Striatal input from the ventrobasal complex of the rat thalamus

We have analyzed whether caudal regions of the caudate putamen receive direct projections from thalamic sensory relay nuclei such as the ventrobasal complex. To this aim, the delivery of the retrograde neuroanatomical tracer Fluoro-Gold into the caudal caudate putamen resulted in the appearance of retrogradely labeled neurons in the ventral posteromedial and ventral posterolateral thalamic nuclei. These projections were further confirmed with injections of the anterograde tracers biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into these thalamic nuclei, by showing the existence of axonal terminal fields located in the caudal striatum. These results support the existence of direct projections linking the thalamic ventrobasal complex and the caudal striatum in the rat, probably via collateralization of thalamocortical axons when passing through the caudate putamen, and therefore supporting the putative involvement of the caudal striatum in sensory-related functions.     

Role of the mediodorsal nucleus of the thalamus in the function of generalization in the cat

In chronic experiments on cats contribution of the thalamic mediodorsal nucleus to the organization of the processes of generalization and abstraction was studied by means of alternation method. Electrolytic ablation of the nucleus retarded the learning and impeded the initial stages of formation of the generalization function. Mechanisms of thalamic dementia are discussed.     

The significance of the thalamic parafascicular complex in tracking processes in cats

In conditions of free behaviour, trace processes of various complexity degrees (conditioned reflexes to time, temporal prognostication and delayed behaviour) were studied in cats in norm and following electrolytic destruction of some thalamic structures. It has been shown that parafascicular nuclei of the cat thalamus take part in the control of temporal reactions characteristics, and the thalamus median centre provides for generalization function proceeding on the basis of complicated trace phenomena connected with events prognostication.     

Distribution of directionally selective neurons within the thalamic nuclei and striopallidal complex of the human brain

Spike activity was analyzed in the course of visual testing for directional sensitivity in 177 neuronal populations in different thalamic nuclei and the striopallidal complex in the brain of nine parkinsonian patients, diagnosed and treated using implanted intracerebral electrodes. Directionally selective neurons were discovered in the centrum medianum, the thalamic zona incerta and reticular nucleus, the caudate nucleus, and the central area of the globus pallidus. Proportions and distribution of neurons with different properties were investigated in the thalamic nuclei and striopallidal complex.Institute of Experimental Medicine, Academy of Medical Sciences of the USSR, Leningrad. Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 21, No. 5, pp. 652–660, September–October, 1989.     

Afferent connections of the basolateral division of the amygdaloid complex of the cat brain

Injection of horseradish peroxidase into the basal macrocellular and lateral nuclei of the amygdaloid complex (BLAC) in the cat brain has revealed their rich thalamic afferentation. On the BLAC there are massive projections of: a) nuclei of the middle line of the precommissural pole of the dorsal thalamus (anterior parts of the paratenial, interanteromedial and reunial nuclei), as well as the whole anterior paraventricular nucleus, medial part of the ventral posteromedial nucleus; b) postcommissural nuclei of the dorsal thalamus; some "nonacustical" nuclei of the internal geniculate body (ventrolateral nucleus, medial and macrocellular parts and the most caudal end of the internal geniculate body). Rather essential are projections of the "posterior group nuclei", those of the suprageniculate nucleus, of some parts of the ventral thalamus (subparafascicular nucleus, marginal and peripeduncular nuclei) and parabrachial nucleus. Scattered single projections are obtained from all hypothalamic parts (most of all the ventromedial nucleus), reticular nuclei of the septum, substantia innominata, substantia nigra, truncal nuclei of the raphe. Variety of the dorsal thalamic nuclei, sending their fibers to the BLAC reflects variety of sensory information, that gets here, according to its modality, degree of its differentiation and integrity. A number of the dorsal thalamus nuclei, owing to abundance of labelled neurons, can be considered as special relay thalamic nuclei for the BLAC resembling corresponding relay nuclei for the new cortex.     

