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Doktorsavhandling vid Karolinska Institutet
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Macoveanu, Julian
Neural mechanisms underlying working memory : computational and neuroimaging studies
Fredagen den 29 september 2006, kl. 13.00.
Skandiasalen, Astrid Lindgrens Barnsjukhus, Plan 1, Stockholm.
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ISBN: 91-7140-901-7
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Diss: 06:231
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Abstract:
The performance on various cognitive tasks, from language to selective attention
and guidance of future actions depends on working memory (WM), the ability to
hold and manipulate limited items of information for a period of up to a few
seconds. During childhood development, WM capacity, the number of items one
can maintain in WM, increases. However, the neural correlates of WM capacity,
distractibility and maturational processes underlying WM development are still
unclear. The present work addresses these issues by the integration of
computational modeling, functional magnetic resonance imaging (fMRI) and
behavioral methods. In the first study we used distracting visual stimuli in
order
to identify cellular mechanisms that account for the observed behavioral
decrease in mnemonic accuracy as a function of the spatial distance to
distractors. The computational model suggests that independently of the
cellular and synaptic properties, increased neuronal firing rates accounted for
higher mnemonic accuracy and resistance against distractors. In the second
study we performed fMRI experiments on adults and children to monitor brain
activity during a WM task. We isolated the delay-related activity and analyzed
group differences and the distractor influence both behaviorally and in terms of
changed brain activity. Accompanying the higher WM capacity and lower
distractibility of adults, the fMRI study showed higher brain activity in middle
frontal gyrus and intraparietal cortex in adults compared to children during the
delay periods of WM tasks. In a subsequent study we addressed the cellular
changes during WM development. The study combined a computational
analysis with FMRI in order to establish putative maturation processes
governing developmental changes in brain activity. We found that the increase in
activity together with higher resistance against distractions could be explained
by stronger connectivity between network areas. The final study addressed the
limited storage capacity of previous WM models. Implementing structural and
connectivity changes likely to occur during WM development in a biophysical
WM model we have obtained multiple-item storage capacity similar to human
WM performance. Furthermore, by using fMRI we found that the
informationactivity curve predicted by the model corresponds to that in the
human posterior parietal cortex during performance of WM tasks.
In conclusion, in the context of neural networks dominated by reverberatory
synaptic input, our studies demonstrate the correlation between a higher WM
capacity, resistance to distractors, mnemonic accuracy, BOLD response and
average firing rate of the neuronal populations responsible for the memory
maintenance.
List of papers
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Neuronal population firing rate predicts distance dependent distractor
effects on mnemonic accuracy in a visuo-spatial working memory task
Macoveanu J, Klingberg T, Tegnér J
Manuscript
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Brain Activity Related to Working Memory and Distraction in Children and Adults
Olesen PJ, Macoveanu J, Tegner J, Klingberg T
Cereb Cortex,
2006
In Print
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Stronger synaptic connectivity as a mechanism behind development of
working memory-related brain activity during childhood
Edin F, Macoveanu J, Olesen P, Tegnér J, Klingberg T
Manuscript
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A biophysical model of multiple-item working memory: a computational and neuroimaging study
Macoveanu J, Klingberg T, Tegner J
Neuroscience,
2006;
141(3):
1611-8
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