Navigating Social Space
2018; Cell Press; Volume: 100; Issue: 2 Linguagem: Inglês
10.1016/j.neuron.2018.10.006
ISSN1097-4199
AutoresMatthew Schafer, Daniela Schiller,
Tópico(s)Memory and Neural Mechanisms
ResumoCognitive maps are encoded in the hippocampal formation and related regions and range from the spatial to the purely conceptual. Neural mechanisms that encode information into relational structures, up to an arbitrary level of abstraction, may explain such a broad range of representation. Research now indicates that social life can also be mapped by these mechanisms: others’ spatial locations, social memory, and even a two-dimensional social space framed by social power and affiliation. The systematic mapping of social life onto a relational social space facilitates adaptive social decision making, akin to social navigation. This emerging line of research has implications for cognitive mapping research, clinical disorders that feature hippocampal dysfunction, and the field of social cognitive neuroscience. Cognitive maps are encoded in the hippocampal formation and related regions and range from the spatial to the purely conceptual. Neural mechanisms that encode information into relational structures, up to an arbitrary level of abstraction, may explain such a broad range of representation. Research now indicates that social life can also be mapped by these mechanisms: others’ spatial locations, social memory, and even a two-dimensional social space framed by social power and affiliation. The systematic mapping of social life onto a relational social space facilitates adaptive social decision making, akin to social navigation. This emerging line of research has implications for cognitive mapping research, clinical disorders that feature hippocampal dysfunction, and the field of social cognitive neuroscience. The hippocampal formation performs functions that include spatial representation and episodic memory. These functions may reflect a multi-dimensional “cognitive map” that organizes previous experience to support flexible navigation (Tolman, 1948Tolman E.C. Cognitive maps in rats and men.Psychol. Rev. 1948; 55: 189-208Crossref PubMed Scopus (2471) Google Scholar). The discovery of spatially modulated cells in the hippocampus and entorhinal cortex led to the view that these regions encode spatial cognitive maps (O’Keefe and Nadel, 1978O’Keefe J. Nadel L. The Hippocampus as a Cognitive Map. Clarendon Press, 1978Google Scholar, Eichenbaum and Cohen, 2014Eichenbaum H. Cohen N.J. Can we reconcile the declarative memory and spatial navigation views on hippocampal function?.Neuron. 2014; 83: 764-770Abstract Full Text Full Text PDF PubMed Google Scholar). Subsequent research has shown that these regions are also sensitive to a variety of non-spatial and even abstract features, such as sound, time, reward, and concepts (Schiller et al., 2015Schiller D. Eichenbaum H. Buffalo E.A. Davachi L. Foster D.J. Leutgeb S. Ranganath C. Memory and space: towards an understanding of the cognitive map.J. Neurosci. 2015; 35: 13904-13911Crossref PubMed Scopus (65) Google Scholar). The hippocampal formation maps and stores this information relationally, enabling inference and decision making by utilizing stored memory elements (Eichenbaum and Cohen, 2014Eichenbaum H. Cohen N.J. Can we reconcile the declarative memory and spatial navigation views on hippocampal function?.Neuron. 2014; 83: 764-770Abstract Full Text Full Text PDF PubMed Google Scholar). Emerging research suggests that the hippocampus also represents social stimuli within physical space, information about specific individuals, and abstract social dimensions, such as power and affiliation (Montagrin et al., 2017Montagrin A. Saiote C. Schiller D. The social hippocampus.Hippocampus. 