How Memories Form, Change, and Persist in the Brain

Memories are a fundamental aspect of human existence, allowing individuals to revisit past moments and shaping their identity and future decisions. However, memories are not static; they are dynamic and can change each time they are recalled. Understanding how memories are formed, stored, and retrieved reveals the intricate workings of the human brain and the fluid nature of our personal histories.

The Brain's Complex Architecture

The human brain, an incredibly complex system, consists of approximately 86 billion neurons. These neurons communicate through synapses, tiny gaps where electrical signals are converted into chemical signals, transmitted, and then converted back into electricity. A typical neuron can connect with up to 10,000 other neurons, forming a vast network of hundreds of trillions of connections—far exceeding the number of galaxies in the observable universe.

Within this intricate network, purpose emerges as certain connections strengthen. When two neurons fire simultaneously, their synaptic connection becomes more robust, making them more likely to fire together in the future.

Zooming out, patterns of activity become apparent. Dozens or thousands of neurons organize into local columns, which serve as basic information processing units. These columns process specific sensory inputs, such as:

• Dark and light
• Spatial location
• Texture sensations
• Sounds of words

Different columns are dedicated to various senses (sound, images, touch) and are located in distinct brain regions. These columns form the "gears" of the cerebral cortex, the fundamental hardware from which consciousness emerges.

Every sensory experience—what you see, hear, or feel—activates these gears, causing neurons to fire together. For a coherent experience, these diverse gears must be interconnected. A perceived moment is a composite of various parts. For instance, watching a video activates:

• Visual cortex columns for vision and color
• Auditory cortex for sound information
• Language areas for decoding words
• Other networks for tracking body and emotions

These signals are processed in deeper brain areas that evaluate their importance, boosting relevant information and filtering out distractions. This synchronized activity of interconnected neural gears creates a "new structure" within the brain: an "assembly." This assembly is a unique pattern of synchronized neural activity that gives rise to the experience of being a human in a specific moment. For example, the assembly active in your brain right now represents you watching this video, hearing the voice, and learning about memories.

However, this assembly is initially just fleeting activity, like ripples on a pond, leaving no permanent trace. To transcend the present and form lasting memories, these temporary sensory inputs must be etched into something physical.

How the Past is Stored: A Deadly Competition

The brain is constantly processing multiple assemblies, not just one. It cannot give full attention to every internal and external stimulus. Therefore, different assemblies compete for dominance. The winning assembly is deemed most important by the brain and represents what you are consciously aware of at any given moment.

When an assembly wins this competition (e.g., processing a spoken sentence), two key processes occur to form a new memory:

1. Chemical Changes: The neurons within the winning assembly are bathed in chemicals that increase their susceptibility to change, strengthening their synaptic connections.
2. Hippocampal Activation: The hippocampus, the brain's memory center and "librarian," becomes active. While the exact process is still being researched, the hippocampus essentially creates a blueprint, saving the rough configuration of the assembly. This blueprint is then indexed with other associated memories (e.g., "every time I was confused by something in a Kurzgesagt video").

This results in a new, albeit fragile, memory: an activation pattern of millions of neurons across various brain regions. Activating any part of this pattern can trigger the entire assembly, allowing you to "relive" a past moment.

However, this initial memory is temporary. Without reinforcement, the hippocampus's blueprint will fade, and the synapses will weaken. This explains why most daily experiences, like the taste of coffee weeks ago, are forgotten. Most of life is experienced only once, in the moment.

Making Memories Stick: Novelty, Repetition, and Emotion

For a memory to become truly long-term, the assembly must "fight for its life" through various mechanisms:

• Novelty: Routine experiences often result in weak neural signals and are quickly forgotten. However, a novel or unusual event (e.g., a crow, squirrel, and mouse fighting over a nut) causes a strong assembly firing due to its uniqueness, increasing the likelihood of memory formation.
• Repetition/Reactivation: Thinking about a memory repeatedly or recounting it to others reinforces the neural connections, etching it deeper into the brain. This is analogous to the repetition needed for learning.
• Emotion: Emotions are powerful, ancient mechanisms that guide behavior, signaling what is important for survival. Strong emotions, whether positive or negative, cause the brain to deem the associated experience significant. This is why many of our strongest memories are emotionally charged, such as:
  • Humiliation from a social blunder
  • Joy from a personal victory
  • Devastation from loss
  • Overwhelming love for a child

Strong activation, repetition, and emotions all contribute to drastic changes in the neurons involved in a memory. The "gears" of the assembly grow new teeth, forming more synapses and firing together more efficiently, making the connections closer and more solid.

Much of this consolidation occurs during sleep. The hippocampus replays the assembly repeatedly, solidifying it and making it easier to retrieve. Insufficient sleep can lead to greater memory loss, highlighting its critical role in learning and memory retention. Ultimately, these processes lead to a long-term memory—a "diorama of hardened wax" etched into the brain's connection patterns.

The Ever-Changing Nature of Memory

While long-term memories can be remembered "forever," the act of remembering itself changes them. To recall a memory, a cue (a smell, sound, word, or image) activates the hippocampus, which searches its index for the corresponding assembly and triggers its firing. The past experience is retrieved, and the "diorama" appears in the mind.

However, recalling a memory is not like playing a static video. Under the "spotlight of attention," parts of the neural connections become soft and moldable again. As the memory is experienced, the involved neurons are bathed in chemicals that allow their structure to change.

The hippocampus organizes memories based on context. When a memory is retrieved, the current context is often different from the original one. For example, recalling a surprising animal fight while telling a humorous story to friends at night is a different context than the original experience of seeing it while tired before work. This new context infiltrates the memory:

• New connections form.
• Some synapses weaken, others reconfigure.
• The "diorama" changes, potentially becoming funnier or more absurd.

This is not a conscious alteration; the brain naturally incorporates the new context into the memory. Once the attention shifts, the "diorama" hardens again, but in its new, modified form. The next time the memory is recalled, it will reflect these changes.

This process applies to all memories: retrieval involves adding new information, forgetting some details, and incorporating current emotions and expectations. In essence, the brain updates past experiences to align with the present narrative of one's life. Over time, core memories can shift, merge with others, or even generate entirely new elements.

The memory system is deeply intertwined with learning mechanisms and is not designed for perfect accuracy. Memories are continuously updated with new experiences and information. Ironically, the more actively a memory is recalled, the less of the original experience may remain. Therefore, a vivid memory does not necessarily equate to an accurate one; it simply indicates a strong and active neural assembly.

This is a gradual process, not a sudden shift, ensuring that one's identity remains stable day-to-day. However, it implies that one's future self will perceive and feel differently about current experiences. This dynamic nature of memory also explains why therapy can be effective. By revisiting painful memories in a safe and introspective context, individuals can literally change their brain's wiring, offering a chance to rewrite their personal narrative and foster greater happiness.