation cues. In addition, we argue that extended surfaces or boundaries can act as landmarks by providing a frame of reference for encoding spatial information. The present review provides a concise taxonomy of the use of visual objects as landmarks in navigation and should serve as a useful reference for future research into landmark-based spatial navigation.

Temporary inactivation of the dorsal entorhinal cortex impairs acquisition and retrieval of spatial information

We tested the effects of temporary inactivation of the dorsal entorhinal cortex on spatial discrimination using a conditioned cue preference (CCP) paradigm. The three phases of the procedure were: pre-exposure: unreinforced exploration of the center platform and two adjacent arms of an eight-arm radial maze; training: rats were confined to the ends of the two arms on alternate days - one arm always contained food and the other never contained food; testing: unreinforced exploration of the center platform and the two arms. Rats that received bilateral infusions of saline into the dorsal entorhinal cortex before the training trials or before the test trial spent significantly more time in the arm that previously contained food than in the arm that never contained food, demonstrating that they had acquired and were able to express information that discriminated between the two adjacent maze arms. In contrast, rats that received bilateral, intra-entorhinal infusions of muscimol, a gamma-aminobutyric acid(a) (GABA(a)) agonist, before either training or testing spent equal amounts of time in the two arms, indicating that they failed to acquire and were unable to express this information. Interactions between the entorhinal cortex and hippocampus in the acquisition and expression of the information required for this discrimination are discussed.

Entorhinal cortex and cognition

Understanding the function of the entorhinal cortex (EC) has been an important subject over the years, not least because of its cortical intermediary to and from the hippocampus proper, and because of electrophysiological advances which have started to reveal the physiology in behaving animals. Clearly, a lot more needs to be done but is clear to date that EC is not merely a throughput station providing all hippocampal subfields with sensory information, but that processing within EC contributes significantly to attention, conditioning, event and spatial cognition possibly by compressing representations that overlap in time. These are transmitted to the hippocampus, where they are differentiated again and returned to EC. Preliminary evidence for such a role, but also their possible pitfalls are summarised.

Neural representation of object location and route direction: an event-related fMRI study

The human brain distinguishes between landmarks placed at navigationally relevant and irrelevant locations. However, to provide a successful wayfinding mechanism not only landmarks but also the routes between them need to be stored. We examined the neural representation of a memory for route direction and a memory for relevant landmarks. Healthy human adults viewed objects along a route through a virtual maze. Event-related functional magnetic resonance imaging (fMRI) data were acquired during a subsequent subliminal priming recognition task. Prime-objects either preceded or succeeded a target-object on a preciously learned route. Our results provide evidence that the parahippocampal gyri distinguish between relevant and irrelevant landmarks whereas the inferior parietal gyrus, the anterior cingulate gyrus as well as the right caudate nucleus are involved in the coding of route direction. These data show that separated memory systems store different spatial information. A memory for navigationally relevant object information and a memory for route direction exist.

Flexible use of proximal objects and distal cues by hippocampal place cells

The purpose of the present experiment was to examine how distal cues and proximal objects interact to control firing fields. In a previous study, Shapiro et al. (1997) Hippocampus 7:624-642, suggested that hippocampal place cell firing is controlled by distal cues and proximal floor inserts in a flexible and hierarchical fashion. Control exerted by the combined set of cues prevailed over control by distal cues, which itself prevailed over control by proximal cues. Here, we examined the generality of this hierarchy in the use of cues. Place cells were recorded as rats performed a pellet chasing task on a platform containing three proximal objects, surrounded by a curtain where three visual stimuli were hung. A double rotation of distal and proximal cue sets producing a 180 degrees mismatch revealed noncoherent responses of place cells. Most fields were controlled by the configuration of proximal and distal cues (i.e., remapped). Less often, fields were controlled by specific cues with a majority being controlled by proximal cues, thus suggesting that response hierarchy is modulated by the environment. We finally examined the effect of removing one set of cues after the double rotation session. Half of the fields were controlled by the remaining cues while the other half remapped, thus suggesting a competition between pattern completion and pattern separation processes. Furthermore, cells that were controlled by the remaining cues were mainly those that had remapped in the double rotation session. Our results are compatible with the idea that the flexibility of the place cell system results from an interaction between the sensory properties of individual cell and the attractor networks properties of the whole place cell population.

