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At least 379 records · Page 21Linked to original sources
PubMed2001

Hyperexcitatory activity in visual cortex in homonymous hemianopia after stroke.

OBJECTIVES: Damage to and destruction of neural afferents result in a disruption of sensory input, which causes reduced activity in the corresponding cortical areas. Conversely, there is also evidence that lesions in the sensory pathway induce changes in the intracortical connectivity resulting in augmented cortical activity due to disinhibition. As disinhibition is assumed to be involved in the reconfiguration of neural networks, its appearance after brain lesions might be relevant for the restitution of impaired brain functions. METHODS: The effects of lesions in the visual pathway on the activity in visual cortex were studied using magnetoencephalography. In order to compare the neural activity affected by the lesion with the activity associated with intact visual processing, only patients with unilateral, post-chiasmatic lesions resulting in homonymous hemianopia were examined. RESULTS: Stimulation within the scotoma resulted in reduced magnetic activity compared to the stimulation of the intact hemifield. Increased activity was observed when the border region of the scotoma was stimulated. CONCLUSIONS: It is concluded that the magnetic hyperactivity reflects cortical disinhibition induced by lesions in the visual system. Furthermore, the possible role of cortical disinhibition as a basis for cortical reorganization and as a precondition for the recovery of impaired visual functions is discussed.

Hemianopsia↗
PubMed2006

Discrete and analogue quantity processing in the parietal lobe: a functional MRI study.

The human intraparietal sulcus (IPS) is implicated in processing symbolic number information and possibly in nonsymbolic number information. Specific IPS activity for discrete quantities (numerosities) as compared with continuous, analogue quantity has not been demonstrated. Here we use a stimulus-driven paradigm to distinguish automatic estimation of "how many things" from "how much" and "how long." The discrete analogue response task (DART) uses the perception of hues which can change either abruptly (discrete, numerous stimuli) or smoothly (analogue, nonnumerous stimuli) in space or in time. Subjects decide whether they saw more green or more blue. A conjunction analysis of spatial and temporal conditions revealed that bilateral IPS was significantly more active during the processing of discrete stimuli than during analogue stimuli, as was a parietal-occipital transition zone. We suggest that processing numerosity is a distinct process from processing analogue quantity, whether extended in space or time, and that an intraparietal network connects objects' segmentation to the estimation of their numerosity.

Adolescent↗
PubMed2001

The small world of human language.

Words in human language interact in sentences in non-random ways, and allow humans to construct an astronomic variety of sentences from a limited number of discrete units. This construction process is extremely fast and robust. The co-occurrence of words in sentences reflects language organization in a subtle manner that can be described in terms of a graph of word interactions. Here, we show that such graphs display two important features recently found in a disparate number of complex systems. (i) The so called small-world effect. In particular, the average distance between two words, d (i.e. the average minimum number of links to be crossed from an arbitrary word to another), is shown to be d approximately equal to 2-3, even though the human brain can store many thousands. (ii) A scale-free distribution of degrees. The known pronounced effects of disconnecting the most connected vertices in such networks can be identified in some language disorders. These observations indicate some unexpected features of language organization that might reflect the evolutionary and social history of lexicons and the origins of their flexibility and combinatorial nature.

Biological Evolution↗
PubMed1996

The development of nerve connections in the peripheral nervous system.

The structural basis for the functions of the brain is the wiring of nerve cells. The wiring is achieved by 100 billion neurons via some 100,000 billion synaptic connections. Formation of this network is a formidable task which requires intricate, but also reliable mechanisms at work. It is generally recognized that the precise connections between nerve cells are accomplished by two different sorts of mechanisms. First, various molecular guidance cues lead outgrowing axons from specific areas to particular target regions which are rather broadly defined. The second stage includes an exact, point-to-point matching between each axon and the specific target neurons. This is achieved by an intricate interaction between the involved nerve cells. In the following, the attention will primarily regard the second type of events, but some aspects on the initial outgrowth of axons and axon guidance mechanisms will also be given. A large part of the description will focus on the neuromuscular junction, since it has been extensively used as a model for the study of synapse development.

Axons↗
PubMed2004

In search of the sky compass in the insect brain.

