Time and space in the hippocampus

The storage and recall of memories in the hippocampo-cortical system

A quantitative computational theory of the operation of the hippocampus as an episodic memory system is described. The CA3 system operates as a single attractor or autoassociation network (1) to enable rapid one-trial associations between any spatial location (place in rodents or spatial view in primates) and an object or reward and (2) to provide for completion of the whole memory during recall from any part. The theory is extended to associations between time and object or reward to implement temporal order memory, which is also important in episodic memory. The dentate gyrus performs pattern separation by competitive learning to create sparse representations producing, for example, neurons with place-like fields from entorhinal cortex grid cells. The dentate granule cells generate, by the very small number of mossy fibre connections to CA3, a randomizing pattern separation effect that is important during learning but not recall and that separates out the patterns represented by CA3 firing as being very different from each other. This is optimal for an unstructured episodic memory system in which each memory must be kept distinct from other memories. The direct perforant path input to CA3 is quantitatively appropriate for providing the cue for recall in CA3 but not for learning. The CA1 recodes information from CA3 to set up associatively learned backprojections to the neocortex to allow the subsequent retrieval of information to the neocortex, giving a quantitative account of the large number of hippocampo-neocortical and neocortical-neocortical backprojections. Tests of the theory including hippocampal subregion analyses and hippocampal NMDA receptor knockouts are described and support the theory.

Human Episodic Memory Retrieval Is Accompanied by a Neural Contiguity Effect

Cognitive psychologists have long hypothesized that experiences are encoded in a temporal context that changes gradually over time. When an episodic memory is retrieved, the state of context is recovered-a jump back in time. We recorded from single units in the medial temporal lobe of epilepsy patients performing an item recognition task. The population vector changed gradually over minutes during presentation of the list. When a probe from the list was remembered with high confidence, the population vector reinstated the temporal context of the original presentation of that probe during study, a neural contiguity effect that provides a possible mechanism for behavioral contiguity effects. This pattern was only observed for well remembered probes; old probes that were not well remembered showed an anti-contiguity effect. These results constitute the first direct evidence that recovery of an episodic memory in humans is associated with retrieval of a gradually changing state of temporal context, a neural "jump back in time" that parallels the act of remembering.SIGNIFICANCE STATEMENT Episodic memory is the ability to relive a specific experience from one's life. For decades, researchers have hypothesized that, unlike other forms of memory that can be described as simple associations between stimuli, episodic memory depends on the recovery of a neural representation of spatiotemporal context. During study of a sequence of stimuli, the brain state of epilepsy patients changed slowly over at least a minute. When the participant remembered a particular event from the list, this gradually changing state was recovered. This provides direct confirmation of the prediction from computational models of episodic memory. The resolution of this point means that the study of episodic memory can focus on the mechanisms by which this representation of spatiotemporal context is maintained and sometimes recovered.

The Medial Prefrontal Cortex Shapes Dopamine Reward Prediction Errors under State Uncertainty

Animals make predictions based on currently available information. In natural settings, sensory cues may not reveal complete information, requiring the animal to infer the "hidden state" of the environment. The brain structures important in hidden state inference remain unknown. A previous study showed that midbrain dopamine neurons exhibit distinct response patterns depending on whether reward is delivered in 100% (task 1) or 90% of trials (task 2) in a classical conditioning task. Here we found that inactivation of the medial prefrontal cortex (mPFC) affected dopaminergic signaling in task 2, in which the hidden state must be inferred ("will reward come or not?"), but not in task 1, where the state was known with certainty. Computational modeling suggests that the effects of inactivation are best explained by a circuit in which the mPFC conveys inference over hidden states to the dopamine system. VIDEO ABSTRACT.

Electrical Stimulation in Hippocampus and Entorhinal Cortex Impairs Spatial and Temporal Memory

