The hippocampus is a small, curved structure located deep within the brain’s temporal lobe. As a core component of the limbic system, this paired structure—one on each side of the brain—is fundamentally involved in processing new information and learning. Its primary role is to serve as a temporary processing center for memory. Damage to this sensitive region leads to distinct and often severe impairments in a person’s ability to create, store, and navigate their world.
Disruptions to Memory Formation and Recall
Damage to the hippocampus most prominently interferes with declarative memory, which is the memory of facts, events, names, and places. This type of impairment is often categorized into two major forms of amnesia, depending on the timing of the memory loss relative to the injury.
Anterograde amnesia is the inability to form new declarative memories after the point of damage. Individuals can recall events that happened before the injury but struggle to retain new information, such as remembering a new acquaintance’s name or what they ate for lunch. The hippocampus acts as the transfer point that converts unstable short-term experiences into permanent long-term memories, and its dysfunction prevents this consolidation process.
Retrograde amnesia is the loss of memories that were formed before the damage occurred. When the damage is isolated to the hippocampus, this memory loss is often temporally graded, meaning that recently acquired memories are lost while very old memories remain intact. This occurs because, over time, memories are gradually transferred from the hippocampus to long-term storage areas in the cerebral cortex.
While declarative memory is significantly affected, the ability to learn and perform skills—known as procedural memory—is preserved. Procedural memories involve motor skills, like riding a bike or tying a shoe, and are processed and stored in other brain regions, such as the basal ganglia and cerebellum. A person with hippocampal damage may be unable to recall meeting their physical therapist but can still learn a new physical exercise routine well. This distinction highlights the hippocampus’s specialized role in processing explicit facts and events, not unconscious motor learning.
Loss of Spatial Orientation and Navigation
Beyond its role in memory, the hippocampus is indispensable for spatial memory and the ability to navigate the environment. This function is closely tied to the existence of specialized neurons within the structure known as “place cells.”
Place cells are neurons that fire specifically when an individual is in a particular location in their environment, effectively creating an internal representation of their surroundings. Collectively, these cells form what researchers call a “cognitive map”—a mental blueprint of space that allows for efficient navigation.
Damage to the hippocampus disrupts the function of these place cells, leading to symptoms of spatial disorientation. A person may experience difficulty following familiar routes or struggle to create new mental maps of unknown locations. Even in a highly familiar setting, such as their own home or neighborhood, they may become disoriented or unable to judge their location relative to objects or landmarks. This impairment affects the brain’s internal GPS system.
Common Medical Conditions That Affect the Hippocampus
A variety of medical conditions and injuries can specifically target and compromise hippocampal function, resulting in characteristic cognitive symptoms. Neurodegenerative diseases are recognized causes, with Alzheimer’s disease frequently beginning its course in the hippocampal region. The resulting atrophy, or shrinkage, is often one of the earliest signs of the disease, leading to a noticeable decline in the ability to form new memories.
Oxygen deprivation, or ischemia, is another significant cause, occurring during events like severe stroke, cardiac arrest, or near-drowning incidents. Hippocampal neurons are highly sensitive to a lack of oxygen and are therefore easily damaged, even after only a few minutes of interrupted blood flow.
Chronic, high levels of the stress hormone cortisol can also negatively affect the hippocampus, causing a measurable reduction in its volume over time. Conditions associated with sustained psychological distress, such as severe depression or post-traumatic stress disorder, have been linked to this atrophy because the hippocampus contains a high density of receptors for stress hormones.
Traumatic Brain Injury (TBI), including severe concussions, can cause direct structural damage or secondary vascular insults to the hippocampus. The resulting inflammation and scarring can lead to long-term cognitive deficits, especially if the injury is severe.
Certain forms of epilepsy, particularly mesial temporal lobe epilepsy, are strongly associated with hippocampal damage. Patients with this condition often develop hippocampal sclerosis, which involves a loss of neurons and scarring in the structure. This damage is thought to contribute to both the seizure activity and the memory difficulties experienced by those with the disorder.