Habituation in the somatosensory cortex

Responses of neurons without spontaneous activity ("silent") to prolonged stimulation of the mesenteric nerves were studied in cats anesthetized with chloralose (65–70 mg/kg) and immobilized with flaxedil. The results showed that neurons of the association and primary projection areas are characterized by habituation to prolonged stimulation of visceral nerves. The rate of development of habituation depends on the parameters of stimulation and on the cortical region studied. Habituation developed more rapidly in the association area and was slower to develop at the focus of maximal activity of the second somatosensory area. The special features of cortical habituation during visceral stimulation are discussed.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 11, No. 5, pp. 412–418, September–October, 1979.     

Electrophysiological analysis of the functional organization of cortico-cerebellar connections

The effect of stimulation of cortical association (orbito-frontal, parietal) and projection (auditory, sensomotor) areas on the activity of Purkinje neurons of the cerebellar cortex was studied in adult cats anesthetized with pentobarbital, with or without chloralose. These responses were compared with those to peripheral stimuli. Definite similarity was found between the responses of Purkinje cells to different cortical (association and projection) stimuli as regards both the types of responses of the neurons and their ability to respond. No similarity was observed in the responses of Purkinje cells to peripheral (visual, auditory, electrodermal) stimulation. Whereas almost identical numbers of neurons (over 50%) were excited in response to the different forms of cortical stimulation, the ability of the neurons to respond to peripheral stimuli differed considerably: 44.6% of neurons responded to electrodermal stimulation, 34.2% to auditory, and 18.8% to visual.Medical Institute, Kemerovo. Translated from Neirofiziologiya, Vol. 8, No. 5, pp. 483–489, September–October, 1976.     

Effect of stimulus intensity on unit responses in the cat frontal cortex

Unit activity of the frontal cortex during changes in stimulus intensity in the near-threshold range (15–16 dB above the threshold for the combined evoked potential) was investigated by an extracellular recording method in acute experiments on cats anesthetized with chloralose (70 mg/kg). Comparative analysis of unit responses in specific (SI) and nonspecific projection areas revealed basically similar changes in pattern during an increase in stimulus intensity: A decrease in the latent period, an increase in the total frequency and the phasic character of the discharge, and an increase in the probability of response. However, a relatively stable latent period and probability of response were observed in specific projection neurons for a stimulus intensity of 3–5 threshold units, whereas for the nonspecific projection neurons it was observed for a stimulus intensity of 10–15 threshold units. All sensory projections in the frontal cortex are formed by two inputs: short-latency low-threshold and long-latency high-threshold. Analysis of modality-dependent differences in the threshold of sensitivity and the latent period of response of the polysensory neurons suggests that stimuli of different modalities converge directly on cortical neurons.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 8, No. 6, pp. 606–612, November–December, 1976.     

Comparison of spontaneous cortical unit activity in several brain areas of unanesthetized cats

Spontaneous unit activity in association area 5 and some projection areas of the cortex (first somatosensory, first and second auditory areas) were studied in cats immobilized with D-tubocurarine in which the index of specific spontaneous activity, the mean frequency, types of spontaneous activity, and statistical parameters — distribution of interspike intervals and autocorrelation function — were determined. The results showed that spontaneous unit activity in the association area differs from that in the projection areas in both intensity and character. A special feature of the spontaneous activity of the auditory areas was a well-marked volley distribution of activity. In the somatosensory area the level of spontaneous activity as reflected in all indices was the lowest. In the association cortex the largest number of neurons with spontaneous activity lay at a depth of 500–1000 µ corresponding to cortical layers III–IV. In the first auditory area neurons with spontaneous activity were concentrated at a depth of 1400 µ (layer V) and in the somatosensory area at a depth of 1000–1400 µ (alyers IV–V). The possible functional significance of these differences is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 13–21, January–February, 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.     

Some characteristics of unit activity in the frontal cortex of alert monkeys

Spontaneous activity of frontal cortical neurons (middle part of sulcus principalis) and their responses to stimuli of different biological significance were studied in alert monkeys. The region studied is characterized by a lower level of spontaneous activity than the motor cortex. Fluctuations in the level of spontaneous activity are connected with changes in the experimental situation. Two types of unit responses were distinguished to repetitive stimulation: preservation of responses and habituation. The type of unit response depends on the relationship of the stimuli to the animal's motivational sphere.A. A. Ukhtomskii Physiological Research Institute, A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 12, No. 1, pp. 3–12, January–February, 1980.     

