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 in the auditory cortex to stimulation of geniculocortical fibers

Extracellular and intracellular single unit responses of neurons of the auditory cortex to electrical stimulation of geniculocortical fibers (GCF) were recorded in experiments on cats immobilized with tubocurarine. The latent period of responses of 15% of neurons to GCF stimulation was 0.3–1.5 msec. It is postulated that they were excited anti-dromically. The latent period of spikes generated by neurons responding to GCF stimulation orthodromically varied from 1.6 to 12 msec. In 28.6% of neurons the latent period was 1.6–2.5 msec. It is postulated that these neurons were excited monosynaptically. Intracellular recording revealed primary IPSPs in response to GCF stimulation in 63.3% of neurons, a brief EPSP followed by a prolonged IPSP in 17.7%, an EPSP-spike-IPSP complex in 12.3%, and subthreshold EPSPs in 7% of neurons. The latent period of the primary IPSPs varied from 1.8 to 11 msec, being 1.8–3.7 in 72%, 3.8–5.7 in 20.0%, and 5.8–11 msec in 8.0% of neurons. The latent period of responses beginning with an EPSP was 1–4 msec (mean 1.8 msec). Orthodromic responses arising 3–10 msec after the antidromic response, and consisting of 3–5 spikes, were recorded in some antidromically excited neurons. Hypotheses regarding the functional organization of the auditory cortex and mechanisms of inhibition in its neurons are put forward on the basis of the results obtained.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 3, pp. 227–235, May–June, 1972.     

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.     

Reaction of neurons of the medial geniculate body to acoustic stimulation

Responses of medial geniculate body (MGB) neurons to pure tones and clicks were studied in acute experiments in immobilized cats, preliminary operations being performed under calypsol anaesthesia. MGB units were identified by their reactions to cortical zone AI and brachium of inferior colliculus stimulations. When tonal stimuli were applied relay neurons of pars principalis of MGB usually demonstrated either unimodal tuning curves with narrow frequency band or fragmental ones with several narrow bands. On-response with subsequent inhibition of the background activity or without such an inhibitory period was most frequent type of the reaction (66.6%) of relay MGB neurons to tonal stimulation. The group of relay neurons with the tonic type of reaction (9.1%) was classified for which the duration of tonic response depends on the duration of tonal stimulus. Change of the excitatory reaction to the inhibitory one when the characteristic tone frequency is changed by non-characteristic++ ones is supposed to be a mechanism supplying sharpness of tuning at relay MGB neurons. It is concluded that responses of acoustic cortical neurons to sound stimulation depend to a great extent on the pattern of impulsation that comes from MGB relay units.     

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.     

Specificity of neuronal responses evoked in the medial geniculate body by stimulation of its different main inputs:In vivo andin vitro investigations

Responses evoked in single neurons of the medial geniculate body (MGB) by electrical stimulation of auditory cortex and fibers of the brachium of the inferior colliculus (BIC) were investigated in vivo and in vitro. In vivo experiments were performed on cats anaesthetized by kalipsol. In vitro experiments were carried out on surviving slices of the rat brain using MGB intranuclear simulation. It has been found that the responses to cortical and nuclear local stimulations show similar peculiarity: an increase in stimulation rate is followed by potentiation and summation of slow EPSPs. At the same time, BIC stimulation evokes mainly fast EPSPs (both in vivo and in vitro) which are remarkably suppressed when rate and intensity of BIC stimulation increase. Distinct features of the MGB neuronal responses to activation of ascending pathways and corticofugal fibers are probably due to differences in anatomical structural among the tested inputs and in chemical mechanisms of the synaptic processes.Neirofiziologiya/Neurophysiology, Vol. 25, No. 3, pp. 203–211, May–June, 1993.     

