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How Does The Astonishing Hippocampus Store Memories In Your Brain?

Surgical destruction of the hippocampus caused Henry Gustav Molaison (HM) to lose his ability to remember events, which occurred even a few seconds earlier. But, HM retained memories for events long past.

While it is not the primary store of human memory, the hippocampus plays a crucial role in the consolidation of memories in extensive regions of the cortex. The hippocampus also supports a working memory, which enables the post-perceptual processing of information by the prefrontal regions.

The organ records memories of significant experiences of the mind in the sensory and recognition regions of the cortex, while providing strong space/time book marks for the recall of such memories. The organ recirculates the transient perceptions of the immediate past to the prefrontal regions for its current evaluation. This becomes a working memory, which protects the system from distraction and emotional reactivity and ensures quick and considered decisions and action plans.

During REM sleep, the hippocampus replays the space and time context of significant waking experiences. LTP circuits within the organ increase synaptic strength for such links. Neuronal reverberation, where linked nerve cells fire in rhythm, record the combinatorial patterns in all the linked groups of cells. Over many sleep/wake cycles, the organ spreads associative learning to extensive regions of the nervous system. With damage to the hippocampus, the nervous system loses its ability to bookmark, store and consolidate its episodic memories.

  • A brief description of the organ.
  • The size of the organ is related to a capacity for spacial memory.
  • Working memory, implicit memory, procedural memories and episodic memories.
  • Various ways, including combinatorial coding, LTP and neuronal reverberation of storing memories.
  • Place cells store complex spacial contexts.
  • Neuronal reverberation, where nerve cells fire in rhythm store memories.
  • The hippocampus has links to the working memory.
  • Sleep/wake cycles have relationships to the storage of long term memory.
  • Eye movements during REM sleep may be linked to memory storage and recall.

The Hippocampus And Severe Memory Loss
What Is The Effect Of The Hippocampus?

The hippocampus is a major component of the limbic system in the brains of humans and other vertebrates. Like the cerebral cortex, with which it is closely associated, it is a paired structure, with mirror-image halves in the left and right sides of the brain. In humans and other primates, the hippocampus is located inside the medial temporal lobe, beneath the cortical surface. It resembles the folded back forelimbs and webbed feet of the classical hippocampus - a sea monster with a horse's forequarters and a fish's tail.

Damage to the hippocampus can occur through age and disease. It is one of the first regions to atrophy in Alzheimer's disease, causing memory problems and disorientation. Damage can also result from oxygen starvation, encephalitis, or medial temporal lobe epilepsy. In rats, stress shortly after birth affects hippocampal function throughout life.

Humans who have experienced severe, long-lasting traumatic stress, suffer atrophy of the hippocampus. In such cases, high levels of cortisol in the blood stream damage the large population of glucocorticoid receptors in the hippocampus. The effects show up in post-traumatic stress disorder, and contribute to the hippocampal atrophy in schizophrenia and severe depression. Damage to the hippocampus does not affect the ability to learn to play a musical instrument, or to solve certain types of puzzles, which involve learning new motor or cognitive skills.

The Hippocampus And Severe Memory Loss
Does Utilization Impact The Size Of The Hippocampus?

The earliest role of the hippocampus was to remember the position of an animal in space and to enable it to find the path to a hidden goal. The organ helps to find shortcuts and new routes between familiar places. As an example, London's taxi drivers are required by law to know the most direct routes between numerous places in the city. A study revealed that the relevant regions of the hippocampus were larger in these drivers than in the general public. The organ also had larger volumes in more experienced drivers.

In similar species of animals, those with greater capacities for spatial memory have larger hippocampal volumes. Bird species, which cache food, also have a larger hippocampus. In humans, there is a reliable relationship between the size of the hippocampus and memory performance. Elderly people, suffering hippocampal shrinkage, tend to perform less well on several memory tasks. Chores, which require memory, tend to produce less hippocampal activation in the elderly. While the hippocampus appears to be designed for spatial navigation, the organ plays a significant role in enabling the storage of long term memories.

The Hippocampus And Severe Memory Loss
What Are The Types Of Human Memory?

Imagine that the physical and mental experiences of the mind are stored by nerve cells as memories for contextual combinatorial patterns. While most such memory is implicit, only a small proportion of this vast store remains available for conscious recall. Most people can indicate familiarity (implicit memory) with any one of 10,000 images displayed to them at 1 second intervals, without being able to recall them (declarative memory). Amnesic patients show implicit memory for experiences, while lacking a conscious knowledge of them. They may guess which of two faces they saw most recently, while claiming not to have seen them at all.

