Brain Areas Involved in Memory and Learning
Memory and Learning Require the Cerebral Cortex and Limbic System
Memory Opens in new window and learning are inextricably linked because part of the learning process involves the assimilation of new information and its commitment to memory.
The most likely sites of learning in the human brain are the large association areas of the cerebral cortex, in coordination with subcortical structures deep in the temporal lobe, including the hippocampus and amygdala.
The association areas draw on sensory information received from the primary visual, auditory, somatic sensory, and olfactory cortices and on emotional feelings transmitted via the limbic system.
This information is integrated with previously learned skills and stored memory, which presumably also reside in the association areas.
The learning process itself is poorly understood, but it can be studied experimentally at the synaptic level in isolated slices of mammalian brain or in more simple invertebrate nervous systems.
Synapses subjected to repeated presynaptic neuronal stimulation show changes in the excitability of postsynaptic neurons. These changes include the facilitation of neuronal firing, altered patterns of neurotransmitter release, second messenger formation and, in intact organisms, evidence that learning occurred.
The phenomenon of increased excitability and altered chemical state on repeated synaptic stimulation is known as long-term potentiation, a persistence beyond the cessation of electrical stimulation, as is expected of learning and memory.
Ca2+ entry through activation of NMDA receptors is critical to the development of long-term potentiation. An early event in long-term potentiation is a series of protein phosphorylations induced by receptor-activated second messengers and leading to activation of a host of intracellular proteins and altered excitability.
In addition to biochemical changes in synaptic efficacy associated with learning at the cellular level, structural alterations occur. The number of connections between sets of neurons increases as a result of experience.
Much of our knowledge about human memory formation and retrieval is based on studies of patients in whom stroke, brain injury, or surgery resulted in memory disorders.
Such knowledge is then examined in more rigorous experiments in nonhuman primates capable of cognitive functions.
From these combined approaches, we know that the prefrontal cortex Opens in new window is essential for coordinating the formation of memory. Starting from a learning experience in the cerebral cortex, then processing the information and communicating it to the subcortical limbic structures.
The prefrontal cortex receives sensory input from the parietal, occipital, and temporal lobes and emotional input from the limbic system. Drawing on skills such as language and mathematical ability, the prefrontal cortex integrates these inputs in light of previously acquired learning.
The prefrontal cortex Opens in new window can thus be considered the site of working memory, where new experiences are processed, as opposed to sites that consolidate the memory and store it.
The processed information is then transmitted to the hippocampus Opens in new window, where it is consolidated over several hours into a more permanent form that is stored in, and can be retrieved from, the association cortices.
Categorizing Memory Based on Retention Time
Memory can be divided into that which can be recalled for only a brief period (seconds to minutes) and that which can be recalled for weeks to years.
Newly acquired learning experiences can be readily recalled for only a few minutes or more using short-term memory Opens in new window. An example of short-term memory is looking up a telephone number, repeating it mentally until you finish dialing the number, then promptly forgetting it as you focus your attention on starting the conversation.
Short-term memory is a product of working memory; the decision to process information further for permanent storage is based on judgment as to its importance or on whether it is associated with a significant event or emotional state. An active process involving the hippocampus must be employed to make a memory more permanent.
The conversion of short-term to long-term memory Opens in new windowis facilitated by repetition, by adding more than one sensory modality to learn the new experience (e.g., writing down a newly acquired fact at the same time one hears it spoken) and, even more effectively, by tying the experience (through the limbic system) to a strong, meaningful emotional context.
The role of the hippocampus Opens in new window in consolidating the memory is reinforced by its participation in generating the emotional state with which the new experience is associated.
As determined by studying patients such as H.M. Opens in new window, the most important regions of the medial temporal lobe for long-term declarative memory formation are the hippocampus and parahippocampal cortex.
Once a long-term memory is formed, the hippocampus is not required for subsequent retrieval of the memory. Thus, H.M. Opens in new window showed no evidence of a loss of memories laid down prior to surgery; this type of memory loss is known as retrograde amnesia Opens in new window.
Nor was there loss of intellectual capacity, mathematical skills, or other cognitive functions.
An extreme example of H.M.’s memory loss is that Dr. Milner, who worked with him for years, had to introduce herself to her patient every time they met, even though he could readily remember people he had met and events that had occurred before his surgery.
