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Biomedical subjects

J Parisi

Publications and source records attributed to J Parisi.

At least 19 recordsLinked to original sources
PubMed2000

Dynamics and control of a multimode laser: Reduction of space-dependent rate equations to a low-dimensional system.

We suggest a quantitatively correct procedure for reducing the spatial degrees of freedom of the space-dependent rate equations of a multimode laser that describe the dynamics of the population inversion of the active medium and the mode intensities of the standing waves in the laser cavity. The key idea of that reduction is to take advantage of the small value of the parameter that defines the ratio between the population inversion decay rate and the cavity decay rate. We generalize the reduction procedure for the case of an intracavity frequency doubled laser. Frequency conversion performed by an optically nonlinear crystal placed inside the laser cavity may cause a pronounced instability in the laser performance, leading to chaotic oscillations of the output intensity. Based on the reduced equations, we analyze the dynamical properties of the system as well as the problem of stabilizing the steady state. The numerical analysis is performed considering the specific system of a Nd:YAG (neodymium-doped yttrium aluminum garnet) laser with an intracavity KTP (potassium titanyl phosphate) crystal.

Journal Article↗
PubMed2000

Heterogeneity of multiple sclerosis lesions: implications for the pathogenesis of demyelination.

Multiple sclerosis (MS) is a disease with profound heterogeneity in clinical course, neuroradiological appearance of the lesions, involvement of susceptibility gene loci, and response to therapy. These features are supported by experimental evidence, which demonstrates that fundamentally different processes, such as autoimmunity or virus infection, may induce MS-like inflammatory demyelinating plaques and suggest that MS may be a disease with heterogeneous pathogenetic mechanisms. From a large pathology sample of MS, collected in three international centers, we selected 51 biopsies and 32 autopsies that contained actively demyelinating lesions defined by stringent criteria. The pathology of the lesions was analyzed using a broad spectrum of immunological and neurobiological markers. Four fundamentally different patterns of demyelination were found, defined on the basis of myelin protein loss, the geography and extension of plaques, the patterns of oligodendrocyte destruction, and the immunopathological evidence of complement activation. Two patterns (I and II) showed close similarities to T-cell-mediated or T-cell plus antibody-mediated autoimmune encephalomyelitis, respectively. The other patterns (III and IV) were highly suggestive of a primary oligodendrocyte dystrophy, reminiscent of virus- or toxin-induced demyelination rather than autoimmunity. At a given time point of the disease--as reflected in autopsy cases--the patterns of demyelination were heterogeneous between patients, but were homogenous within multiple active lesions from the same patient. This pathogenetic heterogeneity of plaques from different MS patients may have fundamental implications for the diagnosis and therapy of this disease.

Adolescent↗
PubMed2000

Stabilization of an unstable steady state in intracavity frequency-doubled lasers

We predict theoretically that it is possible to stabilize the steady state in multimode, intracavity doubled, diode pumped Nd:YAG (neodymium-doped yttrium aluminum garnet) lasers using two output signals, namely, the sum intensities of the infrared laser modes polarized in two different orthogonal directions (X and Y) and one feedback input parameter, the pump rate. The stabilization is possible for arbitrarily large numbers of modes polarized in the X and Y direction. Different strategies of stabilization based on proportional feedback, derivative control, and their combination are discussed. The analytical and numerical results of the linear control theory are illustrated with numerical simulations of the underlying nonlinear differential equations. We show that one can maintain the stable steady state of the laser output for an arbitrarily large pump rate by taking advantage of a tracking procedure.

Journal Article↗
PubMed1999

A quantitative analysis of oligodendrocytes in multiple sclerosis lesions. A study of 113 cases.

