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Results for “Cortical Synchronization”

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At least 235 records · Page 13Linked to original sources

PubMed1999

Asynchronous pentobarbital-induced burst suppression with corpus callosum hemorrhage.

OBJECTIVE: We describe the electroencephalographic (EEG) findings in a 9-year-old girl, who presented with generalized tonic-clonic status epilepticus requiring pentobarbital anesthesia, and correlate these findings with clinicoradiologic evidence of a ruptured AVM with hemorrhage into the body of the corpus callosum. METHODS: EEG analysis accompanied by clinical assessment, CT and MRI scans, and cerebral angiography were performed. RESULTS: With pentobarbital coma, the EEG showed burst suppression with prominent interhemispheric asynchrony. Suppression epochs >2 s in duration and with amplitude <20 microV in all channels were identified. In 12 min of the EEG analyzed, 6 unilateral and 20 bilateral epochs occurred. Of the 20 bilateral suppression epochs, interhemispheric asynchrony of >1 s was noted at onset for 5 epochs and at offset for one. Chi-square analysis revealed an equal tendency for unilateral suppressions to occur over either hemisphere, and for suppression in one hemisphere to begin before the other. CONCLUSIONS: We conclude that the corpus callosum plays a critical role in interhemispheric synchronization of cortical neuronal electrical activity and propose that: (1) normally, the corpus callosum modulates interhemispheric synchronization of cortical inhibition; and (2) with corpus callosal disruption, cortical areas are 'released' from such synchronization.

Cerebral Hemorrhage↗

PubMed1998

Spike-wave complexes and fast components of cortically generated seizures. III. Synchronizing mechanisms.

The intracortical and thalamocortical synchronization of spontaneously occurring or bicuculline-induced seizures, consisting of spike-wave (SW) or polyspike-wave (PSW) complexes at 2-3 Hz and fast runs at 10-15 Hz, was investigated in cats under ketamine-xylazine anesthesia. We used single and dual simultaneous intracellular recordings from cortical areas 5 and 7, and extracellular recordings of unit firing and field potentials from neocortical areas 5, 7, 17, 18, as well as related thalamic nuclei. The evolution of time delays between paroxysmal depolarizing events in single neurons or neuronal pools recorded from adjacent and distant sites was analyzed by using 1) sequential cross-correlations between field potentials, 2) averaged activities triggered by the spiky component of cortical SW/PSW complexes, and 3) time histograms between neuronal discharges. In all instances, the paroxysmal activities recorded from the dorsal thalamus lagged the onset of seizures in neocortex. The time lags between simultaneously impaled cortical neurons were significantly smaller during SW complexes than during the prior epochs of slow oscillation. During seizures, as during the slow oscillation, the intracortical synchrony was reduced with increased distance between different cortical sites. Dual intracellular recordings showed that, during the same seizure, time lags were not constant and, instead, reflected alternating precession of the recorded foci. After transection between areas 5 and 7, the intracortical synchrony was lost, but corticothalamocortical volleys could partially restore seizure synchrony. These data show that the neocortex leads the thalamus during SW/PSW seizures, that time lags between cortical foci are not static, and that thalamus may assist synchronization of SW/PSW seizures after disconnection of intracortical synaptic linkages.

Anesthesia↗

PubMed1996

Event-related synchronization (ERS) in the alpha band--an electrophysiological correlate of cortical idling: a review.

EEG desynchronization is a reliable correlate of excited neural structures of activated cortical areas. EEG synchronization within the alpha band may be an electrophysiological correlate of deactivated cortical areas. Such areas are not processing sensory information or motor output and can be considered to be in an idling state. One example of such an idling cortical area is the enhancement of mu rhythms in the primary hand area during visual processing or during foot movement. In both circumstances, the neurons in the hand area are not needed for visual processing or preparation for foot movement. As a result of this, an enhanced hand area mu rhythm can be observed.

Alpha Rhythm↗

PubMed1983

EEG effects of microinjection of naloxone in the region of the nucleus tractus solitarius of the rat.