Features of the spontaneous unit activity in the human thalamic parafascicular (CM-Pf) complex and its modification in functional cerebral changes

New data on neural organization of the human thalamic parafascicular (CM-Pf) complex were revealed with microelectrode technique during 11 stereotaxic operations in alert diskinetic patients suffering from the tonic forms of spasmodic torticollis. The data were obtained as follows: the functional heterogeneity in cellular organization of the human CM-Pf thalamic nuclei and the existence of three (A, B and C) different types of neurons in these thalamic nuclei: with the irregular discharges (A-type, 18%); with short (10-20 ms) bursts characterised by unstable rhythmic 2-5 Hz pattern and by the low threshold Ca2+ dependent K+ conductance (B-type, 77%); with long-lasting (0.1-2.0 s) bursts of high-frequency trains and constant interburst intervals (C-type, 5%). The functional cerebral changes after motor test performances were shown to be accompanied by appearance of transient modifications of the background unit activity pattern and by tendency towards an increase of neural activity local synchronisation with some stabilising of oscillatory rhythm of discharging B-type neurons. For the first time, a direct relationship between functional characteristics of the human thalamic CM-Pf units and the motor deviations was found in spasmodic torticollis patients.     

Role of the human thalamic parafascicular (CM-Pf) complex in neuronal mechanisms of selective attention

Responses of 93 single units of the human thalamic CM-Pf complex to relevant and irrelevant verbal (or sensory) stimuli were studied using microelectrode technique in alert diskinetic patients suffering from the tonic forms of spasmodic torticollis during 11 stereotaxic operations. The response patterns of two types units with irregular unitary (A-type) and low-threshold bursting Ca(2+)-dependent (B-type) spike activity were studied. Three main conclusions emerge from the studies: 1) high reactivity of both A- and B-units to presentation of relevant verbal stimuli with differences of their response patterns as determined by the type of constituent elements; 2) close functional connectivity of these neuronal changes with the level of selective attention; 3) at the moment of attention activation, the appearance of transient interneuronal interactions between adjacent A and B cells characterized by the local synchronization and stabilization of rhythmic oscillations. These data point to considerable contribution of the thalamic CM-Pf complex and its neuronal mechanisms into organization of the human selective attention and triggering verbal-related processing during performance of purposive speech-provoked voluntary acts.     

Evoked potentials in the parafascicular complex of the rabbit thalamus during peripheral noxious stimulation

Systematic research was conducted into the parafascicular complex of the nonspecific nociceptive system of the rabbit hypothalamus using a technique of evoked potentials. Two types of evoked response were recorded during electrocutaneous stimulation of the paw; a compound response consisting of early and late positive-negative potentials in the lateral region and a simple positive-negative evoked potential in the medial area. Evidence suggests a more complex organization of the thalamic parafascicular complex in leporines than previously supposed.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 18, No. 6, pp. 787–793, November–December, 1986.     

Pulvina-lateralis posterior nucleus complex of mammals and the visual function

It is now well established that the lateral posterior-pulvinar (LP-P) complex of mammals is involved in visual processing. However, the actual function of these large nuclei of the thalamus remains unknown. In contrast to the nearby lateral geniculate nucleus, the LP-P complex does not receive any substantial direct projections from the retina. Its main visual inputs come from the mesencephalon and the neocortex. Most cells in the LP-P complex behave like cortical units. They are tuned to the orientation, direction, spatial and temporal frequencies of the visual stimulus. In addition, most units are binocular and sensitive to relative retinal disparity. Despite their multiple inputs, the LP-P complex cells form an homogeneous population and their overall properties do not reflect those of a given cortical or subcortical area. On the basis of its afferent and efferent connectivity, it has been proposed that the LP-P complex may serve as a relay of an extrageniculate ascendant pathway which originates from the superior colliculus, and/or provide another route for the geniculo-striate input to reach the extrastriate areas. Despite the fact that there is some electro-physiological evidence of such functions, it is now often suggested that the LP-P complex may integrate its multiple inputs and be involved in functions which go beyond those of a simple thalamic relay. Recent findings suggest that the LP-P complex might play a role in visual spatial attention.     

