Learning and memory Flashcards
1
Q
What does it mean for neural pathways to be hardwired?
A
these are neural pathways
that establish connections during development that subsequently remain unaltered. the generic term for those processes that ensure the pathway is properly connected is specificity.
2
Q
Which neural pathways are referred to as plastic?
A
pathways subject to continual re-wiring, either during development or as a result of experience, are referred to as plastic, and the re-wiring processes described as plasticity.
3
Q
What is developmental plasticity?
A
It is wiring that is conditional on early sensory experience and shapes subsequent perception.
4
Q
What is learning?
A
it is also plasticity and is the acquisition of reproducible alterations in behavior as a result of particular experiences. The storage of
the altered behavior over time is memory.
5
Q
How can learning and memory be tested operationally in animals?
A
by recall, in which the previously learned behavior is elicited by the appropriate stimuli.
6
Q
What is declarative (explicit) memory?
A
It is memory for facts. Declarative learning is fast, it requires few trials, requires conscious recall, and may be readily forgotten. It has two components that are dissociable in patients with cortical damage.
7
Q
What is episodic (recollection) memory?
A
It is memory for specific events, in which associations are established at a specific time and place
8
Q
What is spatial navigational learning?
A
It is an extensively studied example of episodic learning. It is the ability of rats
to navigate through a maze in which they must learn to associate their positions in the maze with cues in their surroundings.
9
Q
What do the extensive connections of the medial temporal lobe allow in terms of episodic memory?
A
to act as a convergence zone for the different information streams. The different components of an episodic memory are distributed across disparate regions of the brain and must be bound together for recall of the episode.
10
Q
What is semantic (familiarity) memory?
A
it is a component of declarative memory. is memory of facts unrelated to events; that Bali is an Indonesian island can be recalled without ever having been there, so semantic memory is about “knowing that.”
11
Q
How are semantic memories sorted?
A
sorted into categories (sets of related objects), which appear to be stored in different areas of brain. Recall of specific items seems to need activation of multiple brain sites, each of which codes for a given attribute (e.g., color, function, name) of the item.
12
Q
What is procedural (implicit) memory)?
A
memory is memory for skills, such as learning to walk. It is “knowing how” memory. Procedural memory is slow, it needs many trials and it is incremental in that improvement occurs gradually over time. Performance of procedural tasks does not involve conscious recall. For this reason procedural memory (like emotional memory) is described as implicit memory. Once established, procedural memories are not forgotten even after many years without rehearsal.
13
Q
Declarative memory has at least two phases categorized by their time course. What is Working memory (short-term memory)?
A
It is temporary, limited in capacity and requires continuous rehearsal to keep it.
14
Q
What is Long-term (remote) memory (LTM)?
A
It is, if not permanent, at least long-lasting, has no obvious upper limit to its capacity and does not require continual rehearsal.
15
Q
Amnesias (loss of memory) due to brain damage can affect working memory and LTM independently. What are the two types of LTM?
A
It depends on whether memories are lost for
events and facts acquired before, retrograde amnesia, or after, anterograde amnesia, the brain damage.
16
Q
What is consolidation?
A
is the process that makes both declarative and procedural memories increasingly resistant to disruption or interference from similar learning over time.
17
Q
How does consolidation of memories occur?
A
Serial models postulate that elements may be selected from working memory for consolida-
tion into LTM by attention and arousal mechanisms. Experiments in which animals are injected shortly after training on a novel task with antibiotics which inhibit protein syn-
thesis show that consolidation requires protein synthesis.
18
Q
What is interference?
A
recalling a consolidated memory returns it to a labile state in which it becomes sensitive to interference. This is where a previously established memory can no longer be retrieved because of competition from new learning.
19
Q
How is interference prevented?
A
the recalled memory must now be reconsolidated. For example, if established auditory fear memories are reactivated in rats, protein synthesis is required for the memory to be retained.
20
Q
Procedural memory is learning to produce a motor response to a particular input. It is
divided into two types; nonassociative and associative, what is non-associative?
A
occurs in response to only a single kind of stimulus. Two examples are habituation, in which repeated exposure to a weak stimulus results in a reduction or a loss of the response normally seen with occasional presentation of the stimulus, and sensitization, which is an exaggerated response to innocuous stimuli following a strong noxious (unpleasant) stimulus
21
Q
What is associative learning?