In recent years, we have carried out several studies on brainstem mechanisms associated with pain modulation. Of particular interest has been the region of the nucleus tractus solitarius (NTS) at the level of the area postrema (AP) in the dorsomedial medulla. This region has been found to be rich in opiate receptors and to be involved in pain modulation. Having previously demonstrated the EEG and behavioral effects of systemic administration of morphine, we initiated the present study to determine whether microinjection of naloxone directly into the region of the NTS will modify any or all of the EEG effects induced by subsequent systemic administration of morphine. Our results have shown that microinjections of naloxone into the region of the NTS block the cortical EEG synchronizing effects of systemically administered morphine. These results indicate that morphine-induced cortical EEG synchronization is mediated through the region of the NTS. This region may, therefore, comprise part of an opiate sensitive system which can influence cortical EEG activity.

Animals↗

PubMedDate not supplied

Correlation of preoptic neuronal activity with spontaneous and induced cortical EEG changes.

Preoptic area has neurons which change their firing rate along with spontaneous alterations of the cortical EEG between synchronization and desynchronization. The cortical EEG synchronization and desynchronization could be induced by stimulation of the caudal and the rostral brain stem respectively. This study was aimed at finding out whether stimulation of the brain stem structures produce the same change in the unit activity as that occurring during spontaneous alteration of the EEG. The changes in unit activity showed some concordance and dissociation between spontaneous and induced EEG alterations. The possible interaction of inputs from cortex and brain stem at the level of the preoptic area is discussed.

Animals↗

PubMed2004

Glutamate regulates the frequency of spontaneous synchronized Ca2+ spikes through group II metabotropic glutamate receptor in cultured mouse cortical networks.

1. Synchronized spontaneous intracellular Ca2+ spikes in networked neurons are believed to play a major role in the development and plasticity of neural circuits. Glutamate-induced signals through the ionotropic glutamate receptors (iGluRs) are profoundly involved in the generation of synchronized Ca2+ spikes. 2. In this study, we examined the involvement of metabotropic glutamate receptors (mGluRs) in cultured mouse cortical neurons. We pharmacologically revealed that glutamate-induced signals through inclusive mGluRs decreased the frequency of Ca2+ spikes. Further experiments indicated that this suppressive effect on the spike frequency was mainly due to the signal through group II mGluR, inactivation of adenylate cyclase-cAMP-PKA signaling pathway. Group I mGluR had little involvement in the spike frequency. 3. Taken together, glutamate generates the synchronized Ca2+ spikes through iGluRs and modulates simultaneously their frequency through group II mGluR-adenylate cyclase-cAMP-PKA signaling pathway in the present in vitro neural network. These results provide the evidence of the profound role of group II mGluR in the spontaneous and synchronous neural activities.

Action Potentials↗

PubMed1995

Synchronized oscillations in interneuron networks driven by metabotropic glutamate receptor activation.

Partially synchronous 40-Hz oscillations of cortical neurons have been implicated in cognitive function. Specifically, coherence of these oscillations between different parts of the cortex may provide conjunctive properties to solve the 'binding problem': associating features detected by the cortex into unified perceived objects. Here we report an emergent 40-Hz oscillation in networks of inhibitory neurons connected by synapses using GABAA (gamma-aminobutyric acid) receptors in slices of rat hippocampus and neocortex. These network inhibitory postsynaptic potential oscillations occur in response to the activation of metabotropic glutamate receptors. The oscillations can entrain pyramidal cell discharges. The oscillation frequency is determined both by the net excitation of interneurons and by the kinetics of the inhibitory postsynaptic potentials between them. We propose that interneuron network oscillations, in conjunction with intrinsic membrane resonances and long-loop (such as thalamocortical) interactions, contribute to 40-Hz rhythms in vivo.

Action Potentials↗

PubMed2003

Induced gamma-band activity and human brain function.

Oscillatory activity in the gamma-band range has been related both to gestalt perception and to cognitive functions such as attention, learning, and memory. After giving a brief account of recent findings from electroencephalography and intracortical recordings, the present review will focus on spectral activity in the magnetoencephalogram. Here, gamma-band effects are topographically more local and involve higher frequencies than in the electroencephalogram. Bottom-up-driven auditory spatial mismatch detection elicits gamma-band activity over posterior parietal cortex, whereas auditory pattern mismatch processing leads to gamma-band enhancements over anterior temporal and inferior frontal regions. These topographies support representations of auditory spatial and pattern information in the putative dual auditory "where" and "what" pathways, respectively. During top-down-guided auditory spatial and pattern-working memory tasks, prefrontal gamma-band increases are observed in addition to activations over putative auditory stream areas. Moreover, stimulus maintenance in working memory is accompanied by coherence increases between sensory and prefrontal regions. Gamma-band topographies in magnetoencephalogram are highly comparable with hemodynamic brain imaging studies but yield additional information on the temporal dynamics of activations and connectivity patterns. In summary, magnetoencephalographic gamma-band activity revealed both local synchronization patterns and cortico-cortical interactions accompanying cognitive processes at a good spatial and high temporal resolution.

