Control of spatiotemporal coherence of a thalamic oscillation by corticothalamic feedback

Long-range connections synchronize rather than spread intrathalamic oscillations: computational modeling and in vitro electrophysiology

A thalamic network model was developed based on recent data regarding heterogeneous thalamic reticular (RE) cell axonal arborizations that indicate at least two projection patterns, short-range cluster projections and long-range diffuse projections. The model was constrained based on expected convergence and the biophysical properties of RE and thalamocortical (TC) cells and their synapses. The model reproduced in vitro synchronous slow (3-Hz) oscillatory activity and the known effects of T-channel blockade and cholecystokinin (CCK) application on this activity. Whereas previous models used the speed at which approximately 3-Hz oscillations propagate in vitro to infer the spatial extent of intrathalamic projections, we found that, so long as the gamma-aminobutyric acid-B synaptic conductance was adjusted appropriately, a network with only short-range projections and another network with both short- and long-range projections could both produce physiologically realistic propagation speeds. Although the approximately 3-Hz oscillations propagated at similar speeds in both networks, phase differences between oscillatory activity at different locations in the network were much smaller in the network containing both short- and long-range projections. We measured phase differences in vitro and found that they were similar to those that arise in the network containing both short- and long-range projections but are inconsistent with the much larger phase differences that occur in the network containing only short-range projections. These results suggest that, although they extend much further than do short-range cluster projections, long-range diffuse projections do not spread activity over greater distances or increase the speed at which intrathalamic oscillations propagate. Instead, diffuse projections may function to synchronize activity and minimize phase shifts across thalamic networks. One prediction of this hypothesis is that, immediately after a collision between propagating oscillations, phase gradients should vary smoothly across the thalamic slice. The model also predicts that phase shifts between oscillatory activity at different points along a thalamic slice should be unaffected by T-channel blockers and decreased by suppression of synaptic transmission or application of CCK.

Spike-wave complexes and fast components of cortically generated seizures. IV. Paroxysmal fast runs in cortical and thalamic neurons

In the preceding papers of this series, we have analyzed the cellular patterns and synchronization of neocortical seizures occurring spontaneously or induced by electrical stimulation or cortical infusion of bicuculline under a variety of experimental conditions, including natural states of vigilance in behaving animals and acute preparations under different anesthetics. The seizures consisted of two distinct components: spike-wave (SW) or polyspike-wave (PSW) at 2-3 Hz and fast runs at 10-15 Hz. Because the thalamus is an input source and target of cortical neurons, we investigated here the seizure behavior of thalamic reticular (RE) and thalamocortical (TC) neurons, two major cellular classes that have often been implicated in the generation of paroxysmal episodes. We performed single and dual simultaneous intracellular recordings, in conjunction with multisite field potential and extracellular unit recordings, from neocortical areas and RE and/or dorsal thalamic nuclei under ketamine-xylazine and barbiturate anesthesia. Both components of seizures were analyzed, but emphasis was placed on the fast runs because of their recent investigation at the cellular level. 1) The fast runs occurred at slightly different frequencies and, therefore, were asynchronous in various cortical neuronal pools. Consequently, dorsal thalamic nuclei, although receiving convergent inputs from different neocortical areas involved in seizure, did not express strongly synchronized fast runs. 2) Both RE and TC cells were hyperpolarized during seizure episodes with SW/PSW complexes and relatively depolarized during the fast runs. As known, hyperpolarization of thalamic neurons deinactivates a low-threshold conductance that generates high-frequency spike bursts. Accordingly, RE neurons discharged prolonged high-frequency spike bursts in close time relation with the spiky component of cortical SW/PSW complexes, whereas they fired single action potentials, spike doublets, or triplets during the fast runs. In TC cells, the cortical fast runs were reflected as excitatory postsynaptic potentials appearing after short latencies that were compatible with monosynaptic activation through corticothalamic pathways. 3) The above data suggested the cortical origin of these seizures. To further test this hypothesis, we performed experiments on completely isolated cortical slabs from suprasylvian areas 5 or 7 and demonstrated that electrical stimulation within the slab induces seizures with fast runs and SW/PSW complexes, virtually identical to those elicited in intact-brain animals. The conclusion of all papers in this series is that complex seizure patterns, resembling those described at the electroencephalogram level in different forms of clinical seizures with SW/PSW complexes and, particularly, in the Lennox-Gastaut syndrome of humans, are generated in neocortex. Thalamic neurons reflect cortical events as a function of membrane potential in RE/TC cells and degree of synchronization in cortical neuronal networks.

