The saccadic spike artifact in MEG

Electro- and magnetoencephalography (EEG/MEG) are the means to investigate the dynamics of neuronal activity non-invasively in the human brain. However, both EEG and MEG are also sensitive to non-neural sources, which can severely complicate the interpretation. The saccadic spike potential (SP) at saccade onset has been identified as a particularly problematic artifact in EEG because it closely resembles synchronous neuronal gamma band activity. While the SP and its confounding effects on EEG have been thoroughly characterized, the corresponding artifact in MEG, the saccadic spike field (SF), has not been investigated. Here we provide a detailed characterization of the SF. We simultaneously recorded MEG, EEG, gaze position and electrooculogram (EOG). We compared the SF in MEG for different saccade sizes and directions and contrasted it with the well-known SP in EEG. Our results reveal a saccade amplitude and direction dependent, lateralized saccadic spike artifact, which was most prominent in the gamma frequency range. The SF was strongest at frontal and temporal sensors but unlike the SP in EEG did not contaminate parietal sensors. Furthermore, we observed that the source configurations of the SF were comparable for regular and miniature saccades. Using distributed source analysis we identified the sources of the SF in the extraocular muscles. In summary, our results show that the SF in MEG closely resembles neuronal activity in frontal and temporal sensors. Our detailed characterization of the SF constitutes a solid basis for assessing possible saccadic spike related contamination in MEG experiments.the European Union ; the German Federal Ministry of Education and Researchpublisher versio

High frequency oscillations as a correlate of visual perception

“NOTICE: this is the author’s version of a work that was accepted for publication in International journal of psychophysiology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in International journal of psychophysiology , 79, 1, (2011) DOI 10.1016/j.ijpsycho.2010.07.004Peer reviewedPostprin

Sequential Activation of Human Oculomotor Centers During Planning of Visually-Guided Eye Movements: A Combined fMRI-MEG Study

We used magneto-encephalography (MEG) to measure visually evoked activity in healthy volunteers performing saccadic eye movements to visual targets. The neuromagnetic activity was analyzed from regions of cortical activation identified in separate functional magnetic resonance imaging (fMRI) studies. The latency of visual responses significantly increased from the Middle Temporal region (MT+) to the Intraparietal Sulcus (IPS) to the Frontal Eye Field (FEF), and their amplitude was greater in the hemisphere contralateral to the visual target. Trial-to-trial variability of oculomotor reaction times correlated with visual response latency across cortical areas. These results support a feedforward recruitment of oculomotor cortical centers by visual information, and a model in which behavioral variability depends on variability at different neural stages of processing

Manifestation of ocular-muscle EMG contamination in human intracranial recordings

It is widely assumed that intracranial recordings from the brain are only minimally affected by contamination due to ocular-muscle electromyogram (oEMG). Here we show that this is not always the case. In intracranial recordings from five surgical epilepsy patients we observed that eye movements caused a transient biphasic potential at the onset of a saccade, resembling the saccadic spike potential commonly seen in scalp EEG, accompanied by an increase in broadband power between 20 and 200 Hz. Using concurrently recorded eye movements and high-density intracranial EEG (iEEG) we developed a detailed overview of the spatial distribution and temporal characteristics of the saccade-related oculomotor signal within recordings from ventral, medial and lateral temporal cortex. The occurrence of the saccadic spike was not explained solely by reference contact location, and was observed near the temporal pole for small (< 2 deg) amplitude saccades and over a broad area for larger saccades. We further examined the influence of saccade-related oEMG contamination on measurements of spectral power and interchannel coherence. Contamination manifested in both spectral power and coherence measurements, in particular, over the anterior half of the ventral and medial temporal lobe. Next, we compared methods for removing the contaminating signal and found that nearest-neighbor bipolar re-referencing and ICA filtering were effective for suppressing oEMG at locations far from the orbits, but tended to leave some residual contamination at the temporal pole. Finally, we show that genuine cortical broadband gamma responses observed in averaged data from ventral temporal cortex can bear a striking similarity in time course and band-width to oEMG contamination recorded at more anterior locations. We conclude that eye movement-related contamination should be ruled out when reporting high gamma responses in human intracranial recordings, especially those obtained near anterior and medial temporal lobe

