Limits...
Insights into the mechanisms of absence seizure generation provided by EEG with functional MRI.

Carney PW, Jackson GD - Front Neurol (2014)

Bottom Line: The main component displaying an increase in blood oxygen level dependent (BOLD) signal relative to the resting state, in group studies, is the thalamus while the most consistent cortical change is reduced BOLD signal in the DMN.This region also shows altered FC in patients with AS.Hence, it appears that engagement of this network is central to AS.

View Article: PubMed Central - PubMed

Affiliation: The Florey Institute for Neuroscience and Mental Health , Heidelberg, VIC , Australia ; The University of Melbourne , Parkville, VIC , Australia ; Austin Health , Heidelberg, VIC , Australia.

ABSTRACT
Absence seizures (AS) are brief epileptic events characterized by loss of awareness with subtle motor features. They may be very frequent, and impact on attention, learning, and memory. A number of pathophysiological models have been developed to explain the mechanism of absence seizure generation, which relies heavily on observations from animal studies. Studying the structural and functional relationships between large-scale brain networks in humans is only practical with non-invasive whole brain techniques. EEG with functional MRI (EEG-fMRI) is one such technique that provides an opportunity to explore the interactions between brain structures involved in AS generation. A number of fMRI techniques including event-related analysis, time-course analysis, and functional connectivity (FC) have identified a common network of structures involved in AS. This network comprises the thalamus, midline, and lateral parietal cortex [the default mode network (DMN)], caudate nuclei, and the reticular structures of the pons. The main component displaying an increase in blood oxygen level dependent (BOLD) signal relative to the resting state, in group studies, is the thalamus while the most consistent cortical change is reduced BOLD signal in the DMN. Time-course analysis shows that, rather than some structures being activated or inactivated during AS, there appears to be increase in activity across components of the network preceding or following the electro-clinical onset of the seizure. The earliest change in BOLD signal occurs in the DMN, prior to the onset of epileptiform events. This region also shows altered FC in patients with AS. Hence, it appears that engagement of this network is central to AS. In this review, we will explore the insights of EEG-fMRI studies into the mechanisms of AS and consider how the DMN is likely to be the major large-scale brain network central to both seizure generation and seizure manifestations.

No MeSH data available.


Related in: MedlinePlus

GSW-related networks identified using event-related ICA. Each row represents a different network, labeled from (A–F). The plots on the right show the mean time course of fMRI signal change within each network with error bars indicating the standard error, over the time period from −32 to +32 s relative to the GSW onset. The vertical dotted line in each plot represents the time of GSW onset, and the horizontal dotted line represents the baseline fMRI signal level. Asterisks indicate where the BOLD signal is significantly different to baseline (p < 0.05, uncorrected). The images on the left are z-statistic maps, thresholded to show significant (p < 0.05) clusters of voxels, overlaid upon a reference anatomical image. The hot and cool colors in the images indicate whether the brain region shows a positive or negative modulation with respect to the network time course, i.e., they are analogous to activations and deactivations except with respect to the network-specific time course instead of a canonical HRF (67) (published with permission from Epilepsia, copyright 2013, ILAE/Willey Blackwell).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4150362&req=5

Figure 3: GSW-related networks identified using event-related ICA. Each row represents a different network, labeled from (A–F). The plots on the right show the mean time course of fMRI signal change within each network with error bars indicating the standard error, over the time period from −32 to +32 s relative to the GSW onset. The vertical dotted line in each plot represents the time of GSW onset, and the horizontal dotted line represents the baseline fMRI signal level. Asterisks indicate where the BOLD signal is significantly different to baseline (p < 0.05, uncorrected). The images on the left are z-statistic maps, thresholded to show significant (p < 0.05) clusters of voxels, overlaid upon a reference anatomical image. The hot and cool colors in the images indicate whether the brain region shows a positive or negative modulation with respect to the network time course, i.e., they are analogous to activations and deactivations except with respect to the network-specific time course instead of a canonical HRF (67) (published with permission from Epilepsia, copyright 2013, ILAE/Willey Blackwell).

Mentions: As stated above, the thalamus has retained a central role in models of absence generation given its role as a relay station for information transfer in the brain with strong reciprocal connection to the cortex. A robust positive thalamic BOLD response has been consistently observed associated with AS (54–58, 66) and interictal GSW (59–63, 65). It has been suggested that the spatial extent of thalamic involvement extends beyond the thalamus into the nearby striatal structures (67). Using event-related independent components analysis (eICA), it has been possible to identify two thalamic components, one located in the midline, which may reflect the local venous drainage into thalamostriate veins, while the other component involves the lateral thalamic nuclei and lentiform nuclei bilaterally (Figure 3). The spatial extent of thalamic involvement as identified using EEG-fMRI, however, is uncertain. Given requirements for spatial smoothing in the analysis, functional imaging may simplify more complex BOLD change within discrete thalamic nuclei.