Spike-wave complexes and fast components of cortically generated seizures. III. Synchronizing mechanisms

The intracortical and thalamocortical synchronization of spontaneously occurring or bicuculline-induced seizures, consisting of spike-wave (SW) or polyspike-wave (PSW) complexes at 2-3 Hz and fast runs at 10-15 Hz, was investigated in cats under ketamine-xylazine anesthesia. We used single and dual simultaneous intracellular recordings from cortical areas 5 and 7, and extracellular recordings of unit firing and field potentials from neocortical areas 5, 7, 17, 18, as well as related thalamic nuclei. The evolution of time delays between paroxysmal depolarizing events in single neurons or neuronal pools recorded from adjacent and distant sites was analyzed by using 1) sequential cross-correlations between field potentials, 2) averaged activities triggered by the spiky component of cortical SW/PSW complexes, and 3) time histograms between neuronal discharges. In all instances, the paroxysmal activities recorded from the dorsal thalamus lagged the onset of seizures in neocortex. The time lags between simultaneously impaled cortical neurons were significantly smaller during SW complexes than during the prior epochs of slow oscillation. During seizures, as during the slow oscillation, the intracortical synchrony was reduced with increased distance between different cortical sites. Dual intracellular recordings showed that, during the same seizure, time lags were not constant and, instead, reflected alternating precession of the recorded foci. After transection between areas 5 and 7, the intracortical synchrony was lost, but corticothalamocortical volleys could partially restore seizure synchrony. These data show that the neocortex leads the thalamus during SW/PSW seizures, that time lags between cortical foci are not static, and that thalamus may assist synchronization of SW/PSW seizures after disconnection of intracortical synaptic linkages.

Spike-wave complexes and fast components of cortically generated seizures. I. Role of neocortex and thalamus

We explored the relative contributions of cortical and thalamic neuronal networks in the generation of electrical seizures that include spike-wave (SW) and polyspike-wave (PSW) complexes. Seizures were induced by systemic or local cortical injections of bicuculline, a gamma-aminobutyric acid-A (GABAA) antagonist, in cats under barbiturate anesthesia. Field potentials and extracellular neuronal discharges were recorded through arrays of eight tungsten electrodes (0.4 or 1 mm apart) placed over the cortical suprasylvian gyrus and within the thalamus. 1) Systemic injections of bicuculline induced SW/PSW seizures in cortex, whereas spindle sequences continued to be present in the thalamus. 2) Cortical suprasylvian injection of bicuculline induced focal paroxysmal single spikes that developed into full-blown seizures throughout the suprasylvian cortex. The seizures were characterized by highly synchronized SW or PSW complexes at 2-4 Hz, interspersed with runs of fast (10-15 Hz) activity. The intracellular aspects of this complex pattern in different types of neocortical neurons are described in the following paper. Complete decortication abolished the seizure, leaving intact thalamic spindles. Injections of bicuculline in the cortex of athalamic cats resulted in similar components as those occurring with an intact thalamus. 3) Injection of bicuculline in the thalamus decreased the frequency of barbiturate spindles and increased the synchrony of spike bursts fired by thalamocortical and thalamic reticular cells but did not induce seizures. Decortication did not modify the effects of bicuculline injection in the thalamus. Our results indicate that the minimal substrate that is necessary for the production of seizures consisting of SW/PSW complexes and runs of fast activity is the neocortex.

Characterization of sensory and corticothalamic excitatory inputs to rat thalamocortical neurones in vitro

1. Using an in vitro slice preparation of the rat dorsal lateral geniculate nucleus (dLGN), the properties of retinogeniculate and corticothalamic inputs to thalamocortical (TC) neurones were examined in the absence of GABAergic inhibition. 2. The retinogeniculate EPSP evoked at low frequency (>= 0.1 Hz) consisted of one or two fast-rising (0.8 +/- 0.1 ms), large-amplitude (10.3 +/- 1.6 mV) unitary events, while the corticothalamic EPSP had a graded relationship with stimulus intensity, owing to its slower-rising (2.9 +/- 0.4 ms), smaller-amplitude (1.3 +/- 0.3 mV) estimated unitary components. 3. The retinogeniculate EPSP exhibited a paired-pulse depression of 60.3 +/- 5.6 % at 10 Hz, while the corticothalamic EPSP exhibited a paired-pulse facilitation of > 150 %. This frequency-dependent depression of the retinogeniculate EPSP was maximal after the second stimulus, while the frequency-dependent facilitation of the corticothalamic EPSP was maximal after the fourth or fifth stimulus, at interstimulus frequencies of 1-10 Hz. 4. There was a short-term enhancement of the >= 0.1 Hz corticothalamic EPSP (64.6 +/- 9.2 %), but not the retinogeniculate EPSP, following trains of stimuli at 50 Hz. 5. The >= 0.1 Hz corticothalamic EPSP was markedly depressed by the non-NMDA antagonist 1-(4-amino-phenyl)-4-methyl-7,8-methylene-dioxy-5H-2, 3-benzodiazepine (GYKI 52466), but only modestly by the NMDA antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid ((RS)-CPP), and completely blocked by the co-application of GYKI 52466, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), (RS)-CPP and (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801). Likewise, the corticothalamic responses to trains of stimuli (1-500 Hz) were greatly reduced by this combination of ionotropic glutamate receptor antagonists. 6. In the presence of GYKI 52466, CNQX, (RS)-CPP and MK-801, residual corticothalamic responses and slow EPSPs, with a time to peak of 2-10 s, could be generated following trains of five to fifty stimuli. Neither of these responses were occluded by 1S,3R-1-aminocyclopentane-1, 3-dicarboxylic acid (1S,3R-ACPD), suggesting they are not mediated via group I and II metabotropic glutamate receptors.

