Functional significance of cortical evoked potentials

Experimental results indicating a limited and indirect dependence of the amplitudes of the first two phases of the primary response (PR) of the cat auditory cortex on the information content of the stimulus are described. The PR amplitude is slightly reduced during the action of positive acoustic stimuli. This is due, primarily, to activation of the EEG in response to positive stimuli, for a similar decrease in amplitude of the cortical PR against the background of an activated EEG is also observed to fine differential stimuli. No PRs to acoustic stimulation are found in the sensomotor cortex either before or after motor-food conditioning; they are recorded only in the auditory projection zone. The amplitude of the PRs falls regularly during fast (25–30/min) and prolonged repetitive acoustic stimulation, and the rate of fall is greater when the interval between stimuli is shortened. The appearance of PRs of high amplitude in response to infrequent stimuli of whatever quality indicates that these responses are dependent on a component of the orienting reaction. It is concluded that the role of the first two phases of the PR as an indicator of fine analysis of information by the brain is limited.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 2, No. 4, pp. 423–428, July–August, 1970.     

Reticular unit responses in rats during functional blocking of cortical representation of the stimulated paw

Responses of single reticular units to electrodermal stimulation were studied in unanesthetized, immobilized rats during cold blocking of the cortical representation of the stimulated limbs. Local cooling of the somatosensory cortex caused reversible and opposite changes in responses of 60 of the 86 neurons tested. In 25 cells responses only to stimulation of the limb whose sensory projection was in the cooled zone were modified. In 31 neurons changes in responses to this stimulation predominated and in 22 they were comparable with changes in responses of the same neurons to electrodermal stimulation of the other limb, whose cortical representation was intact. Cold blocking of the cortical response to presentation of one of the stimuli thus modifies the conditions for information processing in the neuron net of the reticular formation selectively for the response to presentation of that same stimulus.I. M. Sechenov Institute of Evolutionary Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 2, pp. 179–186, March–April, 1981.     

Monosynaptic inhibitory postsynaptic potentials of cortical neurons

Monopolar intracortical stimulation of the auditory cortex was carried out in cats immobilized with D-tubocurarine. A macroelectrode (tip diameter 100 µ) or a microelectrode (tip diameter 10–15 µ) was used for stimulation. In both cases, besides excitatory responses, primary IPSPs with latent periods of 0.4–1.2 and 1.4–6.0 msec were recorded in cortical neurons close to the point of stimulation. The first group of IPSPs are considered to be generated in response to direct stimulation of bodies or axons of inhibitory cortical neurons, i.e., monosynaptically. The amplitude of these IPSPs varied in different neurons from 3 to 15 mV, and their duration from 4 to 150 msec. Additional later inhibitory responses were superposed on many of them. Of the IPSPs generated in auditory cortical neurons in response to stimulation of geniculocortical fibers 1.5% had a latency of 0.8–1.3 msec. They also are assumed to be monosynaptic. It is concluded that the duration of synaptic delay of IPSPs in cortical neurons and spinal motoneurons is the same, namely 0.3–0.4 msec. Axons of auditory cortical inhibitory neurons may be 1.5 mm long. The velocity of impulse conduction along these axons is 1.6–2.8 m/sec. The genesis of some special features of IPSPs of cortical neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 7, No. 5, pp. 458–467, September–October, 1975.     

Corticofugal influences on segmental responses of lumbar spinal ventral horn interneurons in cats

Experiments on cats using extra- and intracellular recording methods showed that stimulation of the motor cortex of both hemispheres leads to considerable modulation of responses to stimulation of cutaneous and muscular lower limb afferents in spinal ventral horn interneurons in segments L6, 7. Three types of conditioning corticofugal effect were observed: facilitation, inhibition, and facilitation followed by inhibition (biphasic effect), and inhibitory effects predominated. The duration of facilitation of responses did not exceed 30–40 msec. The characteristics of the time course of inhibition varied: in some cases it began with relatively short intervals (8–15 msec), in other cases with an interval of 30–40 msec; its duration was 125–500 msec, or sometimes more. The effect of cortical stimulation on responses to stimulation of various afferent inputs of the same interneuron was shown to differ. The character of the conditioning corticofugal effect correlated with the latent period of segmental responses: facilitation was observed only in responses with a relatively short latent period (under 5 msec); responses with a longer latent period were mainly inhibited. The type of cortical effect also depended on the function performed by the activated afferent input. It is suggested that differential descending control of segmental polysynaptic responses recorded in ventral horn interneurons with wide convergence of afferent influences takes place in the initial stages of the reflex are. The mechanism of this control is discussed.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neiorofizologiya, Vol. 14, No. 6, pp. 563–571, November–December, 1982.     

