Phase-shift gradients of dominant cortical potentials in rabbits

Phase shifts of cortical potentials were studied in rabbits before and during photic stimulation and their importance for irradiation of excitation from the visual to the motor area was examined. Both before and during stimulation variations in phase relationships of various kinds were observed. In most cases, however, with an increase in distance between the electrodes, the phase shift of the theta-waves gradually increased. Most motor responses of the rabbits took place in the presence of a marked phase shift gradient. It is suggested that the state of the brain in which a spatial phase shift gradient of the theta-waves is recorded in the EEG is the optimal condition for realization of the motor response. Photic stimulation increases the number of in-phase waves in the EEG recorded from closely situated points and it reduces scatter of the phase shift values between components of the sensomotor and visual cortical EEG, i.e., it leads to an increase in coherence. Motor responses of the rabbits to stimulation take place in the presence of theta-waves with the most constant phase shift in the EEG of the sensomotor and visual cortex.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 4, pp. 512–519, July–August, 1984.     

Spatial synchronization of rabbit brain biopotentials upon repeated electrical stimulation of the cortex]

A study was made on 23 rabbits of the degree of spatial synchronization in response to repeated electrical stimulations of one or simultaneously two cortical areas: the sensorimotor or visual, or simultaneously the sensorimotor and visual, or sensorimotor and precentral respectively. Enhancement of the synchronization induced by the stimulation, globally spread over the entire cortex and was followed by its decrease. However, in the two directly stimulated areas of the cortex, the similarity of bioelectrical oscillations persisted for a long time, which testifies to the establishment of selective relations between them.     

Phase relationships of cortical potentials in rabbits in different functional states

Phase shifts in EEG potentials were investigated in the rabbit cortex during photic stimulation and in controls. Degree of phase shift in the predominating theta waves was found to increase gradually with increasing distance between recording electrodes both with and without photic stimulation, pointing to the existence of a phase gradient — the conditions appropriate to the greater proportion of motor reactions. Photic stimulation induces an increase in numbers of non-phasic EEG waves recorded from close-lying sites as well as reduced scatter in levels of phase shift between EEG of the sensorimotor and visual cortex, thus rendering phase shifts more stable. Irradiation of excitation from the visual to the motor analyzer in response to photic stimulation occurs against a background of high correlation coefficient and coherent function levels and with a phase shift from 0 to 10–11°.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR. Moscow. Translated from Neirofiziologiya, Vol. 21, No. 4, pp. 507–513, July–August, 1989.     

Spectral correlation analysis of the potentials of the neocortex and certain subcortical structures during low-frequency electrical stimulation of nonspecific thalamic nuclei]

The spectral-correlation analysis of biopotentials in the cortex and some other brain structures (the anteroventral thalamic nucleus, dorsal hippocampus, lateral geniculate body, mid-brain reticular formation), in chronic experiments on alert rabbits, revealed that during electrical stimulation of thalamic mid-line nuclei within the ranges of 1-3, 4-7 and 8-10 c/s, there occured a rearrangement of the EEG frequencies; a dominant, narrow-band peak at the stimulation frequency, appeared. The coherence of the biopotentials of different cortical areas, of the cortex and subcortical formations increased during the stimulation at the frequency of the stimulation, reaching maximum values between the potentials of the visual and sensorimotor cortical areas.     

The mechanism of action of a reinforcing stimulus]

Experiments on alert non-immobilized rabbits revealed that electrical cutaneous stimulation of a limb, used as a reinforcing agent in elaboration of a conditioned reflex to photic flashes, weakened slow polyrhythmic oscillations of background EEG and late components of evoked potentials in the visual cortex to photic flashes. Against this background, the connection between slow potentials and spike activity in both the visual and sensorimotor cortical areas considerably diminished. During EEG activation, induced by the reinforcing stimulus, inhibitory pauses and post-inhibitory activation in the firing of the neocortical units weakened and protracted, ordered spike activity appeared. The data obtained are in agreement with the hypothesis that weakening of the recurrent inhibition system is one of the basic mechanisms in the action of the reinforcing stimulus in conditioning.     

Modulation of the neuronal activity and evoked potentials of the cerebral cortex by stimulation of the dorsal hippocampus]

In experiments on alert rabbits high frequency stimulation of the CA1 field of the dorsal hippocampus reduced the peak latency of the main negative component of the evoked potential (EP) to a light flash in the sensorimotor and occipital tests areas of the cerebral cortex. A single stimulation of the same part of the hippocampus resulted in a gradually developing facilitation of secondary negativity of the EP 5th component, predominantly in the sensorimotor cortex. Investigation of neuronal responses in the same cortical areas to a stimulation of the CA1 field with different parameters has shown that the effects of EP modulation are due to dynamic reorganizations of cortical neuronal activity.     