Coherence analysis of the human sleep electroencephalogram

Animal studies have shown that the sleep-related oscillations in the frequency range of spindles and slow-waves, and in the gamma band occur synchronously over large parts of the cerebral cortex. Coherence analysis was used to investigate these oscillations in the human sleep electroencephalogram. In all-night electroencephalogram recordings from eight young subjects power and coherence spectra within and between cerebral hemispheres were computed from bipolar derivations placed bilaterally along the antero-posterior axis. The 0.75-50 Hz range was examined with a resolution of 0.25 Hz. Distinct peaks in coherence were present in non-rapid eye movement sleep but not in rapid eye movement sleep. The most prominent and consistent peak was seen in the range of sleep spindles (13-14 Hz), and additional peaks were present in the alpha band (9-10 Hz) and low delta band (1-2 Hz). Whereas coherence in the spindle range was highest in stage 2, the alpha peak was most prominent in slow-wave sleep (stages 3 and 4). Interhemispheric coherence at 30 Hz was higher in rapid eye movement sleep than in non-rapid eye movement sleep. There were also marked sleep state-independent regional differences. Coherence between homologous interhemispheric derivations was high in the low frequency range and declined with increasing frequencies, whereas coherence of intrahemispheric and non-homologous interhemispheric derivations was at a low level throughout the spectra. It is concluded that coherence analysis may provide insights into large-scale functional connectivities of brain regions during sleep. The high coherence of sleep spindles is an indication for their widespread and quasi-synchronous occurrence throughout the cortex and may point to their specific role in the sleep process.

The influence of stimulus properties, complexity, and contingency on the stability and variability of ongoing and evoked activity in human auditory cortex

The real-time, single-trial activity in the human auditory cortex was extracted from magnetoencephalographic signals. A predictor of single-trial activity was defined as the sum of the average response and a mean-free base level computed over a range of base times. For simple stimuli the residual (predicted-actual) activity had a stimulus-independent oscillatory (10 Hz) component. This component was larger and more durable in trained subjects, reaching saturation only in the most trained of the five subjects studied (S1). Changes in variability and associated reduction of the absolute value and duration of the oscillations were evident in experiments with stimuli loaded with information, saliency, or task contingency. Repetition reintroduces stimulus-independent oscillations very slowly. For S1, after training, the stimulus-independent oscillations were reestablished in the auditory cortices to the level seen for simple stimuli, except for the time periods and in the hemisphere associated with the combination of task demands and stimulus processing.

Viewpoint: the core and matrix of thalamic organization

The integration of the whole cerebral cortex and thalamus during forebrain activities that underlie different states of consciousness, requires pathways for the dispersion of thalamic activity across many cortical areas. Past theories have relied on the intralaminar nuclei as the sources of diffuse thalamocortical projections that could facilitate spread of activity across the cortex. A case is made for the presence of a matrix of superficially-projecting cells, not confined to the intralaminar nuclei but extending throughout the whole thalamus. These cells are distinguished by immunoreactivity for the calcium-binding protein, D28K calbindin, are found in all thalamic nuclei of primates and have increased numbers in some nuclei. They project to superficial layers of the cerebral cortex over relatively wide areas, unconstrained by architectonic boundaries. They generally receive subcortical inputs that lack the topographic order and physiological precision of the principal sensory pathways. Superimposed upon the matrix in certain nuclei only, is a core of cells distinguished by immunoreactivity for another calcium-binding protein, parvalbumin, These project in highly ordered fashion to middle layers of the cortex in an area-specific manner. They are innervated by subcortical inputs that are topographically precise and have readily identifiable physiological properties. The parvalbumin cells form the basis for sensory and other inputs that are to be used as a basis for perception. The calbindin cells, especially when recruited by corticothalamic connections, can form a basis for the engagement of multiple cortical areas and thalamic nuclei that is essential for the binding of multiple aspects of sensory experience into a single framework of consciousness.

