Effects of selective blockade of synaptic transmission in the parafascicular complex on activity of sensorimotor cortex neurons

Neuronal firing activity pattern in the sensorimotor complex and conditioned reflexes produced by blockade of noradrenergic and serotoninergic input to the parafascicular complex (CM-PF) were investigated during chronic experiments on cats in which lever-pressing reflex had previously been set up. Micro-injection of obsidan, a beta-adrenergic blocker and lysergide, a serotonin blocker, were found to produce intensification and inhibition, respectively, of neuronal spontaneous firing activity in the sensorimotor cortex, with both blockers having a similar (inhibitory) effect on neuronal response induced by conditioned acoustic stimuli. Blockade of monoaminergic inputs to the CM-PF temporarily suppressed execution of the conditioned reflex and the non-conditioned reflex of lever-pressing (for 30–60 min). The study concludes that the pattern of effect produced by the monoaminergic system on the sensorimotor cortex is largely determined by indirect as well as direct agents and that such effects are mediated by neurons belonging to the CM-PF complex of the thalamic nuclei.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 3, pp. 327–336, May–June, 1990.     

Effects of serotonin on neuronal response induced in the sensorimotor cortex by tactile and conditioned acoustic stimuli

Neuronal firing response in the sensorimotor cortex to tactile (non-conditioned) and acoustic (conditioned) stimuli was investigated in trained cats before and after iontophoretic application of serotonin and lysergide. Three functionally distinct groups of neurons were identified from the response produced by presenting tactile and acoustic stimuli. Applying serotonin was found to facilitate preliminary and residual spike response induced by tactile stimulation; it also facilitates and modulates response in many cortical neurons to conditioned stimuli. Facilitation takes the form of reduced latency of response and increased numbers of spikes in response to conditioned stimulus presentation, especially at the initial phase of response to sound and immediately after the onset of conditioned reflex motion. Additional neurons formerly unresponsive to acoustic stimuli joined in the reaction under the effects of serotonin. Changed response patterns often evolve following minor fluctuations in background activity level. It is suggested that facilitation of response following iontophoretic serotonin application in the neocortex is associated with activation of excitatory serotonin receptors (S₂). The lysergide-induced increase in background and evoked activity noted during experimentation can apparently be put down to blockade of inhibitory serotonon (S_1B) receptors.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 3, pp. 337–347, May–June, 1990.     

Effects of acetylcholine and cholinergic transmission blockers on neuronal spike activity in the cat motor cortex associated with conditioned reflex

The effects of direct application of acetylcholine (ACh) and m- and n-cholinoreceptor blockers on test cells were investigated in waking cats having developed instrumental lever-pressing conditioned reflex. Changes were recorded in both spontaneous and invoked firing activity in a functionally homogeneous group of motor cortex cells, in which increased discharge rate usually preceded the start of conditioned reflex movements. It was found, however, that ACh increased spontaneous activity considerably in some of the neurons tested and reduced it moderately in others. Atropine sharply reduced background activity in cortical neurons while preserving spike response to presentation of a conditioned stimulus and n-cholino-blockers such as hexonium and (occasionally) tubocurarine inhibited spike response produced by conditioned stimuli; background activity was slightly inhibited by hexonium and reinforced by tubocurarine. It was concluded that ACh put out by cholinergic fibers helps to maintain background firing activity level in cortical neurons under naturally occurring conditions, acting via m-cholinoreceptors, whereas factors influencing generation of spike discharges associated with performance of conditioned reflex movements are mediated by n-cholinoreceptors.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 5, pp. 579–589, September–October, 1989.     

