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Spatial memory comprises different representational systems that are sensitive to different environmental cues, like proximal landmarks or local boundaries. Here we examined how sleep affects the formation of a spatial representation integrating landmark-referenced and boundary-referenced representations. To this end, participants (n = 42) were familiarized with an environment featuring both a proximal landmark and a local boundary. After nocturnal periods of sleep or wakefulness and another night of sleep, integration of the two representational systems was tested by testing the participant's flexibility to switch from landmark-based to boundary-based navigation in the environment, and vice versa. Results indicate a distinctly increased flexibility in relying on either landmarks or boundaries for navigation, when familiarization to the environment was followed by sleep rather than by wakefulness. A second control study (n = 45) did not reveal effects of sleep (vs. wakefulness) on navigation in environments featuring only landmarks or only boundaries. Thus, rather than strengthening isolated representational systems per se, sleep presumably through forming an integrative representation, enhances flexible coordination of representational subsystems.

Advances in virtual reality (VR) technology have greatly benefited spatial navigation research. By presenting space in a controlled manner, changing aspects of the environment one at a time or manipulating the gain from different sensory inputs, the mechanisms underlying spatial behaviour can be investigated. In parallel, a growing body of evidence suggests that the processes involved in spatial navigation extend to non-spatial domains. Here, we leverage VR technology advances to test whether participants can navigate abstract knowledge. We designed a two-dimensional quantity space—presented using a head-mounted display—to test if participants can navigate abstract knowledge using a first-person perspective navigation paradigm. To investigate the effect of physical movement, we divided participants into two groups: one walking and rotating on a motion platform, the other group using a gamepad to move through the abstract space. We found that both groups learned to navigate using a first-person perspective and formed accurate representations of the abstract space. Interestingly, navigation in the quantity space resembled behavioural patterns observed in navigation studies using environments with natural visuospatial cues. Notably, both groups demonstrated similar patterns of learning. Taken together, these results imply that both self-movement and remote exploration can be used to learn the relational mapping between abstract stimuli. [ABSTRACT FROM AUTHOR]

The extent to which navigational spatial memory depends on hippocampal integrity in humans is not well documented. We investigated allocentric spatial recall using a virtual environment in a group of patients with severe hippocampal damage (SHD), a group of patients with "moderate" hippocampal damage (MHD), and a normal control group. Through four learning blocks with feedback, participants learned the target locations of four different objects in a circular arena. Distal cues were present throughout the experiment to provide orientation. A circular boundary as well as an intra-arena landmark provided spatial reference frames. During a subsequent test phase, recall of all four objects was tested with only the boundary or the landmark being present. Patients with SHD were impaired in both phases of this task. Across groups, performance on both types of spatial recall was highly correlated with memory quotient (MQ), but not with intelligence quotient (IQ), age, or sex. However, both measures of spatial recall separated experimental groupsbeyondwhatwouldbe expected based on MQ, a widely used measure of general memory function. Boundary-based and landmark-based spatial recall were both strongly related to bilateral hippocampal volumes, but not to volumes of the thalamus, putamen, pallidum, nucleus accumbens, or caudate nucleus. The results show that boundary-based and landmark-based allocentric spatial recall are similarly impaired in patients with SHD, that both types of recall are impaired beyond that predicted by MQ, and that recall deficits are best explained by a reduction in bilateral hippocampal volumes. [ABSTRACT FROM AUTHOR]

We explore the environment not only by navigating, but also by viewing our surroundings with our eyes. Here we review growing evidence that the mammalian hippocampal formation, extensively studied in the context of navigation and memory, mediates a representation of visual space that is stably anchored to the external world. This visual representation puts the hippocampal formation in a central position to guide viewing behavior and to modulate visual processing beyond the medial temporal lobe (MTL). We suggest that vision and navigation share several key computational challenges that are solved by overlapping and potentially common neural systems, making vision an optimal domain to explore whether and how the MTL supports cognitive operations beyond navigation. [ABSTRACT FROM AUTHOR]

The article reports that cognitive spaces provide a domaingeneral format for processing in the hippocampalentorhinal region, in line with its involvement beyond navigation and memory. It mentions that technological advances such as calcium imaging will clarify coding principles on the population level and facilitate the translation to human cognitive neuroscience.

Summary Grid cells are one of the core building blocks of spatial navigation [ 1 ]. Single-cell recordings of grid cells in the rodent entorhinal cortex revealed hexagonal coding of the local environment during spatial navigation [ 1 ]. Grid-like activity has also been identified in human single-cell recordings during virtual navigation [ 2 ]. Human fMRI studies further provide evidence that grid-like signals are also accessible on a macroscopic level [ 3–7 ]. Studies in both non-human primates [ 8 ] and humans [ 9, 10 ] suggest that grid-like coding in the entorhinal cortex generalizes beyond spatial navigation during locomotion, providing evidence for grid-like mapping of visual space during visual exploration—akin to the grid cell positional code in rodents during spatial navigation. However, electrophysiological correlates of the grid code in humans remain unknown. Here, we provide evidence for grid-like, hexadirectional coding of visual space by human high-frequency activity, based on two independent datasets: non-invasive magnetoencephalography (MEG) in healthy subjects and entorhinal intracranial electroencephalography (EEG) recordings in an epileptic patient. Both datasets consistently show a hexadirectional modulation of broadband high-frequency activity (60–120 Hz). Our findings provide first evidence for a grid-like MEG signal, indicating that the human entorhinal cortex codes visual space in a grid-like manner [ 8–10 ], and support the view that grid coding generalizes beyond environmental mapping during locomotion [ 4–6, 11 ]. Due to their millisecond accuracy, MEG recordings allow linking of grid-like activity to epochs during relevant behavior, thereby opening up the possibility for new MEG-based investigations of grid coding at high temporal resolution. Highlights • Hexadirectional modulation of human high-frequency electrophysiological activity • Grid-like mapping of visual space in the human entorhinal cortex • Grid-coding beyond environmental mapping during locomotion Staudigl et al. show grid-like modulation of human high-frequency activity in non-invasive magnetoencephalographic and intracranial EEG recordings. The results indicate that the human entorhinal cortex codes visual space in a grid-like manner, supporting the view that grid coding generalizes beyond environmental mapping during locomotion. [ABSTRACT FROM AUTHOR]