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PubMed1985

Antiepileptic drug evaluation in a new animal model: spontaneous petit mal epilepsy in the rat.

One-third of Wistar rats bred in our laboratory present recurrent seizures whose EEG and clinical symptomatology resemble those of human petit mal. Bilateral cortical synchronous spike- and wave discharges (7-11 c/s; 200-600 microV, lasting 0.5 to 40 s) accompany behavioral arrest and are associated frequently with facial myoclonia. These seizures, observed as long as the animals survive, appear spontaneously and seem to be unrelated to surgical procedures. Antiepileptics in common clinical use were tested. Ethosuximide (greater than 12.5 mg/kg), diazepam (greater than 0.5 mg/kg), trimethadione and sodium valproate (greater than 50 mg/kg) suppressed these discharges in a dose related manner. Carbamazepine and phenytoin were ineffective or aggravated the seizures. Phenobarbital, effective at 2.5 to 10 mg/kg, was ineffective at 20 mg/kg. The similar effects of these antiepileptics on both the rats' seizures and human petit mal confirm the hypothesis that this phenomenon constitutes a valid pharmacological model of petit mal epilepsy. Its predictive value appears to be superior to that of other currently used models.

Animals↗
PubMed1980

Dynamic interactions of evoked potentials in the association cortex of the cat.

Interactions of acoustic (Ac) and somatosensory (SS) evoked potentials were studied in the suprasylvian association cortex of cats anaesthetized with chloralose and pentobarbital. Ac and SS stimuli were synchronously applied so as to their phases of identical polarity overlapped each other completely. The amplitude of the summated potential was compared with the algebraic sum of the separately recorded Ac and SS potentials. Three kinds of phenomena were observed in these conditions: (a) algebraic summation; (b) occlusion and (c) facilitatory interaction of the two sorts of evoked potentials. Under chloralose anaesthesia only algebraic summation occurred, while use of barbiturate favoured the appearance of occlusion and facilitatory interactions. Synchronous activation of the Ac and SS inputs with 2 cps frequency for 2 min enhanced the occlusion for several minutes. Samples taken during synchronized cortical activity also showed a marked tendency for occlusive interactions. Direct stimulation of the visual cortex or optic flashes at a frequency of 10--20/sec and stimulation of the thalamic association nuclei enhanced the occlusion of the Ac and SS evoked potentials as well. The observed phenomena are explained with heterosynaptic facilitation taking place on the cortical pyramidal cells and interneurons.

Animals↗
PubMed1998

Induced gamma-band activity during the delay of a visual short-term memory task in humans.

It has been hypothesized that visual objects could be represented in the brain by a distributed cell assembly synchronized on an oscillatory mode in the gamma-band (20-80 Hz). If this hypothesis is correct, then oscillatory gamma-band activity should appear in any task requiring the activation of an object representation, and in particular when an object representation is held active in short-term memory: sustained gamma-band activity is thus expected during the delay of a delayed-matching-to-sample task. EEG was recorded while subjects performed such a task. Induced (e.g., appearing with a jitter in latency from one trial to the next) gamma-band activity was observed during the delay. In a control task, in which no memorization was required, this activity disappeared. Furthermore, this gamma-band activity during the rehearsal of the first stimulus representation in short-term memory peaked at both occipitotemporal and frontal electrodes. This topography fits with the idea of a synchronized cortical network centered on prefrontal and ventral visual areas. Activities in the alpha band, in the 15-20 Hz band, and in the averaged evoked potential were also analyzed. The gamma-band activity during the delay can be distinguished from all of these other components of the response, on the basis of either its variations or its topography. It thus seems to be a specific functional component of the response that could correspond to the rehearsal of an object representation in short-term memory.

Adult↗
PubMed1998

Information about movement direction obtained from synchronous activity of motor cortical neurons.

Although neuronal synchronization has been shown to exist in primary motor cortex (MI), very little is known about its possible contribution to coding of movement. By using cross-correlation techniques from multi-neuron recordings in MI, we observed that activity of neurons commonly synchronized around the time of movement initiation. For some cell pairs, synchrony varied with direction in a manner not readily predicted by the firing of either neuron. Information theoretic analysis demonstrated quantitatively that synchrony provides information about movement direction beyond that expected by simple rate changes. Thus, MI neurons are not simply independent encoders of movement parameters but rather engage in mutual interactions that could potentially provide an additional coding dimension in cortex.

