How does our brain go about making memories? And where are such memories kept for later retrieval?
These might seem like simple questions but, in reality, they are anything but. To begin with, all memory isn’t that same; some detail the events of our lives, others inform us that the capital of France is Paris or that three times three equals nine. Others, still, are rarely brought into conscious awareness, such as the process of riding a bike or driving from our home to our place of work.
Indeed, not all memory is conscious memory, in fact, most of it isn’t. Conscious memory (or declarative memory) is the kind of memory that we consciously refer to, such as autobiographical events, a list of names or numbers or any other such knowledge that we acquire. I know the capital of New Zealand is Wellington and I know that I know (I actually lived in a suburb of Wellington when I was very young, so it’s also linked to my autobiographical memory).
A great deal of our day-to-day memory isn’t conscious; it’s what psychologists call non-declarative memory (see figure 1). Even though these memories aren’t conscious, they still influence our behaviour. It might take a great deal of conscious effort to learn to drive a car, but when you’ve been driving for a while, you rarely have to think about it. It’s not just driving a car or riding a bike, however, because memory and emotion are intricately entwined, and we often exhibit emotional responses without fully understanding why.
Some theories of memory present a single system (unitary) approach. Early models such as that proposed by Richard Atkinson and Richard Shiffrin, postulated that information flows through a system, from sensory to short-term and long-term memory stores, while other models (such the Working Memory Model of Alan Baddeley and Graham Hitch) take a more non-unitary approach with separate components dealing with different kinds of information.
What is clear is that not only are there two distinct processes involved in memory (long-term and short-term) these processes also rely on different areas of the brain.
Short-term memory, as its name implies, is anything that we need to hold in our minds temporarily and that can easily be lost should we become distracted. Temporarily means a few seconds; anything beyond that is long-term memory.
Damage to our brains, therefore, can lead to the inability to form new memories (because our short-term memory has become impaired) yet leave our pre-existing long-term memories intact.
And then there’s prospective memory; our ability to carry out specific tasks at a particular time: buying milk because we’ve run out, picking up the kids from school and all those other little tasks we need to carry out during the day. Some people are really bad at remembering to do these things, yet have no actual memory impairment to speak of, they’re just forgetful, or absent-minded. Indeed, damage to the prefrontal cortex can lead to problems with prospective memory but seemingly no problems with other aspects of memory, suggesting that memory is either distributed throughout the brain or at least contained within a number of specific regions.
How memories form in the brain
The first systematic attempt to discover the location of memory in the brain was made by behavioural psychologist Karl Lashley (1890-1958). Lashley was on a quest to find what he called an engram, that is, an individual memory trace. He taught rats to navigate a maze (a common pastime amongst psychologists back in the day). He would then cut out bits of the rat’s brain in an attempt to discover the parts that would prevent the rats from completing the maze. The more brain he cut out, the more lost the rats would become (hardly surprising, one would think).
It was left to his student, Donald Hebb, to continue the work, concluding (in 1949) that memory wasn’t stored in a specific location but, rather, stored across the cortex as networks of neurons (or cell assemblies).
Hebb also concluded that the more we repeat associations, the stronger the connections between neurons become. More specifically, the more frequently a synapse is activated, the more easily it will be triggered in the future. This led to the view that neurons that fire together, wire together. This is why repetition and self-testing make it easier for us to recall information in the future. I can still recite I must endeavour to remember to furnish myself with all necessary scholastic requisites, more than thirty years after my English teacher last gave me the line to write out one hundred times. Not very useful perhaps, but a pretty good example of Hebbian learning. Incidentally, I can also still recite the serial number I was given at 16 when I joined the army, even though I didn’t stick around for all that long.
This means that each time we revisit an event in our mind, we strengthen the connection. We can do this by actively remembering (for example, testing ourselves or practising a new to be learned skill) or by recalling via a cue in the environment.
