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PubMed2006

Neuron participation in a synchrony-encoding assembly.

BACKGROUND: Synchronization of action potentials between neurons is considered to be an encoding process that allows the grouping of various and multiple features of an image leading to a coherent perception. How this coding neuronal assembly is configured is debated. We have previously shown that the magnitude of synchronization between excited neurons is stimulus-dependent. In the present investigation we compare the levels of synchronization between synchronizing individual neurons and the synchronizing pool of cells to which they belong. RESULTS: Even though neurons belonged to their respective pools, some cells synchronized for all presented stimuli while others were rather selective and only a few stimulating conditions produced a significant synchronization. In addition the experiments show that one synchronizing pair rarely replicates the level of synchrony between corresponding groups of units. But when synchronizing clusters of neurons increase in number, the correlation (measured as a coefficient of determination) between unit synchronization and the synchronization between the entire pools of cells to which individual neurons belong improves. CONCLUSION: These results prompt the hypothesis that random or spontaneous synchronization becomes progressively less important, whereas coincident spikes related to encoding properties of targets gain significance because a particular configuration of an image biases the excitatory inputs in favor of connections driven by the applied features of the stimulus.

Action Potentials↗
PubMedDate not supplied

[EEG study of the functional organization of the right and left hemisphere during solution of verbal and spatial problems].

In adult healthy right-handed subjects, the expression and degree of synchronization of the EEG alpha-range rhythmic components in different areas of the right and left hemispheres, were studied in a state of quiet wakefulness and during solving of verbal and spatial tasks presented in the visual field. The EEG of quiet wakefulness was characterized by different distribution of the alpha-range rhythmic components in the right and left hemispheres; in the right hemisphere low frequencies (7.5-10.5 c/s) were more expressed and more coherent; in the left one--the high frequencies (10.5-13.5 c/s). The solving of tasks was accompanied--along with a decrease of the whole alpha-range power spectra both in the right and the left hemispheres--by a local increase of synchronization of certain components of this range; the increase was specific to the hemisphere and the kind of task. The increase of synchronization of low-frequency components was observed in the right hemisphere during solving of the spatial task and that of the high-frequency components was noticed in the left hemisphere during solving of the verbal task. On the basis of the data on hemispheric specificity of electric activity synchronization of the alpha-rhythm, a suggestion is made about a different character of the functional integration of the structures of the right and left hemispheres in the process of solving of spatial and verbal tasks.

Adult↗
PubMed2004

Acute cortisol administration promotes sleep intensity in man.

The neuronal mechanisms of sleep generation, in particular synchronization of brain activity in the process of non-rapid-eye movement (non-REM) sleep, has been elucidated in the past decade. A previous study of our group showed that acute administration of cortisol is known to increase slow-wave sleep and suppress rapid-eye movement (REM) sleep in man. To further elucidate the non-REM sleep-promoting effects of cortisol with respect to the synchronization of cortical activity, it is important to establish a sleep-state-specific quantitative EEG analysis. We therefore investigated the effects of repetitive injections of hydrocortisone on spectral composition of sleep EEG in 10 healthy male young volunteers. In addition, we performed high-frequency blood samplings to assess the relation between changes in the sleep EEG and sleep-associated secretion of growth hormone (GH). Cortisol administration resulted in a significant increase in highly synchronized EEG activity including delta and theta frequencies, according to a higher amount of slow-wave sleep. This effect predominated in the first few hours of night sleep. REM sleep was decreased, which appeared to be secondary to the lengthened first sleep cycle. The cortisol-induced stimulation of GH release did not occur in correspondence with the increased slow-wave activity. In view of the sleep impairing properties of corticotropin-releasing hormone (CRH) and the sleep-promoting function of GH-releasing hormone, it appears likely that a negative feedback inhibition of endogenous CRH was the key mechanism mediating the observed results. The cortisol-induced effects on sleep intensity and sleep-associated GH secretion appeared to be driven by different mechanisms.

Adult↗
PubMed1976

Modulating systems of hippocampal EEG.

