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PubMed1998

The synchronization principle in modelling of binding and attention.

The paper presents some mathematical models in support of the hypothesis that there is a general principle of information processing at the levels of both preattention and attention. It is claimed that at both levels, information processing is based on the coherent (synchronous) activity of neurons, neural populations, and brain structures. The difference between the two levels is presumed to relate to how synchronization is realized. At the level of preattention, synchronization results from the self-organization of the cortical activity, while at the level of attention, synchronous activity is controlled by special structures that act as a central executive. Two types of oscillatory neural networks are developed to model preattention and attention phenomena. In the preattention modelling, we concentrate on the binding problem. To solve this problem, a two-layer network of neural oscillators is developed. The network is able to generate two-frequency envelope oscillations, in which the amplitude of high-frequency oscillations is modulated by a lower frequency. This network synchronizes regions of oscillatory activity at high and low frequencies according to the type of stimulation. Such a synchronization gives feature binding for both simple and complex stimuli. Networks of phase oscillators with a central element are used to described different dynamic behaviour associated with the attention focus formation and switching. For the input to the attention system represented by two stimuli, we give a complete description of conditions when a specific attention focus can be formed. The results are interpreted and discussed in terms of attention modelling. This includes the interpretation of psychological experiments on visual selective attention, the problem of attention focus formation and the possibility of spontaneous attention switching .

Attention↗
PubMed1997

Spike synchronization and rate modulation differentially involved in motor cortical function.

It is now commonly accepted that planning and execution of movements are based on distributed processing by neuronal populations in motor cortical areas. It is less clear, though, how these populations organize dynamically to cope with the momentary computational demands. Simultaneously recorded activities of neurons in the primary motor cortex of monkeys during performance of a delayed-pointing task exhibited context-dependent, rapid changes in the patterns of coincident action potentials. Accurate spike synchronization occurred in relation to external events (stimuli, movements) and was commonly accompanied by discharge rate modulations but without precise time locking of the spikes to these external events. Spike synchronization also occurred in relation to purely internal events (stimulus expectancy), where firing rate modulations were distinctly absent. These findings indicate that internally generated synchronization of individual spike discharges may subserve the cortical organization of cognitive motor processes.

Action Potentials↗
PubMed2003

Corticofugal modulation of amplitude domain processing in the midbrain of the big brown bat, Eptesicus fuscus.

Recent studies have shown that the corticofugal system systematically modulates and improves subcortical signal processing in the frequency, time and spatial domains. The present study examined corticofugal modulation of amplitude sensitivity of 113 corticofugally inhibited neurons in the central nucleus of the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus. Cortical electrical stimulation decreased the number of impulses and increased the response latency of these neurons. They had an average of 5.9+/-4.4 kHz best frequency (BF) differences between collicular and electrically stimulated cortical neurons. Cortical electrical stimulation synchronized with sound stimulation for 30 min compressed the rate-amplitude functions of half (56, 49.6%) of these collicular neurons and shifted their minimum thresholds (MT) and dynamic ranges (DR) toward that of electrically stimulated cortical neurons for as long as 40 min. These collicular neurons had an average of 1.6+/-1.4 kHz BF differences. The shift in collicular MT and DR significantly increased with differences in MT and DR between collicular and cortical neurons. Cortical electrical stimulation also shifted the BF and best amplitude (BA) of collicular neurons toward that of cortical neurons. The BF shift increased with BF differences and the BA shift increased with BA differences. These data suggest that the corticofugal system modulates collicular responses on the basis of topographic projections between the IC and auditory cortex. However, corticofugal modulation of collicular amplitude sensitivity is primarily dependent upon the difference but not the absolute amplitude sensitivity between collicular and cortical neurons.

Acoustic Stimulation↗
PubMed1999

Brainstem triggers absence seizures in human generalized epilepsy.

