Memory is not one thing but many: there is our store of facts, our skills or ‘how to’ abilities, personal recollections of events, unconscious knowledge, and our continuous monitoring of the immediate past. Different parts of the brain handle each of these memory functions.
The human memory system evolved in a rough-and-ready fashion. New skills built onto older ones, taking over and then prompting growth in whichever brain area was most convenient. As a result, various types of memory, although constantly interacting – are located in different regions of the brain.
Apart from instincts, which are genetically encoded memories that arise mostly from a part of the brain called the caudate nucleus, all conscious memory involves the hippocampus. This arching seahorse-shaped organ is part of the limbic system – the primitive, unconscious area of the brain lying beneath the brain’s outer layer, the cortex. When we are paying attention to things that are happening around us, the information we take in through our senses is sent to the hippocampus along nerve pathways from the cortex. Most of it causes a fleeting ripple of activity, and then disappears, but memorable events create changes that cause their neural patterns to be retained in the network of hippocampal connections. This process takes about two hours, and in a day, millions of these traces are left. Later, we may replay some of them by going over the most important events of the day, and this rehearsing will etch them deeper. At night, when we sleep, this process continues. During rapid eye movement (REM ) sleep, some of the neural records in the hippocampus are replayed, producing those curious, fractured snatches of past activity that we experience in dreams. This replaying etches them deeper.
Storing events and facts
The play of hippocampal memories is transmitted to the neurons that lie in the cortex, especially in the temporal lobes on each side of the brain. Events that are particularly exciting or significant tend to be replayed regularly and, after up to three years of replay, become lodged permanently in both the hippocampus and the cortex. These are our long-term ‘ episodic’ memories – events that we can recall almost as though we are living them all over again. If events are replayed less frequently, usually because they are mundane and unremarkable, the patterns that represent them in the hippocampus eventually fade away and are lost or become inaccessible.
However, even when the memory of an event has disappeared, some of its factual elements may remain, encoded in the cortex as semantic memories. For example, long after the hippocampal memory of, say, your first meeting with Mr. Jaya Deva has disappeared, you may be left with a long-term knowledge of who Mr. Jayadevais is but no recollection of when you were first introduced.
The hippocampus plays a crucial role in consolidating long-term memories during the two- to three-year period it takes to store them permanently in the cortex. Once firmly lodged there, however, they are no longer dependent on the hippocampus for retrieval. One exception is long-term spatial memory – the cognitive maps of places and spaces that help us to find our way around. These seem to be held permanently in the hippocampus, and people who suffer damage in this part of the brain are often permanently disoriented.
Working memory and unconscious skills
As well as holding long-term knowledge, the brain has a short-term’ notice board, ‘or working memory, to keep track of information that needs to be held for just long enough to be acted upon. Regions of the left hemisphere have a larger role in verbal working memory, while the right hemisphere is more involved in spatial working memory tasks. However, the critical area for processing short-term information is the prefrontal cortex, an area of the brain just behind the forehead that expanded greatly during human evolution.
Procedural or ‘ how to’ memories are stored in the putamen. This structure is located above the hippocampus and has close links with the motor cortex, which controls movement. When you access a procedural memory – for example, when you set out to cycle to the shops – the put a man is included in a circuit of activity that runs through the premotor cortex and cerebellum (a structure at the back of the brain concerned, with balance, coordination, tinting and incoming sensory information). This circuit integrates all the functions you need to begin pedaling.
Body states such as fear and anxiety are laid down with the emotionally charged events that they accompany and are thought to be stored in the amygdala – the part of the limbic system that registers moods and feelings. Emotional memories cannot be accessed consciously but may suddenly pop up in the form of panic attacks or flashbacks.
Locating memory in the brain
The brain’s memory stores were discovered gradually, largely by observing people who developed memory problems after a head injury or stroke and then comparing the size of the damage with the type of memory loss. For example, the ability to lay down new memories and access recent ones is lost after damage to the hippocampus. At the same time, problems with concentration and planning often result from damage to the prefrontal cortex.
Revealing Memory In Action
Today, new methods of brain imaging allow investigators to see the precise brain areas that are activated during the recall of different types of information. Although the general areas that process the main types of memory are now well mapped, no two individuals encode information in precisely the same way. Musicians, for example, might store the memory of a piece of music in a different area from someone who is not a musician because they ‘read’ the information as a skilled memory rather than just a sound. The exact memory ‘geography of every individual brain is unique.
