The Experts below are selected from a list of 117 Experts worldwide ranked by ideXlab platform
Massimo Nespolo - One of the best experts on this subject based on the ideXlab platform.
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Structural rationale for the occurrence of the elbow twins in cassiterite and rutile
Journal of Mineralogical and Petrological Sciences, 2015Co-Authors: Massimo Nespolo, Bernd SouvignierAbstract:Elbow twins are a distinct feature of cassiterite and rutile. Their occurrence is justified by the Reticular Theory of twinning because of the existence of a common sublattice- the twin lattice - formed by the overlap of a large fraction of the lattice nodes of the individuals. Yet, the atomic basis for the existence of twins falls outside the possibilities of the Reticular Theory. We present here the analysis of the pseudo-eigensymmetry of the crystallographic orbits building the structure of the elbow twins and show that their occurrence is justified by the complete restoration of the structure near the composition surface, with a deviation of less than 0.7 angstrom.
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The staurolite enigma solved
Acta Crystallographica Section A, 2014Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:Staurolite has been long considered an enigma because of its remarkable pseudosymmetry and the frequent twinning. Staurolite gives two twins whose occurrence frequency seems to contradict the condition of lattice restoration requested by the Reticular Theory of twinning, in that the more frequent one (Saint Andrews cross twin) has a twin index of 12, whereas the less frequent one (Greek cross twin) has a twin index of 6. The hybrid Theory of twinning shows that the former is actually a hybrid twin with two concurrent sublattices and an effective twin index of 6.0. However, this is still not sufficient to explain the observed higher occurrence frequency of the Saint Andrews twin. The (pseudo)-eigensymmetry of the crystallographic orbits of staurolite has been analysed and it was found that the whole substructure built on anions is restored (with small deviations) by both twin laws, which explains why twinning is frequent in staurolite. On the other hand, 45% of the cation sites are quasi-restored in the Saint Andrews cross twin, against only 19% for the Greek cross twin: this difference finally explains the different occurrence frequencies of the two twins.
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Analysis of the structural continuity in twinned crystals in terms of pseudo-eigensymmetry of crystallographic orbits
IUCrJ, 2013Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:The Reticular Theory of twinning gives the necessary conditions on the lattice level for the formation of twins. The latter are based on the continuation, more or less approximate, of a substructure through the composition surface. The analysis of this structural continuity can be performed in terms of the eigensymmetry of the crystallographic orbits corresponding to occupied Wyckoff positions in the structure. If {\cal G} is the space group of the individual and {\cal H} a space group which fixes the twin lattice obtained as an intersection of the space groups of the individuals in their respective orientations, then a structural continuity is obtained if (1) the eigensymmetry of an orbit under {\cal G} contains the twin operation; (2) the eigensymmetry of a union of orbits under {\cal G} contains the twin operation; (3) the eigensymmetry of a split orbit under {\cal H} contains the twin operation; or (4) the eigensymmetry of a union of split orbits under {\cal H} contains the twin operation. The case of the twins in melilite is analysed: the (approximate) restoration of some of the orbits explains the formation of these twins.
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Analysis of the structural continuity in twinned crystals in terms of pseudo-eigensymmetry of crystallographic orbits.
IUCrJ, 2013Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:The Reticular Theory of twinning gives the necessary conditions on the lattice level for the formation of twins. The latter are based on the continuation, more or less approximate, of a substructure through the composition surface. The analysis of this structural continuity can be performed in terms of the eigensymmetry of the crystallographic orbits corresponding to occupied Wyckoff positions in the structure. If [Formula: see text] is the space group of the individual and [Formula: see text] a space group which fixes the twin lattice obtained as an intersection of the space groups of the individuals in their respective orientations, then a structural continuity is obtained if (1) the eigensymmetry of an orbit under [Formula: see text] contains the twin operation; (2) the eigensymmetry of a union of orbits under [Formula: see text] contains the twin operation; (3) the eigensymmetry of a split orbit under [Formula: see text] contains the twin operation; or (4) the eigensymmetry of a union of split orbits under [Formula: see text] contains the twin operation. The case of the twins in melilite is analysed: the (approximate) restoration of some of the orbits explains the formation of these twins.
