Daniel L. Schacter

Harvard University

Episodic Simulation of Future Events

and the Medial Temporal Lobe

Banbury Center, April 13, 2009

Amnesic Patient KC: No Past, No Future

Tulving, Canadian Psychology (1985)

* Amnesic patients: Difficulties imagining personal futures/novel scenes(Tulving, 1985; Klein, Loftus, & Kihlstrom, 2002; Hassabis et al., 2007)

* Depressed patients/older adlts: Reduction in episodic specificity of past events and future events significantly correlated (Williams et al., 1996; Addis, Wong, & Schacter, 2008)

* Cognitive studies: Manipulations/individual differences similarly influence past and future events

(D’Argembeau & van der Linden, 2004; 2006; Spreng & Levine, 2006)

• Neuroimaging: Similar areas active when remembering past and imagining future (Addis, Wong, & Schacter, 2007; Hassabis et al., 2007; Okuda et al.,

2003; Szpunar, Watson, & McDermott, 2007)

Commonalities between Past and Future Events

Reviewed by Schacter, Addis, & Buckner, Nature Reviews Neuroscience (2007);Year in Cognitive Neuroscience, Annals of the NY Academy of Sciences (2008)

Core Network of Regions Involved in Remembering thePast, Imagining the Future, & Related Forms of Mental Simulation

Schacter, Addis, & Buckner, Nature Reviews Neuroscience (2007)

– Episodic memory involves constructive processes: *Details are stored as fragments in cortical regions; during retrieval they

are reactivated by these cortices and reintegrated by hippocampus into a coherent event.

– The constructive nature of episodic memory makes it well-suited to building simulations of possible future events: *Enables one to extract details from past events

*Enables the flexible recombination of details from past events into coherent future scenarios – which likely relies heavily on relational processing capacities supported by the hippocampal region.

– Though well-suited to simulating future events, constructive nature of episodic memory has a cost:

*Miscombination of details can result in memory errors such as false recognition.

Constructive Episodic Simulation Hypothesis (Schacter & Addis, 2007, Nature; Phil.Trans. Royal Society B)

• We used event-related fMRI to examine the neural substrates of past and future event construction and elaboration; focus on everyday autobiographical events

• Instructions (14 young adult participants):–Cued to recall past event or imagine future event–Future events should be novel and plausible–Three time periods for both past and future:last/next week, last/next year, last/next 5-20 yrs.

Subjects describe events in post-scan interview

Addis, Wong, & Schacter (Neuropsychologia, 2007)

Past & Future: A Neuroimaging Approach

Past and Future: Common Neural Substrates?

CUECUE CONSTRUCTIONCONSTRUCTION ELABORATIONELABORATION

PAST event

Last 5-20 yrs

CAR

FUTURE event

Next year

DRESS

time-period

task

cue

0

• Event cue screen presented for 20 s

• Button press made when event in mind – signifies end of construction & beginning of elaboration

•24 past and 24 future event task trials

20 time

OR

2 RT

RATINGSRATINGS

•3 scales, shown consecutively each for 5 s

35

Detail

1 2 3 4 5

Emotion

1 2 3 4 5

Perspective

field/observer

CUECUE CONSTRUCTIONCONSTRUCTION ELABORATIONELABORATION

items to generate

task

cue

0

• Event cue screen presented for 20 s

• Button press made when event in mind – signifies end of construction & beginning of elaboration

•24 semantic and 24 visuospatial task trials

20 time

OR

2 RT

RATINGSRATINGS

•3 scales, shown consecutively each for 5 s

35

Detail

1 2 3 4 5

Relatedness

1 2 3 4 5

Difficulty

easy / difficult

WORDS-sentence

2 related words

CABLE

OBJECTS- triangle

bigger / smaller

LEAF

Past and Future: Control Tasks

CUE CONSTRUCTIONCONSTRUCTION ELABORATION

FUSIFORM & R. MIDDLE OCCIPITAL L. HIPPOCAMPUS

Cue processing / object recognition Initial retrieval

past events future events control tasks

OVERLAP:

Past and Future Events: Common Neural Substrates?

Construction: Neural differentiation

CUE CONSTRUCTIONCONSTRUCTION ELABORATION

future > past

R. FRONTAL POLE (BA 10)

Novelty / Recombining details

R. HIPPOCAMPUS

Novelty encoding?

Recombining details

to form specific episodes ?

past events future events control tasks

CUE CONSTRUCTION ELABORATIONELABORATION

L. FRONTAL POLE L. HIPPOCAMPUS

Self referential processing Reintegrate/recombine event details

L. TEMPORAL POLE MEDIAL PARIETAL / PARAHIPPOCAMPUS

Conceptual details Imagery / context

past events future events control tasks

OVERLAP:

Past and Future Events: Common Neural Substrates?

Are hippocampal responses to detail similar for past and future events?

