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Lior Segev

Publications and source records attributed to Lior Segev.

7 recordsLinked to original sources
PubMed2006

Radiation damage to alkyl chain monolayers on semiconductor substrates investigated by electron spectroscopy.

Monolayers of alkyl chains, attached through direct Si-C bonds to Si(111), via phosphonates to GaAs(100) surfaces, or deposited as alkyl-silane monolayers on SiO2, are investigated by ultraviolet and inverse photoemission spectroscopy and X-ray absorption spectroscopy. Exposure to ultraviolet radiation from a He discharge lamp, or to a beam of energetic electrons, leads to significant damage, presumably associated with radiation- or electron-induced H-abstraction leading to carbon-carbon double-bond formation in the alkyl monolayer. The damage results in an overall distortion of the valence spectrum, in the appearance of (occupied) states above the highest occupied molecular orbital of the alkyl molecule, and in a characteristic (unoccupied state) pi resonance at the edge of the carbon absorption peak. These distortions present a serious challenge for the interpretation of the electronic structure of the monolayer system. We show that extrapolation to zero damage at short exposure times eliminates extrinsic features and allows a meaningful extraction of the density of state of the pristine monolayer from spectroscopy measurements.

Journal Article↗
PubMed2005

Single polymer chains as specific transducers of molecular recognition in scanning probe microscopy.

A new approach for the specific detection and mapping of single molecule recognition is presented, based on the nonlinear elastic behavior of a single polymer chain. The process of molecular recognition between a ligand and a receptor is inherently accompanied by a decrease in the translational and rotational degrees of freedom of the two molecules. We show that a polymeric tether linked to the ligand can effectively transduce the configurational constraint imposed by molecular recognition into a measurable force, which is dominated by the entropic elasticity of the polymer. This force is specifically characterized by a strong nonlinearity when the extension of the polymer approaches its contour length. Thus, a polymer chain tethering the ligand to an oscillating cantilevered tip gives rise to a highly anharmonic motion upon ligand-receptor binding. Higher-harmonics atomic force microscopy allows us to detect this phenomenon in real time as a specific signature for the probing and mapping of single-molecule recognition.

Entropy↗
PubMed2003

Conformational rearrangements associated with the gating of the G protein-coupled potassium channel revealed by FRET microscopy.

G protein-coupled potassium channels (GIRK/Kir3.x) are key determinants that translate inhibitory chemical neurotransmission into changes in cellular excitability. To understand the mechanism of channel activation by G proteins, it is necessary to define the structural rearrangements in the channel that result from interaction with Gbetagamma subunits. In this study we used a combination of fluorescence spectroscopy and through-the-objective total internal reflection microscopy to monitor the conformational rearrangements associated with the activation of GIRK channels in single intact cells. We detect activation-induced changes in FRET consistent with a rotation and expansion of the termini along the central axis of the channel. We propose that this rotation and expansion of the termini drives the channel to open by bending and possibly rotating the second transmembrane segment.

Animals↗
PubMed2003

Localization of function via lesion analysis.

This article presents a general approach for employing lesion analysis to address the fundamental challenge of localizing functions in a neural system. We describe functional contribution analysis (FCA), which assigns contribution values to the elements of the network such that the ability to predict the network's performance in response to multilesions is maximized. The approach is thoroughly examined on neurocontroller networks of evolved autonomous agents. The FCA portrays a stable set of neuronal contributions and accurate multilesion predictions that are significantly better than those obtained based on the classical single lesion approach. It is also used for a detailed synaptic analysis of the neurocontroller connectivity network, delineating its main functional backbone. The FCA provides a quantitative way of measuring how the network functions are localized and distributed among its elements. Our results question the adequacy of the classical single lesion analysis traditionally used in neuroscience and show that using lesioning experiments to decipher even simple neuronal systems requires a more rigorous multilesion analysis.

Algorithms↗
PubMed2003

High-dimensional analysis of evolutionary autonomous agents.

This article presents a new approach to the important challenge of localizing function in a neurocontroller. The approach is based on the basic functional contribution analysis (FCA) presented earlier, which assigns contribution values to the elements of the network, such that the ability to predict the network's performance in response to multi-unit lesions is maximized. These contribution values quantify the importance of each element to the tasks the agent performs. Here we present a generalization of the basic FCA to high-dimensional analysis, using high-order compound elements. Such elements are composed of conjunctions of simple elements. Their usage enables the explicit expression of sets of neurons or synapses whose contributions are interdependent, a prerequisite for localizing the function of complex neurocontrollers. High-dimensional FCA is shown to significantly improve on the accuracy of the basic analysis, to provide new insights concerning the main subsets of simple elements in the network that interact in a complex nonlinear manner, and to systematically reveal the types of interactions that characterize the evolved neurocontroller.

Artificial Intelligence↗
PubMedDate not supplied

Drug abuse in travelers to southeast Asia: an on-site study.

BACKGROUND: Drug abuse constitutes a major sociomedical problem throughout the world. A unique subgroup with a higher potential of drug abuse are young travelers to Southeast Asia. Less than a handful of studies have focused on this population, and even fewer have been carried out on site. Our aim was to characterize the phenomenon of drug abuse among Israelis and other nationals, and to define risk factors that would predict which travelers are prone to abusing drugs. METHODS: Data was collected through questionnaires that were distributed in Southeast Asia to 430 travelers. Medical students administered the questionnaires across India, Thailand, Nepal, Vietnam, and Laos during 2002 and 2003. RESULTS: Questionnaires from 231 Israelis and 199 other nationals (mostly from the United Kingdom, Sweden, Australia, and Germany) were analyzed. These travelers had a mean age of 25.3 years. We found that 54.3% of the travelers abused drugs during the trip. Israelis (66.2%) abused drugs more frequently than did non-Israelis (40.7%, p < .001). Males abused drugs significantly more than females did, as did secular more than religious people; however, those with an academic degree abused drugs less than others. For 23.5% of the Israelis, the trip was their first encounter with drugs. Of the entire cohort, 72% abused cannabis products, and most of them (49.6%) did it on daily basis. The use of "hard" drugs (eg, lysergic acid diethylamide) was more common among non-Israelis than among the Israelis (37% and 20%, respectively; p < .006). Much higher rates of drug abuse (70.1%) were found in India than in other Southeast Asian countries. Logistic regression identified that prior use of drugs, Israeli nationality, travel to India, cigarette smoking, and traveling alone were significant predictors of drug abuse. CONCLUSIONS: There is a disturbingly high rate of drug abuse in travelers to certain Southeast Asian countries, both among Israeli and other nationals. For many youngsters, this is their first encounter with drugs, and many plan to continue abusing drugs upon their repatriation. Travelers to Southeast Asia should be a major target group for primary, preventive, antidrug campaigns worldwide.

Adult↗