Hippocampus

Principal cells in the rodent hippocampus often fire in response to traversal through a specific spatial location (place cells), as well as elapsed time during an imposed temporal delay or after stimulus offset (time cells). Sequences of time cells unfold rapidly at first, with many time cells with narrow time fields. As the triggering event recedes into the past, time cells are fewer and have broader fields. This means that the representation of time in the hippocampus is compressed with greater resolution for time points near the present. Using tetrode recordings we measured individual CA1 units while rats traveled along a track that could be changed in length. Consistent with previous results, most place cells coded for distance from the starting point of the trajectory. Critically, place cells became less numerous and showed gradually widening fields with distance from the starting location. These results suggest that as the animal leaves a landmark, the hippocampal place code forms a compressed representation of distance from the starting location. The representation of time and space in the hippocampus have similar properties suggesting that they arise from similar computational mechanisms. Significance StatementThe hippocampus represents relationships between events in time and space. It has been hypothesized that temporal and spatial relationships are the result of a common computational mechanism. Previous work has shown that the representation of time in the hippocampus is compressed, with less neural resolution for more temporally remote events, consistent with the observation that temporal memory is worse for events further in the past. This paper shows an analogous result for spatial relationships. Place cells coded for distance from the start of a journey. As distance increased, place fields became broader and less numerous, showing a decrease in spatial resolution. This result suggests a unified coding scheme for the dimensions of time and space in the rodent hippocampus.

High-resolution in vivo magnetic resonance imaging (MRI) of hippocampal subfields is a rapidly advancing field due to their implication in cognition, disorder, and disease. Hippocampal subfield segmentation on in vivo MRI is generally guided by postmortem reference material, which has been limited by small sample sizes that preclude comprehensive characterization of subfield border locations and their variability. Addressing this, we characterized hippocampal subfield border variability in two ultra-high-resolution postmortem MRI datasets with combined annotated histological sections, including cases with and without dementia. We examined: 1) the order of appearance and disappearance of subfields along the long axis of the hippocampus; 2) the order of appearance and disappearance of subicular subregions; 3) the medial-lateral position of subicular subregional boundaries along the hippocampal body; 4) the location of the CA3 relative to hippocampal head digitations; 5) the subfield borders in the hippocampal body relative to a volume proportion of the dark band; and 6) the association of hippocampal length and subiculum-CA1 border location with diagnosis, demographic factors, and factors related to postmortem imaging. Our findings reaffirmed that there is a consistent order of appearance and disappearance of subfields in the hippocampal head and tail, respectively. The subicular subregions exhibited a first in, last out order of appearance and disappearance, and pre/parasubiculum consistently occupied half of the subicular complex in coronal slices throughout the hippocampal body. Hippocampal head digitations were not a reliable landmark for CA3 appearance, but SRLM proportionality did offer a potentially consistent approach for estimating CA2 and CA3 subfield borders in relation to the hippocampal border. No clear relationship was found between the anatomical features and diagnosis, demographic factors, and factors related to postmortem imaging. These findings have implications for the development and harmonization of hippocampal subfield segmentation protocols and interpretation of high-resolution functional MRI studies of the human hippocampus.

A theory and neurocomputational model are presented that explain grid cell responses as the byproduct of equally dissimilar hippocampal memories. On this account, place and grid cells are best understood as the natural consequence of memory encoding and retrieval. In the proposed memory model, place cells represent memories that are conjunctions of both spatial and non-spatial attributes, and grid cells primarily represent the non-spatial attributes (e.g., sounds, surface texture, etc.) found throughout the two-dimensional recording enclosure. Place cells support memories of the locations where non-spatial attributes can be found (e.g., positions with a particular sound), which are arranged in a hexagonal lattice owing to memory encoding and consolidation processes (pattern separation) as applied to situations in which the non-spatial attributes are found at all locations of a two-dimensional surface. Grid cells exhibit their spatial firing pattern owing to feedback from hippocampal place cells (i.e., a hexagonal pattern of remembered locations for the non-spatial attribute represented by a grid cell). The model explains: 1) grid fields that appear to be centered outside the box; 2) the toroidal nature of grid field representations; 3) grid field alignment with the enclosure borders; 4) modules in which grid cells have the same orientation and spacing but different phases; 5) head direction conjunctive grid cells that become simple head direction cells in the absence of hippocampal feedback; 6) the instant existence of grid fields in a novel environment; 7) the slower learning of place cells; 8) the manner in which head direction sensitivity of place cells changes near borders and in narrow passages; 9) the kinds of changes that underlie remapping of place cells; and 10) grid-like responses for two-dimensional coordinate systems other than navigation.