Centrifugal and centripetal connections of the cat visual cortex

In experiments on curarized cats unit responses in the dorsal lateral geniculate body to stimulation of various zones in area 17 of the visual cortex were analyzed. Of all cells tested 69% were found to respond antidromically and 8% orthodromically; in 7.6% of cells IPSPs occurred either after an initial antidromic spike or without it. The velocities of conduction of excitation along the corticopetal fibers of the optic radiation varied from 28 to 4.3 m/sec, but the three commonest groups of fibers had conduction velocities of 28–19, 14–12, and 10–9.5 m/sec. A difference between latent periods of antidromic responses of the same neurons was found to stimulation of different zones of the visual cortex; this indicates that axons of geniculo-cortical fibers split into several branches which form contacts with several neurons in area 17 of the visual cortex. The degree and possible mechanisms of cortical influences on neurons of the lateral geniculate body are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 243–249, May–June, 1976.     

Conduction of excitation in the cat visual system

Unit responses in area 17 of the visual cortex to stimulation of the lateral geniculate body and optic tract were studied in experiments on unanesthetized cats immobilized with D-tubocurarine. Of the neurons tested, 53.6% responded to stimulation of the lateral geniculate body. In 92% of these cells the responses were orthodromic with latent periods of between 2 and 12.5 msec. Most cells responded with latent periods of 2.0–2.5, 3.0–3.5, and 4.0–4.5 msec, corresponding to latent periods of the components of the electropositive wave of the primary response. Antidromic responses to stimulation of the lateral geniculate body were given by 8% of neurons. The difference between the latent periods of responses of the same visual cortical neurons to stimulation of the optic tract and lateral geniculate body was 0.1–1.8 msec, but for most neurons (55.8%) it was 0.5–1 msec. The histograms of response latencies of visual cortical neurons to stimulation of the above-mentioned formations were found to be similar. It is concluded that the optic radiation contains three principal groups of fibers with conduction velocities of 28.5–16.6, 11.7–8.9, and 7.4–6.0 m/sec, respectively.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 7, No. 6, pp. 589–596, November–December, 1975.     

Unit responses in the cat auditory cortex to medial geniculate body stimulation

Responses of 98 auditory cortical neurons to electrical stimulation of the medial geniculate body (MGB) were recorded (45 extracellulary, 53 intracellularly) in experiments on cats immobilized with tubocurarine. Responses of the same neurons to clicks were recorded for comparison. Of the total number of neurons, 75 (76%) responded both to MGB stimulation and to clicks, and 23 (24%) to MGB stimulation only. The latent period of extracellularly recorded action potentials of auditory cortical neurons in response to clicks varied from 7 to 28 msec (late responses were disregarded), and that to MGB stimulation varied from 1.5 to 12.5 msec. For EPSPs these values were 8–13 and 1–4 msec respectively. The latent period of IPSPs arising in response to MGB stimulation varied from 2.2 to 6.5 msec; for 34% of neurons it did not exceed 3 msec. The difference between the latent periods of responses to clicks and to MGB stimulation varied for different neurons from 6 to 21 msec. Responses of 11% of neurons to MGB stimulation, recorded intracellularly, consisted of sub-threshold EPSPs, while responses of 23% of neurons began with an EPSP which was either followed by an action potential and subsequent IPSP or was at once cut off by an IPSP; 66% of neurons responded with primary IPSPs. Neurons responding to MGB stimulation by primary IPSPs are distributed irregularly in the depth of the cortex: there are very few in layers III and IV and many more at a depth of 1.6–2 mm. Conversely, excited neurons are predominant in layer III and IV, and they are few in number at a depth of 1.6–2 mm. It is concluded that the afferent volley reaching the auditory cortex induces excitation of some neurons therein and, at the same time, by the principle of reciprocity, induces inhibition of others. This afferent inhibition takes place with the participation of inhibitory interneurons, and in some cells the inhibition is recurrent. The existence of reciprocal relationships between neurons in different layers of the auditory cortex is postulated.A. A. Bogomolets' Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 1, pp. 23–31, January–February, 1972.     

