Papers by Andreas Ioannides
Current Research in Physiology, 2022
High amplitude electroencephalogram (EEG) events, like unitary K-complex (KC), are used to partit... more High amplitude electroencephalogram (EEG) events, like unitary K-complex (KC), are used to partition sleep into stages and hence define the hypnogram, a key instrument of sleep medicine. Throughout sleep the heart rate (HR) changes, often as a steady HR increase leading to a peak, what is known as a heart rate surge (HRS). The hypnogram is often unavailable when most needed, when sleep is disturbed and the graphoelements lose their identity. The hypnogram is also difficult to define during normal sleep, particularly at the start of sleep and the periods that precede and follow rapid eye movement (REM) sleep. Here, we use objective quantitative criteria that group together periods that cannot be assigned to a conventional sleep stage into what we call REM0 periods, with the presence of a HRS one of their defining properties. Extended REM0 periods are characterized by highly regular sequences of HRS that generate an infra-low oscillation around 0.05 Hz. During these regular sequence of HRS, and just before each HRS event, we find avalanches of high amplitude events for each one of the mass electrophysiological signals, i.e. related to eye movement, the motor system and the general neural activity. The most prominent features of long REM0 periods are sequences of three to five KCs which we label multiple K-complexes (KCm). Regarding HRS, a clear dissociation is demonstrated between the presence or absence of high gamma band spectral power (55–95 Hz) of the two types of KCm events: KCm events with strong high frequencies (KCmWSHF) cluster just before the peak of HRS, while KCm between HRS show no increase in high gamma band (KCmNOHF). Tomographic estimates of activity from magnetoencephalography (MEG) in pre-KC periods (single and multiple) showed common increases in the cholinergic Nucleus Basalis of Meynert in the alpha band. The direct contrast of KCmWSHF with KCmNOHF showed increases in all subjects in the high sigma band in the base of the pons and in three subjects in both the delta and high gamma bands in the medial Pontine Reticular Formation (mPRF), the putative Long Lead Initial pulse (LLIP) for Ponto-Geniculo-Occipital (PGO) waves.
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European Journal of Neuroscience, 1998
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Biomag 96, 2000
A general method for obtaining continuous solutions to the biomagnetic inverse problem was first ... more A general method for obtaining continuous solutions to the biomagnetic inverse problem was first presented during the 1989 Biomagnetism conference. From the outset, the method was developed for three-dimensional source spaces [1, 2] but, partly to reduce computational demands and partly for ease of presentation, the first major publication of the method [3] used two-dimensional reconstructions. Soon afterwards the algorithms were ported to a transputer, making possible the analysis of large sets of MEG data. The output (estimates of the primary current density in a three-dimensional source space) was displayed by taking slices (tomes) through the source space leading to a series of MFT (Magnetic Field Tomographie) images. Later forms of representation included time integrals and activation curves, describing the evolution of activity within specified Regions of Interest (ROI). Animations run on the transputer array and recorded on video, were by far the most effective presentation tool, showing in colour-coded form the changes in the activity against a background of coregistered MRI slices[4]. Early reconstructions used averaged data but the emphasis has recently shifted to analysis of single trial or continuous data [5, 6]. Each three-dimensional study was underpinned by thorough tests with Computer generated data which, however, have been only superficially reported because of space limitations. We have previously described our philosophy and method [3], the transputer based implementation [7] and the logic in the various algorithmic steps [8]. However, the lack of published material on the background tests must be the cause of misrepresentations of MFT in some recent publications dealing with distributed source analysis, e.g. [9]. The present paper makes good this omission and uses computer-generated data to illustrate the power and limitations of MFT as a 3D imaging method, using realistic sensor configurations and signals that are neurophysiologically realistic in terms of location and temporal characteristics.
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Encyclopedia of Neuroscience
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Frontiers in Applied Mathematics and Statistics
In this article, we present a unified framework for the analysis and characterization of a comple... more In this article, we present a unified framework for the analysis and characterization of a complex system and demonstrate its application in two diverse fields: neuroscience and astrophysics. The framework brings together techniques from graph theory, applied mathematics, and dimensionality reduction through principal component analysis (PCA), separating linear PCA and its extensions. The implementation of the framework maps an abstract multidimensional set of data into reduced representations, which enable the extraction of its most important properties (features) characterizing its complexity. These reduced representations can be sign-posted by known examples to provide meaningful descriptions of the results that can spur explanations of phenomena and support or negate proposed mechanisms in each application. In this work, we focus on the clustering aspects, highlighting relatively fixed stable properties of the system under study. We include examples where clustering leads to sem...
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Proceedings of the International Joint Conference on Neural Networks, 2003.
... [IO]Tzclcpi A., loannidcs A. A. and Poghosyan V. Early (N70m) [IIlMoradi F., Liu LC, ... Clin... more ... [IO]Tzclcpi A., loannidcs A. A. and Poghosyan V. Early (N70m) [IIlMoradi F., Liu LC, ... Clin. Neurophysiol. 52: 298 - 305 (1981) [24]Ress D., Benjamin T., Backus T. and Heeger D. J. Activity in primary visual cortex predicts performance in a visual detecting task. Norure Neurosci. ...
