Differential Representation of Landmark and Self-Motion Information along the CA1 Radial Axis: Self-Motion Generated Place Fields Shift toward Landmarks during Septal Inactivation

Development of differential sublaminar feedforward inhibitory circuits in CA1 hippocampus requires Satb2

Pyramidal cells (PCs) in CA1 hippocampus can be classified by their radial position as deep or superficial and organize into subtype-specific circuits necessary for differential information processing. Specifically, superficial PCs receive fewer inhibitory synapses from parvalbumin (PV)-expressing interneurons than deep PCs, resulting in weaker feedforward inhibition of input from CA3 Schaffer collaterals. Using mice, we investigated mechanisms underlying PC differentiation and the development of this inhibitory circuit motif. We found that expression of the transcriptional regulator SATB2 is biased towards superficial PCs during early postnatal development and necessary to suppress PV+ interneuron synapse formation. In the absence of SATB2, the number of PV+ interneuron synaptic puncta surrounding superficial PCs increases during development to match deep PCs. This results in equivalent inhibitory current strength observed in paired whole-cell recordings, and equivalent feedforward inhibition of Schaffer collateral input. Thus, SATB2 is necessary for superficial PC differentiation and biased feedforward inhibition in CA1.

Bidirectional synaptic changes in deep and superficial hippocampal neurons following in vivo activity

Neuronal activity during experience is thought to induce plastic changes within the hippocampal network that underlie memory formation, although the extent and details of such changes in vivo remain unclear. Here, we employed a temporally precise marker of neuronal activity, CaMPARI2, to label active CA1 hippocampal neurons in vivo, followed by immediate acute slice preparation and electrophysiological quantification of synaptic properties. Recently active neurons in the superficial sublayer of stratum pyramidale displayed larger post-synaptic responses at excitatory synapses from area CA3, with no change in pre-synaptic release probability. In contrast, in vivo activity correlated with weaker pre- and post-synaptic excitatory weights onto pyramidal cells in the deep sublayer. In vivo activity of deep and superficial neurons within sharp-wave/ripples was bidirectionally changed across experience, consistent with the observed changes in synaptic weights. These findings reveal novel, fundamental mechanisms through which the hippocampal network is modified by experience to store information.

Opto-juxtacellular interrogation of neural circuits in freely moving mice

Neural circuits are assembled from an enormous variety of neuronal cell types. Although significant advances have been made in classifying neurons on the basis of morphological, molecular and electrophysiological properties, understanding how this diversity contributes to brain function during behavior has remained a major experimental challenge. Here, we present an extension to our previous protocol, in which we describe the technical procedures for performing juxtacellular opto-tagging of single neurons in freely moving mice by using Channelrhodopsin-2-expressing viral vectors. This method allows one to selectively target molecularly defined cell classes for in vivo single-cell recordings. The targeted cells can be labeled via juxtacellular procedures and further characterized via post-hoc morphological and molecular analysis. In its current form, the protocol allows multiple recording and labeling attempts to be performed within individual animals, by means of a mechanical pipette micropositioning system. We provide proof-of-principle validation of this technique by recording from Calbindin-positive pyramidal neurons in the mouse hippocampus during spatial exploration; however, this approach can easily be extended to other behaviors and cortical or subcortical areas. The procedures described here, from the viral injection to the histological processing of brain sections, can be completed in ~4-5 weeks.This protocol is an extension to: Nat. Protoc. 9, 2369-2381 (2014): https://doi.org/10.1038/nprot.2014.161.

A local circuit-basis for spatial navigation and memory processes in hippocampal area CA1

The hippocampus is a multi-stage neural circuit that is critical for memory formation. Its distinct anatomy has long inspired theories that rely on local interactions between neurons within each subregion in order to perform serial operations important for memory encoding and storage. These local computations have received less attention in CA1 area, the primary output node of the hippocampus, where excitatory neurons are thought to be only very sparsely interconnected. However, recent findings have demonstrated the power of local circuitry in CA1, with evidence for strong functional interactions among excitatory neurons, regulation by diverse inhibitory microcircuits, and novel plasticity rules that can profoundly reshape the hippocampal ensemble code. Here we review how these properties expand the dynamical repertoire of CA1 beyond the confines of feedforward processing, and what implications they have for hippocampo-cortical functions in memory formation.

