Deep within the brain’s temporal lobes are two seahorse-shaped structures called the hippocampus. As a component of the limbic system, a group of structures involved in emotion and learning, the hippocampus is particularly associated with forming new memories. It functions as a central hub, connecting with other brain regions to process our daily experiences.
Primary Functions of the Hippocampus
The hippocampus is most recognized for its role in converting short-term experiences into long-term memories. This process, known as consolidation, involves the hippocampus acting as a processing center to take in new information and prepare it for long-term storage in other brain areas. This consolidation is particularly active during sleep.
Another primary function is spatial navigation, which allows us to understand our position in relation to our surroundings. The rear part of the hippocampus creates internal cognitive maps of our environment. Specialized “place cells” within the hippocampus fire in response to specific locations, acting as an internal GPS that helps determine direction and distance.
The hippocampus also contributes to the regulation of emotional responses. It works closely with the amygdala, another part of the limbic system, to connect memories with emotions. This connection helps generate an appropriate emotional response to a situation based on past experiences. For example, a positive past experience with a friendly dog can lead to feelings of calm when encountering dogs later.
Understanding Hippocampal Volume
There is no single “correct” size for the hippocampus, as its volume naturally varies among individuals. Scientists measure this volume using magnetic resonance imaging (MRI) scans, which provide detailed images of the brain’s structure. From these detailed scans, researchers can calculate the volume of the hippocampus.
The hippocampus has neuroplasticity, meaning its structure and size are not fixed and can change throughout a person’s life in response to various experiences. It is one of the few areas in the adult brain where neurogenesis, the creation of new neurons, occurs. This capacity for change means that both positive and negative influences can alter its physical structure over time.
Changes in hippocampal volume are associated with various neurological and psychiatric conditions. Studies show that a smaller hippocampal volume can be a biomarker for conditions like Alzheimer’s disease, depression, and post-traumatic stress disorder (PTSD). The ability to track these changes provides a valuable tool for understanding brain health and the progression of certain diseases.
Factors That Alter Hippocampus Size
A range of factors can lead to a decrease in hippocampal volume, a process often referred to as atrophy. Chronic stress is a major contributor, as prolonged exposure to the stress hormone cortisol can be neurotoxic to hippocampal cells. This long-term stress can damage these cells and reduce the structure’s overall volume. Conditions like Cushing’s syndrome, which involves excessive cortisol production, are also associated with a smaller hippocampus.
Conditions such as major depression and PTSD are also linked to reduced hippocampal volume. While it is still being researched whether a smaller hippocampus is a cause or a result of depression, studies show an association. In individuals with depression, a greater decrease in volume can be linked to longer-lasting depressive episodes. Neurodegenerative diseases, most notably Alzheimer’s, are characterized by significant hippocampal atrophy, which is an early sign of the disease.
Conversely, certain activities can help maintain or even increase hippocampal volume. Aerobic exercise is a well-documented method for promoting growth by increasing brain-derived neurotrophic factor (BDNF). This protein supports the survival of existing neurons and encourages the growth of new ones. A study of older adults found that consistent aerobic exercise increased hippocampal volume by 2%, reversing age-related loss by one to two years.
Engaging in continuous and complex learning also helps preserve hippocampal size by promoting the survival of new neurons within the hippocampus. Mindfulness meditation may also increase hippocampal size, largely by reducing stress and its harmful effects. Furthermore, getting adequate sleep is important, as chronic sleep deprivation can reduce neurogenesis by elevating stress hormone levels.
The Link Between Hippocampus Size and Health Outcomes
The size of the hippocampus is directly linked to tangible health and cognitive outcomes. A smaller hippocampal volume is frequently correlated with memory deficits and mood disorders. It can also serve as a biomarker indicating an increased risk for developing dementia. In Alzheimer’s disease, the degree of hippocampal shrinkage is used to help diagnose the progression of the illness.
On the other hand, a larger hippocampus is associated with better cognitive resilience and memory function. The hippocampus can adapt in response to cognitive demands, as shown in a well-known study of London taxi drivers. To become licensed, these drivers must memorize “The Knowledge,” a complex layout of 25,000 streets and thousands of landmarks. Structural MRI scans revealed that these drivers had a larger posterior hippocampus compared to control subjects, and the volume correlated positively with the amount of time spent working as a driver.