Learning and Memory. 17 Learning and Memory. Functional Perspectives on Memory There Are Several Kinds of Memory and Learning Memory Has Temporal Stages: Short, Intermediate, and Long Successive Processes Capture, Store, and Retrieve Information in the Brain
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Functional Perspectives on Memory
Neural Mechanisms of Memory
Neural Mechanisms of Memory (continued)
Learning is the process of acquiring new information.
Patient H.M., Henry Molaison, suffered from severe epilepsy.
Because his seizures began in the temporal lobes, a decision was made to remove the anterior temporal lobes on both sides.
H.M.’s surgery removed the amygdala, the hippocampus, and some cortex.
H.M. had normal short-term memory but had severe anterograde amnesia.
Damage to the hippocampus can produce memory deficits.
H.M. was able to show improvement with motor skills but could not remember performing them (i.e. he could not recall the tasks verbally.).
H.M.’s memory deficit was confined to describe the tasks he performed.
Two kinds of memory:
Damage to other areas can also cause memory loss.
Patient N.A. has amnesia due to accidental damage to the left dorsal thalamus, bilateral damage to the mammillary bodies (limbic structures in the hypothalamus), and probable damage to the mammillothalamictract.
Like Henry Molaison, he has short-term memory but cannot form declarative long-term memories.
Korsakoff’s syndrome is a memory deficiency caused by lack of thiamine—seen in chronic alcoholism.
Patients often confabulate—fill in a gap in memory with a falsification which they accept as true.
Brain damage occurs in mammillary bodies and dorsomedial thalamus, similar to N.A., and to the basal frontal cortex.
Two subtypes of declarative memory:
Patient K.C. cannot retrieve personal (episodic) memory due to accidental damage to the cortex and severe shrinkage of the hippocampus and parahippocampal cortex; his semantic memory is good.
Three subtypes of nondeclarative memory:
Short-term memory is also known as working memory.
Working memory can be subdivided into three components, all supervised by an executive control module:
Mechanisms differ for STM and LTM storage but are similar across species.
Long-term memory has a large capacity.
Information can also be forgotten or recalled inaccurately.
A functional memory system incorporates three aspects:
Multiple brain regions are involved in encoding, as shown by fMRI.
For recalling pictures, the right prefrontal cortex and parahippocampal cortex in both hemispheres are activated.
For recalling words, the left prefrontal cortex and the left parahippocampal cortex are activated.
Thus, the prefrontal cortex and parahippocampal cortex are important for consolidation.
These mechanisms reflect hemispheric specializations(left hemisphere for language and right hemisphere for spatial ability).
The engram, or memory trace, is the physical record of a learning experience and can be affected by other events before or after.
Each time a memory trace is activated and recalled, it is subject to changes.
Consolidation of memory involves the hippocampus, but the hippocampal system does not store long-term memory.
LTM storage occurs in the cortex, near where the memory was first processed and held in short-term memory.
In posttraumatic stress disorder (PTSD, characterized as reliving and being preoccupied by traumatic events), memories produce stress hormones that further reinforce the memory.
GABA, ACh, and opioid transmission can also enhance memory formation in animal models.
Treatments that can block chemicals acting on the basolateral amygdala may alter the effect of emotion on memories.
The process of retrieving information from LTM can cause memories to become unstable and susceptible to disruption or alteration.
Reconsolidation is the return of a memory trace to stable long-term storage after it’s temporarily volatile during recall.
Reconsolidation can distort memories.
Successive activations can deviate from original information.
New information during recall can also influence the memory trace.
Leading questions can lead to ‘remembering’ events that never happened.
‘Recovered memories’ and ‘guided imagery’ can have false information implanted into the recollection.
Testing declarative memories in monkeys:
Medial temporal lobe damage causes impairment on the delayed nonmatching-to-sample task.
The amygdala is not necessary for declarative memory tasks.
The hippocampus (in conjunction with the entorhinal, parahippocampal) and perirhinal cortices, is important for these tasks.
Imaging studies confirm the importance of medial temporal (hippocampal) and diencephalic regions in forming long-term memories.
Both are activated during encoding and retrieval, but long-term storage depends on the cortex.
Episodic and semantic memories are processed in different areas.
Episodic (autobiographical) memories cause greater activation of the right frontal and temporal lobes.
Early research indicated that animals form a cognitive map—a mental representation of spatial relationships.
Latent learning is when acquisition has taken place but has not been demonstrated in performance tasks.
The hippocampus is also important in spatial learning.
It contains place cells that become active when in, or moving toward, a particular location.
Place cells remap when a rodent is placed in a new environment.