Progression of change in NMDA, non-NMDA, and metabotropic glutamate receptor function at the developing corticothalamic synapse

The development of receptor function at corticothalamic synapses during the first 20 days of postnatal development is described. Whole cell excitatory postsynaptic currents (EPSCs) were evoked in relay neurons of the ventral posterior nucleus (VP) by stimulation of corticothalamic fibers in in vitro slices of mouse brain from postnatal day 1 (P1). During P1-P12, excitatory postsynaptic conductances showed strong voltage dependence at peak current and at 100 ms after the stimulus and were almost completely antagonized by -2-amino-5-phosphonopentoic acid (APV), indicating that N-methyl--aspartate (NMDA) receptor-mediated currents dominate corticothalamic EPSCs at this time. After P12, in 42% of cells, excitatory postsynaptic conductances showed no voltage-dependence at peak current but still showed voltage-dependence 100-ms poststimulus. This voltage-dependent conductance was antagonized by APV. The nonvoltage-dependent component was APV resistant, showed fast decay, and was antagonized by the nonNMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In the remaining 58% of cells after P12, excitatory postsynaptic conductances showed moderate voltage dependence at peak conductance and strong voltage dependence 100 ms after the stimulus. Analysis of EPSCs before and after APV showed a significant increase in the relative contribution of the non-NMDA conductance after the second postnatal week. From P1 to P16, there was a significant decrease in the time constant of decay of the NMDA EPSC but no change in the voltage dependence of the NMDA response. After P8, slow EPSPs, 1.5-30 s in duration and mediated by metabotropic glutamate receptors (mGluRs), could be evoked by high-frequency stimulation of corticothalamic fibers in the presence of APV and CNQX. Similar slow depolarizations could be evoked by local application of the mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) but from P0. Both conductances were blocked by the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine. Hence functional mGluR receptors are present on VP cells from birth, but their synaptic activation at corticothalamic synapses can only be detected after P8. In voltage clamp, the extrapolated reversal potential of the t-ACPD current, with potassium gluconate-based internal solution, was +12 +/- 10 (SE) mV, and the measured reversal potential with cesium gluconate-based internal solution was 1.5 +/- 9.9 mV, suggesting that the mGluR-mediated depolarization was mediated by a nonselective cation current. Replacement of NaCl in the external solution caused the reversal potential of the current to shift to -18 +/- 2 mV, indicating that Na+ is a charge carrier in the current. The current amplitude was not reduced by application of Cs+, Ba2+, and Cd2+, indicating that the t-ACPD current was distinct from the hyperpolarization-activated cation current (IH) and distinct from certain other previously characterized mGluR-activated, nonselective cation conductances.

Extrastriate feedback to primary visual cortex in primates: a quantitative analysis of connectivity

Knowledge-based or top-down influences on primary visual cortex (area V1) are believed to originate from information conveyed by extrastriate feedback axon connections. Understanding how this information is communicated to area V1 neurons relies in part on elucidating the quantitative as well as the qualitative nature of extrastriate pathway connectivity. A quantitative analysis of the connectivity based on anatomical data regarding the feedback pathway from extrastriate area V2 to area V1 in macaque monkey suggests (i) a total of around ten million or more area V2 axons project to area V1; (ii) the mean number of synaptic inputs from area V2 per upper-layer pyramidal cell in area V1 is less than 6% of all excitatory inputs; and (iii) the mean degree of convergence of area V2 afferents may be high, perhaps more than 100 afferent axons per cell. These results are consistent with empirical observations of the density of radial myelinated axons present in the upper layers in macaque area V1 and the proportion of excitatory extrastriate feedback synaptic inputs onto upper-layer neurons in rat visual cortex. Thus, in primate area V1, extrastriate feedback synapses onto upper-layer cells may, like geniculocortical afferent synapses onto layer IVC neurons, form only a small percentage of the total excitatory synaptic input.

A model of cortically induced synchronization in the lateral geniculate nucleus of the cat: a role for low-threshold calcium channels

Recently Sillito et al. (Nature 1994;369:479-82) discovered correlations in the spike trains of a relatively distant pair of cat lateral geniculate nucleus cells when simultaneously stimulated by a drifting grating; no such correlation occurs when the visual cortex is removed. In a further analysis of the data, we have found that short, high-frequency bursts contribute substantially to the synchronization and we hypothesize that the origin of the bursts is the low-threshold calcium spike. Guided by this hypothesis, our model of the corticogeniculate pathway and early visual system reproduces the experimental data in nearly every detail, as well as making predictions about cortical activity during the synchronizing process. We also discuss the possible behavioral relevance of correlations in the geniculo-cortical loop as well as other neural systems.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

Responsiveness of human cortical activity to rhythmical stimulation: a three-modality, whole-cortex neuromagnetic investigation

