“In Lewis Carroll’s The Hunting of the Snark (1876), the hapless snark-hunting crew found themselves presented with a navigational challenge by their captain, the Bellman:
He had bought a large map representing the sea,
Without the least vestige of land:
And the crew were much pleased when they found it to be
A map they could all understand.”
“Refreshingly uncluttered, the Bellman’s map had nothing on it:
‘Other maps are such shapes, with their islands and capes!
But we’ve got our brave Captain to thank’
(So the crew would protest) ‘that he’s bought us the best –
A perfect and absolute blank!’”
“Even a perfectly blank map, however, could be useful if it had a grid reference, something to indicate compass direction, a scale, and a marker for current position. Sadly, the Bellman’s map had none of these things:
‘What’s the good of Mercator’s North Poles and Equators,
Tropics, Zones, and Meridian Lines?’
So the Bellman would cry: and the crew would reply
‘They are merely conventional signs! …’”
“So, the map was truly blank and not much use for navigating.”
“We humans have been mapping space for many centuries, and our maps have greatly improved over this time so that now they are very useful for navigating.”
“Maps are for humans, but how do animals, which began navigating millions of years before parchment was invented, manage to find their way around?”
“These questions have intrigued biologists for many decades, particularly because animals can perform astonishing feats such as navigating their way from the North Pole to the South and back again, like the Arctic tern; or returning home after being transported hundreds of miles away, like the homing pigeon.”
“How animals (both human and non-human) work out their location is just beginning to be understood by brain scientists.”
“There are maps in the brain, as it happens. The properties of these maps, which neuroscientists call ‘cognitive maps’, have turned out to be highly intriguing, and are helping us to understand not just how animals navigate, but also more general principles about how the brain forms, stores and retrieves knowledge.”
“Might the hippocampus be a memory organ buried deep in the brain?”
“Instead of memory, O’Keefe seemed to have stumbled on a map in the brain, and he said as much in The Hippocampus as a Cognitive Map (1978), co-written with the neuropsychologist Lynn Nadel.”
“Their basic idea was that neurons in the hippocampus form a memory of the animal’s environment, so that when the animal goes to a particular place, neurons representing that place become active, as if reminded about that place.”
“However, this idea of a map in the hippocampus was not well-received, and generated a great deal of controversy. In fact, the idea that the brain could make a map pre-dated O’Keefe by some decades, and it had not been well-received back then either.”
“The American psychologist Edward Tolman had already suggested this in 1948, on the basis of his studies of how rats find their way around mazes.”
“we need to turn to the critical aspect of a map that makes it useful for navigation, which is that it needs to have metric information concerning distance and direction – the keystones that the Bellman’s map in The Hunting of the Snark lacked.”
“No amount of chaining together stimulus-response associations could inject these spatial components into a representation: the brain needs to have a way of calculating distances and directions based on its incoming sensory data.”
“This type of inference is exactly the type of ‘magical’ internal process that Behaviourists dislike, and it wasn’t until two further crucial discoveries came along that they had to admit defeat and concede that the brain might in fact be capable of metric inference.”
“The first discovery was made by the American physiologist Jim Ranck, who in 1984 was recording neurons from a part of the brain near the hippocampus and found that some of them fired when the animal faced in a particular direction.”
“It was as if the rat had an internal compass, except one with a different ‘North’ set for different rooms. Importantly, these ‘head direction cells’ worked the same way whether in the light or in the dark, with eyes open or closed, and anywhere in the room, so it didn’t matter exactly what the rat could see, smell or touch, it mattered only which way it faced.”
“Also, importantly, every cell always had the same firing direction relationship to every other cell: if cell A’s preferred firing direction was to the left of cell B’s in one room, this was so in every room.”
“Thus, direction seems to be an intrinsic property of the system – it seems to be only the orientation of the whole representation relative to the outside world that depends on sensations and perceptions.”
“So, there is metric information about direction in the brain.”
“What about distance? This brings us to the second major challenge to Behaviourism: the discovery of grid cells by Torkel Hafting and Marianne Fyhn, then part of a team at the Norwegian University of Science and Technology headed by Edvard and May-Britt Moser.”
“The Mosers had been fascinated by the hippocampus since the very early days of their doctoral studies. Not long after qualifying, they visited O’Keefe’s lab in London to learn how to record these mysterious place cells that were so exercising neuroscientists.”
“They then returned to Norway to establish a new lab of their own, where they imported O’Keefe’s technique and started studying inputs to the hippocampus. Their reasoning was that, to understand how place cells ‘know’ where the rat is, it is necessary to see what the upstream neurons are saying to them.”
“What the team found was rather unexpected. In the entorhinal cortex, which is the part of the brain that sends more information to the hippocampus than almost anywhere else, they found a new type of spatial cells. These cells, like place cells, fired only when the rat went into specific places in the environment, which in itself was not so surprising. What was surprising, though, was that they didn’t just fire in one place, they fired in many places.”
“More surprisingly still, these places together formed a remarkable, regular pattern in which each firing location was the same distance from all the neighbouring ones. This meant that the entire array of fields formed a regular hexagonal pattern, like oranges when packed in a box”
“The distance between the fields could be small (30 cm or so) or large, depending on the cell, and was so regular that its discoverers named the cells ‘grid cells’.”
“The importance of grid cells lies in the apparently minor detail that the patches of firing (called ‘firing fields’) produced by the cells are evenly spaced.”
“This even spacing of firing fields is something that couldn’t possibly have arisen from building up a web of stimulus associations over the life of the animal, because 30 cm (or whatever) isn’t an intrinsic property of most environments, and therefore can’t come through the senses – it must come from inside the rat, through some distance-measuring capability such as counting footsteps, or measuring the speed with which the world flows past the senses.”
“In other words, metric information is inherent in the brain, wired into the grid cells as it were, regardless of its prior experience.”
“Studies of other animals, including humans, have revealed place, head direction and grid cells in these species too, so this seems to be a general (and thus important) phenomenon and not just a strange quirk of the lab rat.”
“Grid cells were discovered in 2005, and more than a decade later we still don’t know exactly what they are for, but they are believed to be the brain’s equivalent of the grid reference on a map.”
“Whatever their function, their existence does prove, however, that these structures in the brain – hippocampus, entorhinal cortex and a host of their neighbours – collaborate in forming a metric representation of space.”
“But where does this leave memory? This is where research in the hippocampus began: do place cells have anything to do with memory?”
“Yes, we think they do, and research now aims to uncover precisely what. One of the most important implications for humans, arising from study of the hippocampus, is its involvement in Alzheimer’s disease, which begins in the entorhinal cortex (where the grid cells are) and spreads throughout the hippocampus and thence to the rest of the brain.”
“Scientists now know that the hippocampus is both a map and a memory system.”
“For some reason, nature long ago decided that a map was a handy way to organise life’s experiences. This makes a lot of sense, since knowing where things happened is a critical part of knowing how to act in the world. The quest now is to understand how memories get attached to this map.”
“Armed with this knowledge about memory, we might one day be able to study memories directly, and even, perhaps, manipulate them – to soften traumatic memories, for example, or repair damaged ones such as those affected in Alzheimer’s disease.”