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How the past becomes present: neural mechanisms governing retrieval from episodic memory
Umeå University, Faculty of Medicine, Radiation Sciences, Diagnostic Radiology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Remembering previously experienced events can happen as a result of an effortful retrieval attempt. At other occasions, a memory can enter our minds without any apparent effort – or, indeed, intention - to retrieve. Although it has long been appreciated that retrieval from episodic memory is intertwined with cognitive control, the neural mechanisms of memory-control interactions remain unclear.

In this thesis I have used functional magnetic resonance imaging (fMRI) and scalp-recorded event-related potentials (ERP) to study the neural basis of episodic retrieval at varying levels of cognitive control. The dorsolateral prefrontal cortex (dlPFC) has been suggested to support a cognitive control mechanism (context processing) which is relevant during various situations that demand maintenance of current goals and rules. Although increased dlPFC recruitment with increasing context processing demands has been demonstrated during episodic retrieval, there are relatively few studies directly comparing the engagement of dlPFC during episodic retrieval with that during other task domains.

In Study I, context processing demands were amplified in episodic retrieval, auditory attention and emotion regulation tasks. This led to overlapping dlPFC recruitment in the first two domains and a divergent reliance on ventromedial prefrontal cortex in the emotion domain. Thus, when selection between competing representations needs to be carried out in accordance with the currently relevant goals and task rules, the episodic memory system interacts with domain-general cognitive control mechanisms.

Studies II and III explored the reactive nature of retrieval-specific control mechanisms: can we flexibly switch between semantic and episodic retrieval based on the information extracted from a retrieval cue? This was studied using a recognition memory task where the relevant information could with equal probability be supplied by the semantic or the episodic memory system. The fMRI results (Study II) showed that the brain activation during the ‘episodic’ but not the ‘semantic’ trials was expressed in the right prefrontal cortex. As the order of trials was unpredictable, the corresponding changes in brain activation might be evoked by differences in early cue-trace interactions. An event-related potential study (Study III) with the same experimental protocol as in Study II showed that neural processing corresponding to the two trial types diverged as early as in the time window 100-140 ms post-cue onset, thus highlighting the importance of early cue-trace matching in the selection of further retrieval processing.

Study IV explored incidental episodic retrieval. Although this form of retrieval is a common experience in everyday life and a disturbing symptom in some psychiatric conditions, it is not clear how such spontaneous expressions of memory are initiated and to what extent the prefrontal cortex is engaged. The fMRI results showed, consistent with Study I, that dlPFC is specifically associated with the intention to retrieve, independently of success. Retrieval success engaged similar networks for incidentally as well as intentionally retrieved memories, comprising the hippocampus, precuneus, ventrolateral PFC, and the anterior cingulate cortex. Collectively, the fMRI and ERP results indicated that incidental retrieval was initiated by early (< 200 ms) oldness estimation carried out on the semantic information extracted from the retrieval cues.

Taken together, the results of this thesis indicate that episodic retrieval can be initiated via two routes:  a bottom-up input rising early during the cue processing, and a top-down input provided by the cognitive control processes mediated by the prefrontal cortex.

Place, publisher, year, edition, pages
Umeå: Umeå university , 2010. , 71 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1332
Keyword [en]
episodic retrieval, cognitive control, retrieval intention, prefrontal cortex, functional magnetic resonance imaging, event-related potentials
National Category
Physiology
Research subject
Diagnostic Radiology
Identifiers
URN: urn:nbn:se:umu:diva-31873ISBN: 978-91-7264-948-4 (print)OAI: oai:DiVA.org:umu-31873DiVA: diva2:297969
Public defence
2010-03-18, sal 260, Inst. för strålningsvetenskaper, NUS (byggnad 3A), Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2010-02-26 Created: 2010-02-19 Last updated: 2010-02-26Bibliographically approved
List of papers
1. Distinct control networks for cognition and emotion in the prefrontal cortex
Open this publication in new window or tab >>Distinct control networks for cognition and emotion in the prefrontal cortex
Show others...
2009 (English)In: Neuroscience Letters, ISSN 0304-3940, E-ISSN 1872-7972, Vol. 467, no 2, 76-80 p.Article in journal (Refereed) Published
Abstract [en]

The activation of dorsolateral prefrontal cortex (dlPFC) has been suggested to reflect the engagement of a control mechanism for top-down biasing of context processing in resource-demanding memory tasks. Here we tested the hypothesis that the dlPFC subserves a similar function also in attention and emotion tasks. 18 healthy young adults were tested in a functional magnetic resonance imaging (fMRI) study where the demands for context processing were manipulated in three different cognitive domains: auditory attention, episodic retrieval, and emotion regulation. We found that the right dlPFC was jointly sensitive to increased cognitive demands in the attention and memory tasks. By contrast, increased demands in the emotion task (reappraisal) were associated with increased activity in ventromedial PFC along with decreased amygdala activity. Our findings of divergent prefrontal control networks for cognitive and emotional control extend previous separations of cognition and emotion in the anterior cingulate cortex.

