Directional learning, but no spatial mapping by rats performing a navigational task in an inverted orientation

Simulated vestibular spatial disorientation mouse model under coupled rotation revealing potential involvement of Slc17a6

Spatial disorientation (SD) is the main contributor to flight safety risks, but research progress in animals has been limited, impeding a deeper understanding of the underlying mechanisms of SD. This study proposed a method for constructing and evaluating a vestibular SD mouse model, which adopted coupled rotational stimulation with visual occlusion. Physiological parameters were measured alongside behavioral indices to assess the model, and neuronal changes were observed through immunofluorescent staining. The evaluation of the model involved observing decreased colonic temperature and increased arterial blood pressure in mice exposed to SD, along with notable impairments in motor and cognitive function. Our investigation unveiled that vestibular SD stimulation elicited neuronal activation in spatially associated cerebral areas, such as the hippocampus. Furthermore, transcriptomic sequencing and bioinformatics analysis revealed the potential involvement of Slc17a6 in the mechanism of SD. These findings lay a foundation for further investigation into the molecular mechanisms underlying SD.

The Neural Correlates of Spatial Disorientation in Head Direction Cells

While the brain has evolved robust mechanisms to counter spatial disorientation, their neural underpinnings remain unknown. To explore these underpinnings, we monitored the activity of anterodorsal thalamic head direction (HD) cells in rats while they underwent unidirectional or bidirectional rotation at different speeds and under different conditions (light vs dark, freely-moving vs head-fixed). Under conditions that promoted disorientation, HD cells did not become quiescent but continued to fire, although their firing was no longer direction specific. Peak firing rates, burst frequency, and directionality all decreased linearly with rotation speed, consistent with previous experiments where rats were inverted or climbed walls/ceilings in zero gravity. However, access to visual landmarks spared the stability of preferred firing directions (PFDs), indicating that visual landmarks provide a stabilizing signal to the HD system while vestibular input likely maintains direction-specific firing. In addition, we found evidence that the HD system underestimated angular velocity at the beginning of head-fixed rotations, consistent with the finding that humans often underestimate rotations. When head-fixed rotations in the dark were terminated HD cells fired in bursts that matched the frequency of rotation. This postrotational bursting shared several striking similarities with postrotational "nystagmus" in the vestibulo-ocular system, consistent with the interpretation that the HD system receives input from a vestibular velocity storage mechanism that works to reduce spatial disorientation following rotation. Thus, the brain overcomes spatial disorientation through multisensory integration of different motor-sensory inputs.

Keep a level head to know the way ahead: How rodents travel on inclined surfaces?

Animals traveling on a horizontal surface stabilize their head in relation to the substrate in order to gather spatial information and orient. What, however, do they do when traveling on an incline? We examined how three rodent species differing in motor abilities and habitats explore a platform tilted at 0–90°, hypothesizing that they would attempt to maintain bilateral vestibular cues. We found that traveling up or down was mainly straight vertically rather than diagonally, which results in identical bilateral vestibular cues. This was also achieved when traveling horizontally through rotating the head to parallel the horizontal plane. Traveling diagonally up or down was avoided, perhaps due to different bilateral vestibular cues that could hinder orientation. Accordingly, we suggest that maintaining identical bilateral cues is an orientational necessity that overrides differences in motor abilities and habitats, and that this necessity is a general characteristic of animals in motion.

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Three rodent species were tested on a platform inclined at 0°–90°

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Increased inclination results in traveling straight vertically or horizontally

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Both these shapes of trajectories feature a horizontal leveled head

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We suggest that such posture is required for spatial orientation when in motion

On the absence or presence of 3D tuned head direction cells in rats: a review and rebuttal

