Dynamics of single unit activity in the association cortex of waking cats during defensive conditioning

Responses of neurons in association area 5 during defensive conditioning to acoustic stimulation were studied in chronic experiments on cats. As a rule the neurons responded by excitation to presentation of conditioned and unconditioned stimuli. During the conditioned reflex unit responses usually appeared in the first 50 msec after the beginning of acoustic stimulation, i.e., they were connected with the action of the conditioned stimulus and not with manifestations of conditioned-reflex motion. The most significant changes in responses of cortical association units were observed in the initial period of conditioning. During stabilization of the conditioned reflex, responses of some neurons became stabilized, whereas in other neurons the spontaneous activity and intensity of responses increased, and in a third group the response to one of the stimuli disappeared. This last result indicates a switch during conditioning from polysensory unit responses to monosensory specialized responses. Extinctive inhibition was found to consist of a gradual decrease in the level of the spike discharge and its approximation to spontaneous activity, i.e., to be passive in character.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 563–572, November–December, 1978.     

Analysis of spike discharge of a neuron population in the cat motor cortex during a conditioned change of posture reflex

Spike responses of area 4 neurons in the projection area of the contralateral forelimb to acoustic stimulation (1 sec), which became the conditioned stimulus after training, and to dropping of the platform beneath the test limb, which served as reinforcing stimulus, were studied in trained and untrained cats. Responses only of those neurons which were activated during a passive movement caused by dropping of the platform were studied. In trained animals the number of these neurons which responded to the conditioned stimulus if a reflex occurred was 100%, and in the absence of conditioned-reflex movements to the conditioned stimulus it was 70%, much greater than the number of neurons responding to the same acoustic stimulus in untrained animals (45%). On peristimulus histograms of responses of the test neuron population in untrained and trained animals to acoustic stimulation (in the absence of movements) only the initial spike response with a latent period of under 50 msec and a duration of up to 100 msec could be clearly distinguished. In the presence of reflex movement multicomponent spike responses were observed: an initial spike response and early and late after-responses linked with performance of conditioned-reflex limb flexion. Early after-responses 100–200 msec in duration, appearing after a latent period of 100–150 msec, were linked to the time of application of the conditioned stimulus, whereas the appearance and duration of late after-responses were determined by the time of onset of conditioned-reflex movement. The magnitude of the neuronal response to reinforcement in trained animals does not depend on the appearance of the conditioned movement.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 17, No. 1, pp. 93–102, January–February, 1985.     

Involvement of mesencephalic reticular formation neurons in cat conditioned reflexes

Traditional defensive and operant food reflexes were used to investigate neuronal responses of the mesencephalic reticular formation. It was found that these neurons may be divided into different groups according to function, depending on how they respond to positive conditioning stimuli. Of the two main groups of neurons with sustained tonic reactions one is activated in response to positive acoustic conditioning stimulation; it no longer reacts to the same stimulus after extinction of the reflex, while the other only becomes involved in response to positive stimulation accompanying the initiation of movement. Neurons belonging to the second group begin to respond directly to acoustic stimulation after extinction of the conditioned reflex. Neurons of the mesencephalic reticular formation can thus exercise additional tonic ascending effects both in the production and inner inhibition of the conditioned reflex. The group of neurons with a phasic reaction, i.e., a double response (a direct response to sound and another produced by movement) displayed a drop in spontaneous activity during the shaping of inhibition of differentiation and of extinction in particular. It was found that the initial changes in the spike response of reticular formation neurons during conditioning and pseudo-conditioning are similar. There are thus grounds for stating that neurons of the mesencephalic reticular formation participate in the shaping, production, and inner inhibition of traditional and operant conditioned reflexes in a differentiated capacity rather than as a population reacting identically.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 2, pp. 161–171, March–April, 1986.     

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.     