Structured AbstractO_ST_ABSObjectiveC_ST_ABSMemory problems are comorbid with Temporal Lobe Epilepsy (TLE). Animal models of TLE reveal impairments in spatial firing fields of hippocampal place cells, providing a potential neural substrate for memory problems. Each subfield of the hippocampus carries out unique aspects of spatial memory, yet little is known about how individual subfields are perturbed. Here, we investigated the spatial coding properties of the three major subfields of the hippocampus. MethodsSingle unit recordings were made from CA1, CA3 and the dentate gyrus (DG) of mice (N = 10, 6M/4F) induced with epilepsy using the supra-hippocampal kainate model and in control mice injected with saline (N = 6, 3M/3F). Place cell activity was measured while mice foraged in highly familiar environments to assess basic place cell properties and in novel environments to assess remapping. ResultsA lower percentage of cells were classified as place cells in CA1 of epileptic mice, whereas percentages were similar in CA3 and DG compared to control. Place fields of CA1 were less coherent, place fields of CA3 were less stable, and place fields in DG has smaller differences between in-field and out of field firing. All regions constructed new distinct maps within the first session of exposure to a novel environment, however new maps in CA3 trended toward instability. SignificanceThese results point to specific deficits within subfields of the hippocampus, which may indicate that there are different cellular and network mechanisms at play. Such heterogeneity would be predicted to contribute differently to memory deficits. Key PointsO_LICA1 exhibits place map quality problems; CA1 has fewer place cells and those remaining have reduced spatial coherence C_LIO_LICA3 exhibits place map stability problems; CA3 has lower spatial correlation within sessions and trends toward not forming stable new maps in a novel environment. C_LIO_LIDG exhibits reduced signal to noise; DG has smaller differences between in-field and out of field firing rates. C_LI

Hippocampal place cells form a map of an animals environment. When the animal moves to a new environment, place field locations and firing rates change, a phenomenon known as remapping. Different animals can have different remapping responses to the same environments. This variability across animals in remapping behavior is not well understood. In this work, we analyzed electrophysiological recordings from Alme et al. (2014), in which five male rats were exposed to 11 different environments. To compare the hippocampal maps in two rooms, we computed average rate map correlation coefficients. We discovered that the heterogeneity in animals remapping behavior is structured: animals remapping behavior is consistent across a range of independent comparisons. Our findings highlight that remapping behavior between repeated environments depends on animal-specific factors.

Action potentials, the primary information units of the nervous system, are usually generated at the axon initial segment (AIS). Changes in the length and position of the AIS are associated with alterations in neuronal excitability but there is only limited information about the baseline structural variability of the AIS. This work provides a comprehensive atlas of the diversity of proximal cell geometries across all anatomical axes of the murine hippocampus, encompassing dorsal-ventral, superficial-deep, and proximal-distal regions. We analyzed the morphology of 3,936 hippocampal pyramidal neurons in 12 animals of both sexes, focusing on AIS length, position, and their association with proximal cellular features such as the soma and dendritic geometries. Notably, neurons with axon-carrying dendrites were significantly more common in ventral compared to dorsal hippocampal areas, suggesting a functional adaptation to regional demands. Validation of this finding in human samples confirms the translational relevance of our murine model. We employed NEURON simulations to assess the functional implications of this variability. Here, variation in proximal geometry only minimally contributed to neuronal homeostasis, but instead increased heterogeneity of response patterns across neurons.

A challenge in both modern and historic neuroscience has been achieving an understanding of neuron circuits, and determining the computational and organizational principles that underlie these circuits. Deeper understanding of the organization of brain circuits and cell types, including in the hippocampus, is required for advances in behavioral and cognitive neuroscience, as well as for understanding principles governing brain development and evolution. In this manuscript, we pioneer a new method to analyze the spatial clustering of active neurons in the hippocampus. We use calcium imaging and a rewarded navigation task to record from 100s of place cells in the CA1 of freely moving rats. We then use statistical techniques developed for and in widespread use in geographic mapping studies, global Morans I and local Morans I to demonstrate that cells that code for similar spatial locations tend to form small spatial clusters. We present evidence that this clustering is not the result of artifacts from calcium imaging, and show that these clusters are primarily formed by cells that have place field around previously rewarded locations. We go on to show that, although cells with similar place fields tend to form clusters, there is no obvious topographic mapping of environmental location onto the hippocampus, such as seen in the visual cortex. Insights into hippocampal organization, as in this study, can elucidate mechanisms underlying motivational behaviors, spatial navigation, and memory formation.