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Hybrid twinning - - a cooperative type of oriented crystal association
Zeitschrift Fur Kristallographie, 2005Co-Authors: Massimo Nespolo, Giovanni FerrarisAbstract:The Reticular Theory of twinning is extended to include the contribution of different, coexisting sublattices that consist of quasi-restored nodes (cooperative sublat- tices) and are characterized by a specific superposition in- dex. The overall degree of quasi-restoration of the twin lattice is a function of the superposition index of the coop- erative sublattices, and is expressed by a new parameter, called effective twin index, which is the generalization of the classical twin index. Twins where the effective twin index is lower than the classical twin index are termed hybrid twins. Several twins (e.g. twins of pyrite and of forsterite) that could not so far be explained by the reticu- lar Theory because of their too high twin index are inter- preted as hybrid twins. The effect of hybrid twins on dif- fraction patterns is discussed.
Anton M.l. Coenen - One of the best experts on this subject based on the ideXlab platform.
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Evolving concepts on the pathophysiology of generalised absence seizures: the cortical focus Theory
2020Co-Authors: H.k.m. Meeren, E.l.j.m. Van Luijtelaar, F.h. Lopes Da Silva, Anton M.l. CoenenAbstract:Three major theories concerning the pathophysiology of generalised absence seizures have been proposed. Penfield and Jasper (1947) put forward the 'centrencephalic' Theory, suggesting that the discharges originate from a deep-seated central subcortical pacemaker in the midline thalamus, which diffusely projects to the cortex. This concept was refined by Buzsaki (1991) who proposed that the Reticular thalamic nucleus contains the pacemaker cells for the thalamic clock, which imposes its rhythm to the cortex. According to proponents of the cortical Theory (Gibbs and Gibbs, 1952; Bancaud, 1969; Luders et al., 1984; Niedermeyer, 1969), however, the cortex plays a leading role. They found evidence that generalised spike-wave discharges have a focal onset in the frontal cortex. They suggested that seizures became secondarily generalised through a rapid propagation over the cortex. The cortico-Reticular Theory, postulated by Gloor (1968), formed a reconciliation between the thalamic and cortical Theory. Work on the 'feline penicillin generalised epilepsy' model showed that the mechanisms responsible for the spike-wave discharges were linked to the thalamocortical mechanisms that generate spindles. Gloor (1968) demonstrated that rhythmic spindle oscillations generated in the thalamus could be transformed into spike-wave discharges when the cortex was made hyperexcitable. In that case the cortex initiates abnormal hypersynchronous oscillations in the thalamocortical network. Experiments performed in genetically epileptic rats by Meeren et al. (2002) confirmed that a functionally intact thalamocortical network is required for the generation of generalised spike-wave discharges. They investigated the cortico-cortical, intrathalamic and cortico-thalamic interrelationships during spontaneous absence seizures using the advanced signal analysis method of nonlinear association analysis. The analyses revealed a consistent cortical 'focus' within the peri-oral region of the somatosensory cortex. From here, seizure activity spreads rapidly over the cortex, which gives the discharges their generalized appearance. During the first few cycles of the seizure the cortex drives the thalamus, which subsequently becomes entrained into the oscillation. Thereafter cortex and thalamus form a unified network in which both structures drive each other, thus amplifying and maintaining the rhythmic paroxysmal discharges. In this way Meeren's et al. (2002) cortical focus Theory for generalised absence epilepsy forms a synthesis between the cortical and the cortico-Reticular Theory.
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The cortico-thalamic Theory for generalised spike-wave discharges
Uspekhi fiziologicheskikh nauk, 2020Co-Authors: H.k.m. Meeren, E.l.j.m. Van Luijtelaar, F.h. Lopes Da Silva, S.a. Chepurnov, R.k. Berdiev, Chepurnova Ne, Anton M.l. CoenenAbstract:The origin of generalized absence epilepsy is still not known. In the last century, four theories have dominated the debate about the origin of the bilateral synchronous generalized spike-wave discharges associated with absence seizures: the "centrencephalic" Theory [Penfield and Jasper], the "cortical" [Bancaud, Niedermeyer, Luders], the "cortico-Reticular" Theory [Gloor, Kostop[oulos, Avoli] and the "thalamic clock" Theory [Buzsaki]. There is now some evidence that absence epilepsy, as studied in the WAG/Rij model, is a corticothalamic type of epilepsy. A new hypothesis is proposed which suggests that a cortical focus in the somatosensory cortex is driving the widespread corticothalamic networks during spontaneous absence seizures. This modern Theory was given the name "hot spot' Theory" [Meeren et al., 2002]. According to the present view three brain structures are critically involved and their integrity seems a minimal and sufficient condition for the occurrence of spike-wave discharges. Firstly, the Reticular thalamic nucleus is involved and most likely its rostral pole. Secondly, the thalamocortical relay cells in the ventrobasal complex play a role and, thirdly and most importantly, the cerebral cortex with its epileptic zone. The zone in which the epileptic focus seems to be localised is located on the somato-sensory cortex, and more precisely in the area on which the peri-oral region including the upper lip, projects.