Grid cells and border cells are neurons that fire when animal is at an intersection and at the perimeter of an abstract grid map, respectively.
In rats, place cells in the hippocampus are more active as the animal moves toward a particular location.
In monkeys, spatial view cells in the hippocampus respond to what the animal is looking at.
Comparisons of behaviors and brain anatomy show that increased demand for spatial memory results in increased hippocampal size (relative to the rest of the brain) in mammals and birds.
In food-storing species of birds, the hippocampus is larger but only if used to retrieve stored food.
Spatial memory and hippocampal size can change within the life span.
In some species, there can be sex differences in spatial memory, depending on behavior.
Polygynous male meadow voles travel further (to find females) and have a larger hippocampus than female meadow voles or males of monogamous pine voles.
Imaging studies help to understand learning and nondeclarative memory for different skills:
All three of these depend on functional basal ganglia; the motor cortex and cerebellum are also important for some skills.
Imaging studies of repetition priming show reduced bilateral activity in the occipitotemporal cortex, related to perceptual priming.
Perceptual priming reflects prior processing of the form of the stimulus.
Conceptual priming (priming based on word meaning) is associated with reduced activation of the left frontal cortex.
Imaging of conditioned responses can show changes in activity.
PET scans made during eye-blink tests show increased activity in several brain regions, but not all may be essential.
Patients with unilateral cerebellar damage can acquire the conditioned eye-blink response only on the intact side.
Different brain regions are involved with different attributes of working memories such as space, time, or sensory perception.
Memory tasks assess the contributions of each brain region.
The eight-arm radial maze is used to test spatial location memory.
Rats must recognize and enter an arm that they have entered recently to receive a reward.
Only lesions of the hippocampus produce a deficit in this predominantly spatial task.
In a memorytest of motor behavior, the animal must remember whether it made a left or right turn previously.
If it turns the same way as before, it receives a reward.
Only animals with lesions to the caudate nucleus showed deficits.
Sensory perception can be measured by the object recognition task.
Rats must identify which stimulus in a pair is novel.
This task depends on the extrastriate cortex.
Interim summary of brain regions involved in learning and memory:
Molecular, synaptic, and cellular events store information in the nervous system.
New learning and memory formation can involve new neurons, new synapses, or changes in synapses in response to biochemical signals.
Neuroplasticity (or neural plasticity) is the ability of neurons and neural circuits to be remodeled by experience or the environment.
Sherrington speculated that alterations in synapses were the basis for learning.
Synaptic changes can be measured physiologically, and may be presynaptic, postsynaptic, or both.
Changes include increased neurotransmitter release and/or a greater effect due to changes in neurotransmitter-receptor interactions.
Changes in the rate of inactivation of transmitter would also increase effects.
Inputs from other neurons might increase or decrease neurotransmitter release.
Structural changes at the synapse may provide long-term storage.
New synapses could form or some could be eliminated with training.
Training might also lead to synaptic reorganization.
Lab animals living in a complex environment demonstrated biochemical and anatomical brain changes from those living in simpler environments.
Three housing conditions:
Animals housed in EC, compared to those in IC, developed:
Aplysia is used to study plastic synaptic changes in neural circuits.
The advantages of Aplysia:
Invertebrates demonstrate nonassociative learning which involves a single stimulus presented once or repeated.
Three types of nonassociative learning:
Habituation is studied in Aplysia.
Squirts of water on its siphon causes it to retract its gill.
After repeated squirts, the animal retracts the gills less; it has learned that the water poses no danger.
The habituation is caused by synaptic changes between the sensory cell in the siphon and the motoneuron that retracts the gill.
Less transmitter released in the synapse results in less retraction.
Over several days, the animal habituates faster, representing long-term habituation.
The number of synapses between the sensory cell and the motoneuron is reduced.
Hebb proposed that when two neurons are repeatedly activated together, their synaptic connection will become stronger.
Cell assemblies—ensembles of neurons— linked via Hebbian synapses could store memory traces.
Hebb’s idea was supported when researchers used tetanus (a brief increase of electrical stimulation that triggers thousands of axon potentials) on the hippocampus.
Long-term potentiation (LTP)—a stable and enduring increase in the effectiveness of synapses.
A weakening of synaptic efficacy—termed long-term depression—can also encode information.
Synapses in LTP behave like Hebbian synapses:
LTP can be generated in conscious and freely behaving animals, in anesthetized animals, and in tissue slices and that LTP is evident in a variety of invertebrate and vertebrate species.
LTP can also last for weeks or more.
Superficially, LTP appears to have the hallmarks of a cellular mechanism of memory.