A
it learning needs the pairing of two different types of stimulus within a short time and in the correct order. It enables animals to behave as if they can predict relationships of the kind: if A then B.
22
Q
What is classical conditioning characterised by?
A
temporal contiguity, the requirement that the CS must be presented before the US, and contingency, that animals learn that a predictive relationship exists between the CS and the US.
23
Q
When does extinction of a conditioned response occur?
A
It occurs if the CS is repeatedly presented without the US or if the temporal pairing of the CS and US is disrupted (i.e., if they are presented randomly). Extinction is not the same as forgetting. If after extinction the pairing of CS and US is restored the CR returns much more rapidly than it does in naive animals.
24
Q
What is aversive conditioning?
A
It is classical conditioning in which the US is noxious and which results in fear responses to normally neutral stimuli
25
What is operant (instrumental) conditioning?
an animal learns an association between a
motor activity it performs (e.g., pressing a lever) and the arrival of a stimulus, termed the reinforcer (e.g., a food pellet). Reinforcers may be positive, in which case they increase
the probability that an animal will act to obtain it, or negative (an aversive stimulus, such as an electric foot shock) in which case the animal will work to avoid it. Operant conditioning is used to investigate motivated behaviors.
26
What modulates the likelihood of specific memories being consolidated?
The arousal levels associated with an event. The arousal signals to which the brain memory circuits respond are adrenal hormones (both catecholamines and steroids), and several CNS peptide neurotransmitters released in response to stress.
27
what is the evidence of the involvement of catecholamines in memory circuit?
● Enhanced recall of emotionally neutral learning tasks by noradrenaline (norepinephrine) or adrenaline (epinephrine) given within a short time of the learning trials.
● No better recall of an emotionally charged version of a story compared with the neutral
version after administration of the b-adrenoceptor antagonist propranolol.
People with higher levels of sympathetic activity are more likely to suffer from post-traumatic stress disorder after a traumatic experience.
28
What is the shape of catecholamine-dose-response curve?
has an inverted U shape; moderate concentrations are more effective enhancers of memory than either high or low levels.
29
How do catecholamines exert their effect on memory?
neither of these hormones crosses the blood–brain barrier their actions on the CNS must be exerted. The catecholamines act at b-adrenoceptors on visceral afferents that run in the vagus (X) nerve to the nucleus of the solitary tract. This results in activation of noradrenergic neurons of the locus coeruleus that are part of a brain arousal system. This system projects to the amygdala and hippocampus to modulate learning.
30
How do glucocorticoids released by activation of the hypothalamic–pituitary adrenal axis in stress have effects on learning and memory?
These hormones readily cross the blood–brain barrier to act on steroid receptors that are located in high density in the amygdala and hippocampus. Low doses of glucocorticoids enhance, while high doses (or chronic exposure in long-term stress), impairs memory. Low concentrations occupy the high-affinity mineralocorticoid receptors (MR) and this facilitates strengthening of synapses thought to be crucial for learning. In contrast, high glucocorticoid concentrations fully saturate the low-affinity glucocorticoid receptors (GR) and this blocks the synaptic strengthening necessary for learning.
31
The anterior pituitary corticotrophs manufacture, from a single precursor, adrenocorticotrophic hormone (ACTH) and the opioid peptide, b endorphin, enkephalins are released by adrenal medulla, what is their effect on memory?
both of which impair learning by direct action on the CNS. Enkephalins, also opioid peptides, are co-released impair memory by a peripheral action. Naloxone, an antagonist of opioid receptors facilitates memory.
32
Cholinergic enhancement of memory is well documented. Muscarinic receptor antago-
nists impair memory, while inhibitors of acetylcholinesterase improve it. What is the mechanism of this?
Acetylcholine modulation of memory is mediated by the septohippocampal pathway and the cholinergic nuclei of the basal forebrain. The amygdala may enhance consolidation by activating the cholinergic attentional system in the basal forebrain.