Responses of limbic cortical neurons during instrumental conditioned reflexes in cats

Unit activity in cortical areas 24 and 32 was studied during conditioned placing reflex formation in cats. Neuronal responses in the limbic cortex of trained animals correlated with acoustic stimulation, the motor response, and also with the presentation of food reinforcement. In untrained animals 16% of neurons responded to acoustic stimulation. After training the number of neurons responding to sound in area 32 increased to 51.3%. Of the total number of neurons, 34.6% responded by initial excitation and 26.7% by inhibition of spike activity. The latent period of these responses was about 50 msec and their duration up to 200 msec. Similar but weaker responses were observed in area 24. Short-latency activation responses to conditioned and differential stimulation were similar in character. It is suggested that after training processes taking place in the limbic cortex may contribute to better perception of both conditioned and differential acoustic stimuli, irrespective of their functional significance.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 2, pp. 201–208, March–April, 1984.     

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.     

Neuronal responses in the limbic cortex of awake cats during defensive conditioning

Spike activity was investigated in limbic cortex neurons during defensive conditioning to acoustic stimulation in chronic experiments on cats. A relationship was found between the numbers of neurons responding, their contribution to formation of a temporal connection, and the duration of the acoustic stimulus. Phasic responses of 50–500 msec duration with latencies of 15–50 msec were observed for the most part. Intensive spike response with a minimum latency of 15 msec and a duration of between 200 msec and 2.5 sec evolved in most cells (95.1% in field 24 and 83% in field 32) in response to electrical stimulation. Response to acoustic stimulation rose during defensive conditioning in 33.3% cells and declined and finally disappeared in 13.3%, but response at the site where reinforcement was abolished was reproduced in all these cells. It was thus found that the numbers of limbic cortex neurons responding to sound not only fails to increase but actually decreases after training. The limbic cortex is thought to play its most active part in conditioning response to a recognized signal during the period preceding the awaited painful reinforcement.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 660–669, September–October, 1986.     

Unit responses of the somatosensory cortex of the waking cat to stimulation of the ventro-posterolateral thalamic nucleus

Unit responses in the primary somatosensory projection cortex to stimulation of the ventro-posterolateral thalamic nucleus were investigated by extra- and intracellular recording in chronic experiments on cats. Short-latency spike responses of 71.3% of recorded neurons appeared after not more than 4 msec. It is concluded that activation of neurons in this area of the cortex is chiefly monosynaptic and disynaptic. Besides participating in the initial response to the stimulus, one-quarter of the neurons generated after-discharges 120–314 msec later. These after-discharges are based on rebound after IPSPs and additional synaptic activation. Initial inhibition may appear 1.5 msec after stimulation of the ventro-posterolateral nucleus, evidence against the participation of recurrent collaterals in the formation of these IPSPs.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 5, No. 4, pp. 348–354, July–August, 1973.     

Characteristics of unit responses of the cat cerebellar cortex to acoustic stimuli

Investigation of unit responses of the cerebellar cortex (lobules VI–VII of the vermis) to acoustic stimulation showed that the great majority of neurons responded by a discharge of one spike or a group of spikes with a latent period of 10–40 msec and with a low fluctuation value. Neurons identified as Purkinje cells responded to sound either by inhibition of spontaneous activity or by a "climbing fiber response" with a latent period of 40–60 msec and with a high fluctuation value. In 4 of 80 neurons a prolonged (lasting about 1 sec or more), variable response with a latent period of 225–580 msec was observed. The minimal thresholds of unit responses to acoustic stimuli were distributed within the range from –7 to 77 dB, with a mode from 20 to 50 dB. All the characteristics of the cerebellar unit responses studied were independent of the intensity, duration, and frequency of the sound, like neurons of short-latency type in the inferior colliculi. In certain properties — firing pattern, latent period, and threshold of response — the cerebellar neurons resemble neurons of higher levels of the auditory system: the medial geniculate body and auditory cortex.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 5, No. 1, pp. 3–12, January–February, 1973.     