Unit responses of the first and second somatosensory cortical areas to peripheral,thalamic, and cortical stimulation

Unit responses of the first (SI) somatosensory area of the cortex to stimulation of the second somatosensory area (SII), the ventral posterior thalamic nucleus, and the contralateral forelimb, and also unit responses in SII evoked by stimulation of SI, the ventral posterior thalamic nucleus, and the contralateral forelimb were investigated in experiments on cats immobilized with D-tubocurarine or Myo-Relaxin (succinylcholine). The results showed a substantially higher percentage of neurons in SII than in SI which responded to an afferent stimulus by excitation brought about through two or more synaptic relays in the cortex. In response to cortical stimulation antidromic and orthodromic responses appeared in SI and SII neurons, confirming the presence of two-way cortico-cortical connections. In both SI and SII intracellular recording revealed in most cases PSPs of similar character and intensity, evoked by stimulation of the cortex and nucleus in the same neuron. Latent periods of orthodromic spike responses to stimulation of nucleus and cortex in 50.5% of SI neurons and 37.1% of SII neurons differed by less than 1.0 msec. In 19.6% of SI and 41.4% of SII neurons the latent period of response to cortical stimulation was 1.6–4.7 msec shorter than the latent period of the response evoked in the same neuron by stimulation of the nucleus. It is concluded from these results that impulses from SI play an important role in the afferent activation of SII neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 4, pp. 351–357, July–August, 1976.     

Neuronal and synaptic mechanisms of cortical inhibition

The latent periods, amplitude, and duration of IPSPs arising in neurons in different parts of the cat cortex in response to afferent stimuli, stimulation of thalamocortical fibers, and intracortical microstimulation are described. The duration of IPSPs evoked in cortical neurons in response to single afferent stimuli varied from 20 to 250 msec (most common frequency 30–60 msec). During intracortical microstimulation of the auditory cortex, IPSPs with a duration of 5–10 msec also appeared. Barbiturates and chloralose increased the duration of the IPSPs to 300–500 msec. The latent period of 73% of IPSPs arising in auditory cortical neurons in response to stimulation of thalamocortical fibers was 1.2 msec longer than the latent period of monosynaptic EPSPs evoked in the same way. It is concluded from these data that inhibition arising in most neurons of cortical projection areas as a result of the arrival of corresponding afferent impulsation is direct afferent inhibition involving the participation of cortical inhibitory interneurons. A mechanism of recurrent inhibition takes part in the development of inhibition in a certain proportion of neurons. IPSPs arise monosynaptically in 2% of cells. A study of responses of cortical neurons to intracortical microstimulation showed that synaptic delay of IPSPs in these cells is 0.3–0.4 msec. The length of axons of inhibitory neurons in layer IV of the auditory cortex reaches 1.5 mm. The velocity of spread of excitation along these axons is 1.6–2.8 msec (mean 2.2 msec).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 394–403, May–June, 1984.     

Analysis of evoked potentials from the limbic cortex

In acute experiments on cats we investigated evoked potentials from the cingulate gyrus developed in response to stimulation of somatic and visceral nerves; also potentials from various parts of the hypothalamus, and midbrain reticular formation. We showed that the nonspecific afferent system influences electrical activity in the limbic cortex through hypothalamic pathways. We consider the limbic cortex to be part of the association area of the neocortex and that the associative responses of the cortex are more complex in nature than is usually thought to be the case, and that they are formed under the influence of impulses arriving at the cortex along many specific and nonspecific pathways. The hypothalamo—cingulate system is one of the main systems of cortico—subcortical integration. It plays an important part in regulation of autonomic, somatic, and emotional responses.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 2, No. 5, pp. 451–459, September–October, 1970.     

Unit responses of the cat caudate nucleus to cortical stimulation before and after destruction of nonspecific thalamic nuclei

Responses of caudate neurons to stimulation of the anterior sigmoid and various parts of the suprasylvian gyrus were studied in acute experiments on cats. The experiments consisted of two series: on animals with an intact thalamus and on animals after preliminary destruction of the nonspecific thalamic nuclei. Stimulation of all cortical areas tested in intact animals evoked complex multicomponent responses in caudate neurons with (or without) initial excitation, followed by a phase of inhibition and late activation. The latent periods of the initial responses to stimulation of all parts of the cortex were long and averaged 14.5–25.5 msec. Quantitative and qualitative differences were established in responses of the caudate neurons to stimulation of different parts of the cortex. Considerable convergence of cortical influences on neurons of the caudate nucleus was found. After destruction of the nonspecific thalamic nuclei all components of the complex response of the caudate neurons to cortical stimulation were preserved, and only the time course of late activation was modified.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 5, pp. 464–471, September–October, 1980.     