Bernd Souvignier - One of the best experts on this subject based on the ideXlab platform.
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Structural rationale for the occurrence of the elbow twins in cassiterite and rutile
Journal of Mineralogical and Petrological Sciences, 2015Co-Authors: Massimo Nespolo, Bernd SouvignierAbstract:Elbow twins are a distinct feature of cassiterite and rutile. Their occurrence is justified by the Reticular Theory of twinning because of the existence of a common sublattice- the twin lattice - formed by the overlap of a large fraction of the lattice nodes of the individuals. Yet, the atomic basis for the existence of twins falls outside the possibilities of the Reticular Theory. We present here the analysis of the pseudo-eigensymmetry of the crystallographic orbits building the structure of the elbow twins and show that their occurrence is justified by the complete restoration of the structure near the composition surface, with a deviation of less than 0.7 angstrom.
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The staurolite enigma solved
Acta Crystallographica Section A, 2014Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:Staurolite has been long considered an enigma because of its remarkable pseudosymmetry and the frequent twinning. Staurolite gives two twins whose occurrence frequency seems to contradict the condition of lattice restoration requested by the Reticular Theory of twinning, in that the more frequent one (Saint Andrews cross twin) has a twin index of 12, whereas the less frequent one (Greek cross twin) has a twin index of 6. The hybrid Theory of twinning shows that the former is actually a hybrid twin with two concurrent sublattices and an effective twin index of 6.0. However, this is still not sufficient to explain the observed higher occurrence frequency of the Saint Andrews twin. The (pseudo)-eigensymmetry of the crystallographic orbits of staurolite has been analysed and it was found that the whole substructure built on anions is restored (with small deviations) by both twin laws, which explains why twinning is frequent in staurolite. On the other hand, 45% of the cation sites are quasi-restored in the Saint Andrews cross twin, against only 19% for the Greek cross twin: this difference finally explains the different occurrence frequencies of the two twins.
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Analysis of the structural continuity in twinned crystals in terms of pseudo-eigensymmetry of crystallographic orbits
IUCrJ, 2013Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:The Reticular Theory of twinning gives the necessary conditions on the lattice level for the formation of twins. The latter are based on the continuation, more or less approximate, of a substructure through the composition surface. The analysis of this structural continuity can be performed in terms of the eigensymmetry of the crystallographic orbits corresponding to occupied Wyckoff positions in the structure. If {\cal G} is the space group of the individual and {\cal H} a space group which fixes the twin lattice obtained as an intersection of the space groups of the individuals in their respective orientations, then a structural continuity is obtained if (1) the eigensymmetry of an orbit under {\cal G} contains the twin operation; (2) the eigensymmetry of a union of orbits under {\cal G} contains the twin operation; (3) the eigensymmetry of a split orbit under {\cal H} contains the twin operation; or (4) the eigensymmetry of a union of split orbits under {\cal H} contains the twin operation. The case of the twins in melilite is analysed: the (approximate) restoration of some of the orbits explains the formation of these twins.
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Analysis of the structural continuity in twinned crystals in terms of pseudo-eigensymmetry of crystallographic orbits.
IUCrJ, 2013Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:The Reticular Theory of twinning gives the necessary conditions on the lattice level for the formation of twins. The latter are based on the continuation, more or less approximate, of a substructure through the composition surface. The analysis of this structural continuity can be performed in terms of the eigensymmetry of the crystallographic orbits corresponding to occupied Wyckoff positions in the structure. If [Formula: see text] is the space group of the individual and [Formula: see text] a space group which fixes the twin lattice obtained as an intersection of the space groups of the individuals in their respective orientations, then a structural continuity is obtained if (1) the eigensymmetry of an orbit under [Formula: see text] contains the twin operation; (2) the eigensymmetry of a union of orbits under [Formula: see text] contains the twin operation; (3) the eigensymmetry of a split orbit under [Formula: see text] contains the twin operation; or (4) the eigensymmetry of a union of split orbits under [Formula: see text] contains the twin operation. The case of the twins in melilite is analysed: the (approximate) restoration of some of the orbits explains the formation of these twins.
E.l.j.m. Van Luijtelaar - One of the best experts on this subject based on the ideXlab platform.