A comparison of the spike train activity of the cortical neurons and of the spatial synchronization of the EEG

In chronic experiments EEG coherence and conjugation of impulse activity were compared of neurones of the visual and sensorimotor areas of rabbits neocortex simultaneously recorded with the same electrodes. Connection was revealed between the presence and properties of conjugated neurones activity and EEG coherence at various frequencies. At correlated neurones activity a greater EEG coherence was observed on frequencies of 3-4,5 Hz than at the independent activity. At the highest level of the EEG coherence the neurones discharged with less delay of one after the other in pairs, and in their synchronization a common source participated more often than at the lowest level of the EEG coherence.     

Functional interrelation of the brain structures of the cat during the generation of rhythmic activity. I. The frequency characteristics of the electroencephalogram

The similarity of frequency parameters of electroencephalograms (EEG) recorded from visual, associative and sensorimotor cortical area, caudate nucleus and several thalamic nuclei have been analysed using the period analysis of EEG and cluster analysis of data obtained to clarify the functional interrelationships between these parts of the brain during generation of rhythmic activity of different types. Functional interrelationships between brain structures in freely moving cats during the states of drowsiness and slow-wave sleep have been shown to differ from "classical" thalamo-cortical pacemaker relations.     

Neuronal impulse trains of the visual and sensorimotor areas of the rabbit neocortex in calm wakefulness and pseudoconditioning

The conjugation of unit activity in the neocortical visual and sensorimotor areas during calm wakefulness and in intersignal intervals, in two groups of rabbits at pseudoconditioning was studied. The first group was presented in a random order with flashes and electrocutaneous stimuli, the second one--with sounds and electrocutaneous stimuli. The number of neurones pairs working in correlation during calm wakefulness is significantly less (35%) than during pseudoconditioning (49 and 50% in the first and second rabbits groups, respectively). During calm wakefulness and in both groups during pseudoconditioning, the number of pairs with delays of discharges of the visual area neurones after the sensorimotor one, and of the sensorimotor after visual up to 120 ms was equal. Comparison of the data on delayed neuronal discharges during calm wakefulness and pseudoconditioning with those obtained earlier with conditioned reflexes testifies that forestalling of visual area neuronal discharges by sensorimotor discharges is characteristic only for the activity of cortical projections of conditioned and unconditioned stimuli.     

Dynamics of driven rhythm in neuronal assemblies in sensorimotor and visual cerebral cortices in rabbit

Coincident activity of pairs of neurons in the sensorimotor and visual areas of the cerebral cortex was studied in naive, learning, and trained rabbits during the formation of a hidden excitation focus in their central nervous system (a defensive dominanta) of the rhythmic nature. In the trained rabbits (as compared to the naive animals), percent of neuronal pairs (both neighboring and distant) in whose coincident activity the rhythm of stimulation prevailed was higher. In the visual cortex, percent of such pairs was significantly higher only for the distant neurons. Analysis of interaction between neurons in the visual and sensorimotor cortices revealed increasing the number of neuronal pairs with the driven rhythm while training. Such an increase was observed when both sensorimotor and visual neurons were considered as leading.     

Spectral correlation parameters of neocortical potentials in the rabbit undergoing paired isorhythmic stimulation

At pairing of isorhythmic stimuli beyond the theta-rhythm frequency limits (3 and 8 Hz), in power spectra of EEGs of the sensorimotor and visual neocortical areas of rabbits, the frequencies are present both of the theta-range and of the stimulation frequency, in the background activity as well as during the stimulation. Both rhythms are in reciprocal relations. The frequency of the theta-rhythm approaches the frequency divisible by that of the stimulation. Under the action of the conditioned stimulus, crosscorrelation coefficients (CC) between the potentials of the areas under study decrease in most cases in comparison to their background values. Combination of the conditioned stimulus with the unconditioned one, leads approximately in equal number of cases to an increase or decrease of CC. After elimination of the stimuli, in most cases CC increases. CC of the background activity does not increase in the course of paired stimuli presentation though a conditioned response is being formed. At presence of stimuli frequency fluctuations simultaneously in the potentials of both areas, the rise of coherence function at this frequency does not occurs always. Thus, the above spectral-correlation parameters of rabbit's cortical potentials differ from those which arise at pairing of continuous nonrhythmic stimuli. This difference is probably due to different characteristics of the stimuli presented.     