2017; (Published online August 26, 2017)https://doi.org/10.1002/hipo.22797Crossref Scopus (5) Google Scholar). The hippocampus and related regions may thus perform social functions and encode “social space” in the form of a cognitive map. This Perspective argues that social cognitive mapping occurs and is supported by mechanisms that map physical space. The argument presents evidence for spatial mapping, followed by evidence that these same mechanisms also map non-spatial and abstract information and enable the use of cognitive maps in decision making. To support the idea that the hippocampus is involved in social cognitive mapping, we highlight research showing that spatially sensitive cells in the hippocampus encode social information and discuss how this may relate to the role of the hippocampus in social memory. Finally, the “social space” is presented: abstract dimensions of social power and affiliation may be mapped by the same regions that map physical space and thus facilitate social navigation (Figure 1). We end with a discussion on how social mapping research advances social cognitive neuroscience. Tolman, 1948Tolman E.C. Cognitive maps in rats and men.Psychol. Rev. 1948; 55: 189-208Crossref PubMed Scopus (2471) Google Scholar formed the cognitive map hypothesis when he observed that animals infer relationships between locations that had never been directly experienced together, a phenomenon that stimulus-response learning could not easily explain. In one illustrative study, rats were trained to travel down a path to a food reward. After training, the rats were placed in a similar environment, where the learned path to the food reward was now blocked. Now, there were new, alternative routes, one of which led to the reward location. In a single-trial test, the rats chose the correct path that angled toward the reward despite never having been trained to take that specific path. The animals may have retained a representation of the reward’s location and used it to flexibly navigate toward the reward despite obstacles, perhaps by searching an internal “cognitive map” of the relationships within the environment to predict decision outcomes. The discovery of hippocampus pyramidal cells that encode the animal’s current location (“place cells”) led to the speculation that cognitive maps are encoded in the hippocampal formation (O’Keefe and Nadel, 1978O’Keefe J. Nadel L. The Hippocampus as a Cognitive Map. Clarendon Press, 1978Google Scholar), defined as entorhinal cortex, dentate gyrus, hippocampus CA subfields, and subicular complex. Subsequent research has clarified many of the properties of place cells, which are consistent with the basic idea of a cognitive map. Forming these neurons’ preferred spatial locations (“place fields”) requires environmental experience but, once stabilized, spiking continues to be spatially specific absent external cues for a period of time (McNaughton et al., 2006McNaughton B.L. Battaglia F.P. Jensen O. Moser E.I. Moser M.B. Path integration and the neural basis of the ‘cognitive map’.Nat. Rev. Neurosci. 2006; 7: 663-678Crossref PubMed Scopus (871) Google Scholar), suggesting that these representations are learned and at least in part internally maintained. This spiking activity is stable and location specific within spatial contexts but highly variable across contexts, indicating context dependence (“remapping”; Colgin et al., 2008Colgin L.L. Moser E.I. Moser M.B. Understanding memory through hippocampal remapping.Trends Neurosci. 2008; 31: 469-477Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). Place fields also get continuously larger from the dorsal to ventral hippocampal poles (corresponding to posterior to anterior in humans, respectively), suggesting a functional gradient that may allow for both precise and continuous spatial representations (Strange et al., 2014Strange B.A. Witter M.P. Lein E.S. Moser E.I. Functional organization of the hippocampal longitudinal axis.Nat. Rev. Neurosci. 2014; 15: 655-669Crossref PubMed Scopus (330) Google Scholar). The ability to learn and (in part) internally maintain location specific activity within spatial contexts provides some of the machinery necessary to encode the physical environment into a cognitive map. Regions connected with the hippocampus also contain spatially modulated cell types. In superficial layers of medial entorhinal cortex, the main cortical input to the hippocampus, there are “grid cells” that fire at multiple, regular positions within the environment and are clustered in modules, with distinct scales and orientations (e.g., Hafting et al., 2005Hafting T. Fyhn M. Molden S. Moser M.