Autobiographical events are often cued by external sources such as smells, sights and sounds. For example, a particular song might bring to mind a specific event from our past, with all the emotional components attached. In a way, we do not only recall the event in our minds, we also feel the event. Recall might make us laugh or cry or experience the pangs of melancholy. Cognitive neuroscientist Dr Catherine Loveday has conducted some fascinating research in this area. To complicate matters further, our autobiographical memories are never really accurate because the very act of recalling them and then reconsolidating them back into long-term memory changes them.
The Role of the Environment
We often hear stories in the press about certain activities reshaping the structure of the brain. These might be related to the use of computers, mobile phones or social media. Or it might be meditation, learning a new language or playing chess. The truth is that all experiences are re-moulding our brains because they result in synaptic changes.
Back in 1960, Mark Rosenzweig decided to examine the role the environment played in brain development using three groups of genetically identical rats. Each group was reared under different conditions:
Group 1 was raised under impoverished conditions
Group 2 was raised under simple conditions
Group 3 was raised under enriched conditions
Rosenzweig discovered that rats in the impoverished group had fewer synapses and lower levels of acetylcholine (a memory neurotransmitter) than the other two groups. This early study indicated that the environment has a major role to play in brain development.
Many of these studies have, of course, used non-human animals (from rats to sea slugs) but the main principles are still thought to apply.
Where are memories stored?
This is far from an easy question to answer. While there is no memory centre of the brain, certain regions play a major role in the creation and storage of memories. For example, Endel Tulving discovered that the brain appears to treat episodic and semantic memories differently (see here for a description of this study).
In a study conducted by Eleanor Maguire in 2000, the navigational abilities of London taxi drivers were strongly correlated to the amount of grey matter in an area of the brain known as the hippocampus. Maguire theorised that due to the need for taxi drivers to recall large amounts of geographical information, the physical structure of this part of the brain appeared larger than would have been expected in the average person. This, again, supports the view that learning something new has the power to physically change the brain, adding further weight to the theory of neuroplasticity.
Indeed, these findings are somewhat consistent with in-depth case studies of individuals who have suffered some kind of brain trauma. For example, one of the most widely studied amnesiacs (a man named Henry Molaison, or simply H.M.) had two-thirds of his hippocampi removed in an attempt to cure him of epilepsy, resulting in his inability to form and retain new memories.
Of course, memory loss needn’t be associated with severe brain trauma and occurs through the normal ageing process. In addition, few people can recall very much about their life before the age of about four, suggesting that the brain isn’t yet able to lay down episodic memories.
The hippocampus (or more accurately, the hippocampal formation, consisting of a number of brain regions) is certainly important for memory. This is why damage to the region can affect the ability to lay down new episodic memories and why it’s more developed in taxi drivers, even if the process by which it does this is still not fully understood.
Other areas of the brain important to memory include the pre-frontal cortex, the amygdala and the cerebellum (see figure 2)
The pre-frontal cortex is involved in executive control. It carries out a number functions, including the organisation of retrieval strategies, time stamping events, verifying accuracy and deciding which memories are permitted to enter consciousness.
The amygdala works closely with the hippocampus and provides our memories with the all-important emotional component. Memories are also emotional and damage to the amygdala can strip emotion from them. This is one explanation for a condition known as Capgras syndrome, were sufferers become convinced that a loved one has been replaced by an imposter because that emotional kick we feel when seeing someone we are close to is absent.
The cerebellum is the seat of what is called muscle memory and motor learning. This would include tasks such as riding a bike that, when learned, require no conscious effort.
It is clear that certain brain regions are involved (rather than a single unitary store) and that the strength of the connection between neurons increases the likelihood that we will recall memories more readily in the future. This is certainly supported by cognitive psychology, in that information that is recalled more often becomes easier to recall in the future (the so-called testing effect). However, there is much we don’t understand about these processes, so asking questions such as where are memories stored? or how does the brain make memories? isn’t as straightforward as one would hope.