High-frequency brain activity and muscle artifacts in MEG/EEG: A review and recommendations

In recent years high-frequency brain activity in the gamma-frequency band (30–80 Hz) and above has become the focus of a growing body of work in MEG/EEG research. Unfortunately, high-frequency neural activity overlaps entirely with the spectral bandwidth of muscle activity (~20–300 Hz). It is becoming appreciated that artifacts of muscle activity may contaminate a number of non-invasive reports of high-frequency activity. In this review, the spectral, spatial, and temporal characteristics of muscle artifacts are compared with those described (so far) for high-frequency neural activity. In addition, several of the techniques that are being developed to help suppress muscle artifacts in MEG/EEG are reviewed. Suggestions are made for the collection, analysis, and presentation of experimental data with the aim of reducing the number of publications in the future that may contain muscle artifacts

Atypical disengagement from faces and its modulation by the control of eye fixation in children with Autism Spectrum Disorder

By using the gap overlap task, we investigated disengagement from faces and objects in children (9–17 years old) with and without autism spectrum disorder (ASD) and its neurophysiological correlates. In typically developing (TD) children, faces elicited larger gap effect, an index of attentional engagement, and larger saccade-related event-related potentials (ERPs), compared to objects. In children with ASD, by contrast, neither gap effect nor ERPs differ between faces and objects. Follow-up experiments demonstrated that instructed fixation on the eyes induces larger gap effect for faces in children with ASD, whereas instructed fixation on the mouth can disrupt larger gap effect in TD children. These results suggest a critical role of eye fixation on attentional engagement to faces in both groups

Synchronization of medial temporal lobe and prefrontal rhythms in human decision-making

Optimal decision making requires that we integrate mnemonic information regarding previous decisions with value signals that entail likely rewards and punishments. The fact that memory and value signals appear to be coded by segregated brain regions, the hippocampus in the case of memory and sectors of prefrontal cortex in the case of value, raises the question as to how they are integrated during human decision making. Using magnetoencephalography to study healthy human participants, we show increased theta oscillations over frontal and temporal sensors during nonspatial decisions based on memories from previous trials. Using source reconstruction we found that the medial temporal lobe (MTL), in a location compatible with the anterior hippocampus, and the anterior cingulate cortex in the medial wall of the frontal lobe are the source of this increased theta power. Moreover, we observed a correlation between theta power in the MTL source and behavioral performance in decision making, supporting a role for MTL theta oscillations in decision-making performance. These MTL theta oscillations were synchronized with several prefrontal sources, including lateral superior frontal gyrus, dorsal anterior cingulate gyrus, and medial frontopolar cortex. There was no relationship between the strength of synchronization and the expected value of choices. Our results indicate a mnemonic guidance of human decision making, beyond anticipation of expected reward, is supported by hippocampal–prefrontal theta synchronization

Looking for a face in the crowd: Fixation-related potentials in an eye-movement visual search task