-B. Moser E.I. Microstructure of a spatial map in the entorhinal cortex.Nature. 2005; 436: 801-806Crossref PubMed Scopus (1553) Google Scholar, Stensola et al., 2012Stensola H. Stensola T. Solstad T. Frøland K. Moser M.B. Moser E.I. The entorhinal grid map is discretized.Nature. 2012; 492: 72-78Crossref PubMed Scopus (213) Google Scholar). Like place cells, grid cells require some experience with the environment and their spiking remaps in new spatial contexts. Other medial entorhinal cortex cells also provide spatial information, including head direction (Sargolini et al., 2006Sargolini F. Fyhn M. Hafting T. McNaughton B.L. Witter M.P. Moser M.-B. Moser E.I. Conjunctive representation of position, direction, and velocity in entorhinal cortex.Science. 2006; 312: 758-762Crossref PubMed Scopus (636) Google Scholar), environmental borders (Solstad et al., 2008Solstad T. Boccara C.N. Kropff E. Moser M.B. Moser E.I. Representation of geometric borders in the entorhinal cortex.Science. 2008; 322: 1865-1868Crossref PubMed Scopus (445) Google Scholar), speed (Kropff et al., 2015Kropff E. Carmichael J.E. Moser M.B. Moser E.I. Speed cells in the medial entorhinal cortex.Nature. 2015; 523: 419-424Crossref PubMed Scopus (134) Google Scholar), and vectors (distance and direction information), connecting the current location to objects (Høydal et al., 2018Høydal Ø.A. Skytøen E.R. Moser M. Moser E.I. Object-vector coding in the medial entorhinal cortex.bioRxiv. 2018; https://doi.org/10.1101/286286Crossref Google Scholar). Retrosplenial cortex, an area that is densely interconnected with the hippocampal formation, also contains place and head direction cells (Miller et al., 2014Miller A.M.P. Vedder L.C. Law L.M. Smith D.M. Cues, context, and long-term memory: the role of the retrosplenial cortex in spatial cognition.Front. Hum. Neurosci. 2014; 8: 586Crossref PubMed Scopus (45) Google Scholar), as well as other spatial cell types. For example, some retrosplenial cells encode sub-routes within a larger environment at multiple spatial scales, and others map the distance between current location and every other environmental position, giving relational information about complex paths (Alexander and Nitz, 2017Alexander A.S. Nitz D.A. Spatially periodic activation patterns of retrosplenial cortex encode route sub-spaces and distance traveled.Curr. Biol. 2017; 27: 1551-1560.e4Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar). These neuronal subpopulations, among others, may cooperate within a spatially distributed network to support cognitive mapping. Place cells encode more than the animal’s current location: decoded place cell network activity reveals that place cells can represent prospective locations as well as locations the animal just left (Ferbinteanu and Shapiro, 2003Ferbinteanu J. Shapiro M.L. Prospective and retrospective memory coding in the hippocampus.Neuron. 2003; 40: 1227-1239Abstract Full Text Full Text PDF PubMed Scopus (311) Google Scholar). These cells bind recently experienced and to-be-experienced locations together into an orderly trajectory. Place cell sequences can also represent spatial locations that were visited further in the past. During rest or sleep, place cell ensembles often reactivate in patterns that correlate with sequences that were active during environmental experience in both reverse and forward order (Pfeiffer, 2017Pfeiffer B.E. The content of hippocampal “replay”.Hippocampus. 2017; (Published online December 20, 2017)https://doi.org/10.1002/hipo.22824Crossref PubMed Scopus (2) Google Scholar). These “replay” sequences may reflect a variety of network level phenomena related to cognitive mapping, such as memory consolidation and memory-based planning (Pfeiffer, 2017Pfeiffer B.E. The content of hippocampal “replay”.Hippocampus. 2017; (Published online December 20, 2017)https://doi.org/10.1002/hipo.22824Crossref PubMed Scopus (2) Google Scholar). Grid cell replay-like activity has recently been observed as well, suggesting that replay might be common across the cognitive map network (O’Neill et al., 2017O’Neill J. Boccara C.N. Stella F. Schoenenberger P. Csicsvari J. Superficial layers of the medial entorhinal cortex replay independently of the hippocampus.Science. 