We developed numerical indicators to quantify stimulus-related changes in cortical magnetic signals recorded from nine healthy subjects while they received 1- to 2.5-s trains of 15 stimuli (somatosensory, visual, or auditory in separate runs) at rates from 6 to 14 Hz, intermingled with 1.6-s pauses. A locking index (L) was introduced to quantify how well the responses are time locked to the stimuli and a global change factor (GC) to indicate changes in the whole-cortex oscillatory activity in the 5- to 25-Hz frequency range. The responses were visualized with color-coded images illustrating cortical reactivity for all stimulus rates simultaneously. These color maps clearly showed that the modality-specific cortical signals were enhanced at frequencies corresponding to the stimulus rate during the trains. For somatosensory stimulation the activity in the vicinity of the somatosensory hand area was enhanced at most stimulus rates, suggesting mainly superposition of evoked responses. In individuals with strong posterior resting rhythm, visual stimuli typically entrained activity in the parietooccipital sulcus at stimulus rates close to the main frequency of the spontaneous activity, probably reflecting driving of the intrinsic cortical rhythm, whereas in subjects with little spontaneous parietooccipital rhythm the cortical signal appeared to be composed mainly of visual evoked responses. No modality-specific enhancement was observed during auditory stimulation. During the pauses between the trains, the cortical signals were significantly suppressed compared with the resting condition: The peak activity (7-13 Hz) was modulated within, but also outside, the modality-specific areas, and the signals outside the frequency peaks of maximum power were consistently and reproducibly suppressed over the whole cortex by all stimuli.

Relations between cortical and thalamic cellular activities during absence seizures in rats

In a rat model of generalized absence epilepsies (Genetic Absence Epilepsy Rats from Strasbourg, GAERS), multiunit activity was recorded simultaneously at different sites of the thalamocortical system under neurolept anaesthesia (fentanyl-droperidol). Under these conditions, bilaterally synchronized spike-and-wave-discharges (SWDs) occurred spontaneously on the electroencephalogram (EEG) that were in principle identical to those reported earlier from unanaesthetized preparations. The generation of SWDs on the EEG was associated with spike-concurrent, rhythmic burst-like activity in (mono-)synaptically connected regions of specific (somatosensory) thalamic regions and layers IVN of the somatosensory cortex, and the reticular thalamic nucleus. Precursor activity was typically recorded in cortical units, concomitant with 'embryonic' SW seizures on the EEG, before the paroxysm was evident on the gross EEG and in the thalamus. On average, SWD-correlated activity in layers IVN of the somatosensory cortex started significantly earlier than correlated burst-like firing in reticular and in ventrobasal thalamic neurons. Cellular peak firing in thalamus and cortex during bilaterally synchronized SWDs was related to the spike component on the gross EEG with the temporal rank order ventroposteromedial > ventrolateral > or = ventroposterolateral thalamic > > rostral reticular thalamic nuclei > or = cortex (layers IVN) = caudal reticular thalamic nucleus. A spike-related depression and wave-related increase in firing was recorded in anteroventral ventrolateral thalamic areas, presumably reflecting their peculiar anatomical arrangement within the thalamus. These results from an in vivo preparation with intact synaptic connections that spontaneously produces SWDs indicate that SWDs spread within the thalamocortical network, involving short and long delays. The order of concurrent rhythmic firing observed in thalamocortical circuits during SW seizures are supportive of the hypothesis that the processes of rhythmogenesis recruit local thalamic networks, while cortical mechanisms appear to synchronize rhythmic activities on a larger spatiotemporal scale, thereby providing an important contribution to the generalization of epileptiform activity and expression of SWDs on the EEG.

Mechanisms underlying the synchronizing action of corticothalamic feedback through inhibition of thalamic relay cells

Early studies have shown that spindle oscillations are generated in the thalamus and are synchronized over wide cortical territories. More recent experiments have shown that this large-scale synchrony depends on the integrity of corticothalamic feedback. Previously proposed mechanisms emphasized exclusively intrathalamic mechanisms to generate the synchrony of these oscillations. In the present paper, we propose a cellular mechanism in which the synchrony is dependent of a mutual interaction between cortex and thalamus. This cellular mechanism is tested by computational models consisting of pyramidal cells, interneurons, thalamic reticular (RE) and thalamocortical (TC) relay cells, on the basis of voltage-clamp data on intrinsic currents and synaptic receptors present in the circuitry. The model suggests that corticothalamic feedback must operate on the thalamus mainly through excitation of GABAergic RE neurons, therefore recruiting relay cells essentially through inhibition and rebound. We provide experimental evidence for such dominant inhibition in the lateral posterior nucleus. In these conditions, the model shows that cortical discharges optimally evoked thalamic oscillations. This feature is essential to the present cellular mechanism and is also consistently observed experimentally. The model further shows that, with this type of corticothalamic feedback, cortical discharges recruited large areas of the thalamus because of the divergent cortex-to-RE and RE-to-TC axonal projections. Consequently, the thalamocortical network generated patterns of oscillations and synchrony similar to in vivo recordings. The model also emphasizes the important role of the modulation of the Ih current by calcium in TC cells. This property conferred a relative refractoriness to the entire network, a feature also observed experimentally, as we show here. Further, the same property accounted for various spatiotemporal features of oscillations, such as systematic propagation after low-intensity cortical stimulation, local oscillations, and more generally, a high variability in the patterns of spontaneous oscillations, similar to in vivo recordings. We propose that the large-scale synchrony of spindle oscillations in vivo is the result of thalamocortical interactions in which the corticothalamic feedback acts predominantly through the RE nucleus. Several predictions are suggested to test the validity of this model.