Hippocampal EEG changes in response to stimulation of several mesodiencephalic inputs (mesodiencephalic reticular formation, MRF; medial and lateral hypothalamus, MH and LH; medial septum, MS) were quantitatively investigated by means of spectral analysis in cats with chronically indwelling electrodes. The rhythmic activity of the hippocampus, in the theta frequency range, increased as a function of stimulus strength wherever the stimulation took place in MRF, LH and MH. Characteristic response patterns were found for each input area. These differed as regards the occurrence of shifts in theta peak frequency and the decrease in one or more theta frequency components below baseline levels at high stimulation voltage. Desynchronization (EEG flat spectrum) did not take place at low stimulation strength of the MRF, LH and MH except in a few cases, where the electrodes were placed in the neighbourhood of fibre tracts (fornix and medial forebrain bundle). Thus evidence for the existence of "synchronizing" and "desynchronizing" systems was not found. A model of the origin of the hippocampal EEG is discussed in the light of the results, by means of which several hypotheses concerning the mechanisms of desynchronization are considered.

Animals↗
PubMed2005

Dynamics of the EEG slow-wave synchronization during sleep.

OBJECTIVE: To study the dynamics of spatial synchronization of the slow-wave activity recorded from different scalp electrodes during sleep in healthy normal controls. METHODS: We characterized the different levels of EEG synchronization during sleep (in the 0.25-2.5 Hz band) of five healthy subjects by means of the synchronization likelihood (SL) algorithm and analyzed its long-range temporal correlations by means of the detrended fluctuation analysis (DFA). RESULTS: We found higher levels of interregional synchronization during 'cyclic alternating pattern' (CAP) sleep than during nonCAP with a small but significant difference between its A and B phases. SL during CAP showed fluctuations probably corresponding to the single EEG slow-wave elements. DFA showed the presence of two linear scaling regions in the double-logarithmic plot of the fluctuations of SL level as a function of time scale. This indicates the presence of a characteristic time scale in the underlying dynamics which was very stable among the different subjects (1.23-1.33 s). We also computed the DFA exponent of the two scaling regions; the first, with values approximately 1.5, corresponded to fluctuations with period 0.09-0.75 s and the second, with values approximately 1, corresponded to fluctuations with period 1.5-24.0 s. Only the first exponent showed different values during the different sleep stages. CONCLUSIONS: All these results indicate a different role for each sleep stage and CAP condition in the EEG synchronization processes of sleep which show a complex time structure correlated with its neurophysiological mechanisms. SIGNIFICANCE: Very slow oscillations in spatial EEG synchronization might play a critical role in the long-range temporal EEG correlations during sleep which might be the chain of events responsible for the maintenance and correct complex development of sleep structure during the night.

Adult↗
PubMed2006

Regional scalp EEG slow-wave synchronization during sleep cyclic alternating pattern A1 subtypes.

The levels of EEG synchronization, in the 0.25-2.5 Hz band, during the A1 subtypes of the sleep "cyclic alternating pattern" (CAP) were measured in five healthy subjects by means of the synchronization likelihood (SL) algorithm. SL was measured for seven electrode pairs (F4-F3, C4-C3, P4-P3 for the analysis of interhemispheric SL and F4-C4, C4-P4, F3-C3, and C3-P3, for the analysis of intrahemispheric SL). During the A1 CAP subtypes, SL tended to be highest between pairs of electrodes situated over different hemispheres; in particular, SL obtained from F4-F3 was the highest, followed by that of P4-P3. These results indicate that the transient high level of synchronization in the slow-wave EEG range, during the sleep A1 CAP subtypes, is a phenomenon involving mostly the anterior parts of the brain and is probably based on interhemispheric interactions, possibly mediated by transcallosal connections.

Adult↗
PubMedDate not supplied

Synchronization in a network of fast-spiking interneurons.

Experimental results revealed that in neocortex inhibitory fast-spiking (FS) interneurons interact also by electrical synapses (gap-junctions). They receive sensory information from thalamus and transfer it to principal cells by feedforward inhibition. Moreover, their synchronous discharge enhances their inhibitory control of pyramidal neurons. By using a biophysical model of FS interneurons the synchronization properties of a network of two synaptically coupled units are investigated. In the case they interact only by inhibitory synapses, well defined regions exist in the parameters space described by the strength and duration of the synaptic current, where synchronous regimes occur. Then an empirical protocol is proposed to determine approximately the borders of the synchronization manifold (SM). When electrical synapses are included, the region of synchronous discharge of the two interneurons becomes larger. In both cases, the coherent states are characterized by discharge frequencies in the gamma range. Lastly, the effects of heterogeneity, either obtained by using different stimulation currents or unidirectional inhibitory coupling, are studied.

