Learning and Memory Chapter 13

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The Nature of Learning

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Classical conditioning is a form of learning in which an unimportant stimulus acquires

the properties of an important one. It involves an association between two stimuli. A stimulus that previously had little effect on behavior becomes able to evoke a reflexive, species-typical behavior. •

classical conditioning

• A learning procedure; when a stimulus that initially produces no particular response is followed several times by an unconditional stimulus (US) that produces a defensive or appetitive response (the unconditional response—UR), the first stimulus (now called a conditional stimulus—CS) itself evokes the response (now called a

conditional response—CR).

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The Nature of Learning

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Hebb rule

• The hypothesis proposed by Donald Hebb that the cellular basis of learning involves strengthening of a synapse that is repeatedly active when the postsynaptic neuron fires.

• When Hebb formulated his rule, he was unable to determine whether it was true or false. • Now, finally, enough progress has been made in laboratory techniques that the strength of individual synapses can be determined, and investigators are studying the physiological bases of learning. 4

The Nature of Learning

• The second major class of stimulus-response learning is instrumental conditioning (also called operant conditioning).

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instrumental conditioning

• A learning procedure whereby the effects of a particular behavior in a particular situation increase (reinforce) or decrease (punish) the probability of the behavior; also called operant conditioning. • Whereas classical conditioning involves automatic, species-typical responses, instrumental conditioning involves behaviors that have been learned.

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The Nature of Learning

• Collectively, “ favorable consequences “ unfavorable consequences ” ” are referred to as reinforcing stimuli, and are referred to as punishing stimuli. • For example, a response that enables a hungry organism to find food will be reinforced, and a response that causes pain will be punished. (Psychologists often refer to these terms as reinforcers and punishers.) •

reinforcing stimulus

• An appetitive stimulus that follows a particular behavior and thus makes the behavior become more frequent.

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punishing stimulus

• An aversive stimulus that follows a particular behavior and thus makes the behavior become less frequent.

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Synaptic Plasticity: Long Term Potentiation and Long-Term Depression

• Induction of Long-Term Potentiation • Electrical stimulation of circuits within the hippocampal formation can lead to long-term synaptic changes that seem to be among those responsible for learning.

• Lømo (1966) discovered that intense electrical stimulation of axons leading from the entorhinal cortex to the dentate gyrus caused a long-term increase in the magnitude of excitatory postsynaptic potentials in the postsynaptic neurons; this increase has come to be called long-term potentiation (LTP). (The word potentiate means “ to strengthen, to make more potent.

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long-term potentiation (LTP)

• A long-term increase in the excitability of a neuron to a particular synaptic input caused by repeated high-frequency activity of that input.

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Synaptic Plasticity: Long Term Potentiation and Long-Term Depression

• Role of NMDA Receptors •

AP5

• 2-Amino-5-phosphonopentanoate, a drug that blocks NMDA receptors.

• These results indicate that the activation of NMDA receptors is necessary for the first step in the process events that establishes LTP: the entry of calcium ions into dendritic spines. 15

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Synaptic Plasticity: Long Term Potentiation and Long-Term Depression

• Long-Term Depression • I mentioned earlier that low-frequency stimulation of the synaptic inputs to a cell can decrease rather than increase their strength.

• This phenomenon, known as long-term depression (LTD), also plays a role in learning. •

long-term depression (LTD)

• A long-term decrease in the excitability of a neuron to a particular synaptic input caused by stimulation of the terminal button while the postsynaptic membrane is hyperpolarized or only slightly depolarized.

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Synaptic Plasticity: Long Term Potentiation and Long-Term Depression

• Long-Term Depression • Apparently, neural circuits that contain memories are established by strengthening some synapses and weakening others.

• Dudek and Bear (1992) stimulated Schaffer collateral inputs to CA1 neurons in hippocampal slices with 900 pulses of electrical current, delivered at rates ranging from 1 to 50 Hz.

• They found that frequencies above 10 Hz caused long-term potentiation, whereas those below 10 Hz caused long-term depression. Both of these effects were blocked by application of AP5, the NMDA receptor blocker; thus, both effects require the activation of NMDA receptors. (See Figure 13.15.) 19

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Perceptual Learning

• Learning to Recognize Stimuli • In mammals with large and complex brains, objects are recognized visually by circuits of neurons in the visual association cortex.

• Visual learning can take place very rapidly, and the number of items that can be remembered is enormous.

• In fact, Standing (1973) showed people 10,000 color slides and found that they could recognize most of the slides weeks later.

• Other primates are capable of remembering items that they have seen for just a few seconds, and the experience changes the responses of neurons in their visual association cortex (Rolls, 1995b).

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Perceptual Learning

• Perceptual Short-Term Memory •

delayed matching-to-sample task

• A task that requires the subject to indicate which of several stimuli has just been perceived.