Like many vertebrate species, insects rely on a sun compass for spatial orientation and long- range navigation. In addition to the sun, however, insects can also use the polarization pattern of the sky as a reference for estimating navigational directions. Recent analysis of polarization vision pathways in the brain of orthopteroid insects sheds some light onto brain areas that might act as internal navigation centers. Here I review the significance, peripheral mechanisms, and central processing stages for polarization vision in insects with special reference to the locust Schistocerca gregaria. As in other insect species, polarization vision in locusts relies on specialized photoreceptor cells in a small dorsal rim area of the compound eye. Stages in the brain involved in polarized light signaling include specific areas in the lamina, medulla and lobula of the optic lobe and, in the midbrain, the anterior optic tubercle, the lateral accessory lobe, and the central complex. Integration of polarized-light signals with information on solar position appears to start in the optic lobe. In the central complex, polarization-opponent interneurons form a network of interconnected neurons. The organization of the central complex, its connections to thoracic motor centers, and its involvement in the spatial control of locomotion strongly suggest that it serves as a spatial organizer within the insect brain, including the functions of compass orientation and path integration. Time compensation in compass orientation is possibly achieved through a neural pathway from the internal circadian clock in the accessory medulla to the protocerebral bridge of the central complex.

Animals↗
PubMed2002

Creatine kinase B-driven energy transfer in the brain is important for habituation and spatial learning behaviour, mossy fibre field size and determination of seizure susceptibility.

Creatine kinases are important in maintaining cellular-energy homeostasis, and neuroprotective effects have been attributed to the administration of creatine and creatine-like compounds. Herein we examine whether ablation of the cytosolic brain-type creatine kinase (B-CK) in mice has detrimental effects on brain development, physiological integrity or task performance. Mice deficient in B-CK (B-CK-/-) showed no gross abnormalities in brain anatomy or mitochondrial ultrastructure, but had a larger intra- and infrapyramidal mossy fibre area. Nuclear magnetic resonance spectroscopy revealed that adenosine triphosphate (ATP) and phosphocreatine (PCr) levels were unaffected, but demonstrated an apparent reduction of the PCr left arrow over right arrow ATP phosphorus exchange capacity in these mice. When assessing behavioural characteristics B-CK-/- animals showed diminished open-field habituation. In the water maze, adult B-CK-/- mice were slower to learn, but acquired the spatial task. This task performance deficit persisted in 24-month-old, aged B-CK-/- mice, on top of the age-related memory decline normally seen in old animals. Finally, a delayed development of pentylenetetrazole-induced seizures (creating a high-energy demand) was observed in B-CK-/- mice. It is suggested that the persistent expression of the mitochondrial isoform ubiquitous mitochondrial CK (UbCKmit) in the creatine/phospho-creatine shuttle provides compensation for the loss of B-CK in the brain. Our studies indicate a role for the creatine-phosphocreatine/CK circuit in the formation or maintenance of hippocampal mossy fibre connections, and processes that involve habituation, spatial learning and seizure susceptibility. However, for fuelling of basic physiological activities the role of B-CK can be compensated for by other systems in the versatile and robust metabolic-energy network of the brain.

Animals↗
PubMed2005

Impact of repetitive transcranial magnetic stimulation of the parietal cortex on metabolic brain activity: a 14C-2DG tracing study in the cat.

Transcranial magnetic stimulation (TMS) is increasingly utilized in clinical neurology and neuroscience. However, detailed knowledge of the impact and specificity of the effects of TMS on brain activity remains unresolved. We have used 14C-labeled deoxyglucose (14C-2DG) mapping during repetitive TMS (rTMS) of the posterior and inferior parietal cortex in anesthetized cats to study, with exquisite spatial resolution, the local and distant effects of rTMS on brain activity. High-frequency rTMS decreases metabolic activity at the primary site of stimulation with respect to homologue areas in the unstimulated hemisphere. In addition, rTMS induces specific distant effects on cortical and subcortical regions known to receive substantial efferent projections from the stimulated cortex. The magnitude of this distal impact is correlated with the strength of the anatomical projections. Thus, in the anesthetized animal, the impact of rTMS is upon a distributed network of structures connected to the primary site of application.

Anesthesia↗
PubMed1990

Role of excitatory amino acid neurotransmission in synaptic plasticity and pathology. An integrative hypothesis concerning the pathogenesis and evolutionary advantages of schizophrenia-related genes.

N-methyl-D-aspartate (NMDA) receptors are involved in long-term potentiation, burst firing and the generation of patterned activity in neuronal networks; in use-dependent stabilization of synaptic connectivity in developing animals; in some forms of learning in mature animals; and in pathologies as found in brain aging. A number of these characteristics are reminiscent of several manifestations of schizophrenia and therefore we present the hypothesis that one of the genes modified in schizophrenia is directly or indirectly linked to the control of excitatory neurotransmission; possibly the normal switching on of the expression of the adult form of the NMDA receptor is delayed. Alternatively the adult form of the NMDA receptor is altered, resulting in inappropriate functioning of this receptor. The delayed or faulty expression of the adult form of the NMDA receptor, in turn, should confer a series of evolutionary advantages including protection against aging-associated brain pathologies.

