Chapter 12 - Learning and Memory Flashcards
THREE CATEGORIES of BEHAVIOUR
The behaviour of an organism can be separated in three major categories:
1) REFLEXES, inevitable, involuntary quick responses to stimuli - they are a prime example of evolutionism;
2) INSTINCT, species-specific, inborn patterns of behaviour. They are unconscious and automatic, but result in behavioral patterns rather than in reactions. They include mating and parenting behaviour.
3) LEARNED BEHAVIOURS, or behaviours shaped by experience - the most flexible means for responding to the environment.
TYPES of LEARNING
LEARNING is a relatively permanent change in behaviour due to experience which occurs in one of two ways:
1) NONASSOCIATIVE learning - sensitisation and habituation - involves changes in the magnitude of responses to stimuli.
2) ASSOCIATIVE learning - classical and operant conditioning - occurs when an organism forms a connection between two features of its environment.
NONASSOCIATIVE LEARNING
NONASSOCIATIVE learning comprises sensitisation and habituation.
1) SENSITIZATION occurs when repeated exposure
to a strong stimulus increases responses to other
environmental stimuli;
2) HABITUATION occurs when an organism reduces its response to unchanging, harmless stimuli.
ASSOCIATIVE LEARNING
ASSOCIATIVE LEARNING comprises classical and operant conditioning.
1) In CLASSICAL CONDITIONING - first studied by PAVLOV - organisms learn that stimuli act as signals that predict the occurrence of other important events.
In the classical conditioning paradigm there are conditioned and unconditioned STIMULI and RESPONSES.
2) in OPERANT CONDITIONING - first studied by SKINNER - organisms form connections between a BEHAVIOUR and its consequences that impact the subsequent frequency of that behavior.
TYPES of MEMORY
Critical steps along memory pathways are:
1) ENCODING, transformation of stimuli in information the brain can process;
2) STORAGE, organization of memory information;
2) RETRIEVAL, recovery of stored information.
According to INFORMATION PROCESSING MODELS, information is processed in stages:
1) any information sensed initially enters the SENSORY MEMORY, which has a large capacity but a very limited duration;
2) selected information then enters SHORT-TERM MEMORY, which contains all the data about which we are currently thinking - STM has limited capacity (5 to 9 items) and duration (a few seconds).
3) the final destination is LONG-TERM MEMORY, which has few, if any, limitations on capacity or duration.
LONG-TERM MEMORIES can be classified as:
1) DECLARATIVE - or explicit - memories, which include EPISODIC and SEMANTIC memories;
2) NONDECLARATIVE - or implicit - memories, which include CLASSICAL CONDITIONING, PRIMING and PROCEDURAL memories (information about motor skills)
the SEA SLUG, or Aplysia Californica
The SEA SLUG -or Aplysia Californica - has been the most used animal model in research on learning and memory - KANDEL won a Nobel prize for studying the neurobiology of learning in sea slugs. This invertebrate has been chosen because of the simplicity of its nervous system and because its large nervous cells can be cultured and studied in vitro.
Anatomical features of the sea slug:
- the GILL, a structure through which the animal breathes;
- the SHIPON, a tube through which the animal expels waste;
- the MANTLE, which covers the gill.
Touching the siphon produces the GILL-WITHDRAWAL REFLEX, in which the gill is retracted. The touch receptors of the siphon form synapses directly with the motor neurons serving the gill.
HABITUATION in APLYSIA
HABITUATION occurs when an organism reduces its response to unchanging, harmless stimuli.
In Aplysia, repeated touching of the SIPHON produces a weak GILL-WITHDRAWAL REFLEX.
1) the siphon is stimulated;
2) sensory neuron releases less neurotransmitter;
3) EPSP in the motor neuron size is reduced;
4) motor neuron releases less neurotransmitter;
5) gill shows weak withdrawal.
Habituation is caused by the closing of CALCIUM CHANNELS in the presynaptic membrane of sensory neurons. Reduced calcium influx reduces EXOCYTOSIS, hence neurotransmitter release.
Not only does habituation influences the strength of synapses, but it also decreases the number of synapses of a specific neural pathway.
SENSITIZATION in APLYSIA
SENSITIZATION occurs when repeated exposure
to a strong stimulus increases responses to other
environmental stimuli.
In Aplysia, repeated pairing of tactile stimuli to the SIPHON and electric shocks to the TAIL produce an enhanced GILL-WITHDRAWAL REFLEX.
1) the siphon is stimulated and the tail is shocked;
2) the sensory neuron serving the tail synapses with an interneuron, which in turn synapses with the sensory neuron of the siphon;
3) the facilitating interneuron releases SEROTONIN onto the sensory neuron of the siphon;
4) serotonin causes the sensory neuron of the siphon to release increased amounts of neurotransmitter onto the motor neuron;
5) in turn, the motor neuron releases more neurotransmitter;
6) gill shows increased withdrawal reflex.
Not only does sensitization influences the strength of synapses, but it also increases the number of synapses of a specific neural pathway.
