Memory Part II Memory Stages and Processes

Neural Basis of Memory

Overview

• Memory processes – encoding, storage, consolidation, and retrieval

• Capacity & duration of memory stages

– sensory memory – short-term memory – long-term memory

• Working memory

• Neural basis of memory

Memory Processes

• Acquisition or “encoding” – turning sensory stimuli into a form that can be stored

in memory • Storage

– retention of information in a memory trace • Consolidation

– Strengthening of existing memories • Retrieval

– bringing stored information into awareness where it can be reviewed/manipulated

Sensory Memory

Decay

Short-term Memory

Response

Rehearsal

Encoding

Retrieval

Long-Term Memory

Attention

Stage theory of memory (Atkinson & Shiffrin, 1968)

Properties of Memory Stages

• Capacity: how much information can be maintained

• Duration: how long information stays there

Sensory Memory

Decay

Short-term Memory

Response

Rehearsal

Encoding

Retrieval

Long-Term Memory

Attention

Stage theory of memory

Sensory Memory • Impressions of sensory information that persist

after the original stimulus has ended

Experiments by Sperling on Sensory Memory

• Presented array consisting of three rows of

four letters for 50msec

• Participants were cued to report part of or whole display

X M R J C N K P V F L B

George Sperling Demo: http://psiexp.ss.uci.edu/research/teaching9B/IconicMemory.pps

Full report condition

• Instruction: report any letter from the array

M N X L R T J S K Y G B

Results: participants report 4 out of 12 letters But… they saw more letters than that

array flashed for 50msec blank screen

T K J G

Example response

Partial Report Condition

• Instruction: report only cued letters from the array

M N X L R T J S K Y G B

Results: participants can report 3.3 of the 4 cued letters (on average)

array flashed for 50msec auditory tone or visual cue indicates which set of letters to report (in this example: top row)

M N X L

Example response

Implication for the Memory Capacity

• Participants did not know which letters would be cued

• By extrapolation, reporting 3.3 of 4 cued letters

corresponds to reporting 10 letters out of 12 letters from whole display

• Participants retain most or all of the items displayed very briefly in a visual sensory register high capacity

Rapid Decay

• Vary length of time between turning off the display and onset of the cue (tone/visual cue)

• Result: memory of display decays rapidly and is gone within one second if not attended

• Similar result for echoic memory (auditory information) – gone within 4 seconds

Remember this array of letters:

Sensory Memory

Decay

Short-term Memory

Response

Rehearsal

Encoding

Retrieval

Long-Term Memory

Attention

Stage theory of memory

Short-term memory

• Short-term memory is a limited capacity store for information -- place to rehearse new information from sensory buffers

• George Miller (1956): capacity of short-term memory is seven plus or minus two items

Measuring Digit Span

• Remember the following digit sequences

(6 digits) 9 6 5 2 8 3

(4 digits) 6 1 9 4

(7 digits) 4 2 6 9 8 5 1

(8 digits) 8 1 6 3 7 2 4 9

(10 digits) 9 3 8 2 4 7 1 5 3 6

Working Memory

• More contemporary concept of short-term retention of information (Baddeley)

• Working memory may be the current memory files we have open or are still working on -- things that we are keeping in mind

• Working memory allows for active manipulation of information

• Coding of verbal material is not in terms of individual letters/numbers but phonetically

Rehearsal of Words

• Verbal information is coded phonetically

• We use our “inner voice” when using silent rehearsal

• Memory span is influenced by pronunciation time

Working Memory Capacity influenced by word length

• Experiment:

• Word length effect – mean number of words recalled in order (list 1 4.2 words; list 2 2.8 words)

LIST 1: Burma Greece Tibet

Iceland Malta Laos

LIST 2: Switzerland Nicaragua

Afghanistan Venezuela Philippines

Madagascar

(Baddeley)

Working memory and Language Differences

• Recall for numbers are different across languages

• E.g. memory for English number sequences is better than Spanish or Arabic sequences

(Naveh-Benjamin & Ayres, 1986)

Growth in digit-span for one subject (S.F.) with practice

Chunking

• Capacity of short-term memory is not fixed by number of digits, but by chunks

• A chunk is a meaningful way to group information

Remember this sequence:

Dividing the sequence into chunks

Video: Chunking Digit Sequences (~2 min)

full video clip available at: http://www.youtube.com/watch?v=6vsYCSmBcM0 (relevant section starts at 3:50)

Chase & Simon (1973)

Number of pieces successfully recalled by chess players after the first study of a chess board.

(Chase & Simon, 1973)

(Chase & Simon, 1973)

Conclusion from Chase & Simon (1973)

• Chess masters only expert with real chess positions. They do not have better memory in general

• Expertise allows chunking of salient information to

promote memory of good moves

Sensory Memory

Decay

Short-term Memory

Response

Rehearsal

Encoding

Retrieval

Long-Term Memory

Attention

Role of Rehearsal

Items need to be rehearsed in short-term memory to entering long-term memory

a memory test...

TABLE CANDLE MAPLE SUBWAY PENCIL COFFEE TOWEL SOFTBALL CURTAIN PLAYER KITTEN DOORKNOB FOLDER CONCRETE RAILROAD DOCTOR SUNSHINE LETTER TURKEY HAMMER

Serial Position Effects

• In free recall, more items are recalled from start of list (primacy effect) and end of the list (recency effect)

• Distractor task (e.g. counting) after last item removes recency effect

distractor task

no distractor

task

Explaining Serial Position Effects

• Explanation from stage theory of memory

– primacy effect: early items can be rehearsed more often: more likely to be transferred to long-term memory

– recency effect: last items of list are still in short-term memory (with no distractor task)

Neural Basis of Memory

• Two general questions: – Where are memories stored? – How are memories formed?

Types of Representations

Localist

Distributed

Attempts to localize memories

• Karl Lashley searched for a localized memory trace or engram

• Believed that memory was localized – specific memory stored in a specific area.

• Removed parts of rat’s cerebral cortex but found no one area contained the memory of the maze

• Found that maze-learning in rats was distributed throughout the brain (equipotentiality)

Memories’ Physical Location • Memory is distributed among different brain regions. Memory does

not “live” in one part of the brain.

• Memory involves multiple regions of the brain, but not all brain regions are equally involved. A great deal of neural specialization occurs.

• The middle section of the temporal lobes, called the medial temporal lobes, is responsible for the formation of new memories.

• The actual storage occurs in the particular brain regions engaged during the perception, processing, and analysis of the material being learned.

Synaptic Plasticity

• Changes in neural connections responsible for learning and memory

• Donald O. Hebb: “Neurons that fire together, wire together”

Neural connections between simultaneously active neurons are strengthened

Existing neural connections can help to complete patterns of activation (“memory retrieval”)

Long Term Potentiation (LTP)

• Biological basis for Hebbian learning

• LTP is an experimentally observed increase in synaptic strength that lasts for hours or days

How about improving LTP?

• NMDA receptor is involved in LTP

• Joe Tsien (Tang et al. 1999) and colleagues bred mice with extra NMDA receptors

• These transgenic mice performed better on memory tasks (e.g. Morris water maze)

• Evidence from Delayed Match to Sample Tasks – Correct response requires keeping location of

food in mind.

• Monkeys and humans w/lesions of PFC fail these tasks.

Prefrontal Cortex is needed for Working Memory

Overview

• Memory processes – encoding, storage, consolidation and retrieval

• Information-processing models of memory

– sensory memory – short-term memory – long-term memory

• Working memory

• Neural basis of memory