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Memories: The Enigma of the Human Brain

What is memory? Is it something tangible, like blood and bone? Is it something which you can hold physically, maybe a ball of some complex chemical compound? Or is it an abstract concept, like love and anger? This has been one of the most pressing scientific questions for centuries.

Memory is managed by 2 components- the brain and the neurons. The brain controls everything while the neurons connect everything. The neurons cannot regenerate- once a neuron dies, then you are short of one neuron for the rest of your life. The hippocampus region in the brain is responsible for the production of neurons. As memories run through the hippocampus, the connections between neurons associated with a memory eventually become a fixed combination. For example, if you hear a piece of music, you will probably be flooded with other memories associated with an episode where you heard the same music. The hippocampus helps to solidify the pattern of connections that form a memory, but the memory depends on the solidity of the connections between individual brain cells.

Production of neurons occurs continuously in the hippocampus till the age of 3 and stops by the time one turns 7. Do you remember your first birthday? Probably not. That is because infantile amnesia prevents most humans from recollecting memories up to 3 years of age.

In 1953, Henry Molaison had his hippocampus removed and suffered from heavy anterograde amnesia and temporally graded retrograde amnesia. This patient did not remember what happened in his life. Retrograde amnesia means losing all existing memories, and anterograde amnesia refers to the inability to retain new information. Henry could remember up to a few minutes of anything which happened but could not retrieve it after an hour. But he could still recall his motor memories (how to write, walk, etc.) which rely on the basal ganglia and cerebellum. The hippocampus, along with the neocortex and amygdala, stores episodic memories of events in your life, like a birthday memory, a traumatic accident, your school days, etc. When the temporal lobes were removed, Henry only lost the episodic memories and retained everything else.

You probably remember a happy moment, or a time when you were angry at your best friend, with better clarity. Ever wondered why you don’t recall other memories with similar strength? That is where the amygdala comes in. The amygdala is a part of your hippocampus which gives emotions. When you think of a good memory, that emotion of happiness is due to your amygdala, strengthening the neurons associated with a single memory.

How exactly are these memories stored? The most acceptable theory with substantial supporting evidence is that a certain set of neurons are associated with a particular memory. In an experiment, scientists genetically engineered the neurons in mice with a fluorescent marker. The mice were trained to react to a shock on their left foot while a sound rang nearby. They connected the sound with the shock and expected a shock the next time the sound came. When their brain activity was scanned, the same set of neurons lit up whenever they heard the sound.

Another crucial part of memory is its retrieval- the process of accessing stored memories. Long-term memory retrieval happens in four ways:

  1. Recall: One remembers things without any cue.

  2. Recollection: This type of retrieval reconstructs different bits and pieces, similar to writing an essay.

  3. Recognition: This includes identifying information one has already encountered, like choosing the correct answer in an MCQ.

  4. Relearning: This involves retrieving what one has already learnt, which helps in strengthening a memory.

But, like everything has its pros and cons, even memory retrieval is not perfect. Remember how many times you couldn’t exactly say something which was on the tip of your tongue? It is a common phenomenon, known as the tip of the tongue phenomenon. Retrieval failure is another reason why we forget things. The memories are stored, but we cannot access them because we don’t have the appropriate cues to trigger the memory.

Our brain is a complex piece of engineering, and we have not even started unveiling its secrets and abilities. The story of how our memory works and how it is processed, stored, and retrieved is a very intricate and mysterious process. But it is the key to what makes us human and what makes us tick.

- written by Harshavardini

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