Hugo J. Spiers

Approaching the edge of a towering cliff is an unnerving experience. Watching another person do so can be equally unsettling. Our brain’s capacity to process these boundaries is important — not only for avoiding such dangers, but also for navigation in general, because borders that divide spaces can help to locate resources. For instance, a steep ravine might be risky to amble through, but also useful for locating food or friends. How does our brain keep track of such information? Writing in Nature, Stanglet al.1 report that brain rhythms at a particular frequency increase when navigating near boundaries. This rhythm is also present when watching another person navigate.

Read the paper: Boundary-anchored neural mechanisms of location-encoding for self and others

Our ability to navigate depends on regions in the brain’s medial temporal lobe (MTL), such as the entorhinal cortex and hippocampus2. Neurons in these areas provide an internal signal similar to a ‘you are here’ marker on a map2, allowing other brain regions to associate experiences with space (‘don’t ever come back to this terrible bar’, for instance). Some of these neurons specifically signal proximity to borders3,4.

Owing to the challenges of recording from neurons in humans while they are awake and mobile, much of our understanding of how borders are represented in the brain has come from rodents. For rats scurrying around, the activity of neurons that signal borders occurs alongside the theta oscillation — a rolling change in the overall electrical activity of the broader brain region, caused by the co-activity of many neurons. The theta oscillation occurs at a frequency of between 8 and 12 hertz5.

In humans, insights have been gleaned from people with epilepsy awaiting neurosurgery, who have electrodes implanted into the MTL. Recordings of neuronal activity can be filtered to account for epileptic discharge, revealing neuronal-activity patterns in deep brain structures. For example, experiments in which seated people navigate virtual environments have helped to reveal that theta oscillations in the human MTL are enhanced when the brain is encoding locations near boundaries6. However, it has been unclear whether such activity would occur when walking.

Stangl et al. overcame the challenge of recording from mobile humans using a wireless recording system7. Participants wearing the device had to alternate between navigating to unmarked goal locations in a room and walking towards visually cued targets on the walls (Fig. 1a). The unmarked goal locations were learnt in an initial exploring phase, and participants then had to remember the positions of these goals in the experiment itself.

Figure 1 | Walkers and watchers share neuronal activity patterns. a, Stangl et al.1 designed an experiment in which one person explored a room while another watched. The walker navigated towards hidden goal locations they remembered from an earlier exploration phase, and walked towards visible goals marked on the walls (only one of each type of goal is shown here for simplicity, although the experiment used several). Dashed lines indicate the threshold over which the walker was considered to be near a wall for the analysis, and arrows indicate different phases of walking (towards a hidden goal while near a wall is in black, for instance). b, The authors analysed electrical activity in the brain’s medial temporal lobe (MTL) in both walkers and watchers as the walkers navigated the room. They observed a strong oscillating pattern of brain activity called a theta oscillation in walkers as they navigated towards hidden goals — but only if they were also near walls. The oscillation was weak when navigating towards visible cues. Watchers showed the same activity patterns, implying that theta oscillations are part of our internal representation of space that helps track other people.