A major question in the field of spatial cognition is how animals represent three-dimensional (3D) space. Different results have been obtained across various species and may depend on whether the species inhabits a 3D environment or is terrestrial (land dwelling). The head direction (HD) cell system is an attractive candidate to study in terms of 3D representations. HD cells fire as a function of the animal's directional heading in the horizontal plane, independent of the animal's location and on-going behavior. Another issue concerns whether HD cells are tuned in 3D space or tuned to the 2D horizontal plane. Shinder and Taube (Shinder ME, Taube JS. J Neurophysiol 121: 4-37, 2019) addressed this issue by manipulating a rat's orientation in 3D space while monitoring responses from classic HD cells in the rat anterodorsal thalamus. They reported that HD cells did not display conjunctive firing with pitch or roll orientations. Direction-specific firing was primarily derived from horizontal semicircular canal information and that the gravity vector played an important role in influencing the cell's firing rate and its preferred firing direction. Laurens and Angelaki (Laurens J, Angelaki DE. J Neurophysiol 122: 1274-1287, 2019) challenged this view by performing a mathematical analysis on the Shinder and Taube data and concluded that they would not have seen 3D tuning based on their experimental approach. We provide a historical review of these issues followed by a summary of the experiments, which includes additional analyses. We then define what it means for a HD cell to be tuned in 3D and finish by rebutting the reanalyses performed by Laurens and Angelaki.

"It's all in their head": hierarchical exploration of a three-dimensional layered pyramid in rats

Wayfinding in a three-dimensional (3D) environment is intricate, and surface-bounded animals may overcome this complexity by breaking it down into horizontal layers along with the vertical location of each layer. Here, we examined how rats explored a layered pyramid placed in a large open field. We found that exploration presented a hierarchical (or fractal) shape of three types of roundtrips: (1) from the primary home-base to the open-field floor; (2) from the floor up and down the pyramid levels; and (3) from local home-base on each pyramid level. Ascent was slow and interrupted, whereas descent was fast. This difference was a result of level altitude, remaining after data were normalized proportionally to level area. In contrast, the time spent and the distance traveled on each level were dependent on level area, not on level altitude. This structure of spatial behavior accords with multilevel exploration, presenting a relatively independent exploration of each level. The vertical dimension in this experiment thus did not alter the typical spatiotemporal behavior, and the 3D environment was explored by application of the same spatiotemporal approach as that of a horizontal open field. We suggest that this lack of alteration is due to the horizontal posture of the animal's head and trunk during progression on the pyramid. This behavior also seems to fit the bicoding hypothesis, in which the vertical information is virtually contextual (non-metric), and so, when the rat progresses to a new level, it explores it as a newly accessed horizontal floor area.

The Growing Evidence for the Importance of the Otoliths in Spatial Memory

Many studies have demonstrated that vestibular sensory input is important for spatial learning and memory. However, it has been unclear what contributions the different parts of the vestibular system - the semi-circular canals and otoliths - make to these processes. The advent of mutant otolith-deficient mice has made it possible to isolate the relative contributions of the otoliths, the utricle and saccule. A number of studies have now indicated that the loss of otolithic function impairs normal spatial memory and also impairs the normal function of head direction cells in the thalamus and place cells in the hippocampus. Epidemiological studies have also provided evidence that spatial memory impairment with aging, may be linked to saccular function. The otoliths may be important in spatial cognition because of their evolutionary age as a sensory detector of orientation and the fact that velocity storage is important to the way that the brain encodes its place in space.

Why Isn't the Head Direction System Necessary for Direction? Lessons From the Lateral Mammillary Nuclei

Complex spatial representations in the hippocampal formation and related cortical areas require input from the head direction system. However, a recurrent finding is that behavior apparently supported by these spatial representations does not appear to require input from generative head direction regions, i.e., lateral mammillary nuclei (LMN). Spatial tasks that tax direction discrimination should be particularly sensitive to the loss of head direction information, however, this has been repeatedly shown not to be the case. A further dissociation between electrophysiological properties of the head direction system and behavior comes in the form of geometric-based navigation which is impaired following lesions to the head direction system, yet head direction cells are not normally guided by geometric cues. We explore this apparent mismatch between behavioral and electrophysiological studies and highlight future experiments that are needed to generate models that encompass both neurophysiological and behavioral findings.