Subtle visual change in a virtual environment induces heterogeneous remapping systematically in CA1, but not CA3

Environmental change may lead to new memories or modify old ones, but the underlying neural mechanisms are largely unclear. We recorded hippocampal place cells simultaneously from CA1 and CA3 in a virtual reality environment. Compared with CA1, place cells in CA3 are more tolerant of individual landmark changes but undergo orthogonal changes to code distinctively different environments. As visual noise (virtual fog) is introduced to a visually enriched environment, place cells in CA1 split into two subpopulations: in one, place cells maintain their field locations while changing their firing rates to reflect sensory changes; in the other, place cells exhibit global remapping in response to the contextual change. In contrast, place cells in CA3 exhibit mainly rate remapping under the same conditions. Our results suggest that CA1 may simultaneously represent heterogeneous maps of the same environment when subtle visual noise induces both sensory and contextual changes.

Preconfigured dynamics in the hippocampus are guided by embryonic birthdate and rate of neurogenesis

The incorporation of new information into the hippocampal network is likely to be constrained by its innate architecture and internally generated activity patterns. However, the origin, organization and consequences of such patterns remain poorly understood. In the present study we show that hippocampal network dynamics are affected by sequential neurogenesis. We birthdated CA1 pyramidal neurons with in utero electroporation over 4 embryonic days, encompassing the peak of hippocampal neurogenesis, and compared their functional features in freely moving adult mice. Neurons of the same birthdate displayed distinct connectivity, coactivity across brain states and assembly dynamics. Same-birthdate neurons exhibited overlapping spatial representations, which were maintained across different environments. Overall, the wiring and functional features of CA1 pyramidal neurons reflected a combination of birthdate and the rate of neurogenesis. These observations demonstrate that sequential neurogenesis during embryonic development shapes the preconfigured forms of adult network dynamics.

Juxtacellular opto-tagging of hippocampal CA1 neurons in freely moving mice

Neural circuits are made of a vast diversity of neuronal cell types. While immense progress has been made in classifying neurons based on morphological, molecular, and functional properties, understanding how this heterogeneity contributes to brain function during natural behavior has remained largely unresolved. In the present study, we combined the juxtacellular recording and labeling technique with optogenetics in freely moving mice. This allowed us to selectively target molecularly defined cell classes for in vivo single-cell recordings and morphological analysis. We validated this strategy in the CA1 region of the mouse hippocampus by restricting Channelrhodopsin expression to Calbindin-positive neurons. Directly versus indirectly light-activated neurons could be readily distinguished based on the latencies of light-evoked spikes, with juxtacellular labeling and post hoc histological analysis providing ‘ground-truth’ validation. Using these opto-juxtacellular procedures in freely moving mice, we found that Calbindin-positive CA1 pyramidal cells were weakly spatially modulated and conveyed less spatial information than Calbindin-negative neurons – pointing to pyramidal cell identity as a key determinant for neuronal recruitment into the hippocampal spatial map. Thus, our method complements current in vivo techniques by enabling optogenetic-assisted structure–function analysis of single neurons recorded during natural, unrestrained behavior.

Distal CA1 Maintains a More Coherent Spatial Representation than Proximal CA1 When Local and Global Cues Conflict

Entorhinal cortical projections show segregation along the transverse axis of CA1, with the medial entorhinal cortex (MEC) sending denser projections to proximal CA1 (pCA1) and the lateral entorhinal cortex (LEC) sending denser projections to distal CA1 (dCA1). Previous studies have reported functional segregation along the transverse axis of CA1 correlated with the functional differences in MEC and LEC. pCA1 shows higher spatial selectivity than dCA1 in these studies. We employ a double rotation protocol, which creates an explicit conflict between the local and the global cues, to understand the differential contributions of these reference frames to the spatial code in pCA1 and dCA1 in male Long-Evans rats. We show that pCA1 and dCA1 respond differently to this local-global cue conflict. pCA1 representation splits as predicted from the strong conflicting inputs it receives from MEC and dCA3. In contrast, dCA1 rotates more in concert with the global cues. In addition, pCA1 and dCA1 display comparable levels of spatial selectivity in this study. This finding differs from the previous studies, perhaps because of richer sensory information available in our behavior arena. Together, these observations indicate that the functional segregation along proximodistal axis of CA1 is not of the amount of spatial selectivity but that of the nature of the different inputs used to create and anchor spatial representations.SIGNIFICANCE STATEMENT Subregions of the hippocampus are thought to play different roles in spatial navigation and episodic memory. It was previously thought that the distal part of area CA1 of the hippocampus carries lesser information about space than proximal CA1 (pCA1). We report that distal CA1 (dCA1) spatial representation moves more in concert with the global cues than pCA1 when the local and the global cues conflict. We also show that spatial selectivity is comparable along the proximodistal axis in this experimental protocol. Thus, different parts of the brain receiving differential outputs from pCA1 and dCA1 receive spatial information in different spatial reference frames encoded using different sets of inputs, rather than different amounts of spatial information as thought earlier.