Hippocampal place cells are the functional units of spatial navigation and are present in all subregions-CA1, CA2, CA3 and CA4. Recent studies on CA2 have indicated its role in social and contextual memory, but its contribution towards spatial novelty detection and consolidation remains largely unknown. The current study aims to uncover how CA1 and CA2 detect, process, assimilate and consolidate spatial novelty. Accordingly, a novel 3-day paradigm was designed where the animal was introduced to a completely new environment on the first day and to varying degrees of familiarity and novelty on subsequent days, as the track was extended in length and modified in shape, keeping other environmental constraints fixed. Detection of spatial novelty was found to be a dynamic and complex phenomenon, characterized by different responses from hippocampal place cells, depending on when novelty was introduced. Therefore, the study concludes that early novelty detection (the first time a novel space is introduced in a relatively familiar environment) and subsequent novelty detection are not processed in the same way. Additionally, while neuronal responses to spatial novelty detection (early and subsequent) were found to be the same in CA1 and CA2 ensembles, their responses differed in spatial consolidation mechanisms during subsequent sleep replays. For CA1, spatial coverage of prior behaviour was found to be closely reflected in subsequent sleep for that particular day, but CA2 showed no such coherent response, highlighting mnemonic processing differences between CA2 and CA1 with respect to spatial novelty.

Elucidating the fundamental neural mechanisms of hippocampal information processing is necessary for understanding memory formation and related brain disorders. Differences in hippocampal genetics, anatomy, and connectivity across the longitudinal axis suggest functional heterogeneity in this structure. The dorsal pole is suggested to be primarily involved in spatial processing, whereas the ventral pole is implicated in emotional processing. Connectivity and genetic studies show this functional segregation is more prominent near their respective poles and weaker toward the intermediate region. The current study compares the firing properties of CA1 cells in the dorsal and intermediate regions of the hippocampus during spatial or social/odor novelty. We measured basic spatial properties, firing rate response, and remapping to the spatial re-configuration of a linear track or the social/odor presentation of a novel male conspecific, female bedding, or coyote urine. Behaviorally, the average rat exhibited slower maze running latencies during the novel spatial manipulation and spent more time adjacent to the chamber containing the novel social/odor stimulus. As previously shown, dorsal cells had fewer, smaller place fields, and higher spatial information content than intermediate cells in the hippocampus. Despite the differences in place field characteristics, cells in both regions responded similarly to spatial and social/odor manipulations. Taken together, these data support the differentiation of some functions, together with an overlap of other functions progressing along the longitudinal axis, which may facilitate the integration of information throughout the hippocampus.

Animals predominantly use salient visual cues (landmarks) for efficient navigation over other sensory modalities. When the relative position of the visual cues is altered, the hippocampal population exhibits heterogeneous responses and constructs context-specific spatial maps. Another critical factor that can strongly modulate spatial representation is the presence of reward. Reward features can drive behavior and are known to bias spatial attention. However, it is unclear whether reward flavors are used for spatial reference in the presence of distal cues and how the hippocampus population dynamics changes when the association between reward flavors and distal cues is altered. We investigated these questions by recording place cells from the CA1 while the rats ran in an environment with the conflicting association between reward flavors and distal cues. We report that the hippocampal place cells coherently and dynamically bind to reward flavors or distal cues across sessions, but not simultaneously suggesting the use of a single spatial map. We found that place cells maintained their spatial offset in the cue conflict conditions, thus showing a robust spatial coupling featuring an attractor-like property in the CA1. When the textures were added on the track, the coherency of the CA1 is degraded, as the hippocampus showed a heterogeneous response and weak spatial coupling of co-recorded cells suggesting a break away from the attractor network. These results indicate that reward flavors alone may be used for spatial reference but may not cause sufficient input difference to create context-specific spatial maps in the CA1.

