When we are under stress, our sense of direction also suffers – we get lost more easily. An experiment has now revealed why this is so and what happens in the brain. In this test subjects were given the stress hormone cortisol or a placebo and then solved virtual orientation tasks. Analyzes of their brain activity showed that the stress hormone primarily affects the brain’s “GPS”: It disrupts the function of the grid cells, which track our spatial position in a kind of brain-specific coordinate system – our mental map. Orientation towards landmarks, which is controlled by another area of the brain, was less disturbed, but only works if there are landmarks.
Normally it’s easy for us to find our way around familiar environments: we know how to get to the nearest supermarket and find our daily route to work in our sleep. This is possible because our brain creates a mental map of our surroundings and familiar places, which allows us to orientate ourselves. This brain-internal navigation system consists of two components located in different areas of the brain. The first is formed by the grid cells in the entorhinal cortex, an area in the temporal lobe near the hippocampus. As the “GPS” of our brain, they are responsible for the so-called path integration and store our position and that of possible destinations in a kind of brain-specific coordinate system. The second component are place cells in the caudate nucleus of the hippocampus, which primarily remember landmarks. Thanks to them, we remember, for example, that the bakery is in the direction of the church tower.
Stress hormone disrupts grid cells
But how does stress affect these navigation systems in our brain? Observations have already shown that increased stress can affect our orientation and we get lost more easily. However, what happens at the neuronal level was still unclear. A team led by Osman Akan from the Ruhr University Bochum has now examined this in more detail in an experiment. To do this, the researchers gave 39 test subjects either 20 milligrams of the stress hormone cortisol or a placebo. They then completed a virtual navigation task while their brain activity was recorded using functional magnetic resonance imaging (fMRI). The task was to head for certain, only sometimes visible trees in a wide, flat landscape, collect apples and find your way back to the starting point. In some passages there were permanent orientation aids, in others a lighthouse served as a landmark. After a week, the groups were swapped and the tests were repeated.
The evaluation showed: Under the influence of the stress hormone cortisol, the test subjects performed significantly worse in the navigation test – especially when landmarks were missing. This was also reflected in the brain activity of the test subjects. Under the influence of cortisol, the grid cells in the entorhinal cortex no longer fired in the ordered grid pattern as is typical. Instead, this order disappeared: “We found the grid-like representations under placebo, but not under cortisol,” report Akan and his colleagues. The grid cells, which are equipped with cortisol receptors, lose their ability to generate the mental map due to the stress hormone. “Under stress, the brain loses the ability to effectively use its internal navigation maps,” explains Akan.
Landmark orientation can only partially compensate
This failure of the internal “GPS system” also affects the second component of our internal navigation system, as the fMRI images showed. If the test subjects received cortisol, activity in the caudate nucleus, the brain area responsible for landmark recognition, increased during the orientation task. This became significantly more active under cortisol, especially when navigating with the lighthouse landmark, than under placebo. “This suggests that the brain is trying to compensate for the failure of the main navigation system in the entorhinal cortex through alternative strategies,” explains Akan. However, this compensation only works if there are landmarks and then only in parts. Because path integration through the enthorinal cortex is impaired by stress, people therefore have greater difficulty navigating under stress.
The results are also interesting for another reason: studies show that the entorhinal cortex is one of the first areas of the brain to be damaged in Alzheimer’s dementia. That’s why deficits in spatial orientation and getting lost are among the first symptoms of dementia. The effect of the stress hormone cortisol on this area could possibly explain why this is the case. “Since chronic stress is considered a risk factor for dementia, our study provides a crucial mechanism for how stress hormones destabilize this sensitive region,” says Akan. This is where further studies could begin.
Source: Osman Akan (Ruhr University Bochum) et al., PLOS Biology, doi: 10.1371/journal.pbio.3003661