Control mechanisms in the human binocular oculomotor system

A study of human eye movements was made in order to elucidate the nature of the control mechanism in the binocular oculomotor system.

We first examined spontaneous eye movements during monocular and binocular fixation in order to determine the corrective roles of flicks and drifts. It was found that both types of motion correct fixational errors, although flicks are somewhat more active in this respect. Vergence error is a stimulus for correction by drifts but not by flicks, while binocular vertical discrepancy of the visual axes does not trigger corrective movements.

Second, we investigated the non-linearities of the oculomotor system by examining the eye movement responses to point targets moving in two dimensions in a subjectively unpredictable manner. Such motions consisted of hand-limited Gaussian random motion and also of the sum of several non-integrally related sinusoids. We found that there is no direct relationship between the phase and the gain of the oculomotor system. Delay of eye movements relative to target motion is determined by the necessity of generating a minimum afferent (input) signal at the retina in order to trigger corrective eye movements. The amplitude of the response is a function of the biological constraints of the efferent (output) portion of the system: for target motions of narrow bandwidth, the system responds preferentially to the highest frequency; for large bandwidth motions, the system distributes the available energy equally over all frequencies. Third, the power spectra of spontaneous eye movements were compared with the spectra of tracking eye movements for Gaussian random target motions of varying bandwidths. It was found that there is essentially no difference among the various curves. The oculomotor system tracks a target, not by increasing the mean rate of impulses along the motoneurons of the extra-ocular muscles, but rather by coordinating those spontaneous impulses which propagate along the motoneurons during stationary fixation. Thus, the system operates at full output at all times.

Fourth, we examined the relative magnitude and phase of motions of the left and the right visual axes during monocular and binocular viewing. We found that the two visual axes move vertically in perfect synchronization at all frequencies for any viewing condition. This is not true for horizontal motions: the amount of vergence noise is highest for stationary fixation and diminishes for tracking tasks as the bandwidth of the target motion increases. Furthermore, movements of the occluded eye are larger than those of the seeing eye in monocular viewing. This effect is more pronounced for horizontal motions, for stationary fixation, and for lower frequencies.

Finally, we have related our findings to previously known facts about the pertinent nerve pathways in order to postulate a model for the neurological binocular control of the visual axes.

The ecological impact of different mechanisms of chronic sub-lethal toxicity on feeding and respiratory physiology

Sub-lethal toxicity tests, such as the scope-for-growth test, reveal simple relationships between measures of contaminant concentration and effect on respiratory and feeding physiology. Simple models are presented to investigate the potential impact of different mechanisms of chronic sub-lethal toxicity on these physiological processes. Since environmental quality is variable, even in unimpacted environments, toxicants may have differentially greater impacts in poor compared to higher quality environments. The models illustrate the implications of different degrees and mechanisms of toxicity in response to variability in the quality of the feeding environment, and variability in standard metabolic rate. The models suggest that the relationships between measured degrees of toxic stress, and the maintenance ration required to maintain zero scope-for-growth, may be highly nonlinear. In addition it may be possible to define critical levels of sub-lethal toxic effect above which no environment is of sufficient quality to permit prolonged survival.

Resveratrol: Anti-Obesity Mechanisms of Action

Resveratrol is a non-flavonoid polyphenol which belongs to the stilbenes group and is produced naturally in several plants in response to injury or fungal attack. Resveratrol has been recently reported as preventing obesity. The present review aims to compile the evidence concerning the potential mechanisms of action which underlie the anti-obesity effects of resveratrol, obtained either in cultured cells lines and animal models. Published studies demonstrate that resveratrol has an anti-adipogenic effect. A good consensus concerning the involvement of a down-regulation of C/EBPa and PPAR. in this effect has been reached. Also, in vitro studies have demonstrated that resveratrol can increase apoptosis in mature adipocytes. Furthermore, different metabolic pathways involved in triacylglycerol metabolism in white adipose tissue have been shown to be targets for resveratrol. Both the inhibition of de novo lipogenesis and adipose tissue fatty acid uptake mediated by lipoprotein lipase play a role in explaining the reduction in body fat which resveratrol induces. As far as lipolysis is concerned, although this compound per se seems to be unable to induce lipolysis, it increases lipid mobilization stimulated by beta-adrenergic agents. The increase in brown adipose tissue thermogenesis, and consequently the associated energy dissipation, can contribute to explaining the body-fat lowering effect of resveratrol. In addition to its effects on adipose tissue, resveratrol can also acts on other organs and tissues. Thus, it increases mitochondriogenesis and consequently fatty acid oxidation in skeletal muscle and liver. This effect can also contribute to the body-fat lowering effect of this molecule.

