Forgetting is usually seen as a weakness of the human brain; it leaves us struggling to recall names, misplacing objects, or overlooking details we once knew well. However, modern neuroscience suggests that forgetting is not merely a passive decay of information, but an active and biologically important process that allows the brain to maintain cognitive efficiency and prioritise relevant information.
Most of us probably know that memory is stored in a network of neurons in the brain connected by junctions known as synapses. When we experience something new, sensory information from the environment is processed in different regions of the brain, and neurons in these regions fire in specific patterns that represent a memory. Once a memory is formed, it undergoes a process known as consolidation, where it becomes more stable and is gradually transferred to the cerebral cortex. The cortex plays a key role in long-term memory storage, particularly episodic memories (personal experiences) and semantic memories (recalling words, concepts, or numbers). The connections between these neurons are strengthened every time a memory is recalled, a process known as long-term potentiation (LTP). This strengthening allows memories to become even more stable and easier to recall. This is why learning something like vocabulary over a long period of time is more effective than learning it all at once.
Traditionally, forgetting is simply interpreted as the inverse process of long-term potentiation, which is known as synaptic depression. This is where the strength of connections between neurons decreases due to the memory not being recalled or used frequently. However, in some cases the brain can go a step further through a process called synaptic pruning, which is the selective removal of weak or redundant neural connections. This is most active during early development/childhood, when the brain is constantly making new neural connections and older ones become redundant more quickly. Synaptic pruning is carried out by microglia, the brain’s immune cells, which act like clean-up crews and are typically active during sleep. These cells detect and remove redundant synapses through phagocytosis, effectively consuming the connection and the memory. This pruning can be incredibly
important in shaping life after early development, although it continues later in life as well. For example, studies have recently shown that a deficiency in synaptic pruning can lead to an excess of connections in the brain, which is thought to disrupt brain function by creating too much “noise” from too many signals, potentially contributing to sensory and social processing differences seen in people with autism. On the other hand, excess synaptic pruning is also linked to conditions like Alzheimer’s, where the over-activation of the brain’s microglia becomes a key driver of the memory loss and cognitive decline commonly seen in the condition.
Synaptic pruning and synaptic depression are guided by neurotransmitters such as dopamine, acetylcholine, and glutamate, which play a key role in determining whether a synaptic connection should be strengthened or weakened. One type of dopamine signal helps tag emotionally significant or rewarding memories, making them less likely to be forgotten and more likely to be consolidated into long-term memory. For example, you are more likely to remember something exciting or scary because dopamine is released when the memory is first made. However, a recent rodent study by Hallo et al. (2022) has found that blocking the receptors for a different type of dopamine in the prefrontal cortex prevented normal forgetting of competing memories, while activating the receptors restored it. This shows that dopamine activity can also promote active forgetting, helping the brain suppress irrelevant information.
These two mechanisms for forgetting to ensure that only the most relevant and frequently used memories are maintained, while outdated or unnecessary information is removed to conserve energy and prioritise important information. Researchers theorise that this could allow for clearer thinking, quicker decision-making, and improved learning capacity, which can lead to better recall of required memories and creativity. Furthermore, forgetting can combat some of the problems of interference, when different
memories disrupt the recall or formation of other memories. The constant reshaping of neural pathways driven by the formation of new memories and loss of older memories is also vital for neuroplasticity, which is the brain’s ability to adapt to new experiences and be creative.
This system of forgetting is so important that a new theory has emerged stating that PTSD could even be the result of an injury to the forgetting function of the brain. The theory suggests that PTSD develops as a result of an overgrowth of synaptic connections in the amygdala, the part of the brain that stores memories of fear after intense or repeated exposure to frightening stimuli. Overall, this shows that forgetting is not simply a loss, but a crucial and complicated process that allows the brain to remain efficient, adaptable, and resilient.
Synaptic pruning genes networks in Alzheimer’s disease: correlations with neuropathology and cognitive decline (2023)
Microglia in the pathogenesis of autism spectrum disorders (2015)
Dopamine Modulates Adaptive Forgetting in Medial Prefrontal Cortex (Gallo et al., 2022)
Microglia mediate forgetting via complement-dependent synaptic elimination (Filipello et al., Science 2018)
Long-term memory, synaptic plasticity and dopamine in rodent PFC (Sheynikhovich et al., 2023)
The Biology of Forgetting – A Perspective (Davis & Zhong, 2015)
Failing to forget: inhibitory-control deficits compromise memory suppression in posttraumatic stress disorder (2015)