Transient structures of the human fetal brain: subplate, thalamic reticular complex, ganglionic eminence

Morphological features of the subplate, the thalamic reticular complex and the ganglionic eminence, which represent three major transient structures of the human fetal forebrain, are summarized with special reference to their functional roles. The subplate harboring various neuronal types is an outstandingly wide zone subjacent to the cortical plate in the human fetal brain. Within the subplate various cortical afferents establish synaptic contacts for a prolonged period before entering the cortical plate. Therefore, the subplate is regarded as a "waiting compartment" which is required for the formation of mature cortical connections. Next to the thalamic reticular nucleus, within the fibers of internal capsule, the perireticular nucleus is located which has been established as a distinct entity during development. Its various neuronal types express a number of different neuroactive substances. Perinatally, the perireticular nucleus is drastically reduced in size. It is involved in the guidance of corticofugal and thalamocortical fibers. The ganglionic eminence is a conspicuous proliferative area that persists throughout nearly the entire fetal period. In the human fetal brain it extends medially upon the dorsal thalamic nuclei which receive precursor cells from the ganglionic eminence. Postmitotic cells in the marginal zone of the ganglionic eminence serve as an intermediate target for growing axons. On the whole, all three structures establish transient neural circuitries that may be essential for the formation of adult projections. The characteristics of the three transient structures are particularly relevant for developmental neuropathology as these structures may be damaged in disorders that preferentially occur in preterm infants.     

A Possible Mechanism of Blocking of Limbic Motor Seizure Reactions Induced by Activation of the Thalamic Reticular Nucleus

Experiments were directed toward elucidation of the role of the thalamic reticular nucleus (R) in the modulation of generalized seizure reactions under kindling conditions and of the mechanisms mediating the effects of stimulation of the above nucleus on seizure activity. It was shown that activation of the thalamic R in rats limits generalization of the seizure reactions both in the course of development of seizures of limbic genesis (evoked by stimulation of the hippocampus) and under conditions of the existence of a pre-formed epileptic nidus. Tetanic stimulation of the R in cats under conditions of acute experiments induced significant facilitation of IPSPs in thalamo-cortical neurons of the ventrolateral thalamic nucleus. This effect is rather long-lasting and may be considered a mechanism providing blocking of generalized seizures under kindling conditions. Neirofiziologiya/Neurophysiology, Vol. 37, No. 4, pp. 352–361, July–August, 2005.     

A quantitative comparison of the hominoid thalamus: III. A motor substrate—the ventrolateral complex

Nuclear volumes, nerve cell densities, numbers of neurons, and volumes of nerve cell perikarya of the thalamic ventrolateral complex (VL), a neural substrate for movement, were measured in specimens from two gibbons, one gorilla, one chimpanzee, and three humans, and the values were compared. The human VL had about one-and-a-half times as many neurons as did those of the great apes. The relative frequencies of the sizes of nerve cell perikarya differed slightly in the ventrolateral segment of VL; no differences were noted in the rest of VL. Compared with findings from other parts of the thalamus, the differences in the volumes of VL were greater than those found in the thalamic sensory nuclei, similar to those of rest of the thalamus, and less than those found in the whole brain. The increased number of neurons in human VL was similar to that of the somatosensory relay complex, but greater than those of the auditory and visual nuclei and less than those of the limbic and association nuclei. In human evolution, the numbers of neurons in the VL appeared to increase at a faster rate than did neurons of the pyramidal tract, whereas the motor cortex apparently increased at a rate greater than VL.     

Generalization mediates sensitivity to complex odor features in the honeybee

Animals use odors as signals for mate, kin, and food recognition, a strategy which appears ubiquitous and successful despite the high intrinsic variability of naturally-occurring odor quantities. Stimulus generalization, or the ability to decide that two objects, though readily distinguishable, are similar enough to afford the same consequence, could help animals adjust to variation in odor signals without losing sensitivity to key inter-stimulus differences. The present study was designed to investigate whether an animal's ability to generalize learned associations to novel odors can be influenced by the nature of the associated outcome. We use a classical conditioning paradigm for studying olfactory learning in honeybees to show that honeybees conditioned on either a fixed- or variable-proportion binary odor mixture generalize learned responses to novel proportions of the same mixture even when inter-odor differences are substantial. We also show that the resulting olfactory generalization gradients depend critically on both the nature of the stimulus-reward paradigm and the intrinsic variability of the conditioned stimulus. The reward dependency we observe must be cognitive rather than perceptual in nature, and we argue that outcome-dependent generalization is necessary for maintaining sensitivity to inter-odor differences in complex olfactory scenes.     

A neuropharmacological analysis of the compensatory-recovery processes in organic failure of the parafascicular complex of the thalamus]

On the model of biological precursors of thinking in animals (cats), in conditions of free behaviour it has been shown that after neurosurgical lesion of various parts of the parafascicular complex restoration is possible of the disturbed functions of generalization and abstraction by neuropharmacological drugs acting on cholinergic, dopaminergic and GABA-ergic systems. Complex interactions are observed between transmitter structures.     