Animals↗

PubMed1995

The mechanisms of generation and propagation of synchronized bursting in developing networks of cortical neurons.

The characteristics and mechanisms of synchronized firing in developing networks of cultured cortical neurons were studied using multisite recording through planar electrode arrays (PEAs). With maturation of the network (from 3 to 40 d after plating), the frequency and propagation velocity of bursts increased markedly (approximately from 0.01 to 0.5 Hz and from 5 to 100 mm/sec, respectively), and the sensitivity to extracellular magnesium concentration (0-10 mM) decreased. The source of spontaneous bursts, estimated from the relative delay of onset of activity between electrodes, varied randomly with each burst. Physical separation of synchronously bursting networks into several parts using an ultraviolet laser, divided synchronous bursting into different frequencies and phases in each part. Focal stimulation through the PEA was effective at multiple sites in eliciting bursts, which propagated over the network from the site of stimulation. Stimulated bursts exhibited both an absolute refractory period and a relative refractory period, in which partially propagating bursts could be elicited. Periodic electrical stimulation (at 1 to 30 sec intervals) produced slower propagation velocities and smaller numbers of spikes per burst at shorter stimulation intervals. These results suggest that the generation and propagation of spontaneous synchronous bursts in cultured cortical neurons is governed by the level of spontaneous presynaptic firing, by the degree of connectivity of the network, and by a distributed balance between excitation and recovery processes.

Animals↗

PubMed1996

Synchronized chaos in local cortical circuits.

Neurons in cortical slices emit spikes or bursts of spikes regularly in response to a suprathreshold current injection. This is in marked contrast to the behavior of cortical neurons in-vivo, whose response to electrical or sensory input displays a strong degree of irregularity. Correlation measurements show significant degree of synchrony in the temporal fluctuations of neuronal activities in cortex. We explore the hypothesis that these phenomena are the result of the synchronized chaos generated by the deterministic dynamics of local cortical networks. A model of a "hypercolumn" in the visual cortex is studied. It consists of two populations of neurons, one inhibitory and one excitatory. The dynamics of the neurons is based on a Hodgkin-Huxley type model with several cellular and synaptic conductances. The pattern of connectivity is correlated with the internal organization of hypercolumns in orientation columns. Numerical simulations of the model show that in an appropriate parameter range, the network settles in a synchronous chaotic state, characterized by a strong temporal variability of the neural activity which is correlated across the hypercolumn. Strong inhibitory feedback is essential for the stabilization of this state. Auto-correlation and cross-correlation functions of neuronal spike trains are computed, and analyzed. The relation between the results of the model and experiments in visual cortex is discussed.

Animals↗

PubMed2001

Temporal and rate representations of time-varying signals in the auditory cortex of awake primates.

Because auditory cortical neurons have limited stimulus-synchronized responses, cortical representations of more rapidly occurring but still perceivable stimuli remain unclear. Here we show that there are two largely distinct populations of neurons in the auditory cortex of awake primates: one with stimulus-synchronized discharges that, with a temporal code, explicitly represented slowly occurring sound sequences and the other with non-stimulus-synchronized discharges that, with a rate code, implicitly represented rapidly occurring events. Furthermore, neurons of both populations displayed selectivity in their discharge rates to temporal features within a short time-window. Our results suggest that the combination of temporal and rate codes in the auditory cortex provides a possible neural basis for the wide perceptual range of temporal information.

Acoustic Stimulation↗

PubMed2006

Delayed synchronization of activity in cortex and subthalamic nucleus following cortical stimulation in the rat.