CLASSICAL CONDITIONING in APLYSIA
In CLASSICAL CONDITIONING organisms learn that stimuli act as signals that predict the occurrence of other important events.
In Aplysia, electric shock of the tail produces the gill-withdrawal reflex. Repeated pairing of electric shocks to the TAIL and tactile stimulations of the MANTLE produce an association between the two stimuli. After learning, tactile stimulation of the mantle produces the gill-withdrawal reflex on its own.
1) the mantle is stimulated and the tail is shocked;
2) the sensory neuron serving the tail synapses with an interneuron, which in turn synapses with the sensory neuron of the mantle;
3) the facilitating interneuron releases SEROTONIN onto the sensory neuron of the mantle;
4) serotonin causes the sensory neuron of the mantle to release increased amounts of neurotransmitter onto the motor neuron responsible of the gill-withdrawal reflex;
5) the reflex is shown in response to stimulation of the mantle even when the tail is not shocked.
MEMORY PATHWAYS in the HIPPOCAMPUS
The HIPPOCAMPUS and its surrounding structures are the main areas associated with long-term memory.
1) information is processed in the ASSOCIATION AREAS;
2) information pass through the PARAHIPPOCAMPAL CORTEX and the RHINAL CORTEX (ventral to the hippocampus);
3) information is passed to the HIPPOCAMPUS
4) output travels through the FORNIX, the pathway that connects the hippocampus to the hypothalamus;
5) information reach the HYPOTHALAMUS.
LTP and LTD are memory mechanisms that have been demonstrated in slices of hippocampus.
LTP and LTD
LONG-TERM POTENTIATION (LTP) is a memory mechanism which consists in change in responsiveness of target cells after a rapid series of shocks. LTP occurs in ways predicted by the cellular learning model proposed HEBB:
“when an axon of cell A is near enough to excite a cell B and repeatedly fires it, some metabolic change takes place in one or both cells such that A’s efficacy is increased”.
LTP relies on 2 principles or essential requisites:
1) ASSOCIATIVITY, or simultaneous activity of presynaptic and postsynaptic cells;
2) COOPERATIVITY, or simultaneous activity of several synapses of the target postsynaptic cell.
Although LTP was originally observed in the hippocampus, the phenomenon has also been demonstrated throughout the central nervous system, from the cerebral cortex to the spinal cord.
The opposite effect, LONG-TERM DEPRESSION (LTD), or the permanent weakening of synaptic strength due to poor electric stimulation, has been demonstrated.
LTP MECHANISMS
(LTP) is a memory mechanism which consists in change in responsiveness of target cells after substantial synaptic firing.
It takes place in a series of step which ultimately affect both presynaptic and postsynaptic cells:
1) GLUTAMATE is released by the presynaptic neuron;
2) AMPA glutamate receptors on the postsynaptic membrane are activated;
3) SODIUM IONS enter the postsynaptic cell through AMPA channels and depolarize it;
4) depolarization reaches NMDA glutamate receptors;
5) NMDA receptors release a molecule of MAGNESIUM which usually block their channels;
6) now that NMDA receptors are free, GLUTAMATE binds to them and activate their channels;
7) CALCIUM IONS enter the postsynaptic cell through NMDA channels;
8) calcium ions activate PROTEIN KINASES, which in turn:
A) increase the number of postsynaptic glutamate receptors;
B) increase their sensitivity;
C) activate RETROGRADE MESSENGERS that increase glutamate release in the presynaptic cell.
NEURAL SYSTEMS of CLASSICAL CONDITIONING in vertebrates.
The paradigm of classical conditioning has been used to locate learning networks in vertebrates - two brain areas have been found to be associated to this type of learning:
1) the CEREBELLUM, which is involved in classical conditioning of MOTOR RESPONSES;
2) the AMYGDALA, which is involved in classical conditioning EMOTIONAL RESPONSES, especially FEAR - lesion studies with rats show that, without amygdala, classical conditioning of threatening stimuli is not possible.
NEURAL SYSTEMS of LTM
LTM is associated to several brain areas:
1) the TEMPORAL LOBES, especially the HIPPOCAMPUS;
2) the DIENCEPHALON ( thalamus and hypothalamus), especially the DORSOMEDIAL NUCLEUS of the THALAMUS;
3) the BASAL GANGLIA, especially the PUTAMEN and the NUCLEUS CAUDATUS - is associated with LTM of MOTOR SEQUENCES.
SEMANTIC and EPISODIC memory are associated with large networks - different areas are activated depending on the target concept.
the DIENCEPHALON in LTM
The diencephalon, comprised of THALAMUS and HYPOTHALAMUS, is associated with LTM.
The DORSOMEDIAL NUCLEUS seems to be especially relevant - lesions to this nucleus in both animals and humans lead to ANTEROGRADE AMNESIA, which is an impairment in the formation of new EXPLICIT memories.
The DORSOMEDIAL NUCLEUS is also damaged in KORSAKOFF’S SYNDROME, a disorder that produces anterograde amnesia as a result of chronic alcoholism.