Constructive Episodic Simulation Hypothesis:

– Past events: reintegration of relevant event details– Future events: recombination of various details into novel event

Past and Future Detail

Background

RECALL PAST

Last 5-20 yrs

CAR

IMAGINE FUTURE

Next year

DRESS

0 2 20 timeRT 25

RATE DETAIL

1 2 3 4 5

RATE DETAIL

1 2 3 4 5

– Past & future detail did not differ on average

– Parametric modulation: what regions vary with amount of detail?

Past and Future Detail

Method

CUE CONSTRUCTION ELABORATIONELABORATION RATE DETAIL

Posterior HC activity correlates with past AND future detail:

- Retrieval of details from past events?

More anterior HC activity correlates with future detail

- Flexible recombination of details from past events? Cf. Preston et al. (2004)

Past and Future Detail

Addis and Schacter (2008, Hippocampus)

Hippocampal Response to Recombined Details

Preston et al. (2004): Recombined details engage anterior hippocampus

Constructive Episodic Simulation: Two Conceptual Issues

1. Comparison has focused on remembering the past vs. imagining the future, but past/future confounded with remembering/imagining:

Are observed patterns specific to imagining future events orassociated with more general imagination/simulation?

According to constructive episodic simulation hypothesis,critical process of recombining event details should occurregardless of whether individuals imagine an event as occurringin the future, present, or past.

Constructive Episodic Simulation: Two Conceptual Issues

2. Constructive episodic simulation hypothesis emphasizes recombining of details across events, but it is possible thatsubjects simply remember entire events and recast themin the future.

In previous studies, future simulations could be based on recasting, recombining, or some combination of the two.

Are main effects still observed when individuals are requiredto recombine elements of different episodes?

fMRI Paradigm: Experimental Recombination of Details

Fall outside library

PERSON: Katie

LOCATION: Widener

OBJECT: Hat

Graduation Day

PERSON: Mom

LOCATION: Harvard Yard

OBJECT: Gown

Meeting Cathy

PERSON: Cathy

LOCATION: Filipes

OBJECT: Fajita

MEMORY POOL

Imagine PAST event:

Mom: Graduation Day

Filipes: Meeting Cathy

Gown: Graduation Day

IMAGINE PAST

RECALL TASK

RECALL memories:

Cathy: Meeting Cathy

Widener: Fall outside library

Hat: Fall outside library

Imagine FUTURE event:

Katie: Fall outside library

Harvard Yard: Graduation Day

Fajita: Meeting Cathy

IMAGINE FUTURE

CONTROL TASKCREATE SENTENCE

Start with smallest:

Cat: Include cat in sentence

Bulb: Include bulb in sentence

Tree: Include tree in sentence

fMRI Paradigm:

CUE CONSTRUCTION ELABORATION

Imagine FUTURE event :

Katie: Fall outside library

Harvard Yard: Graduation

Fajita: Meeting Cathy

Button press when event is in mind

0 2 24 timeRT 25

RATE DETAIL

RATE DETAIL

1 2 3 4 5

RATE DETAIL

1 2 3 4 5

Imagine PAST event :

Mom: Graduation Day

Filipes: Meeting Cathy

Gown: Graduation Day

Recall MEMORIES:

Cathy: Meeting Cathy

Widener: Fall outside library

Hat: Fall outside library

RATE DETAIL

1 2 3 4 5

Experimental Recombination Task

Common Core Network

Addis, Pan, Vu, Laiser, & Schacter (in press, Neuropsychologia)

Future-Imagine, Past-Imagine & Past-Recall > Control Task

Lateral temporal lobeBilateral hippocampus

CuneusRetrosplenial / precuneusMedial prefrontal / frontal poles

Lateral parietal lobeRetrosplenial / precuneus

Experimental Recombination Task

Imagining Subsystem?

Future-Imagine and Past-Imagine > Control Task

Lateral temporal lobeBilateral hippocampus

Retrosplenial / precuneusMedial prefrontal / frontal poles

Lateral parietal lobeRetrosplenial / precuneus

Experimental Recombination Task

Remembering Subsystem?

Past-Recall > Past-Imagine, Future-Imagine & Control Task

CuneusMiddle/inferior occipital gyrus

• Constructive nature of the episodic memory system: during retrieval, various elements of past experiences are reintegrated and recombined, allowing us to draw on the past to imagine the future.

• Imaging data support the constructive episodic simulation hypothesis - and the possibility that simulation of future events is a primary function of a constructive episodic memory.

• The hippocampus plays an important role in recombining and encoding details from past episodes into

simulations of the future. Converging evidence from work on prospective coding/preplay of event sequences.

Conclusions(see Schacter & Addis, 2009, Phil. Trans. Roy. Soc.)

Acknowledgements

Memory Lab, Harvard Collaborators

Brendan Gaesser Donna Addis

Kathy Gerlach Randy Buckner

Adrian Gilmore Theresa Cheng Yoko Okado Elizabeth Chua Ling Pan Noa Laiser

Jessica Payne Ling Pan

Nathan Spreng Alana Wong

Dale Stevens

Gagan Wig

Supported by NIMH & NIA