Unit responses of the first and second somatosensory areas to stimulation of the ventroposterior thalamic nucleus

Single unit responses of the first (SI) and second (SII) somatosensory areas to stimulation of the ventroposterior thalamic nucleus (VP) were investigated in cats immobilized with D-tubocurarine. In response to VP stimulation 12.0% of reacting SI neurons and 9.5% of SII neurons generated an antidromic spike. In most antidromic responses of both SI and SII neurons the latent period did not exceed 1.0 msec. The minimal latent period of spike potentials during orthodromic excitation was 1.5 msec in SI and 1.7 msec in SII. Neurons with an orthodromic spike latency of not more than 3.0 msec were more numerous in SI than those with a latency of 3.1–4.5 msec. The ratio between the numbers of neurons of these two groups in SII was the opposite. In SII there were many more neurons with a latency of 5.6–8.0 msec than in SI. EPSPs appeared after a latent period of 1.1–9.0 msec in SI and of 1.4–6.6 msec in SII. The latent period of IPSPs was 1.5–6.8 msec in SI and 2.2–9.4 msec in SII. The relative importance of different pathways for excitatory and inhibitory influences of VP on SI and SII neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 2, pp. 115–121, March–April, 1976.     

Responses of cat cerebellar cortical neurons to stimulation of the caudate nucleus,globus pallidus,and substantia nigra

Stimulation of the head of the caudate nucleus in cats anesthetized with chloralose and pentobarbital evoked spike responses of the Purkinje cells and other cerebellar cortical neurons in the paramedian lobes, lobulus simplex, and the tuber of the vermis. Phasic responses in the form of simple discharges (on account of activation of the neurons through mossy fibers) appeared mainly after a latent period of 5–12 and 14–20 msec; the latent period of responses consisting of complex discharges (on account of activation of Purkinje cells through climbing fibers) was 5–6, 9–22 msec, or more. Depending on the latent period, the spike responses differed in their rhythm of generation. In response to stimulation of the caudate nucleus with a frequency of 4–6/sec recruiting responses were found. An inhibitory pause was an invariable component of the tonic responses. During stimulation of the globus pallidus responses of the same types (phasic and tonic) appeared as during stimulation of the caudate nucleus, but they differed in the distribution of the neurons by latent period of spike responses. The minimal latent period was 4 msec. Recruiting also was observed during repetitive stimulation of the globus pallidus. During stimulation of the substantia nigra Pukinje cells activated by climbing fibers responded. Evoked complex discharges appeared after a stable latent period of 8.5±0.3 msec. Arguments are put forward regarding the role of the substantia nigra, the globus pallidus, nuclei of the inferior olive, and also the thalamic nuclei in the mechanism of caudato-cerebellar oligosynaptic and polysynaptic connections.N. I. Pirogov Medical Institute, Vinnitsa. Translated from Neirofiziologiya, Vol. 10, No. 4, pp. 375–384, July–August, 1978.     

Neuronal responses in area 7 of the cat cortex during defensive conditioning

Responses of neurons in area 7 of the parietal association cortex during and after formation of a defensive conditioned reflex to sound were recorded in waking cats. Changes in spike responses of the neurons as a result of the onset of conditioned reflex limb movements were observed in 68% of neurons. Spike responses of neurons formed as a result of learning appeared only if conditioned-reflex limb movements appeared, and they were not observed if, for some reason or other, movements were absent after presentation of the positive conditioned stimulus or on extinction of the reflex. Responses of 46% neurons to conditioned stimulation preceded the conditioned-reflex motor responses by 50–450 msec. The remaining responding neurons were recruited into the response after the beginning of movement. Characteristic spike responses of neurons to the conditioned stimulus appeared 500–900 msec before the beginning of movement and, in the case of appearance of special, "prolonged" motor responses of limb withdrawal, evoked by subsequent reinforcing stimulation.     

The neuronal organization of a focus of conditioned excitation in the cat sensory-motor cortex]

The activity of single units in the focus of conditioned excitation was studied during defensive conditioning to direct electrical stimulation of the cat sensorimotor cortex. Reorganizations of spike activity set in during the period of reflex elaboration, were manifest in the increased number of excited neurones and those which respond both to the conditioned and unconditioned stimuli. In the course of elaboration, the inhibitory phase of unit responses to direct electrical stimulation of the sensorimotor cortex was reduced, while the frequency of background unit spike activity was enhanced. Acute extinction of the reflex restored the initial duration of the inhibitory phase and reduced the frequency of the background activity.     

