8 research outputs found
S-cone signals invisible to the motion system can improve motion extraction via grouping by color
Peer reviewedPublisher PD
Rhythmic Pulsing: Linking Ongoing Brain Activity with Evoked Responses
The conventional assumption in human cognitive electrophysiology using EEG and MEG is that the presentation of a particular event such as visual or auditory stimuli evokes a âturning onâ of additional brain activity that adds to the ongoing background activity. Averaging multiple event-locked trials is thought to result in the cancellation of the seemingly random phased ongoing activity while leaving the evoked response. However, recent work strongly challenges this conventional view and demonstrates that the ongoing activity is not averaged out due to specific non-sinusoidal properties. As a consquence, systematic modulations in ongoing activity can produce slow cortical evoked responses reflecting cognitive processing. In this review we introduce the concept of ârhythmic pulsingâ to account for this specific non-sinusoidal property. We will explain how rhythmic pulsing can create slow evoked responses from a physiological perspective. We will also discuss how the notion of rhythmic pulsing provides a unifying framework linking ongoing oscillations, evoked responses and the brain's capacity to process incoming information
Experience Drives Synchronization : The phase and Amplitude Dynamics of Neural Oscillations to Musical Chords Are Differentially Modulated by Musical Expertise
Musical expertise is associated with structural and functional changes in the brain that underlie facilitated auditory perception. We investigated whether the phase locking (PL) and amplitude modulations (AM) of neuronal oscillations in response to musical chords are correlated with musical expertise and whether they reflect the prototypicality of chords in Western tonal music. To this aim, we recorded magnetoencephalography (MEG) while musicians and non-musicians were presented with common prototypical major and minor chords, and with uncommon, non-prototypical dissonant and mistuned chords, while watching a silenced movie. We then analyzed the PL and AM of ongoing oscillations in the theta (4-8 Hz) alpha (8-14 Hz), beta- (14-30 Hz) and gamma- (30-80 Hz) bands to these chords. We found that musical expertise was associated with strengthened PL of ongoing oscillations to chords over a wide frequency range during the first 300 ms from stimulus onset, as opposed to increased alpha-band AM to chords over temporal MEG channels. In musicians, the gamma- band PL was strongest to non-prototypical compared to other chords, while in non-musicians PL was strongest to minor chords. In both musicians and non-musicians the long-latency (> 200 ms) gamma-band PL was also sensitive to chord identity, and particularly to the amplitude modulations (beats) of the dissonant chord. These findings suggest that musical expertise modulates oscillation PL to musical chords and that the strength of these modulations is dependent on chord prototypicality.Peer reviewe
Influence of ongoing alpha rhythm on the visual evoked potential
The relationship between ongoing occipital alpha rhythm (8â12 Hz) and the generation of visual evoked potentials (VEPs) has been discussed controversially. While the âevoked theoryâ sees no interaction between VEP generation and the alpha rhythm, the âoscillatory theoryâ (also known as âphase-reset theoryâ) postulates VEP generation to be based on alpha rhythm phase resetting. Previous experimental results are contradictory, rendering a straightforward interpretation difficult. Our approach was to theoretically model the implications of the evoked and oscillatory theory also incorporating stimulus-induced alpha-rhythm desynchronization. As a result, the model based on the oscillatory theory predicts alpha-band dependent VEP amplitudes but constant phase locking. The model based on the evoked theory predicts unaffected VEP amplitudes but alpha-band dependent phase locking. Subsequently, we analyzed experimental data in which VEPs were assessed in an âeyes openâ and âeyes closedâ condition in 17 subjects. For early components of the VEP, findings are in agreement with the evoked theory, i.e. VEP amplitudes remain unaffected and phase locking decreases during periods of high alpha activity. Late VEP component amplitudes (> 175 ms), however, are dependent on pre-stimulus alpha amplitudes. This interaction is contradictory to the oscillatory theory since this VEP amplitude difference is not paralleled by a corresponding difference in alpha-band amplitude in the affected time window. In summary, by using a model-based approach we identified early VEPs to be compatible with the evoked theory, while results of late VEPs support a modulatory but not causative role â the latter implied by the oscillatory theory â of alpha activity for EP generation
Spatiotemporal dynamics of spontaneous and evoked large-scale signals in the human brain
RĂ€umlich und zeitlich koordinierte AktivitĂ€ten groĂer Neuronenpopulationen im
Gehirn bilden die Voraussetzung fĂŒr das Entstehen von Kognition und Verhalten.
