We Cannot Learn Without Forgetting

Forgetting has traditionally been thought of as the polar opposite of learning. However, every day there is more evidence that the two are part of the same process, that they go together. To a large extent, our experience is the product of what we have let go. On the other hand, without forgetting and releasing resources that this phenomenon implies, it would be very difficult for us to distinguish relevant information from that which is not, to solve problems or simply to remember.

To live in an adapted way we are constantly learning from the environment and through experience: which way is better, how the orange blossom smells or whether to add salt or sugar to the food. In these situations, the chosen option prevails because we discard and forget the others.

As a rule, the absence of processing of a certain content does not mean that there has been an error when storing, but that the brain is giving way to other more relevant information.

Oblivion

Like learning, which consists of a neurobiological process by which connections between neural networks are established, forgetting is not a passive mechanism. It is not about the simple fading due to lack of use of the connections that were made at some point. Rather, it works as a filter: the mind discards that which is not salient or that we do not use frequently. Thus, for example, we are better at remembering information than sources of information.

In this way, by discarding unhelpful knowledge, we lighten our cognitive load. Because we forget we can process new information quickly and efficiently. However, there are times when this system fails in its process of discarding memories and we cannot access a memory that we need or we cannot get a song out of our head.

Forgetting, then, also consists of a neurobiological process complementary to that of learning, hence we cannot learn without forgetting. This chemical process, called long-term depression (DLP), also takes place in the same regions and involves the same proteins and receptors.

Long-term depression

This cellular mechanism is in charge of making us forget. As indicated above, it occurs in neurons in the hippocampus and other regions, such as the amygdala, which are involved in storing new information. In that case, the mechanism would be long-term potentiation (PLP), which is responsible for transferring data from short-term memory to long-term memory.

Thus, PLP produces physical changes in neurons. When we memorize something, a first neuron is activated and releases neurotransmitters (glutamate) in the gap that separates it from another. This glutamate activates AMPA receptors and, as a result, sodium enters the cell causing the electrical charge to change completely and what is called an action potential is generated.

Then, the magnesium leaves nearby receptors (NMDA) while another action potential arrives and the whole process is repeated. In this way, a large amount of calcium enters and more receptors are inserted. Therefore, over time, the intensity of these synapses becomes stronger and learning occurs.

In long-term depression, the hippocampal fibers receive very little stimulation, but for a long period or they receive delayed stimuli. This causes few neurotransmitters to flow and less calcium reaches the second neuron, therefore, the number of receptors and the size of the cell are reduced. Thus, it is less and less likely that these two neurons receive stimulation at the same time.

In summary, to remember, high amounts of calcium are needed to cause the connection between neurons, while forgetting requires the reduction of both and, with it, the weakening of the synapse.

Erased or difficult to access?

If there is one thing that is certain about the brain today, it is that it is plastic from the moment we are born. For example, when a child begins to speak, his brain uses many resources (and connections) for it, but it will optimize and reduce the number of synapses needed. We even stop perceiving phonemes that could have been useful being minors.

In this sense, it is known that, to store information, it is written on numerous occasions over another. Just as neurons change functionally and structurally. If not, we would accumulate a large amount of useless information that would make it difficult to access another.

However, scientists are still not really clear if forgetting consists of a permanent erasure of information or if it is only difficult to access those memories.

As mentioned before, DLP consists of a weakening of communication between neurons. Therefore, if we extinguish a learning, the brain could replace that association with a new one. However, it could also be that there was some trace of that connection, structurally or functionally. In such a way, we would never forget.

The future

Not being able to learn without forgetting raises questions that, in addition to being interesting, may be relevant for certain pathologies. For example, in the case of post-traumatic stress disorder, it happens that the memory is so strong that it does not change or forget over time, as is often the case. In addition, the connections of that memory are so ingrained that any small stimulus can activate the memory very quickly.

In this line, the researchers, already knowing the forgetting mechanism, are studying the possibility of designing interventions to facilitate the forgetting of certain events. Like the movie Forget About Me  , researchers have tested the efficacy of applying a series of memory-related proteins to the brain in animals.

However, this is not feasible in humans, so they study when a psychological intervention would be most effective. Likewise, the usefulness of building new perceptions about memories each time they are recovered. That is to say, learning without forgetting, only making use of the natural mechanisms of one’s own memory.