Task-related coherence and task-related spectral power changes during sequential finger movements

In order to investigate the activity of cortical regions in the control of complex movements, we studied task-related coherence (TRCoh) and task-related spectral power (TRPow) changes in 8 right-handed subjects during the execution of 4 different finger movement sequences of increasing complexity. All sequences were performed with the right hand and were paced by a metronome at 2 Hz. EEG power spectra and coherence values were computed within alpha (8-12 Hz) and beta (13-20 Hz) frequency bands for 29 scalp EEG positions during the execution of the sequences and were compared with values obtained during a rest (control) condition. Movement sequences were associated with TRPow decreases in the alpha and beta frequency bands over bilateral sensorimotor and parietal areas, with a preponderance over the contralateral hemisphere. Increases of TRCoh occurred over bilateral frontocentral regions. TRCoh decreases were present over the temporal and occipital areas. The spatial extent and the magnitude of TRPow decreases and TRCoh increases in both frequency bands were greater for sequential movements of higher complexity than for simpler ones. These results are consistent with previous findings of bilateral activation of sensorimotor areas during sequential finger movements. Moreover, the present results indicate an active intercommunication between bilateral and mesial central and prefrontal regions which becomes more intense with more complex sequential movements.

Plateau potentials in cat neocortical association cells in vivo: synaptic control of dendritic excitability

The dendrites of neocortical pyramidal cells are bombarded by myriads of synaptic inputs and express active conductances generating prominent plateau potentials. We have examined in vivo the possibility that spontaneous synaptic inputs trigger or terminate plateau potentials after blockage of K+ currents. Under barbiturate anaesthesia, pairs of cortical cells were intracellularly recorded with sharp electrodes from the cat's association cortex (areas 5-7). In one pyramidal cell, K+ channels were blocked with intracellular Cs+, while in the simultaneously impaled pyramidal cell the K+ conductances were left intact to act as a control; this second cell allowed recognition of spontaneous spindle-related synaptic activity. Depolarizing current pulses elicited single, all-or-none plateau potentials (60-70 mV, 0.1-0.4 s). Plateau potentials slowly repolarized towards a break point of fast repolarization around -20 mV. Thalamic-evoked inhibitory postsynaptic potentials consistently shut off the plateaus. Synchronized spontaneous activity, as occurring during thalamic-generated spindle oscillations, either triggered or blocked the plateaus. These results suggest that spontaneously occurring synaptic activation during synchronized oscillatory states, such as those that occur during sleep spindles in vivo, may exert a strong control over the dendritic excitability in neocortical pyramidal cells.

The brain as a symbol-processing machine

The knowledge accumulated about the biochemistry of the synapsis in the last decades completely changes the notion of brain processing founded exclusively over an electrical mechanism, toward that supported by a complex chemical message exchange occurring both locally, at the synaptic site, as well as at other localities, depending on the solubility of the involved chemical substances in the extracellular compartment. These biochemical transactions support a rich symbolic processing of the information both encoded by the genes and provided by actual data collected from the surrounding environment, by means of either special molecular or cellular receptor systems. In this processing, molecules play the role of symbols and chemical affinity shared by them specifies the syntax for symbol manipulation in order to process and to produce chemical messages. In this context, neurons are conceived as message-exchanging agents. Chemical strings are produced and stored at defined places, and ionic currents are used to speed up message delivery. Synaptic transactions can no longer be assumed to correspond to a simple process of propagating numbers powered by a factor measuring the presynaptic capacity to influence the postsynaptic electrical activity, but they must be modeled by more powerful formal tools supporting both numerical and symbolic calculations. It is proposed here that formal language theory is the adequate mathematical tool to handle such symbolic processing. The purpose of the present review is therefore: (a) to discuss the relevant and recent literature about trophic factors, signal transduction mechanisms, neuromodulators and neurotransmitters in order (b) to point out the common features of these correlated processes; and (c) to show how they may be organized into a formal model supported by the theory of fuzzy formal languages (d) to model the brain as a distributed intelligent problem solver.