Simultaneous analysis of brainstem auditory evoked potentials (BAEPs) with reference to electroencephalography (EEG) was designed to examine the brainstem function corresponding to the EEG event. With this method, we investigated the brainstem function pre- and during the paroxysmal discharge in human absence seizures classified as primary generalized epilepsy (PGE). Two types of functional change in the lower brainstem were revealed as parameters of wave-III components (amplitude and area) of BAEPs without significant change in the upper brainstem. One was long-range biphasic fluctuation (acceleration followed by abrupt deceleration with the maximum -6.4+/-3.2 s before the seizure onset), and the other was rhythmic oscillation with 3 Hz. The latter, synchronized with the cortical spike-and-wave complex, imposed on the descending slope of the former. One important point is that both preceded the onset of cortical paroxysmal discharge. The results reappraise the classical hypothesis of "centrencephalic system" on seizure generating mechanism in human PGE. The results prove the primary triggering role of the lower brainstem that is independent of sleep-related synchronizations. The method is applicable to other types of EEG event for the investigation of brainstem involvement.

Adolescent↗
PubMedDate not supplied

The lateral spread of ictal discharges in neocortical brain slices.

The in vitro brain slice technique was used to examine the lateral propagation of spontaneous electrographic ictal episodes across adjacent areas of guinea pig neocortex. Epileptiform activity was induced by perfusing slices with Mg-free artificial CSF. Simultaneous field potential recordings of ictal episodes were obtained from 4 micropipettes placed 1-3 mm apart across coronal slices in middle-cortical layers. Two types of lateral spread were characterized. Ictal episodes often developed focally and then spread as a slowly moving wavefront traveling at less than 0.3 mm/sec into adjacent, uninvolved cortex. By contrast, other episodes began nearly synchronously at all cortical sites. The individual afterdischarges that composed each ictal episode propagated rapidly across the cortex at greater than 30 mm/sec and were triggered by multiple pacemakers. Ictal episodes always terminated abruptly across the entire slice. The NMDA-receptor antagonist, 2-amino-phosphono-valerate, applied focally between recording sites, blocked rapid propagation across treated areas and resulted in the emergence of spatially separate, independent pacemakers. Pacemaker failure is the proposed mechanism for simultaneous and generalized termination of ictal episodes in this in vitro model of epileptogenesis.

2-Amino-5-phosphonovalerate↗
PubMed1990

General pharmacology of recombinant human tumor necrosis factor. 2nd communication: effects on central nervous system functions.

Effects of recombinant human Tumor Necrosis Factor (rHu-TNF, PT-050) administered s.c. and i.v. on the central nervous system were investigated behaviorally and physiologically in different animal species. With i.v. administration at doses of 10(4) U/kg and less, PT-050 produced no significant changes in most of behavioral and physiological tests, except that a transient increase in rectal temperature in dogs occurred with 0.765 x 10(4) U/kg and acetic acid-induced writhing syndrome in mice was inhibited with 10(4) U/kg. Locomotor activity in mice was inhibited after 3 x 10(4) U/kg. With 10(5) U/kg and more, in addition to the above effects, diverse effects were observed as follows: various symptoms were induced in general behavior in mice such as piloerection, catalepsy, lowering of body position, ptosis and pupil dilatation; rectal temperature was increased in rats and decreased in mice; cortical EEG was synchronized in hippocampal theta waves were disturbed in gallamine-immobilized cats. On the other hand, with s.c. administration, the effect on rectal temperature was seen at 10(4) U/kg in dogs and at 10(5) U/kg in rats. Cortical EEG in conscious rabbits was affected with 10(6) U/kg. At higher dose of 10(7) U/kg. PT-050 (s.c.) further influenced locomotor activity, rectal temperature and acetic acid-induced writhing syndrome in mice. However, even with 10(7) U/kg (s.c.), no effect was found in general behavior, rotarod performance and hexobarbital narcosis in mice. Thus, it is concluded that PT-050 has weaker effects on the central nervous system by s.c. route than by i.v. route. From the present findings and the efficacy of PT-050 as TNF, it is suggested that PT-050, when administered intratumorally, may be useful for cancer therapy without severe side effects.

