PubMed Health⌕ Search

Biomedical subjects

R Linsker

Publications and source records attributed to R Linsker.

10 recordsLinked to original sources
PubMed2006

Biased diffusion and universality in model queues.

We study the structure and robustness of universality classes for queueing, deriving analytic results for priority-based models with continuous-valued priorities. By mapping one model onto the problem of biased diffusion, we show that its distribution of waiting times, P(tau), decreases for large times tau as P(tau) approximately tau;{-3/2} or as P(tau) approximately tau;{-5/2}exp(-tau/tau_{0}) in different parameter regimes. In a second model, introducing a cost for switching between different classes of tasks substantially changes the asymptotic behavior of P(tau).

Journal Article↗
PubMed1988

Laser-induced fluorescence detection of atherosclerotic plaque with hematoporphyrin derivative used as an exogenous probe.

Clinical use of laser angioplasty is limited by the lack of an adequate guidance system. As a first step toward developing a reliable guidance system, laser-induced surface fluorescence and a fluorescent probe were used to differentiate plaque from normal arterial wall. The aortas from four normal New Zealand white rabbits and six atherosclerotic rabbits were studied in vitro. Rabbits from each group received 2.5 mg/kg of hematoporphyrin derivative intravenously 24 hours before death. Segments of the aortas were irradiated with a helium-cadmium laser, and the tissue surface fluorescence spectra were recorded with an optical multichannel analyzer. A plaque index, based on the resulting spectra, was calculated for each specimen of aorta. The spectra from normal aorta without hematoporphyrin derivative, normal aorta with hematoporphyrin derivative, and from plaque of atherosclerotic rabbits without hematoporphyrin derivative showed the same wavelength dependence. The plaque index values were not significantly different from one another. However, in plaque from atherosclerotic rabbits given hematoporphyrin derivative, the spectrum was markedly different, showing a broad spectral peak at 632 nm. This spectral peak corresponds to the spectral peak of hematoporphyrin derivative and was only seen in the plaque of atherosclerotic rabbits given hematoporphyrin derivative. The plaque index for these specimens was significantly different from all other specimens (p less than 0.001). This difference in fluorescence spectra and plaque index could be incorporated into a guidance system for laser angioplasty.

Angioplasty, Balloon↗
PubMed1986

A measure of DNA periodicity.

A Fourier transform g(n) of a sequence of bases along a given stretch of DNA is defined. The transform is invariant to the labelling of the bases and can therefore be used as a measure of periodicity for segments of DNA with differing base content. It can also be conveniently used to search for base periodicities within large DNA data bases.

Base Sequence↗
PubMed1986

From basic network principles to neural architecture: emergence of spatial-opponent cells.

The functional architecture of mammalian visual cortex has been elucidated in impressive detail by experimental work of the past 20-25 years. The origin of many of the salient features of this architecture, however, has remained unexplained. This paper is the first of three (the others will appear in subsequent issues of these Proceedings) that address the origin and organization of feature-analyzing (spatial-opponent and orientation-selective) cells in simple systems governed by biologically plausible development rules. I analyze the progressive maturation of a system composed of a few layers of cells, with connections that develop according to a simple set of rules (including Hebb-type modification). To understand the prenatal origin of orientation-selective cells in certain primates, I consider the case in which there is no external input, with the first layer exhibiting random spontaneous electrical activity. No orientation preference is specified to the system at any stage, and none of the basic developmental rules is specific to visual processing. Here I introduce the theory of "modular self-adaptive networks," of which this system is an example, and explicitly demonstrate the emergence of a layer of spatial-opponent cells. This sets the stage for the emergence, in succeeding layers, of an orientation-selective cell population.

Animals↗
PubMed1986

From basic network principles to neural architecture: emergence of orientation-selective cells.