Three-dimensional tuning of head direction cells in rats

Head direction (HD) cells fire when the animal faces that cell's preferred firing direction (PFD) in the horizontal plane. The PFD response when the animal is oriented outside the earth-horizontal plane could result from cells representing direction in the plane of locomotion or as a three-dimensional (3D), global-referenced direction anchored to gravity. To investigate these possibilities, anterodorsal thalamic HD cells were recorded from restrained rats while they were passively positioned in various 3D orientations. Cell responses were unaffected by pitch or roll up to ~90° from the horizontal plane. Firing was disrupted once the animal was oriented >90° away from the horizontal plane and during inversion. When rolling the animal around the earth-vertical axis, cells were active when the animal's ventral surface faced the cell's PFD. However, with the rat rolled 90° in an ear-down orientation, pitching the rat and rotating it around the vertical axis did not produce directionally tuned responses. Complex movements involving combinations of yaw-roll, but usually not yaw-pitch, resulted in reduced directional tuning even at the final upright orientation when the rat had full visual view of its environment and was pointing in the cell's PFD. Directional firing was restored when the rat's head was moved back-and-forth. There was limited evidence indicating that cells contained conjunctive firing with pitch or roll positions. These findings suggest that the brain's representation of directional heading is derived primarily from horizontal canal information and that the HD signal is a 3D gravity-referenced signal anchored to a direction in the horizontal plane. NEW & NOTEWORTHY This study monitored head direction cell responses from rats in three dimensions using a series of manipulations that involved yaw, pitch, roll, or a combination of these rotations. Results showed that head direction responses are consistent with the use of two reference frames simultaneously: one defined by the surrounding environment using primarily visual landmarks and a second defined by the earth's gravity vector.

3-D Maps and Compasses in the Brain

The world has a complex, three-dimensional (3-D) spatial structure, but until recently the neural representation of space was studied primarily in planar horizontal environments. Here we review the emerging literature on allocentric spatial representations in 3-D and discuss the relations between 3-D spatial perception and the underlying neural codes. We suggest that the statistics of movements through space determine the topology and the dimensionality of the neural representation, across species and different behavioral modes. We argue that hippocampal place-cell maps are metric in all three dimensions, and might be composed of 2-D and 3-D fragments that are stitched together into a global 3-D metric representation via the 3-D head-direction cells. Finally, we propose that the hippocampal formation might implement a neural analogue of a Kalman filter, a standard engineering algorithm used for 3-D navigation.

A dual-axis rotation rule for updating the head direction cell reference frame during movement in three dimensions

In the mammalian brain, allocentric (Earth-referenced) head direction, called azimuth, is encoded by head direction (HD) cells, which fire according to the facing direction of the animal's head. On a horizontal surface, rotations of the head around the dorsoventral (D-V) axis, called yaw, correspond to changes in azimuth and elicit appropriate updating of the HD "compass" signal to enable large-scale navigation. However, if the animal moves through three-dimensional (3D) space then there is no longer a simple relationship between yaw rotations and azimuth changes, and so processing of 3D rotations is needed. Construction of a global 3D compass would require complex integration of 3D rotations, and also a large neuronal population, most neurons of which would be silent most of the time since animals rarely sample all available 3D orientations. We propose that, instead, the HD system treats the 3D space as a set of interrelated 2D surfaces. It could do this by updating activity according to both yaw rotations around the D-V axis and rotations of the D-V axis around the gravity-defined vertical axis. We present preliminary data to suggest that this rule operates when rats move between walls of opposing orientations. This dual-axis rule, which we show is straightforward to implement using the classic one-dimensional "attractor" architecture, allows consistent representation of azimuth even in volumetric space and thus may be a general feature of mammalian directional computations even for animals that swim or fly. NEW & NOTEWORTHY Maintaining a sense of direction is complicated when moving in three-dimensional (3D) space. Head direction cells, which update the direction sense based on head rotations, may accommodate 3D movement by processing both rotations of the head around the axis of the animal's body and rotations of the head/body around gravity. With modeling we show that this dual-axis rule works in principle, and we present preliminary data to support its operation in rats.

