learning / memory study guide

Difference Between Habituation and Sensitization:

• Habituation: a decrease in response to a repeated, non-threatening, or irrelevant stimulus. 

  • The organism learns to ignore the stimulus because it is not significant. 

  • Reduces attention to unimportant stimuli.

• Sensitization: An increase in response to a stimulus, often due to it being intense, harmful, or arousing.

  • The organism becomes more alert or reactive after exposure to a strong or noxious stimulus.

  • Enhances response to potential threats.

Kandel’s Three Hypotheses:

1. Sensory neurons become less responsive.

   • The sensory neurons stop firing as strongly after repeated stimulation. 

2. Motor neurons become fatigued.

   • The motor neurons that control the gill reflex eventually get tired and stop responding.

3. Changes occur between the sensory and motor neurons. 

   • The weakening of the synapse between sensory and motor neurons. 

Which Hypothesis Is Correct?:

• Changes are occurring between the sensory and motor neurons. 

• Normally, sensory neurons depolarize as an influx of calcium enters the presynaptic terminal, triggering the release of glutamate into the synapse.

• After repeated stimulation, fewer calcium channels open due to inactivation. 

  • Therefore, less calcium is released in the sensory neuron, causing less glutamate release into the synapse. 

• With less glutamate, ESPSs become smaller and weaker.

  • Therefore, motor neurons will fire less strongly. 

Neural Mechanism for Habituation/Sensitization in Aplysia:

• Aplysia is a good animal to study because it has fewer, but larger, neurons. 

• The gill-withdrawal reflex explains:

  • The Aplysia’s gill is used for breathing and is covered with a mantle shelf, a protective tissue. 

  • Waste and seawater are released through the siphon, a tube-like structure.

  • Touching the siphon causes the gill to retract. 

• Touching the siphon repeatedly leads to a less pronounced reflex or retraction of the gill. 

• After an electrical shock to the head or tail, an Aplysia will show an increased gill-withdrawal response.

  • Sensory neurons release more neurotransmitters onto the interneurons and motor neurons, leading to a more intense response. 

What Are The Long-Term Changes?:

• Typically, Aplysias show 1300 axon terminals synapsed with sensory neurons.

  • However, Aplysias that have experienced sensitization had 2800 terminals.

  • Aplysias that have experienced habituation have 800 terminals. 

Long-Term Potentiation:

• The long-lasting increase in synaptic strength.

• Thought to be associated with memory formation. 

• LTP makes signal transmission faster and stronger.

• NMDA and AMPA are receptors for glutamate.

• Typically, only NMDA receptors are available to depolarize the postsynaptic membrane.

  • When the connection is strengthened, AMPA receptors are added to the membrane.

  • More AMPA receptors allow for a larger response to glutamate.

  • More AMPA receptors make it easier to reach depolarization (reaching threshold faster). 

  • This is the cellular basis for LTP.

Long-Term Depression:

• The long-lasting decrease in synaptic strength that lasts hours or days.

• Thought to be associated with learning. 

Evidence For LTP As A Mechanism For Long-Term Memory:

• Both LTP and memories can last indefinitely.

• Both LTP and memories can result from very brief input or events.

• LTP is consistent with models proposed by Donald Hebb.

• LTP is found in cells thought to be associated with memory formation, such as those in the hippocampus and cerebellum.

• NMDA receptors (linked to LTP) are also linked to memory.

Amnesias:

• Retrograde amnesia: loss of memories from the past.

• Anterograde amnesia: inability to form new memories.

Patient H.M.:

• Large areas of H.M.’s hippocampi and temporal lobes were surgically removed. 

• His personality and IQ were not impacted.

• However, he experienced profound retrograde amnesia as well as anterograde amnesia.

• His short-term memory was ok.

  • “Working memory”

  • Could hold information for a very short amount of time when undistracted.

• He could still learn procedural tasks.

  • Mirror drawing. He improved with practice despite not remembering ever doing it before. 

• His deficits were in explicit memory tasks.

• Concluded that the ability to store memories and the ability to access previously stored memories are in different brain areas.

• Procedural memories are dealt with in different brain areas than explicit memories. 

Thalamus:

• Korsakoff’s Syndrome:

  • Damage to the thalamus and the mammillary bodies

  • Caused by a thiamine deficiency due to alcoholism. 

• Patient N.A.

  • A fencing foil went up his nose and caused a lesion to their left thalamus.

  • He had similar amnesia to H.M.

• Delayed Non-Matching to Sample Task:

  • Monkeys with temporal lobe or thalamus damage could not do the DNMS task.

  • Requires the ability to transfer memories from short-term systems into long-term systems. 

Hippocampus:

• The right hippocampus is active during spatial memory processing.

  • May include a 3D representation of the world around us.

• The left hippocampus is more active during verbal memory tasks.

Prefrontal Cortex:

• Patients with prefrontal cortex damage have issues with the Wisconsin card-sorting task.

  • Short-term memory issues.

• Object permanence studies:

  • Adult monkeys with prefrontal damage don’t seem to grasp object permanence.

  • Prefrontal cortex is constantly developing throughout youth and childhood.

  • Object permanence usually develops in the first year of life. 

Basal Ganglia:

• Lesions of the basal ganglia impact procedural memories, but not explicit memories.

  • The opposite of H.M.

• Diseases that lead to damage to the basal ganglia typically also have procedural memory deficits.

  • Huntington’s and Parkinson’s Disease

Cerebellum:

• Possibly key to procedural memories.

  • Motor learning

• It is unclear and not currently agreed upon whether damage to the cerebellum leads to deficits in motor learning or simply the performance of that learned task.

Papez’s Circuit:

• A network that connects the thalamus, mammillary bodies, and hippocampus.

  • Believed to be key to forming long memories. 

Stress and Memory:

• Stress impacts the amygdala.

  • Has connections to the hippocampus.

• Cortisol may damage the hippocampus.

  • Repression

  • Flashbacks/flashbulb memories