Oscillations may play a role in the functional organization of cortico-basal ganglia-thalamocortical circuits, and it is important to understand their underlying mechanisms. The cortex often drives basal ganglia (BG) activity, and particularly, oscillatory activity in the subthalamic nucleus (STN). However, the STN may also indirectly influence cortex. The aim of this study was to characterize the delayed (>200 ms) responses of STN neurons to synchronized cortical inputs, focusing on their relationship with oscillatory cortical activity. We recorded the short-latency and delayed responses of STN units and frontal electrocorticogram (ECoG) to cortical stimulation in anaesthetized rats. Similar to previous studies, stimulation of ipsilateral frontal cortex, but not temporal cortex, evoked a short-latency triphasic response, followed by a sustained reduction or pause in firing, in rostral STN units. Caudal STN units did not show the short-latency triphasic response but often displayed a prolonged firing reduction. Oscillations in STN unit activity and ECoG were common after this sustained firing reduction, particularly between 200 and 600 ms after frontal cortical stimulation. These delayed oscillations were significantly coherent in a broad frequency band of 5-30 Hz. Coherence with ECoG at 5-15 Hz was observed throughout STN, though coherence at 15-30 Hz was largely restricted to rostral STN. Furthermore, oscillatory responses at 5-30 Hz in rostral STN predominantly led those in cortex (mean latency of 29 ms) after frontal cortical stimulation. These findings suggest that STN neurons responding to corticosubthalamic inputs may provide a delayed input to cortex, via BG output nuclei, and thence, thalamocortical pathways.

Action Potentials↗

PubMed2003

Cortical operational synchrony during audio-visual speech integration.

Information from different sensory modalities is processed in different cortical regions. However, our daily perception is based on the overall impression resulting from the integration of information from multiple sensory modalities. At present it is not known how the human brain integrates information from different modalities into a unified percept. Using a robust phenomenon known as the McGurk effect it was shown in the present study that audio-visual synthesis takes place within a distributed and dynamic cortical networks with emergent properties. Various cortical sites within these networks interact with each other by means of so-called operational synchrony (Kaplan, Fingelkurts, Fingelkurts, & Darkhovsky, 1997). The temporal synchronization of cortical operations processing unimodal stimuli at different cortical sites reveals the importance of the temporal features of auditory and visual stimuli for audio-visual speech integration.

Cerebral Cortex↗

PubMed1998

State-dependent receptive-field restructuring in the visual cortex.

To extract important information from the environment on a useful timescale, the visual system must be able to adapt rapidly to constantly changing scenes. This requires dynamic control of visual resolution, possibly at the level of the responses of single neurons. Individual cells in the visual cortex respond to light stimuli on particular locations (receptive fields) on the retina, and the structure of these receptive fields can change in different contexts. Here we show experimentally that the shape of receptive fields in the primary visual cortex of anaesthetized cats undergoes significant modifications, which are correlated with the general state of the brain as assessed by electroencephalography: receptive fields are wider during synchronized states and smaller during non-synchronized states. We also show that cortical receptive fields shrink over time when stimulated with flashing light spots. Finally, by using a network model we account for the changing size of the cortical receptive fields by dynamically rescaling the levels of excitation and inhibition in the visual thalamus and cortex. The observed dynamic changes in the sizes of the cortical receptive field could be a reflection of a process that adapts the spatial resolution within the primary visual pathway to different states of excitability.

Animals↗

PubMed2005

Attentional modulation of SSVEP power depends on the network tagged by the flicker frequency.

Modulation of the steady-state visual evoked potential (SSVEP) by attention was studied in detail using 15 'tag' frequencies in the range of 2.5-20 Hz. The stimuli were two series of random disc search arrays superimposed on two concentric color-marked annuli respectively. Two series of arrays were updated independently; one updated at one fixed frequency (flicker) and the other updated randomly according to a white noise distribution (random broadband flicker, rbbf). On each trial, the observer was instructed to attend one annulus and to detect a target (a triangle) that occasionally appeared in a random disc array in the attended annulus. The SSVEP results show that the choice of flicker frequency selects which cortical network synchronizes to the flicker two distinct cortical networks showed different effects of attention. SSVEP power and the effects of attention on SSVEP power strongly depend on both flicker frequency and radial position of rbbf annulus. At flicker frequencies in the delta band (2-4 Hz), and in the upper alpha band (10-11 Hz), an occipital-frontal network appears to phase-lock to the flicker when attending to the flicker, increasing the magnitude of the SSVEP. At flicker frequencies in the lower alpha band (8-10 Hz), a global response to a peripheral flickering stimulus, that includes parietal cortex and posterior frontal cortex, has higher amplitude when attention is directed away from the flickering peripheral stimulus and towards a competing rbbf stimulus in the fovea. Increases in SSVEP power when attention is directed to peripheral flicker are always associated with increases in phase locking. By contrast, at frequencies in the lower alpha band, increases in SSVEP power when attention is directed away from the flicker and towards foveal stimuli are not associated with changes in phase-locking. Thus, whether attention to a flicker stimulus increases or decreases SSVEP amplitude and phase locking depends on which of two cortical networks, which have distinct spatial and dynamic properties, is selected by the flicker frequency.

