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      Neural Synchrony in Cortical Networks: History, Concept and Current Status

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          Following the discovery of context-dependent synchronization of oscillatory neuronal responses in the visual system, the role of neural synchrony in cortical networks has been expanded to provide a general mechanism for the coordination of distributed neural activity patterns. In the current paper, we present an update of the status of this hypothesis through summarizing recent results from our laboratory that suggest important new insights regarding the mechanisms, function and relevance of this phenomenon. In the first part, we present recent results derived from animal experiments and mathematical simulations that provide novel explanations and mechanisms for zero and nero-zero phase lag synchronization. In the second part, we shall discuss the role of neural synchrony for expectancy during perceptual organization and its role in conscious experience. This will be followed by evidence that indicates that in addition to supporting conscious cognition, neural synchrony is abnormal in major brain disorders, such as schizophrenia and autism spectrum disorders. We conclude this paper with suggestions for further research as well as with critical issues that need to be addressed in future studies.

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          Most cited references 141

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          Regulation of synaptic efficacy by coincidence of postsynaptic APs and EPSPs.

          Activity-driven modifications in synaptic connections between neurons in the neocortex may occur during development and learning. In dual whole-cell voltage recordings from pyramidal neurons, the coincidence of postsynaptic action potentials (APs) and unitary excitatory postsynaptic potentials (EPSPs) was found to induce changes in EPSPs. Their average amplitudes were differentially up- or down-regulated, depending on the precise timing of postsynaptic APs relative to EPSPs. These observations suggest that APs propagating back into dendrites serve to modify single active synaptic connections, depending on the pattern of electrical activity in the pre- and postsynaptic neurons.
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            Dynamic predictions: oscillations and synchrony in top-down processing.

            Classical theories of sensory processing view the brain as a passive, stimulus-driven device. By contrast, more recent approaches emphasize the constructive nature of perception, viewing it as an active and highly selective process. Indeed, there is ample evidence that the processing of stimuli is controlled by top-down influences that strongly shape the intrinsic dynamics of thalamocortical networks and constantly create predictions about forthcoming sensory events. We discuss recent experiments indicating that such predictions might be embodied in the temporal structure of both stimulus-evoked and ongoing activity, and that synchronous oscillations are particularly important in this process. Coherence among subthreshold membrane potential fluctuations could be exploited to express selective functional relationships during states of expectancy or attention, and these dynamic patterns could allow the grouping and selection of distributed neuronal responses for further processing.
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              Theta oscillations in the hippocampus.

              Theta oscillations represent the "on-line" state of the hippocampus. The extracellular currents underlying theta waves are generated mainly by the entorhinal input, CA3 (Schaffer) collaterals, and voltage-dependent Ca(2+) currents in pyramidal cell dendrites. The rhythm is believed to be critical for temporal coding/decoding of active neuronal ensembles and the modification of synaptic weights. Nevertheless, numerous critical issues regarding both the generation of theta oscillations and their functional significance remain challenges for future research.

                Author and article information

                Front Integr Neurosci
                Front. Integr. Neurosci.
                Frontiers in Integrative Neuroscience
                Frontiers Research Foundation
                20 March 2009
                30 July 2009
                : 3
                1simpleDepartment of Neurophysiology, Max Planck Institute for Brain Research Frankfurt am Main, Germany
                2simpleLaboratory for Neurophysiology and Neuroimaging, Department of Psychiatry, Johann Wolfgang Goethe Universität Frankfurt am Main, Germany
                3simpleFrankfurt Institute for Advanced Studies, Johann Wolfgang Goethe Universität Frankfurt am Main, Germany
                Author notes

                Edited by: Rui M. Costa, Champalimaud Neuroscience Programme, Instituto Gulbenkian de Ciência, Portugal

                Reviewed by: Miles A. Whittington, Newcastle University, UK; Shih-Chieh Lin, Duke University Medical Center, USA

                *Correspondence: Wolf Singer, Department of Neurophysiology, Max Planck Institute for Brain Research, Deutschordenstr. 46, Frankfurt am Main 60528, Germany. e-mail: singer@
                Copyright © 2009 Uhlhaas, Pipa, Lima, Melloni, Neuenschwander, Nikolić and Singer.

                This is an open-access article subject to an exclusive license agreement between the authors and the Frontiers Research Foundation, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are credited.

                Page count
                Figures: 7, Tables: 0, Equations: 0, References: 188, Pages: 19, Words: 18281
                Original Research


                gamma, oscillations, cortex, synchrony, cognition


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