Modulatory effects of noradrenaline on the spike activity of cat motor cortex neurons under conditions of skin electrostimulation and action of extra stimuli

Effects of iontophoretic application of ephedrine (an indirect adrenomimetic that evokes noradrenaline liberation) and obsidane (a blocker of -adrenoreceptors) on extra stimulus-related modifications of evoked unit spike activity in cortical field 4 were studied in chronic experiments on cats. Neuronal responses were evoked by skin electrostimulation (SES) of extremities; a flow of air blowing on the muzzle 1.0 sec before SES was used as the extra stimulus. In most cases, the SES-evoked unit activity was depressed by ephedrine application, whereas obsidane enhanced it. Extra stimulus-related depression of the SES-evoked responses was removed by obsidane but it did not change or become even more pronounced during ephedrine applications. It is concluded that depressive effects caused by the extra stimulus on the SES-evoked responses of cortical neurons are essentially determined by noradrenergic mechanisms.Translated from Neirofiziologiya, Vol. 25, No. 2, pp. 119–125, March–April, 1993.     

Effects of applying electrical stimulation to the locus coeruleus on neuronal activity in the parietal association cortex

It was shown during experiments on cats undergoing surgery under ketamine-induced anesthesia and immobilized with myorelaxin that applying trains of stimuli to the locus coeruleus (LC) produces an effect on 79% of parietal cortex neurons. This manifests as inhibition lasting 300–700 msec or a 16–32% decline in the activity rate of neurons with background activity. Hyperpolarization of 5–7 mV lasting 120–500 msec preceded by a latency of 30–90 msec was noted in such neurons as well as "silent" cells during intracellular recording. Duration of the inhibitory pause in neuronal background activity induced by transcallosal stimulation (TCS) increased by 50–200 msec under the effects of conditioned stimuli applied to the LC. Duration of the IPSP triggered by TCS likewise increased (by 50–100 msec) under the effects of LC stimulation. It was concluded that the effects of stimulating the LC on neuronal activity in the parietal cortex may manifest either directly, as inhibition of background activity and hyperpolarization, or else as modulation of influences exerted by other neurotransmitters.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 486–494, July–August, 1990.     

Inhibitory effects of external stimulation on neuronal spike response in the cat association cortex during conditioning

The effects of external stimulus presentation on neuronal spike response induced in area 7 by conditioned and unconditioned stimuli were investigated in waking cats. Pointing reaction arising in response to the action of external stimuli led to the phasing out of spike response to unconditioned cutaneous stimulation, as well as depression or complete suppression of response in area 7 neurons to conditioned stimulus presentation, despite occurrence of conditioned reflex motion. External stimuli no longer exerted an inhibitory effect on the neuronal spike response induced by protracted use of conditioned and unconditioned stimuli. The fact that the conditioned lever-pressing reflex could occur either following or without neuronal spike response in area 7 would raise the question of whether neurons belonging to this area always participate directly in the triggering and occurrence of conditioned reflex motion.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziogiya, Vol. 22, No. 2, pp. 147–155, March–April, 1990.     

Specific effects by the psychotomimetic drugsd-amphetamine and phencyclidine on the performance of an aversely motivated successive visual discrimination in the rat

Summary Rats were trained to perform a conditioned avoidance response to white noise in a conventional two-compartment shuttle-box. The partition between the compartments had two openings, however, and the correct passage (leftor right) was signalled by changes in background illumination. In this situation the psychotomimetic compoundsd-amphetamine (4 mg kg^–1 IP) and phencyclidine (PCP) (2 mg kg^–1 SC) were found to selectively disrupt the visual discrimination. Thed-amphetamine-induced abnormal behavior in this situation has previously been linked to excessive dopamine (DA) receptor stimulation, not controlled by nerve impulse flow and its regulation by important local feed-back mechanisms. Thus, the psychotomimetic effects produced by this compound should not only by due to increased DA receptor activationper se, but also to a disruption of normal patterns of firing and release in dopaminergic neurons. There is evidence to suggest that PCP via an excitatory amino acid (EAA) receptor produces a similar net effect on brain meso-limbic dopaminergic neurotransmission via an increased rate of firing, accompanied by regularization of firing (loss of burst activity). In support for a mediation of PCP-induced effects via EAA receptors, the local application of kynurenic acid into the ventral forebrain (4.7µg, bilaterally) was found also to produce a selective disruption of discriminative performance. It should be noted, however, thatd-amphetamine-induced loss of discriminative behavior, but not that induced by PCP, was antagonized by haloperidol (0.1–0.2 mg kg^–1 IP) administration. It is thus possible that at least some effects of PCP in this situation are mediated on the efferent side of the dopaminergic neuron. It is suggested that the abnormal behavior, as evidenced by a loss of discriminative (but not avoidance) behavior, is due to disruption of normal, feed-back regulated, nerve impulse flow.     