Altered Neuronal Intrinsic Properties and Reduced Synaptic Transmission of the Rat's Medial Geniculate Body in Salicylate-Induced Tinnitus

Sodium salicylate (NaSal), an aspirin metabolite, can cause tinnitus in animals and human subjects. To explore neural mechanisms underlying salicylate-induced tinnitus, we examined effects of NaSal on neural activities of the medial geniculate body (MGB), an auditory thalamic nucleus that provides the primary and immediate inputs to the auditory cortex, by using the whole-cell patch-clamp recording technique in MGB slices. Rats treated with NaSal (350 mg/kg) showed tinnitus-like behavior as revealed by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. NaSal (1.4 mM) decreased the membrane input resistance, hyperpolarized the resting membrane potential, suppressed current-evoked firing, changed the action potential, and depressed rebound depolarization in MGB neurons. NaSal also reduced the excitatory and inhibitory postsynaptic response in the MGB evoked by stimulating the brachium of the inferior colliculus. Our results demonstrate that NaSal alters neuronal intrinsic properties and reduces the synaptic transmission of the MGB, which may cause abnormal thalamic outputs to the auditory cortex and contribute to NaSal-induced tinnitus.     

Responses of bulbar neurons to stimulation of locomotor and inhibitory sites in the cat brainstem

Bulbar locomotor and inhibitory sites were located in the pons of mesencephalic decerebellate cats. Rhythmic stimulation of locomotor sites through microelectrodes at the rate of 60 Hz elicited stepping movements in the forelimbs which were halted when the inhibitory sites were rhythmically stimulated. Neuronal response was elicited by single or paired stimulation of locomotor sites at the rate of 1.5 Hz or by applying a series of 2–4 stimuli spaced 2 msec apart to the inhibitory site. Medial neurons generated synaptic responses (postsynaptic potentials or action potentials) to stimulation of the inhibitory site twice as frequently as when the locomotor site was stimulated. Responses in lateral neurons, however, occurred twice as frequently to stimulation of the locomotor site, while IPSP were only observed half as often as EPSP in neurons of both groups. In neurons excited by stimulation of the locomotor site, stimulation of the inhibitory site did not normally produce IPSP. Possible mechanisms underlying the halt of locomotion occurring in response to stimulation of the inhibitory site are discussed.Information Transmission Institute, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 4, pp. 525–533, July–August, 1986.     

Functional properties distinguishing individual auditory cortical neurons

Responses of 246 auditory cortical neurons to paired and repetitive stimulation of geniculo-cortical fibers were studied in experiments on cats immobilized with tubocurarine. The refractory period (RP) varied from 1 to 200 msec in different neurons. For neurons excited antidromically it varied from 1 to 3 msec. Among neurons excited monosynaptically there were some with a short (1.3–6 msec), medium, (8–16 msec) or long (30–100 msec) refractory period. Most neurons excited polysynaptically had a RP of mean length. RPs 30–200 msec in length were due to inhibition arising in the neuron after conditioning stimulation. In some neurons, after a short (1.5–2.0 msec) initial period of refractoriness there was a temporary (for 2–3 msec) recovery of responsiveness, followed by another period of ineffectiveness of the testing stimulus lasting 30–100 msec. Barbiturates selectively inhibited long-latency unit responses in the auditory cortex and during their action the number of responding neurons with a mean RP decreased sharply. The results demonstrate functional heterogeneity of auditory cortical neurons responding to an incoming volley of afferent impulses.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 5, No. 3, pp. 236–245, May–June, 1973.     

Responses of medial geniculate body neurons to auditory cortical stimulation

Extracellular and intracellular unit responses of thepars principalis of the medial geniculate body to stimulation of the first (AI), second (AII), and third (AIII) auditory cortical areas were studied in cats immobilized with D-tubocurarine. In response to auditory cortical stimulation both antidromic (45–50%) and orthodromic (50–55%) responses occurred in the geniculate neurons. The latent period of the antidromic responses was 0.3–2.5 msec and of the orthodromic 2.0–18.0 msec. Late responses had a latent period of 30–200 msec. Of all neurons responding antidromically to stimulation of AII, 63% responded antidromically to stimulation of AI also, confirming the hypothesis that many of the same neurons of the medial geniculate body have projections into both auditory areas. Orthodromic responses of geniculate neurons consisted either of 1 or 2 spikes or of volleys of 8–12 spikes with a frequency of 300–600/sec. It is suggested that the volleys of spikes were discharges of inhibitory neurons. Intracellular responses were recorded in the form of antidromic spikes, EPSPs, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP, and primary IPSP. Over 50% of primary IPSP had a latent period of 2.0–4.0 msec. It is suggested that they arose through the participation of inhibitory interneurons located in the medial geniculate body.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 5–12, January–February, 1976.     

