Hippocampus and Formation of New Engrams
For a long time, the exact structural organization of memory remained entirely elusive, but modern physiological data have identified key regions. One of the most critical structures is the hippocampus.
A fundamental feature of the hippocampus is that it acts as a "gateway" for new memories. Damage to this area results in a specific amnesiac syndrome: the ability to form new long-term memory is completely lost. Meanwhile, previously established old memories remain entirely intact.
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 CA1 cells causes severe functional deficits:
- Both short-term and intermediate memory are impaired.
- Behaviorally, this manifests as extreme distractibility.
- The subject loses the ability to maintain a goal over time and pursue it sequentially.
Role of the Amygdala and Eliciting Centers
The amygdaloid complex, or amygdala, plays a specific role in information consolidation. Its primary function is the determination and fixation of the emotional component of memory. This process is mediated by close and extensive connections between the amygdala and the emotion-generating centers of the hypothalamus.
Interestingly, isolated, targeted destruction of the amygdala alone practically does not disrupt the basic process of factual memorization. However, the clinical picture changes radically with combined injuries. If the amygdala and hippocampus are damaged simultaneously, it causes significantly more pronounced and severe short-term memory impairments than destruction of the hippocampus alone.
Cerebral Cortex: From the Temporal Lobe to the Neocortex
The cerebral cortex assumes the functions of higher-order analysis and long-term storage of memory traces. Significant contributions to understanding these processes were made by the classic clinical observations of neurosurgeons W. Penfield and H. Jasper.
Analyzing the results of brain surgeries (specifically interventions for temporal lobe epilepsy), they established that resection of areas of the temporal cortex leads to a complete loss of the ability to memorize newly incoming information. The patient's preoperative memory, however, remains intact.
Regarding the new cortex (neocortex), its functions are strictly differentiated:
- Association areas play a massive, defining role in global memorization and long-term information storage.
- Medial prefrontal cortex serves as the substrate for working memory functions, allowing real-time data manipulation.
Subcortical Structures: Thalamus and Cerebellum
In addition to the cortex and limbic system, other parts of the central nervous system are involved in mnestic processes.
For example, experiments on monkeys have convincingly demonstrated the importance of the anterior thalamus. Their surgical damage or inactivation inevitably leads to severe short-term memory impairment.
A completely different type of memory is localized in the hindbrain. The cerebellum is responsible for the reliable storage of all automated motor skills (movement patterns executed without conscious thought).