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.     

Analysis of evoked discharges in hypothalamic neurons to somatosensory,acoustic, and photic stimulation

Single unit activity in the supramammillary, mammillary, and anterior hypothalamic areas in response to acoustic, photic, and sciatic nerve stimulation was recorded in cats anesthetized with chloralose and immobilized with succinylcholine. In response to sensory stimulation the spontaneous firing rate was increased or decreased, and silent neurons were activated. Evoked potentials of the silent neurons had the shortest latent period to acoustic and somatosensory stimulation (15 msec), and rather longer to photic stimulation (30 msec); in some cases their latent period was 200 msec. Histograms of interspike interval distribution showed a maximum for intervals of up to 50 msec. Histograms of spike distribution relative to the beginning of stimulation showed maximal density between 100 and 200 msec. A high degree of convergence of excitation was found on units of the anterior as well as the posterior hypothalamus. Unit responses in the hypothalamus to sensory stimuli of all three modalities are regarded as being of secondary, nonspecific type.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 592–598, November–December, 1971.     

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

Neuronal responses in an isolated slab (area AI) to intracortical pulsed electrical stimulation at the level of layer IV were investigated extracellularly in acute experiments on cats immobilized with D-tubocurarine. Responding neurons were found in all layers of the slab. The character of their distribution by depth in the slab depended on the distance between recording and stimulating electrodes. The latent period of responses of different neurons ranged from 0.8 to 25 msec. With interelectrode distances of 0.5–2 mm most neurons responded mono- and disynaptically. However, responses of many neurons had a latent period of over 4 msec, i.e., they were polysynaptic. This indicates the complex character of interneuronal interactions, even in a limited area of the cortex. After intracortical stimulation no after-discharges with a latent period of over 40 msec could be recorded in the isolated slab of auditory cortex.I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 14, No. 1, pp. 85–93, January–February, 1982.     

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.     

Unit responses of the auditory cortex of waking cats at rest and after defensive conditioning

In chronic experiments with glass microelectrodes responses of 288 spontaneously active neurons in the auditory cortex were investigated in cats at rest (123 neurons) and after defensive conditioning to sound (165 neurons). In the first situation 43% of neurons did not respond to acoustic stimulation. Most (about 60%) responses of the reacting neurons showed marked inhibition. Conditioning caused an increase (up to 72%) in the number of neurons responding to acoustic stimulation, the appearance of tonic responses, a severalfold increase in the amplitude of the responses, an increase in the number of responses of activation type, and stabilization of their form. The results point to increased excitability of neurons in this cortical area.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 1, pp. 25–34, January–February, 1979.     

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.     

Responses of units in the magnocellular part of the medial geniculate body to acoustic and somatosensory stimulation

Responses of 150 neurons in the magnocellular part of the medial geniculate body to clicks and to electrodermal stimulation of the contralateral forelimb were investigated in cats immobilized with myorelaxin. Of the total number of neurons 65% were bimodal, 16.6% responded only to clicks, and 15.4% only to electrodermal stimulation. The unitary responses were excitatory (spike potentials) and inhibitory (inhibition of spontaneous activity). Responses beginning with excitation occurred more frequently to stimulation by clicks than to electrodermal stimulation, whereas initial inhibition occurred more often to electrodermal stimulation. The latent period of the initial spike potentials in response to clicks and to electrodermal stimulation was 5–27 and 6–33 (mean 11.6 and 16.2) msec respectively. Positive correlation was found between the latent periods of spike potentials recorded in the same neurons in response to clicks and to electrodermal stimulation, and also to electrodermal stimulation and to stimulation of the dorsal funiculus of the spinal cord. It is concluded that the magnocellular division of the medial genicculate body is a transitional structure between the posterior ventral nucleus and the parvocellular division of the medial geniculate body, and that in addition, it is connected more closely with the auditory than with the somatosensory system. It is suggested that the somatosensory input into the magnocellular division of the medial geniculate body is formed mainly by fibers of the medial lemniscus.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 133–141, March–April, 1978.