Representations in working memory decay unless they are refreshed. Ben-Yakov and Dudai link the hippocampus to sustained increases in neural activity, creating a working memory, in contextual regions of the nervous system, which are involved in execution of the task. If the mind is engaged elsewhere, the task is less well remembered. The attention load depends on the speed of the processing task. Adding digits every half second places a higher load on the system than when adding them every two seconds. The working memory does not require any inputs other than attention.

Procedural memories enable a person to play a musical instrument, or to ride a bike. Such memories, which directly empower the motor system in real time, are acquired through practice. Repetitive activity records the combinatorial memories in nerve cells. Such memories cannot be consciously recalled, but are available as a remembered ability. These memories assemble without the assistance of the hippocampus.

Conscious recall becomes possible, when attention is paid to a new and novel experience. The hippocampus assists in the consolidation and storage of the declarative memories of such experiences. Such memories may be both for events and experiences as well as for semantic concepts (ideas converted into words and sentences). The hippocampus uses its spacial navigation competence to provide context for converting implicit memories into episodic memories, which can be consciously recalled after months and years. Damage to the hippocampus causes a loss of the ability to acquire such memories.

The Hippocampus And Severe Memory Loss
What Is The Basis For Neural Intelligence?

When nerve cells fire to support a remembered mental function, the presence of human memory is proved. A person who recalls a number during calculation is recalling a working memory. The motor systems in one who rides a bike recalls procedural memories. A person, who recalls an event from the past uses his episodic memory.

To understand human memory formation, imagine that complex intelligence depends on the memories of nerve cells for dendritic firing combinations for signals from numerous regions of the nervous system. Imagine that a neuron fires, when it recognizes a combinatorial pattern in the array of its receiving dendrites. The pattern may be a single signal in the array, signals in a channel in the array, or a specific combinatorial pattern of signals in the array.  Nerve cells have memories for such combinatorial patterns.

Such memories were discovered to be applied for hundreds of millions of years by the olfactory sense (Nobel Prize 2004) for the instant identification of odors. Imagine that implicit memory for an odor is assembled, when nerve cells routinely record the related firing combinations. Such memories, which subsequently cause the cell to fire, may be further consolidated through LTP, neural plasticity, or neuronal reverberation.

Long term potentiation (LTP) enables neurons to become sensitive to a single contextual signal. A neuron may also grow new dendrites (neural plasticity), increasing accessibility to active communication channels. Procedural memory is saved when linked neurons fire repetitively during physical practice of the procedure. Declarative memories for complex events are assembled through neuronal reverberations, where groups of linked nerve cells fire in rhythm. Later, contextual perceptual or emotional links recall those memories.

The Npas4 gene may play a role in the recording of such combinatorial memories. Researchers at MIT discovered that by knocking out the Npas4 gene in the DNA in the hippocampus, neuroscientists created mice which kept entering cage compartments, in spite of continued foot shocks. They had interfered with the sequence of processes, which recorded memories of those painful experiences. The motor memories, which enabled the animal to run, remained.

The Hippocampus And Severe Memory Loss
What Is The Role Of Place Cells?

The nervous system constantly monitors its current location. The hippocampus uses eye movement and head direction data as an inertial compass to chart geographic movement and position. Visual and sound information triangulate the location. These eye and ear coordinates are mapped by head direction cells, grid cells, and border cells in the entorhinal cortex and the closely linked hippocampus. These regions were discovered to contain a neural map of the spatial environment in rats. The firing cells in the arrays lack any spatial topography in the representation. Cells lying next to each may have uncorrelated spatial firing patterns.

While the location of place fields may be random, it was discovered that finite array of those cells chart the location of an animal in a chamber. These are combinatorial patterns. Place cells are typically almost silent when a rat is moving around outside the place field, but fire faster when a rat reaches a place represented by the cell. The arrays of place fields in young rats alter if the rat is moved into a different environment, but reverts if the animal is returned to the same place. But, the place fields frequently fail to "remap" in aged rats.

The place fields record the place looked at by an animal in four dimensions, including time. In humans, the cells even indicate one's position in virtual reality spaces. The time dimensioned combinatorial pattern of firing by the place cells in the hippocampus trace the directions, objectives and movements of an individual in his environment. Damage to the hippocapmus causes a loss of this key reference point for episodic memories.

