Light micrograph of a sagittal (side view) section through the hippocampus of a rat brain showing the nerve cells within. Photo by Thomas Deerinck/NCMIR/SPL
How animals navigate their way around provides clues to how the brain forms, stores and retrieves memories
In Lewis Carroll’s TheHunting 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. In antiquity, maps such as the beautiful Mappa Mundi did have features of the landscape – islands and capes – that were arranged in roughly the right relationship to each other; but the scale was wrong, the shapes of the coastlines were wrong, and there was no convention to place North at the top of the page as we do nowadays. They were exquisite as illustrations, but little use for navigating because they lacked metric (measurement) information such as distance and direction. They also had nothing to measure distances and directions from, which in modern maps are called reference points, lines and planes. As maps evolved over the centuries, however, map-makers added metric information in the form of direction (North and South) and distances (latitude and longitude lines), as well as reference points (North and South poles), reference lines (the Greenwich Meridian and the Equator) and, eventually, when we started to move above and below ground, a reference plane (sea level). A map with all this information is useful even without islands and capes.
Maps are for humans, but how do animals, which began navigating millions of years before parchment was invented, manage to find their way around? Do animal (and human) brains contain a map, and if so does it have islands and capes, North Poles and Equators, reference lines and so on? And if they do, where is it, and how does it work? How could a jelly-like blob of protoplasm contain anything as structured as a map?
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.
The neuroscientist John O’Keefe discovered cognitive maps in the 1970s. His research ultimately won him a share of the 2014 Nobel Prize in Physiology or Medicine. O’Keefe was not looking for maps in the brain. Newly arrived at University College London (UCL) from McGill University in Canada, he was actually interested in memory. Specifically, he was trying to understand a discovery that had been made a few years earlier by the neurosurgeon William Beecher Scoville and his colleague, the neuropsychologist Brenda Milner, which seemed to provide clues about how memories are formed and stored. Scoville and Milner were fascinated by one of Scoville’s neurosurgical patients, a young man known to the world by his initials ‘HM’. He had undergone surgery to cure his epilepsy and, tragically, developed amnesia as a result. Scoville and Milner thought his amnesia might be due to damage to his hippocampus. The idea that the hippocampus might be linked with memory was very exciting to brain scientists, as they had started to think that memory might not have a special location in the brain…