Localisation of the origin of self-sustained after-discharges (SSADs) in the rat: the serrated wave (SerW) type of SSAD

The self-sustained after-discharges (SSADs) characterised by the EEG pattern of serrated waves (SerW) were induced by rhythmic low frequency electrical stimulation of thalamic nuclei and the hippocampus of Wistar albino male rats in acute experiments. We used spreading depression to eliminate functionally the cortex and the hippocampus. Suction ablation of the cortical somatosensory projection area was also used to test its involvement in the SerW SSAD induction. The hippocampal spreading depression but not the cortical one abolished the SerW SSAD induced by the stimulation of the thalamic nuclei. The animals with the suction ablation of the somatosensory projection area also produced SerW SSADs when the stimulation electrodes were placed in the thalamic ventrobasal complex (in intact animals this stimulation induces spike-and-wave SSADs but not SerW-SSAD). The crucial importance of the hippocampus in the SerW SSAD generation and its possible use as a model of partial seizures with complex symptomatology is discussed.     

Features of corticofugal effects of the motor area of the cortex on processing of heterogeneous afferentation in the parafascicular complex of the thalamus in the cat

Evoked potentials to somatic and visual stimuli were recorded in the parafascicular complex (parafascicular nuclei--centrum medianum--Pf--CM) of the thalamus of cats anaesthetized by nembutal. Cooling of the motor cortex was also used. The influence of the motor cortex on processing of the visual and somatic afferent signals at the thalamic level was found to be direct but different by its character. The motor cortex exerted unidirectional facilitatory modulatory action of a tonic type on the processing of visual afferentation and general facilitatory influence against the background of which particular excitatory and inhibitory effects were seen which it exerted on the processing of somatic signals. Episodically the motor cortex completely controlled the afferent inputs activated by somatic impulses. The motor cortical area non-equally influenced afferentation of the same modality forming the different components of the evoked potentials in Pf--CM. On the base of our present and earlier obtained data an idea has been formed of existence of a general principle of differentiated influences of polysensory areas on heterogeneous afferentation on nonspecific and association thalamic nuclei, and of realization of these influences through separation of functionally isolated subsystems in descending pathways. Each of the subsystems by closing separate thalamo-cortical circuits might transmit signals of a single modality.     

Involvement of thalamic neurons and the striopallidal complex in maintaining constancy of visual perception in the human brain

Neuronal activity was investigated in different thalamic nuclei and the striopallidal complex in parkinsonian patients with long-term implantation of intracerebral electrodes in the structures concerned for diagnostic and therapeutic purposes. Directionally selective neurons were found with consistent response to presentation of visual stimuli oriented at the same angle in a variety of (spatial) head positions differing by 90°.Institute of Experimental Medicine, Academy of Medical Sciences of the USSR, Leningrad. Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 21, No. 1, pp. 93–101, January–February, 1989.     

The Role of Thalamic Population Synchrony in the Emergence of Cortical Feature Selectivity

In a wide range of studies, the emergence of orientation selectivity in primary visual cortex has been attributed to a complex interaction between feed-forward thalamic input and inhibitory mechanisms at the level of cortex. Although it is well known that layer 4 cortical neurons are highly sensitive to the timing of thalamic inputs, the role of the stimulus-driven timing of thalamic inputs in cortical orientation selectivity is not well understood. Here we show that the synchronization of thalamic firing contributes directly to the orientation tuned responses of primary visual cortex in a way that optimizes the stimulus information per cortical spike. From the recorded responses of geniculate X-cells in the anesthetized cat, we synthesized thalamic sub-populations that would likely serve as the synaptic input to a common layer 4 cortical neuron based on anatomical constraints. We used this synchronized input as the driving input to an integrate-and-fire model of cortical responses and demonstrated that the tuning properties match closely to those measured in primary visual cortex. By modulating the overall level of synchronization at the preferred orientation, we show that efficiency of information transmission in the cortex is maximized for levels of synchronization which match those reported in thalamic recordings in response to naturalistic stimuli, a property which is relatively invariant to the orientation tuning width. These findings indicate evidence for a more prominent role of the feed-forward thalamic input in cortical feature selectivity based on thalamic synchronization.