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PubMed2005

Multiple sclerosis patients show a highly significant decrease in alpha band interhemispheric synchronization measured using MEG.

MEG data were acquired from a group of relapsing-remitting multiple sclerosis (MS) patients and a group of healthy controls, using an eyes-closed no-task condition. An interhemispheric coherence measure (IHCM), reflecting the synchronization between the left and right hemispheres, showed a decrease in the patients, particularly in the alpha band. No comparable differences were seen in the alpha band power or its distribution over the head. The observed difference is in agreement with a reduced long-range connectivity in the brains of MS patients. The IHCM was found to be reproducible in controls over a period of more than 15 months. Further studies should investigate whether MEG derived synchronization measures may be useful as markers for MS disease load.

Adult↗
PubMed2000

Gamma rhythms and beta rhythms have different synchronization properties.

Experimental and modeling efforts suggest that rhythms in the CA1 region of the hippocampus that are in the beta range (12-29 Hz) have a different dynamical structure than that of gamma (30-70 Hz). We use a simplified model to show that the different rhythms employ different dynamical mechanisms to synchronize, based on different ionic currents. The beta frequency is able to synchronize over long conduction delays (corresponding to signals traveling a significant distance in the brain) that apparently cannot be tolerated by gamma rhythms. The synchronization properties are consistent with data suggesting that gamma rhythms are used for relatively local computations whereas beta rhythms are used for higher level interactions involving more distant structures.

Animals↗
PubMed2005

Cortical oscillatory activity and the dynamics of auditory memory processing.

Oscillatory activity in the gamma-band range (>30 Hz) has been proposed as a correlate of cortical network synchronization. In human electroencephalogram (EEG), enhanced gamma-band activity (GBA) has been found in relation to processes ranging from visual gestalt perception to selective attention, learning and memory. We used statistical probability mapping to investigate oscillatory signals in magnetoencephalogram (MEG) during different types of auditory processing. GBA at frequencies between 50-90 Hz was increased over posterior parietal areas during auditory spatial processing and over anterior temporal/inferior frontal regions during auditory pattern processing. GBA followed early auditory cortex responses after about 130 ms, suggesting serial processing along the putative auditory dorsal and ventral streams. During short-term memory tasks, additional frontal gamma amplitude enhancements were observed, and coherence was increased between putative sensory storage regions and prefrontal networks. These empirical findings have raised some questions concerning research on oscillatory activity. In contrast to EEG, in which gamma responses are spectrally and topographically widely distributed, activity in MEG shows more narrow distribution. This could be attributable to the higher sensitivity of MEG to smaller, more local networks that may synchronize at higher frequencies. In addition, the significance of different dominant frequencies within the gamma range still has to be elucidated. Finally, there have been suggestions of a link between GBA and hemodynamic measures of brain activity which should be explored further. In summary, oscillatory activity in MEG may provide unique information about both the activity of local networks and cortico-cortical integration.

Auditory Perception↗
PubMed2001

Propofol anesthesia induces phase synchronization changes in EEG.

OBJECTIVE: Phase coupling between EEG channel pairs in various frequency bands was evaluated during propofol anesthetic induction and recovery periods. METHODS: Twenty-three patients participated in the study. Phase synchronization indices based on the Hilbert transform were investigated on frequency bands 0.05-1 Hz, 1-4 Hz, 4-8 Hz, 8-12 Hz and 12-16 Hz for all pairs of the 9 EEG channels covering midline and frontal areas. A straight line was used to approximate the index values as a function of time and the Sign Test statistics were applied to the slope parameters. RESULTS: Systematic phase synchronization changes were detected. Generally, phase synchronization in the sub-delta band decreased during the induction and increased during the recovery, while the directions were reversed in the alpha band. The changes were dependent on the channel pair. In the delta, theta and beta bands, the changes were aligned more irregularly than in the sub-delta or in the alpha bands. Highly asymmetric behavior between the induction and the recovery periods was also observed in these bands. CONCLUSIONS: Induction and recovery from propofol anesthesia changes the phase synchronization between the EEG channels. The passband and location-specific behavior of these changes reveals the effects of the anesthetic to the different neural mechanisms.

Adult↗
PubMed1980

[Formation of determinant structures and functional complexes in the neocortex when the brain is transected at different levels].

A strychnine focus created in the cat neocortex preparations with mediopontine section and encephale isolé, increased activity in other weaker foci, synchronized discharges in them, united them into a functional complex and determined the character of the whole complex activity. Such an hyperactive focus plays an active role of the determinant structure. In cortex isolé and preparations with mesencephalic section of the stem, a more rapid formation of functional complexes occurred under the effect of determinant focus. Complete elimination of the afferent and thalamo-cortical interactions seems to hinder neither the hypersynchronization of cortex neuronal elements, nor the formation of functional complexes, nor the generalization of seizure activity over the cortex.