Neural encoding of large-scale three-dimensional space-properties and constraints

How the brain represents represent large-scale, navigable space has been the topic of intensive investigation for several decades, resulting in the discovery that neurons in a complex network of cortical and subcortical brain regions co-operatively encode distance, direction, place, movement etc. using a variety of different sensory inputs. However, such studies have mainly been conducted in simple laboratory settings in which animals explore small, two-dimensional (i.e., flat) arenas. The real world, by contrast, is complex and three dimensional with hills, valleys, tunnels, branches, and—for species that can swim or fly—large volumetric spaces. Adding an additional dimension to space adds coding challenges, a primary reason for which is that several basic geometric properties are different in three dimensions. This article will explore the consequences of these challenges for the establishment of a functional three-dimensional metric map of space, one of which is that the brains of some species might have evolved to reduce the dimensionality of the representational space and thus sidestep some of these problems.

Navigating in a three-dimensional world

The study of spatial cognition has provided considerable insight into how animals (including humans) navigate on the horizontal plane. However, the real world is three-dimensional, having a complex topography including both horizontal and vertical features, which presents additional challenges for representation and navigation. The present article reviews the emerging behavioral and neurobiological literature on spatial cognition in non-horizontal environments. We suggest that three-dimensional spaces are represented in a quasi-planar fashion, with space in the plane of locomotion being computed separately and represented differently from space in the orthogonal axis - a representational structure we have termed "bicoded." We argue that the mammalian spatial representation in surface-travelling animals comprises a mosaic of these locally planar fragments, rather than a fully integrated volumetric map. More generally, this may be true even for species that can move freely in all three dimensions, such as birds and fish. We outline the evidence supporting this view, together with the adaptive advantages of such a scheme.

The vestibular contribution to the head direction signal and navigation

Spatial learning and navigation depend on neural representations of location and direction within the environment. These representations, encoded by place cells and head direction (HD) cells, respectively, are dominantly controlled by visual cues, but require input from the vestibular system. Vestibular signals play an important role in forming spatial representations in both visual and non-visual environments, but the details of this vestibular contribution are not fully understood. Here, we review the role of the vestibular system in generating various spatial signals in rodents, focusing primarily on HD cells. We also examine the vestibular system's role in navigation and the possible pathways by which vestibular information is conveyed to higher navigation centers.

Spatial memory extinction: a c-Fos protein mapping study

While the neuronal basis of spatial memory consolidation has been thoroughly studied, the substrates mediating the process of extinction remain largely unknown. This study aimed to evaluate the functional contribution of selected brain regions during the extinction of a previously acquired spatial memory task in the Morris water maze. For that purpose, we used adult male Wistar rats trained in a spatial reference memory task. Learning-related changes in c-Fos inmunoreactive cells after training were evaluated in cortical and subcortical regions. Results show that removal of the hidden platform in the water maze induced extinction of the previously reinforced escape behavior after 16 trials, without spontaneous recovery 24h later. Extinction was related with significantly higher numbers of c-Fos positive nuclei in amygdala nuclei and prefrontal cortex. On the other hand, the lateral mammillary bodies showed higher number of c-Fos positive cells than the control group. Therefore, in contrast with the results obtained in studies of classical conditioning, we show the involvement of diencephalic structures mediating this kind of learning. In summary, our findings suggest that medial prefrontal cortex, the amygdala complex and diencephalic structures like the lateral mammillary nuclei are relevant for the extinction of spatial memory.