Insights into the mechanisms of absence seizure generation provided by EEG with functional MRI.

Carney PW, Jackson GD - Front Neurol (2014)

GSW-related networks identified using event-related ICA. Each row represents a different network, labeled from (A–F). The plots on the right show the mean time course of fMRI signal change within each network with error bars indicating the standard error, over the time period from −32 to +32 s relative to the GSW onset. The vertical dotted line in each plot represents the time of GSW onset, and the horizontal dotted line represents the baseline fMRI signal level. Asterisks indicate where the BOLD signal is significantly different to baseline (p < 0.05, uncorrected). The images on the left are z-statistic maps, thresholded to show significant (p < 0.05) clusters of voxels, overlaid upon a reference anatomical image. The hot and cool colors in the images indicate whether the brain region shows a positive or negative modulation with respect to the network time course, i.e., they are analogous to activations and deactivations except with respect to the network-specific time course instead of a canonical HRF (67) (published with permission from Epilepsia, copyright 2013, ILAE/Willey Blackwell).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4150362&req=5

Figure 3: GSW-related networks identified using event-related ICA. Each row represents a different network, labeled from (A–F). The plots on the right show the mean time course of fMRI signal change within each network with error bars indicating the standard error, over the time period from −32 to +32 s relative to the GSW onset. The vertical dotted line in each plot represents the time of GSW onset, and the horizontal dotted line represents the baseline fMRI signal level. Asterisks indicate where the BOLD signal is significantly different to baseline (p < 0.05, uncorrected). The images on the left are z-statistic maps, thresholded to show significant (p < 0.05) clusters of voxels, overlaid upon a reference anatomical image. The hot and cool colors in the images indicate whether the brain region shows a positive or negative modulation with respect to the network time course, i.e., they are analogous to activations and deactivations except with respect to the network-specific time course instead of a canonical HRF (67) (published with permission from Epilepsia, copyright 2013, ILAE/Willey Blackwell).
Mentions: As stated above, the thalamus has retained a central role in models of absence generation given its role as a relay station for information transfer in the brain with strong reciprocal connection to the cortex. A robust positive thalamic BOLD response has been consistently observed associated with AS (54–58, 66) and interictal GSW (59–63, 65). It has been suggested that the spatial extent of thalamic involvement extends beyond the thalamus into the nearby striatal structures (67). Using event-related independent components analysis (eICA), it has been possible to identify two thalamic components, one located in the midline, which may reflect the local venous drainage into thalamostriate veins, while the other component involves the lateral thalamic nuclei and lentiform nuclei bilaterally (Figure 3). The spatial extent of thalamic involvement as identified using EEG-fMRI, however, is uncertain. Given requirements for spatial smoothing in the analysis, functional imaging may simplify more complex BOLD change within discrete thalamic nuclei.

Bottom Line: The main component displaying an increase in blood oxygen level dependent (BOLD) signal relative to the resting state, in group studies, is the thalamus while the most consistent cortical change is reduced BOLD signal in the DMN.This region also shows altered FC in patients with AS.Hence, it appears that engagement of this network is central to AS.

View Article: PubMed Central - PubMed

Affiliation: The Florey Institute for Neuroscience and Mental Health , Heidelberg, VIC , Australia ; The University of Melbourne , Parkville, VIC , Australia ; Austin Health , Heidelberg, VIC , Australia.

ABSTRACT
Absence seizures (AS) are brief epileptic events characterized by loss of awareness with subtle motor features. They may be very frequent, and impact on attention, learning, and memory. A number of pathophysiological models have been developed to explain the mechanism of absence seizure generation, which relies heavily on observations from animal studies. Studying the structural and functional relationships between large-scale brain networks in humans is only practical with non-invasive whole brain techniques. EEG with functional MRI (EEG-fMRI) is one such technique that provides an opportunity to explore the interactions between brain structures involved in AS generation. A number of fMRI techniques including event-related analysis, time-course analysis, and functional connectivity (FC) have identified a common network of structures involved in AS. This network comprises the thalamus, midline, and lateral parietal cortex [the default mode network (DMN)], caudate nuclei, and the reticular structures of the pons. The main component displaying an increase in blood oxygen level dependent (BOLD) signal relative to the resting state, in group studies, is the thalamus while the most consistent cortical change is reduced BOLD signal in the DMN. Time-course analysis shows that, rather than some structures being activated or inactivated during AS, there appears to be increase in activity across components of the network preceding or following the electro-clinical onset of the seizure. The earliest change in BOLD signal occurs in the DMN, prior to the onset of epileptiform events. This region also shows altered FC in patients with AS. Hence, it appears that engagement of this network is central to AS. In this review, we will explore the insights of EEG-fMRI studies into the mechanisms of AS and consider how the DMN is likely to be the major large-scale brain network central to both seizure generation and seizure manifestations.

No MeSH data available.


Related in: MedlinePlus