Amino Acids↗
PubMed2000

The dual network of GABAergic interneurons linked by both chemical and electrical synapses: a possible infrastructure of the cerebral cortex.

To know the structural feature of individual nerve cells and of the network they form is essentially important for understanding how the brain works. We have recently shown that a certain subpopulation of hippocampal GABAergic neurons that contain a calcium-binding protein parvalbumin form the dual network connected by both chemical synapses and gap junctions. The mutual chemical synaptic contacts are formed between their axon terminals and somata whereas gap junctions are located between their dendrites. In this article, we demonstrate that the dual network of parvalbumin-containing GABAergic interneurons is not restricted to the hippocampus but found also in the neocortex and, therefore, appears to be a fundamental structure of the cerebral cortex, possibly having some relevance to the synchronized activities observed broadly in various cortical areas.

Animals↗
PubMed2004

Emanuel Miller lecture: confusions and controversies about Asperger syndrome.

BACKGROUND: Hans Asperger drew attention to individuals who show the core symptoms of autism in the presence of high verbal intelligence. METHODS: A review of the literature explores current issues concerning the diagnosis and nature of Asperger syndrome. RESULTS: The behavioural and neurophysiological evidence to date suggests that Asperger syndrome is a variant of autism typically occurring in high-functioning individuals, and not a separate disorder. One of the problems of diagnosis is that the typical impairment of social communication may be difficult to identify in early childhood, and can be camouflaged in adulthood by compensatory learning. The range and nature of the social impairments in Asperger syndrome are still in need of investigation, but appear to be less severe than in autism. Experimental evidence suggests that individuals with Asperger syndrome may lack an intuitive theory of mind (mentalising), but may be able to acquire an explicit theory of mind. Brain imaging studies pinpoint a network that links medial prefrontal and temporal cortex as the neural substrate of intuitive mentalising. This network shows reduced activation and poor connectivity in Asperger syndrome. While some individuals with Asperger syndrome have written eloquently about their lives, their ability to talk about their own emotions appears to be impaired (alexithymia). This impairment may be linked to depression and anxiety, which is common in adulthood. Little is as yet known about the often considerable cognitive strengths in Asperger syndrome, or about the difficulties observed in higher-level executive skills. CONCLUSIONS: Studies are needed that define the developmental course of the disorder and the nature of the strengths and weaknesses in both social and non-social domains. This requires more sensitive assessment instruments than are currently available. Questions about the prevalence of Asperger syndrome, about associated and secondary features, and about optimal education and management, urgently call for such studies.

Adolescent↗
PubMed2002

Functional MRI in human motor control studies and clinical applications.

Functional magnetic resonance imaging (fMRI) has been a useful tool for the noninvasive mapping of brain function associated with various motor and cognitive tasks. Because fMRI is based on the blood oxygenation level dependent (BOLD) effect, it does not directly record neural activity. With the fMRI technique, distinguishing BOLD signals created by cortical projection neurons from those created by intracortical neurons appears to be difficult. Two major experimental designs are used in fMRI studies: block designs and event-related designs. Block-designed fMRI presupposes the steady state of regional cerebral blood flow and has been applied to examinations of brain activation caused by tasks requiring sustained or repetitive movements. By contrast, the more recently developed event-related fMRI with time resolution of a few seconds allows the mapping of brain activation associated with a single movement according to the transient aspects of the hemodynamic response. Increasing evidence suggests that multiple motor areas are engaged in a networked manner to execute various motor acts. In order to understand functional brain maps, it is important that one understands sequential and parallel organizations of anatomical connections between multiple motor areas. In fMRI studies of complex motor tasks, elementary parameters such as movement length, force, velocity, acceleration and frequency should be controlled, because inconsistency in those parameters may alter the extent and intensity of motor cortical activation, confounding interpretation of the findings obtained. In addition to initiation of movements, termination of movements plays an important role in the successful achievement of complex movements. Brain areas exclusively related to the termination of movements have been, for the first time, uncovered with an event-related fMRI technique. We propose the application of fMRI to the elucidation of the pathophysiology of movement disorders, particularly dystonia, which exhibits involuntary co-contraction of agonist and antagonist muscles and manifests abnormal posture or slow repetition of movements.

Brain Mapping↗
PubMed2001

Recurrent network interactions underlying flow-field selectivity of visual interneurons.