2017; 355: 184-188Crossref PubMed Scopus (16) Google Scholar). Consistent with the encoding of trajectories that span the past, present, and future, the hippocampus can track time (Eichenbaum, 2014Eichenbaum H. Time cells in the hippocampus: a new dimension for mapping memories.Nat. Rev. Neurosci. 2014; 15: 732-744Crossref PubMed Scopus (299) Google Scholar). In addition to place cells, the hippocampus contains “time cells” and dual-function cells that encode both time and place (Eichenbaum, 2014Eichenbaum H. Time cells in the hippocampus: a new dimension for mapping memories.Nat. Rev. Neurosci. 2014; 15: 732-744Crossref PubMed Scopus (299) Google Scholar). Independent of spatial information or behavioral factors, time cells fire when an animal is at a particular point in time in a temporally structured experience. For example, when location is held constant, hippocampus CA1 neuronal activity is explained by delay duration, as individual cells fire at different time points to cover the entire delay period (e.g., MacDonald et al., 2011MacDonald C.J. Lepage K.Q. Eden U.T. Eichenbaum H. Hippocampal “time cells” bridge the gap in memory for discontiguous events.Neuron. 2011; 71: 737-749Abstract Full Text Full Text PDF PubMed Scopus (370) Google Scholar), similar to how place cells fire to cover a linear track. Like place cell remapping, this activity is context dependent: firing patterns change in new contexts such as when a delay period is increased. Therefore, this activity does not reflect generic time tracking and instead likely reflects temporal context. That space and time can be encoded similarly and simultaneously within the same or neighboring cells suggests that they may be integrated into a unified spatiotemporal representation. Neural signal at the resolution of functional magnetic resonance imaging (fMRI) supports this interpretation. In a virtual navigation task, hippocampal multivoxel pattern similarity reflected events on both spatial and temporal dimensions, with spatial and temporal similarity having an additive effect on neural similarity (Deuker et al., 2016Deuker L. Bellmund J.L. Navarro Schröder T. Doeller C.F. An event map of memory space in the hippocampus.eLife. 2016; 5: 1-26Crossref Scopus (6) Google Scholar). Spatial grid cells can also track temporal features (Kraus et al., 2015Kraus B.J. Brandon M.P. Robinson 2nd, R.J. Connerney M.A. Hasselmo M.E. Eichenbaum H. During running in place, grid cells integrate elapsed time and distance run.Neuron. 2015; 88: 578-589Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar), bolstering the view that the cognitive map incorporates an abstract temporal dimension. The representation of space and time likely underpins the role of the hippocampus and related regions in spatial and episodic memory (i.e., memory for events in a specific place and time). Hippocampal neural reinstatement of an item’s spatial or temporal context is key to successful memory retrieval (Flegal et al., 2014Flegal K.E. Marín-Gutiérrez A. Ragland J.D. Ranganath C. Brain mechanisms of successful recognition through retrieval of semantic context.J. Cogn. Neurosci. 2014; 26: 1694-1704Crossref Scopus (8) Google Scholar), as is differentiating competing spatial or temporal context representations between memories (Copara et al., 2014Copara M.S. Hassan A.S. Kyle C.T. Libby L.A. Ranganath C. Ekstrom A.D. Complementary roles of human hippocampal subregions during retrieval of spatiotemporal context.J. Neurosci. 2014; 34: 6834-6842Crossref PubMed Scopus (35) Google Scholar). Other connected regions have similar memory-encoding properties as the hippocampus. Retrosplenial cortex is important to both spatial and episodic memory and contains ensembles that can encode context-specific engrams (Milczarek et al., 2018Milczarek M.M. Vann S.D. Sengpiel F. Spatial memory engram in the mouse retrosplenial cortex.Curr. Biol. 2018; 28: 1975-1980.e6Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). This region connects the hippocampus to posterior cingulate cortex (PCC), an area involved in autobiographical memory (Maddock et al., 2001Maddock R.J. Garrett A.S. Buonocore M.H. Remembering familiar people: the posterior cingulate cortex and autobiographical memory retrieval.Neuroscience. 