Organization of afferent inputs of the parietal association (area 7) and Clare-Bishop areas of the cat neocortex

Unit activity was recorded from two parietal areas of the cat neocortex in semichronic experiments. Cell responses to presentation of adequate stimuli of different modalities and to direct electrical stimulation of various cortical zones were studied. About 4% of neurons of the Clare-Bishop area did not respond to visual stimulation. Cells responding to stimuli of different modalities were found in the Clare-Bishop area. A high percentage of cells in this area responded to direct electrical stimulation of area 17. In the association area (area 7) 27% of neurons tested responded to visual stimuli, but only a very small relative number of cells (compared with responding neurons of the Clare-Bishop area) responded to stimulation of the primary sensory areas. Electrical stimulation of area 7 inhibited evoked and spontaneous unit activity in the Clare-Bishop area. The hypothesis that these areas are the association representation of two different sections of the visual system — retino-geniculocortical and retino-tecto-thalamocortical — is discussed.Institute of Experimental Medicine, Academy of Medical Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 612–620, November–December, 1981.     

Visual thalamic afferents to area 29 of the rat limbic cortex

Afferent connections of the retrosplenial area of the rat limbic cortex were investigated by the retrograde horseradish peroxidase axon transport method. After injection of horseradish peroxidase (HRP) into area 29 of the cortex, HRP-labeled cells were found in the dorsal part of the lateral geniculate body and the posterolateral, pretectal, and anterior dorsal thalamic nuclei. Connections were found between cortical area 29 and visual projection areas (areas 17 and 18a) and with area 29 on the contralateral side of the brain. The results are evidence that all the principal visual structures of the thalamus and the visual cortical projection area form direct projections to the retrosplenial cortex.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 14, No. 2, pp. 135–139, March–April, 1982.     

A chronic experimental study to compare the electrocorticogram of certain brain areas in the drowsy cat

The spontaneous electrocorticogram of the first somatosensory (area 53), first auditory (area 22), visual (area 17), association (area 5), and second auditory (area 52) projection areas was studied in chronic experiments on drowsy unanesthetized cats. In the intermediate periods between waking and sleep and vice versa, maximal differences expressed as heterogeneity of the spindle component and low coefficients of cross correlation (0.21±0.04) were found in the slow-wave activity of the areas studied. During deepening of sleep the synchronization of the potentials recorded from the different areas increased and the coefficients of cross correlation rose to 0.40±0.18. The level of differences between the coefficients of correlation compared in the different phases of sleep was 0.19 and was significant by Student's criterion. It is postulated that functional changes in the thalamic pacemaker mechanisms take place during changes in the depth of sleep. In response to afferent stimulation of different modalities no differences were found in the desynchronization response in the projection areas. After extinction of the orienting reflexes and application of stimulation of adequate strength, specific differences in the development of the desynchronization response to adequate stimulation were found in the projection zones. The need for certain conditions being present before signs of specificity of the responses in the projection areas can be detected is emphasized.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 6, pp. 583–596, November–December, 1976.     

Spatial principle of organization of specific afferent projections and generation of evoked potentials in the somatosensory cortex

Here we investigate the functional organization of structures involved in sensory analysis in a restricted region of a cortical projection area. We have shown that stimulation of somatosensory areas I and II (SI and SII) may block an afferent volley at the level of the thalamic relay nucleus, and that SII may be selectively blocked by stimulation of SI. Also definite somatosensory connections have been demonstrated between SII, SI, and the motor cortex. We suggest that common mechanisms underlie the generation of focal reactions in projection areas of the cortex induced by stimulation of various structures. The properties of two groups of neurones from area SII are described: those having a short latency and receiving direct projections from the thalamic relay nucleus, and those of long latent period with a well-marked convergence, and reacting to stimulation of various afferent pathways. It is suggested that each path to a local point of a cortical projection areas terminates with its relay element. The signal is then directed to a common intracortical system of neurones where signals from various sources occurs (afferent, interhemispherical, subcortico-cortical, and intracortical) converge and interact. All groups of neurones are involved in the formation of the common components of evoked potentials.Presented to the All-Union Symposium: "Electrical responses of the cerebral cortex to afferent stimuli," Kiev, October, 1969.Institute of Normal and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 2, No. 2, pp. 155–165, March–April, 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.     

