The hippocampus is a paired structure, with one copy residing deep within the medial temporal lobe of each brain hemisphere. This small, curved region is a core component of the limbic system, a network involved in processing emotion, motivation, and memory. The question of what happens when this structure is removed is rooted in historical neurosurgery. Early attempts to treat severe, intractable epilepsy sometimes involved the removal of large parts of the temporal lobe, which included the hippocampus. Damage can also occur due to stroke, prolonged oxygen deprivation, or disease processes like Alzheimer’s, making its study a major area of human cognition research.
The Hippocampus: Architect of Memory and Space
The primary function of the healthy hippocampus is to act as a temporary processing station for new information, serving as a gateway for memory consolidation. It transforms transient sensory and cognitive input from short-term memory into long-term memory that can be stored elsewhere in the cortex. This role is fundamental to declarative memory, which includes the conscious recall of facts, events, and knowledge.
Declarative memory is subdivided into semantic memory (general knowledge) and episodic memory (recollection of specific personal events and their context). The hippocampus binds together the various elements of an experience—like the time, location, and emotional content—into a unified, retrievable memory. This binding allows a person to consciously “relive” a moment from the past.
Beyond memory for events, the hippocampus plays a specialized role in spatial navigation and memory. This function involves creating and maintaining a cognitive map of the environment, allowing an individual to orient themselves and find their way through space. Specific neurons within the structure, known as place cells, fire only when an individual is in a particular location, providing the neural basis for this internal mapping system.
The Immediate Consequence: Anterograde Amnesia
The most dramatic consequence of removing or severely damaging both hippocampi is a profound and debilitating condition known as anterograde amnesia. An individual loses the ability to form any new long-term declarative memories after the point of injury. While they can understand a conversation in the moment, they are unable to consolidate that information for later recall, essentially trapping them in a perpetual present.
The severity of this deficit was first documented through the famous case of Henry Molaison (H.M.), who underwent bilateral medial temporal lobe resection in 1953 to control severe epilepsy. Molaison could not remember new faces, facts, or events, forgetting who he was speaking to moments after they left the room. He would re-read the same magazines and solve the same puzzles without any conscious awareness of having encountered them before.
The memory loss also affects memories formed immediately before the damage, a condition known as temporally graded retrograde amnesia. Very old memories, such as those from childhood, are typically preserved because they have been fully consolidated and transferred to the neocortex. However, recent memories from the years leading up to the surgery are often lost. This pattern confirms the hippocampus’s role as the temporary staging ground for memory before it becomes permanent and independent of the structure.
The loss of the hippocampus also severely impairs the ability to navigate or form new spatial memories. An individual may struggle to learn the layout of a new home or hospital ward, as the cognitive mapping function is compromised.
Memory Systems That Remain Functional
Despite the devastating loss of the ability to form new conscious memories, other memory and cognitive functions remain entirely intact, demonstrating that memory is not a single, centralized process. The ability to hold a small amount of information in mind for a short duration, known as short-term or working memory, is spared because it does not rely on hippocampal function. Patients can hold a phone number in their mind long enough to dial it, but they will forget the number the moment their attention shifts.
Crucially, the ability to learn new motor skills, referred to as procedural memory, is also unaffected. This type of learning, which governs activities like riding a bicycle, typing, or tracing a pattern, is mediated by brain regions such as the basal ganglia and cerebellum, not the hippocampus. In tests, Molaison was able to learn and improve on complex motor tasks, showing clear skill acquisition over days, even though he had no conscious recollection of ever having performed the task before.
This dissociation between intact skill learning and abolished conscious memory for the learning event was a landmark discovery that redefined the understanding of memory. It proved that the brain operates with multiple, independent memory systems, where the declarative system is uniquely dependent on the hippocampus. The preservation of these non-declarative functions highlights the distributed nature of the brain’s processes.