CA1 pyramidal cell diversity is rootedin the time of neurogenesis

Cellular diversity supports the computational capacity and flexibility of cortical circuits. Accordingly, principal neurons at the CA1 output node of the murine hippocampus are increasingly recognized as a heterogeneous population. Their genes, molecular content, intrinsic morpho-physiology, connectivity, and function seem to segregate along the main anatomical axes of the hippocampus. Since these axes reflect the temporal order of principal cell neurogenesis, we directly examined the relationship between birthdate and CA1 pyramidal neuron diversity, focusing on the ventral hippocampus. We used a genetic fate-mapping approach that allowed tagging three groups of age-matched principal neurons: pioneer, early-, and late-born. Using a combination of neuroanatomy, slice physiology, connectivity tracing, and cFos staining in mice, we show that birthdate is a strong predictor of CA1 principal cell diversity. We unravel a subpopulation of pioneer neurons recruited in familiar environments with remarkable positioning, morpho-physiological features, and connectivity. Therefore, despite the expected plasticity of hippocampal circuits, given their role in learning and memory, the diversity of their main components is also partly determined at the earliest steps of development.

Olfactory landmarks and path integration converge to form a cognitive spatial map

The convergence of internal path integration and external sensory landmarks generates a cognitive spatial map in the hippocampus. We studied how localized odor cues are recognized as landmarks by recording the activity of neurons in CA1 during a virtual navigation task. We found that odor cues enriched place cell representations, dramatically improving navigation. Presentation of the same odor at different locations generated distinct place cell representations. An odor cue at a proximal location enhanced the local place cell density and also led to the formation of place cells beyond the cue. This resulted in the recognition of a second, more distal odor cue as a distinct landmark, suggesting an iterative mechanism for extending spatial representations into unknown territory. Our results establish that odors can serve as landmarks, motivating a model in which path integration and odor landmarks interact sequentially and iteratively to generate cognitive spatial maps over long distances.

The Prefrontal-Hippocampal Comparator: Volition and Episodic Memory

This review describes recent research that is relevant to the prefrontal-hippocampal comparator model with the following conclusions: 1. Hippocampal area CA1 serves, at least in part, as an associative match-mismatch comparator. 2. Voluntary movement strengthens episodic memories for goal-directed behavior. 3. Hippocampal theta power serves as a prediction error signal during hippocampal dependent tasks. 4. The self-referential component of episodic memory in humans is mediated by the corollary discharge (the efference copy of the action plan developed by prefrontal cortex and transmitted to hippocampus where it is stored as a working memory; CA1 uses this efference copy to compare the expected consequences of action to the actual consequences of action). 5. Impairments in the production or transmission of this corollary discharge may contribute to some of the symptoms of schizophrenia. Unresolved issues and suggestions for future research are discussed.

How development sculpts hippocampal circuits and function

In mammals, the selective transformation of transient experience into stored memory occurs in the hippocampus, which develops representations of specific events in the context in which they occur. In this review, we focus on the development of hippocampal circuits and the self-organized dynamics embedded within them since the latter critically support the role of the hippocampus in learning and memory. We first discuss evidence that adult hippocampal cells and circuits are sculpted by development as early as during embryonic neurogenesis. We argue that these primary developmental programs provide a scaffold onto which later experience of the external world can be grafted. Next, we review the different sequences in the development of hippocampal cells and circuits at anatomical and functional levels. We cover a period extending from neurogenesis and migration to the appearance of phenotypic diversity within hippocampal cells, and their wiring into functional networks. We describe the progressive emergence of network dynamics in the hippocampus, from sensorimotor-driven early sharp waves to sequences of place cells tracking relational information. We outline the critical turn points and discontinuities in that developmental journey, and close by formulating open questions. We propose that rewinding the process of hippocampal development helps understand the main organization principles of memory circuits.