Lack of regulation by intracellular Ca2+ of the hyperpolarization-activated cation current in rat thalamic neurones

1. The regulation of the hyperpolarization-activated cation current, Ih, in thalamocortical neurones by intracellular calcium ions has been implemented in a number of mathematical models on the waxing and waning behaviour of synchronized rhythmic activity in thalamocortical circuits. In the present study, the Ca2+ dependence of Ih in thalamocortical neurones was experimentally investigated by combining Ca2+ imaging and patch-clamp techniques in the ventrobasal thalamic complex (VB) in vitro. 2. Properties of Ih were analysed before and during rhythmic stimulation of Ca2+ entry by trains of depolarizing voltage pulses. Despite a significant increase in intracellular Ca2+ concentration ([Ca2+]i) from resting levels of 74 +/- 23 nM to 251 +/- 78 nM upon rhythmic stimulation, significant differences in the voltage dependence of Ih activation did not occur (half-maximal activation at -86.4 +/- 1.3 mV vs. -85.2 +/- 2.9 mV; slope of the activation curve, 11.2 +/- 2.4 mV vs. 12.5 +/- 2.5 mV). Recording of Ih with predefined values of [Ca2+]i (13.2 nM or 10.01 microM in the patch pipette) revealed no significant differences in the activation curve or the fully activated I-V relationship of Ih. 3. In comparison, stimulation of the intracellular cyclic adenosine monophosphate (cAMP) pathway induced a significantly positive shift in Ih voltage dependence of +5.1 +/- 1.9 mV, with no alteration in the fully activated I-V relationship. 4. These data argue against a direct regulation of Ih by intracellular Ca2+, and particularly do not support a primary role of Ca(2+)-dependent modulation of the Ih channels in the waxing and waning of sleep spindle oscillations in thalamocortical neurones.

High-frequency brain activity: its possible role in attention, perception and language processing

Coherent high-frequency neuronal activity has been proposed as a physiological indicator of perceptual and higher cognitive processes. Some of these processes can only be investigated in humans and the use of non-invasive recording techniques appears to be a prerequisite for investigating their physiological substrate in the healthy human brain. After addressing methodological issues in the non-invasive recording of high-frequency responses, we summarize studies indicating co-occurrence of neuronal synchrony of single cells exhibiting rhythmic activity at high frequencies, oscillations in the local field potential and dynamics in high frequencies recorded using high-resolution electroencephalography (EEG) and magnetoencephalography (MEG). We then review EEG and MEG studies of attention, perception, and language processing in humans indicating that dynamics in the high-frequency range > 20 Hz reflect specific cognitive processes. Types of high-frequency (HF) activity can be distinguished according to their latency after stimulus onset, stimulus-locking, cortical topography and frequency. There appears to be a systematic relationship between specific cognitive processes and types of HF activity. The findings are related to recent theories about the generation of HF activity and their possible role in binding of stimulus features. Dynamics of HF cortical activity reflecting higher cognitive processes can be accounted for based on the assumption that the elements of cognitive processing, e.g. visual objects and words, are organized in the brain as distributed neuronal assemblies with defined cortical topographies generating well-timed spatio-temporal activity patterns.

Synchronous oscillatory activity in sensory systems: new vistas on mechanisms

The origin and nature, as well as the functional role, of synchronous oscillatory activity in the cortex are among the major unresolved issues in systems neurobiology. Recent advances in understanding the mechanisms underlying oscillations include the description of intrinsically bursting pyramidal cells in striate cortex in vivo and the discovery of inhibitory interneurons that fire spike doublets to induce synchrony. The behavioral consequences of coordinated activity in cortical neurons remain poorly understood.

Visual stimuli induce waves of electrical activity in turtle cortex

The computations involved in the processing of a visual scene invariably involve the interactions among neurons throughout all of visual cortex. One hypothesis is that the timing of neuronal activity, as well as the amplitude of activity, provides a means to encode features of objects. The experimental data from studies on cat [Gray, C. M., Konig, P., Engel, A. K. & Singer, W. (1989) Nature (London) 338, 334-337] support a view in which only synchronous (no phase lags) activity carries information about the visual scene. In contrast, theoretical studies suggest, on the one hand, the utility of multiple phases within a population of neurons as a means to encode independent visual features and, on the other hand, the likely existence of timing differences solely on the basis of network dynamics. Here we use widefield imaging in conjunction with voltage-sensitive dyes to record electrical activity from the virtually intact, unanesthetized turtle brain. Our data consist of single-trial measurements. We analyze our data in the frequency domain to isolate coherent events that lie in different frequency bands. Low frequency oscillations (<5 Hz) are seen in both ongoing activity and activity induced by visual stimuli. These oscillations propagate parallel to the afferent input. Higher frequency activity, with spectral peaks near 10 and 20 Hz, is seen solely in response to stimulation. This activity consists of plane waves and spiral-like waves, as well as more complex patterns. The plane waves have an average phase gradient of approximately pi/2 radians/mm and propagate orthogonally to the low frequency waves. Our results show that large-scale differences in neuronal timing are present and persistent during visual processing.

