Brain Electrical Activity (ERPs) during Memory Encoding and Retrieval

Investigators: C. Trott, D. Friedman, W. Ritter, M. Fabiani, J.G. Snodgrass

Memory Components A general consensus holds that memory can be

broadly categorized into either declarative or non-declarative.

Within each of these broad categories, additional sub-processes have been discovered.

For example, item (content) and source (context) memory within episodic memory.

These may depend upon unique brain systems.

Medial Temporal Lobes (MTL) = item memory; Prefrontal Cortex (PFCx) = source memory.

Memory Tree

(Semantic) (Episodic)

Item Source

(MTL) (PFCx)

Familiarity & Recollection

• Familiarity - relatively automatic (i.e., item or content memory devoid of contextual details).

• Recollection - more effortful, strategic process; retrieval of initial learning context (i.e., source memory).

• Both aspects required for a completely integrated memory.

The Remember/Know Paradigm (Described by Tulving, 1985)

• Know = no contextual details; just a feeling of familiarity with the item.

• Remember = contextual details (e.g., associations).

• Behavioral studies suggest they are equivalent to familiarity and recollection.

• A previous ERP study (Smith, 1993) found no differences during encoding as a function of subsequent retrieval; however, at retrieval, several studies have shown larger ERP episodic memory (EM) effects for items given remember compared to those given know judgments.

• Other than the Smith study, no other encoding investigations exist.

• The current study was performed to assess this issue at both encoding and retrieval.

Study by Trott, Friedman, Ritter, Fabiani, & Snodgrass (1999)

• Used Remember/Know Paradigm.

• Words presented in sentences at study.

• Each sentence contained 2 unassociated nouns.

• Participants asked to memorize the nouns and the list they were presented in.

• At test, subjects (N = 16 young adults) made old/new judgments which were followed by Remember/Know judgments (source judgments also obtained).

• ERPs Recorded during study (encoding) and test phases (retrieval).

Study Phase Instructions for the Example in the Next Slide

• Each list will be demarcated.• Click your mouse to advance the words.• Memorize the nouns and the list they come

from.• Advance to the Test Phase when the last

period is presented.

STUDY PHASE

THE

LIST 1PLOWCREATEDTHEDELAY.LIST 2THELYRICSREFLECTEDHISWIT.

Test Phase Instructions for the Example in the Next Slide

• Words will be sequentially presented in pairs.

• Judge whether they are old or new.

• After, you will be asked to judge whether you “remember” or “know” them.

RECOGNITION TEST

PLOWDOOMPLOW

Remember? Know?

(Old/New then R/K)

LYRICSWITLYRICS

Remember? Know?

WIT

Remember? Know?

ERP Effects at Study• The ERPs at study were averaged as a function of

whether they were subsequently judged “remember” or “know.”

• Only subsequent hits were used to compute these averages.

• They were compared with the ERPs to study items that were subsequently missed.

• The next slide shows the ERPs at study averaged according to subsequent performance at test (Hits vs. Misses; i.e., averaged across ERPs associated with “remember” and “know” judgments).

• The difference between these 2 ERPs has been labeled “Dm”).

Encoding-Related Effects

5 V+

-500 1000

Subsequent Hit

Subsequent Miss

Dm effect (Difference in subsequent memory)

ms

Dm for Remember vs. Know

• The next slide shows the ERPs associated with Hits, categorized into those that were given remember and know judgments.

• The Dm effect for items subsequently associated with remember judgments is reliably larger than that subsequently associated with a know judgment.

ERPs and Subsequent R/K Judgments

5 V

+

-

Dm Effect

Subsequent Remember

Subsequent Know

Subsequent Miss

Remember

Know

500 1000

ms

These same kinds of effects can be observed in the medial temporal lobe, particularly within the hippocampus, as shown in the next slide.

Medial Temporal Lobe and Encoding

Subsequently Recalled

HippocampusCoronal Saggital

Subsequently Unrecalled

-20

+20

V400 2000

-

+

msFrom Fernandez et al., Science, 1999, 285, 1582-1585.

Retrieval-Related ERPs

• Frontally-oriented episodic memory (EM) effect (300-500 ms) Same amplitude associated with remember and

know judgments (familiarity?)

• Parietally-oriented EM effect (300-700) Larger amplitude in association with remember

judgments (recollection?)

• Right prefrontal EM effect Same amplitude associated with remember and

know judgments (post-retrieval monitoring?)

Episodic Memory (EM) Effects

R/K Judgments

5 V+

-500 1500 500 1500

Remember

KnowNew

Posterior Prefrontal

Right prefrontal old/new

Left Parietal

Medial Prefrontal

Left Parietal

Medial Prefrontal

REMEMBER KNOW

Right Prefrontal

ms

+0.1

-0.1

+0.2

-0.2

Similar EM effects have been recorded in the Medial Temporal Lobe, as shown in next slide

Medial Temporal Lobe and Retrieval

Recognition Memory

1st Presentation

2nd

100 V

Amygdala

Anterior Hippocampus

Mid Hippocampus

Posterior Hippocampus

Posterior Hippocampus

Pol

arity

Rev

ersa

l

Coronal Sections Adapted from Johnson, 1995

Adapted from Smith et al., EEG J, 1986, 63, 145-149.

Amnesics do not show EM effect

Parietal EM effect

Intracranial EM effect

Conclusions• ERP activity at study reflects contextual encoding.• Familiarity (Medial prefrontal EM effect) and

recollection (Left parietal EM effect) reflected in ERP waveform at retrieval.

• Right prefrontal EM effect occurs after the old/new decision (as reflected by reraction time) and is likely to reflect post-retrieval monitoring.

• Remember and Know operate at encoding as well as retrieval.

• Each EM effect is likely to reflect the activity of unique brain structures.