Lesions of the dorsal tegmental nuclei disrupt control of navigation by distal landmarks in cued, directional, and place variants of the Morris water task

Navigation depends on a network of neural systems that accurately monitor an animal's spatial orientation in an environment. Within this navigation system are head direction (HD) cells which discharge as a function of an animal's directional heading, providing an animal with a neural compass to guide ongoing spatial behavior. Experiments were designed to test this hypothesis by damaging the dorsal tegmental nucleus (DTN), a midbrain structure that plays a critical role in the generation of the rodent HD cell signal, and evaluating landmark based navigation using variants of the Morris water task. In Experiments 1 and 2, shams and DTN-lesioned rats were trained to navigate toward a cued platform in the presence of a constellation of distal landmarks located outside the pool. After reaching a training criteria, rats were tested in three probe trials in which (a) the cued platform was completely removed from the pool, (b) the pool was repositioned and the cued platform remained in the same absolute location with respect to distal landmarks, or (c) the pool was repositioned and the cued platform remained in the same relative location in the pool. In general, DTN-lesioned rats required more training trials to reach performance criterion, were less accurate to navigate to the platform position when it was removed, and navigated directly to the cued platform regardless of its position in the pool, indicating a general absence of control over navigation by distal landmarks. In Experiment 3, DTN and control rats were trained in directional and place navigation variants of the water task where the pool was repositioned for each training trial and a hidden platform was placed either in the same relative location (direction) in the pool or in the same absolute location (place) in the distal room reference frame. DTN-lesioned rats were initially impaired in the direction task, but ultimately performed as well as controls. In the place task, DTN-lesioned rats were severely impaired and displayed little evidence of improvement over the course of training. Together, these results support the conclusion that the DTN is required for accurate landmark navigation.

The head-direction signal is critical for navigation requiring a cognitive map but not for learning a spatial habit

Head-direction (HD) cells fire as a function of an animal's directional heading in the horizontal plane during two-dimensional navigational tasks [1]. The information from HD cells is used with place and grid cells to form a spatial representation (cognitive map) of the environment [2, 3]. Previous studies have shown that when rats are inverted (upside down), they have difficulty learning a task that requires them to find an escape hole from one of four entry points but that they can learn it when released from one or two start points [4]. Previous reports also indicate that the HD signal is disrupted when a rat is oriented upside down [5, 6]. Here we monitored HD cell activity in the two-entry-point version of the inverted task and when the rats were released from a novel start point. We found that despite the absence of direction-specific firing in HD cells when inverted, rats could successfully navigate to the escape hole when released from one of two familiar locations by using a habit-associated directional strategy. In the continued absence of normal HD cell activity, inverted rats failed to find the escape hole when started from a novel release point. The results suggest that the HD signal is critical for accurate navigation in situations that require a flexible allocentric cognitive mapping strategy, but not for situations that utilize habit-like associative spatial learning.

Is navigation in virtual reality with FMRI really navigation?

Identifying the neural mechanisms underlying spatial orientation and navigation has long posed a challenge for researchers. Multiple approaches incorporating a variety of techniques and animal models have been used to address this issue. More recently, virtual navigation has become a popular tool for understanding navigational processes. Although combining this technique with functional imaging can provide important information on many aspects of spatial navigation, it is important to recognize some of the limitations these techniques have for gaining a complete understanding of the neural mechanisms of navigation. Foremost among these is that, when participants perform a virtual navigation task in a scanner, they are lying motionless in a supine position while viewing a video monitor. Here, we provide evidence that spatial orientation and navigation rely to a large extent on locomotion and its accompanying activation of motor, vestibular, and proprioceptive systems. Researchers should therefore consider the impact on the absence of these motion-based systems when interpreting virtual navigation/functional imaging experiments to achieve a more accurate understanding of the mechanisms underlying navigation.