Action Potentials↗
PubMed2000

Stimulus-specific oscillatory responses of the brain: a time/frequency-related coding process.

OBJECTIVES: To review the coherent, rhythmic oscillations above approximately 20 Hz that occur in response to sensory inputs in the firing rate and membrane or local field potentials of distributed neuron aggregates of CNS layered structures. RESULTS: Oscillatory activity at approximately 20-80 Hz occurs in response to either olfactory, auditory and visual (contrast) stimuli; oscillations at frequencies centered on 100-120 Hz or 600 Hz are recorded, respectively, from the visual system (luminance stimulation) and from the somatosensory cortex. Experimental evidence suggests sources/mechanisms of generation that depend on inhibitory interneurons and pyramidal cells and are partially independent from those of conventional (broadband) evoked responses. In the olfactory and visual systems, the oscillatory responses reflect the global stimulus properties. A time/phase correlation between firing rate, spiking coincidence and oscillatory field responses has been documented. The oscillatory responses are postsynaptic both in cortex and in precortical structures (e.g. retina; LGN). Evidence indicates intracortical and thalamocortical interacting mechanisms of regulation as well as GABAergic and cholinergic modulation. In the visual cortex the oscillatory responses are driven by oscillations in the synaptic input. Oscillatory potentials are dependent on resonance phenomena and produce narrow-band synchronization of activated neurons. They may have a role in the 'binding' of separate neuronal aggregates into sensory units. CONCLUSIONS: Oscillatory responses contribute as a time/frequency coding mechanism to pacing neurons selectively for the physical properties of stimulus and are involved in sensory information processing.

Animals↗
PubMed2004

Chaotic itinerancy as a mechanism of irregular changes between synchronization and desynchronization in a neural network.

We investigate the dynamic character of a network of electrotonically coupled cells consisting of class I point neurons, in terms of a finite dimensional dynamical system. We classify a subclass of class I point neurons, called class I* point neurons. Based on this classification, we use a reduced Hindmarsh-Rose (H-R) model, which consists of two dynamical variables, to construct a network model consisting of electrotonically coupled H-R neurons. Although biologically simple, the system is sufficient to extract the essence of the complex dynamics, which the system may yield under certain physiological conditions. The network model produces a transitory behavior as well as a periodic motion and spatio-temporal chaos. The transitory dynamics that the network model exhibits is shown numerically to be chaotic itinerancy. The transitions appear between various metachronal waves and all-synchronization states. The network model shows that this transitory dynamics can be viewed as a chaotic switch between synchronized and desynchronized states. Despite the use of spatially discrete point neurons as basic elements of the network, the overall dynamics exhibits scale-free activity including various scales of spatio-temporal patterns.

Action Potentials↗
PubMed2005

Theta activity in neurons and networks of the amygdala related to long-term fear memory.

With a combined in vitro/in vivo electrophysiological and behavioral approach, we have correlated conditioned fear behavior to electrophysiological activities in the lateral amygdala and the hippocampal formation in rodents. Data indicate that projection neurons in the lateral amygdala display a continuum of spike patterns including accommodating patterns, regular firing, and oscillatory activity at theta frequencies. The firing pattern is controlled to an important part by the intracellular cAMP system, in that an increase in intracellular cAMP concentration facilitates regular firing and theta oscillations. Oscillatory electrical activity, in turn, provides an important cellular element of synchronized theta activity at 4-8 Hz (indicating atropine-sensitive type 2 theta) occurring in amygdalo-hippocampal pathways during conditioned fear responses. This type of rhythmic network activity is associated with the retrieval of long-term fear memory following cued and contextual fear conditioning, but is not related to the expression of fear behavior per se or to short-term fear memory. Synchronization at theta frequencies is suggested to represent activity in amygdalo-hippocampal pathways associated with system consolidation of fear memory, which is supported by the cholinergic system.

Action Potentials↗
PubMed2003

Phase synchronization of the ongoing EEG and auditory EP generation.

