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PubMed2000

Top-down processing mediated by interareal synchronization.

Perception and cortical responses are not only driven "bottom-up" by the external stimulus but are altered by internal constraints such as expectancy or the current behavioral goal. To investigate neurophysiological mechanisms of such top-down effects, we analyzed the temporal interactions of neurons on different levels of the cortical hierarchy during perception of stimuli with varying behavioral significance. We found that interareal interactions in a middle-frequency range (theta and alpha; 4-12 Hz) strongly depend on the associated behavior, with a phase relationship and a layer specificity indicating a top-down-directed interaction. For novel unexpected stimuli, presumably processed in a feed-forward fashion, no such interactions occurred but high-frequency interactions (gamma; 20-100 Hz) were observed. Thus corticocortical synchronization reflects the internal state of the animal and may mediate top-down processes.

Animals↗
PubMedDate not supplied

Alpha-frequency rhythms desynchronize over long cortical distances: a modeling study.

Neocortical networks of excitatory and inhibitory neurons can display alpha(a)-frequency rhythms when an animal is in a resting or unfocused state. Unlike some gamma- and beta-frequency rhythms, experimental observations in cats have shown that these alpha-frequency rhythms need not synchronize over long cortical distances. Here, we develop a network model of synaptically coupled excitatory and inhibitory cells to study this asynchrony. The cells of the local circuit are modeled on the neurons found in layer V of the neocortex where alpha-frequency rhythms are thought to originate. Cortical distance is represented by a pair of local circuits coupled with a delay in synaptic propagation. Mathematical analysis of this model reveals that the h and T currents present in layer V pyramidal (excitatory) cells not only produce and regulate the alpha-frequency rhythm but also lead to the occurrence of spatial asynchrony. In particular, these inward currents cause excitation and inhibition to have nonintuitive effects in the network, with excitation delaying and inhibition advancing the firing time of cells; these reversed effects create the asynchrony. Moreover, increased excitatory to excitatory connections can lead to further desynchronization. However, the local rhythms have the property that, in the absence of excitatory to excitatory connections, if the participating cells are brought close to synchrony (for example, by common input), they will remain close to synchrony for a substantial time.

Action Potentials↗
PubMed2005

Long-range EEG phase synchronization during an arithmetic task indexes a coherent cortical network simultaneously measured by fMRI.

An open question lies in whether or not distributed activities in the distant brain regions are integrated into a coherent ensemble for cognitive information processing. Long-range phase synchronization is often observed by scalp EEG measurements during cognitive tasks and is considered to provide a possible neural principle for the functional integration of distributed neural activities. Synchronization could be reflected at the neuron firing level or at the local field potential and could appear in the scalp EEG under certain conditions on neural spatial and temporal coherence. To examine if phase synchronization is concerned with the integration of distant regions, we proposed a method to extract brain activities associated with task-dependent phase synchronization by combining simultaneous fMRI and EEG. By applying this method in a mental arithmetic task, we found a dominant task-dependent increase of phase synchronization around 14 Hz (in beta frequency) across bilateral parietal sites that were associated with both negative and positive BOLD responses. Functional connectivity analyses of these regions demonstrated that an increase in hemispheric beta synchronization was associated with a linking between the cross-hemispheric regions (left angular gyrus and right superior parietal gyrus) and also among the anterior-posterior regions (right dorsolateral prefrontal cortex, putamen, and right superior temporal gyrus). These findings indicate that the positive BOLD regions (dorsolateral prefrontal cortex and superior parietal lobule) are linked with other negative BOLD regions. We also discussed the possible importance of beta synchronization in the formation of a working memory network.

Adult↗
PubMedDate not supplied

[Features of thalamocortical relations in the phantom pain syndrome (electrophysiologic studies during stereotaxic surgery)].

Corticographic effects are analysed in 14 patients with severe phantom-pain syndrome from data obtained in electric stimulation of specific sensory and nonspecific nuclear structures of the thalamus. The findings of EEG, ECoG and VP analysis reflect reorganization of the thalamo-cortical relationships in the phantom-pain syndrome in the direction of disorganization of the regulating mechanisms. Evidence of this are an increased excitability level in the thalamic nuclei and the central cortex, disorder of the regional differences of the weakly modulated alpha-rhythm, and marked changes in the amplitude and configuration of VP of the parietal and central cortical areas. Various frequencies of stimulating current were used to differentiate the specific sensory and nonspecific thalamic nuclei. It was found that for the identification of the relay sensory nucleus, stimulation with a frequency range of 1-20 Hz is most effective, in which there is local synchronization of the cortical rhythm corresponding with the duration of the stimulation. Electric stimulation with a frequency of 50 Hz and more induces reactions characteristic of nonspecific nuclei: general desynchronization of the cortical rhythm during stimulation with the development of characteristic "spindles" in the after-action.