On the nature of three-dimensional encoding in the cognitive map: Commentary on Hayman, Verriotis, Jovalekic, Fenton, and Jeffery

A recent article by Hayman, Verriotis, Jovalekic, Fenton, and Jeffery titled Anisotropic encoding of three-dimensional space by place cells and grid cells (2011) explored how place and grid cells respond when rats locomote vertically above the ground. From their results the authors concluded a number of points about rats' abilities to orient and navigate in three dimensions. Here, we review evidence revolving around several issues including: (1) what reference frame rats use when locomoting vertically, (2) whether rats can perceive their height above the ground, (3) whether rats can estimate vertical distance and have a cognitive map in the vertical domain, (4) whether rats can path integrate in the vertical domain, and (5) does processing 3-dimensional representations require a large number of neurons. We argue that the Hayman et al. results can be accounted for by considering the reference frame the animals used in the tasks. Had the rats been facing inward with their limbs in contact with the vertical surface when moving, it is possible that different patterns of place and grid cell activity would have been observed. Further, there is good evidence to indicate that rats can orient and navigate effectively in the vertical domain.

A role for the head-direction system in geometric learning

Several recent models of episodic memory have highlighted a potential contribution from the head-direction system; there is, however, surprisingly little known about the behavioural effects of selective lesions within the head-direction system. To address this issue, and determine what aspects of spatial memory are dependent on the head-direction system, rats with selective lateral mammillary body lesions were tested on tasks that required the use of specific spatial cues, including direction, visual allocentric, and geometric cues. Animals were first tested on a modified version of the T-maze alternation task that enabled the systematic removal of intramaze and visual allocentric cues. Rats were next tested on a geometric task that required the use of the shape of the environment to locate a hidden platform in the water-maze. The lesion rats were impaired on one stage of the T-maze alternation task and on the acquisition of the geometric task; these results are consistent with the head-direction system contributing to the processing of, and/or subsequent use, of visual allocentric and geometric cues. From the pattern of impairments it also appears that, with training, rats with lateral mammillary body lesions are able to recruit other navigational systems or that there is some degree of redundancy within the head-direction system.

Active and passive movement are encoded equally by head direction cells in the anterodorsal thalamus

The head direction (HD) system is composed of cells that represent the direction in which the animal's head is facing. Each HD cell responds optimally when the head is pointing in a particular, or preferred, direction. Although vestibular system input is necessary to generate the directional signal, motor/proprioceptive inputs can also influence HD cell responses. Previous studies comparing active and passive movement have reported significant suppression of the HD signal during passive restraint. However, in each of these studies there was considerable variability across cells, and the animal's head was never completely fixed. To address these issues, we developed a passive restraint system that more fully prevented head and body movement. HD cell responses in the anterodorsal thalamus (ADN) were evaluated during active and passive movement with this new system. Contrary to previous reports, HD cell responses were not affected by passive restraint. Both head-fixed and hand-held restraint failed to produce significant inhibition of the active HD cell response. Furthermore, direction-specific firing was maintained regardless of 1) the animal's previous experience with restraint, 2) whether it was tested in the light or dark, or 3) the position of the animal relative to the axis of rotation. The maintenance of a stable directional signal without appropriate motor, proprioceptive, or visual input indicates that vestibular input is necessary and sufficient for the generation of the HD signal. Motor and proprioceptive influences may therefore be important for the control of the preferred firing direction of HD cells, but not the generation of the signal itself.

Head direction cell firing properties and behavioural performance in 3-D space

Previous studies have identified a population of neurons in the rat brain that discharge as a function of the animal's directional heading in the horizontal plane, independent of their location and on-going behaviour. Most studies on head direction (HD) cells have explored how they respond in two-dimensional environments within the horizontal plane. Many animals, however, live and locomote in a three-dimensional world. This paper reviews how HD cells respond when the animal locomotes on a vertical surface or inverted on a ceiling. We found that HD cells fire in a normal, direction-dependent manner when the rat is in the vertical plane, but not when the animal is inverted. Recent behavioural studies reported that rats are capable of accurately performing a navigational task when inverted, but only when the task was simple and started from not more than one or two entry points. Probe trials found that they did not have a flexible, map-like representation of space when inverted. The loss of the directional signal when the animal is in an inverted orientation may account for the absence of the map-like representation. Taken together, these findings indicate that a normal otolith signal contributes an important role to HD cell discharge.