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At least 1,135 records · Page 63Linked to original sources
PubMed1993

An efficient computer model of three-dimensional speckle patterns in dynamically focused annular array.

This paper describes an efficient computer method for generating three-dimensional (3-D) ultrasonic speckle patterns for sector scans of an annular array with dynamic focusing in both transmit and receive. Assuming random scatterers in an attenuating medium, the system synthesizes the waveform for each scan line using echo data received at all annular elements of the transducer when short pulses are transmitted by one annulus after another. The amount of echo data needed to synthesize one waveform is tremendously reduced by reducing the 3-D distributions of scatterers to line distributions and by representing scatterers on a small line segment by a single equivalent scatterer taking into account the two-way travel time differences. Furthermore, by a judicious 3-D arrangement of the scatterer lines and scan lines, it is possible to synthesize the waveform for a new scan line with negligible computation overhead. The waveforms of the scan lines are detected to obtain a B-mode image or speckle patterns. The patterns obtained on several differently-oriented image planes showed a good statistical agreement with experimentally obtained patterns.

Computer Simulation↗
PubMed1999

Weighted combination of size and disparity: a computational model for timing a ball catch.

How do we time hand closure to catch a ball? Binocular disparity and optical looming provide two sources of information about an object's motion in depth, but the relative effectiveness of the two cues depends on ball size. Based on results from a virtual reality ball-catching task, we derive a simple model that uses both cues. The model is sensitive to the relative effectiveness of size and disparity and implicitly switches its response to the cue that specifies the earliest arrival and away from a cue that is lost or below threshold. We demonstrate the model's robustness by predicting the response of participants to some very unusual ball trajectories in a virtual reality task.

Cues↗
PubMed1979

A computer model for hydrodynamic shearing of DNA (Gibb's phenomenon): Part II.

This paper presents an extension of the distribution proposed during hydrodynamic shearing of DNA. In the Fourier least square approximation of uniform random distribution or a square wave, Gibb's phenomena occur at the boundaries. The same behavior occurs at the boundaries of the DNA strands undergoing hydrodynamic shearing of DNA. This hypothesis is corroborated by results from a computer simulation of hydrodynamic shearing of DNA. The evidence helps confirm the kinetic experiments of Britten and Kohne [4]. Furthermore the results predict that sequence very near the end (0.2 LAMBDA) should be the most intact.

Computers↗
PubMed1993

Computer modeling and folding of four-helix bundles.

In the context of simplified models of globular proteins, the requirements for the unique folding to a four-helix bundle have been addressed through a new Monte Carlo procedure. In particular, the relative importance of secondary versus tertiary interactions in determining the nature of the folded structure is examined. Various cases spanning the extremes where tertiary interactions completely dominant to that where tertiary interactions are negligible have been explored. Not surprisingly, the folding to unique four-helix bundles is found to depend on an adequate balance of the secondary and tertiary interactions. Moreover, because the simplified model is composed of spheres representing alpha-carbons and side chains, the geometry of the latter being based on small real amino acids, the role played by the side chains, and the problems associated with packing and hard-core repulsions, are considered. Also, possible folding intermediates and their relationship with the experimentally observed molten globule state are explored. From these studies, a general set of rules is extracted which should aid in the further design of more detailed protein models adequate to more fully investigate the protein folding problem. Finally, the relationship between our conclusions and experimental work with specifically designed sequences is briefly discussed.

Amino Acid Sequence↗