Studying Memory Encoding with fMRI Event-related vs. Blocked Designs Aneta Kielar

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 ms NS = 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 : 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: LIFG B: 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 +CA+CAAC X msISI = 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

Frontal RegionsTemporal Regions

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 2.88 ms OIOIIOOI X 4 ISI = ms

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 Scenes > Fixations DLPF, Thalamus, fusiform, parahippocampus, visual areas

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

RDLPF Bilateral Parahippocampal Cortex Activation during Encoding & Subsequent memory

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

Summed parahippocampal activation

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.

Conclusions Interactions between Right Frontal WM processes and parahippocampal long term- memory processes right frontal & parahippocampal activation encoding memory