The medial temporal lobe (MTL) is widely implicated in supporting episodic memory and navigation, but its precise functional role in organizing memory across time and space remains elusive. Here we examine the specific cognitive processes implemented by MTL structures (hippocampus and entorhinal cortex) to organize memory by using electrical brain stimulation, leveraging its ability to establish causal links between brain regions and features of behavior. We studied neurosurgical patients of both sexes who performed spatial-navigation and verbal-episodic memory tasks while brain stimulation was applied in various regions during learning. During the verbal memory task, stimulation in the MTL disrupted the temporal organization of encoded memories such that items learned with stimulation tended to be recalled in a more randomized order. During the spatial task, MTL stimulation impaired subjects' abilities to remember items located far away from boundaries. These stimulation effects were specific to the MTL. Our findings thus provide the first causal demonstration in humans of the specific memory processes that are performed by the MTL to encode when and where events occurred.SIGNIFICANCE STATEMENT Numerous studies have implicated the medial temporal lobe (MTL) in encoding spatial and temporal memories, but they have not been able to causally demonstrate the nature of the cognitive processes by which this occurs in real-time. Electrical brain stimulation is able to demonstrate causal links between a brain region and a given function with high temporal precision. By examining behavior in a memory task as subjects received MTL stimulation, we provide the first causal evidence demonstrating the role of the MTL in organizing the spatial and temporal aspects of episodic memory.

Memory as Perception of the Past: Compressed Time inMind and Brain

In the visual system retinal space is compressed such that acuity decreases further from the fovea. Different forms of memory may rely on a compressed representation of time, manifested as decreased accuracy for events that happened further in the past. Neurophysiologically, "time cells" show receptive fields in time. Analogous to the compression of visual space, time cells show less acuity for events further in the past. Behavioral evidence suggests memory can be accessed by scanning a compressed temporal representation, analogous to visual search. This suggests a common computational language for visual attention and memory retrieval. In this view, time functions like a scaffolding that organizes memories in much the same way that retinal space functions like a scaffolding for visual perception.

Hippocampal corticotropin-releasing hormone neurons support recognition memory and modulate hippocampal excitability

Corticotropin-releasing hormone (CRH) signaling in the hippocampus has been established to be important for mediating the effects of stress on learning and memory. Given our laboratory’s recent characterization of a subset of hippocampal CRH neurons as a novel class of GABAergic interneurons, we hypothesized that these local GABAergic hippocampal CRH neurons may influence hippocampal function. Here we applied an array of molecular tools to selectively label and manipulate hippocampal CRH neurons in mice, in order to assess this interneuron population’s impact on hippocampus-dependent behaviors and hippocampal network excitability. Genetically-targeted ablation of hippocampal CRH neurons in vivo impaired object recognition memory and substantially enhanced the severity of kainic acid-induced seizures. Conversely, selective activation of CRH neurons in vitro suppressed the excitability of the mossy fiber-CA3 pathway. Additional experiments are needed to reconcile the functions of GABA and CRH signaling of hippocampal CRH neurons on hippocampal function. However, our results indicate that this interneuron population plays an important role in maintaining adaptive network excitability, and provide a specific circuit-level mechanism for this role.

Contributions of primate prefrontal cortex and medial temporal lobe to temporal-order memory

Neuropsychological and neurophysiological studies have emphasized the role of the prefrontal cortex (PFC) in maintaining information about the temporal order of events or items for upcoming actions. However, the medial temporal lobe (MTL) has also been considered critical to bind individual events or items to their temporal context in episodic memory. Here we characterize the contributions of these brain areas by comparing single-unit activity in the dorsal and ventral regions of macaque lateral PFC (d-PFC and v-PFC) with activity in MTL areas including the hippocampus (HPC), entorhinal cortex, and perirhinal cortex (PRC) as well as in area TE during the encoding phase of a temporal-order memory task. The v-PFC cells signaled specific items at particular time periods of the task. By contrast, MTL cortical cells signaled specific items across multiple time periods and discriminated the items between time periods by modulating their firing rates. Analysis of the temporal dynamics of these signals showed that the conjunctive signal of item and temporal-order information in PRC developed earlier than that seen in v-PFC. During the delay interval between the two cue stimuli, while v-PFC provided prominent stimulus-selective delay activity, MTL areas did not. Both regions of PFC and HPC exhibited an incremental timing signal that appeared to represent the continuous passage of time during the encoding phase. However, the incremental timing signal in HPC was more prominent than that observed in PFC. These results suggest that PFC and MTL contribute to the encoding of the integration of item and timing information in distinct ways.