The dorsal subiculum lies among a network of interconnected brain regions that collectively map multiple spatial and orientational relationships between an organism and the boundaries and pathways composing its environment. A unique role of the subiculum in spatial information processing has yet to be defined despite reports of small neuron subpopulations that encode relationships to specific boundaries, axes of travel, or locations. We examined the activity patterns among populations of subiculum neurons during performance of a spatial working memory task performed within a complex network of interconnected pathways. Compared to neurons in hippocampal sub-region CA1, a major source of its afferents, subiculum neurons were far more likely to exhibit multiple firing fields at locations that were analogous with respect to path structure and function. Subiculum neuron populations were also found to exhibit a greater dynamic range in scale of spatial representation and for persistent patterns of spiking activity to be aligned to transitions between maze segments. Together, the findings indicate that the subiculum plays a unique role in spatial mapping, one that complements the location-specific firing of CA1 neurons with the encoding of emergent and recurring structural features of a complex path network.

Theta phase precession is thought to confer key computational advantages (e.g. temporal compression suiting spike-timing related plasticity, cognitive relations as phase distances, and population-level coding for directions and sequences). However, direct evidence speaking to: 1) its widely-theorised role in enhancing memorability; 2) its dependence upon sensory input, is lacking. We leveraged the Vector trace cell (VTC) phenomenon to examine these issues. VTCs in subiculum show a simple, unambiguous memory correlate: VTCs remember the distance and direction to a cue after the cue is removed, with a new trace field which was not present before the cue was inserted. Regarding memorability, here we show that subsequently-remembered cue fields (those which become trace fields) exhibit higher levels of phase precession than subsequently-forgotten cue fields (those which produce no trace). Thus, phase precession does appear to enhance memorability, consistent with long-established theory. The second issue concerns the extent of phase precession in sensory-elicited vs memory-dependent firing. Phase precession in CA1 is strongly disrupted following deprivation of its Entorhinal, but not CA3, inputs; this could indicate that theta phase precession is largely sensory-driven and absent in memory-dependent fields. Here, however, we show that phase precession is robust in subicular VTC trace fields, i.e. when the cue that originally elicited the new vector field is no longer present. Thus, the much-theorised benefits of phase precession likely apply to memory-dependent fields. These findings have wide implications for oscillatory-based models of memory.

The representation of episodes is a fundamental requirement for forming episodic memories, but the specific electrophysiological mechanisms supporting episode construction in the human hippocampus remain unknown. Experiments in rodent models indicate that a population of neurons sensitive to edges of an environment, termed border or boundary neurons in spatial navigation, fulfills a role analogous to episode demarcation. We hypothesized that such boundary neurons could be identified in the human mesial temporal lobe, with firing rates sensitive specifically to the beginning and end of mnemonically-relevant episodes in the free recall task. Using a generalized linear model to control for factors such as encoding success and item onset times along with other variables, we found 44 Boundary neurons out of a total 736 single neurons recorded across 27 subjects. We distinguish boundary neurons from a separate population of ramping neurons, which are time-sensitive neurons whose activity provides complementary but distinct information during episodic representation. We also describe evidence that the firing of boundary neurons within the preferred windows (at the beginning and end of episodes) is organized by hippocampal theta oscillations, using spike-field coherence metrics.

The hippocampus is thought to combine "what" and "where" information from the cortex so that objects and events can be represented within the spatial context in which they occur. Surprisingly then, these distinct types of information remain partially segregated in the output region of the hippocampus, area CA1. In this region, objects preferentially activate neurons in the distal segment (adjacent to the subiculum) while spatial locations are precisely represented by neurons in the proximal segment (adjacent to CA2). This difference likely results from distinct anatomical connections; proximal CA1 receives direct input from the medial entorhinal cortex (which encodes spatial context) whereas distal CA1 has reciprocal connections with the lateral entorhinal cortex (which encodes objects and events). Based on these findings, it has been proposed that CA1 contains two distinct representations; one that encodes the animals spatial location and another that encodes objects that are present in the environment. The current study aimed to determine the role of distal CA1 in learning the location of objects in an environment. To do this, we first examined c-Fos expression in proximal and distal CA1 to see if we could replicate previous findings and confirm that neurons in these distinct segment are responsive to different stimuli. As previous studies indicate that catecholamines can regulate the activity of segments of CA1, we then investigate the role of catecholamines on learning object locations using 6-OHDA or SCH23390 to lesion catecholaminergic input and block D1/D5 receptors, respectively. Finally, we monitored calcium activity with fiber photometry while animals performed a hippocampal-dependent object location memory task.