Intracellular and computational characterization of the intracortical inhibitory control of synchronized thalamic inputs in vivo

We investigated the presence and role of local inhibitory cortical control over synchronized thalamic inputs during spindle oscillations (7-14 Hz) by combining intracellular recordings of pyramidal cells in barbiturate-anesthetized cats and computational models. The recordings showed that 1) similar excitatory postsynaptic potential (EPSP)/inhibitory postsynaptic potential (IPSP) sequences occurred either during spindles or following thalamic stimulation; 2) reversed IPSPs with chloride-filled pipettes transformed spindle-related EPSP/IPSP sequences into robust bursts with spike inactivation, resembling paroxysmal depolarizing shifts during seizures; and 3) dual simultaneous impalements showed that inhibition associated with synchronized thalamic inputs is local. Computational models were based on reconstructed pyramidal cells constrained by recordings from the same cells. These models showed that the transformation of EPSP/IPSP sequences into fully developed spike bursts critically needs a relatively high density of inhibitory currents in the soma and proximal dendrites. In addition, models predict significant Ca2+ transients in dendrites due to synchronized thalamic inputs. We conclude that synchronized thalamic inputs are subject to strong inhibitory control within the cortex and propose that 1) local impairment of inhibition contributes to the transformation of spindles into spike-wave-type discharges, and 2) spindle-related inputs trigger Ca2+ events in cortical dendrites that may subserve plasticity phenomena during sleep.

Short-term plasticity during intrathalamic augmenting responses in decorticated cats

The intrathalamic mechanisms of frequency-dependent augmenting responses were investigated in decorticated cats by means of intracellular recordings from thalamocortical (TC) neurons in ventrolateral (VL) nucleus, including simultaneous impalements from two TC neurons. Pulse trains (10 Hz) applied to VL nucleus elicited two types of augmenting responses: (1) in 68% of cells, the incremental responses occurred on a progressive depolarization associated with the decrease in IPSPs produced by preceding stimuli in the train; (2) in the remaining cells, progressively growing low-threshold (LT) responses resulted from the enhancement of Cl--dependent IPSPs, giving rise to postinhibitory rebound bursts, followed by a self-sustained sequence of spindle waves. Although in some TC cells the augmenting responses developed from LT responses once the latter reached a given level of depolarization, other neurons displayed augmenting responses immediately after the early antidromic spike that depolarized the neuron to the required level, without an intermediate step of LT rebound. Repeated pulse trains led to a progressive and persistent increase in slow depolarizing responses of TC cells, as well as to a persistent and prolonged decrease in the amplitudes of the IPSPs. On the basis of parallel experiments, we propose that the two types of augmentation in TC cells are a result of contrasting responses of thalamic reticular neurons evoked by repetitive thalamic stimuli: decremental responses, which may account for disinhibition leading to depolarizing responses in TC cells, and incremental responses, explaining the progressive hyperpolarization of TC cells. These data demonstrate that frequency-dependent changes in neuronal excitability are present in the thalamus of a decorticated hemisphere and suggest that short-term plasticity processes in the gateway to the cerebral cortex may decisively influence cortical excitability during repetitive responses.

Spatiotemporal patterns of spindle oscillations in cortex and thalamus

Spindle oscillations (7-14 Hz) appear in the thalamus and cortex during early stages of sleep. They are generated by the combination of intrinsic properties and connectivity patterns of thalamic neurons and distributed to cortical territories by thalamocortical axons. The corticothalamic feedback is a major factor in producing coherent spatiotemporal maps of spindle oscillations in widespread thalamic territories. Here we have investigated the spatiotemporal patterns of spontaneously occurring and evoked spindles by means of multisite field potential and unit recordings in intact cortex and decorticated animals. We show that (1) spontaneous spindle oscillations are synchronized over large cortical areas during natural sleep and barbiturate anesthesia; (2) under barbiturate anesthesia, the cortical coherence is not disrupted by transection of intracortical synaptic linkages; (3) in intact cortex animals, spontaneously occurring barbiturate spindle sequences occur nearly simultaneously over widespread thalamic territories; (4) in the absence of cortex, the spontaneous spindle oscillations throughout the thalamus are less organized, but the local coherence (within 2-4 mm) is still maintained; and (5) spindling propagation is observed in intact cortex animals only when elicited by low intensity cortical stimulation, applied shortly before the initiation of a spontaneous spindle sequence; propagation velocities are between 1 and 3 mm/sec, measured in the anteroposterior axis of the thalamus; increasing the intensity of cortical stimulation triggers spindle oscillations, which start simultaneously in all leads. We propose that, in vivo, the coherence of spontaneous spindle oscillations in corticothalamic networks is attributable to the combined action of continuous background corticothalamic input initiating spindle sequences in several thalamic sites at the same time and divergent corticothalamic and intrathalamic connectivity.