Learning and Memory
Memory, in its broadest sense, refers to an individual's ability to retain learned information. The formation of memory traces is a complex task which all organisms appear to be able to do. Fundamentally, such an ability requires a series of distinct steps: Information must be encoded as meaningful associations are assessed, the resulting information must be recorded in some form, and the memory record must be retrieved when needed.
As humans exhibit little conscious awareness of mental processes, such as memory, these were long considered inaccessible to objective, scientific study. Inferences about the course of encoding, consolidation and storage of memory thus mostly rely on indirect methods such as testing for retrieval abilities.
Consolidation of memory appears to involve independent paths at different time frames. Such evidence derives from the existence of different critical time frames for its formation, and its susceptibility to disruption with different pharmacological tools.
Subjects are able to report great detail about a complex stimulus immediately following its presentation. This ability forms within a few tens of milliseconds and decays again rapidly within a few hundred milliseconds.
Short-term memory allows the recall of something from several seconds to as long as a minute. Its strength does appear to depend primarily on attention and not rehearsal. It is thus highly vulnerable to disruption when attention shifts elsewhere. The amount of information that can be held is on the order of 4-6 numbers. This amount can be boosted by grouping them into distinct chunks as for telephone numbers (e.g., 202-456-1414).
The ability to recall such information is contingent on transient patterns of neuronal activity in regions of the frontal and the parietal lobe.
This type of memory, lasting hours to months, critically depends on a transfer of the information from short term memory using repeated rehearsal.
The hippocampus appears to be an essential structure in such routing. Sleep is thought to improve the consolidation of information, possibly by hippocampal replaying of activity from the previous wake period. Electrical activation of hippocampal circuits reportedly are linked to feelings of Deja Vu.
Declarative memory refers to the ability to become conscious of, or declare, facts and experiences. It is also referred to as Explicit memory, when it involves direct recalling information that had been obtained from the external world. Representing knowledge of standard textbook material or events, it is best formed by actively recalling the material in spaced intervals. Compared to other forms or memory it is more volatile; more easily formed and more easily forgotten.
The primary neural basis appears to reside in the medial temporal lobe. Bilateral damage to this area results in anterograde amnesia, as in the famous case of the anonymous memory-impaired patient HM. A surgical procedure for epilepsy left him with damage to his brain in the medial temporal lobe on both sides. the hippocampal formation, parahippocampal gyrus, the entorhinal cortex, and the amygdala. He subsequently suffered from severe anterograde amnesia, where transfer of new events into long-term memory was impaired. He was unable to recall events once his attention had focused elsewhere.
Semantic Memory is the ability to consciously recall knowledge of facts that are independent of a specific time and place. medial temporal lobe, diencephalon
Episodic Memory refers to the ability to explicitly recall information about a specific event that has occurred at a specific time and place, medial temporal lobe, diencephalon
Implicit memory, which does not need to involve conscious awareness in the act of recollection
Procedural Memory regards the learning of motor skills and habits. Formation requires repeated reinforcement, repetition and practice over many trials rather than recollection. Once formed it is less likely to be forgotten. It is also less easily transferred to related tasks than declarative learning
Striatum, basal ganglia, Deficits can be assessed using a serial reaction time (SRT) task, backwards reading, mirror drawing, probabilistic classification, artificial grammar learning, or prototype abstraction.
Motor Responses with Classical Conditioning, Cerebellum
Emotional Responses with Fear conditioning involves an organism's ability to acquire fear responses to a previously neutral stimulus. This occurs when it becomes paired with an aversive stimulus, such as a shock or loud noise. James Wattson's Little Albert experiment illustrated that children learned fear of objects when encounters with them where paired with loud noises.
Priming refers to a context-dependent activations of clusters of neocortical neurons. As they become more activated, they are more likely to come into consciousness.
Even reflex pathways are capable of surprising plasticity. For example, circuits that control motor patterns for walking will be subjected to optimization with sensory input.
months to lifetime
Patient HM suffered severe memory impairment following bilateral damage to his medial temporal lobes, hippocampal formation, parahippocampal gyrus, entorhinal cortex, and amygdala. His short-term memory remained intact, he was able to largely recall past events, and his ability to learn new motor skills was not disrupted. However, he suffered from severe anterograde amnesia, i.e., the ability to commit new events to long-term memory.
Memory mechainsms critically depend on changes in synaptic functioning. Hebb Synapses and their role in classical conditioning. If a synapse succeeds at driving a postsynaptic neuron above threshold, its subsequent effectiveness is strengthened.
Long-term potentiation is based on changes in neural signals which will potentiate a neural response for 1-2 weeks. potentiation involves increased release of the excitatory neurotransmitter glutamate.
Molecular changes at the synaptic level
During habituation, repeated stimulation of a sensory neuron leads to a smaller activation of the postsynaptic motor neuron. The primary cause for this is a progressive reduction of Ca++ inflow into the presynaptic terminal, decreased transmitter release, and a smaller activation of the postsynaptic target. In sensitization as a result of electrical head shock, release of serotonin activates second messenger systems and phosphorylation of key target molecules. Long-Term Memory depends on new protein synthesis and the formation of new synaptic connections.