Habituation of postsynaptic unit responses of the cat motor cortex to stimuli of different modalities

Habituation (extinction) of postsynaptic unit responses of the cat motor cortex to repetitive electrodermal, photic, acoustic, and combined bimodal stimulation was investigated by intracellular recording. Habituation was shown by a decrease in the number of spikes per grouped discharge and a decrease in the amplitude and duration of the EPSPs, and sometimes IPSPs, on repetition of the stimulus. The way in which the course of habituation depends on the modality and duration of stimulation (at a constant frequency of 1/sec) is examined. Habituation of postsynaptic responses to sensory stimuli is observed with neurons of different functional groups, namely identified neurons of pyramidal tract and unidentified neurons, some of which were evidently pyramidal neurons and interneurons. The hypothesis is put forward that the habituation of PSPs of the cortical neurons is based on processes taking place mainly at the subcortical level.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 4, No. 5, pp. 545–553, September–October, 1972.     

Habituation in the rabbit visual cortex during development and after septal ablation

In chronic experiments on adult rabbits with injury to the septum pellucidum and on newborn rabbits the dynamics of evoked responses to repeated flashes was studied. Experimental blocking of the hippocampal theta-rhythm by electrolytic destruction of the medial septum is followed by marked facilitation of habituation of the primary responses. This process takes place twice as fast in rabbits with damage to the septum as in intact animals. Very rapid habituation of the primary responses was observed in newborn rabbits before acquiring vision and in rabbits with vision but under the age of 15–16 days, after 10–20 presentations of the stimulus. When the animals reached this age the time required for habituation increased sharply. Starting from the age of 17 days, the primary responses of the animals were just as stable as in normal adult rabbits. The significance of the hippocampal theta-rhythm as a factor preventing the development of habituation is discussed.Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 4, No. 2, pp. 123–129, March–April, 1972.     

Comparative electrophysiological characteristics of afferent representation in the cortical and striatal divisions of the turtle forebrain

In experiments on immobilized, lightly anesthetized turtles the presence of visual and somatic representation was established in the subcortical striatal division of the forebrain — the pallial thickening, the dorsal ventricular ridge, and the putamen. In their physiological characteristics they are similar to the corresponding representation in the general cortex. The absence of significant differences between the latent periods of cortical and striatal evoked potentials to flashes and to stimulation of the dorsal thalamus indicates that visual projection fibers (from the lateral geniculate body) terminate at both cortical and striatal levels. Differences in the distribution of latent periods of unit responses in the cortex to visual and thalamic stimulation are due to the presence of a rotundo-telencephalic visual channel, with direct connections with the striatal and polysynaptic connections with the general cortex, as well as the geniculo-telencephalic tract. Considerable differences between the latent periods of the evoked potentials and also between unit responses to electrodermal stimulation in the cortical and striatal structures indicate that somatic projection fibers relay in the striatum on their path to the general cortex. Consequently, the somatosensory system of turtles is less corticalized than the visual system. Comparison of the results described with those obtained by workers studying other vertebrates suggests that the afferent supply of the striatum may be reorganized in the transition from premammals to mammals.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 7, No. 2, pp. 184–193, March–April, 1973.     

Characteristics and peculiarities of cortical evoked potentials following stimulation of visceral afferent systems

A detailed characterization is given of the evoked potential of the cat cortex following stimulation of various visceral nerves. It is shown that the visceral analyzer has two kinds of projections: strictly local and diffuse. Its stimulation evokes in the cortex two types of potentials — primary and secondary. Primary responses (PR) of the projection zone show where the signal is addressed, indicating only that the focus of excitation arose at the periphery. Therefore, various attempts to increase the information value of PR are not prospective. The information value of secondary responses (SR) are apparently important for understanding cortical processes developing in it after the occurrence of PR. These reactions are diverse and appear at the same point of the cortex following stimulation of different modalities; at the same time their distribution is limited to four cortical areas: association, limbic, motor, and orbital. The convergence of sensory inputs observed in secondary regions probably reflects elementary integration accomplished in them.The material was reported at the All-Union symposium "Electrical Responses of the Cortex to Afferent Stimulation," Kiev. October, 1969.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 2, No. 2, pp. 113–125, March–April, 1970.     