Unreinforced spatial (latent) learning is mediated by a circuit that includes dorsal entorhinal cortex and fimbria fornix

The relationship of the entorhinal cortex (EC) and fimbria-fornix (FF) in unreinforced spatial (latent) learning was studied using the conditioned-cue-preference task on an eight-arm radial maze. The maze was turned before every trial to eliminate the use of local cues. During three pre-exposure sessions, food-deprived rats explored the center platform and two adjacent arms of the maze. Since most of the same cues were visible from both arm locations, discriminating them required spatial learning. The rats were then alternately confined to the end of each arm over several days: one arm always contained food, the other was empty. Finally, the rats were allowed free access to both arms with no food present. Normal rats spent more time in their food-paired than in their unpaired arms showing that they learned to discriminate between the arm locations. Bilateral micro-injections of muscimol into the dorsal, but not into the ventral EC, given before the pre-exposure sessions only, impaired the discrimination. The discrimination was also impaired in rats with unilateral lesions of FF and contralateral injections of muscimol into the dorsal EC given before the pre-exposure sessions. Ipsilateral FF lesions and entorhinal inactivation had no effect. These results indicate that the acquisition of information during unreinforced exploration of a novel environment requires an intact circuit involving the dorsal EC and fimbria fornix. Together with previous reports, that this form of learning does not require a functional hippocampus, (Gaskin et al. (2005) Hippocampus 15:1085-1093) the findings also suggest that the acquisition of certain kinds of unreinforced information by this circuit is independent of the hippocampus.

Spatial memory in the rat requires the dorsolateral band of the entorhinal cortex

The extensive connections of the entorhinal cortex with the hippocampus and the neocortex point to this region as a major interface in the hippocampal-neocortical interactions underlying memory. We asked whether hippocampal-dependent recall of spatial memory depends on the entorhinal cortex, and, if so, which parts are critical. After training in a Morris water maze, rats received fiber-sparing lesions in the dorsolateral band of the entorhinal cortex, which mediates much of the visuospatial input to the dorsal hippocampus. These lesions entirely disrupted retention and retarded new learning. Spatial memory was spared by lesions in the ventromedial band, which connects primarily with ventral hippocampus, but these lesions reduced defensive behavior on an elevated plus maze, mirroring the effects of damage to ventral hippocampus. The results suggest that the functional differences between dorsal and ventral hippocampus reflect their connectivity with modules of the entorhinal cortex that are differently linked to the rest of the cortex.

Impairment in long-term retention but not short-term performance on a water maze reversal task following hippocampal or mediodorsal striatal n-methyl-d-aspartate receptor blockade

Male Long-Evans rats were injected with 32 ng/mul of the N-methyl-D-aspartate (NMDA) receptor antagonist 3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid (CPP) or vehicle and trained to locate a hidden platform in a different location (reversal training) than used on the initial 4 days of training. Rats treated with vehicle or CPP into the dorsal hippocampus, basolateral amygdala, or mediodorsal striatum had similar latencies to locate the platform on the reversal day. Rats infused with CPP into the dorsal hippocampus or mediodorsal striatum failed to search preferentially in the novel location during a 24-hr, drug-free retention test, whereas all other groups searched preferentially in this location. Therefore, blocking dorsal hippocampal or mediodorsal striatal NMDA receptors selectively blocked long-term spatial retention without producing short-term performance deficits.

Entorhinal cortex lesions impair the use of distal but not proximal landmarks during place navigation in the rat

Rats with entorhinal cortex lesions were trained in two versions of the place navigation task in the Morris water maze. In the distal condition, they had to locate the hidden platform on the basis of remote landmarks, while in the proximal condition, they had to rely only on a configuration of proximal objects, placed directly in the pool. Entorhinal rats were impaired in using distal landmarks but were able to use proximal landmarks to navigate toward the platform. These results suggest that the use of distal and proximal landmarks during navigation involves activation of different neural structures. They also suggest, in agreement with previous data, that there are two distinct landmark-processing systems, one devoted to the processing of proximal landmarks and the other to the processing of distal landmarks.

Discrimination of social odors and their locations: role of lateral entorhinal area

Discrimination of individual conspecifics by their odors has been reported for many mammalian species, but little information is available on the brain mechanisms underlying such discrimination. A previous study reported that large parahippocampal lesions, centered on entorhinal cortex but extending into adjacent areas of the brain, eliminated female hamsters' ability to discriminate the flank gland odors of different individuals, as tested with habituation-dishabituation methods. The current study examined the effects of lesions restricted to the lateral entorhinal area on such discriminations. Female hamsters were tested in several types of habituation procedure that differed across a sequence of trials in the locations of familiar and novel social odors. Discrimination of two individuals' odors depended on the sequences of locations of the odors, indicating that odor identity and location were simultaneously salient to female hamsters. Lesions of lateral entorhinal area interfered with this spatial-olfactory discrimination. When confounding spatial cues were eliminated, hamsters did discriminate between novel and familiar odors, and lesions in the entorhinal area did not eliminate this ability. Thus, although the lateral entorhinal area is not necessary for individual odor discrimination, it is involved in processing odor-place combinations.