Keyword
Cognitive control, Memory, Attention, Emotion
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:umu:diva-30962 (URN)10.1016/j.neulet.2009.10.005 (DOI)19818382 (PubMedID)
Available from: 2010-01-24 Created: 2010-01-24 Last updated: 2017-12-12Bibliographically approved
2. Dynamic switching between semantic and episodic memory systems
Open this publication in new window or tab >>Dynamic switching between semantic and episodic memory systems
2009 (English)In: Neuropsychologia, ISSN 0028-3932, E-ISSN 1873-3514, Vol. 47, no 11, 2252-2260 p.Article in journal (Refereed) Published
Abstract [en]

It has been suggested that episodic and semantic long-term memory systems interact during retrieval. Here we examined the flexibility of memory retrieval in an associative task taxing memories of different strength, assumed to differentially engage episodic and semantic memory. Healthy volunteers were pre-trained on a set of 36 face-name pairs over a 6-week period. Another set of 36 items was shown only once during the same time period. About 3 months after the training period all items were presented in a randomly intermixed order in an event-related fMRI study of face-name memory. Once presented items differentially activated anterior cingulate cortex and a right prefrontal region that previously have been associated with episodic retrieval mode. High-familiar items were associated with stronger activation of posterior cortices and a left frontal region. These findings fit a model of memory retrieval by which early processes determine, on a trial-by-trial basis, if the task can be solved by the default semantic system. If not, there is a dynamic shift to cognitive control processes that guide retrieval from episodic memory.

Keyword
memory, episodic, prefrontal, fMRI, learning
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:umu:diva-19436 (URN)10.1016/j.neuropsychologia.2008.11.031 (DOI)19109984 (PubMedID)
Available from: 2009-03-05 Created: 2009-03-05 Last updated: 2017-12-13Bibliographically approved
3. Early analysis of retrieval cues guides selection of retrieval processing
Open this publication in new window or tab >>Early analysis of retrieval cues guides selection of retrieval processing
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Human long-term memory holds semantic and episodic memories. Retrieval from these two memory systems occurs independently. As required information may be held in either of these systems, the question arises how and when is the choice of retrieval processing (episodic/semantic) determined. Here we report results from an ERP study on healthy young adults during a forced-choice associative recognition task, designed to test the hypothesis that early processing of retrieval cues influences subsequent retrieval processing. The test items had previously been encoded repeatedly (6x) or only once (1x) during pre-experimental training period, thereby influencing the reliance on semantic or episodic retrieval processes.  Differences between the two conditions were observed for the familiarity-sensitive FN400 component as well as for a late (>1000 ms) component indexing post-retrieval processing. Most critically, we found that a difference between successfully retrieved 6x and 1x items emerged already during the 100-140 ms time window. These results indicate that choice of retrieval processes (episodic/semantic) depends on early matching between retrieval cues and memory traces.

Keyword
erp, episodic memory, semantic memory
National Category
Physiology
Identifiers
urn:nbn:se:umu:diva-31870 (URN)
Available from: 2010-02-19 Created: 2010-02-19 Last updated: 2010-02-26Bibliographically approved
4. Multimodal imaging of incidental retrieval: the low route to memory
Open this publication in new window or tab >>Multimodal imaging of incidental retrieval: the low route to memory
2011 (English)In: Journal of cognitive neuroscience, ISSN 0898-929X, E-ISSN 1530-8898, Vol. 23, no 4, 947-960 p.Article in journal (Refereed) Published
Abstract [en]

Memories of past episodes frequently come to mind incidentally, without directed search. It has remained unclear how incidental retrieval processes are initiated in the brain. Here we used fMRI and ERP recordings to find brain activity that specifically correlates with incidental retrieval, as compared to intentional retrieval. Intentional retrieval was associated with increased activation in the dorsolateral prefrontal cortex. By contrast, incidental retrieval was associated with a reduced fMRI signal in posterior brain regions, including extrastriate and parahippocampal cortex, and a modulation of a posterior ERP component 170 ms after the onset of visual retrieval cues. Successful retrieval under both intentional and incidental conditions was associated with increased activation in hippocampus, precuneus and ventrolateral prefrontal cortex, as well as increased amplitude of the P600 ERP component. These results demonstrate how early bottom-up signals from the posterior cortex can lead to reactivation of episodic memories in the absence of strategic retrieval attempts.

Keyword
incidental retrieval, episodic memory, fMRI, ERP
National Category
Physiology
Identifiers
urn:nbn:se:umu:diva-31871 (URN)10.1162/jocn.2010.21494 (DOI)20350180 (PubMedID)
Available from: 2010-02-19 Created: 2010-02-19 Last updated: 2017-12-12Bibliographically approved

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