Hippocampus (brain)
Molaison in 1953 before his surgery
Most of Molaison's two hippocampi were removed bilaterally.

One extensively studied anterograde amnesiac patient, codenamed H.M., demonstrated that despite his amnesia preventing him from learning new declarative information, procedural memory consolidation was still possible, albeit severely reduced in power.

- Anterograde amnesia

After the surgery, which was partially successful in controlling his seizures, Molaison developed severe anterograde amnesia: although his working memory and procedural memory were intact, he could not commit new events to his explicit memory.

- Henry Molaison
Hippocampus (brain)

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Overall

Humans have two hippocampi, one in each hemisphere of the brain. They are located in the medial temporal lobes of the cerebrum. In this lateral view of the human brain, the frontal lobe is at the left, the occipital lobe at the right, and the temporal and parietal lobes have largely been removed to reveal one of the hippocampi underneath.

Hippocampus

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Major component of the brain of humans and other vertebrates.

Major component of the brain of humans and other vertebrates.

Humans have two hippocampi, one in each hemisphere of the brain. They are located in the medial temporal lobes of the cerebrum. In this lateral view of the human brain, the frontal lobe is at the left, the occipital lobe at the right, and the temporal and parietal lobes have largely been removed to reveal one of the hippocampi underneath.
Image 1: The human hippocampus and fornix (left) compared with a seahorse (right)
Image 2: Cross-section of cerebral hemisphere showing structure and location of hippocampus
Image 3: Coronal section of the brain of a macaque monkey, showing hippocampus (circled)
Image 4: Basic circuit of the hippocampus, as drawn by Cajal DG: dentate gyrus. Sub: subiculum. EC: entorhinal cortex
Image 5: Hippocampal location and regions
Rats and cognitive maps
Image 6: Spatial firing patterns of 8 place cells recorded from the CA1 layer of a rat. The rat ran back and forth along an elevated track, stopping at each end to eat a small food reward. Dots indicate positions where action potentials were recorded, with color indicating which neuron emitted that action potential.
Image 7: Examples of rat hippocampal EEG and CA1 neural activity in the theta (awake/behaving) and LIA (slow-wave sleep) modes. Each plot shows 20 seconds of data, with a hippocampal EEG trace at the top, spike rasters from 40 simultaneously recorded CA1 pyramidal cells in the middle (each raster line represents a different cell), and a plot of running speed at the bottom. The top plot represents a time period during which the rat was actively searching for scattered food pellets. For the bottom plot the rat was asleep.
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Image 9: An EEG showing epilepsy right-hippocampal seizure onset
Image 10: An EEG showing epilepsy left-hippocampal seizure onset
Image 11: Drawing by Italian pathologist Camillo Golgi of a hippocampus stained using the silver nitrate method
thumb|Hippocampus highlighted in green on coronal T1 MRI images
thumb|Hippocampus highlighted in green on sagittal T1 MRI images
thumb|Hippocampus highlighted in green on transversal T1 MRI images

People with extensive, bilateral hippocampal damage may experience anterograde amnesia: the inability to form and retain new memories.

Although it had historical precursors, this idea derived its main impetus from a famous report by American neurosurgeon William Beecher Scoville and British-Canadian neuropsychologist Brenda Milner describing the results of surgical destruction of the hippocampi when trying to relieve epileptic seizures in an American man Henry Molaison, known until his death in 2008 as "Patient H.M."

The line processes to make information memory

Memory consolidation

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Category of processes that stabilize a memory trace after its initial acquisition.

Category of processes that stabilize a memory trace after its initial acquisition.

The line processes to make information memory
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Systematic studies of anterograde amnesia started to emerge in the 1960s and 1970s.

The case of Henry Molaison, formerly known as patient H.M., became a landmark in studies of memory as it relates to amnesia and the removal of the hippocampal zone and sparked massive interest in the study of brain lesions and their effect on memory.

Retrograde amnesia

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Loss of memory-access to events that occurred or information that was learned in the past.

Loss of memory-access to events that occurred or information that was learned in the past.

Anterograde amnesia is a similar condition that deals with the inability to form new memories following the onset of an injury or disease.

Henry Molaison had epilepsy that progressed and worsened by his late twenties.

Hippocampus as seen in red

Explicit memory

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One of the two main types of long-term human memory, the other of which is implicit memory.

One of the two main types of long-term human memory, the other of which is implicit memory.

Hippocampus as seen in red
Amygdala as seen in red
The Morris water maze

Henry Molaison, previously known as H.M., had parts of both his left and right medial temporal lobes (hippocampi) removed which resulted in the loss of the ability to form new memories.

Guy Pearce plays an ex-insurance investigator suffering from severe anterograde amnesia caused by a head injury.

Short-term memory

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Capacity for holding a small amount of information in an active, readily available state for a short interval.

Capacity for holding a small amount of information in an active, readily available state for a short interval.

One form of evidence cited in favor of the existence of a short-term store comes from anterograde amnesia, the inability to learn new facts and episodes.

Patients with this form of amnesia have an intact ability to retain small amounts of information over short time scales (up to 30 seconds) but have little ability to form longer-term memories (illustrated by patient HM).