Alpha Rhythm↗
PubMed2004

Effects of co-activation on cortical organization and discrimination performance.

We used fMRI to investigate the effects of tactile co-activation on the topographic organization of the human primary somatosensory cortex (SI). Behavioral consequences of co-activation were studied in a psychophysical task assessing the mislocalization of tactile stimuli. Co-activation was applied to the index, middle and ring fingers of the right hand either synchronously or asynchronously. Cortical representations for synchronously co-activated fingers moved closer together, whereas cortical representations for asynchronously co-activated fingers became segregated. Behaviorally, this pattern coincided with an increased and reduced number of mislocalizations between synchronously and asynchronously co-activated fingers, respectively. Thus, both synchronous and asynchronous coupling of passive tactile stimulation is able to induce short-term cortical reorganization associated with functionally relevant changes.

Adult↗
PubMed1995

Brain activation, then (1949) and now: coherent fast rhythms in corticothalamic networks.

The hypothesis of forebrain activation elicited by brainstem reticular core stimulation, formulated almost a half century ago, is now fully substantiated at the intracellular level of thalamic and neocortical neurons. Data show that stimulation of mesopontine cholinergic nuclei induces a prolonged muscarinic depolarization of thalamocortical neurons, associated with an increase in their apparent input resistance (that explains the enhanced probability of thalamic responses to incoming volleys upon arousal) and accompanied by a long-lasting activation of cortical rhythms. Activation also includes the preservation or even enhancement of short-lasting sculpturing inhibitory processes in thalamic and neocortical cells, a basic requirement for discrimination purposes. The notion of activation, that was erroneously termed as a "desynchronized" activity in thalamocortical networks, is now demonstrated to include spontaneously occurring, synchronous fast (20-40 Hz) rhythms. While the spatial coherence of sleep rhythms extends over wide territories, fast oscillations during brain arousal are synchronous within a cortical column and among closely spaced cortical areas, thalamic nuclei, and corticothalamic systems. Fast oscillations do not exclusively characterize brain-active states of waking and REM sleep, as they are also present during the depolarizing phase of the slow sleep oscillation in both cortical and thalamic cells. The subthreshold fast depolarizing spontaneous oscillations may bias thalamic and cortical cells to respond synchronously, at fast frequencies, to external stimuli in the wake state and to internal drives (such as ponto-geniculo-occipital signals) during REM sleep.

Acetylcholine↗
PubMed2000

Low frequency damped oscillations of cat visual cortical neurones.

A synchronizing volley of excitation was delivered to cells from the primary visual cortex of the anaesthetized cat by an electrical pulse stimulation of the optic radiations. In intracellular records, this resulted in an initial, fast depolarization of the membrane potential followed by a longer lasting (266 ms) hyperpolarization, and then to small amplitude damped oscillations of the membrane potential. In different cells the number of peaks ranged from 3 to 6, and the amplitude of the first oscillation averaged 3.5 mV. If the pulse stimulation was repeated during visual drive with an optimal bar stimulus, the membrane potential fluctuations were non-linearly enhanced with spike bursts recorded during the depolarized states of the membrane potential. The amplitude of the first oscillation in these cases averaged 6.9 mV. The mean frequency of damped oscillations was 9.9 Hz. The results suggest that these oscillations are mediated by the waxing and waning of the excitatory drive to the recorded neurone.

Action Potentials↗
PubMed1999

Spatiotemporal coding in the cortex: information flow-based learning in spiking neural networks.

We introduce a learning paradigm for networks of integrate-and-fire spiking neurons that is based on an information-theoretic criterion. This criterion can be viewed as a first principle that demonstrates the experimentally observed fact that cortical neurons display synchronous firing for some stimuli and not for others. The principle can be regarded as the postulation of a nonparametric reconstruction method as optimization criteria for learning the required functional connectivity that justifies and explains synchronous firing for binding of features as a mechanism for spatiotemporal coding. This can be expressed in an information-theoretic way by maximizing the discrimination ability between different sensory inputs in minimal time.

