Format:
Online-Ressource
Content:
Zusammenfassung: Rationale:The Default Mode Network (DMN) as the most prominent resting state network of the brain has been associated with internally oriented thinking (e.g. thinking about the past, i.e. autobiographical memory (AM)). An aberrant organization and activity of this network has been discussed to underlie rumination in depression via an increased internal orientation due to reduced capacities to down-regulate network's activity. The prevalence of AM disturbances and depressive symptoms is significantly increased in temporal lobe epilepsy (TLE) patients when compared to patients with other focal epilepsies and general population. Depression in TLE is largely assumed to be evoked by disease-related disturbances of brain networks. Since, the DMN has been found to be altered at a global and at a nodal level in TLE patients, the issue under investigation is to clarify to what extent network properties of the DMN might be able to account for cognitive and psychological symptoms frequently observed in TLE patients and to what extent AM disturbances and depressive symptoms might have common underlying correlates within the DMN, and how they are related to seizure freedom. For this purpose, three studies were conducted:In Study 1, associations of seizure freedom and DMN properties in TLE patients were investigated. In Study 2, it was assessed how DMN properties associated with seizure freedom are related to AM performance in these patients. In a third study, it was examined how DMN properties associated with seizure freedom and AM performance are connected to the prevalence of depressive symptoms in TLE patients. A graphtheoretical approach based upon resting state - functional magnetic resonance imaging (rs-fMRI) data was used to investigate these questions.Methods:27 TLE patients were recruited from the Epilepsy Center of the Freiburg University Hospital. Twelve of them had been seizure free for at least one year of duration, 15 patients had ongoing active TLE. In addition, 15 healthy volunteers participated in the study. AM was assessed using the autobiographical part C of the Inventar fuer Gedaechtnisdiagnostik (IGD). Depressive symptoms were assessed using Beck Depression Inventory-II (BDI-II). An eight minute rs-fMRI was conducted using the following parameters: TR = 1750 ms, TE = 30 ms, flip angle = 70°, 3 x 3 x 5 m3 voxel size, 28 oblique axial slices. DMN was extracted using 13 predefined regions of interest (ROI). A graphtheoretical approach was used to construct the DMN as functional brain network. Standard graphtheoretical parameters were calculated. Rank transform procedure as well as classical non-parametric statistics were used to assess associations between functional and behavioral data.Results:Study 1: Seizure free and not seizure free TLE patients did not differ in their global network topology from healthy controls, but seizure free TLE patients exhibited longer paths within the network than not seizure free TLE patients. Central nodes of the DMN (Precuneus/PCC, mesial prefrontal cortex (mPFC), inferior parietal lobe (IPL)) had significantly less functional connections in seizure free TLE patients than in not seizure free TLE patients and healthy controls. Seizure free TLE patients revealed a significantly higher number of functional connections of the right anterior superior temporal gyrus (STG) than not seizure free TLE patients and healthy controls. Network's local efficiency was in trend reduced in seizure free TLE patients.Study 2: Seizure free TLE patients performed significantly worse on qualitative episodic AM than not seizure free TLE patients and tended to perform worse than healthy controls: Worse AM was significantly associated with worse local efficiency of the DMN and in trend with a lower number of functional connections of the Precuneus/PCC. Focus lateralization was associated with semantic, but not episodic AM aspects. Study 3: Seizure free and not seizure free TLE patients did not differ from healthy controls in their depression scores. But if exceeding the cut-off score of BDI-II, seizure free TLE patients tended to report higher depression scores than not seizure free patients. No differences in depression scores were obtained as a function of focus lateralization. Qualitative episodic AM performance was initially inversely related to depression scores. When DMN properties were additionally considered, higher depression scores were significantly associated with better qualitative episodic AM and in trend with a higher number of functional connections of Precuneus/PCC and right IPL. Then, seizure free TLE patients tended to report less depressive symptoms than not seizure free patients.Conclusion:Seizure freedom seems to be associated with longer paths within the network, a trend of a reduced local efficiency of the DMN and hub redistribution processes. The present findings indicate that DMN properties do not seem to be able to account for the association between AM disturbances and depressive symptoms in TLE patients. Network properties associated with seizure freedom were found to be associated with worse AM performance, but do also seem to have positive implications on mood. The right (posterior) hemisphere has largely been associated with negative affect, more negative thinking styles, rumination and withdrawal behavior. An association between an aberrant functionality of the DMN has moreover been proposed to possibly underlie typical symptoms of Major Depression such as an increased internal orientation and rumination tendency. Thus, seizure freedom might release TLE patients from a possibly increased negatively biased internal orientation. This might particularly be mediated by a reduction of functional connections of the right IPL within the DMN. In addition to the positive implications of seizure freedom on quality of life, this may positively contribute to the cessation of depressive symptoms in seizure free TLE patients. Moreover, the findings indicate that mechanisms underlying depression might be different in seizure free and not seizure free TLE patients
Note:
Dissertation Albert-Ludwigs-Universität Freiburg 2015
Language:
English
Keywords:
Hochschulschrift
URN:
urn:nbn:de:bsz:25-freidok-105700
URL:
https://doi.org/10.6094/UNIFR/10570
URL:
https://nbn-resolving.org/urn:nbn:de:bsz:25-freidok-105700
URL:
https://d-nb.info/1119452988/34
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