Motion-sensitive large-field neurons found at higher processing stages in many species often exhibit a remarkable selectivity for particular flow fields. However, the underlying neural mechanisms are not yet understood. We studied this problem in the so-called lobula plate tangential cells (LPTCs) of the fly. Investigating the connectivity between LPTCs by means of dual recordings, we find two types of connections: (1) heterolateral connections between LPTCs of both hemispheres and (2) ipsilateral connections between LPTCs within one lobula plate. The circuit is suitable to amplify incoming, dendritic signals in the case of rotatory flow fields and to reduce them in the case of other flow-field structures. In addition to feedforward connectivity, thus, the flow-field selectivity of LPTCs may be significantly attributable to recurrent excitation involving the network of large-field neurons in both brain hemispheres.

Action Potentials↗
PubMedDate not supplied

Vein of Galen malformation combined with atrial septal defect in a neonate.

An arteriovenous fistula (AVF) is an abnormal connection between an artery and a vein, whereby the interconnecting capillary network is missing. Such a malformation frequently occurs in the deep midline regions of the brain, and the subsequent increased flow into the draining vein of Galen substantially dilates in an aneurysmal manner. Congenital forms of the aneurysmal dilatation of the vein of Galen (AVG) often lead to death in the neonatal period, predominantly due to cardiac failure caused by the increased venous inflow as a consequence of the intracerebral arteriovenous shunting. In the presented case a male baby suffered from a rare combination of a cerebral AVF and an atrial septal defect (ASD). He was born at week 38 of pregnancy and subsequently developed tachydyspnoe. Ultrasound (US) and CT scans revealed a large bilateral AVF with dilated basal venous sinuses, hydrocephalus and brain atrophy. In the heart, severe right ventricular hypertrophy, patent ductus arteriosus and an ASD were detectable by US. Neurosurgical consultation rejected the possibility of an operative treatment due to size and localization of the lesion and the existing irreversible brain damage. The child died because of cardiac failure 6 days after birth. Autopsy examination in the brain demonstrated a large conglomerate of dilated blood vessels predominantly in the midline and left occipital lobe, edema and hydrocephalus. In the heart, the ASD detected by US proved to be an ostium secundum-type lesion. Histologically, the conglomerate of vessels revealed features of an AVF and matched the characteristics of AVG. Consequences of chronic ischemic brain injury were also present, with ferruginated neurons suggesting intrauterine damage caused by a congenital AVF. Based on data in the literature, we assume that the left-to-right shunt due to increased venous influx into the heart caused not only cardiomegaly, but may have also interfered with the normal development of the atrial septum leading to an ASD, contributing to the rapid progression of the cardiac failure.

Abnormalities, Multiple↗
PubMed1996

The role of activity in the development of long-range horizontal connections in area 17 of the ferret.

Horizontal connections in area 17 of adult cats and ferrets link cells with similar preferred orientations by a patchy network of projections extending several millimeters across the cortex. The maturation of orientation selectivity in ferret area 17 has been demonstrated previously by quantitative single-unit recording and optical imaging to begin at approximately postnatal days (P) 32-P36. We therefore made restricted injections of cholera toxin B-subunit (CTB) or CTB-gold into ferret area 17 at a series of developmental ages and statistically quantified the degree of clustering in plots of retrogradely labeled cells in tangential sections through layer III for comparison to the published values for orientation tuning at each age. At P21, horizontal connections within area 17 lacked patchiness entirely, although clear patches of labeled cells were present in extrastriate areas. By P27, significant clustering of horizontal connections within area 17 was present. A second phase of cluster refinement was observed to occur at approximately P34-P36, coinciding with the emergence of mature orientation tuning and maps. Continuous silencing of cortical action potentials by chronic tetrodotoxin infusion from P21 resulted in a spatially random distribution of retrogradely labeled cells at P34. In contrast, bilateral enucleation from P21 did not prevent the initial development of clustered horizontal connections. We conclude, based on our findings and those of others, that the anatomical specificity of long-range horizontal connections results from an activity-dependent process that initially can use spontaneous activity in the cortical and thalamic networks to establish crude periodic connections and later uses visual cues to refine these connections.

Animals↗
PubMed1996

Gamma-aminobutyric acid type B receptor-dependent burst-firing in thalamic neurons: a dynamic clamp study.

