Deep within the brain’s temporal lobes are two curved structures known as the hippocampi. Each is a convex elevation of gray matter that folds into the temporal lobe. The name is derived from the Greek words for horse and sea monster, as its C-shape resembles a seahorse. Each hippocampus is about five centimeters long and is part of the limbic system, a collection of brain structures involved in behavioral and emotional responses. It receives and sends information to the rest of the brain primarily through a structure called the entorhinal cortex.
Role in Memory Formation
The hippocampus is widely recognized for its function in memory, specifically in consolidating short-term memories into lasting long-term ones. It does not store memories itself but acts like a librarian, organizing and indexing memories stored in other regions of the cerebral cortex. The hippocampus is particularly active in forming new episodic memories, which are the recollections of personal experiences and specific events.
Its role is less pronounced in forming procedural memories, such as learning a new skill like playing an instrument. These motor-based memories are handled by other brain areas, including the basal ganglia and cerebellum. This highlights its role in declarative memories—facts and events that can be consciously recalled.
The case of Patient H.M., Henry Molaison, illustrates this function. In 1953, to treat severe epilepsy, he underwent surgery to remove large portions of his medial temporal lobes, including both hippocampi. While the surgery controlled his seizures, it left him with profound anterograde amnesia. He was unable to form new episodic memories, yet his procedural memory remained intact.
The neural mechanism for memory consolidation involves a process known as long-term potentiation (LTP), which strengthens the connections between neurons. When a new experience occurs, a specific pattern of neurons is activated. Through LTP, these connections become more efficient, creating a durable memory trace that can be reactivated.
Spatial Navigation and Mental Mapping
The hippocampus is also responsible for our ability to navigate the world. It creates and maintains internal representations of our environment, often referred to as cognitive maps. These mental maps allow us to understand our position in space, remember routes, and find our way to different locations. This navigational function is a specialized form of memory, intertwined with our experiences of places.
Neurons within the hippocampus, known as place cells, are responsible for this process. These cells become active when an individual is in a particular location. Different sets of place cells fire for different locations, creating a neural blueprint of an area. This internal mapping system is dynamically updated as we explore new environments or as familiar ones change.
Studies of London taxi drivers demonstrate the hippocampus’s role in spatial navigation. To earn their license, these drivers must memorize the city’s complex layout. Research has shown that experienced taxi drivers have a larger posterior hippocampus compared to control groups. This difference correlates with their years on the job, suggesting that extensive use of spatial memory can lead to physical changes in the brain.
Connection to Emotion and Mood
As part of the limbic system, the hippocampus works with other brain regions to regulate emotional responses. It has dense connections with the amygdala, a structure associated with processing emotions like fear and pleasure. The hippocampus helps place emotional experiences into context, linking a feeling to the specific event or environment where it occurred. This process is why a particular place can trigger a strong emotional memory.
This brain region is also highly sensitive to stress, as it has a high concentration of receptors for cortisol, the body’s primary stress hormone. During periods of acute stress, cortisol can enhance memory formation, which is why we often have vivid recollections of frightening events. This response can be adaptive, helping us to remember and avoid potential dangers in the future.
However, prolonged exposure to high levels of stress hormones can have a detrimental effect. Chronic stress can impair the birth of new neurons and may lead to a reduction in the overall volume of the hippocampus. This link is a focus of research into conditions like major depressive disorder and post-traumatic stress disorder (PTSD).
Impact of Damage and Disease
When the hippocampus is damaged or affected by disease, the consequences for memory and cognitive function can be severe. One of the most documented outcomes is anterograde amnesia, the inability to create new memories after the event that caused the damage. This condition highlights the hippocampus’s role in memory consolidation, as individuals with this amnesia may still retain memories from before the injury.
In neurodegenerative diseases, the hippocampus is particularly vulnerable. Alzheimer’s disease, for instance, often begins in the medial temporal lobe, and atrophy of the hippocampus is one of the earliest biomarkers for the condition. This deterioration leads to the characteristic memory loss and disorientation of Alzheimer’s, starting with difficulties recalling recent events and progressing to more severe deficits.
Direct physical injury, stroke, or oxygen deprivation can also cause significant harm to the hippocampus. Temporal lobe epilepsy, a condition with seizures originating in the temporal lobe, is frequently associated with hippocampal sclerosis, a specific pattern of cell loss and scarring. Damage from these conditions can lead to a range of memory impairments.