Analgesics↗
PubMed2003

Epileptiform synchronization in the human dysplastic cortex.

Taylor's focal cortical dysplasia corresponds to a localized disruption of the normal cortical lamination with an excess of large, aberrant cells. Sustained epileptic discharges originate from the dysplastic neocortex and this tissue retains sufficient connectivity for expressing seizure abnormalities. In this brief review, we summarize the findings obtained by analyzing surgically-resected human tissue with focal cortical dysplasia that was maintained in vitro in a brain slice preparation. These data have been compared with those obtained from human cortex with normal structural organization; such tissue was available from patients undergoing surgery for a variety of neurological disorders, most often for mesial temporal lobe epilepsy. These studies have shown that: (i). slices obtained from focal cortical dysplastic tissue have an intrinsic ability to generate ictal-like epileptiform events when challenged with the convulsant drug 4-aminopyridine; (ii). 4-aminopyridine-induced ictal discharges are not seen in neocortical slices obtained from neocortical samples with no or minimal structural lesion; (iii). these ictal discharges are caused by the activation of excitatory amino acid receptors, and in particular those of the N-methyl-D aspartate type; (iv). focal cortical dysplastic tissue also generates synchronous potentials that are mainly contributed by GABA(A) receptor-mediated conductances.

4-Aminopyridine↗
PubMed1990

[Electroencephalographic study with buspirone, a novel nonbenzodiazepine anxiolytic, and its major metabolite, 1-(2-pyrimidinyl)piperazine (1-PP), in rabbits].

The electroencephalographic (EEG) effects of buspirone and its major metabolite, 1-(2-pyrimidinyl)piperazine (1-PP), were investigated in rabbits with chronic electrode implants, and the effects were compared with those of diazepam. Intravenous administration of buspirone at 0.1 to 1.0 mg/kg evoked an increase in the arousal EEG pattern period (low amplitude fast waves) in the cortical EEG and synchronization of the hippocampal theta waves with decreased voltages, whereas both 1-PP (0.5 to 2.0 mg/kg, i.v.) and diazepam (1.0 and 2.0 mg/kg) evoked an increase in the drowsy EEG pattern periods: high voltage slow waves and spindle bursts in the cortical EEG and desynchronization of the hippocampal theta waves. Buspirone at higher doses caused behavioral excitation in rabbits, whereas both 1-PP and diazepam produced sedation. Buspirone did not affect EEG arousal responses to both auditory stimulation (2,000 Hz, monotone) and electrical stimulation (100 Hz, 0.1 msec, 3-6 V) of the midbrain reticular formation or the posterior hypothalamus. However, 1-PP tended to inhibit the EEG arousal response to auditory stimulation but not brain stimulation, and diazepam markedly suppressed the responses induced by both stimulations. The recruiting response induced by centromedian thalamic stimulation at a low frequency (7 Hz, 0.1 msec, 4-8 V) was not affected by buspirone, 1-PP and diazepam. Neither buspirone nor 1-PP had an effect on the photic driving response to a flash light (2 Hz) in the occipital cortex of the rabbit, whereas the response was suppressed by diazepam. Both buspirone and 1-PP enhanced the duration of after discharges induced by electrical stimulation (50 Hz, 0.5 msec, 4-15 V) of the dorsal hippocampus, whereas diazepam markedly inhibited the afterdischarges. These results suggest that the EEG effect of buspirone is quite different to those of 1-PP and diazepam in qualitative aspects. It is also suggested that buspirone, unlike diazepam, is an effective anxiolytic drug without a sedative effect.

Animals↗
PubMedDate not supplied

[Interaction between neurons of the visual and sensomotor areas of the neocortex during elaboration and extinction of a conditioned defense reflex].

