How Your Memory Works

Sherlock Holmes, the renowned detective, proposed a theory likening the brain to an attic, with limited storage for memories. When Dr. Watson mentioned that the Earth orbits the sun, Holmes amusingly responded, "Now that I do know it, I shall do my best to forget it." Holmes believed that cluttering the mind with trivial facts hinders the ability to focus on important matters, such as discerning subtle differences between lethal poisons. But was Holmes correct? Does our memory have limits, similar to a computer's storage capacity? Or is our memory boundless? Furthermore, what would life be like if we never forgot anything?

In the animated film "Inside Out," memories were depicted as glowing spheres arranged like books in a library inside the brain. However, the reality is more complex. Our memories are not confined to a single location in the brain; they are scattered throughout. Numerous brain cells in various regions work together to form a single memory. For example, recalling the memory of eating grandma's apple pie involves different brain cells responsible for visualizing the appearance of the pie, remembering the aroma of cinnamon, and even sensing the delightful taste.

In actuality, a memory is not a tangible entity residing within a specific brain cell. It is an action, not an object. Just as a wave in a stadium requires coordinated movement from all fans, a memory is formed when interconnected neurons fire in a specific pattern. Since the same cells can generate multiple unique patterns, a single group of neurons can encode multiple memories, expanding the brain's storage capacity. Deep within the brain lies the hippocampus, a group of seahorse-shaped cells that play a vital role in memory formation.

Our understanding of memories owes much to an infamous patient known as H.M. In 1953, H.M. underwent epilepsy surgery that removed most of his hippocampus. As a result, he suffered from severe amnesia, unable to form new memories of facts or events. However, he retained the ability to remember information learned prior to the surgery. This demonstrated the hippocampus's importance in memory creation, although it is not the storage site for memories.

So, how do experiences transform into memories? By observing the brain activity of a mouse navigating a maze, we can identify the active brain cells and create a map. Later, we observe these same cells repeatedly firing in a fast-forwarded manner, reinforcing the connections between them. This process, known as consolidation, enables animals (including humans) to transfer new memories into long-term storage. Days or weeks later, a specific smell might trigger the same pattern of cell firing in the mouse's brain, recalling memories associated with the maze. Similarly, the scent of cinnamon can evoke memories of our grandmothers.

Nevertheless, the brain's memory formation is not flawless. Sometimes, vivid mental imagery can feel as real as actual experiences. If we vividly imagine a crime scene based on someone's description, our brain activates similar networks as if we were truly there. The more we replay such scenes in our minds, the more they resemble genuine memories. This phenomenon explains how leading questions from detectives can inadvertently implant false memories in witnesses.

While our memory capacity is vast, we also forget a great deal. Forgetfulness occurs naturally, but our brains also intentionally forget. We have three distinct ways of forgetting. The first is passive obliviscence, where memories gradually fade over time as connections between brain cells weaken. Alternatively, the memory may still exist, but the triggers required for retrieval, such as sights or smells, are lost. The second type, targeted forgetting, takes place during sleep. Useless information learned during the day is cleared out, and outdated memories are erased. Lastly, motivated forgetting occurs when individuals consciously suppress unpleasant memories. This intentional forgetting helps regulate our emotions, focus on the present.