Neuroscientists have discovered that memories migrate between different regions of the brain, but what do they actually consist of? 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.
More information:
http://www.guardian.co.uk/lifeandstyle/2012/jan/14/what-are-memories-made-of
More information:
http://www.guardian.co.uk/lifeandstyle/2012/jan/14/what-are-memories-made-of