Distinct place cell dynamics in CA1 and CA3 encode experience in new environments

When exploring new environments animals form spatial memories that are updated with experience and retrieved upon re-exposure to the same environment. The hippocampus is thought to support these memory processes, but how this is achieved by different subnetworks such as CA1 and CA3 remains unclear. To understand how hippocampal spatial representations emerge and evolve during familiarization, we performed 2-photon calcium imaging in mice running in new virtual environments and compared the trial-to-trial dynamics of place cells in CA1 and CA3 over days. We find that place fields in CA1 emerge rapidly but tend to shift backwards from trial-to-trial and remap upon re-exposure to the environment a day later. In contrast, place fields in CA3 emerge gradually but show more stable trial-to-trial and day-to-day dynamics. These results reflect different roles in CA1 and CA3 in spatial memory processing during familiarization to new environments and constrain the potential mechanisms that support them.

Subcircuits of Deep and Superficial CA1 Place Cells Support Efficient Spatial Coding across Heterogeneous Environments

The hippocampus is thought to guide navigation by forming a cognitive map of space. Different environments differ in geometry and the availability of cues that can be used for navigation. Although several spatial coding mechanisms are known to coexist in the hippocampus, how they are influenced by various environmental features is not well understood. To address this issue, we examined the spatial coding characteristics of hippocampal neurons in mice and rats navigating in different environments. We found that CA1 place cells located in the superficial sublayer were more active in cue-poor environments and preferentially used a firing rate code driven by intra-hippocampal inputs. In contrast, place cells located in the deep sublayer were more active in cue-rich environments and used a phase code driven by entorhinal inputs. Switching between these two spatial coding modes was supported by the interaction between excitatory gamma inputs and local inhibition.

Place cell maps slowly develop via competitive learning and conjunctive coding in the dentate gyrus

Place cells exhibit spatially selective firing fields that collectively map the continuum of positions in environments; how such activity pattern develops with experience is largely unknown. Here, we record putative granule cells (GCs) and mossy cells (MCs) from the dentate gyrus (DG) over 27 days as mice repetitively run through a sequence of objects fixed onto a treadmill belt. We observe a progressive transformation of GC spatial representations, from a sparse encoding of object locations and spatial patterns to increasingly more single, evenly dispersed place fields, while MCs show little transformation and preferentially encode object locations. A competitive learning model of the DG reproduces GC transformations via the progressive integration of landmark-vector cells and spatial inputs and requires MC-mediated feedforward inhibition to evenly distribute GC representations, suggesting that GCs slowly encode conjunctions of objects and spatial information via competitive learning, while MCs help homogenize GC spatial representations.

Place cells in the hippocampus fire action potentials at spatially selective firing fields that collectively map the environments. Here, the authors show how these activity patterns develop with experience in mice and determine the importance of competitive learning in this process.

Changing phase relationship of the stepping rhythm to neuronal oscillatory theta activity in the septo-hippocampal network of mice

Movement-related sensory and motor activity in the brain contributes to cognitive processes. We have observed that the frequency of stepping rhythm in head-fixed mice running on a jetball overlaps with the range of frequencies that characterize hippocampal rhythmic slow activity, including theta (~ 3 to 10 Hz). On average, step-cycle troughs (i.e. when the paw touches the ground) were weakly coupled to hippocampal theta oscillations. This weak coupling was sustained during a range of running speeds. In short temporal windows, step-cycle troughs were synchronous with hippocampal theta oscillatory cycle troughs, while during other periods they led or lagged behind theta cycles. Furthermore, simultaneously recorded theta rhythmic medial septal neurons in the basal forebrain were phase-coupled to both step-cycles and theta-cycles. We propose that the weak overall phase relationship of step-cycles with theta-cycles signifies a distinct mode of information processing. Transient synchronization of the step-cycle with theta may indicate the engagement of septo-hippocampal-entorhinal network with the current heading of the animal.

