Brain cells involved in spatial navigation and mapping the environment also fire when we merely imagine moving through familiar surroundings, according to a new study by researchers at University College London. The research, published today in the journal Current Biology, shows that memory and imagination are intimately linked in the brain at the cellular level, and could help to explain some of the changes that occur in the early stages of Alzheimer’s Disease.

Grid cells, discovered in 2005, are one of at least four types of neurons that make up the global positioning system in the mammalian brain. Located deep inside the temporal lobes, in a structure called the entorhinal cortex, grid cells fire periodically as experimental rats traverse a space, acting something like an internal co-ordinate system that helps the animals track their own movements and update their location, and to navigate their environment and form spatial memories of it.

In 2010, Neil Burgess of the Institute of Cognitive Neuroscience at University College London and his colleagues published a brain scanning study showing that humans exhibit grid cell-like activity in the entorhinal cortex while navigating a virtual reality environment, suggesting that grid cells are also present in the human brain, and contribute to spatial memories. (Three years later, others provided the first direct evidence for grid cells in the human brain.)

Burgess and his colleagues now extend their earlier findings to show that healthy volunteers exhibit exactly the same grid cell-like activity during imaginary navigation, providing the first evidence that that this activity occurs in the absence of actual movements.

The new study was led by Aidan Horner, now at the University of York, and involved 26 participants, all of whom underwent functional magnetic resonance imaging while navigating a virtual mountainous environment and memorizing the location of a few objects within it, and also while just imagining that they were moving through it to retrieve those objects.

As expected, the researchers observed a signal that is consistent with grid cell-like activity in the entorhinal cortex during the navigational task, but they also observed the same or a very similar signal during periods when the participants merely imagined navigating through the environment to retrieve they had seen earlier. This suggests that grid cells not only track our movements during spatial navigation, but also contribute to path planning, perhaps by moving the viewpoint of the imagined environment.

Back to the future

Memory enables us to perform mental time travel, not only by recollecting events from the past, but also by imagining events that have not yet occurred, and to predict how they might unfold. Previous research has shown that amnesic patients cannot imagine new experiences, leading some to speculate, somewhat controversially, that memory evolved not to recall the past, but to predict the future, and this new study adds some weight to the idea.

“It is particularly exciting to see the involvement of a specific type of neuron whilst people are simply imagining moving through an environment,” said Horner, “[so] grid cells may contribute to… planning and imagining the future. Evidence that grid cells play a role in imagined navigation, he adds, “bridg[es] the gap between neurons and mental imagery.”

The new findings may also help researchers to gain a better understanding of the cellular changes that occur in Alzheimer’s Disease. The pathologies associated with Alzheimer’s typically begin in the entorhinal cortex, and patients in the early stages usually lose the ability to find their way around, and also have problems remembering and imagining things.

The topographical memory test used in the new study is based on the ‘Four Mountains Task,’ which was developed earlier by Burgess and his colleagues. Performance on this test is closely correlated with hippocampal volume in healthy volunteers, and also with the progression of Alzheimer’s. And last year, researchers in Germany showed that grid cell-like activity in the entorhinal cortex is reduced in people who carry genetic variants associated with an increased risk for Alzheimer’s. All of this suggests that grid cells may die off at the earliest stages of the disease, and that tests like the one used here could help clinicians to diagnose it sooner.


Horner, A. J., et al. (2016). Grid-like Processing of Imagined Navigation. Curr. Biol. 26: 1–6. DOI: 10.1016/j.cub.2016.01.042 [Full text]

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