". . . memory is the process by which that knowledge of the world is encoded , stored , and later retrieved ." Eric Kandel 2000
"Memory is a phase of learning . . .
Encoding -information for each memory is assembled from the different sensory systems and translated into whatever form necessary to be remembered. This is presumably the domain of the association cortices and perhaps other areas.
Consolidation -converting the encoded information into a form that can be permanently stored. The hippocampal and surrounding areas apparently accomplish this.
Storage -the actual deposition of the memories into the final resting places–this is though to be in association cortex.
Retrieval -memories are of little use if they cannot be read out for later use. Less is known about this process.
What is Learning?
"Learning refers to a more or less permanent change in behavior which occurs as a result of practice," Kimble, 1961
“ Learning is the process by which we acquire knowledge about the world.” Eric Kandel 2000
Learning is the strengthening of existing responses or formation of new responses to existing stimuli that occurs because of practice or repetition
Types of learning
Reflex : Basis of all Behavior
There is a decrease in behavioral response to a stimulus after repeated exposure to that stimulus over a duration of time
Progressive amplification of a response follows repeated administrations of a stimulus
Ivan Pavlov: Classical Conditioning 1904
Instrumental Learning Thorndike's (1911)
Law of effect
Thorndike’s theory that behavior consistently rewarded will be “stamped in” as learned behavior, and behavior that brings about discomfort will be “ stamped out.”
Operant conditioning (Skinner )
Individuals learn new behaviors that "operate on" the environment — behaviors that cause the individuals to experience environmental stimuli
A box often used in operant conditioning of animals; it limits the available responses and thus increases the likelihood that the desired response will occur.
Operant conditioning: the type of learning in which behaviors are emitted to earn rewards or avoid punishments.
Operant behavior: behavior designed to operant on the environment in a way that will gain something desired or avoid something unpleasant.
Reinforcer: a stimulus that follows a behavior and increases the likelihood that the behavior will be repeated.
Punisher: a stimulus that follows a behavior and decreases the likelihood that the behavior will be repeated
The antecedent stimulus, which is called the discriminative stimulus, serves as a cue that signals the probable consequence of an operant response
The Operant Conditioning of Drinking Alcohol
Hebb’s Rule 1949
“ When an axon of cell A . . . excites cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells so that A’s efficiency as one of the cells firing B is increased.”
The Organization of Behavior: A Neuropsychological Theory
Aplysia has about 20,000 neurons in the nervous system consisting of nine ganglia -- four pairs of symmetrical ganglia and one large abdominal ganglion consisting of two lobes
Habituation Involves an Activity-Dependent Presynaptic Depression of Synaptic Transmission
Sensitization is produced by applying a noxious stimulus to the tail of the Aplysia's tail, activated sensory neuron 2. This, in turn activates a facilitating interneuron that enhances transmission in the pathway from the siphon to the motor neuron.
Short-term sensitization of the gill-withdrawal reflex in Aplysia involves presynaptic facilitation
A single shock to the tail triggers the release of the neurotransmitter serotonin at the terminals of the interneuron.
Serotonin binds to receptors in the cell body and terminals of the sensory neuron in the siphon-gill pathway.
These are G-protein-coupled receptors (GPCRs) that
activate adenylyl cyclase which catalyzes the formation of the second messenger, cyclic AMP (cAMP).
The rise in cAMP activates a cAMP-dependent protein kinase ( PKA ) which
increases the release of transmitter at its synaptic connection to the motor neurons (red arrow pointing up).
The result: a longer period of gill-withdrawal in response to a light touch to the siphon
Classical conditioning of the gill-withdrawal reflex in Aplysia
Persistent synaptic enhancement with long-term sensitization.
The level of cAMP in the cell becomes still higher.
Some of the activated PKA moves into the nucleus where it
phosphorylates and thus activates CREB-1 ( c AMP R esponse E lement B inding protein-1) which
binds the cAMP Response Element - a DNA sequence in the promoters of many genes whose transcription and translation produce the proteins needed for
forming new synaptic connections between the sensory and motor neurons in the siphon-gill pathway. (The number may be more than doubled.)
Long-term habituation and sensitization in Aplysia
A. When measured 1 day or 1 week after training, the number of presynaptic terminals is highest in sensitized animals (about 2800) compared with control (1300) and habituated animals (800).
B. Long-term habituation leads to a loss of synapses and long-term sensitization leads to an increase in synapses.
Long-term potentiation (LTP)
A short high-frequency train of stimuli to any of the three major synaptic pathways in the hippocampus increases the amplitude of the excitatory postsynaptic potentials in the target hippocampal neurons
Long-Term Potentiation in the Mossy Fiber Pathway Is Nonassociative
The mossy fiber pathway consists of the axons of the granule cells of the dentate gyrus.
The mossy fiber terminals release glutamate as a transmitter, which binds to both NMDA and non-NMDA receptors on the target pyramidal cells.
However, in this pathway the NMDA receptors have only a minor role in synaptic plasticity under most conditions;
blocking the NMDA receptors has no effect on LTP. Similarly, blocking Ca2+ influx into the postsynaptic pyramidal cells in the CA3 region does not affect LTP
Long-Term Potentiation in the Schaffer Collateral and Perforant Pathways Is Associative
The Schaffer collateral pathway connects the pyramidal cells of the CA3 region of the hippocampus with those of the CA1 region.
Like the mossy fiber terminals, the terminals of the Schaffer collaterals also use glutamate as transmitter, but LTP in the Schaffer collateral pathway requires activation of the NMDA-type of glutamate receptor
Therefore, LTP in CA1 cells has two characteristic features that distinguish it from LTP in the mossy fiber pathway, both of which derive from the known properties of the NMDA receptor
Long-Term Potentiation Has a Transient Early and a Consolidated Late Phase
Mice that lack the NMDA receptor in the CA1 region of the hippocampus have a defect in LTP and in spatial memory
Long-Term Depression (LTD)
Slow, weak electrical stimulation of CA1 neurons also brings about long-term changes in the synapses, in this case, a reduction in their sensitivity. This is called long-term depression or LTD. It reduces the number of AMPA receptors at the synapse. .