The impossibility of creating a map on a scale of 1:1 has been a favourite subject for scientists and artists over the years . However we look at the problem, every map must reduce the infinity of reality to a finite representation of the world. The map of the tube in London is not accurate but it is sufficient for its purpose. The question of scaling reality into manageable chunks of data lies at the heart of two neuronal functions that overlap in curious and unexpected ways—navigation and memory. The unusual story of the relationship between these two systems was the topic of a swathe of papers in Nature Neuroscience with a special emphasis on an interesting family of neurons known as ‘grid cells’ .
Grid cells were discovered in 2005 in the entorhinal cortex of the brain by Edvard and May-Britt Moser . Traditionally, neuroscientists have neglected the entorhinal cortex because of the difficulty of capturing electrophysiological recordings from this area. Yet, its location directly upstream from the hippocampus—the key structure responsible for memory formation—suggested to the Mosers that it was worth the effort to learn how to collect data from this area. What they eventually found once they mastered some tricky recording techniques was that when an animal was freely roaming, the firing of a particular kind of neuron, termed a ‘grid cell’, mapped out a hexagonal grid on the floor of the environment that the animal was exploring. To state it another way, if a dot was placed on the floor every time a grid cell fired, the accumulated dots would trace out a grid of hexagons. An adjacent grid cell would trace out a different hexagonal grid. Since the first recordings it has been possible to record from multiple grid cells at once, with results showing how the overlapping grids form the basis of a wonderfully intricate global positioning system .
Originally, the Mosers doubted their own findings. It seemed unlikely that the brain would operate in such a purely mathematical way. But this hexagonal mapping has been replicated by many groups and it is now beyond doubt that grid cells allow for navigation by creating maps that are held online by the combined entorhinal-hippocampal complex.
Another remarkable property of grid cells is that they map the world at different scales. Grids cells located closer to the bottom of the entorhinal cortex (ventral grid cells) create lattices with relatively wide spacing between vertices, whereas grid cells higher up (dorsal grid cells) map out grids that are more packed together . For their work, the Mosers received the Nobel Prize for Physiology or Medicine in 2014 .
Yet, the intricacy of the system seemed too elaborate to be reserved solely for the purposes of navigation. Insects manage quite well with simpler technologies . What the papers in Nature Neuroscience examine is the way in which the evolution of a system that supports the storage of millions of relationships among coordinates can also be used as a framework for recreating the past and imagining the future. Tellingly, the first structure to beaffected by Alzheimer’s disease is the entorhinal cortex . Along with losing our way physically in the world, damage to the entorhinal cortex precipitates a loss of autobiographical memory. But in the absence of pathology, the entorhinal cortex and the hippocampus can sustain memories as well as maps. What can be used to define coordinates inside a mental map can also be used to store relationships among the myriad experiences gathered over a lifetime.
Advances in experimental set-ups now allow researchers to record entire assemblies of neurons during a single experiment. This makes it possible for unique constellations of firing patterns to be directly linked to kinds of different maps. It is even possible to see the moment when one constellation is replaced—in a single burst—by an entirely new constellation. This corresponds to the moment when an animal mentally jumps from one map to another which can be instigated by ‘teleporting’ an animal between two worlds that it is familiar with. Teleporting in this case being simply a matter of alternating between different light patterns that represent two different worlds for the animal. Neuronal recordings clearly indicate the moment when the switch occurs, as the animal selects the correct map from its available store of representations of the different worlds it has explored .
Even more remarkable is the ability to predict in advance an animal’s choice when confronted with two options in a maze. To a reasonable degree researchers have access to the animal’s decision making process as they observe the firing assembly that represents ananimal’s ‘look-see’ through the imagined wing it is about to explore . Researchers can even predict an animal’s correct or incorrect choices before the final decision is made to turn one way or another. One hypothesis is that these neural ‘trajectories’ that first evolved for the purpose of navigation, also support memory recall as well as the planning of goals .
This switching through different reels occurs as we replay the past. Similar to map- making, our replay is always based on compression. We are unable to experience the past on a scale of 1:1. Just as a map is an approximation of reality, so too our autobiographical memory is not a dead-reckoning from one experience to another. There is no straight line through the story of our past.
The current state of neuroscience suggests that our physical wandering through the world may be the origin of our ability to imaginatively flicker through the past and the future, via a navigational system that has been co-opted by memory processes. Studying the brain’s grid cells seems to offer a unique opportunity to decipher not only navigation but also the somewhat more esoteric mechanisms of mental time-travel, getting to the heart of what Paul Valéry noted was one of the deepest and most fundamental functions of the brain; to look ahead, to produce the future.
References and Links
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Laurence O’Dwyer holds a PhD in paradigms of memory formation from Trinity College Dublin. His first book of poetry, Tractography (Templar Poetry, 2018), received the Straid Collection Award. It includes his Poems from Haiti and Poems from Lapland. In 2018, he was a visiting scholar at the Scott Polar Research Institute at the University of Cambridge and a fellow of The Rensing Center (South Carolina). In 2017, he received a fellowship from The MacDowell Colony. In 2016, he won the Patrick Kavanagh Award for Poetry. He has also received a Hennessy New Irish Writing Award. His most recent publications from the Arctic, are Poems from Litløy Fyr and The Lighthouse Journal from Litløy Fyr. “The Old Light”, the last of the Poems from Litløy Fyr won the Yeovil Poetry Prize for 2018.