Action Potentials↗
PubMed1994

Event-related synchronization of mu rhythm in the EEG over the cortical hand area in man.

Spontaneous EEG activity was recorded at 56 electrodes in 3 healthy subjects. All subjects displayed event-related desynchronization (ERD) of mu rhythms over the cortical hand area during discrete finger movement. In contrast to this, foot movement resulted in an enhancement or event-related synchronization (ERS) of mu rhythms over the hand area. This phasic synchronization of mu waves was circumscribed and found at electrodes overlying both cortical hand areas. It is speculated, that this ERS represents a short lasting 'idling state' of hand area neurons when other body parts are moved.

Adult↗
PubMed2000

Sub-second quenched-flow/X-ray microanalysis shows rapid Ca2+ mobilization from cortical stores paralleled by Ca2+ influx during synchronous exocytosis in Paramecium cells.

Though only actual local free Ca2+ concentrations, [Ca2+], rather than total Ca concentrations, [Ca], govern cellular responses, analysis of total calcium fluxes would be important to fully understand the very complex Ca2+ dynamics during cell stimulation. Using Paramecium cells we analyzed Ca2+ mobilization from cortical stores during synchronous (< or = 80 ms) exocytosis stimulation, by quenched-flow/cryofixation, freeze-substitution (modified for Ca retention) and X-ray microanalysis which registers total calcium concentrations, [Ca]. When the extracellular free calcium concentration, [Ca2+]e, is adjusted to approximately 30 nM, i.e. slightly below the normal free intracellular calcium concentration, [Ca2+]i = 65 nM, exocytosis stimulation causes release of 52% of calcium from stores within 80 ms. At higher extracellular calcium concentration, [Ca2+]e = 500 microM, Ca2+ release is counterbalanced by influx into stores within the first 80 ms, followed by decline of total calcium, [Ca], in stores to 21% of basal values within 1 s. This includes the time required for endocytosis coupling (350 ms), another Ca2+-dependent process. To confirm that Ca2+ mobilization from stores is superimposed by rapid Ca2+ influx and/or uptake into stores, we substituted Sr2+ for Ca2+ in the medium for 500 ms, followed by 80 ms stimulation. This reveals reduced Ca signals, but strong Sr signals in stores. During stimulation, Ca2+ is spilled over preformed exocytosis sites, particularly with increasing extracellular free calcium, [Ca2+]e. Cortically enriched mitochondria rapidly gain Ca signals during stimulation. Balance calculations indicate that total Ca2+ flux largely exceeds values of intracellular free calcium concentrations locally required for exocytosis (as determined previously). Our approach and some of our findings appear relevant also for some other secretory systems.

Animals↗
PubMed2000

Long-range EEG synchronization during word encoding correlates with successful memory performance.

Distinct cortical activity during memory encoding of words, which were either recalled or not, was reported by a number of studies. This activity was mainly found at frontal and temporal/parietal brain regions. However, it was not clear if these regions interact with each other or work independently. In order to get a functional measure of the degree of neuronal large-scale cooperation, we calculated EEG coherence, which provides a statistical measure of synchronization between two EEG signals per frequency band. Therefore, coherence enables us to assess the functional interaction between cell assemblies of distant brain regions. The purpose of our study was to investigate if successfully recalled words show enhanced cortical synchronization compared with not recalled ones. Additionally, the influence of stimulus modality and the way different EEG frequencies participate in this process was examined. The EEG of 25 participants was recorded during memory encoding of concrete German nouns, either presented auditorily or visually and stimuli were separated according to the participant's memory performance. Recalled nouns exhibited overall enhanced synchronization but showed typical patterns, especially between anterior and posterior brain regions in all frequency bands except the alpha-1 band (8-10 Hz). Recalling nouns was accompanied by increased synchronization between more distant electrodes in relation to an increase of synchronization between adjacent electrodes. Moreover, the degree of intrahemispheric synchronization was higher for recalled nouns. The pattern of EEG coherence and amplitude changes during verbal memory encoding allowed us to assess the probability that nouns would be recalled or not.