OBJECTIVE: We investigated the role of phase synchronization of the spontaneous electroencephalogram (EEG) in auditory evoked potential (EP) generation in a sample of healthy individuals. METHODS: Auditory responses were obtained from 20 healthy subjects following a double stimulus paradigm, using two identical tone bursts (S1 and S2) separated by 0.5s. Single-trial auditory evoked potentials were decomposed into sinusoidal, exponentially decaying/increasing components using the piecewise Prony method (PPM). Pre- and post-stimulus phase histograms were compared to determine the degree of phase synchronization produced by auditory stimulation. RESULTS: Analysis of single responses revealed that the S1 stimuli produced phase synchronization in the 2-8Hz frequency range, with little or no concomitant amplitude increase. A significantly reduced phase effect was seen in response to S2 stimuli. CONCLUSIONS: Stimulus-induced phase synchronization of the ongoing EEG is a major mechanism for the generation of auditory EP components with a latency in the 50-250ms range. SIGNIFICANCE: The fact that the EP components accessed here are generated through phase synchronization implies that the ensemble-averaged EP will not resemble the single trial response, and it would certainly be misleading to consider the single trial response as an amplitude-scaled version of the ensemble average.

Acoustic Stimulation↗
PubMed2000

Attention modulates synchronized neuronal firing in primate somatosensory cortex.

A potentially powerful information processing strategy in the brain is to take advantage of the temporal structure of neuronal spike trains. An increase in synchrony within the neural representation of an object or location increases the efficacy of that neural representation at the next synaptic stage in the brain; thus, increasing synchrony is a candidate for the neural correlate of attentional selection. We investigated the synchronous firing of pairs of neurons in the secondary somatosensory cortex (SII) of three monkeys trained to switch attention between a visual task and a tactile discrimination task. We found that most neuron pairs in SII cortex fired synchronously and, furthermore, that the degree of synchrony was affected by the monkey's attentional state. In the monkey performing the most difficult task, 35% of neuron pairs that fired synchronously changed their degree of synchrony when the monkey switched attention between the tactile and visual tasks. Synchrony increased in 80% and decreased in 20% of neuron pairs affected by attention.

Action Potentials↗
PubMed1999

Electrophysiological properties of inferior olive neurons: A compartmental model.

As a step in exploring the functions of the inferior olive, we constructed a biophysical model of the olivary neurons to examine their unique electrophysiological properties. The model consists of two compartments to represent the known distribution of ionic currents across the cell membrane, as well as the dendritic location of the gap junctions and synaptic inputs. The somatic compartment includes a low-threshold calcium current (I(Ca_l)), an anomalous inward rectifier current (I(h)), a sodium current (I(Na)), and a delayed rectifier potassium current (I(K_dr)). The dendritic compartment contains a high-threshold calcium current (I(Ca_h)), a calcium-dependent potassium current (I(K_Ca)), and a current flowing into other cells through electrical coupling (I(c)). First, kinetic parameters for these currents were set according to previously reported experimental data. Next, the remaining free parameters were determined to account for both static and spiking properties of single olivary neurons in vitro. We then performed a series of simulated pharmacological experiments using bifurcation analysis and extensive two-parameter searches. Consistent with previous studies, we quantitatively demonstrated the major role of I(Ca_l) in spiking excitability. In addition, I(h) had an important modulatory role in the spike generation and period of oscillations, as previously suggested by Bal and McCormick. Finally, we investigated the role of electrical coupling in two coupled spiking cells. Depending on the coupling strength, the hyperpolarization level, and the I(Ca_l) and I(h) modulation, the coupled cells had four different synchronization modes: the cells could be in-phase, phase-shifted, or anti-phase or could exhibit a complex desynchronized spiking mode. Hence these simulation results support the counterintuitive hypothesis that electrical coupling can desynchronize coupled inferior olive cells.

Calcium Channels↗
PubMed2003

Modulation of excitatory synaptic coupling facilitates synchronization and complex dynamics in a biophysical model of neuronal dynamics.

In this paper, complex dynamical synchronization in a non-linear model of a neural system is studied, and the computational significance of the behaviours is explored. The local neural dynamics is determined by voltage- and ligand-gated ion channels and feedback between densely interconnected excitatory and inhibitory neurons. A mesoscopic array of local networks is modelled by introducing coupling between the local networks via weak excitatory-to-excitatory connectivity. It is shown that with modulation of this long-range synaptic coupling, the system undergoes a transition from independent oscillations to stable chaotic synchronization. Between these states exists a 'weakly' stable state associated with complex, intermittent behaviour in the temporal domain and clusters of synchronous regions in the spatial domain. The paper concludes with a discussion of the putative relevance of such processes in the brain, including the role of neuromodulatory systems and the mechanisms underlying sensory perception, adaptation, computation and complexity.

Action Potentials↗
PubMed2002

Desynchronization of brain rhythms with soft phase-resetting techniques.