Despite the compelling contribution of the study of event related potentials (ERPs) and eye movements to cognitive neuroscience, these two approaches have largely evolved independently. We designed an eye-movement visual search paradigm that allowed us to concurrently record EEG and eye movements while subjects were asked to find a hidden target face in a crowded scene with distractor faces. Fixation event-related potentials (fERPs) to target and distractor stimuli showed the emergence of robust sensory components associated with the perception of stimuli and cognitive components associated with the detection of target faces. We compared those components with the ones obtained in a control task at fixation: qualitative similarities as well as differences in terms of scalp topography and latency emerged between the two. By using single trial analyses, fixations to target and distractors could be decoded from the EEG signals above chance level in 11 out of 12 subjects. Our results show that EEG signatures related to cognitive behavior develop across spatially unconstrained exploration of natural scenes and provide a first step towards understanding the mechanisms of target detection during natural search.Fil: Kaunitz, Lisandro N.. University of Leicester; Reino UnidoFil: Kamienkowski, Juan Esteban. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Neurociencia Integrativa; Argentina. Universidad Diego Portales; Chile;Fil: Varatharajah, Alexander. University of Leicester; Reino UnidoFil: Sigman, Mariano. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto de Física del Sur; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Neurociencia Integrativa; ArgentinaFil: Quian Quiroga, Rodrigo. University of Leicester; Reino UnidoFil: Ison, Matias Julian. University of Leicester; Reino Unid

Varhaisten visuaalisten EEG vasteiden mallintaminen kuva-ärsykkeen ominaisuuksien mukaan

Introduction Experiment consisting of a visual search was conducted in order to investigate the effect of top-down and bottom-up processing with natural stimuli. Gray-scale photographs of nature scenes were used as stimuli. Stimuli had two conditions: natural and scrambled. Natural images were unaltered photographs and in scrambled images the global information was reduced. Aim Aim of the experiment was to form statistical models explaining the effect the amplitude of early visual potentials as functions of visual input, oculomotor variables and top-down factors. Methods Magnetoencephalography (MEG), electroencephalography (EEG) and eye tracking were recorded simultaneously during the experiment. In the scope of this thesis only EEG and eye tracking were analysed. Statistical models were generated using a method called linear mixed (effect) modeling. Results Data analysis produced two models describing the early visual potential as parameters of visual input and oculomotor variables. The effect of top-down processing was investigated as an additional statistical test. Conclusion Out of the two generated models the visual input model was deemed more accurate due to spatial focality and amplitude latency. Results of the study indicate that early visual responses in EEG correlate strongly with low-level visual inputs and to a lesser degree with oculomotor variables. No evidence of correlation between response amplitude and top-down factors were observed.Johdanto Visuaalista tarkkaavaisuutta tutkittiin koeasetelmassa, jossa koehenkilöt etsivät heille esitettyjä kuvan osia suuremmista luontovalokuvista. Kuvia oli kahta tyyppiä: muokkaamattomia, sekä kuvia joista globaali informaatio oli hävitetty. Tavoite Kokeen tavoitteena oli kehittää visuaalisia hermostovasteita kuvaava tilastollinen malli. Mallin avulla oli tarkoitus tutkia kuinka matalan ja korkean tason tarkkaavaisuusprosessit vaikuttavat mitattuun vasteeseen. Menetelmät Kokeen aikana koehenkilöiltä mitattiin aivosähkökäyrää (EEG), aivomagneettikäyrää (MEG) sekä silmänliikettä. Kerätty data analysoitiin mallintamalla katseeseen synkronisoituja EEG-vasteita tilastollisesti. Kokeessa kerättyä MEG dataa ei analysoitu tämän työn puitteissa. Mallintamiseen käytettiin lineaarisia sekamalleja, jotka muodostettiin ärsykekuvien, silmänliikkeiden ja tarkkaavaisuusprosessien avulla. Tulokset Mallintaminen tuotti kaksi erilaista mallia, jotka selittivät syntyneen vasteen visuaali-informaation ja silmänliikkeiden perusteella. Korkean tason tarkkaavaisuuden vaikutusta tutkittiin molemmissa malleissa ylimääräisellä tilastollisella testillä. Johtopäätökset Kahdesta tuotetusta mallista visuaalimalli vaikutti todenmukaisemmalta. Silmänliikemallin tulokset olivat puolestaan epävarmempia sijaintinsa ja esiintymislatenssinsa takia. Kummankaan mallin tapauksessa ei havaittu todisteita korkean tason tarkkaavaisuuden vaikutuksesta vasteisiin