Die resultierenden SummenaktivitÀten treten als dynamische makroskopische
âlarge-scaleâ Signale in Erscheinung, die mit nichtinvasiven Methoden wie der
Elektroenzephalographie (EEG) und der funktionellen Magnetresonanztomographie
(fMRT) am Menschen gemessen werden können. Die Kenntnis der dynamischen
Eigenschaften von neuronalen large-scale Signalen gibt - in Kombination mit
theoretischen Modellen - Aufschluss ĂŒber allgemeine Prinzipien der
Hirnfunktion. In dieser Habilitationsschrift werden sieben Originalarbeiten
vorgestellt, die sich mit der rÀumlich-zeitlichen Dynamik von spontanen und
evozierten large-scale Signalen im Gehirn befassen. Publikation 1 âEvaluating
gradient artifact correction of EEG data acquired simultaneously with fMRIâ
beschreibt die Problematik von Interferenzen beim gleichzeitigen Erfassen von
EEG und fMRT Daten. Simultanes EEG-fMRT erfasst unterschiedliche Dimensionen
neuronaler AktivitÀt und bietet gleichzeitig eine hohe zeitliche und rÀumliche
Auflösung der gemessenen large-scale Signale. Die Kombination beider Methoden
fĂŒhrt jedoch zu schwerwiegenden Artefakten im EEG-Signal. In der Publikation
werden vorhandene und selbst entwickelte Algorithmen zur Entfernung von âfMRT-
Artefaktenâ vorgestellt und bezĂŒglich ihrer LeistungsfĂ€higkeit evaluiert. In
Publikation 2 âHigh frequency (600Hz) population spikes in human EEG delineate
thalamic and cortical fMRI activation sitesâ lokalisieren wir fMRT Korrelate
innerhalb weniger Millisekunden nacheinander ablaufender ultraschneller
Potentiale entlang der thalamo-kortikalen Bahn in den (vermuteten)
Generatorstrukturen. In Publikation 3 âUltrahigh-frequency EEG during fMRI:
Pushing the limits of imaging-artifact correctionâ demonstrieren wir, wie
diese ultraschnellen EEG-Korrelate von Summenaktionspotentialen gleichzeitig
und kontinuierlich mit fMRT und dem klassischen niedrigfrequenten
(postsynaptisch generierten) EEG erfasst werden können. Koordinierte neuronale
large-scale AktivitÀt wird im EEG in Form von Oszillationen bzw. Rhythmen
sichtbar. Diese treten nicht nur als Antwort auf Ă€uĂere Reize oder Aufgaben
auf, sondern werden kontinuierlich - scheinbar spontan â vom Gehirn generiert.
Publikationen 4 âCorrelates of alpha rhythm in functional magnetic resonance
imaging and near infrared spectroscopy â und 5 âRolandic Alpha and Beta EEG
Rhythmsâ Strengths are Inversely Related to fMRI-BOLD Signal in Primary
Somatosensory and Motor Cortexâ zeigen, dass spontane Amplitudenfluktuationen
des klassischen Alpha- und der subtilen Rolandischen Rhythmen negativ zum
blutogygenierungsabhĂ€ngigen (BOLD) fMRT Signal korreliert sind â und zwar
spezifisch in den Kortexarealen, die den Rhythmus jeweils generieren. Das sind
im Fall des klassischen Alpha-Rhythmus visuelle und fĂŒr die Rolandischen
Rhythmen sensomotorische Regionen. Die negativen Korrelationen deuten drauf
hin, dass bei stark ausgeprÀgten Rhythmen, also in Phasen hoher SynchronizitÀt
innerhalb der Neuronenpopulation, die entsprechenden Generatorareale weniger
Energie verbrauchen. Letzteres ist ein Indiz fĂŒr verminderte neuronale
AktivitĂ€t in diesen Arealen und unterstĂŒtzt die Theorie, dass
Hintergrundrhythmen von inhibierten Kortexarealen generiert werden.