Effects of experimentally induced deficiency of catecholaminergic transmission on neuronal activity in the ventrolateral thalamic nucleus

Background activity was investigated in 272 neurons of the ventrolateral thalamic nucleus (VLTN) before and after systemic administration of neuroleptics (haloperidol and droperidol) at cataleptic doses by means of extracellular techniques during chronic experiments on cats. Autocorrelation and spectral analysis revealed regularly-occurring changes in the background activity rate of VLTN neurons, the periodicity of which changed by fractions of seconds (0.2–0.8 sec), seconds (1.5–10 sec), or tens of seconds (12–30 sec). While numbers of neurons with individual types of periodic activity did not exceed 6–8% in intact animals, it did increase to 18–30% after administering neuroleptics. Raised numbers of neurons with two types of regularly occurring processes within a single spike train were also noted. Experimentally-produced data were compared with findings from clinical observations. Quantities of neurons with different variations in the periodicity of their firing activity reached 19–46% in patients with parkinsonism but did not exceed 4–8% in those with torsion dystonia. The genesis of raised rhythmic firing in thalamic neurons occurring with parkinsonism is thought to be associated with impaired catecholaminergic (both dopaminergic and -adrenergic) transmission.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 3, pp. 359–368, May–June, 1990.     

Effects of stimulating the central gray matter on neuronal response in the cat medial thalamic nuclei

This study was performed on the effects of stimulating the midbrain central gray matter (CGM) on neuronal response in the association medial thalamic nuclei evoked by stimulation of A-alpha and A-delta fibers of the infraorbital nerve and the caudal nucleus of the spinal trigeminal tract (CN STT) and tooth pulp stimulation using cats anesthetized by thiopental-chloralose as experimental animals. Stimulating the CGB with trains of stimuli was found to evoke an excitatory response in a percentage of the neurons tested, in which latency fluctuated between 15 and 40 msec. Applying conditioned stimuli to the CGM caused suppression of response to afferent impulses in neurons belonging to the "convergent" and "low" and "high" threshold groups. Responses induced by stimulating tooth pulp and A-delta fibers showed 100% inhibition as compared with 86% during A-alpha fiber and infraorbital nerve stimulation. The fact that stimulating the CGM produces an inhibitory effect on the response of thalamic neurons evoked by stimulation of both peripheral afferents and the CN STT would indicate that the CGM can exert a direct action on thalamic neuronal activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 5, pp. 660–665, September–October, 1987.     

Unit responses of the secondary somatosensory cortex during defensive conditioning in cats

Unit responses in the secondary somatosensory cortex during the formation and extinction of a defensive conditioned reflex to acoustic stimulation were investigated in chronic experiments on cats. In 21 of 28 neurons tested during defensive conditioning the firing pattern changed in accordance with the character of responses to electric shock reinforcement. Two types of conditioned-reflex unit responses were distinguished: excitatory and inhibitory. Most neurons responding to the conditioned stimulus by activation did so during the first 50 msec, which was 80–100 msec before the conditioned motor response. Considerable variability of the unit responses was observed during conditioning. By the time of stabilization of the conditioned-reflex connections the unit response to the conditioned stimulus was stable in form. The pattern of extinction of the conditioned unit activity was expressed as a decrease in the discharge frequency in responses of excitatory type and disinhibition of activity in the case of inhibitory responses.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev, Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 232–238, May–June, 1977.     

