There are several ways information can end up as long-term memory. At a conscious level, we can ensure that short-term memories end up as long-term memories by rehearsing the relevant information, such as a phone number of someone important. We repeat it to ourselves to ensure we can remember it. This is necessary because, rather than patterns of brief activity like short-term memories, long-term memories are based on new connections between neurons, supported by synapses, formation of which can be spurred on by doing something like repeating specific things you want to remember.
Neurons conduct signals, known as “action potentials,” along their length in order to transmit information from the body to the brain or vice versa, like electricity along a surprisingly squishy cable. Typically, many neurons in a chain make up a nerve and conduct signals from one point to another, so signals have to travel from one neuron to the next in order to get anywhere. The link between two neurons (or possibly more) is a synapse. It’s not a direct physical connection; it’s actually a very narrow gap between the end of one neuron and the beginning of another (although many neurons have multiple beginning and end points, just to keep things confusing). When an action potential arrives at a synapse, the first neuron in the chain squirts chemicals known as neurotransmitters into the synapse. These travel across the synapse and interact with the membrane of the other neuron via receptors. Once a neurotransmitter interacts with a receptor, it induces another action potential in this neuron, which travels along to the next synapse, and so on. There are many different types of neurotransmitter, as we’ll see later; they underpin practically all the activity of the brain, and each type of neurotransmitter has specific roles and functions. They also have specific receptors that recognize and interact with them, much like security doors that will open only if presented with the right key, password, fingerprint or retinal scan.
Synapses are believed to be where the real information is “held” in the brain; just as a certain sequence of ones and zeros on a hard drive represents a specific file, so a specific collection of synapses in a specific place represents a memory, which we experience when these synapses are activated. So these synapses are the physical form for specific memories. Just like certain patterns of ink on paper become, when you look at them, words that make sense in a language you recognize, similarly, when a specific synapse (or several synapses) becomes active, the brain interprets this as a memory.
This creation of new long-term memories by forming these synapses is called “encoding”; the process where the memory is actually stored in the brain.
Encoding is something the brain can do fairly quickly, but not immediately, hence short-term memory relies on less permanent but more rapid patterns of activity to store information. It doesn’t form new synapses; it just triggers a bunch of essentially multipurpose ones. Rehearsing something in short-term memory keeps it “active” long enough to give the long-term memory time to encode it.
But this “rehearsing something until I remember it” method isn’t the only way we remember things, and we clearly don’t do it for everything we can remember. We don’t need to. There’s strong evidence to suggest that nearly everything we experience is stored in the long-term memory in some form.
All of the information from our senses and the associated emotional and cognitive aspects is relayed to the hippocampus in the temporal lobe. The hippocampus is a highly active brain region that is constantly combining the never-ending streams of sensory information into “individual” memories. According to a great wealth of experimental evidence, the hippocampus is the place that the actual encoding happens. People with a damaged hippocampus can’t seem to encode new memories; those who have constantly to learn and remember new information have surprisingly large hippocampi (like taxi drivers having enlarged hippocampal regions that process spatial memory and navigation, as we’ll see later), suggesting greater dependence and activity. Some experiments have even “tagged” newly formed memories (a complex process involving injecting detectable versions of proteins used in neuronal formation) and found that they are concentrated at the hippocampus.5 This isn’t even including all the newer scanning experiments that can be used to investigate hippocampal activity in real time.
New memories are laid down by the hippocampus and slowly move out into the cortex as new memories form “behind” them, gradually nudging them along. This gradual reinforcing and shoring up of encoded memories is known as “consolidation.” So the short-term-memory approach of repeating something until it’s remembered isn’t essential for making new long-term memories, but it is often crucial for making sure that a specific arrangement of information is encoded.
Say it’s a phone number. This is just a sequence of numbers that are already in the long-term memory. Why would it need to encode them again? By repeating the phone number, it flags up that this particular sequence of numbers is important and requires a dedicated memory to be retained long term. The repetition is the short-term memory equivalent of taking a bit of information, sticking on a label marked “Urgent!” then sending it to the filing team.
So, if the long-term memory remembers everything, how do we still end up forgetting things? Good question.
The general consensus is that forgotten long-term memories are still technically there in the brain, barring some trauma in which they’re physically destroyed (at which point being unable to remember a friend’s birthday will not seem so important). But long-term memories have to go through three stages in order to be useful: they need to be made (encoded); they need to be effectively stored (in the hippocampus and then the cortex); and they need to be retrieved. If you can’t retrieve a memory, it’s as good as not being there at all. It’s like when you can’t find your gloves; you still have gloves, they still exist, but you have cold hands anyway.