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A study is made of the propagation of a deuteron through a nuclear medium. We consider the effect... more A study is made of the propagation of a deuteron through a nuclear medium. We consider the effect of the Pauli exclusion principle on the internal motion of the deuteron, for the idealized case of a deuteron propagating through infinite nuclear matter, assumed to be adequately described by the Fermi-gas model. This greatly simplifies the calculations and enables us to include explicitly the D-state component of the deuteron wavefunction in our model. It is shown that the action of the Pauli exclusion principle, on the D-state part of the deuteron, endowes various properties of the deuteron with strong dependence on the relative orientation of the deuteron spin and its linear momentum. The dependence of the binding energy of the deuteron on this relative orientation is particularly important, because, it can be associated with a new tensor term, of the T[p] type, in the deuteron optical potential. The strength of this tensor potential is found to be non-negligible at high incident de...
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We investigate where the brain responds to affective and cognitive advertising stimuli. Subjects ... more We investigate where the brain responds to affective and cognitive advertising stimuli. Subjects were exposed to brand advertising whilst patterns of neural activity were being recorded by magnetoencephalography (MEG), a non-invasive technique which provides brain images for later analysis. The hypotheses were, first that more affective ads would be distinguished from more cognitive ads by observable, distinct brain activations and second that the affective sections of the ads would stimulate those parts of the brain shown to be associated with affect in previous neuroscience research unconnected with advertising. The hypotheses were broadly supported.
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Advanced Methods in the Evaluation of Nuclear Scattering Data
ABSTRACT
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NeuroImage, 1998
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Frontiers in Human Neuroscience, 2016
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Frontiers in Human Neuroscience, 2016
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Frontiers in Human Neuroscience, 2016
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Frontiers in Human Neuroscience, 2016
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Advances in Clinical Neurophysiology, 2012
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Cyberphysical Systems for Epilepsy and Related Brain Disorders, 2015
Epilepsy and sleep are characterized by spontaneously occurring large graphoelements in electroen... more Epilepsy and sleep are characterized by spontaneously occurring large graphoelements in electroencephalography (EEG) and magnetoencephalography (MEG) recordings, such as ictal and interictal epileptiform discharges in epilepsy and K-complexes (KC) in sleep. Localization of the neural sources of these graphoelements, which is of immense clinical and research importance, requires application of electromagnetic source analysis methods. A number of such methods are available; however their ability to localize widespread synchronous cortical sources, such as the sources of KCs and widespread epileptiform discharges, is contested. Here, we used KC as an exemplar of large graphoelements with such sources to test the performance of a diverse set of commonly employed source analysis methods. We analyzed segments of sleep MEG data with clear KCs using equivalent current dipole models, beamformer methods, linear distributed source methods, and a non-linear distributed source method—magnetic field tomography (MFT). MFT provided the most robust and steady localization across KCs, which was also highly consistent with the intracranial findings: strong and widespread activations were reliably found in superior aspects of bilateral frontal cortex. Conversely, the localizations provided by the other methods were very variable across KCs and were all inconsistent with the intracranial findings: in many cases, the KCs were incorrectly localized in deep medial brain structures. Our current and earlier results showing the excellent localization accuracy of MFT for focal as well as extended brain sources and the smart uses of MEG and EEG in epilepsy, demonstrate that the MFT analysis of MEG signals may be a powerful tool for the studies of epilepsy, epilepsy monitoring and in pre-surgical evaluation of patients.
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Arousal and valence are the primary dimensions of human emotion. However, the degree to which the... more Arousal and valence are the primary dimensions of human emotion. However, the degree to which these dimensions correlate to complex subcortical structures (i.e. amygdala, cerebellum) that are anatomically homogeneous is still elusive. Magneto-encephalography (MEG) recordings were performed on 12 healthy individuals exposed to affective stimuli from the International Affective Picture System (IAPS) collection using a 2 (Valence levels) x 2 (Arousal levels) design. Source power was estimated using a beamformer within 1-30 and 30-100 Hz bands. Activations referring to the subcortical sub-regions were defined through probabilistic cytoarchitectonic maps (PCMs). Within the 1-30 Hz band, right laterobasal (LB) amygdala activity mediates negative valence (elicited by unpleasant stimuli) while left centromedial (CM) activity correlates to the interaction of valence by arousal (arousing pleasant stimuli). Within the 30-100 Hz band, cerebellar VIIIa lobule of the Vermis and left hemispheric VIIa Crus II lobule activity mediate high arousal while left hemispheric V lobule correlates to the interaction of valence by arousal (arousing pleasant stimuli). Our results support that distinct sub-regional subcortical activity responds specifically to valence and arousal dimensions as well as combinations of the two, pronouncing the sophisticated nature of emotion. Given the anatomical interconnections between amygdala and cerebellum, future studies may focus on the interplay of their specific sub-regions.
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Physics Letters B, 1986
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Papers by Andreas Ioannides