studying memory encoding with fmri event-related vs. blocked designs aneta kielar
TRANSCRIPT
Memory • Memory encoding: Process by which
experience of an event is transformed into memory trace.
• Main Questions:– Why some events are remembered and
others forgotten?– What brain regions/neural processes are
required for formation of durable memories?
Memory
• Remembered Events:– Undivided attention– Semantic vs. nonsemantic processing
• Brain Regions: ERP, fMRI, lesions studies
– Medial Temporal Lobe• Hippocampus & adjacent regions
(parahippocampal cortex)
– prefrontal cortex (modality dependent)
Memory• Problems
– Lesion studies• Encoding = Storage = Retrieval
– fMRI: blocked designs• Trials from each conditions presented sequentially,
inseparable from each other.• Allow comparison between encoding conditions• Do not allow direct trial- by- trial comparison between
encoding trials and subsequent memory
– ERP studies• Allow for trial–by- trial comparison• Limited spatial resolution
Memory
• Role of MTL in memory encoding = ?– MTL essential for memory encoding– MTL activation absent in some studies– Parahippocampal gyrus: memory encoding– Novel > familiar (Gabrielli et al., 1997)
– Novelty detection?
• It has been difficult to determine which regions underlie the process of memory encoding.
Memory and fMRI
• fMRI studies can distinguish between encoding and retrieval by measuring brain activation at each stage of memory
• Event-related designs: separate neural activations are recorded for each stimulus
• Allow direct comparison between specific encoding trials that lead to subsequent remembering and forgetting.
• Wagner et al. (1998) & Brewer et al (1998): whole brain fMRI to investigate whether neural activity in certain brain regions predicts later memory performance.
Building Memories: Wagner et al. (1998)
• Experiment 1: Blocked Design
– Effect of manipulation of encoding task (sem vs. nonsem) on prefrontal and MTL activation
Procedure:Participants: n =12
Encoding Condition (Scanning Session):Semantic processing: abstract vs. concrete (10 w each, 1 sec)Nonsemantic processing: upper of lower case
Alternating blocks: semantic, nonsemantic, visual fixation
Novelty of words was equivalent*
Procedure
Fix=8 msNS = 40sec
Fix=24 ms
Sem = 40sec
Fix=24 ms
NS = 40sec
Fix=24 ms
Sem = 40sec
X 4 scans+ + ++
Blocked Design
Blocked Design*: Segregate different cognitive processes into distinct time periods; many trials of the same type presented sequentially, trials
inseparable from each other during functional scan
Memory Test: 20-40 min later; yes/no recognition
Results• Behavioral:
– Decision RT• Semantic (873 ms) > nonsemantic (539 ms)
– Memory Accuracy• Semantic (85%)> nonsemantic (47%)
• Imaging Results:– Activation
• Word > fixation• Memory encoding, stimulus perception, response
generation
Results: Exp 1
A: LIFG; B: RIFG,
C: LPC
D:LIFG; E: Frontal Operculum; F:
LMTG
G: Visual cortex;
I: fusiform gyrus;
H: Parahippocampal gyrus
WORD > FIXATION
Results: Exp 1
• To identify regions of differential activation during encoding yielding high vs. poor memory
• Semantic vs. nonsemantic conditions– Semantic >nonsemantic
• Left prefrontal cortex• Left parahippocampal & fusiform gyri
Results: Exp 1
SEMANTIC > NONSEMANTIC
A: LIFGB: RIFG
D: LFG
E: bilateral frontal operculum
H: parahippocampal;
I: fusiform
• Blocked Design results: temporal and prefrontal brain processes influence encoding
• But they do not directly specify encoding differences that predict whether a specific word will be remembered or forgotten.
• Difficult to present blocks of forgotten and remembered items.
Experiment 2: Event-Related Design
• Event-related designs: associate brain processes with discrete events, which may occur at any point in the scanning session.
• Allow direct comparison between encoding trials that are remembered or forgotten
• Q: Does trial-by-trial diff. In encoding activation predict subsequent memory for experience?