The structure of the background EEG of the rabbit in the high-frequency portion of the spectrum]

With the purpose of studying the character and structure of high frequency bioelectric activity of rabbits cerebral cortex in the state of calm alertness, the EEG ensembles of different areas of the cortex (sensorimotor, visual, acoustic) and dorsal hippocampus were studied with FFT method. A supposition was made about the presence of systemic organization of the background EEG in rabbits cerebral cortex, reflected, in particular, in the presence of determined components both of chaotic and rhythmic character having different degrees of manifestation. Heterogeneity was revealed in distribution of energies of spectral EEG components in the studied frequency ranges from 14.7 to 100 Hz with predominance of total specific energy value in the band of 14.7-60 Hz. In coherence functions of all the studied pairs of EEG leads rhythmic component, stable in time, was absent. Functions of the mean EEG coherence in the band of 61-100 Hz had significantly greater values in comparison with the values in the band of 14.7-40 Hz.     

Spectral-correlation characteristics of the electrical activity of the brain of the rabbit in a state of calm wakefulness

Inter- and intrahemispheric relations of electrical activity of the pre-motor, sensorimotor (representation of forelimb and blinking) and visual zones of rabbit's cerebral cortex in calm alertness was studied by method of spectral-correlative analysis. Mean coherence levels of the EEG of tested hemispheric symmetric points and symmetric pairs of leads in the left and right hemispheres were characterized by a high temporal stability in the state of calm alertness and during sensory stimulation. A comparison of mean coherence values of EEG in symmetric leads, revealed a tendency to left-side dominance of statistical bonds of electrical processes. A tendency was shown towards interhemispheric asymmetry by mean parameters of EEG power spectra: the left hemisphere of the rabbit is characterized by a lower mean frequency of electrical activity and a more narrow effective frequency of the spectrum.     

Functional asymmetry of electric processes in the rabbit brain cortex at formation of the hunger dominant

The motivational condition of hunger and formation of the hunger dominant after daily food deprivation was studied in the conditions of chronic experiments on rabbits. It was shown, that the hunger condition was accompanied by left sided interhemispher asymmetry on indicators of spectral capacity of EEG frontal and right-hand asymmetry sensorimotor areas of the cortex. A hunger dominant was accompanied by falling of spectral capacity of EEG of areas of both hemispheres. The condition of hunger and a hunger dominant were characterized by right-hand asymmetry on average level of EEG coherence of frontal and sensorimotor areas. At transition of a condition of hunger in a hunger dominant there was an average level of EEG coherence decrease in areas of the right hemisphere. Electric processes of the cortex of the brain at a motivational condition of hunger and a hunger dominant were different.     

Spectral analysis of the human EEG in acutely developing foci of irritation at the diencephalic and brain stem levels

The dynamic study of EEG spectra and coherence in 52 patients in early period after tumor ablation at the diencephalic (27 patients) and brainstem (25 patients) levels with favourable (24 patients) and lethal (28 patients) outcomes revealed a dependence of their changes on the reversibility or irreversibility of pathological irritation of these brain levels. At regressively developing foci and retained compensatory mechanisms characteristic changes of EEG relations were seen in the central cortical areas, i. e. in cortical projection zone of the nonspecific thalamic pathways. The changes observed consisted of EEG synchronization in the band of 7 cps and sharp coherence increase while changes of EEG correlations in other cortical areas were unsimilar. Gradual EEG normalization with alpha-rhythm restoration passed through the stage of sigma-rhythm and slow sleeplike waves which indicated the safety of brainstem-subcortical hypnogenic systems. In patients with destructive foci at the diencephalic and brainstem levels, i. e. under conditions of brain functioning similar to "cerveau isolé" and "encephale isolé",--a general coherence decrease and EEG synchronization in the band of 5 cps, or generalized slow, monomorphic oscillations were observed.     

Formation of plastic connections of the visual system during ontogenesis of the rabbit

Analysis of evoked potentials and unit activity in the visual cortical projection area of rabbits revealed a definite succession of forming of interneuronal connections in ontogeny. In early postnatal period, the neuronal reactions were characterized by stable responses with one excitatory phase corresponding to initially negative surface evoked potential. Similarity of reactions of neurones situated in the same vertical column was observed and explained by functioning of a system of rigid connections of the thalamic relay nuclei afferents with cortical pyramidal neurones. Beginning from the third week of postnatal life of rabbits the neuronal reactions assumed a distinctly expressed phasic character, and variability of responses was seen along the vertical line. The changes revealed correlated with formation of a system of interneurones providing a possibility of plastic neuronal interaction. A study of the influence of preliminary cortical stimulation of the associative areas showed that intercentral cooperation mediated by cortical interneurones providing a systemic analysis of visual information began to form from the third week of postnatal life and reached the definitive level at later stages of development.     

