Path shape impacts the extent of CA1 pattern recurrence both within and across environments

The human brain uses spatial schemas to represent segmented environments

Humans and animals use cognitive maps to represent the spatial structure of the environment. Although these maps are typically conceptualized as extending in an equipotential manner across known space, psychological evidence suggests that people mentally segment complex environments into subspaces. To understand the neurocognitive mechanisms behind this operation, we familiarized participants with a virtual courtyard that was divided into two halves by a river; we then used behavioral testing and fMRI to understand how spatial locations were encoded within this environment. Participants' spatial judgments and multivoxel activation patterns were affected by the division of the courtyard, indicating that the presence of a boundary can induce mental segmentation even when all parts of the environment are co-visible. In the hippocampus and occipital place area (OPA), the segmented organization of the environment manifested in schematic spatial codes that represented geometrically equivalent locations in the two subspaces as similar. In the retrosplenial complex (RSC), responses were more consistent with an integrated spatial map. These results demonstrate that people use both local spatial schemas and integrated spatial maps to represent segmented environment. We hypothesize that schematization may serve as a general mechanism for organizing complex knowledge structures in terms of their component elements.

Graded remapping of hippocampal ensembles under sensory conflicts

Hippocampal place cells are thought to constitute a cognitive map of space derived from multimodal sensory inputs. Alteration of allocentric (visual) cues in a fixed environment is known to induce modulations of place cell activity to varying degrees from rate changes to global remapping. To determine how hippocampal ensembles combine multimodal sensory cues, we examine hippocampal CA1 remapping in Mongolian gerbils in a 1D virtual reality experiment, during which self-motion cues (locomotor, vestibular, and optic flow information) and allocentric visual cues are altered. We observe that self-motion cues are over-represented, but responsiveness to allocentric visual cues, although task-irrelevant, elicits both rate and global remapping in the hippocampal ensemble. We propose that remapping can be reconciled by considering global, partial, and rate remapping on a continuous scale on which the graded change of activity in the entire CA1 population can be interpreted as the expectancy about the animal's spatial environment.

Cognitive Maps: Distortions of the Hippocampal Space Map Define Neighborhoods

In place of continuous overhead satellite views of an environment, the brain often relies on first-person experiences to estimate spatial relationships between locations. Using new methods, a recent study has found the spatial metric observed in hippocampal activity adapts to encode local environmental terrain.

Hippocampal Place Cells Encode Local Surface-Texture Boundaries

The cognitive map is often assumed to be a Euclidean map that isometrically represents the real world (i.e., the Euclidean distance between any two locations in the physical world should be preserved on the cognitive map). However, accumulating evidence suggests that environmental boundaries can distort the mental representations of physical space. For example, the distance between two locations can be remembered as longer than the true physical distance if the locations are separated by a boundary. While this overestimation is observed under different experimental conditions, even when the boundary is formed by flat surface cues, its physiological basis is not well understood. We examined the neural representation of flat surface cue boundaries, and of the space segregated by these boundaries, by recording place cell activity from CA1 and CA3 while rats foraged on a circular track or square platforms with inhomogeneous surface textures. About 40% of the place field edges concentrated near the boundaries on the circular track (significantly above the chance level 33%). Similarly, place field edges were more prevalent near boundaries on the platforms than expected by chance. In both one- and two-dimensional environments, the population vectors of place cell activity changed more abruptly with distance between locations that crossed cue boundaries than between locations within a bounded region. These results show that the locations of surface boundaries were evident as enhanced decorrelations of the neural representations of locations to either side of the boundaries. This enhancement might underlie the cognitive phenomenon of overestimation of distances across boundaries.

Field repetition and local mapping in the hippocampus and the medial entorhinal cortex

Hippocampal place cells support spatial cognition and are thought to form the neural substrate of a global "cognitive map." A widely held view is that parts of the hippocampus also underlie the ability to separate patterns or to provide different neural codes for distinct environments. However, a number of studies have shown that in environments composed of multiple, repeating compartments, place cells and other spatially modulated neurons show the same activity in each local area. This repetition of firing fields may reflect pattern completion and may make it difficult for animals to distinguish similar local environments. In this review we 1) highlight some of the navigation difficulties encountered by humans in repetitive environments, 2) summarize literature demonstrating that place and grid cells represent local and not global space, and 3) attempt to explain the origin of these phenomena. We argue that the repetition of firing fields can be a useful tool for understanding the relationship between grid cells in the entorhinal cortex and place cells in the hippocampus, the spatial inputs shared by these cells, and the propagation of spatially related signals through these structures.

