Biomedical subjects

V V Yakovlev

Publications and source records attributed to V V Yakovlev.

4 recordsLinked to original sources

PubMed2004

Quantitative characterization of aqueous solutions probed by the third-harmonic generation microscopy.

Third-harmonic microscopy is one of the emerging techniques for noninvasive microscopic imaging of biological structures. We use a novel technique for nonlinear optical material characterization and study the effect of different environment and the structural sensitivity of the third harmonic. In particular, a transformation of collagen in solution is observed for the first time using third-harmonic generation. We also study the ultimate limits of the third harmonic to detect micro- and nanoscopic features inside living cells and find that structures as small as 50 nm can be detected using the current level of technology.

Collagen↗

PubMed2004

Efficiency optimization for systems and components in microwave power engineering.

The paper is concerned with one aspect of optimization of microwave thermal processing, namely, with optimization of energy coupling interpreted as a numerical characteristic of system efficiency. Since in computer-aided design coupling can be evaluated through the computed reflections, an optimization scheme is presented that is particularly suitable for minimizing the reflection coefficient in typical systems and elements of microwave power engineering. Based on response surface methodology and the sequential quadratic programming for constrained optimization, the procedure is linked with the full-wave 3-D FDTD electromagnetic simulator QuickWave-3D. Credibility and effectiveness of the method is illustrated by four examples: dimensional optimization is performed for a dry waveguide load, a waveguide T-junction with a partial-height post, a water cylinder in a cavity, and a slotted waveguide-backed radiating element.

Journal Article↗

PubMed1989

Spatial organization of subfields in receptive fields of cells in cat striate cortex.

Spatial organization of receptive fields (RF) of cells in cat striate cortex was investigated with moving and flashing light and dark bars and with grating-patterns of a varying number of cycles. It was shown that the maximum number of subfields in a simple cell is equal to eight or the number of periods in weighting function is equal to four. Quantitative comparison of the data with the results of seven other studies allows us to suggest that the number of periods in linear component of some complex cells is close to this value. The discrepancies between the results of different authors in estimation of the number of subfields are explained by the experimental data.

Animals↗

PubMed1989

Harmonic basis functions for spatial coding in the cat striate cortex.

The number of subregions in the activity profiles of simple cells varies in different cells from 2-8; that is, the number of cycles in the weighting function varies from 1-4. The distribution of receptive-field (RF) sizes at eccentricities of 0-6 deg are clustered at half-octave intervals and form a discrete distribution with maxima at 0.62, 0.9, 1.24, 1.8, 2.48, and 3.4 deg. The spatial frequencies to which the cells are tuned are also clustered at half-octave intervals, forming a discrete distribution peaking at 0.45, 0.69, 0.9, 1.35, 1.88, 2.7, 3.8, and 5.6 cycles/deg. If we divide the RF sizes by the size of the period of the subregions, then the average indices of complexity (really existing) or the number of cycles in the weighting function form (after normalization) the sequences: 1, 1.41, 2.0, 2.9, 4.15. The relation between the bandwidth of the spatial-frequency characteristic and the optimal spatial frequency is in accordance with predictions of the Fourier hypothesis. The absolute bandwidth does not change with the number of cycles/module. This means that inside the module the absolute bandwidth does not change with the number of the harmonic. The results allow us to suggest the following. A module of the striate cortex, which is a group of cells with RFs of equal size projected onto the same area of central visual field, accounts for the Fourier description of the image. The basis functions of the module are composed of four harmonics only, irrespective of size and position of the module. Besides linear cells (sinusoidal and cosinusoidal elements), the module contains nonlinear cells, performing a nonlinear summation of the responses of sinusoidal and cosinusoidal elements. Such cells are characterized by an index of complexity which is more than the number of cycles in the weighting function and by marked overlap of ON and OFF zones. The analysis of organization suggests that the cells can measure the amplitude and phase of the stimulus.

Animals↗