Postsynaptic integrative properties of dorsal CA1 pyramidal neuron subpopulations

The population activity of CA1 pyramidal neurons (PNs) segregates along anatomical axes with different behaviors, suggesting that CA1 PNs are functionally subspecialized based on somatic location. In dorsal CA1, spatial encoding is biased toward CA2 (CA1c) and in deep layers of the radial axis. In contrast, nonspatial coding peaks toward subiculum (CA1a) and in superficial layers. While preferential innervation by spatial vs. nonspatial input from entorhinal cortex (EC) may contribute to this specialization, it cannot fully explain the range of in vivo responses. Differences in intrinsic properties thus may play a critical role in modulating such synaptic input differences. In this study we examined the postsynaptic integrative properties of dorsal CA1 PNs in six subpopulations along the transverse (CA1c, CA1b, CA1a) and radial (deep, superficial) axes. Our results suggest that active and passive properties of deep and superficial neurons evolve over the transverse axis to promote the functional specialization of CA1c vs. CA1a as dictated by their cortical input. We also find that CA1b is not merely an intermediate mix of its neighbors, but uniquely balances low excitability with superior input integration of its mixed input, as may be required for its proposed role in sequence encoding. Thus synaptic input and intrinsic properties combine to functionally compartmentalize CA1 processing into at least three transverse axis regions defined by the processing schemes of their composite radial axis subpopulations.NEW & NOTEWORTHY There is increasing interest in CA1 pyramidal neuron heterogeneity and the functional relevance of this diversity. We find that active and passive properties evolve over the transverse and radial axes in dorsal CA1 to promote the functional specialization of CA1c and CA1a for spatial and nonspatial memory, respectively. Furthermore, CA1b is not a mean of its neighbors, but features low excitability and superior integrative capabilities, relevant to its role in nonspatial sequence encoding.

Cell type, sub-region, and layer-specific speed representation in the hippocampal-entorhinal circuit

It has been hypothesised that speed information, encoded by ‘speed cells’, is important for updating spatial representation in the hippocampus and entorhinal cortex to reflect ongoing self-movement during locomotion. However, systematic characterisation of speed representation is still lacking. In this study, we compared the speed representation of distinct cell types across sub-regions/layers in the dorsal hippocampus and medial entorhinal cortex of rats during exploration. Our results indicate that the preferred theta phases of individual neurons are correlated with positive/negative speed modulation and a temporal shift of speed representation in a sub-region/layer and cell type-dependent manner. Most speed cells located in entorhinal cortex layer 2 represented speed prospectively, whereas those in the CA1 and entorhinal cortex layers 3 and 5 represented speed retrospectively. In entorhinal cortex layer 2, putative CA1-projecting pyramidal cells, but not putative dentate gyrus/CA3-projecting stellate cells, represented speed prospectively. Among the hippocampal interneurons, approximately one-third of putative dendrite-targeting (somatostatin-expressing) interneurons, but only a negligible fraction of putative soma-targeting (parvalbumin-expressing) interneurons, showed negative speed modulation. Putative parvalbumin-expressing CA1 interneurons and somatostatin-expressing CA3 interneurons represented speed more retrospectively than parvalbumin-expressing CA3 interneurons. These findings indicate that speed representation in the hippocampal-entorhinal circuit is cell-type, pathway, and theta-phase dependent.

Calbindin immunostaining in the CA1 hippocampal pyramidal cell layer of the human and mouse: A comparative study

Immunostaining for calbindin (CB) is commonly used to label particular populations of neurons. Recently, it has been shown that the CA1 pyramidal cells in the mouse can be subdivided along the radial axis into superficial and deep pyramidal cells and that this segregation in the radial axis may represent a general principle of structural and functional organization of the hippocampus. One of the most widely used markers of the superficial pyramidal cells is CB. However, this laminar segregation of pyramidal cells has not been reported in the human CA1 using CB immunostaining. The problem is that the different pattern of CB immunostaining observed in the mouse compared to the human could be explained by technical features, of which one of the most important is the postmortem time (PT) delay typical of the brain tissue obtained from humans. In the present study, we have studied the influences of PT delays and fixation procedures and we found that the clear differences found between the CA1 of the human and mouse do not depend on the fixation, but represent actual species-specific differences. These remarkable differences between species should be taken into consideration when making interpretations in translational studies from mouse to human brains.