Spatially Periodic Activation Patterns of Retrosplenial Cortex Encode Route Sub-spaces and Distance Traveled

Traversal of a complicated route is often facilitated by considering it as a set of related sub-spaces. Such compartmentalization processes could occur within retrosplenial cortex, a structure whose neurons simultaneously encode position within routes and other spatial coordinate systems. Here, retrosplenial cortex neurons were recorded as rats traversed a track having recurrent structure at multiple scales. Consistent with a major role in compartmentalization of complex routes, individual retrosplenial cortex (RSC) neurons exhibited periodic activation patterns that repeated across route segments having the same shape. Concurrently, a larger population of RSC neurons exhibited single-cycle periodicity over the full route, effectively defining a framework for encoding of sub-route positions relative to the whole. The same population simultaneously provides a novel metric for distance from each route position to all others. Together, the findings implicate retrosplenial cortex in the extraction of path sub-spaces, the encoding of their spatial relationships to each other, and path integration.

Multivoxel Pattern Analysis Reveals 3D Place Information in the Human Hippocampus

The spatial world is three dimensional (3D) and humans and other animals move both horizontally and vertically within it. Extant neuroscientific studies have typically investigated spatial navigation on a horizontal 2D plane, leaving much unknown about how 3D spatial information is represented in the brain. Specifically, horizontal and vertical information may be encoded in the same or different neural structures with equal or unequal sensitivity. Here, we investigated these possibilities using fMRI while participants were passively moved within a 3D lattice structure as if riding a rollercoaster. Multivoxel pattern analysis was used to test for the existence of information relating to where and in which direction participants were heading in this virtual environment. Behaviorally, participants had similarly accurate memory for vertical and horizontal locations and the right anterior hippocampus (HC) expressed place information that was sensitive to changes along both horizontal and vertical axes. This is suggestive of isotropic 3D place encoding. In contrast, participants indicated their heading direction faster and more accurately when they were heading in a tilted-up or tilted-down direction. This direction information was expressed in the right retrosplenial cortex and posterior HC and was only sensitive to vertical pitch, which could reflect the importance of the vertical (gravity) axis as a reference frame. Overall, our findings extend previous knowledge of how we represent the spatial world and navigate within it by taking into account the important third dimension.

SIGNIFICANCE STATEMENT The spatial world is 3D. We can move horizontally across surfaces, but also vertically, going up slopes or stairs. Little is known about how the brain supports representations of 3D space. A key question is whether horizontal and vertical information is equally well represented. Here, we measured fMRI response patterns while participants moved within a virtual 3D environment and found that the anterior hippocampus (HC) expressed location information that was sensitive to the vertical and horizontal axes. In contrast, information about heading direction, found in retrosplenial cortex and posterior HC, favored the vertical axis, perhaps due to gravity effects. These findings provide new insights into how we represent our spatial 3D world and navigate within it.

Gaze-informed, task-situated representation of space in primate hippocampus during virtual navigation

To elucidate how gaze informs the construction of mental space during wayfinding in visual species like primates, we jointly examined navigation behavior, visual exploration, and hippocampal activity as macaque monkeys searched a virtual reality maze for a reward. Cells sensitive to place also responded to one or more variables like head direction, point of gaze, or task context. Many cells fired at the sight (and in anticipation) of a single landmark in a viewpoint- or task-dependent manner, simultaneously encoding the animal’s logical situation within a set of actions leading to the goal. Overall, hippocampal activity was best fit by a fine-grained state space comprising current position, view, and action contexts. Our findings indicate that counterparts of rodent place cells in primates embody multidimensional, task-situated knowledge pertaining to the target of gaze, therein supporting self-awareness in the construction of space.

In the brain of mammalian species, the hippocampus is a key structure for episodic and spatial memory and is home to neurons coding a selective location in space (“place cells”). These neurons have been mostly investigated in the rat. However, species such as rodents and primates have access to different olfactory and visual information, and it is still unclear how their hippocampal cells compare. By analyzing hippocampal activity of nonhuman primates (rhesus macaques) while they searched a virtual environment for a reward, we show that space coding is more complex than a mere position or orientation selectivity. Rather, space is represented as a combination of visually derived information and task-related knowledge. Here, we uncover how this multidimensional representation emerges from gazing at the environment at key moments of the animal’s exploration of space. We show that neurons are active for precise positions and actions related to the landmarks gazed at by the animals. Neurons were even found to anticipate the appearance of landmarks, sometimes responding to a landmark that was not yet visible. Overall, the place fields of primate hippocampal neurons appear as the projection of a multidimensional memory onto physical space.

