Imagine being unable to remember the past. Like a fading dream, your current consciousness is lost to eternity. This is the experience of someone suffering from amnesia. Despite otherwise being healthy, they are unable to commit new experiences to memory. Studying the brains of amnesic patients has revealed that, while most regions of the brain play a role in memory, some areas are more crucial than others. There appears to be no single memory store, but instead a diverse taxonomy of memory systems, each with its own special circuitry evolved to package and retrieve that type of memory. Memories are not static entities; over time they shift and migrate between different territories of the brain.
At the top of the taxonomical tree, a split occurs between declarative and non-declarative memories. Declarative memories are those you can state as true or false, such as remembering whether you rode a bicycle to work. Non-declarative memories are those that cannot be described as true or false, such as knowing how to ride a bicycle. A central hub in the declarative memory system is a brain region called the hippocampus. This undulating, twisted structure gets its name from its resemblance to a sea horse. Destruction of the hippocampus, through injury, neurosurgery or the ravages of Alzheimer's disease, can result in an amnesia so severe that no events experienced after the damage can be remembered.
However, amnesic patients can show an astounding array of mnemonic abilities, such as learning new skills and habits. For example, repeatedly following a particular route to work can slowly be learned. Such ingrained habits appear to rely on a brain region called the striatum. Amnesic patients can also show an impressive short-term memory. For example, if they concentrate on one piece of information, such as a phone number, they can hold it in mind for many minutes. This ability relies on regions in the neocortex (the convoluted grey matter you see looking at a brain from the outside).
Despite being unable to form new long-term memories, many amnesic patients can still access long-term memories formed before the brain damage was inflicted. The further back in time the memory was created the more likely it is to survive, which results in the uncanny situation where patients cannot remember what they have just done, but are able to reminisce at length about their distant past. It is thought this occurs because the brain doesn't just create, store and retrieve memories; it restructures them.
A popular view is that during sleep your hippocampus "broadcasts" its recently captured memories to the neocortex, which updates your long-term store of past experience and knowledge. Eventually the neocortex is sufficient to support recall without relying on the hippocampus. However, there is evidence that if you need to vividly picture a scene in your mind, this appears to require the hippocampus, no matter how old the memory. We have recently discovered that the hippocampus is not only needed to reimagine the past, but also to imagine the future.
Studying patients has taught us where memories might be stored, but not what physically constitutes a memory. The answer lies in the multitude of tiny modifiable connections between neuronal cells, the information-processing units of the brain. These cells, with their wispy tree-like protrusions, hang like stars in miniature galaxies and pulse with electrical charge. Thus, your memories are patterns inscribed in the connections between the millions of neurons in your brain. Each memory has its unique pattern of activity, logged in the vast cellular network every time a memory is formed.
It is thought that during recall of past events the original activity pattern in the hippocampus is re-established via a process that is known as "pattern completion". During this process, the initial activity of the cells is incoherent, but via repeated reactivation the activity pattern is pieced together until the original pattern is complete. Memory retention is helped by the presence of two important molecules in our brain: dopamine and acetylcholine. Both help the neurons improve their ability to lay down memories in their connections. Sometimes, however, the system fails, leaving us unable to bring elements of the past to mind.
Of all the things we need to remember, one of the most essential is where we are. Becoming lost is debilitating and potentially terrifying. Within the hippocampus, and neighbouring brain structures, neurons exist that allow us to map space and find our way through it. "Place cells" provide an internal map of space; "head-direction cells" signal the direction we are facing, similar to an internal compass; and "grid cells" chart out space in a manner akin to latitude and longitude.
For licensed London taxi drivers, it appears that navigating the labyrinth of London's streets on a daily basis causes the density of grey matter in their posterior hippocampus to increase. Thus, the physical structure of your brain is malleable, depending on what you learn.
With impressive technical advances such as optogenetics, in which light beams excite or silence targeted groups of neurons, scientists are beginning to control memories at an unprecedented level. Whether we'll ever be able to implant or selectively erase memories – to actually manipulate memories – remains to be seen …
• Dr Hugo Spiers is a neuroscientist and lecturer at the institute of behavioural neuroscience in the cognitive, perceptual and brain sciences department at University College London. You can follow him on Twitter at @hugospiers and find out more about his research at ucl.ac.uk/spierslab