Cooperation in activities of pairs of neurones situated in projection cortical areas of conditioned and unconditioned stimuli was studied in rabbits during intertrial intervals at different stages of elaboration and extinction of conditioned defensive response by means of auto- and crosscorrelation analysis of impulse fluxes. At the stage of generalization the number of pairs of neurones discharging in correlation was shown to increase a little (64 per cent) in comparison to that at the initial stage of conditioning (50 per cent) and pseudoconditioning (54 per cent). At the stage of stabilization and during extinction the number of pairs of neurones discharging in correlation decreased correspondingly to 34 and 32 per cent. Parallel analysis of correlation in neuronal discharges and simultaneously recorded electroencephalogram allowed to suppose that excitatory synchronizing influences and inhibitory cortical system play a great part in synchronization of activities of cortical neurones. Participation of these two systems is not the same at different stages of conditioning and extinction.

Animals↗
PubMed1999

Cortically-induced coherence of a thalamic-generated oscillation.

Oscillatory patterns in neocortical electrical activity show various degrees of large-scale synchrony depending on experimental conditions, but the exact mechanisms underlying these variations of coherence are not known. Analysis of multisite local field potentials revealed that the coherence of spindle oscillations varied during different states. During natural sleep, the coherence was remarkably high over cortical distances of several millimeters, but could be disrupted by artificial cortical depression, similar to the effect of barbiturates. Possible mechanisms for these variations of coherence were investigated by computational models of interacting cortical and thalamic neurons, including their intrinsic firing patterns and various synaptic receptors present in the circuitry. The model indicates that modulation of the excitability of the cortex can affect spatiotemporal coherence with no change in the thalamus. The highest level of coherence was obtained by enhancing the excitability of cortical pyramidal cells, simulating the action of neuromodulators such as acetylcholine and noradrenaline. The underlying mechanism was due to cortex-thalamus-cortex loops in which a more excitable cortical network generated a more powerful and coherent feedback onto the thalamus, resulting in highly coherent oscillations, similar to the properties measured during natural sleep. In conclusion, these experiments and models are compatible with a powerful role for the cortex in triggering and synchronizing oscillations generated in the thalamus, through corticothalamic feedback projections. The model suggests that intracortical mechanisms may be responsible for synchronizing oscillations over cortical distances of several millimeters through cortex-thalamus-cortex loops, thus providing a possible cellular mechanism to explain the genesis of large-scale coherent oscillations in the thalamocortical system.

Anesthesia↗
PubMed2003

Changes in oscillatory cortical activity related to a visuomotor task in young and elderly healthy subjects.

OBJECTIVE: In order to better understand the spatio-temporal interaction of the activated cortical areas when the movement is visuo-guided and to assess the age effect on the spatio-temporal pattern of cortical activity, we have compared a proximo-distal movement with visual-motor control and hand-eye coordination (targeting movement) with a distal and a proximal movement. METHODS: Brain's electrical activity was studied using the analysis of event-related (de)synchronizations (ERD/S) of cortical mu and beta rhythms in 17 subjects, 8 young and 9 elderly subjects. RESULTS: In both populations, we found an earlier and broader mu and beta ERD during the preparation of the targeting movement compared to distal and proximal movements, principally involving the contralateral parietal region. During the execution, a spreading over the parietocentral region during proximal movement and over the parietal region during targeting movement was observed. After the execution of proximal and targeting movements, a wider and higher beta ERS was observed only in the young subjects. In the elderly subjects, our results showed a significant decrease of beta ERS during the targeting task. CONCLUSIONS: These results suggest there was a larger recruitment of cortical areas, involving notably the parietal cortex when the movement is visuo-guided. Moreover, cerebral aging-related changes in the spatio-temporal beta ERS pattern suggests an impaired sensory integration.

Adult↗
PubMed2000

The effects of pair-wise and higher order correlations on the firing rate of a post-synaptic neuron.

