EyeTSS

Mitarbeiter der Projekte „EyeTSS“ und „MAMUD“stellten auf der ARVO 2015 Poster zu aktuellen Forschungsergebnissen vor und diskutierten diese mit dem anwensenden Fachpublikum.

Die folgenden Konferenzbeiträge wurden präsentiert:

• Influence of the blood glucose level on FLIO measurements at the human retina in healthy volunteers
  M. Klemm, D. Schweitzer
• Simultaneous dynamic retinal vessel analysis and pulsed anodal transcranial electrical stimulation at the eye
  S. Freitag, A. Hunold, M. Klemm, J. Haueisen
• Sampling paradigms for increasing the repeatability of Static Vessel Analysis by using multiple images
  S. Rieger, W. Vilser , D. Baumgarten

Im Rahmen des23. Treffens des Arbeitskreises Ophthalmische Optik vom 24. bis 26. Oktober 2014 in Ilmenau (Deutschland)

wurden aktuelle Forschungsinhalte aus dem Projekt „EyeTSS“ in einem Vortrag präsentiert:

S. Freitag: Untersuchung der Flickerantwort retinaler Gefäße bei transkranialer Stromstimulation am Auge

Der vorgestellte Beitrag befasst sich mit der transkranialen Stromstimulation in unmittelbarer Nähe zum Auge und deren Effekten auf retinale Gefäßantworten. Die Abbildung zeigt exemplarisch den Augenhintergrund eines Probanden mit verschiedenen Messstellen für die dynamische Gefäßanalyse, die zur Erfassung der retinalen Gefäßreaktionen genutzt wird.

Auf der 4th Conference Information Technologies in Biomedicine (ITiB) stellten drei Mitarbeiter der Projekte EyeTSS und MAMUD aktuelle Forschungsinhalte vor. Die Vorträge waren Teil einer speziellen Session zum Thema „Non-invasive Measurement of Neural and Vessel Activity“:

Enhanced Retinal Arterial Dilation at Transcranial Anodal Electrical Stimulation of the Eye
S. Freitag, A. Hunold, A. Dietzel, M. Klemm, J. Haueisen

Complementary Sensitivity of EEG and MEG to Tangential and Radial Epileptic Spike Activity: Influence of Source Depth
A. Hunold, M. Funke, R. Eichardt, J. Haueisen

Short-term Correlation of Retinal Vessel Width and Blood Pressure
S. Rieger, D. Baumgarten

Transcranial direct current stimulation causes excitation or inhibition of neuronal activity which reflect on vessel dilation and constriction due to different demands in metabolism. This effect is known as neurovascular coupling (NVC) which directly can be examined by dynamic vessel analysis (DVA) in the retina.
We aim to infer on the NVC from diameter changes in retinal vessels induced by electrical stimulation.
We perform retinal vessel analysis with simultaneous transcranial current stimulation. Applied current configurations are simulated in volume conductor models.
Retinal vessels provide enhanced dilation at simultaneously applied flickering light and current stimulation compared to pure flickering light stimulation. Simulated current density distribution follow the expected pattern.

Tuberous sclerosis complex (TSC) is a disorder of tissue growth and differentiation, characterized by benign hamartomas in the brain triggering epilepsy in up to 90% of TSC patients. There is an ongoing debate on whether or not the epileptogenic zone is within the tuber itself or in abnormally developed surrounding tissue.
We examined a four-year old patient with TSC-related refractory epilepsy undergoing magnetoencephalography (MEG) and electroencephalography (EEG) recordings. For MEG, we used a prototype system that offers higher spatial resolution and sensitivity compared to the conventional adult systems. EEG was simultaneously recorded from 32-leads according to the 10-20 international system. The source analysis of interictal activity was performed using both EEG and MEG data. Equivalent current dipoles (ECD) were fitted to the peak of individual interictal spikes. For averaged interictal spike signals, we performed ECD localizations to the spike upslope. Further, we estimated the minimum norm estimates (MNEs) to averaged interictal spike signals.
Multiple cortical tubers were identified in patient’s MRI including one prominent calcified tuber in the right parietal-occipital lobe. The simultaneously recorded spikes in EEG and MEG data provided a time shift of 20 ms between peak latencies. ECDs localized to individual and averaged interictal activity in EEG and MEG consistently clustered in the millimeter vicinity of the large calcified cortical tuber. The ECD trace localized to the averaged EEG data located on the posterior side ~5 mm superior to the tuber. The ECD trace localized to the averaged MEG spike located ~4 mm anterior to the tuber. MNE and ECDs indicated epileptiform activity in the same areas.
Our source analysis indicated generators of epileptiform activity in the millimeter vicinity of the tuber margin outside the tuber volume. Separate EEG and MEG source analysis provided distinct source characteristics.