The Hippocampus And Severe Memory Loss
What Is Neuronal Reverberation?

The tactile, gustatory, olfactory, spatial, and motor activities produced by the free exploration of novel objects trigger precise contextual combinatorial links in multiple brain structures. When these active groups of neurons fire in rhythm, among millions of silent ones, they store combinatorial memories. Ann Graybiel recorded this process in the basal ganglia of a monkey, while it learned to associate the sound of a click with the availability of a sip of juice. Neuronal reverberation, when connected groups of neurons fired rhythmically, converted the action into a remembered drive for the animal. After learning, the task did not require conscious effort for the animal.

After a novel experience for an animal in a cage, the correlation of neuronal reverberation between groups of cells increases dramatically. This process repeats for several hours after the learning experience. At the same, firing patterns related to experiences with less new information (movements along the bare sides of the cage) reduce. Cortical recognition identifies a new geographic feature as being relevant to the goal of learning. Attention to the feature intensifies the activity in the nerve cells, which perceive and recognize. They fire in rhythm, triggering the acquisition of combinatorial memories.

The Hippocampus And Severe Memory Loss
What Is The Link Of The Hippocampus To Working Memory?

Joaquin Fuster recorded the electrical activity of neurons in the PFC of monkeys while they were doing a delayed matching task. In that task, the monkey observed a bit of food being placed under one of two identical looking cups. A shutter was then lowered for a variable delay period, screening off the cups from the monkey's view. When the shutter opened, the monkey was allowed to retrieve the food. Successful retrieval required holding the location of the food in working memory over the delay period. Fuster found neurons in the PFC, the posterior parietal cortex, the thalamus, the caudate, and the globus pallidus that fired mostly during the delay period, suggesting that such firing represented the food location while it was invisible.

Ben-Yakov and Dudai report a “stimulus offset,” where events become “time locked,” after perception, in the absence of sensory stimulation. In their (fMRI) experiments, they identified bilateral hippocampus activity starting immediately after stimulus presentation. Post-perceptual processing in the absence of sensory stimulation was time-locked to the offset of sensory input. The activation was found to increase for subsequently remembered over forgotten content. Working memory maintenance may be part of the long term memory maintenance process by the hippocampus.

The Hippocampus And Severe Memory Loss
How Do Sleep/Wake Cycles Affect Memory Formation?

Mammals and birds enjoy long periods of dreamless slow-wave sleep (SW), followed by short periods of rapid-eye-movement dreaming sleep (REM). Sidarata Ribeiro suggests that the wake-sleep cycle promotes propagation of memories outwards from the early coding sites. Neuronal reverberation is strongest during SW sleep. The correlations increase progressively during SW sleep, strengthening the memory trace. SW sleep shortly after memory acquisition is critical for memory consolidation. Sustained experience-dependent neuronal reverberation can be detected in several cortical areas up to 48 hours after exposure to novel experiences.

Lack of such SW sleep causes an irreversible loss of the recently acquired implicit memory. Incoming sensory inputs during wake periods subdue the neuronal reverberation linked to past novel experience. Memory formation occurs during sleep from the absence of sensory interference. The neocortical reverberation decays rapidly within one hour of memory trace formation. Brain activity after acquisition of new data has been shown to be proportional to memory acquisition in rats and humans and to quantitatively predict learning.

The Hippocampus And Severe Memory Loss
What Is REM Sleep?

A significant consolidation of memory takes place during REM sleep. Nature has provided a mechanism to replay the space/time context through rapid eye movements during sleep, causing persistent neuronal reverberation in the cortex, hippocampus, putamen, and thalamus. Incremental learning continues several nights after memory acquisition due to the progressive recruitment of larger neuronal networks over time. The hippocampus also has mechanisms, which progressively disengage the organ from its older memory consolidation processes.

Neuronal reverberation takes place in both SW and REM sleep. But, the physical movements of the eye distinguish the REM sleep process, critical to the learning experience. The added neural firing by the eye muscles may contribute to access (or store) data by identifying its space/time context. Subconscious eye movements often accompany search processes of the mind - say, the preparation of a shopping list. The eye movements may enable REM sleep to achieve greater knowledge consolidation in less time than SW sleep.

This page was last updated on 08-Sep-2016.