Nonspatial sequence coding varies along the CA1 transverse axis

The hippocampus plays a critical role in the memory for sequences of events, a defining feature of episodic memory. To shed light on the fundamental mechanisms supporting this capacity, we recently recorded neural activity in CA1 as rats performed a nonspatial odor sequence memory task. Our main finding was that, while the animals' location and behavior remained constant, a proportion of CA1 neurons fired differentially to odors depending on whether they were presented in or out of sequence (sequence cells). Here, we further examined if such sequence coding varied along the distal-to-proximal axis of the dorsal CA1 region (distal: toward subiculum; proximal: toward CA3). Differences in information processing along this axis have been suggested by recent anatomical and electrophysiological evidence that odor information may be more strongly represented in the distal segment, whereas spatial information may be more strongly represented in the proximal segment. Recorded neurons were grouped into four arbitrary sections of dorsal CA1, ranging from distal to proximal. We found that, although sequence cell coding was observed across the distal-to-proximal extent of CA1 from which we recorded, it was significantly higher in intermediate CA1, a region with more balanced anatomical input from lateral and medial entorhinal regions. More specifically, in that particular segment of CA1, we observed a significant increase in the magnitude of sequence coding of all cells, as well as in the sequential information content of sequence cells. Importantly, a different pattern was observed when examining the distribution of spatial coding from the same electrodes. Consistent with previous reports, our results suggest that spatial information was more strongly represented in the proximal section of CA1 (higher proportion of cells with place fields). These findings indicate that nonspatial sequence memory coding is not uniformly distributed along the transverse axis of CA1, and that this distribution does not simply follow the expected gradient based on the stimulus modality or the degree of spatial selectivity. Instead, the observed distribution suggests this form of sequence coding may be associated with convergent input from lateral and medial entorhinal regions, which is present throughout the proximodistal axis but greater in intermediate CA1.

Time Regained: How the Human Brain Constructs Memory for Time

Life's episodes unfold against a context that changes with time. Recent neuroimaging studies have revealed significant findings about how specific areas of the human brain may support the representation of temporal information in memory. A consistent theme in these studies is that the hippocampus appears to play a central role in representing temporal context, as operationalized in neuroimaging studies of arbitrary lists of items, sequences of items, or meaningful, lifelike events. Additionally, activity in a posterior medial cortical network may reflect the representation of generalized temporal information for meaningful events. The hippocampus, posterior medial network, and other regions-particularly in prefrontal cortex-appear to play complementary roles in memory for temporal context.

Temporal and spatial context in the mind and brain

Theories of episodic memory have long hypothesized that recollection of a specific instance from one's life is mediated by recovery of a neural state of spatiotemporal context. This paper reviews recent theoretical advances in formal models of spatiotemporal context and a growing body of neurophysiological evidence from human imaging studies and animal work that neural populations in the hippocampus and other brain regions support a representation of spatiotemporal context.

Time (and space) in the hippocampus

There is considerable recent evidence that, in addition to its representation of space, the hippocampus also represents the temporal organization of memories. Time plays a central role in episodic memory, and studies have identified the hippocampus as playing an essential role in the temporal organization of memories in humans and animals. Temporal organization is supported by a gradually changing temporal context signal in the hippocampus, and this changing context signal involves "time cells" in the hippocampus that code sequential moments in temporally organized experiences. Finally, hippocampal temporal context representations mechanisms in intrinsic circuitry and oscillatory patterns throughout hippocampal subfields and likely depend on inputs from parahippocampal cortical areas and a widespread temporal processing system in the neocortex.

Dynamic Hippocampal and Prefrontal Contributions to Memory Processes and Representations Blur the Boundaries of Traditional Cognitive Domains

The hippocampus has long been known to be a critical component of the memory system involved in the formation and use of long-term declarative memory. However, recent findings have revealed that the reach of hippocampal contributions extends to a variety of domains and tasks that require the flexible use of cognitive and social behavior, including domains traditionally linked to prefrontal cortex (PFC), such as decision-making. In addition, the prefrontal cortex (PFC) has gained traction as a necessary part of the memory system. These findings challenge the conventional characterizations of hippocampus and PFC as being circumscribed to traditional cognitive domains. Here, we emphasize that the ability to parsimoniously account for the breadth of hippocampal and PFC contributions to behavior, in terms of memory function and beyond, requires theoretical advances in our understanding of their characteristic processing features and mental representations. Notably, several literatures exist that touch upon this issue, but have remained disjointed because of methodological differences that necessarily limit the scope of inquiry, as well as the somewhat artificial boundaries that have been historically imposed between domains of cognition. In particular, this article focuses on the contribution of relational memory theory as an example of a framework that describes both the representations and processes supported by the hippocampus, and further elucidates the role of the hippocampal–PFC network to a variety of behaviors.