This is the second paper in a series of three that explores the emergence of several prominent features of the functional architecture of visual cortex, in a "modular self-adaptive network" containing several layers of cells with parallel feedforward connections whose strengths develop according to a Hebb-type correlation-rewarding rule. In the present paper I show that orientation-selective cells, similar to the "simple" cortical cells of Hubel and Wiesel [Hubel, D. H. & Wiesel, T. N. (1962) J. Physiol. 160, 106-154], emerge in such a network. No orientation preference is specified to the system at any stage, the orientation-selective cell layer emerges even in the absence of environmental input to the system, and none of the basic developmental rules is specific to visual processing.

Adaptation, Physiological↗
PubMed1986

From basic network principles to neural architecture: emergence of orientation columns.

Orientation-selective cells--cells that are selectively responsive to bars and edges at particular orientations--are a salient feature of the architecture of mammalian visual cortex. In the previous paper of this series, I showed that such cells emerge spontaneously during the development of a simple multilayered network having local but initially random feedforward connections that mature, one layer at a time, according to a simple development rule (of Hebb type). In this paper, I show that, in the presence of lateral connections between developing orientation cells, these cells self-organize into banded patterns of cells of similar orientation. These patterns are similar to the "orientation columns" found in mammalian visual cortex. No orientation preference is specified to the system at any stage, none of the basic developmental rules is specific to visual processing, and the results emerge even in the absence of visual input to the system (as has been observed in macaque monkey).

Cell Communication↗
PubMed1985

Ultraviolet-laser ablation of skin.

We report on the use of pulsed ultraviolet-laser irradiation at 193 nm from an argon-fluoride laser and at 248 nm from a krypton-fluoride laser to ablate skin. In vitro, both wavelengths performed comparably, removing tissue precisely and cleanly, and leaving minimal thermal damage to the surrounding tissue. In vivo, the 193-nm laser radiation failed to remove tissue after bleeding began. The 248-nm radiation, however, continued to remove tissue despite bleeding and left a clean incision with only minimal thermal damage. The krypton-fluoride excimer laser beam at 248 nm, which should be deliverable through a quartz optical fiber, has great potential as a surgical instrument.

Animals↗
PubMed1984

Far-ultraviolet laser ablation of atherosclerotic lesions.

Far-ultraviolet (far-UV) (193 nm) laser radiation ablates arterial wall tissue, including noncalcified atherosclerotic lesions, with no apparent thermal damage to remaining tissue. This effect contrasts sharply with the thermal damage produced by visible-wavelength laser irradiation. The mechanism by which far-UV radiation interacts with tissue is predominantly photochemical rather than photothermal. Potential clinical applications include those in which geometrically precise removal of tissue, without thermal damage to the remaining substrate, is desired. Ultraviolet laser catheterization appears practical with respect to the availability of fiberoptic materials and high-pulse-rate excimer lasers.

Aorta↗
PubMedDate not supplied

Synchronization of the acoustic evidence in the assassination of President Kennedy.

We have revisited the acoustic evidence in the Kennedy assassination--recordings of the two Dallas police radio channels upon which our original NRC report (Ramsey NF et al., Report of the Committee on Ballistic Acoustics. National Research Council (US). Washington: National Academy Press, 1982. Posted at http://www.nap.edu/catalog/10264.html) was based--in response to the assertion by DB Thomas (Echo correlation analysis and the acoustic evidence in the Kennedy assassination revisited. Science and Justice 2001; 41: 21-32) that alleged gunshot sounds (on Channel 1), apparently recorded from a motorcycle officer's stuck-open microphone, occur at the exact time of the assassination (as established by emergency communications on Channel 2). We have critically reviewed these two publications, and have performed additional analyses. In particular we have used recorded 60 Hz hum and correlation methods to obtain accurate speed calibrations for recordings made on both channels, cepstral analysis to seek instances of repeated segments during playback of Channel 2 (which could result from groove jumping), and spectrographic and correlation methods to analyze instances of putative crosstalk used to synchronize the two channels. This paper identifies serious errors in the Thomas paper and corrects errors in the NRC report. We reaffirm the earlier conclusion of the NRC report that the alleged "shot" sounds were recorded approximately one minute after the assassination.

Acoustics↗