Dissociable Perceptual Effects of Visual Adaptation

Neurons in the visual cortex are responsive to the presentation of oriented and curved line segments, which are thought to act as primitives for the visual processing of shapes and objects. Prolonged adaptation to such stimuli gives rise to two related perceptual effects: a slow change in the appearance of the adapting stimulus (perceptual drift), and the distortion of subsequently presented test stimuli (adaptational aftereffects). Here we used a psychophysical nulling technique to dissociate and quantify these two classical observations in order to examine their underlying mechanisms and their relationship to one another. In agreement with previous work, we found that during adaptation horizontal and vertical straight lines serve as attractors for perceived orientation and curvature. However, the rate of perceptual drift for different stimuli was not predictive of the corresponding aftereffect magnitudes, indicating that the two perceptual effects are governed by distinct neural processes. Finally, the rate of perceptual drift for curved line segments did not depend on the spatial scale of the stimulus, suggesting that its mechanisms lie outside strictly retinotopic processing stages. These findings provide new evidence that the visual system relies on statistically salient intrinsic reference stimuli for the processing of visual patterns, and point to perceptual drift as an experimental window for studying the mechanisms of visual perception.     

Chromatin degradation in white blood cells of irradiated rats

A study was made of the dose-time relationship during chromatin degradation in white blood cells of non-irradiated and irradiated rats. There was a linear increase in the release of PDN from leukocytes 1,2 and 3 days after irradiation (1-3 Gy) followed by the deceleration of the chromatin degradation at doses exceeding 3 Gy.     

Determination of the biological activity of Clostridium difficile toxins in in vivo and in vitro experiments

The biological activity of the filtrates of 29 C. difficile strains was studied in vivo (suckling white mice) and in vitro (cell cultures of different species and origin). The action of the filtrates on the experimental models in vivo was evaluated from the cytotoxic effect index, while in vitro the intensity of the cytotoxic effect was evaluated from the percentage of dead cells in the monolayer. The results of the comparative determination of toxicity characteristics in vivo and in vitro demonstrated that cell cultures were more sensitive experimental models than suckling white mice. The use of cell cultures permitted the quantitative evaluation of the cytotoxic activity of the filtrates under study, as well as the detection of their cell-directed action at minimal concentrations.     

The Differential Susceptibility of Vetch (Vicia spp.) to Orobanche aegyptiaca: Anatomical Studies

Vetches (Vicia spp .) are important forage legumes in the MiddleEast and Mediterranean regions. Most vetches are highly susceptibleto the root holoparasites Orobanche aegyptiaca and O. crenata,suffering severe quality and yield losses. However, purple vetch(V. atropurpurea) has shown good resistance to Orobanche. Microscopicstudies were performed to reveal anatomical differences of host-parasiteinteractions between susceptible and resistant vetch. SusceptibleV. sativa‘Yovel’ and resistant V. atropupurea‘Popany’were grown in association with O. aegyptiaca seeds, on glassmicrofibre sheets in a polyethylene-bag system. Whole root observationsusing stereoscopic microscopy detected necrotic lesions surroundingthe attachments of Orobanche radicles on resistant vetch roots.Hand-cut sections examined under the light microscope revealedthat in both susceptible and resistant vetch genotypes the Orobancheseeds germinated, attached and penetrated the host root epidermisand cortex. A reddish-brown secretion was observed in the apoplastat the interface between the parasite haustorium and the hosttissues, including the cell walls of the resistant vetch xylemvessels. Fixed and embedded sections observed under the lightmicroscope showed that in the susceptible genotype the parasitehaustorium penetrated through the endodermis into the host vascularcylinder, successfully forming a continuum with host vascularvessels. However, in the resistant genotype, the parasite haustoriumwas blocked at the root endodermis layer. The blockage was coupledwith secretion of unidentified material, thus preventing theparasite from establishing. These findings indicate that mechanicaland possibly chemical barriers are responsible for the hostdefence mechanism(s) conferring resistance of V. atropurpureato O. aegyptiaca. Copyright 2000 Annals of Botany Company Broomrape, Vicia atropurpurea, Vicia sativa, parasitic plants, plant resistance, tissue sections.