Effect of electrodermal stimulation on single unit responses to acoustic stimulation in the parvocellular part of the medial geniculate body

The effect of electrodermal stimulation of the contralateral forelimb on responses arising in neurons of the parvocellular part of the medial geniculate body (MGB) to clicks was studied in cats anesthetized with thiopental and immobilized with myorelaxin (suxamethonium). Neurons whose responses to clicks were inhibited by electrodermal stimulation were located in zones of the parvocellular part of MGB adjacent to the posterior ventral nucleus and magnocellular part of MGB. Electrodermal stimulation had no effect on unit responses in more lateral zones of the parvocellular part. Intracellularly recorded responses of most neurons to clicks were of the EPSP-spike-IPSP or EPSP-IPSP type, whereas those to electrodermal stimulation were of the IPSP type only. Inhibition of responses under the influence of electrodermal stimulation could arise both in the presence and absence of an IPSP in the neuron. The mechanisms of the inhibitory effect of electrodermal stimulation on responses arising in neurons of the parvocellular part of MGB to stimulation by clicks are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 2, pp. 175–181, March–April, 1980.     

Characteristics of cortical inhibitory neurons

Spikes were recorded extracellularly and IPSPs intracellularly from auditory cortical neurons of cats immobilized with D-tubocurarine in response to stimulation of geniculo-cortical fibers. Fibers whose stimulation induces IPSPs in auditory cortical neurons mainly have low thresholds. When two stimuli, each of which separately evoked an IPSP of maximal amplitude, were applied to them the shortest interval at which the second stimulus evoked an effect was 2.5–3 msec. This effect consisted of an increase in the duration of the integral IPSP, the amplitude of which either remained unchanged or increased under these circumstances by only 5–10%. The interval at which a separate IPSP appeared in response to the second stimulus depended on the duration of the ascending phase of the IPSP and varied from 4 to 22 msec for different neurons. The amplitude of the second IPSP in this case depended on the interval between stimuli. Under moderately deep pentobarbital anesthesia the number of neurons responding to stimulation of the geniculo-cortical fibers by spikes fell sharply but the number of neurons responding by primary IPSPs remained almost unchanged. Under very deep pentobarbital anesthesia, when spike responses of the cortical neurons completely disappeared, the IPSPs also were completely suppressed. It is concluded that inhibitory neurons of the auditory cortex are excited by thick low-threshold fibers, they have a short refractory period, and they are resistant to the narcotic action of pentobarbital.     

Synaptic potentials of digastric-muscle motoneurons

We studied the antidromic and synaptic potentials evoked from 32 digastric-muscle motoneurons by stimulation of the motor nerve to this muscle, different branches of the trigeminal nerve, and the mesencephalic trigeminal nucleus. Antidromic potentials appeared after 1.1 msec and lasted about 2.0 msec. Stimulation of the infraorbital, lingual, and inferior alveolar nerves led to development of excitatory postsynaptic potentials (EPSP) and action potentials in the motoneurons. The antidromically and synaptically evoked action potentials of the digastric-nerve motoneurons were characterized by weak after-effects. We were able to record EPSP and action potentials in two of the motoneurons investigated in response to stimulation of the mesencephalic trigeminal nucleus, the latent period being 1.3 msec. This indicates the existence of a polysynaptic connection between the mesencephalic-nucleus neurons and the digastric-muscle motoneurons. Eight digastric-muscle motoneurons exhibited inhibitory postsynaptic potentials (IPSP), which were evoked by activation of the afferent fibers of the antagonistic muscle (m. masseter). The data obtained indicate the presence of reciprocal relationships between the motoneurons of the antagonistic muscles that participate in the act of mastication.A. A. Bogomol'ts Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 52–57, January–February, 1971.     

Effect of pentobarbital on neuronal inhibitory response evoked in an isolated slab of cat auditory cortex by intracortical stimulation

Neuronal responses of an acutely isolated slab of auditory cortex (area AI) to intracortical electrical stimulation were studied intracellularly in cats anesthetized with pentobarbital. It was found that 77% of responses were primary IPSPs, and allowing for secondary inhibitory responses, an inhibitory response was observed in 92% of neurons. All types of neuronal responses in the slab were short-latency. The maximal response latency did not exceed 5 msec. Neurons responding to stimulation by IPSPs were found at all depths in the slab, with a maximum in layers II–III. Nearly all primary IPSPswere mono- and disynaptic. Pentobarbital increased the duration of individual neuronal inhibitory responses in the isolated slab of auditory cortex without affecting maximal duration of the IPSP. The mechanisms of the effect of pentobarbital on the amplitude and duration of IPSPs are discussed.I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 16, No. 2, pp. 147–152, March–April, 1984.     