33
Working memory (WM) is an on-line memory system. On average how many items can the working memory hold?
up to four items simultaneously in WM, though there are individual differences. EEG, brain imaging, and behavioral studies all suggest that the number of items that can be held is unaffected by the complexity of the items. Hence, the capacity of WM seems to be defined by the number of items rather than the total quantity of information.
34
What is the discrete resource model?
argues that any item represented in WM must be assigned to one of a limited number of slots.
35
What is the flexible resource model?
each item is assigned a share of WM resources and that performance is limited for large numbers of items because each item gets only a small share of WM resources.
36
How does interference between representations held in WM occur?
If items to be remembered are similar to those being processed, accuracy of recall is reduced. Delaying recall has little effect on recall accuracy so a simple decay model, in which item storage in WM fades over time, cannot be the case.
37
Which cognitive skills is WM capacity positively correlated with?
reading comprehension, problem-solving ability, maintaining attention in the face of distractions.
38
What is the phonological loop?
allows speech sounds to be held and rehearsed for long enough to give continuity to spoken language, so that phrases and sentences can be comprehended. It requires the left cerebral hemisphere.
39
What is the visuospatial sketch pad?
temporary store for visual, spatial, and kinesthetic input that brain imaging indicates involves several regions in the right hemisphere
40
What are the slave systems?
The phonological loop and visuospatial sketchpad are together referred to as slave systems.
41
What is the proposed interface between LTM and slave systems?
An episodic buffer via the central executive, which binds information from a variety of sources into coherent episodes. The contents of the episodic buffer are assumed to be consciously retrievable through activity of the central executive.
42
Other than binding information into coherent episodes, what is the central executive also involved in ?
The executive is also postulated to mediate directed attention, which transfers information into the episodic buffer from the slave systems and LTM where it can used to create new cognitive representations and for problem solving. The central executive may plan how to execute complex cognitive activities.
43
What is the prefrontal cortex involved in?
complex problem solving and planning future actions and there are good reasons for supposing that these executive tasks require working memory
44
How does the connectivity of the prefrontal cortex argue for its role in working memory?
Firstly, there are reciprocal connections between the PFC and other cortical areas, so the PFC receives visual, auditory, and somatosensory information. Secondly, the PFC is interconnected with the medial temporal lobe and dorsomedial thalamus that have a well-documented role in learning and memory. Thirdly, the PFC is a component of the executive thalamocortical-basal ganglia circuit which allows it to modulated by reward and salience.
45
What is the evidence for specific subsystems of working memory in the PFC?
Recording from the PFC in alert behaving monkeys reveals cells that fire in predictable ways during delayed-response tasks. For example, many cells fire throughout the delay period, others fire when the food is placed in the location and when the animal is allowed to choose the location. Particular regions of the PFC seem to be modality-specific
46
Which lobe is critical for declarative learning?
The medial temporal lobe and its connections with the hypothalamus and thalamus
47
What do bilateral lesions of the hippocampus, even when confined to the selective loss of pyramidal cells from just one region, result in?
an anterograde amnesia so extreme that patients are no longer able to form new long term memories. Working memory and procedural memory are unharmed, as are remote long-term memories, although some retrograde amnesia is seen.
48
Lesions to the medial temporal lobe have shown anterograde amnesia, damage to which specific structures give the most severe defecit?
damage to the perirhinal and parahippocampal cortex of the temporal lobe
49
What is the major output of the hippocampus?
the fornix which projects largely to the mammillary bodies of the hypothalamus, output from which goes to the anterior thalamus. Furthermore, areas of the temporal cortex and amygdala make connections with the dorsomedial nucleus of the thalamus.
50
Rats given a microinjection of col-
chicine to destroy a specific population of cells (dentate granule cells) in their hippo-
campus at various time points and asked to complete the morris water maze tell us what about the hippocampus and spatial learning?
that the hippocampus is needed for consolidation of spatial learning, but that over successive weeks the site of the memory store is transferred elsewhere, probably the neocortex.
51
How does retrograde amnesia occur in humans with medial temporal lobe amnesia?
the loss of memory not yet transferred from hippocampus to neocortex.
52
other than spatial learning, what is the hippocampus' broader role in humans?
primates it has abroader role in consolidating all episodic memories.
53
What is the cognitive map hypothesis and its postulates?