Retrosplenial cortex maps the conjunction of internal and external spaces

Intelligent behavior demands not only multiple forms of spatial representation, but also coordination among the brain regions mediating those representations. Retrosplenial cortex is densely interconnected with the majority of cortical and subcortical brain structures that register an animal's position in multiple internal and external spatial frames of reference. This unique anatomy suggests that it functions to integrate distinct forms of spatial information and provides an interface for transformations between them. Evidence for this was found in rats traversing two different routes placed at different environmental locations. Retrosplenial ensembles robustly encoded conjunctions of progress through the current route, position in the larger environment and the left versus right turning behavior of the animal. Thus, the retrosplenial cortex has the requisite dynamics to serve as an intermediary between brain regions generating different forms of spatial mapping, a result that is consistent with navigational and episodic memory impairments following damage to this region in humans.

Movement dependence and layer specificity of entorhinal phase precession in two-dimensional environments

As a rat moves, grid cells in its entorhinal cortex (EC) discharge at multiple locations of the external world, and the firing fields of each grid cell span a hexagonal lattice. For movements on linear tracks, spikes tend to occur at successively earlier phases of the theta-band filtered local field potential during the traversal of a firing field - a phenomenon termed phase precession. The complex movement patterns observed in two-dimensional (2D) open-field environments may fundamentally alter phase precession. To study this question at the behaviorally relevant single-run level, we analyzed EC spike patterns as a function of the distance traveled by the rat along each trajectory. This analysis revealed that cells across all EC layers fire spikes that phase-precess; indeed, the rate and extent of phase precession were the same, only the correlation between spike phase and path length was weaker in EC layer III. Both slope and correlation of phase precession were surprisingly similar on linear tracks and in 2D open-field environments despite strong differences in the movement statistics, including running speed. While the phase-precession slope did not correlate with the average running speed, it did depend on specific properties of the animal's path. The longer a curving path through a grid-field in a 2D environment, the shallower was the rate of phase precession, while runs that grazed a grid field tangentially led to a steeper phase-precession slope than runs through the field center. Oscillatory interference models for grid cells do not reproduce the observed phenomena.

Repeating firing fields of CA1 neurons shift forward in response to increasing angular velocity

Self-motion information influences spatially-specific firing patterns exhibited by hippocampal neurons. Moreover, these firing patterns can repeat across similar subsegments of an environment, provided that there is similarity of path shape and head orientations across subsegments. The influence of self-motion variables on repeating fields remains to be determined. To investigate the role of path shape and angular rotation on hippocampal activity, we recorded the activity of CA1 neurons from rats trained to run on spiral-shaped tracks. During inbound traversals of circular-spiral tracks, angular velocity increases continuously. Under this condition, most neurons (74%) exhibited repeating fields across at least three adjacent loops. Of these neurons, 86% exhibited forward shifts in the angles of field centers relative to centers on preceding loops. Shifts were absent on squared-spiral tracks, minimal and less reliable on concentric-circle tracks, and absent on outward-bound runs on circular-spiral tracks. However, outward-bound runs on the circular-spiral track in the dark were associated with backward shifts. Together, the most parsimonious interpretation of the results is that continuous increases or decreases in angular velocity are particularly effective at shifting the center of mass of repeating fields, although it is also possible that a nonlinear integration of step counts contributes to the shift. Furthermore, the unexpected absence of field shifts during outward journeys in light (but not darkness) suggests visual cues around the goal location anchored the map of space to an allocentric reference frame.

The problem of conflicting reference frames when investigating three-dimensional space in surface-dwelling animals

In a surface-dwelling animal like the rat, experimental strategies for investigating the hippocampal correlates of three-dimensional space appear inevitably complicated by the interplay of global versus local reference frames. We discuss the impact of the resulting confounds on present and future empirical analysis of the "bicoded map" hypothesis by Jeffery and colleagues.