We studied quantitatively the fate of oligodendrocytes (OGs) during lesion formation in 395 lesion areas from biopsy and autopsy tissue of 113 multiple sclerosis cases. The density of OGs in multiple sclerosis lesions was variable at all stages of demyelinating activity, ranging from nearly complete loss to values exceeding those in the periplaque white matter (range 0-970 OGs/mm2) determine whether there were distinct patterns of OG pathology in different patients, we restricted our analysis to the 56 cases in which the longitudinal extent of the lesion extended from periplaque white matter into the active demyelinating edge and inactive plaque centre. Two major groups of OG pathology were defined by the presence or absence of increased OGs within the lesion. In 70% (39 out of 56) of the cases, OGs were variably reduced during active stages of myelin destruction, but reappeared within inactive or remyelinating areas. In inactive areas, an increased number of OGs expressing proteolipid protein (PLP) mRNA compared with those expressing myelin oligodendrocyte glycoprotein (MOG) suggested these cells may have been derived from the progenitor pool. In the remaining 30% (17 out of 56) of the cases, extensive destruction of myelinating cells at active sites of demyelination was observed, but OGs were absent in inactive plaque areas without remyelination. In all lesions from a given patient the pattern of OG pathology remained consistent. A highly significant negative correlation was observed between number of macrophages in lesions and number of MOG- and PLP mRNA-labelled OGs (MOG: r = -0.32, P < 0.0000118; PLP mRNA: r = -0.23, P < 0.00238). OG density did not correlate with T-cell and plasma cell inflammation, or axonal loss. The profound heterogeneity in extent and topography of OG destruction in active demyelinating lesions suggests that in subsets of multiple sclerosis patients, myelin, mature OGs and possibly OG progenitors are differentially affected.

Adult↗
PubMed1997

Self-organization in semiconductor physics.

Non-equilibrium dissipative systems from semiconductor physics have prevailed as a paradigmatic testing field for complex non-linear dynamics during the last decade. Especially, low-temperature impact ionization breakdown in extrinsic germanium crystals displays a variety of interesting nonlinear phenomena, such as spontaneous oscillations and filamentary patterns of the current flow. We report on recent experimental results concerning the interplay between spatial and temporal degrees of freedom during the onset of semiconductor breakdown. Quantitative evaluation of characteristic scaling properties supports the applicability of the model of self-organized criticality.

Biophysical Phenomena↗
PubMed1997

Some remarks on the experimental realization of a mind machine.

The brain not only makes use of measuring apparatuses, but perhaps has the potential to serve as one itself. Since Einstein-Podolsky-Rosen correlations must be absent between observer and object in order for a quantum state to become reducible, it is tempting to perturb measurements by changing the quantum state of the brain. The latter would then be part of the measurement. What kind of effects would one expect? It appears that a new psychophysical problem has been opened up, since any observable consequences would be confined to the subjectivity of the observer.

Brain↗
PubMed1996

Periventricular heterotopia: an X-linked dominant epilepsy locus causing aberrant cerebral cortical development.

Periventricular heterotopia (PH) involves dramatic malformations of the human cerebral cortex. Here we show that PH is closely linked to markers in distal Xq28 (maximal two-point lod score = 4.77 for F8C at theta = 0; maximal multipoint lod score = 5.37), so that affected females are obligatory mosaics for the mutation; that PH is lethal to at least some affected males; that PH malformations consist of well-differentiated cortical neurons filling the adult subependymal zone; and that individuals with PH are at high risk for epilepsy, though they have no other neurological or external stigmata. The PH gene may represent an important epilepsy susceptibility locus in addition to playing a key role in normal cortical development.

Abortion, Habitual↗
PubMed1996

The scleral filet technique for secondary orbital implantation surgery.

OBJECTIVE: To describe a simple procedure, the scleral filet technique, to place a secondary implant and take advantage of the scleral remnant. SETTING: Oculoplastic service at a university-affiliated hospital in Ottawa. PATIENTS: Three patients from Middle Eastern countries who had undergone evisceration following trauma without placement of an orbital implant. All three had a scleral remnant, enophthalmos and poor prosthetic motility. INTERVENTIONS: The scleral remnant was incised (fileted) in a crisscross fashion, allowing access to the retrobulbar space. A hydroxyapatite implant was placed, and the scleral remnants were closed without disrupting the muscle attachments, leaving the socket motility intact. RESULTS: All three patients had an uneventful postoperative course and were fitted with a prosthesis between 6 to 8 weeks. There was no implant exposure during the follow-up period of 8 to 15 months. Implant drilling and peg placement was carried out 6 to 10 months postoperatively. One of the patients had excellent motility, and two had good or fair motility. All were very satisfied with the results. CONCLUSIONS: Advantages of the scleral filet technique include simplicity, ability to use the patient's own vascularized scleral remnant to act as a protective cap over the implant and avoidance of donor material. We advocate the use of this procedure when a scleral remnant is present in a patient presenting for consideration of secondary implantation.

Adult↗