Animals↗
PubMed1981

Sleep elicited by olfactory tubercle stimulation and the effect of atropine.

The effects of high-frequency olfactory tubercle (TbOf) stimulations were studied on the EEG and behaviour of freely-moving cats. The results were as follows: (1) TbOf stimulation elicited sleep similar in every respect to physiological sleep; (2) statistical evaluation of the results showed that primarily the appearance and maintenance of slow wave sleep was facilitated by the stimulations. Stimulations during paradoxical sleep had no effect, but they did promote the recurrence of slow wave sleep after the paradoxical sleep. The effect of stimulations during wakefulness depended on the on-going activity; (3) TbOf stimulations brought about synchronization, which was characterized by a similar arousal threshold as for physiological sleep; and (4)atropine treatment prevented the behavioural effects and did not influence the EEG effects of the stimulations.

Animals↗
PubMed2001

Propagation of synchronous epileptiform events from subiculum backward into area CA1 of rat brain slices.

The hippocampal trisynaptic pathway is comprised of superficial entorhinal afferents (part of the perforant path) to dentate granule cells, dentate mossy fiber inputs to CA3 pyramidal neurons, and CA3 cell projections to CA1 pyramidal neurons. This CA1 output is among others to the subiculum, and both CA1 and subiculum project to the entorhinal cortex to close the loop. Smaller circuits involving fewer hippocampal and parahippocampal regions have also been described. We present morphological and electrophysiological evidence from rat brain slices for a projection from subiculum back into area CA1. Axons of neurobiotin-labeled subicular pyramidal neurons were visualized in the apical dendritic region of CA1. Spontaneous activity in isolated subiculum--CA1 slices was produced by bathing slices in reduced magnesium media. Events in CA1 always followed events in proximal subiculum. Disruption of this subiculum--CA1 circuit with a radially oriented knife cut in the apical dendritic region between subiculum and CA1 eliminated afterdischarges in subicular and CA1 events, but did not de-synchronize the two regions. Full transections between CA1 and subiculum were necessary to functionally isolate the two regions. Only subiculum remained spontaneously active. We conclude that a subiculum--CA1 circuit supports afterdischarges in both regions and synchronizes their activity. This circuit may serve to maintain a level of depolarization in subicular and CA1 pyramidal neurons well beyond the duration of excitatory synaptic potentials resulting from activation of the trisynaptic circuitry.

Action Potentials↗
PubMed2004

Functional connectivity patterns of human magnetoencephalographic recordings: a 'small-world' network?

EEG and MEG (magnetoencephalography) are widely used to study functional connectivity between different brain regions. We address the question whether such connectivity patterns display an optimal organization for information processing. MEG recordings of five healthy human subjects were converted to sparsely connected graphs (N=126; k=15) by applying a suitable threshold to the N * N matrix of synchronization strengths. For intermediate frequencies (8-30 Hz) the synchronization patterns were similar to those of an ordered graph with a consistent drop of synchronization strength as a function of distance. For low (<8 Hz) and high (>30 Hz) frequency bands the synchronization patterns displayed the features of a so-called 'small-world' network. This might reflect an optimal organization pattern for information processing, connecting any two brain area by only a small number of intermediate steps.

Action Potentials↗
PubMed2006

Neuronal activity in the primary visual cortex of the cat freely viewing natural images.

Many studies have now demonstrated that neurons in the visual cortex of cats and monkeys change their activity when stimuli are presented beyond their classical receptive field, and that these responses are not readily apparent from their receptive field properties. However few studies have been conducted to investigate the discharge properties of neurons in the visual cortex of animals when they are allow to freely view natural images. We employ tetrodes, which enable simultaneous and separable recordings of small numbers of neighboring neurons, to record 102 single units from 59 sites from areas 17 and 18 of two alert cats. While the animals viewed either natural images or black screens, they made frequent saccadic eye movements and gaze fixations. Fixations onto an image's location increased neuronal firing peaking at 80-100 ms after the fixation onset, to then decrease steadily with time despite continuous fixation. Saccades trigger a fast decrease in firing rate for both images and darkness. When we examined the incidence of correlated firing, we observed significant synchrony during the initial phases of visual fixations when the animals viewed natural scenes. Such synchrony was absent during saccadic eye movements and during eye movements in darkness. Our data revealed that scanning of natural scenes is associated with a rapid succession of distinct fixation-related activation patterns that included transient rate changes and excess coincident firing. The transient nature of these synchronization phenomena suggests a fast acting mechanism, which is in good agreement with the evidence that basic operations of scene analysis must be accomplished within a few tens of milliseconds in primary visual cortex.

