Learning and Memory
What is learning and memory?
Learning: changes in the brain as a result of experience
Facts, events, skills, and behavior patterns
Memory: ability to store and retrieve the changes
Different types of memory and learning
Short-term: seconds → hours (<24)
Limited capacity
Long-term: ~24 hours → ??
Virtually unlimited capacity
Working memory: information that is temporarily stored while we using it
Requires focused attention
Patient H.M. – know his case and where damage is
27 years old in 1953
Severe epilepsy following a childhood head injury
Seizures arouse from the medial parts of H.M. temporal lobes
H.M. underwent a bilateral medial temporal lobectomy to relieve his symptoms
Removed most of hippocampus, amygdala, adjacent cortex
Results:
Seizure almost completely eliminated
Devastating amnesia (pathological memory loss)
Things he could do vs things he could not
Explicit memory impaired
Remote memory intact
Recent episodic memories were impaired
Could not form new semantic or episodic memories
Could not perform explicit memory task (digital span and block-tapping task)
Implicit memory intact
Could learn new movement patterns, motor skills, and classical condition
New implicit memories retained
Perform well on implicit tasks (mirror tracing and incomplete pictures task)
Medial Temporal Lobes – functions, how we test
Hippocampus
Greek for seahorse
Evidence from humans and animals points to role in spatial memory
Memory for places, locations of objects, spatial navigation abilities
Tests;
fMRI studies indicated increased hippocampal activation when people perform spatial tasks
London taxi driver study (2000)
experienced drivers have larger posterior hippocampi than non-drivers
Size was positively correlated with number of years on the job
Follow up study (2011) on cabbies in training
Before training: no difference in hippocampal volume
After training: 3 groups
Trainees who passed (39)
Trainees who failed (32)
Controls (31)
Rhinal cortex (tissue surrounding the hippocampus)
Perihinal cortex, entorhinal cortex, parahippocampal cortex
Important input to hippocampus for spatial memory and object recognition memory
Test:
DNMS Task/Mumby Box
Novel Objection Recognition Test
Amygdala
Strengthens emotionally significant memories stored by other areas
Place cells and grid cells
Place cells are many hippocampal neurons
Fire action potentials during navigation of a familiar environment
Different groups of place cells fire in different locations
Grid cells found in the entorhinal cortex
Fire at regular intervals as animals navigates an open area
Hexagonal “firing fields” help the brain form an internal map of coordinates
Tests learning and memory in other animals
Hippocampus:
Morris water maze: rats learn to find a hidden platform in a murky pool of water
Visual cues around the room signal location or platform
Starting point is changed in each trial
Radial arm maze: rats learn to remember which arms contain food
Must learn not to visit arms without food or revisit arms where they got the food
Rodents with hippocampal lesions perform poorly on both tasks
Rhinal Cortex:
DNSMS Task/Mumby box:
DNMS = delayed nonmatching to sample
Used in nonhuman primates
Monkey shown an object → delay → choice of 2 objects
Reward is underneath the unfamiliar object
Adapted for rats using the Mumby box
Animals with rhinal cortex lesions are severely impaired
Novel Object Recognition Test
Rodent task that has largely replaced DNMS
Phase 1: Animal placed in area with 2 identical objects
Delay: 45 min ~ 1 hr or more
Phase 2: animal placed in area with 1 object from Trail 1 (familiar) and 1 new object (novel)
“NOR score” = time spent exploring novel - time spent exploring familiar
Perianal cortex lesions impair performance
Areas other than MTL
Inferotemporal cortex
Ventral portion of temporal cortex (outer surface of temporal lobe)
Stores visual memories
Fusiform gyrus: important for facial recognition
PFC and working memory
Most rostral (anterior) portion of the frontal lobe
Important for working memory
Delayed response task:
Present a stimulus → delay → make response
PFC neurons are active during the delay
Striatum
Part of the basal ganglia
Caudate nucleus and putamen
Formation and storage of memories about relationship between stimuli and responses
T maze used to test habit formation in rodents
Cerebellum
“Little brain” in the metencephalon of the hindbrain
Simple forms of associative motor learning
Eyeblink conditioning:
air puff (US) → blink
air puff (US) + tone (CS)
tone (CS) → blink (CR)
Neural plasticity in cerebellum linked to motor skill learning
Hebb’s Theory
Axon of neuron A repeatedly stimulates receptors of dendrites of neuron B
In the future, A is more likely to excite B because the connection between A and B has strengthened
“Cells that fire together wire together”
Memories are stored in cell assemblies (networks of connected neurons)
LTP and glutamate – NMDA receptors
LTP shown to depend on changed at synapses that release glutamate
Activation of NMDA-type glutamate receptors is critical
NMDA receptors cause other proteins to be activated, leading to maintenance of LTP
“Doogie” mice – what is special about them
transgenic mice born with higher numbers of NMDA receptors
Superior learners at a young age
Alzheimer’s
Protein tangles – what are they? Why are they controversial in treatment?
Amyloid plaques surround and destroy neurons
Tau tangles form inside neurons and destroy their internal structure
Progression in the brain – why is it so hard to treat?
Pathology starts in MTL, spreads throughout cortex
Symptoms don’t appear until many years after plaques/tangles start forming
Deficits
Deficits in implicit memory for verbal and perceptual material
Risk and protective factors
Genetic factors
Non-genetic factors
Environment and aging