Increased temporal discounting after chronic stress in CHL1-deficient mice is reversed by 5-HT2C agonist Ro 60-0175

Schizophrenia is a neurodevelopmental disorder in which impaired decision-making and goal-directed behaviors are core features. One of the genes associated with schizophrenia is the Close Homolog of L1 (CHL1); CHL1-deficient mice are considered a model of schizophrenia-like deficits, including sensorimotor gating, interval timing and spatial memory impairments. Here we investigated temporal discounting in CHL1-deficient (KO) mice and their wild-type littermates. Although no discounting differences were found under baseline conditions, CHL1-KO mice showed increased impulsive choice following chronic unpredictable stress (fewer % larger-later choices, and reduced area under the discounting curve). Stressed CHL1-KO mice also showed decreased neuronal activation (number of cFos positive neurons) in the discounting task in the prelimbic cortex and dorsal striatum, areas thought to be part of executive and temporal processing circuits. Impulsive choice alterations were reversed by the 5-HT2C agonist Ro 60-0175. Our results provide evidence for a gene x environment, double-hit model of stress-related decision-making impairments, and identify CHL1-deficient mice as a mouse model for these deficits in regard to schizophrenia-like phenotypes.

Dissociations between Spatial and Temporal Order Memory: A Neuropsychological Patient Study

OBJECTIVES

In complex real life situations, memories for temporal and spatial information are naturally linked since sequential events coincide in time and space. Whether this connection is inseparable or instead whether these processes are functionally dissociable was investigated in this patient study.

METHODS

Spatial object-location and temporal order memory tasks were administered to 36 stroke patients and 44 healthy control participants.

RESULTS

On group level, patients with a stroke in the left hemisphere performed worse on temporal order memory, compared to the control participants. On individual level, using a multiple case-study approach, a clear pattern of dissociations was found between memory for temporal and for spatial features.

CONCLUSIONS

These findings indicate that location and temporal order memory contain functionally separable processes. This adds to our understanding of how context information is processed in human memory. (JINS, 2017, 23, 421-430).

Imagining the future: The core episodic simulation network dissociates as a function of timecourse and the amount of simulated information

Neuroimaging data indicate that episodic memory (i.e., remembering specific past experiences) and episodic simulation (i.e., imagining specific future experiences) are associated with enhanced activity in a common set of neural regions, often referred to as the core network. This network comprises the hippocampus, parahippocampal cortex, lateral and medial parietal cortex, lateral temporal cortex, and medial prefrontal cortex. Evidence for a core network has been taken as support for the idea that episodic memory and episodic simulation are supported by common processes. Much remains to be learned about how specific core network regions contribute to specific aspects of episodic simulation. Prior neuroimaging studies of episodic memory indicate that certain regions within the core network are differentially sensitive to the amount of information recollected (e.g., the left lateral parietal cortex). In addition, certain core network regions dissociate as a function of their timecourse of engagement during episodic memory (e.g., transient activity in the posterior hippocampus and sustained activity in the left lateral parietal cortex). In the current study, we assessed whether similar dissociations could be observed during episodic simulation. We found that the left lateral parietal cortex modulates as a function of the amount of simulated details. Of particular interest, while the hippocampus was insensitive to the amount of simulated details, we observed a temporal dissociation within the hippocampus: transient activity occurred in relatively posterior portions of the hippocampus and sustained activity occurred in anterior portions. Because the posterior hippocampal and lateral parietal findings parallel those observed during episodic memory, the present results add to the evidence that episodic memory and episodic simulation are supported by common processes. Critically, the present study also provides evidence that regions within the core network support dissociable processes.

Zebrafish and relational memory: Could a simple fish be useful for the analysis of biological mechanisms of complex vertebrate learning?

Analysis of the zebrafish allows one to combine two distinct scientific approaches, comparative ethology and neurobehavioral genetics. Furthermore, this species arguably represents an optimal compromise between system complexity and practical simplicity. This mini-review focuses on a complex form of learning, relational learning and memory, in zebrafish. It argues that zebrafish are capable of this type of learning, and it attempts to show how this species may be useful in the analysis of the mechanisms and the evolution of this complex brain function. The review is not intended to be comprehensive. It is a short opinion piece that reflects the author's own biases, and it draws some of its examples from the work coming from his own laboratory. Nevertheless, it is written in the hope that it will persuade those who have not utilized zebrafish and who may be interested in opening their research horizon to this relatively novel but powerful vertebrate research tool.