Heterogeneity within classical cell types is the rule: lessons from hippocampal pyramidal neurons

The mechanistic operation of brain regions is often interpreted by partitioning constituent neurons into 'cell types'. Historically, such cell types were broadly defined by their correspondence to gross features of the nervous system (such as cytoarchitecture). Modern-day neuroscientific techniques, enabling a more nuanced examination of neuronal properties, have illustrated a wealth of heterogeneity within these classical cell types. Here, we review the extent of this within-cell-type heterogeneity in one of the simplest cortical regions of the mammalian brain, the rodent hippocampus. We focus on the mounting evidence that the classical CA3, CA1 and subiculum pyramidal cell types all exhibit prominent and spatially patterned within-cell-type heterogeneity, and suggest these cell types provide a model system for exploring the organization and function of such heterogeneity. Given that the hippocampus is structurally simple and evolutionarily ancient, within-cell-type heterogeneity is likely to be a general and crucial feature of the mammalian brain.

Dentate granule and mossy cells exhibit distinct spatiotemporal responses to local change in a one-dimensional landscape of visual-tactile cues

The dentate gyrus (DG) is critical for detecting changes in environments; however, how granule cells (GCs) and mossy cells (MCs), the two excitatory cell types of the DG, respond to small changes in the object layout is unclear. Here, we recorded GCs and MCs, identified by spike feature and optogenetic tagging, as mice ran on a treadmill belt enriched with visual-tactile cues. We observed that fixing a new cue on the belt induced a reconfiguration of GC and MC spatial representations via the emergence, extinction and rate alteration of firing fields. For both GCs and MCs, the response was maximal near the cue and spread over the entire belt. However, compared to the GC response, the MC response was stronger and more immediate, peaked at a slightly earlier belt position, and exhibited a transient component reminiscent of neuromodulatory activity. A competitive neural network model reproduced the GC response contingent on both the introduction of new object-vector inputs and the reconfiguration of MC activity, the former being critical for spreading the GC response in locations distant from the cue. These findings suggest that GCs operate as a competitive network and that MCs precede GCs in detecting changes and help expand the range of GC pattern separation.

Reevaluating the ability of cerebellum in associative motor learning

It has been well established that the cerebellum and its associated circuitry constitute the essential neuronal system for both delay and trace classical eyeblink conditioning (DEC and TEC). However, whether the cerebellum is sufficient to independently modulate the DEC, and TEC with a shorter trace interval remained controversial. Here, we used direct optogenetic stimulation of mossy fibers in the middle cerebellar peduncle (MCP) as a conditioned stimulus (CS) replacement for the peripheral CS (eg, a tone CS or a light CS) paired with a periorbital shock unconditioned stimulus (US) to examine the ability of the cerebellum to learn the DEC and the TEC with various trace intervals. Moreover, neural inputs to the pontine nucleus (PN) were pharmacological blocked to limit the associative motor learning inside the cerebellum. We show that all rats quickly acquired the DEC, indicating that direct optogenetic stimulation of mossy fibers in the left MCP is a very effective and sufficient CS to establish DEC and to limit the motor learning process inside the cerebellum. However, only five out of seven rats acquired the TEC with a 150-ms trace interval, three out of nine rats acquired the TEC with a 350-ms trace interval, and none of the rats acquired the TEC with a 500-ms trace interval. Moreover, pharmacological blocking glutamatergic and GABAergic inputs to the PN from the extra-cerebellar and cerebellar regions has no significant effect on the DEC and TEC learning with the optogenetic CS. These results indicate that the cerebellum has the ability to independently support both the simple DEC, and the TEC with a trace interval of 150 or 350 ms, but not the TEC with a trace interval of 500 ms. The present results are of great importance in our understanding of the mechanisms and ability of the cerebellum in associative motor learning and memory.

Origin and role of path integration in the cognitive representations of the hippocampus: computational insights into open questions

Path integration is a straightforward concept with varied connotations that are important to different disciplines concerned with navigation, such as ethology, cognitive science, robotics and neuroscience. In studying the hippocampal formation, it is fruitful to think of path integration as a computation that transforms a sense of motion into a sense of location, continuously integrated with landmark perception. Here, we review experimental evidence that path integration is intimately involved in fundamental properties of place cells and other spatial cells that are thought to support a cognitive abstraction of space in this brain system. We discuss hypotheses about the anatomical and computational origin of path integration in the well-characterized circuits of the rodent limbic system. We highlight how computational frameworks for map-building in robotics and cognitive science alike suggest an essential role for path integration in the creation of a new map in unfamiliar territory, and how this very role can help us make sense of differences in neurophysiological data from novel versus familiar and small versus large environments. Similar computational principles could be at work when the hippocampus builds certain non-spatial representations, such as time intervals or trajectories defined in a sensory stimulus space.