Action Potentials↗
PubMed2004

"Dynamic" connectivity in neural systems: theoretical and empirical considerations.

The study of functional interdependences between brain regions is a rapidly growing focus of neuroscience research. This endeavor has been greatly facilitated by the appearance of a number of innovative methodologies for the examination of neurophysiological and neuroimaging data. The aim of this article is to present an overview of dynamical measures of interdependence and contrast these with statistical measures that have been more widely employed. We first review the motivation, conceptual basis, and experimental approach of dynamical measures of interdependence and their application to the study of neural systems. A consideration of boot-strap "surrogate data" techniques, which facilitate hypothesis testing of dynamical measures, is then used to clarify the difference between dynamical and statistical measures of interdependence. An overview of some of the most active research areas such as the study of the "synchronization manifold," dynamical interdependence in neurophysiology data and the putative role of nonlinear desynchronization is then given. We conclude by suggesting that techniques based on dynamical interdependence--or "dynamical connectivity"--show significant potential for extracting meaningful information from functional neuroimaging data.

Brain↗
PubMed1984

[Variability of the EEG in the acute period of comas resulting from cerebral circulatory disorders].

Up to now there has been a general consensus in the literature that the development of the comatose state is attended with the appearance on the EEG of high amplitude slow activity. The article shows that the acute period of coma is not always associated with this regularity. A large part of the EBG presents low amplitude irregular activity with occasional rapid fluctuations. In some cases there are phenomena characteristic of the physiological nocturnal sleep. The data obtained make it possible to consider the role of different synchronizing and desynchronizing cerebral systems in the genesis of electrical activity in the period of coma and are of certain interest in terms of studying the neurophysiological mechanisms of comatose states.

Adult↗
PubMed1975

Dreaming sleep attacks and desynchronized sleep enhancement. Report of a case of brain stem signs.

When central neurologic signs were localized to the vestibular region of the brain stem and cerebellum, a 54-year-old man experienced frequent awakenings from nocturnal sleep and daytime sleep attacks with hallucinosis. Sleep attacks were characterized by lid fluttering and closure, upward turning of the eyes, rapid eye movements, myoclonic twitching of all extremities, and loss of consciousness, lasting one or two minutes and aborted by strong sensory stimulation. At their termination, reports of hallucinoid imagery were given. In a sleep record of 6.5 hours, there were 2.2 hours awake (34%), 4.3 hours desynchronized sleep (66%), and a complete absence of synchronized sleep. Reports of hallucinoid imagery were given after awakenings from desynchronized sleep. The findings support the hypotheses that desynchronized sleep is normally under brain stem control and that some types of narcolepsy may be pathophysiologically related to desynchronized sleep.

Brain Diseases↗
PubMed1977

Effect of harmaline on the cerebello-rubral system.

Harmaline induces synchronous rhythms in both the cerebellum and the red nucleus of the rabbit. The level of synchronization is lower in the red nucleus than in the cerebellar cortex, probably because the cerebello-rubral pathway and the red nucleus neurons only participate poorly in the harmaline-induced olivo-cerebellar rhythm.

Alkaloids↗
PubMed2001

Synchronized population oscillation of excitatory synaptic potentials dependent of calcium-induced calcium release in rat neocortex layer II/III neurons.

We examined the roles played by calcium-induced calcium release from ryanodine-sensitive calcium stores in induction of neocortical membrane potential oscillation by using caffeine, an agonist of ryanodine receptors. Intracellular recordings were made from neurons in layer II/III of rat visual cortex slices in a caffeine-containing medium. White matter stimulation initially evoked monophasic synaptic potentials. As low-frequency stimulation continued for over 10 min, an oscillating synaptic potential gradually became evoked, in which a paroxysmal depolarization shift was followed by a 8-10-Hz train of several depolarizing wavelets. This oscillating potential was not induced in a medium containing no caffeine with 2 or 0.5 mM [Mg2+](o). Under blockade of N-methyl-D-aspartate receptors, induction of this oscillating potential failed even with caffeine application. Experiments with the calcium store depletor, thapsigargin, revealed that this oscillating potential is induced in a manner dependent on intracellular calcium release. Dual intracellular recordings revealed that the oscillation was synchronized in pairs of layer II/III neurons. The oscillating potential was detectable by field potential recordings also, suggesting that the present oscillation seems to reflect a network property.

Animals↗
PubMed2000

Sleep EEG synchronization mechanisms and activation of interictal epileptic spikes.