The hippocampus of the rat and associated cortex is thought to provide the rat with a
representation of the space around it and its location within it. Firstly, the map allows the animal to find its way through the environment. Secondly, it is constructed by episodic learning as specific locations come to be associated with particular sensory and motor cues. Thirdly, it does not require reinforcers; and finally, the map is continually updated by exploration.
54
What is the evidence for the cognitive map hypothesis?
the existence of place cells, pyramidal cells in the hippocampus that fire when a rat is in a particular position in its environment. In rat maze experiments, The location in the
maze which causes the place cell to fire is the cell’s place field (analogous to the sensory
field of sensory neurons) and the entire maze is encoded by an array of place cells.
55
What are the characteristics of place cell firing?
Place cells fire at a higher frequency the faster the animal moves, so are encoding loco-
motor cues, although place fields are influenced by sensory cues. The higher the firing
frequency of a place cell, the smaller its place field. This sets an upper limit on distance
coding of about 50 cm; that is, locations and objects > 50 cm ahead will be indistinguishable in a rat’s cognitive map, but will be resolved as the animal moves forward.
56
How are place fields created?
A place cell may have several place fields, each for a different environment. New place fields arise as an animal explores a novel environment and altering familiar surroundings disrupts preexisting place fields. When rats explore a maze, the EEG shows a theta (q) rhythm with a frequency of 4–10 Hz which reflects periodic firing of all hippocampal neurons (including place cells) organized by GABAergic interneurons. During q discharge, place cells encoding new information about the environment are entrained by the q rhythm while all other pyramidal cells are silenced by inhibition from GABAergic interneurons.
57
What is theta phase precession?
It is an example of temporal coding and encodes the distance traveled. The firing of hippocampal place cells shows a systematic phase relationship to the theta rhythm. Individual place cells fire whenever the animal enters a particular region in its environment but this firing is modulated at a slightly higher frequency than the theta
rhythm. Hence, as the animal moves through the place field the place cell fires at a progressively earlier phase of the theta rhythm.
58
What is the hippocampal formation?
folded archaecortex (ancient cortex) consisting of the dentate gyrus and the cornu ammonis (CA)—collectively termed the hippocampus— plus the subiculum.
59
What is the histological characteristics of the hippocampal formation?
The cortex of the dentate gyrus and CA has three layers, while the subiculum is transitional cortex between the hippocampus proper and the six-layered
neocortex of the entorhinal area.
60
What are major inputs to the hippocampus?
A major input from the entorhinal cortex comes via the perforant pathway, axons of which synapse with granule cells of the dentate gyrus or pyramidal cells in the CA3 region of the CA. Axons of the granule cells (mossy fibers) also synapse with CA3 pyramidal cells.
61
The CA3 pyramidal cell axons branch, forming:
● Commissural fibers which pass to the opposite hippocampus
● Efferents which leave the hippocampus via the fornix to terminate largely in the hypo-
thalamus or thalamus
● Collaterals which turn back to form synapses on the same and neighboring CA3 cells
(recurrent collaterals), or which synapse with cells in the CA1 region of the CA (Schaffer collaterals)
62
Where do CA1 axons go to?
to the subiculum and entorhinal cortex.
63
What is the neurotransmitter of the perforant pathway, granule cells, and pyramidal cells?
glutamatergic and excitatory.
64
The hippocampus also harbors inhibitory interneurons that are GABAergic. What do these help with?
help generate intrinsic oscillations (theta and gamma rhythms) that are essential for learning and memory.
65
Other than the glutamatergic input to the hippocampus, what other input does it get?
Other inputs to the hippocampus include a cholinergic pathway from the septum (required for theta rhythm) and modulatory noradrenergic and serotinergic axons from the brainstem reticular system.
66
How does theoretical modeling of the hippocampal circuitry suggest how it might work?