Coincident firing of neurons projecting to a common target cell is likely to raise the probability of firing of this postsynaptic cell. Therefore, synchronized firing constitutes a significant event for postsynaptic neurons and is likely to play a role in neuronal information processing. Physiological data on synchronized firing in cortical networks are based primarily on paired recordings and cross-correlation analysis. However, pair-wise correlations among all inputs onto a postsynaptic neuron do not uniquely determine the distribution of simultaneous postsynaptic events. We develop a framework in order to calculate the amount of synchronous firing that, based on maximum entropy, should exist in a homogeneous neural network in which the neurons have known pair-wise correlations and higher-order structure is absent. According to the distribution of maximal entropy, synchronous events in which a large proportion of the neurons participates should exist even in the case of weak pair-wise correlations. Network simulations also exhibit these highly synchronous events in the case of weak pair-wise correlations. If such a group of neurons provides input to a common postsynaptic target, these network bursts may enhance the impact of this input, especially in the case of a high postsynaptic threshold. The proportion of neurons participating in synchronous bursts can be approximated by our method under restricted conditions. When these conditions are not fulfilled, the spike trains have less than maximal entropy, which is indicative of the presence of higher-order structure. In this situation, the degree of synchronicity cannot be derived from the pair-wise correlations.

Animals↗
PubMed2004

Reciprocal modulation of neuromagnetic induced gamma activity by attention in the human visual and auditory cortex.

For attentional control of behavior, the brain permanently resolves a competition between the impressions supplied by different senses. Here, using a dual-modality temporal order detection task, we studied attentional modulation of oscillatory neuromagnetic activity in the human cerebral cortex. On each trial, after simultaneous exposure to visual and auditory noise, subjects were presented with an asynchronous pair of a visual and an auditory stimulus. Either of the two stimuli could occur first equally often, their order was not cued. Subjects had to determine the leading stimulus in a pair and attentively monitor it to respond upon its offset. With the attended visual or auditory stimuli, spectral power analysis revealed marked enhancements of induced gamma activity within 250 ms post-stimulus onset over the modality-specific cortices (occipital at 64 Hz, right temporal at 53 Hz). When unattended, however, the stimuli led to a significantly decreased (beneath baseline) gamma response in these cortical regions. The gamma decreases occurred at lower frequencies ( approximately 30 Hz) than did the gamma increases. An increase in the gamma power and frequency for the attended modality and their decrease for the unattended modality suggest that attentional regulation of multisensory processing involves reciprocal changes in synchronization of respective cortical networks. We assume that the gamma decrease reflects an active suppression of the task-irrelevant sensory input. This suppression occurs at lower frequencies, suggesting an involvement of larger scale cell assemblies.

Acoustic Stimulation↗
PubMedDate not supplied

[The mechanisms behind the generation of the slow oscillations found in EEG recordings during sleep].

INTRODUCTION: Electroencephalogram (EEG) recordings reflect different oscillatory activities during slow wave sleep (stages 2 and 3-4 of the sleep-waking cycle), namely d oscillations (< 4 Hz), sleep spindles and K complexes. These activities are essentially generated by the activity of the thalamo-cortical relay neurons, the neurons of the thalamic reticular nucleus and by the neurons in the cerebral cortex. DEVELOPMENT: The combination of the intrinsic electrophysiological properties of the thalamic and cortical neurons, together with their synaptic connections, are responsible for the generation of these oscillations. Extra or intracellular recordings of these neurons during spontaneous or anaesthetic-induced sleep show how these neurons change their electrical activity during slow sleep due to the hyperpolarization of their membrane potential. Thus, the thalamic neurons lower their response to sensory stimuli and filter this information towards the cerebral cortex. Glial cells also contribute to the generation of the d waves seen in the EEG and oscillate synchronously with the cortical neurons. In addition, the oscillations of this neuronal and glial network is linked to important changes in the concentration of certain ions in the extracellular space; for instance, the K+ and Ca2+ concentration oscillates synchronously with the neuronal and the glial activity. CONCLUSIONS: The oscillations in the EEG, which are slower than those observed in the waking state or in paradoxical sleep, play a fundamental role in processing the information handled by the central nervous system and filter the irrelevant information towards the cerebral cortex.