Animals↗
PubMed1991

Simulations of cortical pyramidal neurons synchronized by inhibitory interneurons.

1. The interaction between inhibitory interneurons and cortical pyramidal neurons was studied by use of computer simulations to test whether inhibitory interneurons could assist in phase-locking postsynaptic cells. Two models were used: a simplified model, which included only 3 membrane channels, and a detailed 11-channel model. 2. The 11-channel model included most of the ion channels known to be present in neocortical pyramidal neurons as well as calcium diffusion and other membrane mechanisms. The kinetics for the channels were obtained from voltage-clamp studies in a variety of preparations. The parameters were then adjusted to produce repetitive bursting similar to that seen in some cortical pyramidal cells entrained during visual stimulation. 3. Phase-locking to a train of inhibitory postsynaptic potentials (IPSPs) located on or near the soma was observed in the 3-channel model cell subjected to random synaptic bombardment. In the 11-channel model, phase-locking due to multiple IPSPs was compared with phase-locking due to multiple excitatory postsynaptic potentials (EPSPs). Phase-locking began to occur when 20% of the IPSPs (20/100) or 40% of the EPSPs (4,000/10,000) were synchronized. The exact percentages differed with different 11-channel models, but either EPSPs or IPSPs would generally produce entrainment with approximately 40% synchronization. Thus 40 inhibitory boutons had an effect equivalent to 4,000 excitatory boutons in producing phase-locking. 4. Phase-locking with IPSPs in these models was possible because the IPSPs could cause either an increase or a decrease in firing rate over a limited range. The IPSPs served a modulatory role, increasing the rate of firing in some cases and decreasing it in others, depending on the state of the cell. 5. We examined frequency entrainment by IPSPs. In the 3-channel model, frequency entrainment of a postsynaptic cell was observed with a rapid train of strong (20-100 nS), brief, compound IPSPs. A 40-Hz compound IPSP train of 60 nS entrained cells having initial firing rates between 32 and 47 Hz. Below this range, cells could be partially entrained. Above the range, entrainment would fail. Frequency entrainment in the 3-channel model generally occurred on the first cycle after onset of the IPSPs. 6. Phase-locking and frequency entrainment were less robust in the 11-channel model. This was partly because bursts rather than individual spikes were being entrained. A 40-Hz, 90-nS compound IPSP train entrained a model cell upward from 34 Hz. Downward frequency entrainment also occurred.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium Channels↗
PubMed2002

Enhancement of temporal and spatial synchronization of entorhinal gamma activity by phase reset.

The synchronization of cortical gamma oscillatory activity (25-80 Hz) is thought to coordinate neuronal assemblies in the processing and storage of information. The mechanism by which independently oscillating and distantly located cortical zones become synchronized is presumed to involve activity in corticocortical connections, although evidence supporting this conjecture has only been indirect. In the present study, we show that activation of synaptic inputs within and to the medial entorhinal cortex (mEC) of the in vitro isolated guinea pig brain preparation resets the phase of ongoing gamma activity induced by muscarinic receptor agonism with carbachol (frequency: 24 +/- 2 Hz at 32 degrees C). Phase reset was associated with a transient enhancement of the synchronization of gamma activity recorded at distant (>1 mm) mEC sites, across which low coherence (>0.75) was observed before stimulation. This increase in synchronization, as measured by cross-correlation analysis, was restricted to a maximal period of 200 ms after either local mEC or CA1 afferent stimulation. The results provide direct evidence that synaptic activation can enhance the rhythmic synchronization of spatially remote, independently oscillating neuronal assemblies in the mEC through a mechanism of synaptically evoked phase reset. Dynamic functional grouping of oscillatory discharges across long distances in the mEC may underlie coding processes involved in the integration and storage of incoming information and thus may be important for the role of this region in memory processes.

Afferent Pathways↗
PubMed2005

Cortical dynamics and synchronization related to multiple target consolidation under rapid-serial-visual-presentation conditions.