The dentate gyrus may act to keep sensory representations coming from the entorhinal cortex segregated. Intriguingly, recent evidence suggests that this requires new neurons to be born (neurogenesis) in spatial learning. The recurrent connections of CA3 allow associations between entorhinal inputs (i.e., episodic memories) to be rapidly established and then incorporated into neocortical long-term memory (i.e., consolidation) over a period of a few weeks. CA1 acts as a novelty detector by comparing stored memories with ongoing sensory information. Encoding in CA1 corresponds to
input from entorhinal cortex while retrieval from CA1 corresponds to CA3 input. At each
of the synapses in the hippocampus spike timing-dependent plasticity is seen;
67
LTP can be either associative (Hebbian) or nonassociative. What is the LTP at the synapses between CA3 Schaffer collaterals (Scs) and CA1 cells in the hippocampus?
Hebbian
68
Describe LTP at the synapses between CA3 Schaffer collaterals (Scs) and CA1 cells in the hippocampus?
In response to brief, low frequency stimulation of the Scs, the CA1 cells show a brief epsp due to glutamate release. If a brief tetanic burst of high-frequency stimulation is given (typically 100 Hz for 0.5 s), subsequent low frequency pulses now elicit a larger epsp. This is LTP.
69
What is input specificity in terms of Hebbian LTP?
Delivery of low frequency stimuli to the CA1 cell via a different untetanized bundle of Scs does not elicit the enhanced epsp.
70
What is cooperativity in terms of LTP?
The probability of producing LTP increases with the number of afferent fibers (Scs) tetanically stimulated. While weak (i.e., low current) high-frequency stimuli often fail to generate LTP, because they excite only a few afferents, strong tetanic stimuli are successful because they recruit many afferents.
71
What is associativity in terms of LTP?
A given CA1 cell receives Scs from CA3 cells on the same side and commissural axons that come from CA3 cells in the contralateral hippocampus. A weak tetanic stimulus to either pathway that fails to generate LTP will do so if it is paired with a strong tetanic stimulus in the other pathway.
72
What is persistence in terms of LTP?
It lasts for many minutes (in brain slices) to months when induced in vivo. This marks it out from other forms of synaptic plasticity.
73
What are the two phases of LTP?
It lasts for many minutes (in brain slices) to months when induced in vivo. This marks it out from other forms of synaptic plasticity.
74
CA1 cells have AMPA and NMDA glutamate receptors. How are NMDA receptors activated?
NMDA receptors must bind glutamate and experience a depolarization big enough to remove Mg2+ ions from the channel (voltage-dependent blockade). This condition is not provided by low-frequency Sc stimulation. The amount of glutamate released is low, few AMPA receptors are activated, and the resulting epsp is too small to open NMDA receptors. However, high-frequency stimulation opens numerous AMPA receptors and so depolarizes the cell sufficiently to activate NMDA receptors.
75
How does cooperativity and associativity arise in CA1 cells?
Cooperativity arises because the more
afferents activated the greater the depolarization of CA1 cells.Associativity is a property of the NMDA receptor itself: to activate it requires the coincidence of presynaptic glutamate release and postsynaptic neuron depolarization (Hebb’s rule).
76
What does Ca2+ entry through NMDA receptors trigger?
the induction of E-LTP at CA3-CA1
synapses.
77
What do antagonists of NMDA receptors at CA3-CA1 synapses do?
prevent the induction of LTP. Crucially for the
idea that LTP is a cellular substrate of learning, manipulations that impair LTP cause
learning deficits and vice versa. Preventing NMDA receptors from functioning not only
blocks induction of LTP, it also prevents some types of learning. Pharmacological block-
ade of NMDA receptors impairs learning of the Morris water maze by rats.
78
NMDA receptors are required for induction of LTP in most locations in the CNS, what other receptors are required at some synapses?
metabotropic glutamate receptors or kainate receptors are required.
79
What are post-synaptic changes at CA3-CA1 synapses?
● An increase in responsiveness of the postsynaptic membrane as AMPA receptors
become more sensitive to glutamate. The biochemistry of this has been worked out.
Calcium–calmodulin dependent protein kinase II (CaMKII), a major protein of the
postsynaptic density, is thought to be activated by Ca2+ entry through NMDA recep-
tors, and it then phosphorylates AMPA receptors (GluR1 subunits), enhancing their
response to glutamate.
● Activation of silent synapses. These are synapses that harbor only NMDA receptors.