Acoustic Stimulation↗
PubMed2005

Intercolumnar synchronization of neuronal activity in rat barrel cortex during patterned airjet stimulation: a laminar analysis.

We used cross-correlation analysis to characterize the incidence and strength of stimulus-induced neuronal synchronization in different layers of SI barrel cortex and as a function of neuronal location in different barrel columns. To reduce the possibility of evoking responses that were coordinated by simultaneous whisker movements, multiple whiskers were sequentially stimulated with airjets that moved back-and-forth across the peripheral whisker pad. From a sample of 627 neurons, we characterized 1,182 neuron pairs and found that 687 (58.1%) of these displayed significant peaks of synchronized activity that exceeded the 99.9% confidence limits. Whereas 88% of the infragranular neuron pairs were synchronized during whisker stimulation, only 30% of the neuron pairs in the granular or supragranular layers displayed synchronized responses. The strength of synchronization, as measured by the correlation coefficient, was significantly higher in the infragranular layers than in the other layers. These results indicate that synchronized outputs from the infragranular layers do not depend on synchronized inputs from the upper cortical layers. We also found that synchronization varies with the spatial configuration of the neurons and is strongest for neuron pairs residing in the same row. Given the dense local projections between neighboring barrel columns in the same row, our results indicate that neuronal synchronization is greatest when stimuli simultaneously activate those peripheral receptors whose cortical representations are most densely interconnected. Finally, we compared the present results with synchronized responses in somatosensory (SI) barrel cortex that were evoked by controlled, pulsatile whisker movements in a previous study. We conclude that highly-controlled whisker stimulation increases stimulus coordination and may exaggerate the incidence and strength of synchronization among neurons in the granular or supragranular layers.

Action Potentials↗
PubMedDate not supplied

[Intracortical synchronization of the epileptiform field potentials at different stages of ultrastructural reorganizations in neuronally isolates island in rats].

A temporal delay (parameter of synchronization) between the incidence of epileptiform discharges in cortical sited located at a distance of 4 mm from each other was studied in rat intact cortex and neuronally isolated cortical slab using the cross-correlation function. Experiments were carried out at different stages of axonal sprouting. By 30 days of isolation, a significant increase in the number of boutons in the V cortical layer coincided with a significant decrease in the delay, whereas a reduction of the number of boutons by 90 days corresponded to its increase. These findings convincingly testify that the newly formed boutons form a basis for increase in synchronization and thus affect the epileptogenesis. The results obtained in this work and literature data suggest that under pathological conditions large pyramids of the V layer form a neuronal network which provides exclusively cortical synchronization of epileptiform field potentials.

Animals↗
PubMed2002

Synchronized spikes of thalamocortical axonal terminals and cortical neurons are detectable outside the pig brain with MEG.

We show that it is feasible to monitor the synchronized population spikes of the thalamocortical axonal terminals and cortical neurons outside the brain using high-resolution magnetoencephalography (MEG). Electrical stimulation of the snout elicited somatic-evoked magnetic fields (SEFs) above the primary somatosensory cortex (SI) of the piglet. The SEFs contained high-frequency oscillations (HFOs) around 600 Hz similar in many respects to the noninvasively measured HFOs from humans with MEG and electroencephalography (EEG). These HFOs were highly correlated with those in simultaneously measured intracortical somatic-evoked potentials (SEPs) in the snout projection area in SI. Both HFOs in SEFs and SEPs consisted of an initial component insensitive to cortically injected kynurenic acid (Kyna, 20 mM), a nonspecific antagonist of glutamatergic receptors, and a subsequent Kyna-sensitive component. The former was localized in cortical layer IV, indicating that it was due to spikes produced by the specific thalamocortical axonal terminals, whereas the latter was initially localized in layer IV and subsequently in the superficial and deeper layers. These results suggest that it may be possible to study properties of the thalamocortical and cortical spike activities in humans with MEG.

Action Potentials↗
PubMedDate not supplied

[Spectral and coherence EEG analysis of rats differing in the level of seizure readiness].