2001; 104: 667-676Crossref PubMed Scopus (316) Google Scholar). The interconnections and functional similarities of these regions suggest that the hippocampus, retrosplenial cortex, and PCC are nodes within a memory system that builds and stores relational models of the contextual elements of an experience (Ranganath and Ritchey, 2012Ranganath C. Ritchey M. Two cortical systems for memory-guided behaviour.Nat. Rev. Neurosci. 2012; 13: 713-726Crossref PubMed Scopus (342) Google Scholar). These elements include more than the spatiotemporal context (the “where” and the “when”) of an event, however. The hippocampus can also encode information about specific elements within a spatiotemporal context (the “what” of an episode), such as object identity (Manns and Eichenbaum, 2009Manns J.R. Eichenbaum H. A cognitive map for object memory in the hippocampus.Learn. Mem. 2009; 16: 616-624Crossref PubMed Scopus (0) Google Scholar). For example, some hippocampus neurons respond optimally to the combination of a specific object and a specific location and, in some cases, do not fire at all to the object or location alone (e.g., Komorowski et al., 2009Komorowski R.W. Manns J.R. Eichenbaum H. Robust conjunctive item-place coding by hippocampal neurons parallels learning what happens where.J. Neurosci. 2009; 29: 9918-9929Crossref PubMed Scopus (174) Google Scholar). Conjunctive signaling of this sort may encode “what happened where” and is a possible consequence of the “what” and “where” visual pathways converging in the hippocampus (Komorowski et al., 2009Komorowski R.W. Manns J.R. Eichenbaum H. Robust conjunctive item-place coding by hippocampal neurons parallels learning what happens where.J. Neurosci. 2009; 29: 9918-9929Crossref PubMed Scopus (174) Google Scholar). The same mapping mechanisms that encode spatiotemporal contexts and their specific elements might function to organize relationships between environmental elements into a stable, relational framework for visual perception. When individuals visually explored a virtual environment, there was a grid-like BOLD signal in entorhinal cortex, suggesting the presence of visual grid cells with similar properties to spatial grid cells (Julian et al., 2018Julian J.B. Keinath A.T. Frazzetta G. Epstein R.A. Human entorhinal cortex represents visual space using a boundary-anchored grid.Nat. Neurosci. 2018; 21: 191-194Crossref PubMed Scopus (2) Google Scholar), a finding supported by single-cell recordings in monkeys (Killian et al., 2012Killian N.J. Jutras M.J. Buffalo E.A. A map of visual space in the primate entorhinal cortex.Nature. 2012; 491: 761-764Crossref PubMed Scopus (142) Google Scholar). Additionally, single entorhinal and hippocampal neurons can respond preferentially to the visual perception of spatial layouts (Kreiman et al., 2000Kreiman G. Koch C. Fried I. Category-specific visual responses of single neurons in the human medial temporal lobe.Nat. Neurosci. 2000; 3: 946-953Crossref PubMed Scopus (336) Google Scholar). Visual and physical exploration may therefore use some of the same mechanisms to map the physical environment. Other sensory modalities, such as auditory environments, are similarly encoded by the hippocampal formation, with specific place cells and grid cells responding to specific sound frequencies in a manner akin to spatial mapping (Aronov et al., 2017Aronov D. Nevers R. Tank D.W. Mapping of a non-spatial dimension by the hippocampal-entorhinal circuit.Nature. 2017; 543: 719-722Crossref PubMed Scopus (49) Google Scholar). Like a linear track, sound frequency exists on a single dimension, and navigation in these two domains yields very similar neural representations, suggesting that the same mechanism underlies both spatial and non-spatial dimensional mapping. Additionally, professional piano tuners, who navigate a complex auditory landscape, were found to have larger than normal hippocampal volumes, an effect that was experience-dependent as it was larger in tuners who had more years of practice (Teki et al., 2012Teki S. Kumar S. von Kriegstein K. Stewart L. Lyness C.R. Moore B.C.J. Capleton B. Griffiths T.D. Navigating the auditory scene: an expert role for the hippocampus.J. Neurosci. 