They are normally silent because low-frequency stimulation does not activate NMDA receptors. Calcium entry during LTP induction causes them to acquire AMPA receptors and so become responsive to low-frequency input.
80
A presynaptic component to CA3–CA1 synapse LTP has been proposed in which there is increased glutamate release. What is the mechanism of this?
This requires that the NMDA Ca2+
signal generates a retrograde messenger in the postsynaptic cell that travels the “wrong” way across the synaptic cleft. One molecule proposed for this role is nitric oxide (NO). In neurons NO is synthesized by a Ca2+-dependent nitric oxide synthase (NOS). As a small, freely diffusible molecule, NO rapidly diffuses out of the postsynaptic cell, across the cleft and into the presynaptic cell where it stimulates guanylyl cyclase, thereby enhancing the probability of glutamate release.
81
What are the two well documented processes of how E-LTP is maintained? (how are synaptic alterations retained, even while individual molecules are being turned over, after the original signals that brought about the alterations have gone)
● CaMKII consists of four subunits. Ca2+
activation phosphorylates them and once the
Ca2+concentration has fallen to resting levels they remain phosphorylated. This is
because if a subunit becomes dephosphorylated it will immediately become autonomously phosphorylated by one of the other subunits. In this way CaMKII remains persistently active.● Ca2+ stimulates an isoform of adenylyl cyclase and consequently cAMP concentrations increase in LTP. Protein kinase A becomes persistently activated and has effects on gene expression.
82
A number of kinases are activated in E-LTP, but which one is thought to be important in the transition to L-LTP that occurs if several tetanic stimuli are delivered?
extracellular signal-related kinase (ERK)
83
long-lasting L-LTP is blocked by drugs that inhibit mRNA or protein synthesis , what does this show?
it depends on transcription and translation.
84
What morphological and functional changes in L-LTP are seen?
includes the changes to the geom- etry of dendritic spines and the formation of new synapses by the splitting of old ones into two.
85
What does late- phase synaptic plasticity depend on?
on synthesis of proteins that must function only in activated synapses; synaptic tagging. The idea is that a specific protein or perhaps a process (altered mRNA translation or cytoskeletal assembly) marks out an activated synapse so that plasticity-related proteins can then assemble to produce the functional and morphological changes that result in persistent synapse strengthening.
86
What is an example of a synaptic tag which ensures input specificity?
NMDA receptor activation has been shown to trigger input-specific spine entry of specific proteins which could serve as synaptic tags.
87
Which biochemical process is involved in the maintenance of L-LTP and implicated in several learning models?
modification of gene expression by the cyclic AMP second messenger system
88
What is CREB?
In maintenance of L-LTP, The activated catalytic subunit of protein kinase A translocates to the nucleus where it phosphorylates a transcription factor that binds to cAMP response elements (CREs) in upstream regions of genes regulated by cAMP. The transcription factor is termed cAMP response element binding protein (CREB).
89
What does CREB do once phosphorylated?
CREB binds to cre and this engages the other components needed for transcription
90
Other than by PKA, how is CREB phosphorylated?
by calcium–calmodu- lin-dependent kinases (e.g., CaMKIV) which will be activated by Ca2+ entry via NMDA receptors, or any other mechanism that increases cytoplasmic calcium concentration.
91
How is CREB- mediated gene transcription controlled?
● By protein phosphatases that are activated by the calcium-dependent protein calcineurin
● By repressor be proteins termed cAMP response element modulators (CREMs) that bind to cres and so prevent CREBs from doing so
92
Considerable evidence suggests that CREB is implicated in long-term memory in a number of different learning models: what are these models?
procedural learning in a marine snail (Aplysia),
odor discrimination learning in the fruit-fly Drosophila, and several types of learning in mammals. For example, transgenic mice that lack two of the three CREB isoforms are impaired in three different tasks that depend on the hippocampus and fear conditioning that requires the amygdala.
93
What occurs when neurons overexpressing CREB in the lateral amygdala are selectively deleted in mice?
abolished fear memory
94
Why was the importance of brain oscillations for hippocampal learning first suggested?
by the fact that tetanic stimulus protocols effective in producing LTP in vitro are similar to hippocampal theta rhythms in vivo.