Publikationen 6 âInfluence of Ongoing Alpha Rhythm on the Visual Evoked
Potentialâ und 7 âOscillatory brain states interact with late cognitive
components of the somatosensory evoked potentialâ beleuchten die Bedeutung von
Hintergrundrhythmen fĂŒr die Generierung von evozierten Potentialen (EPs). In
der ersten Arbeit haben wir mit selbst entwickelten theoretischen Modellen
Vorhersagen fĂŒr konkurrierende Theorien getroffen und mit experimentellen
Daten verglichen. So konnten wir nachweisen, dass die Beziehung des Alpha
Rhythmus zu frĂŒhen Anteilen des visuellen EPs (VEP) im Einklang steht mit der
so genannten âEvoked Theoryâ. Diese besagt, dass Rhythmus und EP unabhĂ€ngig
voneinander generiert werden und sich linear summieren. Die ebenfalls populÀre
âPhase-Reset Theoryâ wurde durch unsere Daten nicht bestĂ€tigt. Laut dieser
Theorie entsteht das EP durch die Ausrichtung der Phase des zuvor spontanen
Hintergrundrhythmus zum Stimulus. Wir konnten auĂerdem zeigen, dass der Alpha-
Rhythmus mit der spÀten kognitiven Komponente des VEPs interagiert. In der
zweiten Arbeit, in der wir die Beziehung zwischen Rolandischem Rhythmus und
somatosensorischem EP (SEP) untersuchten, fanden wir ein grundsÀtzlich mit der
visuellen ModalitĂ€t ĂŒbereinstimmendes Verhalten. Unsere Ergebnisse deuten auf
eine funktionelle Bedeutung von Hintergrundrhythmen fĂŒr die höhere kognitive
Verarbeitung von Reizen hin. Wir konnten mit den sieben vorgestellten Arbeiten
zeigen, dass mit den von uns optimierten Verfahren die multimodale Erfassung
von large-scale Signalen des Gehirns möglich ist. Funktionelle MRT,
klassisches EEG sowie ultraschnelle EEG-Korrelate von Summenaktionspotentialen
können gleichzeitig nichtinvasiv am Menschen gemessen werden. Das zeitliche
und rĂ€umliche Verhalten dieser Signaturen gibt Aufschluss ĂŒber die der
Hirnfunktion zugrunde liegenden Prinzipien und Mechanismen.Spatially and temporally coordinated activities of large neuronal populations
in the brain constitute the foundation for the emergence of cognition and
behavior. Resulting population activities can be assessed in terms of large-
scale signals by noninvasive methods such as electroencephalography (EEG) and
functional magnetic resonance imaging (fMRI) noninvasively in humans. The
present habilitation thesis presents seven original works that investigate
spatiotemporal dynamics of spontaneous and evoked large-scale signals in the
human brain. In Publication-1 âEvaluating gradient artifact correction of EEG
data acquired simultaneously with fMRIâ, we describe technical challenges
posed by simultaneous EEG-fMRI acquisitions in terms of emerging artefacts in
the EEG signal caused by electromagnetic interferences between the two
methods. We present different approaches of artefact removal and evaluate
their performance. Publication-2 âHigh frequency (600Hz) population spikes in
human EEG delineate thalamic and cortical fMRI activation sitesâ we localize
the putative generator structures of two bursts of ultrafast neuronal
oscillations succeeding within a few milliseconds along thalamocortical
structures. In publication-3 âUltrahigh-frequency EEG during fMRI: Pushing the
limits of imaging-artifact correctionâ we demonstrate how these ultrafast EEG
correlates of population spikes can be assessed simultaneously and
continuously along with fMRI and with the classical low-frequency EEG
spectrum. Publication-4 âCorrelates of alpha rhythm in functional magnetic
resonance imaging and near infrared spectroscopyâ and Publication-5 âRolandic
Alpha and Beta EEG Rhythmsâ Strengths are Inversely Related to fMRI-BOLD
Signal in Primary Somatosensory and Motor Cortexâ demonstrate that spontaneous
amplitude fluctuations of the prominent posterior alpha rhythm and of the more
subtle Rolandic my-rhythm are correlated negatively with the blood oxygenation
level dependent (BOLD) fMRI signal in areas where the respective rhythm is
generated. These findings are in line with the theory that background rhythms
such as alpha and mu rhythms are generated by inhibited cortical areas.
Publication-6 âInfluence of Ongoing Alpha Rhythm on the Visual Evoked
Potentialâ and Publication-7 âOscillatory brain states interact with late
cognitive components of the somatosensory evoked potentialâ illuminate the
role of background rhythms for the generation of evoked potentials (EPs). Our
results indicate that background rhythms are of functional relevance
specifically for the higher cognitive processing of stimuli. Taken together,
we have demonstrated that with our optimized methodological approach of
simultaneous EEG-fMRI acquisition and analysis a wide spectrum of large-scale
signals of the human brain can be assessed noninvasively. The spatiotemporal
behavior of these signals provides insight into the principles and mechanisms
that underlie human brain function