Synaptic activation of thoracic spinal interneurons by reticulospinal pathways

Synaptic processes of 119 thoracic spinal interneurons (T10–11) were investigated in anesthetized cats in response to stimulation of the medial and central zones of the gigantocellular nucleus in the medulla and the ventral columns of the spinal cord. Fast (90–130 m/sec) reticulospinal fibers running in the ventral column were found to produce monosynaptic or disynaptic excitation of interneurons of Rexed's layers VII–VIII, which are connected monosynaptically with group I muscle afferents, and interneurons excited both by group I muscle afferents and low-threshold cutaneous afferents. In most neurons of layer IV, connected monosynaptically with low-threshold cutaneous afferents, and in neurons of layers VII–VIII excited by afferents of the flexor reflex no marked postsynaptic processes were observed during stimulation of the reticular formation. Excitatory, inhibitory, and mixed PS Ps during activation of reticulospinal fibers were found in 14 neurons, high-threshold afferents in which evoked predominantly polysynaptic IPSPs. Seventeen neurons activated monosynaptically by reticulospinal fibers and not responding to stimulation of segmental afferents were found in the medial part of the ventral horn (layers VII–VIII).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 566–578, November–December, 1972.     

Raphe Stimulation-Evoked Modulation of Postsynaptic Responses by Neurons of the Cat Somatosensory Cortex Activated by Stimulation of Nociceptors

We studied the effects of electrical stimulation of the raphe nuclei (RN) of the cat brain on postsynaptic potentials developing in somatosensory cortex neurons activated by nociceptive influences. Intracellular records were obtained from 15 cells, which were either selectively excited by stimulation of nociceptors (intense electrical stimulation of the dental pulp) or activated by both the above nociceptive and non-nociceptive (moderate stimulations of the infraorbital nerve or thalamic ventroposteromedial nucleus, VPMN) influences. In neurons of both groups, stimulation of both nociceptive afferents and the VPMN evoked complex responses (EPSP–AP–IPSP; IPSPs were 200 to 300 msec long). In some studied cortical neurons, isolated electrical stimulation of the RN (which caused the release of serotonin, 5-HT, in the cortex) resulted in relatively short-latency synaptic excitation, while inhibition was observed in other cells. In the case where stimulation of the RN was used as conditioning influence, such stimulation (independently of the kind of the initial response to RN stimulation) led to long-latency and long-lasting suppression of all components of the synaptic reactions evoked by excitation of nociceptors. The maximum of inhibition was observed at test intervals of 300 to 800 msec. The mechanisms underlying modulatory influences coming from the 5-HT-ergic brainstem system to neurons of the somatosensory cortex, which are activated by excitation of high-threshold (nociceptive) afferent inputs, are discussed.     

Intracellular responses of primary auditory cortical neurons to tones of different frequencies and to electrical stimulation of spiral ganglion fibers in cat

Experiments on cats anesthetized with pentobarbital showed that, depending on the intensity and frequency of acoustic stimulation, neurons in auditory area AI give responses of EPSP, EPSP-spike-IPSP, EPSP-IPSP, and IPSP type. Presentation of a tone of characteristic or near-characteristic frequency and above-threshold intensity, and also electrical stimulation of nerve fibers of the spiral ganglion, innervating the central zone of the receptive field of the neuron, evoke in most cases a response of EPSP-spike-IPSP type. Tone differing considerably in frequency from the characteristic, and electrical stimulation of peripheral zones of the receptive field, evoked responses of EPSP-IPSP or IPSP type. The range of frequencies of tones to which, at threshold intensity, an action potential is generated by the neuron is considerably narrower than the range of frequencies of tones evoking an EPSP and IPSP. Above the intensity of tone threshold IPSP is an invariable component of the response of most neurons in area AI. The appearance of an IPSP in the neuron is accompanied by depression of spontaneous activity and the neuronal response to testing stimulation. Two types of IPSP were distinguished: One type is a component of the EPSP-spike-IPSP response and arises during excitation of auditory receptors located in the central part of the receptive field of the neuron, the other arises during excitation of receptors located at the periphery of the field, and which project to neurons with other characteristic frequencies. The former arise after spike excitation of the neuron, the latter after EPSP or primarily.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 123–131, January–February, 1984.     