95
What can provide the synchrony needed to generate LTP in the hippocampus and control
information flow that brings about encoding or retrieval of information?
modulations in gamma frequency (30–100 Hz) and theta frequency (4–8 Hz) bands and in phase relations between them.
96
How do gamma oscillations of hippocampal pyramidal cell membrane potential arise?
from rhythmic inhibition imposed by activity in local GABAergic hub interneurons.
97
When is the effectiveness of incoming excitatory inputs to the pyramidal cell the greatest?
when it occurs out of phase with the inhibition and this synchronizes the cells subjected to the gamma
inhibition to fire within 10 ms of each other.
98
What are the effects of the incoming excitatory inputs to the pyramidal cells synchronizing the cells subjected to the gamma inhibition to fire within 10 ms of each other?
● It can generate STDP (10–20 ms is the necessary time window).
● If the synchronized cells converge onto downstream neurons their near simultaneous activity will result in significant temporal summation, increasing the probability that the downstream neuron will fire.
● Because stronger excitatory inputs overwhelm inhibition more easily than weak ones, more active neurons spike earlier during a gamma band oscillation than weakly activated neurons. Hence the gamma oscillations sort inputs so that the stronger arrive downstream earlier than the weak ones.
99
How are theta oscillations produced by the hippocampal neural networks?
they can be produced in isolation but normally it requires cholinergic input from the septum, which makes pyramidal cells more
excitable.
100
How have the importance of the correlations between gamma and theta oscillations been proven?
In rats learning associations between items and places (i.e., when we assume that place cells are acquiring their place fields) the amplitude of gamma oscillations is frequency modulated by the
theta oscillations (i.e., the size of the gamma spikes rises and falls in phase with the theta rhythm). These correlations can be related to models of how the hippocampus works.
101
Gamma activity in the hippocampus falls into two frequency bands, low and high. When does low frequency gamma occur?
Low-frequency gamma in
CA1 coincides with the falling phase of the theta band oscillations and is synchronous with low-frequency gamma in CA3.
102
When does high frequency gamma activity in the hippocampus occur?
High-frequency gamma in CA1 happens during the
rising phase of the theta band and is synchronized with high-frequency gamma in the entorhinal cortex.
103
How do theta rhythms play a role in controlling information flow through the entorhinal cortex and CA1 and CA3?
inputs to CA1 from memory-related recurrent processing in CA3 occurs at one phase of theta and is characterized by low gamma frequency coherence, while sensory-related input from the entorhinal cortex occurs at a different theta phase and is characterized by high gamma coherence. These two different patterns are thought to correspond to different phases of the theta rhythm being needed for encoding or retrieval in CA1, with encoding corresponding to input from the entorhinal cortex while retrieval corresponds to CA3 input.
104
How do hippocampal oscillations affect spatial learning?
During spatial learning
tasks theta activity coherent with that in the hippocampus is seen in the prefrontal cortex and the ventral striatum.
105
How do hippocampal oscillations affect exploration and REM sleep?
In somatosensory and medial prefrontal regions bursts of locally produced gamma oscillations are briefly coherent with hippocampal theta rhythms
106
How do hippocampal oscillations influence memory tasks?
Synchronous activity in the gamma band is seen in the medal temporal lobe of rodents and primates (including humans)
107
Which phases of sleep are important for consolidation of which types of memory?
Slow-wave sleep seems
to be important for declarative memory of events and facts, whilst REM sleep may be required for procedural learning and emotional learning.
108
Where is episodic memory stored?
It is rapidly stored in the recurrent connections in CA3 and then incorporated into neocortical long-term memory.
109
What is the transfer of episodic memory to long-term mediated by?
It has been proposed that the transfer is mediated by coherent spiking of hippocampal neurons during high-frequency sharp wave/ripple complexes (SPW-Rs) that occur during awake immobility and slow-wave sleep. During SPW-Rs, sequential activation of place cells that occurred during spatial exploration are replayed at a faster rate.
110
How are SPW-Rs generated?
They are generated within CA3, in response to LTP induction there, and spread to CA1 then entorhinal cortex.
111
What is the effect of SPW-Rs?
