The hippocampus is a paired, seahorse-shaped region located in the medial temporal lobe of both the left and right hemispheres of the brain. This bilateral structure is necessary for normal human cognition. The hippocampus acts as the brain’s central processing hub for forming new conscious memories. Removal or severe damage to this area in both hemispheres results in devastating and specific memory loss, fundamentally changing a person’s ability to live a continuous, connected life.
The Hippocampus: A Primer on Function
The healthy hippocampus performs two primary functions. The first is the consolidation of memory, which stabilizes short-term memories into a long-term form. It serves as a temporary organizing center where information from various brain regions is bound together to create a single, coherent memory. This process is responsible for declarative memory, which includes memories of facts (semantic memory) and personal events (episodic memory).
Another element is spatial navigation and memory. The hippocampus contains specialized neurons, known as place cells, that fire when an individual is in a specific location. These cells create a mental map, allowing people and animals to navigate their surroundings and remember routes. The ability to create a cognitive map of a new environment is lost when the hippocampus is removed.
The Inability to Form New Memories
The most dramatic consequence of bilateral hippocampal removal is the complete inability to form new long-term declarative memories, a condition known as anterograde amnesia. This memory loss means that new information or experiences encountered after the damage cannot be transferred into permanent storage. The person is effectively trapped in an eternal present, unable to create new conscious recollections of their life.
The famous case study of Henry Molaison (H.M.) illustrates this condition. He underwent bilateral removal of his medial temporal lobes, including the hippocampi, in 1953 to treat severe epilepsy. Following the procedure, Molaison could not remember meeting new people or events that had occurred minutes earlier. He would greet the same researchers as strangers day after day.
Anterograde amnesia is profound and permanent. While short-term memory remains functional for a few seconds, once attention shifts, the information is lost forever. This deficit extends to both episodic memories (like what they ate for lunch) and semantic memories (like learning a new fact). Because the hippocampus is the necessary bridge for memory consolidation, its absence prevents the formation of a continuous personal history.
The Loss of Past Contextual Memories
Removal of the hippocampus also causes a loss of memories acquired before the damage, known as retrograde amnesia. This loss is typically not total; it is often temporally graded. Memories formed immediately before the damage are lost, but very old memories remain largely intact. This pattern supports the standard model of memory consolidation.
This model suggests the hippocampus is only required temporarily to stabilize a new memory. Over time, through a slow process, the memory is gradually transferred and permanently stored in the neocortex, becoming independent of the hippocampus. For patients like H.M., he could not recall events from the one to two years preceding his surgery, but his childhood memories were preserved.
The severity of retrograde amnesia depends on the extent of the damage to the medial temporal lobe structure. Damage limited strictly to the hippocampus may only result in a loss of memories from the last few years. More extensive lesions that include adjacent cortical areas, like the parahippocampal cortex, can result in memory loss that spans decades.
What Memory Functions Remain Intact
Despite the devastating loss of new declarative memory, several other memory systems remain fully functional following hippocampal removal. Working memory, the ability to hold a small amount of information in mind for a short duration, is unaffected. A person can still hold a phone number in their mind long enough to dial it, even though they will immediately forget the number once the task is complete.
Procedural memory, which governs the learning of skills and habits, also remains intact. This is primarily mediated by structures like the basal ganglia and cerebellum. For example, a patient can learn to solve a new puzzle or master a new motor skill, and their performance will improve with practice. Crucially, they will have no conscious recollection of ever having practiced the task, demonstrating a dissociation between the skill and the learning event.
Emotional memory, particularly fear conditioning, is also spared, as this is largely controlled by the amygdala. A patient may show a normal physiological fear response to a stimulus paired with an unpleasant event. However, they would not consciously remember the pairing event, highlighting the brain’s ability to process and store different types of memory in parallel.