Influence of local objects on hippocampal representations: Landmark vectors and memory

The hippocampus is thought to represent nonspatial information in the context of spatial information. An animal can derive both spatial information as well as nonspatial information from the objects (landmarks) it encounters as it moves around in an environment. In this article, correlates of both object-derived spatial as well as nonspatial information in the hippocampus of rats foraging in the presence of objects are demonstrated. A new form of CA1 place cells, called landmark-vector cells, that encode spatial locations as a vector relationship to local landmarks is described. Such landmark vector relationships can be dynamically encoded. Of the 26 CA1 neurons that developed new fields in the course of a day's recording sessions, in eight cases, the new fields were located at a similar distance and direction from a landmark as the initial field was located relative to a different landmark. In addition, object-location memory in the hippocampus is also described. When objects were removed from an environment or moved to new locations, a small number of neurons in CA1 and CA3 increased firing at the locations where the objects used to be. In some neurons, this increase occurred only in one location, indicating object + place conjunctive memory; in other neurons, the increase in firing was seen at multiple locations where an object used to be. Taken together, these results demonstrate that the spatially restricted firing of hippocampal neurons encode multiple types of information regarding the relationship between an animal's location and the location of objects in its environment.

Hippocampal phase precession from dual input components

Phase precession is a well known phenomenon in which a hippocampal place cell will fire action potentials at successively earlier phases (relative to the theta-band oscillations recorded in the local field potential) as an animal moves through the cell's receptive field (also known as a place field). We present a model in which CA1 pyramidal cell spiking is driven by dual input components arising from CA3 and EC3. The receptive fields of these two input components overlap but are offset in space from each other such that as the animal moves through the model place field, action potentials are driven first by the CA3 input component and then the EC3 input component. As CA3 synaptic input is known to arrive in CA1 at a later theta phase than EC3 input (Mizuseki et al., 2009; Montgomery et al., 2009), CA1 spiking advances in phase as the model transitions from CA3-driven spiking to EC3-driven spiking. Here spike phase is a function of animal location, placing our results in agreement with many experimental observations characterizing CA1 phase precession (O'Keefe and Recce, 1993; Huxter et al., 2003; Geisler et al., 2007). We predict that experimental manipulations that dramatically enhance or disrupt activity in either of these areas should have a significant effect on phase precession observed in CA1.

Activity dynamics and behavioral correlates of CA3 and CA1 hippocampal pyramidal neurons

The CA3 and CA1 pyramidal neurons are the major principal cell types of the hippocampus proper. The strongly recurrent collateral system of CA3 cells and the largely parallel-organized CA1 neurons suggest that these regions perform distinct computations. However, a comprehensive comparison between CA1 and CA3 pyramidal cells in terms of firing properties, network dynamics, and behavioral correlations is sparse in the intact animal. We performed large-scale recordings in the dorsal hippocampus of rats to quantify the similarities and differences between CA1 (n > 3,600) and CA3 (n > 2,200) pyramidal cells during sleep and exploration in multiple environments. CA1 and CA3 neurons differed significantly in firing rates, spike burst propensity, spike entrainment by the theta rhythm, and other aspects of spiking dynamics in a brain state-dependent manner. A smaller proportion of CA3 than CA1 cells displayed prominent place fields, but place fields of CA3 neurons were more compact, more stable, and carried more spatial information per spike than those of CA1 pyramidal cells. Several other features of the two cell types were specific to the testing environment. CA3 neurons showed less pronounced phase precession and a weaker position versus spike-phase relationship than CA1 cells. Our findings suggest that these distinct activity dynamics of CA1 and CA3 pyramidal cells support their distinct computational roles.

Anisotropic encoding of three-dimensional space by place cells and grid cells

The subjective sense of space may result in part from the combined activity of place cells in the hippocampus and grid cells in posterior cortical regions such as the entorhinal cortex and pre- and parasubiculum. In horizontal planar environments, place cells provide focal positional information, whereas grid cells supply odometric (distance measuring) information. How these cells operate in three dimensions is unknown, even though the real world is three-dimensional. We investigated this issue in rats exploring two different kinds of apparatus: a climbing wall (the 'pegboard') and a helix. Place and grid cell firing fields had normal horizontal characteristics but were elongated vertically, with grid fields forming stripes. It seems that grid cell odometry (and by implication path integration) is impaired or absent in the vertical domain, at least when the rat itself remains horizontal. These findings suggest that the mammalian encoding of three-dimensional space is anisotropic.