• Ranganath and his colleagues found that short-term memories of faces activated the fusiform face area and that short-term memories of places activated the parahippocampal place area. 23

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Instrumental Conditioning

• Basal Ganglia • Evidence suggests that as learned behaviors become automatic and routine, they are “ transferred ” to the basal ganglia.

• The process seems to work like this: As we deliberately perform a complex behavior, the basal ganglia receive information about the stimuli that are present and the responses we are making.

• At first the basal ganglia are passive “ observers ” of the situation, but as the behaviors are repeated again and again, the basal ganglia begin to learn what to do. • Eventually, they take over most of the details of the process, leaving the transcortical circuits free to do something else. We need no longer think about what we are doing. 25

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Instrumental Conditioning

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Neural Circuits Involved in Reinforcement

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ventral tegmental area (VTA)

• A group of dopaminergic neurons in the ventral midbrain whose axons form the mesolimbic and mesocortical systems; plays a critical role in reinforcement.

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nucleus accumbens

• A nucleus of the basal forebrain near the septum; receives dopamine-secreting terminal buttons from neurons of the ventral tegmental area and is thought to be involved in reinforcement and attention.

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Instrumental Conditioning

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Section Summary: Instrumental Conditioning

• Olds and Milner discovered that rats would perform a response that caused electrical current to be delivered through an electrode placed in their brain; thus, the stimulation was reinforcing.

• Subsequent studies found that stimulation of many locations had reinforcing effects but that the medial forebrain bundle produced the strongest and most reliable ones.

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Relational Learning

• Human Anterograde Amnesia • In 1889, Sergei Korsakoff, a Russian physician, first described a severe memory impairment caused by brain damage, and the disorder was given his name.

• The most profound symptom of Korsakoff remember old ones.

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s syndrome is a severe anterograde

amnesia: The patients appear to be unable to form new memories, although they can still •

Korsakoff

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s syndrome

• Permanent anterograde amnesia caused by brain damage resulting from chronic alcoholism or malnutrition.

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Relational Learning

• Declarative and Nondeclarative Memories • For example, some investigators (Eichenbaum, Otto, and Cohen, 1992; Squire, 1992) suggest that patients with anterograde amnesia are unable to form declarative

memories, which have been defined as those that are

“ explicitly available to conscious recollection as facts, events, or specific stimuli 218).

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declarative memory

• Memory that can be verbally expressed, such as memory for events in a person's past.

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Relational Learning

• Declarative and Nondeclarative Memories • The other category of memories, often called nondeclarative memories, includes instances of perceptual, stimulus-response, and motor learning that we are not necessarily conscious of. (Some psychologists refer to these two categories as explicit and implicit memories, respectively.) •

nondeclarative memory

• Memory whose formation does not depend on the hippocampal formation; a collective term for perceptual, stimulus-response, and motor memory.

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Relational Learning

• Anatomy of Anterograde Amnesia • The outputs of the hippocampal system come primarily from field CA1 and the subiculum. • Most of these outputs are relayed back through the entorhinal, perirhinal, and parahippocampal cortex to the same regions of association cortex that provide inputs. • The hippocampal formation also receives input from subcortical regions via the fornix. These inputs select and modulate the functions of the hippocampal formation. 36

Relational Learning

• Anatomy of Anterograde Amnesia • The fornix carries dopaminergic axons from the ventral tegmental area, noradrenergic axons from the locus coeruleus, serotonergic axons from the raphe nuclei, and acetylcholinergic axons from the medial septum.

• The fornix also connects the hippocampal formation with the mammillary bodies, located in the posterior hypothalamus.

• The most prominent brain damage seen in cases of Korsakoff bodies. (See Figure 13.28.) ’ s syndrome—and presumably the cause of the anterograde amnesia—is degeneration of the mammillary 37

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Relational Learning

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Place Cells in the Hippocampal Formation

• A particular neuron might fire twenty times per second when the animal was in a particular location but only a few times per hour when the animal was located elsewhere.

• For obvious reasons these neurons were named place cells. •

place cell

• A neuron that becomes active when the animal is in a particular location in the environment; most typically found in the hippocampal formation.

• The red spots indicate the firing of a single hippocampal place cell that, as you can see, fired primarily when the rat was in a particular location (Derdikman and Moser, 2010). (See Figure 13.34.) 42

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Relational Learning

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Reconsolidation of Memories

• Memories can be altered or connected to newer memories.

• In recent years, researchers have been investigating a phenomenon known as

reconsolidation, which appears to involve modification of long-term memories.

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reconsolidation

• A process of consolidation of a memory that occurs subsequent to the original consolidation that can be triggered by a reminder of the original stimulus; thought to provide the means for modifying existing memories.

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