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B Tawfik

Publications and source records attributed to B Tawfik.

8 recordsLinked to original sources
PubMed1997

Parameter estimation by reduced-order linear associative memory (ROLAM).

In a series of papers we have shown that nonlinear parameter estimation by linear association provides accurate estimates of the parameters in complex systems described by nonlinear differential equations even in the presence of additive white noise of considerable power. The technique is based on linearly associating the system's output with a set of parameter values spanning the region of interest. When an actual output is measured, the system's unknown parameters could be estimated by a matrix inversion. The size of the inverted matrix, being equal to the length of the output vector, poses a limiting factor upon the generalization of the technique. In this paper we propose a modification which requires the inversion of a matrix whose dimension equals the number of model parameters. The modified version is called reduced-order associative memory (ROLAM). The technique is applied to two complex lumped-parameter nonlinear models: the Van der Pol relaxation oscillator and the passive neuron model of the granule cells. Results validate ROLAM as a parameter-estimation tool which is especially suited in cases where the number of parameters is large, the number of samples in the observation signal is high, or when on-line parameter estimation is required. It is also shown that ROLAM provides an optimal parameter estimate in the special case of single-parameter nonlinear models.

Algorithms↗
PubMed1994

Nonlinear parameter estimation by linear association: application to a five-parameter passive neuron model.

Linear associative memories (LAM) have been intensely used in the areas of pattern recognition and parallel processing for the past two decades. Application of LAM to nonlinear parameter estimation, however, has only been recently attempted. The process consists in converting the nonlinear function in the parameters into a set of linear algebraic equations. The nature of the linearized system and the factors influencing the accuracy of the parameter estimates have not yet been fully investigated. In this paper, LAM is applied to a nonlinear five-parameter model of the neuron. Ill-conditioning, which is often exhibited in LAM, is treated with the method of regularization as well as by the singular value decomposition (SVD). Simulation results indicate that the parameters estimated by LAM exhibit a remarkable robustness against additive white noise in comparison with the classical gradient optimization technique. Moreover, it is shown that regularization can be superior to SVD under certain conditions. Our results suggest that LAM can be used both as a noise reduction technique and as a stand-alone nonlinear parameter estimation algorithm. The comparison between LAM and a gradient technique show that, for this estimation problem, the LAM method can give more reliable estimates. Further improvements in estimation quality may still be achieved by the use of other forms of regularizing functions.

Linear Models↗
PubMed1993

Atypical fibroxanthoma of the cheek: a case report.

Atypical fibroxanthoma is a cutaneous soft tissue tumor that may mimic squamous cell carcinoma and melanoma on clinical and histologic examination. Immunohistochemical stains for cytokeratin, S100 protein, and vimentin may be helpful in differentiating atypical fibroxanthoma from the latter two diagnoses. We report a patient with atypical fibroxanthoma of the cheek who underwent successful local excision. The clinical and laboratory features of this neoplasm are reviewed.

Aged↗
PubMed1991

A new method to measure nasal impedance in spontaneously breathing adults.

As an alternative to standard rhinomanometric methods, we applied forced oscillations at the mouth in five normal subjects and determined their nasal impedance with a novel method involving flow subtraction. Pressure oscillations of constant amplitude were applied at the mouth of a subject both when the nostrils were open and when they were closed with a noseclip. The airflows measured under the two conditions were subtracted to yield the oscillating nasal airflow at the imposed pressure. The resultant pressure-flow relation defined the nasal impedance of the subject. For frequencies between 3 and 15 Hz, the transnasal pressure-flow relation was well described by a linear lumped parameter model consisting of a resistive and inertial element. Nasal resistance obtained with flow subtraction did not differ significantly from control measurements obtained while the subjects performed the Valsalva maneuver. In contrast, nasal inertance obtained with flow subtraction was approximately twice that obtained with the Valsalva method. The difference between inertances may reflect structural changes in nasopharyngeal dimensions that occur with the Valsalva maneuver. We conclude that the mechanical impedance of the nasal passage may be determined during spontaneous breathing from the response to imposed forced oscillations at the mouth. The noninvasive nature of this method suggests that it may be simpler to implement than traditional rhinomanometric methods.

Adult↗
PubMed1988

A nonlinear model of respiratory mechanics in emphysematous lungs.

Existing mechanical models of chronic obstructive lung disease have failed to explain a number of experimental findings of airway obstruction, e.g., the varying manners of frequency dependence of resistance (FDR). Departing from the parallel-unit concept and attempting to account for the "check-valve" mechanism in the emphysematous lung, we proposed a single-compartment lung model with a nonlinear pressure-flow relationship: P + P* = LV + K1V + K3(V +/- V*)3 + V/c, where P* = K3V*, V* is a constant. The plus and minus signs in the cubic term indicate the expiratory and inspiratory check valves, respectively. The choice of an asymmetric P - V relation reflects several properties of emphysematous lungs such as airflow limitation and higher expiratory resistance. Implementation of the above equation using sine wave, white noise, and step inputs resulted in various forms of FDR at frequencies between 0 and 40 Hz depending on the type of input used. Resistance was most sensitive to changes in input pressure amplitude. The model's results suggest that the P - V nonlinearity can have a significant influence on the impedance construct in obstructed lung disease.

Airway Resistance↗
PubMed1975

Primary oat cell carcinoma of the larynx: an ultrastructural study.

An extrapulmonary oat cell carcinoma arising in the larynx is described. Ultrastructurally the tumor is identical to the oat cell carcinoma of pulmonary origin. The pressure of neurosecretory granules in our case strongly suggests that the tumor is derived from Kulchitsky-like cells, a histogenesis previously proposed for oat cell carcinoma of the lung.

Carcinoma, Small Cell↗
PubMedDate not supplied

Hemangioma of the nasal bones: radiographic appearance.

Four additional cases of hemangioma involving the nasal bones are presented, bringing the reported cases to 15. The radiographic findings of nasal bone hemangioma are distinctive, permitting easy diagnosis. Hemangioma involving the nasal bones may result in a reticular or "soap bubble" osteoporosis, spiculation, bone thickening, or a channelled appearance of the nasal bone. Treatment is by local excision.

Adult↗