Mechanisms involved in Escherichia coli and Serratia marcescens removal during activated sludge wastewater treatment

Wastewater treatment reduces environmental contamination by removing gross solids and mitigating the effects of pollution. Treatment also reduces the number of indicator organisms and pathogens. In this work, the fates of two coliform bacteria, Escherichia coli and Serratia marcescens, were analyzed in an activated sludge process to determine the main mechanisms involved in the reduction of pathogenic microorganisms during wastewater treatment. These bacteria, modified to express green fluorescent protein, were inoculated in an activated sludge unit and in batch systems containing wastewater. The results suggested that, among the different biological factors implied in bacterial removal, bacterivorous protozoa play a key role. Moreover, a representative number of bacteria persisted in the system as free-living or embedded cells, but their distribution into liquid or solid fractions varied depending on the bacterium tested, questioning the real value of bacterial indicators for the control of wastewater treatment process. Additionally, viable but nonculturable cells constituted an important part of the bacterial population adhered to solid fractions, what can be derived from the competition relationships with native bacteria, present in high densities in this environment. These facts, taken together, emphasize the need for reliable quantitative and qualitative analysis tools for the evaluation of pathogenic microbial composition in sludge, which could represent an undefined risk to public health and ecosystem functions when considering its recycling.

Mechanisms of Yb3+ sensitization to Tm3+ for blue upconversion luminescence in fluorophosphate glass

The sensitization mechanisms of Yb3+ to Tm3+ for the blue upconversion luminescence in fluorophosphate glass were studied. Two different mechanisms exist in the sensitization. One is the sequential sensitization that Tm3+ is excited from H-3(6) to (1)G(4) through absorbing three photons transferred from Yb3+ one by one. Another is the cooperative sensitization that two Yb3+ ions form a couple cluster firstly, and then the couple cluster Yb3+ ions transfer their energy to Tm3+ and excite it to (1)G(4). With the increment of the concentration of Yb3+ ions, the sequential sensitization becomes weak and the cooperative sensitization becomes intense, and the transformation trend of sensitization mechanism with the increment of Yb3+ concentration can be clarified by the introduction of Th3+ ions in the glass. (c) 2006 Elsevier B.V. All rights reserved.

Upconversion luminescence and mechanisms of Tm3+/Yb3+-codoped oxyhalide tellurite glasses

To obtain efficient blue upconversion laser glasses, upconversion luminescence and mechanisms of Tm3+/Yb3+-codoped oxyhalide tellurite glasses were investigated under 980nm excitation. The results showed that upconversion blue and red emission intensities of Tm3+ first increase, reach its maximum at TM2O3% =0.1 mol%, and then decrease with increasing Tm2O3 content. The effect of TM2O3 content on upconversion intensity is discussed, and possible effect mechanisms are evaluated. The investigated results were conducing to increase upconversion luminescence efficiency of Tm3+. (c) 2005 Elsevier B.V. All rights reserved.

Mechanisms and concentrations dependence of up-conversion luminescence in Tm3+/Yb3+ codoped oxyfluoride glass-ceramics

The up-conversion properties of Tm3+/Yb3+ codoped oxyfluoride glass-ceramics under 980 nm excitation were investigated. Intense blue up-conversion luminescence due to the Tm3+: (1)G(4) -> H-3(6) transition was observed in the glass-ceramics. The intensity of the blue up-conversion luminescence in a 1 mol% YbF3-containing glass-ceramic was found to be about 40 times stronger than that in the precursor oxyfluoride glass. The up-conversion mechanism is proposed. The reason for the intense Tm3+ up-conversion luminescence in the oxyfluoride glass-ceramics and the concentrations dependence of upconversion luminescence are also discussed. (c) 2005 Elsevier B.V. All rights reserved.

Upconversion luminescence and mechanisms in Yb3+-sensitized Tm3+-doped oxyhalide tellurite glasses

Effect of Yb2O3 content on upconversion luminescence and mechanisms in Yb3+-sensitized Tm3+-doped oxyhalide tellurite glasses were investigated under 980 nm excitation. Intense blue and relatively weak red upconversion emission centered at 476 and 649nm corresponding to the transitions (1)G(4) -> H-3(6) and (1)G(4) -> H-3(4) of Tm3+, respectively, are simultaneously observed at room temperature. The results show that upconversion blue and red emission intensities of Tm3+ first increase, reach its maximum at Yb2O3% = 3 mol%, and then decrease with increasing Yb2O3 content. The effect of Yb2O3 content on upconversion intensity is discussed, and possible effect mechanisms are evaluated. The investigated results were conducing to increase upconversion luminescence efficiency of Tm3+. (c) 2005 Elsevier B.V. All rights reserved.