Spontaneous spiking and synaptic depression underlie noradrenergic control of feed-forward inhibition

Inhibitory interneurons across diverse brain regions commonly exhibit spontaneous spiking activity, even in the absence of external stimuli. It is not well understood how stimulus-evoked inhibition can be distinguished from background inhibition arising from spontaneous firing. We found that noradrenaline simultaneously reduced spontaneous inhibitory inputs and enhanced evoked inhibitory currents recorded from principal neurons of the mouse dorsal cochlear nucleus (DCN). Together, these effects produced a large increase in signal-to-noise ratio for stimulus-evoked inhibition. Surprisingly, the opposing effects on background and evoked currents could both be attributed to noradrenergic silencing of spontaneous spiking in glycinergic interneurons. During spontaneous firing, glycine release was decreased due to strong short-term depression. Elimination of background spiking relieved inhibitory synapses from depression and thereby enhanced stimulus-evoked inhibition. Our findings illustrate a simple yet powerful neuromodulatory mechanism to shift the balance between background and stimulus-evoked signals.     

Participation of midbrain periaqueductal gray matter in defense conditioning in rats

Responses of neurons of the periaqueductal gray matter (PAG) were studied in chronic experiments on cats during formation and extinction of a defensive conditioned reflex to sound and its differential inhibition. In response to conditioned stimulation these neurons developed phasic-tonic spike responses up to 3 sec in duration. During combination of stimuli these responses were formed long before the conditioned reflex and disappeared long after the latter was extinguished. In the case of an established conditioned reflex, the onset of spike responses occurred 100–200 msec before the appearance of motor responses. An increase in spike activity of tonic character in neurons of PAG preceded voluntary movements by 100–500 msec. The responses of these neurons to presentation of a differential stimulus consisted of groups of spikes 150–200 msec in duration. They were formed with difficulty, and their manifestation was made even more difficult by an interruption during the experiment and by preceding positive stimuli. On the basis of the results a conditioned reflex can be regarded as the result of a multilevel hierarchic process of readjustment of unit activity, which begins in the nonspecific structures of the midbrain.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 15, No. 3, pp. 278–287, May–June, 1983.     

Neurophysiological changes and dynamics of bound calcium during the development of associative learning in Helix lucorum

Neurophysiological effects and the dynamics of the content of bound calcium (Ca-b) in command neurons (LP1 and RP1) of defensive behavior during food aversion conditioning are studied inHelix lucorum. In the case of associative learning, there arises in these cells both a response to the conditioned stimulus and a nonspecific facilitation of the reactions to sensory stimuli that is characteristic for the state of sensitization. A response to the presentation of a conditioned stimulus is detected approximately 30 min after the development of long-term sensitization. The use of three or more paired stimuli reveals the characteristic dynamics of the level of Ca-b, which correlates with the neurophysiological effects and differs from the changes in Ca-b content during the development of "pure" sensitization. It is thought that the command neuron of defense behavior exhibits inHelix differences of the molecular-cellular mechanisms lying at the basis of the development of associative learning and sensitization.P. K. Anokhin Institute of Normal Physiology, Russian Academy of Sciences, Moscow. I. P. Pavlov Institute of Physiology, Russian Academy of Sciences, St. Petersburg. Translated from Neirofiziologiya, Vol. 24, No. 6, pp. 691–701, November–December, 1992.     

Rhythmically firing (20–50 Hz) neurons in monkey primary somatosensory cortex: Activity patterns during initiation of vibratory-cued hand movements