GABA-mediated synaptic inhibition of projection neurons in the antennal lobes of the sphinx moth,Manduca sexta

Responses of neurons in the antennal lobe (AL) of the moth Manduca sexta to stimulation of the ipsilateral antenna by odors consist of excitatory and inhibitory synaptic potentials. Stimulation of primary afferent fibers by electrical shock of the antennal nerve causes a characteristic IPSP-EPSP synaptic response in AL projection neurons. The IPSP in projection neurons reverses below the resting potential, is sensitive to changes in external and internal chloride concentration, and thus is apparently mediated by an increase in chloride conductance. The IPSP is reversibly blocked by 100 microM picrotoxin or bicuculline. Many AL neurons respond to application of GABA with a strong hyperpolarization and an inhibition of spontaneous spiking activity. GABA responses are associated with an increase in neuronal input conductance and a reversal potential below the resting potential. Application of GABA blocks inhibitory synaptic inputs and reduces or blocks excitatory inputs. EPSPs can be protected from depression by application of GABA. Muscimol, a GABA analog that mimics GABA responses at GABAA receptors but not at GABAB receptors in the vertebrate CNS, inhibits many AL neurons in the moth.     

Auditory evoked potentials in a patient with a unilateral lesion of the inferior colliculus and medial geniculate body

Brain-stem, middle latency and late auditory evoked potentials (BAEPs, MLAEPs and LAEPs, respectively) were recorded in a patient 2 months after removal of a tumor affecting the quadrigeminal plate. Simultaneously, MRI showed a left unilateral lesion involving the inferior colliculus, brachium colliculi and the medial geniculate body (MGB). On dichotic listening, there was complete extinction of the right ear input, without subjective auditory disturbance. BAEPs were abnormal after stimulation of the right ear alone. Wave V was delayed and reduced in amplitude, and the I–V interval was augmented. Above all, MLAEPs of both ears were very abnormal. The Pa and Na components over the left hemisphere were abolished (Pa) or very reduced in amplitude or abolished (Na) whereas both Pa and Na components over the right hemisphere were normal. LAEPs were asymmetrical, with reduced P1N1P2 complex over the left hemisphere and absence of polarity reversal over the mastoid. It has been demonstrated that a lesion affecting only the inferior colliculus and MGB unilaterally and not extending beyond the MGB can abolish Na and Pa ipsilaterally. Any discussion of Na and Pa sources should take into account the output of the MGB to the auditory radiations, the MGB, the brachium colliculi and the inferior colliculus.     

Responses of neurons in the vibrissae projection area of the cat somatosensory cortex to afferent activation

Responses of 375 primary somatosensory cortical neurons located in the projection area of the vibrissae to electrical stimulation of the infraorbital nerve and also to adequate stimulation of the vibrissae were investigated in unanesthetized cats immobilized with tubocurarine. Stimulation of the nerve and vibrissae most frequently evoked synaptic responses in the neurons, in the form of a short EPSP followed by an IPSP or, less frequently, as a primary IPSP; during extracellular recordings corresponding changes were observed in spike activity. In response to stimulation of the vibrissae, initial inhibition was found more often than to stimulation of the nerve (in 45 and 16% of neurons respectively). The difference between the minimal values of latent periods of IPSP and EPSP evoked by stimulation of the infraorbital nerve was 0.8 msec in different neurons, and the difference between the mean values 1.4 msec. Directional sensitivity of the cortical neurons was demonstrated (to a change in the direction of deflection of the vibrissae). Neurons located close together could differ in the character of their directional sensitivity during stimulation of the same vibrissae. It is concluded that short-latency inhibition arising in the primary projection area of the cat somatosensory cortex is predominantly afferent and not recurrent. The probable mechanisms of directional sensitivity of the neurons studied are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSSR, Kiev. Translated from Neirofiziologia, Vol. 11, No. 6, pp. 550–559, November, 1979.