LTP occurs at several sites in the hippocampal formation—formed by the hippocampus, the dentate gyrus and the subiculum (also called subicular complex or hippocampal gyrus).
The hippocampus has regions called CA1, CA2, and CA3(CA=Cornus Ammon which means Ammon’s Horn).
The CA1 region has two kinds of glutamate receptors:
Glutamate first activates AMPA receptors.
NMDA receptors do not respond until enough AMPA receptors are stimulated, and the neuron is partially depolarized.
NMDA receptors at rest have a magnesium ion (Mg2+) block on their calcium (Ca2+) channels.
After partial depolarization, the block is removed, and the NMDA receptor allows Ca2+ to enter in response to glutamate.
The large Ca2+ influx activates certain protein kinases—enzymes that add phosphate groups to protein molecules.
One protein kinase is CaMKII (calcium-calmodulin kinase II) which affects AMPA receptors in several ways:
These effects all increase the synaptic sensitivity to glutamate.
The activated protein kinases also trigger protein synthesis.
Kinases activate CREB—cAMP responsive element-binding protein.
CREB binds to cAMP responsive elements in DNA promoter regions.
CREB changes the transcription rate of genes.
The regulated genes then produce proteins that affect synaptic function and contribute to LTP.
Strong stimulation of a postsynaptic cell releases a retrograde messenger, often a diffusible gas like carbon monoxide (CO) or nitric oxide (NO) or that travels across the synapse and alters function in the presynaptic neuron.
More glutamate is released and the synapse is strengthened.
LTP can occur without NMDA receptor activation.
There is evidence that LTP may be one part of learning and memory formation:
Associative learning involves relations between events.
Eventually, the neutral stimulus by itself will elicit the response.
Researchers use the eye-blink reflex to study neural circuits in mammals.
An air puff is preceded by an acoustic tone; conditioned animals will blink when just the tone is heard.
A circuit in the cerebellum is necessary for this reflex.
The trigeminal (V) pathway that carries information about the corneal stimulation (the US) to the cranial motor nuclei also sends axons to the brainstem (specifically a structure called the inferior olive).
These brainstem neurons, in turn, send axons called climbing fibers to synapse on cerebellar neurons in a region called the interpositus nucleus .
Blocking GABA in interpositus nucleus stops the behavioral response.
The cerebellum is also important in conditioning of emotions and cognitive learning, as shown by humans with cerebellar damage.
Neurogenesis, or birth of new neurons, occurs mainly in the dentate gyrus in adult mammals.
Neurogenesis and neuronal survival can be enhanced by exercise, environmental enrichment, and memory tasks.
Reproductive hormones and experience are also an influence.
In some studies, neurogenesis has been implicated in hippocampus-dependent learning.
Conditional knockout mice, with neurogenesis selectively turned off in specific tissues in adults, showed impaired spatial learning but were otherwise normal.
Genetic manipulations can increase the survival of newly generated neurons in the dentate, resulting in improved performance.
These animals showed enhanced hippocampal LTP, which was expected since younger neurons display greater synaptic plasticity.
Adult neurogenesis is also seen in the olfactory bulb.
Activation of newly generated neurons in the olfactory bulb enhances olfactory learning and memory.
With age, we tend to show some memory impairment in tasks of conscious recollection that:
We also experience some decreases in spatial memory and navigational skills.
Some causes of memory problems in old age:
Cholinergic pathways to the cortex are lost in Alzheimer’s disease.
Enhancing cholinergic transmission helps with memory tasks.
Nootropics are a class of drugs that enhance cognitive function.
Cholinesterase inhibitors result can have a positive effect on memory and cognition.
Ampakines, which act via glutamate receptors, work to improve LTP in the hippocampus.
One particular protein kinase—PKMζ (ζ is zeta)—is needed for long-term maintenance of both hippocampal LTP and cortical memory traces.
Highly selective memory enhancing drugs could be developed in the near future.
Lifestyle factors can help reduce cognitive decline:
Mice were placed in two contexts:
These mice had also been genetically modified so that whenever neurons in the dentate gyrus (DG) of the hippocampus were active, they would start producing channelrhodopsin, a protein that would excite those cells, and only those cells, when exposed to blue light.
Activity of the subset of DG neurons with channelrhodopsin was responsible for the mice finding context B frightening.
Reactivating those neurons caused the mice to freeze in fear, even when they were in a completely different context.
Turning the light off again caused the animals to resume activity, indicating that they remained unafraid of context A.
It wasn’t just that light-induced activation of any random set of DG neurons induced fear, because when blue light reactivated DG neurons that had been active in a third (nonfearful) context, C, the animals did not freeze.