The present report reviews behavioural, electroencephalographic, and especially magnetoencephalographic findings on the cortical mechanisms underlying attentional processes that separate targets from distractors and that ensure durable target representations for goal-directed action. A common way of investigation is to observe the system's overt and covert behaviour when capacity limitations are reached. Here we focus on the aspect of temporally enhanced processing load, namely on performance deficits occurring under rapid-serial-visual-presentation (RSVP) conditions. The most prominent of these deficits is the so-called "attentional blink" (AB) effect. We first report MEG findings with respect to the time course of activation that shows modulations around 300 ms after target onset which reflect demands and success of target consolidation. Then, findings regarding long-range inter-area phase synchronization are reported that are hypothesized to mediate communication within the attentional network. Changes in synchronization reflect changes in the attentional demands of the task and are directly related to behavioural performance. Furthermore, enhanced vigilance of the system elicits systematically increased synchronization indices. A hypothetical framework is sketched out that aims at explaining limitations in multiple target consolidation under RSVP conditions.

Animals↗
PubMed1999

Stable propagation of synchronous spiking in cortical neural networks.

The classical view of neural coding has emphasized the importance of information carried by the rate at which neurons discharge action potentials. More recent proposals that information may be carried by precise spike timing have been challenged by the assumption that these neurons operate in a noisy fashion--presumably reflecting fluctuations in synaptic input and, thus, incapable of transmitting signals with millisecond fidelity. Here we show that precisely synchronized action potentials can propagate within a model of cortical network activity that recapitulates many of the features of biological systems. An attractor, yielding a stable spiking precision in the (sub)millisecond range, governs the dynamics of synchronization. Our results indicate that a combinatorial neural code, based on rapid associations of groups of neurons co-ordinating their activity at the single spike level, is possible within a cortical-like network.

Action Potentials↗
PubMed2006

Dynamics and effective topology underlying synchronization in networks of cortical neurons.

Cognitive processes depend on synchronization and propagation of electrical activity within and between neuronal assemblies. In vivo measurements show that the size of individual assemblies depends on their function and varies considerably, but the timescale of assembly activation is in the range of 0.1-0.2 s and is primarily independent of assembly size. Here we use an in vitro experimental model of cortical assemblies to characterize the process underlying the timescale of synchronization, its relationship to the effective topology of connectivity within an assembly, and its impact on propagation of activity within and between assemblies. We show that the basic mode of assembly activation, "network spike," is a threshold-governed, synchronized population event of 0.1-0.2 s duration and follows the logistics of neuronal recruitment in an effectively scale-free connected network. Accordingly, the sequence of neuronal activation within a network spike is nonrandom and hierarchical; a small subset of neurons is consistently recruited tens of milliseconds before others. Theory predicts that scale-free topology allows for synchronization time that does not increase markedly with network size; our experiments with networks of different densities support this prediction. The activity of early-to-fire neurons reliably forecasts an upcoming network spike and provides means for expedited propagation between assemblies. We demonstrate this capacity by observing the dynamics of two artificially coupled assemblies in vitro, using neuronal activity of one as a trigger for electrical stimulation of the other.

Action Potentials↗
PubMed1990

Changes in the spatial synchronization of the cortical electrical activity under extreme (hyperbaric) conditions.

Spectral correlation methods of EED-analysis were used to study the local synchronization (by power spectra--FFT) and the spatial synchronization (by assessing the coherence functions) of the cortical electrical activity of persons subjected to prolonged (one-month) extreme impact--hyperbaric conditions. Two neurophysiological mechanisms of system functional organization of the electrical activity of the central nervous system are described, through which the brain compensates its reduced functional capacities under the extreme conditions studied (including boundary physiological states). These mechanisms: (a) act in almost all situations of the extreme impact, irrespective of the scope of the remaining varied transient or lasting readjustments in the EEG, and (b) guarantee spatial interactions in the electrical activity in the brain in frequency ranges which are essential for its functions: alpha (8-12 Hz) and beta-2 (24-36 Hz).

Atmospheric Pressure↗
PubMedDate not supplied

Zero-lag synchronous dynamics in triplets of interconnected cortical areas.

Oscillatory and synchronized activities involving widespread populations of neurons in neocortex are associated with the execution of complex sensorimotor tasks and have been proposed to participate in the 'binding' of sensory attributes during perceptual synthesis. How the brain constructs these coherent firing patterns remains largely unknown. Several mechanisms of intracortical synchronization have been considered, in particular mutual inhibition and reciprocal excitation. These mechanisms fail to account for the zero-lag correlations observed among areas located at different levels in the visual hierarchy because the asymmetric laminar organization of ascending and descending connections in this hierarchy would predict systematic inter-areal phase lags. Here we show through detailed computer simulations that, when triplets rather than pairs of reciprocally connected areas in a cortical hierarchy are considered, zero-lag synchronization emerges naturally from their three-way interactions. These simulations were motivated by the observation that most areas in the cat and macaque monkey visual cortex are organized in such triplets. Our results suggest that patterns of anatomical connections in the mammalian neocortex provide a structural basis for the multi-level synchronization of neuronal activity.

