lecture 25: learning and memory 1

types of memories

skills, episodes, semantic knowledge/facts

memory storage is distributed

• karl lashley lesioned parts of the cortex and measured the impact on memory in mice

• found that a lesion in a single part of the cortex didn’t completely abolish memory, but the more tissue that he lesioned, the more memory was impaired

• conclusion: memories aren’t localized to one part of the cortex, but dist throughout; mechanism where these regions work together to support storage + formation of memory

declarative memory

things you know that you can tell others (explicit) 

• episodic (remembering 1st day of school) - hippocampus

• semantic (facts) - cortex

nondeclarative (procedural) memory

things you know that you can show by doing (implicit) 

• skill learning

• priming (being more likely to use a word you recently heard) 

• conditioning - classical vs pavlovian

cerebellum/BG/cortex

classical conditioning

associating a stimulus with an outcome

• starbucks = coffee

• skunk = smell of a skunk

instrumental conditioning (learning)

associating a particular action with an outcome, OR a stimulus with an action associated with an outcome

• crying kids = no candy

• say please = candy!

semantic memory

• facts!

• associating the word CAKE with the perceptual features of a cake

• cortex!

episodic memory

• hippocampus

• remembering a sequence of events that happened to you

• ex: open presents, playing games, eating cake; IN ORDER!

hippocampus

• deep brain structure

• located close to the temporal lobe, has a horseshoe shape

• next to the amygdala (emotions), mems + emotions closely linked

famous case of HM

• lesion localized to the hippocampus + parts of the medial temporal lobe

• suffered from terrible epileptic seizures, went through surgery to alleviate them where large parts of MTL and hippocampus were removed

• after, seizures stopped, but HM could no longer form new long-term memories (anterograde amnesia)

• little “retrograde amnesia”: memories from before the surgery were largely intact

lesion to HM’s brain

• provided some of the first evidence that the hippocampus and MTL was critical for episodic memory consolidation, but not for all memory storage

• procedural (skill + associative) learning was still intact. HM could learn new skills and action strategies implicitly/subconsciously

  • illustrated that deficits didn’t include the formation of new procedural memories

• deficit didn’t apply to ALL new memories; he would have to do tasks, didn’t remember these, but would get better at them each day

• evidence that there are multiple memory systems in the brain

alzheimer’s disease

• degrades the ability to form and retain recent long-term memories

• involves the degeneration of cells in the hippocampus, and pathology spreads to MTL & affects cortex

• initial pathology is localized in the hippocampus, which is why primary deficit is the inability to form long-term memory

place cells in the hippocampus

• john o’keefe stuck electrodes in the hippocampus of rodents and recorded APs of those neurons

• each dot is AP of a single neuron, different colors indicate diff neurons

• found that as the animal ran down the maze, there were different cells that liked to be active at each location that the animal experienced

• collectively, place cells in the hippocampus represent a “cognitive map” of the environment

properties of place cells

• color indicates amount of firing; each pixel is a location in space; each row is a single neuron

• place cells can maintain firing fields in complete darkness!

• place cells are not just representing visual space, but location within a context

  • ex: have place cells firing at each location as you move through a room → map in your brain of everywhere you’ve been

place cells change their firing in different environments

what does it take to get place cells to change their firing?

• scale change: changes the scale of representation; cells will fire at the same locations relative to each other, but the scale will change

• small change/rate remapping: change color of the box, cause cells to fire in same location, but at diff rates

• large change/global remapping: completely swap out all features of the box, put it in a diff room w/ diff sensory cues; position that the neurons fire in completely change

grid cells

• located in entorhinal cortex, connected to hippocampus

• these cells fire regularly in locations that are spaced at particular intervals

• firing forms the vertices of a grid

• grid cell inputs to hippocampus may be involved in forming place cells

associative properties of the hippocampus

• recorded neurons in the hippocampus of an animal who was receiving tones played at diff frequencies in a certain order

• this order would indicate how close an animal was to getting a reward

• they saw that cells in the hippocampus that fired for particular frequencies in that sequence; those cells tiled the entire sequence of tones from the start to end

• each neuron responded to a different sound frequency

hypothesis of the hippocampus in episodic memory

• hippocampus and MTL are involved in making “associative maps,” allowing you to perceive and process sequential events

• sequential activation of different components of those maps can constitute an episodic experience

hippocampus contribution to episodic memory formation

• receives info from sensory environment, reformats it in a way that allows these sequences to be expressed (encoding from sensory organs to hippocampus)

• sends outputs which allow it to broadcast its message (consolidation from hippocampus to cerebral cortex); turning experience into a memory!

• hippocampus plays a role in “consolidating” episodic events into long term memory

memory consolidation

• when an animal runs down the track, place cells will fire in their particular order

• when they get to the end + get reward, those same cells that were active replay in the animal’s brain in the reverse order

• replay happens way faster than when it’s going down the path

• reverse replay event is the broadcast signal! aka what the hippocampus is telling the other brain regions what it just experienced/episode and to remember it

reverse replay occurs during sleep

• when animals fall asleep, there are replay events happening during certain phases of sleep

• they occur in the same order when the animal was doing the behavior

PTSD

• persistent activation of certain traumatic memories

• remembering things you don’t want to remember

manipulating specific memories

• how to mess with these circuits in the hippocampus that allows you to alter traumatic memories

• using genetic techniques to target neurons that are activated during the encoding/retrieval phases of memory

• goal is to erase pathological memories and/or “implanting” new memories