2012; 32: 12251-12257Crossref PubMed Scopus (0) Google Scholar). This finding mirrors the larger posterior hippocampal volumes found in longtime taxi drivers (Maguire et al., 2000Maguire E.A. Gadian D.G. Johnsrude I.S. Good C.D. Ashburner J. Frackowiak R.S.J. Frith C.D. Navigation-related structural change in the hippocampi of taxi drivers.Proc. Natl. Acad. Sci. USA. 2000; 97: 4398-4403Crossref PubMed Scopus (1526) Google Scholar) and suggests that hippocampal structure changes in response to non-spatial navigation similarly to how it changes in response to spatial navigation. Hippocampal dimensional encoding extends beyond information encoded by the senses to include abstract information—with abstract defined as latent information that is not directly reducible to immediate sensory perception. This can include abstract information embedded within spatial environments, such as reward location. Indeed, place cell activity is altered by the presence of reward: individual place cells increase their firing around rewards (Poucet and Hok, 2017Poucet B. Hok V. Remembering goal locations.Curr. Opin. Behav. Sci. 2017; 17: 51-56Crossref Scopus (3) Google Scholar) and place fields cluster around reward sites (Hok et al., 2007Hok V. Lenck-Santini P.-P. Roux S. Save E. Muller R.U. Poucet B. Goal-related activity in hippocampal place cells.J. Neurosci. 2007; 27: 472-482Crossref PubMed Scopus (106) Google Scholar). There is also a dedicated sub-population of hippocampus ventral CA1 and subicular “reward cells” that encode reward location across environments (Gauthier and Tank, 2018Gauthier J.L. Tank D.W. A dedicated population for reward coding in the hippocampus.Neuron. 2018; 99: 179-193.e7Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar). Their activity was not explained by reward-related behaviors, suggesting that they represent reward location itself. That the hippocampus encodes behaviorally relevant information like reward location is unsurprising: energy resources are limited and information about the environment is not equally important and thus is not attended to with equal probability. Attentional processes may directly engage the hippocampus to support behaviorally relevant encoding, as attention to task goals modulates hippocampal fMRI signal and relates to subsequent task-relevant recall (Aly and Turk-Browne, 2016aAly M. Turk-Browne N.B. Attention promotes episodic encoding by stabilizing hippocampal representations.Proc. Natl. Acad. Sci. USA. 2016; 113: E420-E429Crossref PubMed Scopus (23) Google Scholar, Aly and Turk-Browne, 2016bAly M. Turk-Browne N.B. Attention stabilizes representations in the human hippocampus.Cereb. Cortex. 2016; 26: 783-796PubMed Google Scholar). Additionally, while task-relevant variables, such as spatial context and item identity, can be encoded in the activity of hippocampus CA1 and CA3 ensembles, task irrelevant regularities do not seem to be tracked (McKenzie et al., 2014McKenzie S. Frank A.J. Kinsky N.R. Porter B. Rivière P.D. Eichenbaum H. Hippocampal representation of related and opposing memories develop within distinct, hierarchically organized neural schemas.Neuron. 2014; 83: 202-215Abstract Full Text Full Text PDF PubMed Scopus (109) Google Scholar). Attention-guided encoding may thus preferentially map information relevant to the current task, including information outside of the immediate sensory experience of the animal, such as temporal context, object identity, and reward—in other words, non-spatial and abstract information. The relations between such behaviorally relevant elements is extracted to generate relational models. For example, the hippocampus is important in learning spatial relationships between visual cues (Lavenex et al., 2006Lavenex P.B. Amaral D.G. Lavenex P. Hippocampal lesion prevents spatial relational learning in adult macaque monkeys.J. Neurosci. 2006; 26: 4546-4558Crossref PubMed Scopus (0) Google Scholar), statistical relationships between task items across modalities (Covington et al., 2018Covington N.V. Brown-Schmidt S. Duff M.C. The necessity of the hippocampus for statistical learning.J. Cogn. Neurosci. 