Effect of neurotropic drugs on cortical evoked potentials

In acute experiments on unanesthetized, curarized cats and rabbits and also on animals anesthetized with chloralose, recordings were made of direct cortical and transcallosal responses, responses in the pyramids of the medulla to peripheral stimulation and stimulation of the motor cortex, primary responses in area S-I, and interzonal somatomotor responses. The effect of narcotics on these cortical responses was shown to persist under conditions partially or completely excluding effects mediated through the reticular formation and other subcortical structures (intracarotid injection of the drugs or their local application to the cortex, experiments after premesencephalic section or on the isolated cortex). Neuroleptics have only a slight effect on these cortical evoked responses, mainly due to their blocking action on the reticular formation. Tranquilizers of the benzodiazepine series are active against the cortical responses studied, and this effect is due to their direct action on the cortex.Institute of Pharmacology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 582–591, November–December, 1971.     

Habituation of direct cortical responses in cats

Gradual diminution of direct responses of various cortical areas was studied in acute experiments on cats under barbiturate anesthesia. During repetitive stimulation of the surface of cortical association areas these responses underwent all changes characteristic of the habituation phenomenon. Direct cortical responses in primary projection areas were distinguished by great stability, and habituation was absent in this case. After neuronal isolation of an island of projection cortex, habituation of responses began to develop there just as in the cortical association areas. The possible causes of differences between changes in direct cortical responses in different areas are discussed. It is suggested that projection areas receive strong activating influences from deep brain structures, which exert a sensitizing effect on the direct responses.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 241–246, May–June, 1981.     

Neuronal inhibitory responses to intracortical stimulation of an isolated slab of cat auditory cortex

Neuronal responses of an isolated slab of cortex to intracortical stimulation were studied intracellularly. The predominant responses were primary IPSPs. Their latent periods did not exceed 10 msec. Within the volume of cortex studied, neurons inhibited in response to stimulation were most numerous in the upper layers (II, III). Predominance of disynaptic IPSPs is evidence of the important role of cortical interneurons in their genesis. It is concluded from the results that primary IPSPs limit the spread of excitation primarily in the activated area of cortex. Since involvement of neurons of the isolated slab in the inhibition process takes place for only 10 msec after stimulation, neurons giving spike responses to intracortical stimulation with a longer latent period can transmit information into other brain zones. The role of duration of IPSP in the dynamics of interneuronal interaction in the cerebral cortex is discussed.I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 16, No. 1. pp. 42–49, January–February, 1984.     

Role of cholinergic mechanisms in the genesis of some cortical responses

The dynamics of evoked potentials during blocking of cholinergic cortical structures was investigated in unanesthetized cats. Application of the anticholinergic drug benactyzine inhibits the negative phases of cortical responses to stimulation of the reticular formation and non-specific thalamic nuclei and also of responses to direct cortical stimulation. Direct cortical responses (DCRs), inverted by -aminobutyric acid, are also depressed, indicating the role of cholinergic mechanisms in the genesis of these responses. During blocking of cholinergic synapses, negative phases of the primary response (PR) and response to stimulation of the specific thalamic nucleus are facilitated. A tendency is then observed toward grouping of spontaneous unit discharges and abolition of inhibition of cortical neurons produced by high-frequency stimulation of the reticular formation. One cause of the increase in amplitude of the primary response (PR) to the action of anticholinergic drugs may be widening of the zone of cortical neurons involved in the response because of abolition of the localizing effect of inhibitory neurons.Institute of Physiology, Siberian Division, Academy of Sciences of the USSR, Novosibirsk. Translated from Neirofiziologiya, Vol. 2, No. 4, pp. 406–411, July–August, 1970.     

Role of interoceptive afferent excitation in sensory integration on neurons of the cat visual cortex

Experiments on cats immobilized with diplacin showed that stimulation of the vagus and splanchnic nerves had a significant effect on spontaneous unit activity in area 17 of the cortex. The unit responses most frequently observed were prolonged tonic changes in the discharge frequency. Complex phasic unit responses with stable inhibitory pauses and periods of activation occur less often. The development of phasic responses could begin either with a period of activation or with initial inhibition of activity. As a result of splanchnic nerve stimulation responses of predominantly excitatory type appeared. These also occurred more often in response to somatic stimulation. The degree of recruiting into the responses and the intensity of the responses themselves increased if a series of stimuli was applied to the nerves. The important role of the lateral hypothalamus and mesencephalic reticular formation in the mechanism of the visceral and somatic events observed on visual cortical neurons was demonstrated.Ivano-Frankovsk Medical Institute. Translated from Neirofiziologiya, Vol. 9, No. 4, pp. 361–368, July–August, 1977.     