The dentate gyrus (DG) of the hippocampus exhibits striking anatomical and functional heterogeneity along its dorsoventral axis, yet the intrinsic electrophysiological diversity of its principal excitatory neurons, the granule cells, across the dorsoventral axis remains unexplored. Here, we systematically examined the electrophysiological properties of DG granule cells across the dorsal, intermediate, and ventral regions of the rat hippocampus. We found a progressive increase in input resistance, impedance amplitude, and firing rate of granule cells, accompanied by a gradual slowdown in repolarization kinetics of their action potentials along the dorsal-to-ventral axis. Our analyses demonstrated that granule cells acted as class I integrators that lacked strong resonance properties across the dorsoventral axis. We performed pairwise correlation and dimensionality reduction analyses to reveal weak dependencies across physiological measurements and the absence of distinct clusters for dorsal, intermediate, or ventral granule cells. Importantly, blade-specific analyses of granule cell physiology revealed that all measurements manifested pronounced heterogeneities even within a given dorsoventral section and a specific blade. Strikingly, ventral granule cells in the infrapyramidal blade manifested higher firing rates compared to their counterparts in the suprapyramidal blade. These blade-specific differences were limited to the ventral granule cells, with negligible distinctions between granule cells in the two blades of either dorsal or intermediate hippocampus. Together, our findings unveil a progressive increase in excitability of DG granule cells along the dorsal-to-ventral axis and a blade-specific granularity of firing properties, adding new dimensions to the several known anatomical, molecular, and behavioral differences across the hippocampal dorsoventral axis.

Continuous experiences are constantly segmented into separate events in memory, which is accompanied by increased hippocampal activity at the boundaries between these events. Extracting knowledge across all these experiences about what type of events to expect in a certain environment leads to event schemas being represented in the brain. These event schemas help to predict future events. What are the differences across the various distinct hippocampal subfields in peaks of activity around event boundaries? And what is the influence of event schema learning on these hippocampal subfield peaks? Here, this was investigated using data from an fMRI experiment in which participants learned two (related) event schemas by exposure to many similar animated videos of wedding ceremonies in a novel fictional culture. Results revealed that the hippocampus (CA1, CA2/3, dentate gyrus and subiculum) showed peaks in activity before event boundaries. Interestingly, the CA1, CA2/3 and dentate gyrus pre-boundary peaks attenuated due to event schema learning. This provides evidence for these subfields signaling uncertainty about upcoming events in the seconds prior to event boundaries. These results give a more precise understanding about the temporal dynamics around event boundaries in the distinct hippocampal subfields, and how these hippocampal subfields responses interact with schema learning.

Hippocampal function relies on the anterior thalamic nuclei, but the reasons remain poorly understood. While anterior thalamic lesions disrupt parahippocampal spatial signalling, their impact on the subiculum is unknown, despite the importance of this area for hippocampal networks. We recorded subicular cells in rats with either permanent (N-methyl-D-aspartic acid) or reversible (muscimol) anterior thalamic lesions. Bayesian and other statistical analyses underscored the notable absence of the diverse spatial signals normally found in the subiculum, including place cells, following permanent anterior thalamic lesions. Likewise, there was marked disruption of these diverse spatial signals during transient lesions. By contrast, permanent anterior thalamic lesions had no discernible impact on CA1 place fields. Anterior thalamic lesions reduced spatial alternation performance (permanently or reversibly) to chance, while leaving a non-spatial recognition memory task unaffected. These findings, which help explain why anterior thalamic damage is so deleterious for spatial memory, cast a new spotlight on the importance of subiculum function and reveal its dependence on anterior thalamic signalling. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=184 HEIGHT=200 SRC="FIGDIR/small/928762v2_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@9ae754org.highwire.dtl.DTLVardef@1c96837org.highwire.dtl.DTLVardef@1d91ddaorg.highwire.dtl.DTLVardef@136fa1d_HPS_FORMAT_FIGEXP M_FIG C_FIG