They increase the excitability of hippocampal cells and their neocortical targets dramatically. Most CA1 pyramidal cells are powerfully inhibited during SPW–Rs, but the small number of active cells, those encoding the new spatial learning, increase their firing frequency to be in synchrony with the ripple component of SPW-R spikes. Firing of neocortical neurons by SPW-Rs during the long-lasting depolarizations of slow-wave sleep activity induces long-
term potentiation that allows the neocortical connections to be re-specified.
112
The enhanced recall of emotionally salient memories after particularly REM-rich sleep in humans suggests that REM sleep is also important for emotional memory, what two other facts support this?
● REM sleep is increased in stress generally and specifically in major depression, bereavement, and post-traumatic stress disorder.
● Brain imaging shows that during REM sleep while brain activity is reduced in the dorsolateral prefrontal cortex (part of the executive thalamocortical–basal ganglia circuit involved in working memory, planning, and problem solving), it is increased in the limbic cortex and the amygdala.
113
What is the one problem with the REM sleep hypothesis of memory?
monoamine oxidase inhibi-
tors used for the treatment of depression severely reduce the proportion of REM sleep. However, there is no evidence that patients treated with these agents have any memory deficits even with long-term use.
114
How does motor learning in the cerebellum occur?
involves alterations in the strengths of synapses
between parallel fibers (pf) and Purkinje cells (PC).
115
How does motor learning in the cerebellum occur?
involves alterations in the strengths of synapses
between parallel fibers (pf) and Purkinje cells (PC).
116
What is long term depression in relation to learning in the cerebellum?
a type of spike timing dependent plasticity where synapses that are active at
exactly the same time that there is climbing fiber input to the Purkinje cell, experience a reduction in the synaptic strength
117
According to the Marr-Alber-Ito Model of motor learning - what is the input to the cerebellum?
the frontal cortex (via the corticopontine cerebellar tract) provides the mossy fiber–parallel fiber inputs and the climbing fibers from the inferior olive are thought to transmit error signals.
118
According to the Marr-Alber-Ito model of motor learning, which synapses show LTD?
All the pf–PC synapses that
happen to be activated by a pattern of mossy fiber inputs at the same time as climbing
fiber error signals arrive Synapses not concurrently active are unchanged.
119
According to the Marr-alber-ito model of motor learning what is the overall effect of LTD?
parallel fiber activity at the depressed synapses excites Purkinje cells less,
thereby reducing their inhibitory output on deep cerebellar nuclei. The overall effect is that synapses at which LTD occurs enhance cerebellar output.
120
What is the eye blink reflex?
a puff of air delivered to the eye will produce an eye blink. The eye blink reflex can be conditioned if the air puff is
paired with a tone
121
what is the input and output of the eye blink reflex?
The air puff (US) is sensed by neurons in the spinal nucleus of the trigeminal (5th) cra-
nial nerve. The eye blink reflex (UR) is executed by connections between these cells and motor neurons in the facial (7th cranial) nerve.
122
How does conditioning of the eye blink reflex via the cerebellum occur?
The puff signal is transmitted via climbing fibers that arise from the inferior olivary nucleus. The tone signal goes by way of the ventral cochlear nucleus and pontine nucleus, arriving at the cerebellum in mossy fibers. Activation of the pf–PC synapse by the tone, 250 ms before the arrival of the puff via the climbing fiber, results in LTD of the pf–PC synapse. The effect of the LTD is that any subsequent arrival of the tone produces a smaller excitation of the PC. Hence PC inhibition of the interpositus neurons
is diminished, so these cerebellar nucleus cells drive the eye blink via their connections with the red nucleus.
123
What does induction of LTD in the cerebellum at the pf-PC synapse require?`
coincident Ca2+ influx into the Purkinje cell and activation of AMPA and metabotropic glutamate receptors at the synapse.
124
What provides the CA influx into purkinje cells required for LTD?
by the large depolarization due to climbing fiber activity which opens voltage-dependent Ca2+ channels.
125
What activates the AMPA and glutamate receptors at the pf-PC synapse to induce LTD?
the release of glutamate from the parallel fibers.
126
The final cause of the synaptic depression is desensitization of the AMPA receptors, what is this brought about by?
their phosphorylation by protein kinase C, and nitric oxide-activated protein kinase G.