Analysis of long cortical evoked potentials

Evoked potentials arising in the motor cortex in response to its direct stimulation (dendritic and slow negative potentials), to stimulation of the ventrolateral (primary response) and intralaminar (nonspecific response) thalamic nuclei, and to stimulation of the pyramidal tracts (antidromic response), and also postsynaptic responses of neurons corresponding to them were studied in acute experiments on curarized cats. Evoked potentials arising in response to direct cortical stimulation and also to stimulation of the specific and nonspecific thalamic nuclei and pyramidal tracts were recorded from the same point of the motor cortex, and the corresponding intracellular responses were recorded from the same neuron. Slow negative potentials arising under these conditions of stimulation and the IPSPs corresponding to them were shown to have an identical time course. The results show that slow negative potentials are a reflection of hyperpolarization of pyramidal neurons. It is suggested that the individual components of responses evoked by direct stimulation of the cortex and thalamic nuclei have a common genesis.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 14, No. 2, pp. 115–121, March–April, 1982.     

Responses of cortical neurons to viscero-somatic stimulation

Responses of cortical neurons in the posterior sigmoid gyrus of cats anesthetized with chloralose to electrical stimulation of somatic and visceral nerves were recorded. Bimodal viscero-somatic neurons are predominant in this part of the cortex and some of them also respond to light. Besides the polysensory modally-specific neurons it was also possible to distinguish a group of modally nonspecific cells (27%), whose responses to different stimuli did not differ statistically from each other. Simultaneous stimulation of visceral and somatic nerves led to facilitation of activity of the long-latency neurons; this was reflected in a decrease of 10 msec in the latent period of the response and an increase in the number of spikes per discharge.A. A. Zhdanov Leningrad University. Tadjik University, Dushanbe. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 574–581, November–December, 1971.     

Transformation of the afferent tactile signal into a motor command in the cat motor cortex

Activity of 112 neurons of the precruciate motor cortex in cats was studied during a forelimb placing reaction to tactile stimulation of its distal parts. The latent period of response of the limb to tactile stimulation was: for flexors of the elbow (biceps brachii) 30–40 msec, for the earliest reponses of cortical motor neurons about 20 msec. The biceps response was observed 5–10 msec after the end of stimulation of the cortex with a series of pulses lasting 25 msec. Two types of excitatory responses of the neurons were identified: responses of sensory type observed to each tactile stimulation of the limb and independent of the presence or absence of motion, and responses of motor type, which developed parallel with the motor response of the limb and were not observed in the absence of motion. The minimal latent period of the responses of motor type was equal to the latent period of the sensory responses to tactile stimulation (20±10 msec). Stimulation of the cortex through the recording microelectrode at the site of derivation of unit activity, which increased during active flexion of the forelimb at the elbow (11 stimuli at intervals of 2.5 msec, current not exceeding 25 µA), in 70% of cases evoked an electrical response in the flexor muscle of the elbow.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 9, No. 2, pp. 115–123, March–April, 1977.     

Responses of somatic cortical neurons during the conditioned placing reflex in cats

Unit activity was studied in areas 3 and 4 during the conditioned placing reflex in cats. Responses of somatic cortical neurons in this case were shown to develop comparatively late — 80–100 or, more often, 200–450 msec after the conditioned stimulus. In the motor cortex responses preceded movement by 50–550 msec, whereas in the somatosensory cortex they usually began simultaneously with or after the beginning of the movement. Judging from responses of somatic cortical neurons, the placing reflex is realized by the same neuronal mechanism as the corresponding voluntary movement. The differential stimulus and positive conditioned stimulus, after extinction of the conditioned placing reflex, evoked short-latency spike responses lasting 250–350 msec in the same neurons as took part in the reflex itself. In these types of internal inhibition, responses of the neurons were thus initially excitatory in character. Participation of the neurons in the conditioned placing reflex and its extinction, disinhibition, and differentiation, is the result of a change in the time course of excitatory processes and is evidently connected with differential changes in the efficiency of the various synaptic inputs of the neuron.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 4, pp. 392–401, July–August, 1982.