2018; 30: 680-697Crossref PubMed Scopus (3) Google Scholar), and hierarchical relationships between stimuli but not discrimination between individual items (Dusek and Eichenbaum, 1997Dusek J.A. Eichenbaum H. The hippocampus and memory for orderly stimulus relations.Proc. Natl. Acad. Sci. USA. 1997; 94: 7109-7114Crossref PubMed Scopus (333) Google Scholar). Similarly, hippocampal damage impairs scene perception that relies upon relationships between features, but not perception that relies on individual features (Aly et al., 2013Aly M. Ranganath C. Yonelinas A.P. Detecting changes in scenes: the hippocampus is critical for strength-based perception.Neuron. 2013; 78: 1127-1137Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar), consistent with a role for the hippocampus in constructing orderly relations between information. The hippocampus therefore helps encode behaviorally relevant statistical patterns into a relational framework, which allows generalizations and inferences about relationships. Hippocampal relational mapping likely reflects a general circuit mechanism that allows for any arbitrary dimension to be encoded, accounting for the wide range of information the hippocampal formation can represent. Purely conceptual information, abstract and not spatially embedded, may be mapped by the hippocampal formation in this way. For example, across different images and even the names of the same individual, object, or landmark, a subset of hippocampal formation neurons responded to the concept of the item rather than its sensory details (Quiroga et al., 2005Quiroga R.Q. Reddy L. Kreiman G. Koch C. Fried I. Invariant visual representation by single neurons in the human brain.Nature. 2005; 435: 1102-1107Crossref PubMed Scopus (786) Google Scholar). Entorhinal cortex, home of grid cells and an input to the hippocampus, can also map conceptual dimensions. This region contains grid cells that in the spatial domain are conjunctive for location and direction and fire with a particular spatial periodicity. In fMRI virtual navigation, when participants moved along the orientation of a spatial grid, blood-oxygen-level-dependent (BOLD) signal in entorhinal cortex oscillated in a manner consistent with grid cell activation (Doeller et al., 2010Doeller C.F. Barry C. Burgess N. Evidence for grid cells in a human memory network.Nature. 2010; 463: 657-661Crossref PubMed Scopus (254) Google Scholar). Conceptual knowledge can be organized in a similar manner. When participants had to associate birds of varying leg and neck length with specific objects, creating a two-dimensional conceptual space, BOLD activity within entorhinal cortex and other regions (e.g., medial prefrontal cortex) was modulated in a spatial grid-like fashion (Constantinescu et al., 2016Constantinescu A.O. O’Reilly J.X. Behrens T.E.J. Organizing conceptual knowledge in humans with a gridlike code.Science. 2016; 352: 1464-1468Crossref PubMed Scopus (168) Google Scholar). More grid-like representations correlated with better task performance, suggesting that representational quality related to memory. These findings were strikingly similar to Doeller et al., 2010Doeller C.F. Barry C. Burgess N. Evidence for grid cells in a human memory network.Nature. 2010; 463: 657-661Crossref PubMed Scopus (254) Google Scholar, again suggesting that abstract space is mapped by the same regions and computations as physical space, with resulting representations being similar when the underlying statistical relationships (e.g., same dimensional space) are similar. Relational frameworks may also encode the relationships between individual episodic memories. Temporal organization is crucial in recollection and may help explain how the hippocampus organizes memories for specific events across time (Eichenbaum, 2013Eichenbaum H. Memory on time.Trends Cogn. Sci. 2013; 17: 81-88Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar). The hippocampus is associated with memory for the correct order of sequences (Davachi and DuBrow, 2015Davachi L. DuBrow S. How the hippocampus preserves order: the role of prediction and context.Trends Cogn. 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