Transparency and coherence in human motion perception

When confronted with moving images, the visual system often must decide whether the motion signals arise from a single object or from multiple objects. A special case of this problem arises when two independently moving gratings are superimposed. The gratings tend to cohere and move unambiguously in a single direction (pattern motion) instead of moving independently (component motion). Here we report that the tendency to see pattern motion depends very strongly on the luminance of the intersections (that is, to regions where the gratings overlap) relative to that of the gratings in a way that closely parallels the physics of transparency. When the luminance of these regions is chosen appropriately, pattern motion is destroyed and replaced by the appearance of two transparent gratings moving independently. The observations imply that motion detecting mechanisms in the visual system must have access to tacit 'knowledge' of the physics of transparency and that this knowledge can be used to segment the scene into different objects. The same knowledge could, in principle, be used to avoid confusing shadows with real object boundaries.     

A wide depth distribution of seismic tremors along the northern Cascadia margin

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as M(w) = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements-in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface-are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.     

Sequence and functional expression of the GABA A receptor shows a ligand-gated receptor super-family

Amino-acid sequences derived from complementary DNAs encoding the alpha- and beta-subunits of the GABA/benzodiazepine receptor from bovine brain show homology with other ligand-gated receptor subunits, suggesting that there is a super-family of ion-channel-containing receptors. Co-expression of the in vitro-generated alpha-subunit and beta-subunit RNAs in Xenopus oocytes produces a functional receptor and ion channel with the pharmacological properties characteristic of the GABAA receptor.     

Perception of apparent motion by commissurotomy patients

When two spatially separated light spots are flashed in rapid succession, the spot will appear to move between the two locations--an illusion called apparent motion. We have presented this display to callosum-sectioned human patients and found that they could correctly report the temporal order of a simple apparent motion sequence presented across the vertical meridian. Hence, the forebrain commissures are not required for this function.     

A prolactin-inhibiting factor within the precursor for human gonadotropin-releasing hormone

The cloned complementary DNA sequence encoding the human gonadotropin-releasing hormone (GnRH) precursor protein was used to construct an expression vector for the bacterial synthesis of the 56-amino acid GnRH-associated peptide (GAP). GAP was found to be a potent inhibitor of prolactin secretion and to stimulate the release of gonadotropins in rat pituitary cell cultures. Active immunization with peptides corresponding to GAP sequences led to greatly increased prolactin secretion in rabbits.     

Oriented crystallization of inorganic salts in collagen

Zusammenfassung Wenn gestreckte Kollagenfasern in einer alkalischen Lösung gewisser anorganischer Salze gelassen und dann getrocknet werden, so kristallisieren die Salze mit einer ihrer Achsen in paralleler Orientierung zur Faserachse. Die Periode parallel dieser Achse liegt zwischen 3,2 und 3,75 Å oder einem Vielfachen dieser Werte.     

Regulation of nicotine biogenesis 2: Increased production of nicotine by urea in tobacco tissue cultures

Summary The floral bud callus ofNicotiana tabacum L. was successfully grown on 10 mM urea as sole nitrogen source. Nicotine content in the urea-treated tissues was 3.5 times higher than in the control on the basis of dry weight, and 2.5-fold higher per culture. Contents of ornithine, citrulline and arginine, urea cycle intermediates, and known precursors of nicotine, were higher in urea-treated callus. The implications of the results are discussed.Financial support given to one of us (GAR) by the Maharaja Sayajirao University of Barado is gratefully acknowledged. Reference No. represents part 1 of the series on regulation of nicotine biogenesis.     

Reduced airway surface pH impairs bacterial killing in the porcine cystic fibrosis lung

Cystic fibrosis (CF) is a life-shortening disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Although bacterial lung infection and the resulting inflammation cause most of the morbidity and mortality, how the loss of CFTR function first disrupts airway host defence has remained uncertain. To investigate the abnormalities that impair elimination when a bacterium lands on the pristine surface of a newborn CF airway, we interrogated the viability of individual bacteria immobilized on solid grids and placed onto the airway surface. As a model, we studied CF pigs, which spontaneously develop hallmark features of CF lung disease. At birth, their lungs lack infection and inflammation, but have a reduced ability to eradicate bacteria. Here we show that in newborn wild-type pigs, the thin layer of airway surface liquid (ASL) rapidly kills bacteria in vivo, when removed from the lung and in primary epithelial cultures. Lack of CFTR reduces bacterial killing. We found that the ASL pH was more acidic in CF pigs, and reducing pH inhibited the antimicrobial activity of ASL. Reducing ASL pH diminished bacterial killing in wild-type pigs, and, conversely, increasing ASL pH rescued killing in CF pigs. These results directly link the initial host defence defect to the loss of CFTR, an anion channel that facilitates HCO(3)(-) transport. Without CFTR, airway epithelial HCO(3)(-) secretion is defective, the ASL pH falls and inhibits antimicrobial function, and thereby impairs the killing of bacteria that enter the newborn lung. These findings suggest that increasing ASL pH might prevent the initial infection in patients with CF, and that assaying bacterial killing could report on the benefit of therapeutic interventions.     

Perception of shape from shading

The human visual system can rapidly and accurately derive the three-dimensional orientation of surfaces by using variations in image intensity alone. This ability to perceive shape from shading is one of the most important yet poorly understood aspects of human vision. Here we present several findings which may help reveal computational mechanisms underlying this ability. First, we find that perception of shape from shading is a global operation which assumes that there is only one light source illuminating the entire visual image. This implies that if two identical objects are viewed simultaneously and illuminated from different angles, then we would be able to perceive three-dimensional shape accurately in only one of them at a time. Second, three-dimensional shapes that are defined exclusively by shading can provide tokens for the perception of apparent motion, suggesting that the motion mechanism is remarkably versatile in the kinds of inputs it can use. Lastly, the occluding edges which delineate an object from its background can also powerfully influence the perception of three-dimensional shape from shading.