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Evolving concepts on the pathophysiology of generalised absence seizures: the cortical focus Theory
2020Co-Authors: H.k.m. Meeren, E.l.j.m. Van Luijtelaar, F.h. Lopes Da Silva, Anton M.l. CoenenAbstract:Three major theories concerning the pathophysiology of generalised absence seizures have been proposed. Penfield and Jasper (1947) put forward the 'centrencephalic' Theory, suggesting that the discharges originate from a deep-seated central subcortical pacemaker in the midline thalamus, which diffusely projects to the cortex. This concept was refined by Buzsaki (1991) who proposed that the Reticular thalamic nucleus contains the pacemaker cells for the thalamic clock, which imposes its rhythm to the cortex. According to proponents of the cortical Theory (Gibbs and Gibbs, 1952; Bancaud, 1969; Luders et al., 1984; Niedermeyer, 1969), however, the cortex plays a leading role. They found evidence that generalised spike-wave discharges have a focal onset in the frontal cortex. They suggested that seizures became secondarily generalised through a rapid propagation over the cortex. The cortico-Reticular Theory, postulated by Gloor (1968), formed a reconciliation between the thalamic and cortical Theory. Work on the 'feline penicillin generalised epilepsy' model showed that the mechanisms responsible for the spike-wave discharges were linked to the thalamocortical mechanisms that generate spindles. Gloor (1968) demonstrated that rhythmic spindle oscillations generated in the thalamus could be transformed into spike-wave discharges when the cortex was made hyperexcitable. In that case the cortex initiates abnormal hypersynchronous oscillations in the thalamocortical network. Experiments performed in genetically epileptic rats by Meeren et al. (2002) confirmed that a functionally intact thalamocortical network is required for the generation of generalised spike-wave discharges. They investigated the cortico-cortical, intrathalamic and cortico-thalamic interrelationships during spontaneous absence seizures using the advanced signal analysis method of nonlinear association analysis. The analyses revealed a consistent cortical 'focus' within the peri-oral region of the somatosensory cortex. From here, seizure activity spreads rapidly over the cortex, which gives the discharges their generalized appearance. During the first few cycles of the seizure the cortex drives the thalamus, which subsequently becomes entrained into the oscillation. Thereafter cortex and thalamus form a unified network in which both structures drive each other, thus amplifying and maintaining the rhythmic paroxysmal discharges. In this way Meeren's et al. (2002) cortical focus Theory for generalised absence epilepsy forms a synthesis between the cortical and the cortico-Reticular Theory.
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The cortico-thalamic Theory for generalised spike-wave discharges
Uspekhi fiziologicheskikh nauk, 2020Co-Authors: H.k.m. Meeren, E.l.j.m. Van Luijtelaar, F.h. Lopes Da Silva, S.a. Chepurnov, R.k. Berdiev, Chepurnova Ne, Anton M.l. CoenenAbstract:The origin of generalized absence epilepsy is still not known. In the last century, four theories have dominated the debate about the origin of the bilateral synchronous generalized spike-wave discharges associated with absence seizures: the "centrencephalic" Theory [Penfield and Jasper], the "cortical" [Bancaud, Niedermeyer, Luders], the "cortico-Reticular" Theory [Gloor, Kostop[oulos, Avoli] and the "thalamic clock" Theory [Buzsaki]. There is now some evidence that absence epilepsy, as studied in the WAG/Rij model, is a corticothalamic type of epilepsy. A new hypothesis is proposed which suggests that a cortical focus in the somatosensory cortex is driving the widespread corticothalamic networks during spontaneous absence seizures. This modern Theory was given the name "hot spot' Theory" [Meeren et al., 2002]. According to the present view three brain structures are critically involved and their integrity seems a minimal and sufficient condition for the occurrence of spike-wave discharges. Firstly, the Reticular thalamic nucleus is involved and most likely its rostral pole. Secondly, the thalamocortical relay cells in the ventrobasal complex play a role and, thirdly and most importantly, the cerebral cortex with its epileptic zone. The zone in which the epileptic focus seems to be localised is located on the somato-sensory cortex, and more precisely in the area on which the peri-oral region including the upper lip, projects.
