The hippocampus and formation of new engrams
For a long time, the question of the exact structural organization of memory remained completely open, but modern physiological data have made it possible to identify key areas. One of the most important structures is recognized to be the hippocampus.
A fundamental feature of the hippocampus is that it acts as a "gateway" for new memories. Damage to this area leads to a specific amnesiac syndrome: the ability to form new long-term memory is completely lost. At the same time, previously formed, old memory is completely preserved in the subject.
Special attention in experimental physiology is paid to the ventromedial hippocampus, specifically the neurons of the CA1 field. Detailed studies show that targeted destruction affecting the cells of the CA1 field causes severe functional deficits:
- Both short-term and intermediate memory are impaired.
- In behavior, this manifests as extremely high distractibility.
- The subject loses the ability to maintain a set goal for a long time and move toward it consistently.
The role of the amygdala and emotiogenic centers
The amygdaloid area, or amygdala, plays a specific role in information fixation processes. Its main function is the determination and consolidation of the emotional component of memory. This process is carried out due to the presence of close and numerous connections between the amygdala and the emotiogenic centers of the hypothalamus.
Interestingly, isolated, targeted destruction of the amygdala alone practically does not impair the basic process of memorizing facts. However, the picture changes radically in the case of combined injuries. If simultaneous damage to the amygdala and the hippocampus occurs, it causes significantly more pronounced and severe impairments of short-term memory than if solely the hippocampus were destroyed.
Cerebral cortex: from the temporal lobe to the neocortex
The cerebral cortex assumes the functions of higher analysis and long-term storage of memory traces. A significant contribution to the understanding of these processes was made by the classic clinical observations of neurosurgeons W. Penfield and H. Jasper.
Analyzing the results of brain surgeries (in particular, interventions for temporal lobe epilepsy), they established that the removal of areas of the temporal cortex leads to the complete loss of the ability to memorize newly incoming information. The patient's preoperative memory remains intact in this case.
As for the neocortex, its functions are strictly differentiated:
- Association areas play a colossal, determining role in the processes of global memorization and long-term information storage.
- Medial prefrontal cortex is the substrate for the functions of the so-called working memory, which allows operating with data "here and now".
Subcortical structures: thalamus and cerebellum
In addition to the cortex and the limbic system, other parts of the central nervous system are involved in mnemonic processes.
For example, experiments conducted on monkeys have convincingly proven the importance of the anterior parts of the thalamus. Their surgical damage or inactivation inevitably leads to severe impairment of short-term memory functions.
A completely different type of memory is localized in the rhombencephalon. The structures of the cerebellum are responsible for the reliable storage of all automated motor skills (movement patterns that we perform without thinking).
Briefly: answers to topical questions
What structures other than the hippocampus are part of the Papez circuit and participate in memory consolidation?
In addition to the hippocampus, the Papez circuit includes structures that form a closed pathway for impulse circulation and participate in the transfer of information into long-term memory.
- Fornix — through it, impulses travel from the hippocampus.
- Mammillary bodies — receive impulses from the fornix.
- Mammillothalamic tract (bundle of Vicq d'Azyr) — originates from the nuclei of the mamillary bodies.
- Anterior thalamic nucleus — receives impulses via the mammillothalamic tract.
- Thalamocortical radiation (or thalamo-conical fibers) — fibers extending from the anterior nucleus of the thalamus.
- Cingulate gyrus — receives fibers of the thalamocortical radiation.
- Cingulum — conducts impulses from the cingulate gyrus back to the hippocampus, closing the circuit.
Damage to the structures of the Papez circuit disrupts the transfer of information from short-term to long-term memory.
H-A-T-M: Hippocampus (gateway for new knowledge), Amygdala (emotions), Thalamus (short-term memory), Cerebellum (movements).
Class and exam questions
What happens to memory when the hippocampus is damaged?
The patient loses the ability to form new long-term memory (anterograde amnesia), while all old, previously formed memories are preserved in full.
Which structure is responsible for working memory?
Working memory functions are closely associated with the medial prefrontal cortex (a part of the neocortex).
Is memory impaired in isolated damage to the amygdala?
The very process of memorizing information with isolated destruction of the amygdala is not impaired, but the emotional component of memory is lost. Pronounced deficit occurs only with combined damage to the amygdala and hippocampus.
What else to study on this topic
- Cellular organization of the ventromedial hippocampus (CA1 field)
- Connections of the amygdaloid area with hypothalamic centers
- Clinical observations by W. Penfield and H. Jasper (temporal lobe epilepsy)
- Distribution of neocortical association areas
- Mechanisms of motor skill storage in cerebellar structures
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