Synchronized network responses in thalamus depend on phasic inhibition originating in the thalamic reticular nucleus (nRt) and are mediated by the neurotransmitter gamma-aminobutyric acid (GABA). A suggested role for intra-nRt connectivity in inhibitory phasing remains controversial. Recently, functional GABA type B (GABAB) receptors were demonstrated on nRt cells, and the slow time course of the GABAB synaptic response seems ideally suited to deinactivate low-threshold calcium channels. This promotes burst firing, a characteristic feature of synchronized responses. Here we investigate GABAB-mediated rebound burst firing in thalamic cells. Whole-cell current-clamp recordings were obtained from nRt cells and somatosensory thalamocortical relay cells in rat brain slices. Synthetic GABAB inhibitory postsynaptic potentials, generated by a hybrid computerneuron synapse (dynamic clamp), triggered rebound low-threshold calcium spikes in both cell types when peak inhibitory postsynaptic potential hyperpolarization was greater than -92 mV. The threshold inhibitory postsynaptic potential conductance for rebound burst generation was comparable in nRt (7 nS) and thalamocortical (5 nS) cells. However, burst onset in nRt (1 s) was considerably delayed compared with thalamocortical (0.6 s) cells. Thus, GABAB inhibitory postsynaptic potentials can elicit low-threshold calcium spikes in both relay and nRt neurons, but the resultant oscillation frequency would be faster for thalamocortical-nRt networks (3 Hz) than for nRt-nRt networks (1-2 Hz). We conclude, therefore, that fast (> 2 Hz) GABAB-dependent thalamic oscillations are maintained primarily by reciprocal connections between excitatory and inhibitory cells. These findings further indicate that when oscillatory neural networks contain both recurrent and reciprocal inhibition, then distinct population frequencies may result when one or the other type of inhibition is favored.

Animals↗
PubMed2003

Forward propagating reinforcement learning--biologically plausible learning method for multi-layer networks.

We introduce a biologically plausible method of implementing reinforcement learning to multi-layer neural networks. The key idea is to spatially localize the synaptic modulation induced by reinforcement signals, proceeding downstream from the initial layer to the final layer. Since reinforcement signals are known to be broadcast signals in the actual brain, we need two key assumptions, inhibitory backward connections and bypass to output units, to spatially localize the effect of delayed reinforcement without breaking the basic laws of neurophysiology.

Animals↗
PubMed2001

Small worlds: how and why.

We investigate small-world networks from the point of view of their origin. While the characteristics of small-world networks are now fairly well understood, there is as yet no work on what drives the emergence of such a network architecture. In situations such as neural or transportation networks, where a physical distance between the nodes of the network exists, we study whether the small-world topology arises as a consequence of a tradeoff between maximal connectivity and minimal wiring. Using simulated annealing, we study the properties of a randomly rewired network as the relative tradeoff between wiring and connectivity is varied. When the network seeks to minimize wiring, a regular graph results. At the other extreme, when connectivity is maximized, a "random" network is obtained. In the intermediate regime, a small-world network is formed. However, unlike the model of Watts and Strogatz [Nature 393, 440 (1998)], we find an alternate route to small-world behavior through the formation of hubs, small clusters where one vertex is connected to a large number of neighbors.

Algorithms↗
PubMed1995

Luigi Rolando and his pioneering efforts to relate structure to function in the nervous system.

The fissure separating the motor from the sensory cortex and the substantia gelatinosa capping the posterior horn of the spinal cord are still known by the name of the Italian anatomist Rolando, Luigi Rolando was born in Turin, Italy, in 1773 and died in 1831. His life was not easy, the first of his problems being the death of his father when Rolando was still very young. Three people were to be influential in his life and career: Father Maffei, his maternal uncle who raised him; Dr. Cigna, the anatomy professor who discovered his talent; and Dr. Anformi, a general practitioner who introduced him to the practice of medicine and to the best circles of the city. Forced to leave Turin by the Napoleonic invasion of the country, Rolando first stopped in Florence, where he learned about anatomical dissection, drawing, and engraving and studied the appearance of nervous tissue under the microscope. Later he went to Sardinia where, although cut off from European cultural circles, he developed his major theories. Rolando pioneered the idea that brain functions could be differentiated and located in specific areas and discovered the fixed pattern of cerebral convolutions, highlighting motor and sensory gyri. He demonstrated the complexity of the central gray matter of the spinal cord, describing the "substantia gelatinosa," and he deduced that nervous structures are connected in a network of nervous fibers linked by electrical impulses. Rolando had to struggle for recognition, however, as the priority of his discoveries was challenged by the almost contemporaneous work of Gall and Spurzheim on cerebral localization and of Flourens on cerebellar function. Nevertheless, his efforts contributed greatly to the clarification of brain function. His observations on nervous anatomy have been especially accurate, as shown by the nomenclature "fissure of Rolando."

History, 19th Century↗