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From generalized to focal absence seizures
2020Co-Authors: E.l.j.m. Van Luijtelaar, E.y. SitnikovaAbstract:Correspondence: Dr. E.L.J.M. van Luijtelaar, NICI-Biological Psychology, University of Nijmegen, PO Box 9104, 6500 HE Nijmegen, The Netherlands. Tel: +31 24 361 56 21 Fax: +31 24 361 60 66 e-mail: g.vanluijtelaar@nici.kun.nl Absence seizures were first described by Poupart in 1705, and later, in 1770, Tissot introduced the term “petits acces”. Finally they were called “absence seizures” in 1824 by Calmeil. Absence seizures, also known as petit mal, fundamentally differ from another type of generalized epilepsygrand mal seizures, in that symptomatically there is neiThe leading Theory on the origin of absence seizures, the cortico-Reticular Theory, supposes that a subcortical pacemaker is responsible for normal sleep spindles and pathological spike-wave discharges (SWDs). It postulates that the thalamic relay cells in the basolateral complex of the thalamus, the Reticular thalamic nucleus and the cortex, assembled in a thalamo-cortico-thalamic network, generate both types of EEG oscillations. Recently, Meeren et al. (2002) proposed a focal Theory of absence epilepsy. This Theory, based on experimental findings in the WAG/Rij rats supposes that the somatosensory cortex contains a focus that initiates a cascade of events that lead to the occurrence of bilateral and generalized SWDs. In this review, new data are presented which show that the cortex of WAG/Rij rats contains areas including the somatosensory cortex, which lack parvalbumin. Second, local deactivation of the focal zone with lidocaine reduces the incidence of SWDs. Finally, branching of dendrites of neurons is abnormal in this zone. All these new results provide further support for the focal Theory of absence epilepsy. The question as to whether similar or comparable pathophysiological processes are present in humans needs to be elucidated. The new focal Theory can be easily tested in humans with respect to the location of the origin of the EEG oscillations.
Mohamed Amine Marzouki - One of the best experts on this subject based on the ideXlab platform.
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The staurolite enigma solved
Acta Crystallographica Section A, 2014Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:Staurolite has been long considered an enigma because of its remarkable pseudosymmetry and the frequent twinning. Staurolite gives two twins whose occurrence frequency seems to contradict the condition of lattice restoration requested by the Reticular Theory of twinning, in that the more frequent one (Saint Andrews cross twin) has a twin index of 12, whereas the less frequent one (Greek cross twin) has a twin index of 6. The hybrid Theory of twinning shows that the former is actually a hybrid twin with two concurrent sublattices and an effective twin index of 6.0. However, this is still not sufficient to explain the observed higher occurrence frequency of the Saint Andrews twin. The (pseudo)-eigensymmetry of the crystallographic orbits of staurolite has been analysed and it was found that the whole substructure built on anions is restored (with small deviations) by both twin laws, which explains why twinning is frequent in staurolite. On the other hand, 45% of the cation sites are quasi-restored in the Saint Andrews cross twin, against only 19% for the Greek cross twin: this difference finally explains the different occurrence frequencies of the two twins.
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Analysis of the structural continuity in twinned crystals in terms of pseudo-eigensymmetry of crystallographic orbits
IUCrJ, 2013Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:The Reticular Theory of twinning gives the necessary conditions on the lattice level for the formation of twins. The latter are based on the continuation, more or less approximate, of a substructure through the composition surface. The analysis of this structural continuity can be performed in terms of the eigensymmetry of the crystallographic orbits corresponding to occupied Wyckoff positions in the structure. If {\cal G} is the space group of the individual and {\cal H} a space group which fixes the twin lattice obtained as an intersection of the space groups of the individuals in their respective orientations, then a structural continuity is obtained if (1) the eigensymmetry of an orbit under {\cal G} contains the twin operation; (2) the eigensymmetry of a union of orbits under {\cal G} contains the twin operation; (3) the eigensymmetry of a split orbit under {\cal H} contains the twin operation; or (4) the eigensymmetry of a union of split orbits under {\cal H} contains the twin operation. The case of the twins in melilite is analysed: the (approximate) restoration of some of the orbits explains the formation of these twins.
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Analysis of the structural continuity in twinned crystals in terms of pseudo-eigensymmetry of crystallographic orbits.
IUCrJ, 2013Co-Authors: Mohamed Amine Marzouki, Bernd Souvignier, Massimo NespoloAbstract:The Reticular Theory of twinning gives the necessary conditions on the lattice level for the formation of twins. The latter are based on the continuation, more or less approximate, of a substructure through the composition surface. The analysis of this structural continuity can be performed in terms of the eigensymmetry of the crystallographic orbits corresponding to occupied Wyckoff positions in the structure. If [Formula: see text] is the space group of the individual and [Formula: see text] a space group which fixes the twin lattice obtained as an intersection of the space groups of the individuals in their respective orientations, then a structural continuity is obtained if (1) the eigensymmetry of an orbit under [Formula: see text] contains the twin operation; (2) the eigensymmetry of a union of orbits under [Formula: see text] contains the twin operation; (3) the eigensymmetry of a split orbit under [Formula: see text] contains the twin operation; or (4) the eigensymmetry of a union of split orbits under [Formula: see text] contains the twin operation. The case of the twins in melilite is analysed: the (approximate) restoration of some of the orbits explains the formation of these twins.