Action Potentials↗

PubMed2002

Neural synchronization mediates on-line sentence processing: EEG coherence evidence from filler-gap constructions.

This study investigated cognitive and neural processes involved in gap filling during on-line sentence comprehension. Electroencephalogram (EEG) coherences were used to demonstrate that increases in the synchronization of neural activity in different cortical regions occur during gap filling when load in semantic working memory is high due to semantically unrelated words in the filler-gap interval. Sentences could either require gap filling at a verb or not, and the nouns preceding the verb could be either semantically related or unrelated. In the unrelated but not related condition, coherences in the beta band were larger during verb processing for sentences requiring gap filling compared to sentences not requiring gap filling. The coherence changes involved linkages between frontal and posterior temporal-parietal sites in both hemispheres. These results further indicate that semantic working memory is involved in the process of gap filling.

Adult↗

PubMed1997

Mechanism of synchronized Ca2+ oscillations in cortical neurons.

Dissociated rat cortical neurons reassociate in vitro to form synaptically connected networks. Removal of Mg2+ from the extracellular medium then induces neurons in the network to undergo synchronized oscillations of cytoplasmic calcium. Previous studies have shown that these calcium oscillations involve the activation of NMDA receptors and that the rising phase of each calcium spike is coincident with a brief burst of action potentials (Robinson et al., Jpn. J. Physiol. 43 (Suppl. 1) (1993) S125-130; Robinson et al., J. Neurophysiol. 70 (1993) 1606-1616; Murphy et al., J. Neurosci. 12 (1992) 4834-4845). We have found that these calcium oscillations are dependent on an influx of extracellular calcium but are independent of mobilization of calcium from intracellular stores. The influx of extracellular Ca2+ occurs primarily through L-type voltage-gated calcium channels (VGCCs), since diltiazem inhibits calcium oscillations under all conditions. On the other hand, N-, P/Q-, and T-type VGCCs are not required for calcium oscillations, although inhibitors of these channels may act as partial antagonists. In addition to removal of Mg2+, oscillations can also be induced by the inhibition of voltage-gated K+ channels with 4-aminopyridine (4-AP), a treatment known to increase neurotransmitter release. In the presence of 4-AP, synchronized calcium oscillations become independent of NMDA receptor activation, although they continue to require activation of AMPA/KA receptors. A model for the mechanism of neuronal calcium oscillations and the reason for their synchrony is presented.

Action Potentials↗

PubMed2001

Coincidence detection or temporal integration? What the neurons in somatosensory cortex are doing.

To assess the impact of thalamic synchronization on cortical responsiveness, we used conditional cross-correlation analysis to measure the probability of neuronal discharges in somatosensory cortex as a function of the time between discharges in pairs of simultaneously recorded neurons in the ventrobasal thalamus. Among 26 neuronal trios, we found that thalamocortical efficacy after synchronous thalamic activity was nearly twice as large as the efficacy rate obtained when pairs of thalamic neurons discharged asynchronously. Nearly half of these neuronal trios displayed cooperative effects in which the cortical discharge probability after synchronous thalamic events was larger than could be predicted from the efficacy rate of individual thalamic discharges. In these cases of heterosynaptic cooperativity, thalamocortical efficacy declined to asymptotic levels when the interspike intervals were >6-8 msec. These results indicate that thalamic synchronization has a significant impact on cortical responsiveness and suggest that neuronal synchronization may play a critical role in the transmission of sensory information from one brain region to another.

Cortical Synchronization↗