Elektroenzephalographie (EEG) und Magnetoenzephalographie (MEG) zeigen unterschiedliche Empfindlichkeit gegen-über neuronaler Aktivität. Der Einfluss der Quellorientierung und –tiefenlage wird in der vorliegenden Studie für fokale und ausgedehnte Quellen analysiert. Die Simulationsumgebung besteht aus realistischen drei schaligen Randelementme-thodemodellen als Volumenleitermodell und Gitterrepräsentationen der gefalteten kortikalen Oberfläche. Fokale Einzel-dipole und ausgedehnte Dipolverbundquellen werden anhand der Quellorientierung und –tiefenlage klassifiziert. Simula-tionen von EEG- und MEG-Signalen weisen komplementäre Empfindlichkeitsverteilungen in Abhängigkeit von der Quellorientierung für oberflächennahe Quellen auf. Demnach kann die gleichzeitige Ableitung von EEG und MEG die Wahrscheinlichkeit der Quellerkennung erhöhen

Clinical applications of simultaneous electroencephalography (EEG) and magnetoencephalo-graphy (MEG) recordings of interictal epileptic spikes revealed situations where both modalities showed different sensitivities to the epileptic-form activity. In previous studies we showed that for focal and extended sources EEG and MEG provided varying sensitivities depending on the depth and the orientation of the spike source.

Here we extend this work to effects of background source strength variations depending on the source orientation on the spike detectability in EEG and MEG simulations.

We build realistic three compartment boundary element method head models for two participants with 5120 triangles per layer. The vertices in a triangular grid of the segmented boundary between white and gray matter provided the base points for single dipole sources. For each dipole, the angle between the source vector and the surface normal in the closest point of the inner skull mesh defined the source orientation. The Euclidean distance to the closest scalp vertex defined the source depth. In the baseline simulations, single dipoles generated interictal activity with a maximal strength of 600 nAm and approximately 30,000 randomly distributed dipole contributed to normal brain activity as background noise with an individual strength of 10 nAm. In further simulations, we selectively increased the strength of radial and tangential background sources to 300% of baseline strength. As detectability measure we calculated a linear ratio between spike and background amplitudes as signal-to-noise ratio (SNR) in the EEG/MEG channel with the maximal spike amplitude.

In the baseline simulations, spikes from radially oriented single dipole sources generated the highest SNR in EEG and tangential sources dominated the SNR in MEG responses for superficial source locations. With three times stronger radial background activity, the SNR in EEG responses marginally decreased and the SNR profiles of MEG simulations remained almost stable. Opposing, the increase of tangential background activity strongly lowered the SNR in MEG responses and also affected the SNR in EEG simulations.

Our simulations show complementary sensitivity profiles of EEG and MEG to superficial sources depending on their orientation. Due to the MEG’s orientation selectivity, only increased tangential background activity lowered the SNR in MEG simulations.

The complementary SNR profiles of EEG and MEG to superficial sources indicate the benefit of simultaneous recording of both modalities.

A. Hunold, BaCi 2013, Geneva / Switzerland

Hunold A., Haueisen J., Ahtam B., Doshi C., Harini C., Grant P. E., Okada Y., Papadelis C

Electroencephalography (EEG) is an important clinical and research method. Artifacts overlaying the original signals are a reoccurring problem in EEG. Due to the increasing number of EEG electrodes and high sampling rates, efficient methods for signal analysis and artifact detection are required.

A promising approach enabling online artifact detection is the spatial harmonic analysis (SHA) of EEG data. This method allows signal decomposition based on the eigenspace of the Laplace-Beltrami operator of the triangulated surface of the electrode positions. The resulting signal components are utilized for detecting artifacts in EEG data. We propose a method combining SHA and support vector machines (SVM) for artifact detection. The spatial harmonic signal components are classified binary by a previously trained SVM resulting in one group containing the artifact affected components and one group including the artifact free ones.

Spontaneous EEG signals of ten healthy male volunteers were recorded using a 256-channel cap with an equidistant electrode layout and two coupled 128 channel amplifiers (ANT B.V., Enschede, The Netherlands). Data were sampled at 2048 Hz and band-pass (1 – 300 Hz) and notch (50 Hz and four harmonics) filtered. The recorded resting EEG signals were manually checked for eye blink and swallowing artifacts. Those were individually segmented from the data sets resulting in EEG segments with (134) and without (185) eye blinks as well as with (52) and without (86) swallowing artifacts. The segments consist of 800 ms on average.

The EEG segments for each type of artifact were divided into two-thirds acting as test data and one-third available as training data. To train the SVMs equal amounts of artifact containing and artifact free EEG data were randomly selected from the training data sets.

With a training data set of 5,000 ms a correct classification rate (CCR) of 95 % was achieved for the eye blink test data. The swallowing artifact test data were classified correctly with a CCR of 83 %, using 10,000 ms training data.

The proposed method using SHA and SVM for artifact detection in EEG data enables an efficient identification of artifact containing and artifact free EEG segments, respectively. Due to the low computation times for SHA and SVM, the method is well suited for online artifact detection.

Electroencephalography (EEG) and magneto-encephalography (MEG) non-invasively record human neuronal activity. We aimed to evaluate the influence of source orientation and depth location on the detectability of epileptic spikes in simulated EEG and MEG data for varying levels of background activity. Our realistic three compartment boundary element method models and triangular grids of the convoluted cortical surfaces provided the simulation basis for EEG and MEG signals. Spike dipoles and background sources were quantitatively characterized by depth and orientation properties. We found that spike detectability in EEG signals decreased with increased radial and tangential background activity. SNR in MEG decreased only with increased tangential background activity. Simultaneous application of EEG and MEG improves the probability of spike detection.