An event map of memory space in the hippocampus

The hippocampus has long been implicated in both episodic and spatial memory, however these mnemonic functions have been traditionally investigated in separate research strands. Theoretical accounts and rodent data suggest a common mechanism for spatial and episodic memory in the hippocampus by providing an abstract and flexible representation of the external world. Here, we monitor the de novo formation of such a representation of space and time in humans using fMRI. After learning spatio-temporal trajectories in a large-scale virtual city, subject-specific neural similarity in the hippocampus scaled with the remembered proximity of events in space and time. Crucially, the structure of the entire spatio-temporal network was reflected in neural patterns. Our results provide evidence for a common coding mechanism underlying spatial and temporal aspects of episodic memory in the hippocampus and shed new light on its role in interleaving multiple episodes in a neural event map of memory space.

DOI:http://dx.doi.org/10.7554/eLife.16534.001

The hippocampus: a special place for time

Many findings have demonstrated that memories of past events are temporally organized. It is well known that the hippocampus is critical for such episodic memories, but, until recently, little was known about the temporal organization of mnemonic representations in the hippocampus. Recent developments in human and animal research have revealed important insights into the role of the hippocampus in learning and retrieving sequences of events. Here, we review these findings, including lesion and single-unit recording studies in rodents, functional magnetic resonance imaging studies in humans, and computational models that link findings from these studies to the anatomy of the hippocampal circuit. The findings converge toward the idea that the hippocampus is essential for learning sequences of events, allowing the brain to distinguish between memories for conceptually similar but temporally distinct episodes, and to associate representations of temporally contiguous, but otherwise unrelated experiences.

A Variable Oscillator Underlies the Measurement of Time Intervals in the Rostral Medial Prefrontal Cortex during Classical Eyeblink Conditioning in Rabbits

We were interested in determining whether rostral medial prefrontal cortex (rmPFC) neurons participate in the measurement of conditioned stimulus-unconditioned stimulus (CS-US) time intervals during classical eyeblink conditioning. Rabbits were conditioned with a delay paradigm consisting of a tone as CS. The CS started 50, 250, 500, 1000, or 2000 ms before and coterminated with an air puff (100 ms) directed at the cornea as the US. Eyelid movements were recorded with the magnetic search coil technique and the EMG activity of the orbicularis oculi muscle. Firing activities of rmPFC neurons were recorded across conditioning sessions. Reflex and conditioned eyelid responses presented a dominant oscillatory frequency of ≈12 Hz. The firing rate of each recorded neuron presented a single peak of activity with a frequency dependent on the CS-US interval (i.e., ≈12 Hz for 250 ms, ≈6 Hz for 500 ms, and≈3 Hz for 1000 ms). Interestingly, rmPFC neurons presented their dominant firing peaks at three precise times evenly distributed with respect to CS start and also depending on the duration of the CS-US interval (only for intervals of 250, 500, and 1000 ms). No significant neural responses were recorded at very short (50 ms) or long (2000 ms) CS-US intervals. rmPFC neurons seem not to encode the oscillatory properties characterizing conditioned eyelid responses in rabbits, but are probably involved in the determination of CS-US intervals of an intermediate range (250-1000 ms). We propose that a variable oscillator underlies the generation of working memories in rabbits.

SIGNIFICANCE STATEMENT

The way in which brains generate working memories (those used for the transient processing and storage of newly acquired information) is still an intriguing question. Here, we report that the firing activities of neurons located in the rostromedial prefrontal cortex recorded in alert behaving rabbits are controlled by a dynamic oscillator. This oscillator generated firing frequencies in a variable band of 3-12 Hz depending on the conditioned stimulus-unconditioned stimulus intervals (1 s, 500 ms, 250 ms) selected for classical eyeblink conditioning of behaving rabbits. Shorter (50 ms) and longer (2 s) intervals failed to activate the oscillator and prevented the acquisition of conditioned eyelid responses. This is an unexpected mechanism to generate sustained firing activities in neural circuits generating working memories.