Composite stimulation techniques are presented here which are based on a soft (i.e., slow and mild) reset. They effectively desynchronize a cluster of globally coupled phase oscillators in the presence of noise. A composite stimulus contains two qualitatively different stimuli. The first stimulus is either a periodic pulse train or a smooth, sinusoidal periodic stimulus with an entraining frequency close to the cluster's natural frequency. In the course of several periods of the entrainment, the cluster's dynamics is reset (restarted), independently of its initial dynamic state. The second stimulus, a single pulse, is administered with a fixed delay after the first stimulus in order to desynchronize the cluster by hitting it in a vulnerable state. The incoherent state is unstable, and thus the desynchronized cluster starts to resynchronize. Nevertheless, resynchronization can effectively be blocked by repeatedly delivering the same composite stimulus. Previously designed stimulation techniques essentially rely on a hard (i.e., abrupt) reset. With the composite stimulation techniques based on a soft reset, an effective desynchronization can be achieved even if strong, quickly resetting stimuli are not available or not tolerated. Accordingly, the soft methods are very promising for applications in biology and medicine requiring mild stimulation. In particular, it can be applied to effectively maintain incoherency in a population of oscillatory neurons which try to synchronize their firing. Accordingly, it is explained how to use the soft techniques for (i). an improved, milder, and demand-controlled deep brain stimulation for patients with Parkinson's disease or essential tremor, and for (ii). selectively blocking gamma activity in order to manipulate visual binding.

Animals↗
PubMed2006

Mu rhythm (de)synchronization and EEG single-trial classification of different motor imagery tasks.

We studied the reactivity of EEG rhythms (mu rhythms) in association with the imagination of right hand, left hand, foot, and tongue movement with 60 EEG electrodes in nine able-bodied subjects. During hand motor imagery, the hand mu rhythm blocked or desynchronized in all subjects, whereas an enhancement of the hand area mu rhythm was observed during foot or tongue motor imagery in the majority of the subjects. The frequency of the most reactive components was 11.7 Hz +/- 0.4 (mean +/- SD). While the desynchronized components were broad banded and centered at 10.9 Hz +/- 0.9, the synchronized components were narrow banded and displayed higher frequencies at 12.0 Hz +/- 1.0. The discrimination between the four motor imagery tasks based on classification of single EEG trials improved when, in addition to event-related desynchronization (ERD), event-related synchronization (ERS) patterns were induced in at least one or two tasks. This implies that such EEG phenomena may be utilized in a multi-class brain-computer interface (BCI) operated simply by motor imagery.

Adult↗
PubMed1998

The synchronization between brain areas under motor inhibition process in humans estimated by event-related EEG coherence.

To investigate the functional connection of brain areas under motor inhibition, the event-related coherence (ERCoh) of the electroencephalogram (EEG) was calculated for 10 subjects who were asked to perform a visual discrimination (GO/NO-GO) task. The subjects were instructed to push (GO) or not to push (NO-GO) a micro-switch according to different visual stimuli. Twenty-one-channel scalp EEGs were recorded and the surface Laplacians were calculated at F3, F4, C3, C4, P3 and P4 using the source derivation method. The time-courses of the inter- and intra-hemispheric coherence were calculated using the fast Fourier transform for each condition (GO or NO-GO), and were compared statistically between the two conditions. The results suggest that the ERCoh under the NO-GO condition consisted of two components; alpha band synchronization between bilateral frontal areas and theta band synchronization among bilateral frontal, central and parietal areas. It is likely that the former is related directly to the decision not to move, and the latter is related to the motor inhibition process.

Adult↗
PubMed2002

Spatial direction of attention enhances right hemispheric event-related gamma-band synchronization in humans.

Gamma-band oscillations are related to sensory information processing and attention. To further illuminate the relationship of gamma-band activity and selective somatic attention, we have studied the effects of direction of attention on the stimuli of somatosensory evoked potentials. Responses during focused attention to stimulation of the right median nerve and ignored stimulation at the right tibial nerve were compared with responses of ignored stimulation of the right median nerve but focused attention to stimulation at the right tibial nerve. Subtraction of the data demonstrated a significant predominantly right hemispheric event-related synchronization in the gamma-band by attention to the right median nerve in the time period between 280 and 325 ms post-stimulus. This finding implicates the involvement of a right temporoparietal network in selective spatial attention.

Adult↗