Cerebral Cortex↗
PubMed2001

Fatigue in multiple sclerosis is associated with abnormal cortical activation to voluntary movement--EEG evidence.

Converging evidence is consistent with the view that fatigue in Multiple Sclerosis is independent from pyramidal tract involvement, suggesting a possible involvement of frontal areas. During voluntary movement, changes of the EEG rhythms can be observed over sensorimotor areas. Event-related desynchronization (ERD) of the 10 and 20 Hz frequency bands occurs during motor planning and execution and is followed after movement termination by event-related synchronization (ERS), expressing cortical idling or inhibition. We evaluated the pattern of cortical activation to voluntary movement in MS patients complaining of fatigue assessed using the Fatigue Severity Scale. Fifteen MS patients complaining of fatigue, 18 MS patients without fatigue, and 14 normal controls were studied. The two patients groups were similar for age, sex, disease duration, and were not disabled (score <1.5 at the Expanded Disability Status Scale). Twenty-nine channel EEG was recorded during about 60 self-paced extensions of the right thumb. The onset latency and amount of the contralateral sensorimotor (C3 electrode) 10 and 18--22 Hz ERD were similar in the three groups. ERD was more widespread anteriorly in the fatigue group compared with normal controls (P < 0.01 over Fz electrode). Postmovement contralateral sensorimotor 18--22 Hz ERS was significantly lower in fatigue MS patients compared with normal subjects (P < 0.005) and with nonfatigue MS patients (P = 0.02). These findings are consistent with a central origin of fatigue in MS and indicate cortical dysfunction even during a simple motor task, resulting in hyperactivity during movement execution and failure of the inhibitory mechanisms intervening after movement termination.

Adult↗
PubMed2000

Relationship among discharges of neighboring neurons in the rat prefrontal cortex during spatial working memory tasks.

The relationship among discharges of neurons that were recorded simultaneously with tetrodes in the rat medial prefrontal cortex was analyzed. Spatial working memory tasks were divided into several distinct stages based on the behavioral correlates of individual neurons, and interneuronal correlation of signal (mean discharge rate at each stage) and noise (trial-to-trial deviation from the signal) was calculated. Behavioral correlates of neighboring neurons were quite heterogeneous and, accordingly, average signal correlation was relatively low ( approximately 0.16). Noise correlation was even lower ( approximately 0.06), but neuronal noise was more correlated among the neurons with similar signals. Spikes underlying the signal and noise correlation among the prefrontal cortical neurons were loosely synchronized over a few hundred milliseconds. These results suggest that neighboring prefrontal cortical neurons process largely independent information and have weakly correlated noise and that precisely synchronized spikes play a relatively minor role in producing the correlated signal and noise among these neurons.

Action Potentials↗
PubMed2004

District-related frequency specificity in hand cortical representation: dynamics of regional activation and intra-regional synchronization.

The aim of this work was to study the degree of neuronal synchronization occurring within the portion of the somatosensory cortex devoted to hand control during an external sensory stimulation. In this way, we focused on the properties of the sensory cortical representation, rather than the more investigated motor one. To this aim, we collected magnetoencephalograhic data from healthy subjects during separate stimulation of their thumbs and little fingers and analyzed these data by means of a time-dependent 'synchronization index'. The properties of this index within the beta [16-32 Hz] and gamma [36-44 Hz] frequency bands suggest that the hand representation in the human primary cortex follows a frequency coding, in addition to the somatotopic one, for discriminating different districts. Our results showed that the gamma synchronization is higher following stimulation of the thumb than of the little finger and we suggest that the strength of gamma band synchronization works as a code for functional prevalence. In particular, our comparative analysis of the dynamic synchronization index and the signal amplitude suggests that a prevalent district (thumb) recruits a smaller number of higher-synchronic gamma band tuned neurons than a non-prevalent district (little finger).

Adult↗