Learning by subtraction: Hippocampal activity and effects of ethanol during the acquisition and performance of response sequences

Learning is believed to be reflected in the activity of the hippocampus. However, neural correlates of learning have been difficult to characterize because hippocampal activity is integrated with ongoing behavior. To address this issue, male rats (n = 5) implanted with electrodes (n = 14) in the CA1 subfield responded during two tasks within a single test session. In one task, subjects acquired a new 3-response sequence (acquisition), whereas in the other task, subjects completed a well-rehearsed 3-response sequence (performance). Both tasks though could be completed using an identical response topography and used the same sensory stimuli and schedule of reinforcement. More important, comparing neural patterns during sequence acquisition to those during sequence performance allows for a subtractive approach whereby activity associated with learning could potentially be dissociated from the activity associated with ongoing behavior. At sites where CA1 activity was closely associated with behavior, the patterns of activity were differentially modulated by key position and the serial position of a response within the schedule of reinforcement. Temporal shifts between peak activity and responding on particular keys also occurred during sequence acquisition, but not during sequence performance. Ethanol disrupted CA1 activity while producing rate-decreasing effects in both tasks and error-increasing effects that were more selective for sequence acquisition than sequence performance. Ethanol also produced alterations in the magnitude of modulations and temporal pattern of CA1 activity, although these effects were not selective for sequence acquisition. Similar to ethanol, hippocampal micro-stimulation decreased response rate in both tasks and selectively increased the percentage of errors during sequence acquisition, and provided a more direct demonstration of hippocampal involvement during sequence acquisition. Together, these results strongly support the notion that ethanol disrupts sequence acquisition by disrupting hippocampal activity and that the hippocampus is necessary for the conditioned associations required for sequence acquisition.

A central role for the retrosplenial cortex in de novo environmental learning

With experience we become accustomed to the types of environments that we normally encounter as we navigate in the world. But how does this fundamental knowledge develop in the first place and what brain regions are involved? To examine de novo environmental learning, we created an ‘alien’ virtual reality world populated with landmarks of which participants had no prior experience. They learned about this environment by moving within it during functional MRI (fMRI) scanning while we tracked their evolving knowledge. Retrosplenial cortex (RSC) played a central and highly selective role by representing only the most stable, permanent features in this world. Subsequently, increased coupling was noted between RSC and hippocampus, with hippocampus then expressing knowledge of permanent landmark locations and overall environmental layout. Studying how environmental representations emerge from scratch provided a new window into the information processing underpinning the brain's navigation system, highlighting the key influence of the RSC.

DOI:http://dx.doi.org/10.7554/eLife.09031.001

Throughout our lives, we encounter novel environments that we must learn to find our way around, from a new office to a new city. Studies of brain activity in humans and rodents have revealed that many brain regions are involved in navigation, most notably the hippocampus. However, these experiments have typically involved humans navigating around environments filled with familiar objects and landmarks, and therefore tell us relatively little about how the brain builds up a map of a completely new environment in the first place.

To address this issue, Auger et al. scanned the brains of healthy human volunteers as they experienced an ‘alien’ virtual reality world called ‘Fog World’, so-named because of the dense fog used to precisely control what the volunteers could see. In contrast to previous virtual reality environments, which have contained houses, shops and other recognisable objects, Fog World contains only abstract landmarks that bear little resemblance to anything in the real world. The volunteers watched videos that simulated journeys through Fog World with the goal of learning the layout of the environment so that they could navigate within it. Of note, half of the landmarks in Fog World remained in fixed positions on all learning trials, while the other half changed location from one trial to the next.

After each block of trials, the volunteers were shown single landmarks—some from Fog World and others not—while their brains were scanned. A region called the retrosplenial cortex showed increasing activity that closely tracked the volunteers' growing knowledge of which landmarks had fixed, permanent locations in Fog World. In later trials towards the end of the learning period, the hippocampus also became active, and at this time communication between the retrosplenial cortex and hippocampus was also heightened. By the end of learning, the hippocampal activity was related to the volunteers' knowledge of the locations of the permanent landmarks across Fog World.

As well as revealing that the retrosplenial cortex may be essential for processing permanent landmarks, the work of Auger et al. shows how the hippocampus and retrosplenial cortex could work together to map new environments. These findings might also help us to better understand why some healthy individuals are bad navigators, and why disorientation is a common early symptom in neurodegenerative disorders such as Alzheimer's disease, where the retrosplenial cortex is often one of the first brain regions to become damaged.

DOI:http://dx.doi.org/10.7554/eLife.09031.002