Spatial memory is crucial for navigation and adapting to changing environmental conditions. Known neurophysiological mechanisms of spatial memory center on the importance of hippocampal activity and its spatial tuning. Yet, the behavioral strategies that support adaptive spatial encoding remain poorly understood. We have shown that dorsal hippocampal activity during rearing is necessary for spatial working memory, highlighting a role of information seeking behaviors for spatial memory encoding. Similarly, spatial tuning by dorsal hippocampal neurons is substantially updated during another information seeking behavior: attentive head scanning. However, the functional relationship between these behaviors is unknown. Here, to assess the relevance of environmental context for the expression of these behaviors, we quantified rearing and head scanning in a radial-arm-maze spatial working memory task while manipulating the height of the maze walls. Our goal was to test whether the stereotyped patterns of rearing that rats generate with tall walls are replaced with attentive head scanning when the walls are short enough to reach the top without rearing. We found that rats reared significantly less often when the walls were shortened and, instead, exhibited frequent attentive head scanning. The head scanning was done when and where the rats had previously exhibited stereotyped rearing. These results support the hypothesis that rearing and head scanning are functionally related behaviors. Future work should test two key inferences: 1) Head scanning is a critical epoch of spatial memory encoding, and 2) Spatial tuning by hippocampal neurons is updated during rearing. Significance statementSpatial memory is a core cognitive function, essential for healthy independent living. Though the hippocampus is critical for spatial memory, it remains unclear when and how. Separate prior studies link rearing and lateral head scanning to key periods of hippocampal processing, suggesting both behaviors support sensory information gathering for updating cognitive maps. However, their relationship is unresolved. Here, we test whether these behaviors are functionally interchangeable, with environmental structure determining expression. In a radial-arm maze, rats reared frequently with 21 cm walls but showed reduced rearing when walls were shortened to 4.6 cm, instead increasing head scanning at similar locations. These findings suggest rearing and head scanning share underlying motivations and provide a basis for comparing hippocampal activity during exploration.

The subiculum is positioned at a critical juncture at the interface of the hippocampus with the rest of the brain. However, the exact roles of the subiculum in most hippocampal-dependent memory tasks remain largely unknown. One obstacle to make analytical comparisons of neural firing patterns between the subiculum and hippocampal CA1 is the broad firing fields of the subicular cells. Here, we used spiking phases in relation to theta rhythm to parse the broad firing field of a subicular neuron into multiple subfields to find the unique functional contribution of the subiculum while male rats performed a hippocampal-dependent visual scene memory task. Some of the broad firing fields of the subicular neurons were successfully divided into multiple subfields by using the theta-phase precession cycle. The resulting phase-based fields in the subiculum were more similar to those in CA1 in terms of the field size and phase-precession strength. The new method significantly improved the detection of task-relevant information in subicular cells without affecting the information content represented by CA1 cells. Notably, multiple fields of a single subicular neuron, unlike those in the CA1, could carry heterogeneous task-related information such as visual context and choice response. Our findings suggest that the subicular cells integrate multiple task-related factors by using theta rhythm to associate environmental context with action.

The hippocampus, and particularly the dorsal CA1, is essential for spatial memory consolidation through sharp-wave ripple events (SWRs) that enable the transfer of information to cortical areas. Within the dorsal CA1, distinct pyramidal cell sub-populations of the deep and superficial layer may play distinct roles in the processing and updating of spatial information. Using a Cre-dependent mouseline, we were able to precisely target the superficial, Calbindin (CALB1+) PCs of the CA1. This allowed us to gain a deeper understanding of the connectivity of CALB1+ CA1 PCs to their SWR-propagating excitatory output partners within the Subiculum (SUB), the burst-firing SUB PCs and their functional relevance in spatial memory consolidation and recall. Retrograde rabies tracing revealed heterogeneous innervation of the VGlut2+ bursting PCs by both CA1 PC sub-layers, and showed that the majority of presynaptic inputs are located in the superficial CA1 PC layer. We were able to observe that this anatomically confirmed connection between CALB1+ CA1 PCs and both SUB PC subtypes is able to induce spiking more reliably in burst- than regular-firing SUB PCs. CNO-induced inhibition in a Barnes Maze task revealed that the experimental group showed reduced cognitive flexibility and were slower to adapt to re-location of the goal when CALB1+ CA1 PCs were inhibited during the recall (test), while both groups behaved similarly when consolidation was manipulated (training). Inhibition did not impact overall learning, strategy development or locomotor control. This suggests that CALB1+ CA1 PCs preferentially connect to bursting SUB PCs, and support cognitive flexibility needed to adapt to a changing environment, adding further proof to the functional relevance of laminar segregation of the CA1 and the hippocampus in spatial memory processes.