By Eduardo Zimmer
We are what we decided to forget
The article below answers the question asked by Violeta Reys, 7 years old, for the series “Children’s questions, answers from science”.
Iván Izquierdo, an Argentinean neuroscientist naturalized Brazilian, used to say that “we are what we decide to forget”. For him, forgetting was the most fascinating biological phenomenon in memory. In fact, we need to forget to remember. In a millisecond I can remember my mother’s birth date, but I don’t always need to trigger this memory, it is stored in specific places in the brain. But where?
To understand the neurobiological bases of this phenomenon, two concepts are essential. The first is that the brain is segmented into regions that perform specific functions but are connected in a way that allows us to perform higher cognitive functions such as reading, speaking, and reasoning. The second is related to the types of memory: there is the short-term, which we keep for a few hours (the phone number of a delivery store) and the long-term, which is retained for a long time and can be retrieved (which happened last Christmas), and which can be declarative (“know that”) and non-declarative (“know how”). There are also other classifications, different from a biological point of view, such as semantic and episodic memory. The beginning of the expansion of knowledge about memory deserves to be… remembered.
In 1953, patient Henry Molaison, known as HM, underwent a lobotomy to control epileptic seizures. The epilepsy was contained, but HM could not form new declarative memories, although he could form short-term and non-declarative ones. Thus: HM had a conversation normally, but as soon as the conversation ended and he started another activity, he completely forgot that that conversation had taken place – he would even forget about the person, if he was a “new” person. It was as if he had been subjected to the “neuralizer” of the “Men in Black” trilogy, fictitious equipment used to erase people’s memories.
The study of this case was a watershed. And it was in charge of Brenda Milner, considered by many to be the founder of neuropsychology, who introduced him to the scientific community in 1957. (Today, at 102, Dr. Milner is still active and can be seen in the corridors of the Instituto de Neurology from Montreal, Canada I myself had the honor of conducting part of my doctoral studies at this institution, thanks to the now defunct program “Science without Borders.” An unforgettable memory).
The key to understanding the HM case pointed to the areas that were removed in the lobotomy, especially the hippocampus, the main brain region responsible for short-term and declarative memories. Nowadays neuropsychology suggests that each type of memory is stored in a special place in the brain. That is, areas other than the hippocampus also have the ability to store memories, such as the cortex.
But the understanding of an even more fundamental neurobiological process is needed. It would be intuitive to think that a new neuron would appear with each new memory, or that a neuron could accommodate a limited number of memories. Now, as few neurons are born in adult brains, with the massive amount of information we receive, our “neuronal HD” would already be full.
Both hypotheses are incorrect. The plasticity of the brain is at issue. Neurons form new connections or even strengthen previous connections with other neurons. This connectivity makes the electrical triggers –the synapses– coordinated by a series of neurons to form, retain, “forget” and allow the recall of memories.
Some readers may remember this article for a long time, others may not remember ten seconds after reading it. But then the conversation enters another region of the brain, the amygdala, which coordinates one of the most beautiful neurobiological phenomena in our lives: emotion. It helps to decide which memories to keep. Want to take a test? Who doesn’t remember the first kiss? I know, there are so many emotions…
Eduardo Zimmer is a biochemist and professor at the Department of Pharmacology at the Federal University of Rio Grande do Sul.
We know that children ask the best questions, and that science can have good answers for them. Every month, the series “Children’s Questions, Answers from Science” invites a scientist to answer one of these fundamental questions. Do you have a suggested question? Here’s how to collaborate.