OBJECTIVE: The temporal course of sleep interictal epileptic discharges (IEDs) has been studied focusing their relationship with the temporal course of the main sleep-EEG frequency bands that is thought to reflect the action of different synchronization neural mechanisms. The existence of a mutually exclusive mechanism between spindles and delta waves should be reflected in a mutually exclusive facilitation of IEDs activation by slow wave activity (SWA) and sigma activity (SA) during synchronized NREM sleep. METHODS: We reanalyzed data from 19 children and 15 adult patients affected by different partial epileptic syndromes. The temporal series of SWA, SA and theta band (TB), derived from spectral analysis, were obtained from a spike-free and pathologic alteration-free derivation, controlateral to the most active lead, where the IEDs count was performed. Relationships between SA, SWA and TB and time series of IEDs were tested by means of correlation techniques after data normalization. RESULTS: A positive correlation of spike distribution with SWA time course has been found in the majority of adults. Only a few adult patients showed IEDs that were correlated with SA or TB. Conversely SA was shown to be positively correlated with spiking in many different epileptic syndromes of childhood. Moreover, in the contest of the NREM sleep cycle an inverse relationship between the SWA and SA mode of spike activation has been detected. CONCLUSIONS: Overall results give evidence that 3 main rhythmic spectral components that characterize sleep EEG can exert positive influences on IEDs production. Our studies demonstrate that within NREM sleep the facilitating influences on IEDs production exerted separately by either spindle activity or delta synchronization mechanisms can be detected. Moreover, a mutually exclusive mechanism between SA and SWA oscillations is detectable in the opposite relationship of the correlation between IEDs and the two bands in the central part of the NREM cycle.

Brain↗
PubMed2001

Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling.

In humans, distinct processes within the hippocampus and rhinal cortex support declarative memory formation. But do these medial temporal lobe (MTL) substructures directly cooperate in encoding new memories? Phase synchronization of gamma-band electroencephalogram (EEG) oscillations (around 40 Hz) is a general mechanism of transiently connecting neural assemblies. We recorded depth-EEG from within the MTL of epilepsy patients performing a memorization task. Successful as opposed to unsuccessful memory formation was accompanied by an initial elevation of rhinal-hippocampal gamma synchronization followed by a later desynchronization, suggesting that effective declarative memory formation is accompanied by a direct and temporarily limited cooperation between both MTL substructures.

Action Potentials↗
PubMed2005

Response properties of an integrate-and-fire model that receives subthreshold inputs.

A computational technique is described for calculation of the interspike interval and poststimulus time histograms for the responses of an integrate-and-fire model to arbitrary inputs. The effects of the model parameters on the response statistics were studied systematically. Specifically, the probability distribution of the membrane potential was calculated as a function of time, and the mean interspike interval and PST histogram were calculated for arbitrary inputs. For stationary inputs, the regularity of the output was studied in detail for various model parameters. For nonstationary inputs, the effects of the model parameters on the output synchronization index were explored. The results show that enhanced synchronization in response to low-frequency stimuli required a large number (n > 25) of weak inputs. Irregular responses and a linear input-output rate relationship required strong (but subthreshold) inputs with a small time constant. A model cell with mixed-amplitude synaptic inputs can respond to stationary inputs irregularly and have enhanced synchronization to nonstationary inputs that are phase-locked to low-frequency inputs. Both of these response properties have been reported for some cells in the ventral cochlear nucleus in the auditory brainstem.

Action Potentials↗
PubMed2006

Inhibitory control in high-functioning autism: decreased activation and underconnectivity in inhibition networks.

BACKGROUND: Inhibiting prepotent responses is critical to optimal cognitive and behavioral function across many domains. Several behavioral studies have investigated response inhibition in autism, and the findings varied according to the components involved in inhibition. There has been only one published functional magnetic resonance imaging (fMRI) study so far on inhibition in autism, which found greater activation in participants with autism than control participants. METHODS: This study investigated the neural basis of response inhibition in 12 high-functioning adults with autism and 12 age- and intelligence quotient (IQ)-matched control participants during a simple response inhibition task and an inhibition task involving working memory. RESULTS: In both inhibition tasks, the participants with autism showed less brain activation than control participants in areas often found to be active in response inhibition tasks, namely the anterior cingulate cortex. In the more demanding inhibition condition, involving working memory, the participants with autism showed more activation than control participants in the premotor areas. In addition to the activation differences, the participants with autism showed lower levels of synchronization between the inhibition network (anterior cingulate gyrus, middle cingulate gyrus, and insula) and the right middle and inferior frontal and right inferior parietal regions. CONCLUSIONS: The results indicate that the inhibition circuitry in the autism group is activated atypically and is less synchronized, leaving inhibition to be accomplished by strategic control rather than automatically. At the behavioral level, there was no difference between the groups.

Adaptation, Psychological↗