Conjugation of unit activity in the visual and sensomotor areas of the rabbit neocortex during a conditioned reflex to sound and light

Conjugation of unit activity in the visual and sensorimotor neocortical areas was studied by means of histograms of cross- and autocorrelation in rabbits with conditioned reflex to light (1st group) and sound (2nd group). Relative number of neurones pairs acting in correlation in the areas remote from each other, in intersignal intervals both before and after stimuli did not differ in the 1st and 2nd groups. At the same time delays in neuronal discharges in one area after the other were different. In the 1st group animals there was a predominance of the number of visual area neurones discharging after sensorimotor with a delay up to 125 ms, in comparison with the number of sensorimotor area neurones discharging after the visual one. In the 2nd group rabbits the number of visual area neurones with such a delay of discharges after sensorimotor was less and, on the contrary, a predominance of sensorimotor area neurones was observed discharging after the visual one. The obtained results allow to suggest that neurones of the visual and sensorimotor neocortex areas form a single functional system in cases when conditioned and unconditioned stimuli are addressed to these areas and when only one of the studied areas is the projection zone for the combined stimuli. Organization of the neurones activity in systems in these two cases is different.     

Spreading depression in a waking rabbit with the spectral EEG analysis

It was shown that the manifestation of the SD phenomenon in dynamics of the cortical high-frequency gamma activity is rather prominent after bipolar interpretation of the common reference electrode derivations, i.e. when the modeling of the bipolar signal on the base of monopolar EEG recordings is used. The SD wave was accompanied by significant decrease in the power of the EEG gamma band (37-47 Hz) in all observed cortical areas. A curve of decline of gamma activity power had distinct fore and back fronts, so the time of SD appearance in different cortex areas and it's spread succession could be well determined. In the long-term experiments SD waves were characterized by normal (i.e. successive) spread through the cortex mainly in initial three-four experiments. In the subsequent tests latency of SD waves in different cortex areas changed and disturbance of propagation became obvious. Sometimes SD arose rapidly (due 0.5-2 min) on the whole dorsal neocortical surface, when the standard injection of the KCI was done. In the most of experiments the delay of the SD wave appearance was prolonged till 6-9 min or no SD wave occur at some cortical regions. Week epileptiform activity could conduct abnormalities in the SD. In many instances electrophysiological signs of the cortical excitability changes were absent. However the modified spatial SD characteristic and spontaneous occurrence of the repeated. SD waves indicated the increased functional inhomogeneities of the neighboring cortical areas. So, spectral EEG analysis in awake rabbits made it possible to characterize the SD wave both in case of its normal propagation through the cortex and in unusual forms of this reaction.     

Event-related coherence and event-related desynchronization/synchronization in the 10 Hz and 20 Hz EEG during self-paced movements

To investigate the activity of cortical regions in the control of movement, we studied event-related desynchronization/synchronization (ERD/ERS), event-related coherence (ERC), and phase coherence in 29-channel EEGs from 9 subjects performing self-paced movements of the right index finger. Movement preparation and execution produced ERD over the sensorimotor areas at 10 Hz and 20 Hz, followed by ERS. ERD corresponded spatiotemporally to an increase in coherence over the frontocentral areas. For both frequency bands, ERD began over the left sensorimotor areas and became bilateral at the time of movement onset. The coherence increase with frontal areas began in the left central areas and became symmetrical after EMG onset. The ERD and coherence increase was longer at 10 Hz than at 20 Hz. Phase coherence at 10 Hz showed a lead of anterior regions to posterior regions throughout the time period, and at 20 Hz showed a tendency toward zero phase delay corresponding with the movement. EEG desynchronization parallels functional coupling over sensorimotor and frontal areas. Event-related coherence and phase coherence findings implicate the frontal lobes in control of movement planning and execution. The involvement of different frequency bands with different timings may represent parallel changes in the cortical network.     

Effect of motor deprivation in early ontogeny on evoked potentials of the sensomotor and visual cortex in the rat

The influence of long-term (3 months) locomotor deprivation of rats in a month age, on the evoked potentials (EP) of the sensorimotor and the visual cortex was studied in conditions of presentation of single and paired stimuli. Changes were revealed in both cortical zones. An increase of peak latency of the initial positive EP phase in the sensorimotor cortex, and prolongation of the process of changes in excitability of neural elements, elicited by conditioning stimulus, was revealed both in the sensorimotor and the visual areas. The effect of deprivation on the dynamics of changes in neuronal systems excitability was greater in the visual evoked responses.