Action Potentials↗
PubMed1997

Synchronization of low-frequency rhythms in corticothalamic networks.

We have investigated the degree of synchronization between cortical, thalamic reticular and thalamocortical neurons of cats during low-frequency (< 15 Hz) sleep-like oscillations, as they appear under anaesthesia. We have also studied the effects exerted by cortical stimulation on the synchronization among thalamic units. Parallel experiments [Steriade et al. (1996) J. Neurosci. 16, 392-417] in this laboratory have demonstrated the similarity between the slow oscillation (< 1 Hz) under ketamine-xylazine anaesthesia and that occurring during the natural state of resting sleep. Spontaneous activity was recorded simultaneously, with independent microelectrodes, from groups of two to five physiologically identified neurons. The rhythmicity of spontaneous activity and the temporal relations between cellular discharges were statistically evaluated by auto- and crosscorrelation techniques. We have found no topography in the distribution of synchronization between thalamic reticular and thalamocortical cells. Only the slow, cortical-generated oscillation (< 1 Hz) displayed a stable frequency and correlation among groups of cortical and thalamic cells. The other two sleep oscillations (thalamic-generated spindles at 7-14 Hz and clock-like delta at 1-4 Hz) fluctuated in frequency and the degree of correlation between neurons varied. Cortical volleys entrained and synchronized thalamic cells, and triggered synchronized spindling in the thalamus. These results extend for large populations of cortical and thalamic neurons the phase relations found in intracellular recordings.

Animals↗
PubMed2007

Event-related power modulations of brain activity preceding visually guided saccades.

To analyze the characteristics of the event-related desynchronization (ERD) and synchronization (ERS) of cortical rhythms during the preparation and execution of a lateralized eye movement, EEG was recorded in normal subjects during a visually guided task. Alpha and beta bands were investigated in three temporal intervals: a sensory period, a delay period and a saccade preparation period time locked with saccade onset. Modulations of ERD/ERS power, coupled with the task, reached the largest amplitudes over the frontal and parieto-occipital regions. Differences of oscillatory activity in the alpha bands revealed an intriguing pattern of asymmetry in parieto-occipital areas. Rightward saccades induced a larger desynchronization with respect to the leftward saccades in the left hemisphere, but not in the right. If representative, these findings are congruent to the established right-hemisphere dominance of the brain areas that direct attention. Moreover differences between the two alpha types emerged in the frontal areas before and during the saccade preparation periods, indicative of differential engagement of these areas depending on the task demands. In conclusion, the present approach shows that planning eye movements is linked with covert orienting of spatial attention and may supply a useful method for studying eye movements and selective attention-related processes.

Adult↗
PubMed2005

Ectopic action potential generation in cortical interneurons during synchronized GABA responses.

In the presence of 4-aminopyridine and excitatory amino acid receptor antagonists, individual neurons in brain slice preparations exhibit large gamma aminobutyric acid (GABA)-mediated responses as a consequence of synchronous GABA release from a network of interneurons. These synchronized GABA responses are frequently associated with ectopic action potentials (EAPs), which are thought to be action potentials initiated in distal axon terminals which subsequently travel antidromically toward the soma. Ectopic action potentials feature prominently in some models of epilepsy. Neocortical synchronized GABA responses propagate across the cortex, predominantly in superficial layers. The role that EAPs may play in contributing to laminar differences in the synchronized GABA response has not been addressed. Here we examined the occurrence of EAPs during synchronized GABA responses in neurons within layers I and II/III. EAPs occurred in 78% of layer I interneurons and in 25% of layer II/III interneurons (including chandelier cells). EAPs were not observed in layer II/III pyramidal neurons. The prevalence of EAPs in layer I interneurons provides a mechanism by which layer I can support both the initiation and propagation of synchronized GABA responses. Thus, layer I interneurons are a critical component of a network capable of synchronizing a propagating wave of GABA release across the neocortex.

Action Potentials↗