Experiment 2: Procedure
• N =13 • Performed single incidental encoding task
• Measured neural activity elicited by single item; Direct comparison between encoding trials
• Encoding trials (Scanning Session)
– Semantic decision: abstract vs. concrete
– 40 abstract, 40 concrete word trials, 40 fixation trials pseudo-randomly intermixed
Procedure
• Memory test: 20 min later
• Recognition Test:– 480 studied & 480 unstudied – Confidence judgment: high vs. low, new
+C A+ C AA CX 6
750 ms ISI = 1250 ms
+ + + + + + + +
Results
• Behavioral:– High Confidence RT > Low Confidence RT > Misses– Accuracy:– High Con Hits = Low Con Hits = Misses
• fMRI Results:– Encoding trials categorized based on whether word was
remembered or forgotten on memory test
• Trial types:– High confidence hits; low confidence hits, misses,
fixations
Results
• Activation : Word > Fixation
A: LIFG; B: RIFG; C: LLPC D: anterior/ventral LIFG; E: frontal operculum; F: LMTG; G: visual cortex; fusiform gyrus; H: Parahippocampal gyrus
Results
• Identification of regions with differential activation during encoding of words remembered & forgotten.
• High confidence hits > misses– Left prefrontal regions– Left parahippocampal– Fusiform gyri
More Results
• Effect of processing time: ?– Matched RTs for High confidence hits & missed– Activation for remembered items: left prefrontal
& temporal regions
• Accuracy: ?– Accuracy during encoding same for high
confidence hits & misses.
Summary of Results• Neural signature during encoding differs for
subsequently remembered & forgotten events.
• Engagement of left prefrontal & medial temporal regions during encoding makes experience memorable.
• Activation of left parahippocampal gyrus greater for events later remembered
• Role of parahippocampal gyrus extends beyond novelty detection; general encoding– All stimuli equally novel
Conclusions• Prefrontal cortex:
– Memory encoding– Role of semantic & phonological processing – Effective memory encoding: analysis of meaning of
the item.– Organize info in WM
• Parahippocampal Gyrus:– Novelty detection & more…– Respond to memorable items– Attentional processing (Rugg, 1998)
Conclusions/Questions
• Better memory depends on activation of prefrontal and temporal processes.
• No activation found in hippocampus proper?– What does it mean for memory theories?
• What is the nature of relation between prefrontal & parahippocampal processes?– Independent or serial processing?
Blocked vs. Event-related Designs
• Blocked Designs:• Trials from each conditions presented sequentially,
inseparable from each other.• Allow comparison between encoding conditions• Do not allow direct trial- by- trial comparison
between encoding trials and subsequent memory
• Event-related designs:• Separate neural activations are recorded for each
stimulus• Allow post-hoc trial sorting• Can define experimental conditions based on
participants responses
Making Memories: Brewer et al. (1998)
• Used event-related design to identify brain regions during encoding that differentiated between visual experiences that were later remembered or forgotten.
• Procedure:– n = 6
• Encoding (Scanning Session):– Subject viewed complex color photographs– 24 pictures of outdoor and indoor scenes– Judgment task: indoor or outdoor scene
Procedure
• Memory Test:– 30 min after encoding task– Recognition test: 96 seen pictures & 32 new
pictures– Basis for recognition: remember or know
• Response types:– Remembered, familiar, forgotten
Results• Behavioral Results:
– Remembered: 25%– Familiar: 38%– Forgot: 48%
• fMRI Results:
• Activation Map– Pictures > fixations
Results• Computed correlations between size of
event- related response and memory classification of remembered, familiar and forgotten pictures.
• Regions where event related activity was correlated with greater memory:
– 6 in bilateral parahippocampal cortex– 1 in right dorsolateral prefrontal cortex
Results• Determined source of the correlation between
event-related study phase activation and subsequent memory.
• Summed parahippocampal activation in each individual subject.
• Activation
• Remembered > Familiar
• Familiar > Forgotten
Results• Parahippocampal activation during study
predicted: – When picture would be remembered or
forgotten
– Predicted whether pictures would be more or less centrally remembered.
• Only frontal and parahippocampal activations predicted subsequent memory.
Conclusion
• Degree of Activation in Right Frontal & Bilateral Parahippocampal regions measures how well a particular visual experience is encoded and remembered.