The activity patterns of rhythmically firing neurons in monkey primary somatosensory cortex (SI) were studied during trained wrist movements that were performed in response to palmar vibration. Of 1,222 neurons extracellularly recorded in SI, 129 cells (11%) discharged rhythmically (at 30 Hz) during maintained wrist position. During the initiation of vibratory-cued movements, neuronal activity usually decreased at 25 ms after vibration onset followed by an additional decrease in activity at 60 ms prior to movement onset. Rhythmically firing neurons are not likely to be integrate-and-fire neurons because, during activity changes, their rhythmic firing pattern was disrupted rather than modulated. The activity pattern of rhythmically firing neurons was complimentary to that of quickly adapting SI neurons recorded during the performance of this task (Nelson et al., 1991). Moreover, disruptions of rhythmic activity of individual SI neurons were similar to those reported previously for local field potential (LFP) oscillations in sensorimotor cortex during trained movements (Sanes and Donoghue, 1993). However, rhythmic activity of SI neurons did not wax and wane like LFP oscillations (Murthy and Fetz, 1992; Sanes and Donoghue, 1993). It has been suggested that fast (20–50 Hz) cortical oscillations may be initiated by inhibitory interneurons (Cowan and Wilson, 1994; Llinas et al., 1991; Stern and Wilson, 1994). We suggest that rhythmically firing neurons may tonically inhibit quickly adapting neurons and release them from the inhibition at go-cue onsets and prior to voluntary movements. It is possible that rhythmically active neurons may evoke intermittent oscillations in other cortical neurons and thus regulate cortical population oscillations.     

Effects of transmitters on pyramidal and nonpyramidal neurons in hippocampal slices

Investigations were performed on the effects of acetylcholine (ACh), norepinephrine (NE), 5-hydroxytryptamine (5-HT), and -aminobutyric acid (GABA) on the background firing of the three following groups of field CA₃ neurons in guinea pig hippocampal slices: nonpyramidal neurons of the stratum radiatum moleculare (NSR), stratum pyramidale cells with single spike discharges (SD units), and those with complex discharge patterns (CD units) within the same layer. The action of ACh and NE on presumed interneurons of the pyramidal layer (IPL) was also investigated; CD units were found to differ from the remaining groups, which reacted similarly to the transmitters tested. It was shown that NE, 5-HT, and GABA inhibited the activity of CD cells, while ACh produced inhibitory-activating response in 50% of these units. Both NE and ACh exerted a monophasic activating effect on NSR, ISP, and SD, however, while 5-HT and GABA induced activation in a proportion of NSR and SD cells, as well as inhibitory response. The excitatory effects produced by ACh, NE, and 5-HT on NSR persisted during blockade of synaptic transmission, indicating that associated afferent fibers may be acting directly on these cells.Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 20, No. 1, pp. 64–74, January–February, 1988.     

Role of the midbrain periaqueductal gray matter in conditioned reflexes

Unit activity in the midbrain periaqueductal gray matter (PGM) during an instrumental placing reflex, its extinction, differentiation, and conditioned inhibition, was studied in chronic experiments on cats. Spike responses 1–2 sec in duration in 69 (36.7%) of 182 neurons preceded by 400–800 msec the beginning of conditioned-reflex and voluntary intertrial movements. These advanced responses appeared 200 msec before the corresponding advance responses of motor cortical neurons. Fifty-eight neurons (30.9%) responded directly to acoustic stimulation with a latent period of 10–50 msec for 2–6 sec, 19 neurons (10.1%) generated double responses, linked with both the acoustic stimulus and subsequent conditioned-reflex movement, and 42 neurons (22.3%) did not respond to acoustic stimulation, although individual neurons of this group changed the level of their spontaneous activity in response to repeated conditioned stimulation, and this change was maintained for some tens of minutes. Extinction, differentiation, and conditioned inhibition all abolished conditioned-reflex movements, but each type of internal inhibition was accompanied by its own characteristic changes in the firing pattern of PGM neurons. Functional independence of neurons of the first and second groups was demonstrated during extinction and recovery of the conditioned-reflex. The results indicate the important role of PGM not only in the mechanism of the conditioned reflex, but also in the development of its internal inhibition.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 403–419, May–June, 1984.     