The resting EEGs of several brain structures (motor and visual cortex, caudate nucleus and intralaminar thalamic nuclei) were submitted to spectral and coherence computer analyses in two rat strains. Genetically predisposed to convulsive state KM rats were shown to differ from nonpredisposed Wistar rats in EEG spectral properties. KM rats EEG pattern was characterized by increase of low frequencies (1-2 Hz) power and decrease of faster activity (5-12 Hz) power in cortical spectrograms as well as by decrease of caudate nucleus EEG absolute power. The coherence value between cortical or subcortical structures at below 4 Hz was intensified in KM rats. Reinforcement of cortical auto-oscillating properties manifested by ECoG synchronization in cortical-thalamic resonance interaction as well as weakening of striatal inhibitory system may constitute neurophysiological mechanisms of enhanced convulsive readiness. The probable role of mediator imbalance in these mechanisms is discussed.

Acoustic Stimulation↗
PubMed2005

Antero-posterior functional coupling at sleep onset: changes as a function of increased sleep pressure.

The use of the directed transfer function (DTF), an advanced computational analysis of electroencephalogaphic (EEG) data, which provides an estimation of the direction of the information flow underlying cortico-cortical functional coupling, has shown that the presleep period is characterized by posterior-to-anterior functional cortical coupling, while at sleep onset there is an inversion of that direction. This finding supported the idea that anterior cortical areas first synchronize sleep EEG activity. The aim of the present study was to assess the changes of functional coupling between anterior and posterior midline cortical areas during the sleep onset process when sleep pressure is heightened by a selective slow-wave sleep (SWS) deprivation. The hypothesis was that the anterior-to-posterior direction of the cortical functional coupling at sleep onset is enhanced by SWS deprivation. Ten normal right-handed male students slept for six consecutive nights in the laboratory (1: adaptation, 2: baseline, 3: baseline with awakenings, 4 and 5: SWS deprivations, 6: recovery), with standard polysomnographic recordings. The DTF was computed on data recorded during nights two and six from anterior (Fz(A1)) and posterior (Pz(A1), Oz(A1)) derivations. Results showed that, during the recovery night, the anterior-to-posterior direction of functional cortical coupling is already present in the presleep period, indicating that SWS deprivation advances the shift to an anterior-to-posterior directionality of functional cortical coupling, possibly as a consequence of heightened sleep pressure. These findings support the notion that a spread of synchronizing signals from associative prefrontal to posterior areas play a role in the wake-sleep transition.

Adult↗
PubMed1996

Effects of glucosylceramide synthase inhibitor and ganglioside GQ1b on synchronous oscillations of intracellular Ca2+ in cultured cortical neurons.

To evaluate the role of endogenous gangliosides in synapse formation, the glucosylceramide synthase inhibitor, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), was used to deplete glycosphingolipids (GSLs) of cultured rat cerebral cortical neurons. Synapse formation between the neurons was estimated by the synchronous oscillation of synaptic activity monitored by fura-2 calcium imaging. Treatment with D-PDMP resulted in dose- and time-dependent decreases in the frequency of synchronous oscillations. When a series of gangliosides (GM3, GM1, GD3, GD1b, GT1b, and GQ1b) was supplemented to the GSL depleted cells, only GQ1b was able to normalize the decreased frequency by D-PDMP. These results suggest that de novo synthesis of a particular molecular species of the gangliosides, GQ1b, is essential for synapse formation and synaptic activity.

Animals↗
PubMed2000

Synchronous oscillation in the cerebral cortex and object coherence: simulation of basic electrophysiological findings.

A lumped continuum model for electrocortical activity was used to simulate several established experimental findings of synchronous oscillation which have not all been previously embodied in a single explanatory model. Moving-bar visual stimuli of different extension, stimuli moving in different directions, the impact of non-specific cortical activation upon synchronous oscillation, and the frequency content of EEG associated with synchrony were considered. The magnitude of zero lag synchrony was primarily accounted for by the properties of the eigenmodes of the travelling local field potential superposition waves generated by inputs to the cortex, largely independent of the oscillation properties and associated spectral content. Approximation of the differences in cross-correlation observed with differently moving bar stimuli, and of the impact of cortical activation, required added assumptions on (a) spatial coherence of afferent volleys arising from parts of a single stimulus object and (b) the presence of low-amplitude diffuse field noise, with enhancement of cortical signal/noise ratio with respect to the spatially coherent inputs, at higher levels of cortical activation. Synchrony appears to be a ubiquitous property of cortex-like delay networks. Precision in the modelling of synchronous oscillation findings will require detailed description of input pathways, cortical connectivity, cortical stability, and aspects of cortical/subcortical interactions.

Artifacts↗