Connections of thalamic nucleus lateralis posterior with suprasylvian cortex in cats

Cats were immobilized with D-tubocurarine. Responses of 231 neurons of the thalamic nucleus lateralis posterior to cortical stimulation in areas 5b and 21 of the suprasylvian gyrus were studied. Responses of 34 neurons were antidromic, indicating the existence of a direct projection of this nucleus to the cortical areas studied. This projection was most extensive in area 5b. The long latencies (up to 60 msec) of the antidromic responses of some neurons indicate that axons of certain neurons of thalamic nucleus lateralis posterior conduct excitation very slowly (0.3 m/sec). Orthodromic responses with latencies of 2–3 msec to cortical stimulation point to the presence of direct pathways from cortex to nucleus. The flow of afferent impulses into the nucleus from area 5b is stronger than from area 21. Convergence of impulses from these areas was observed on 44% of neurons of the nucleus. Cortical stimulation of areas 5b and 21 evoked postsynaptic inhibition in most neurons of the nucleus. It is concluded that two-way direct connections exist between nucleus lateralis posterior of the thalamus and the suprasylvian cortex.     

Mechanisms of interaction between the parietal association and projection areas of the cat neocortex

Changes in evoked potentials in the first visual (VI), first somatic (SI), and parietal areas of the cortex during local cooling of each area were investigated under pentobarbital anesthesia. Two types of interaction were distinguished. Type I interaction was found in all areas in the early stages of local cooling and was reflected in a similar decrease in amplitude of evoked potentials in intact parts of the cortex. In the thalamic association nuclei — the pulvinar and posterolateral nucleus — somatic evoked potentials were unchanged but visual were transformed differently from those in the cortex. Type IIinteraction was found in the later stages of cooling and only between the association area and each of the projection areas. It was reflected in a greater change in amplitude of the evoked potentials and also in their configuration. In response to somatic stimulation in the early stage of type II interaction transformation of evoked potentials in the cortex took place sooner than in the nuclei; in the later stage it took place immediately after transformation of the "subcortical" evoked potentials. In response to photic stimulation transformations of cortical evoked potentials were always preceded by the corresponding transformations in the nuclei. It is suggested that type I interaction is formed by intercortical connections and type II by direct and subcortical relay connections. Differences in the role of the association area in interaction of types I and II when activated by stimuli of different modalities are discussed.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 573–581, November–December, 1978.     

Physiological mechanisms of habituation of visual evoked potentials

Experiments on cats showed that complete operative exclusion of the reticular formation by precollicular section prevents the development of habituation of evoked potentials in the primary visual projection area and lateral geniculate body. Similar results were obtained after postcollicular section of the mesencephalon. The phenomenon of habituation of visual evoked potentials is found in posttrigeminal preparations. It is postulated that the tonic inhibitory division of the reticular formation plays an important role in the development of the habituation phenomenon.Odessa State Medical Institute. Translated from Neirofiziologiya, Vol. 4, No. 5, pp. 540–544, September–October, 1972.     

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.     

Dynamics of single unit responses of the rabbit visual cortex to repetitive photic stimulation

Single unit responses in the visual cortex of the waking rabbit to repetitive photic stimulation at a frequency of once every 2.5 sec were studied. Depending on the total number of spikes in the response, the dynamics of the responses could be divided into two types: "fast" and "slow." From 5 to 15 stimuli were required to establish a stable level of response with changes of the first type, but 50 to 100 stimuli were needed for the response with changes of the second type. About 50% of all neurons did not change the characteristics of response. In the group of neurons with changing responses, partial habituation was found in 55–59% of cells; there were 25% of neurons with sensitization of discharge and 17–20% with a humpbacked type of response dynamics. A "slow" dynamics of unit responses in most cases was accompanied by changes in the duration of inhibitory pauses in the response; negative correlation of these values was observed in 65% of neurons.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 10, No. 5, pp. 451–459, September–October, 1978.