Variations in neuronal responses of cat corticocerebral neurons to somatic stimulation on action of additional stimuli

Using alert cats we have investigated the impulse reactions (IR) of corticocerebral neurons (areas 3, 4, 5) in response to electrical stimulation of the contralateral limb (ESL). The short-latency impulse reactions of neurons of an excitatory nature, which were recorded in the animals in the state of rest, decreased considerably in intensity or could not be detected at all after preliminary (after 1.0–1.5 sec) action of a light flash or sound stimulation during the initial periods of their application. When the animals showed a defense or food motivation we observed, apart from a change in the background activity, evoked by the action of the ESL, more complex rearrangements of the impulse reaction, up to the appearance of the periods of inhibition of the impulse activity. Conversely, combined application of ESL with a stimulus of the same modality, did not prevent development of the initially isolated reaction even in cases of decrease in the ESL to a subthreshold value. A subthreshold stimulus "intensified" in this way can become important for the animal and be used as a signal stimulus during the development of the conditioned defense reflex in the animal. We assume that the nature of the reactions of the same neurons to a simple somatic stimulation is not constant. This is due to a preliminary tuning of the analyzers of the animal to the perception of stimuli of a certain modality.A. A. Bogomolets Institute of Physiology, Ukrainian Academy of Sciences, Kiev. Translated from Neirofiziologiya, Vol. 24, No. 1, pp. 27–37, January–February, 1992.     

Learning-related long-term synaptic facilitation in snails: Possible mechanisms of long-term memory formation

The long-term changes in the electrical activity of command neurons related to sensitization and elaboration of associative defensive behavioral habit (food rejection) were studied inHelix snail. The long-term effects consisted of facilitation of synaptic components in neuronal responses to the test stimulations. Variations were found in the dynamics of long-term synaptic facilitation of responses to the applied chemical and tactile stimuli in the course of sensitization, as well as dependence of the degree of long-term facilitation of responses to the test stimulation at the site of its application with respect to the site of the sensitizing stimulation (site-specific sensitization). After conditioning, the synaptic response of command neurons to the conditioning stimulation appeared approximately 30 min later than did the long-term sensitization in these cells. The minimum duration of long-term synaptic facilitation of responses to the test stimulation varied from 1 h (for tactile stimulation) to 3 h (for chemical stimulation). The maximum duration of effects exceeded 4 h. It is suggested that the observed features of the synaptic plasticity in command neurons during learning are based on the selective regulation of synaptic inputs by specific protein regulators, whose lifespan does not exceed 1 h to 3 h.Neirofiziologiya/Neurophysiology, Vol. 25, No. 5, pp. 383–389, September–October, 1993.     

Two patterns of bulbar neuronal response to microstimulation of locomotor and inhibitory brain stem sites

Synaptic responses (postsynaptic potentials and action potentials) were evoked in mesencephalic decerebellated cats by stimulating pontine bulbar locomotor and inhibitory sites (LS and IS, respectively) with a current of not more than 20 µA in "medial" and "lateral" neurons of the medulla. Some neurons even produced a response to presentation of single (actually low — 2–5 Hz — frequency) stimuli. The remaining cells responded to stimulation at a steady rate of 30–60 Hz only. Both groups of medial neurons were more receptive to input from LS. Lateral neurons responding to even single stimuli reacted more commonly to input from LS and those responding to steady stimulation only to input from IS. Many neurons with background activity (whether lateral or medial) produced no stimulus-bound response, but rhythmic stimulation either intensified or inhibited such activity. This response occurs most commonly with LS stimulation. Partial redistribution of target neurons in step with increasing rate of presynaptic input may play a major part in control of motor activity.Institute for Research into Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 257–266, March–April, 1990.     

Investigation of habituation reaction of neurons of the cat motor cortex

The neuronal and total surface activity of the cortical representation of the motor analyzer in the region of the posterior sigmoid gyrus of the cat brain in response to rhythmical light, sound, and electrical stimuli and their complexes was analyzed. Two groups of neurons were found, of which the first is characterized by a gradual decrease in the number of peaks in the response and by their subsequent disappearance and the second by the absence of a discharge in response to stimulation and by its development before the application of the next stimulus. The first group was comprised of neurons which do not have background activity and the second was made up of neurons with a background activity of 0.4–3.7 imp/sec. This reorganization of the activity of cortical neurons in response to rhythmical stimulation is considered to be a habituation phenomenon.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 3, No. 3, pp. 245–251, May–June, 1971.