Saccadic suppression precedes visual motion analysis.

There is now good evidence that perception of motion is strongly suppressed during saccades (rapid shifts of gaze), presumably to blunt the disturbing sense of motion that saccades would otherwise elicit. Other aspects of vision, such as contrast detection of high-frequency or equiluminant gratings, are virtually unaffected by saccades [1] [2] [3] [4] [5]. This has led to the suggestion that saccades may suppress selectively the magnocellular pathway (which is strongly implicated in motion perception), leaving the parvocellular pathway unaffected [5] [6]. Here, we investigate the neural level at which perception of motion is suppressed. We used a simple technique in which an impression of motion is generated from only two frames, allowing precise control over the stimulus [7] [8]. One frame has a certain fixed contrast, whereas the contrast of the other (the test frame) is varied to determine the threshold for motion discrimination (that is, the lowest test-frame contrast level at which the direction of motion can be correctly guessed). Contrast thresholds of the test depended strongly and non-monotonically on the contrast of the fixed-contrast frame, with a minimum at medium contrast. To study the effect of saccadic suppression, we triggered the two-frame sequence by a voluntary saccade. Thresholds during saccades increased in a way that suggested that saccadic suppression precedes motion analysis: when the test frame was first in the motion sequence there was a general depression of sensitivity, whereas when it was second, the contrast response curve was shifted to a higher contrast range, sometimes even resulting in higher sensitivity than without a saccade. The dependence on presentation order suggests that saccadic suppression occurs at an early stage of visual processing, on the single frames themselves rather than on the combined motion signal. As motion detection itself is thought to occur at an early stage, saccadic suppression must take place at a very early phenomenon.     

The perception of visual motion.

Recent developments have led to a greater insight into the complex processes of perception of visual motion. A better understanding of the neuronal circuitry involved and advances in electrophysiological techniques have allowed researchers to alter the perception of an animal with a stimulating electrode. In addition, studies have further elucidated the processes by which signals are combined and compared, allowing a greater understanding of the effects of selective brain damage.     

Flavonoids in the Flowers of Gnaphalium affine D. Don

Thermonsenstivie division mutants were derived from Bacillus subtilis Marburg 168 thy trp₂ by means of membrane filtration after nitrosoguanidine mutagenesis. Among them, ts42 requiring uracil for normal growth at 48°C was investigated.

In the absence of uracil, the mutant cells grew normally at 37°C and stopped dividing after temperature shift to 48°C resulting in filaments of two to four times length of normal rods. The total cell number after temperature shift from 37 to 48°C, increased two to three fold in 90 min and remained constant thereafter. The viable count after the temperature shift to 48°C, increased 1.5 to 2 fold in initial 60 min and then decreased exponentially. A rapid restoration of colony forming ability was shown when the mutant cells were shifted back to the permissive temperature after 120 to 180 min of incubation at 48°C or when uracil was introduced to the culture at 48°C. This recovery of viability was partly observed even in the presence of chloramphenicol. The synthesis of RNA of this mutant was shown to decline 20 min after the temperature shift to 48°C whereas the syntheses of DNA and protein proceeded for more than 80 min at that temperature.

No newly isolated uracil requiring mutants formed filaments in the medium lacking uracil or showed growth pattern like ts42.     

Taxonomic implications of the chemical analysis of the D2 group of corynebacteria

Whole cell acid methanolysates from corynebacteria of the D2 group were found to contain meso-diaminopimelic acid, arabinose and galactose. Among lipids of taxonomic value, saturated and unsaturated straight chain fatty acids (14 to 18 carbon atoms), tuberculostearic acid (10-methyl octadecanoic acid) and mycolic acids were present. The last compounds ranged from 26 to 36 carbon atoms, the predominant types being 28.2, 28.1, 30.3, 30.2, 32.3 and 32.2. By reverse phase thin-layer chromatography the major menaquinone was identified as MK-9(H2)-containing nine isoprene units with two additional hydrogens. Moreover, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol and phosphatidylinositol mannosides were detected among the phospholipids of these bacteria. Thus, on these bases, the D2 group appears to be closely related to the true corynebacteria.     

2D gene expression parameters of wing imaginal disc of Drosophila for developmental analysis

 By using high resolution two-dimensional (2D) gel electrophoresis coupled with computer-analysis we have established a quantitative Drosophila wing imaginal disc protein database of third instar larvae as a reference to be used for comparative purposes in genetic studies. A general catalogue integrated by 1,184 ³⁵S-methionine-labelled polypeptides from wing imaginal disc has been obtained. The level of expression for all the proteins has been quantitatively determined. The quantitative reproducibility of the analysis system has been estimated and all the controls studied as database reference to interpret the results of experiments with mutant discs. One example, corresponding to iro ¹ mutation, has been used to show how some of the changes observed with mutant discs clearly extend out of the limits defined by the controls. This enables us to generate comparative parameters for the study of proliferation, morphogenesis and differentiation of Drosophila and opens the possibility of rapidly defining the nature and quantity of changes in patterns of gene expression in developmental genetic studies. Received: 21 June 1996 / Accepted: 27 September 1996     

Low-level visual processing of biological motion.

Biological motion displays depict a moving human figure by means of just a few isolated points of light attached to the major joints of the body. Naive observers readily interpret the moving pattern of dots as representing a human figure, despite the complete absence of form cues. This paper reports a series of experiments which investigated the visual processes underlying the phenomenon. Results suggest that (i) the effect relies upon responses in low-level motion-detecting processes, which operate over short temporal and spatial intervals and respond to local modulations in image intensity; and (ii) the effect does not involve hierarchical visual analysis of motion components, nor does it require the presence of dots which move in rigid relation to each other. Instead, movements of the extremities are crucial. Data are inconsistent with current theoretical treatments.     

Automated analysis of rabbit sperm motility and the effect of chemicals on sperm motion parameters.

Appropriate software settings and optimum procedures were determined for the measurement of the motion parameters of rabbit spermatozoa by the CellSoft (Cryo Resources Ltd., Montgomery, NY) computer-assisted digital image analysis system. The system was used to follow motion parameter changes occurring in spermatozoa incubated for 6 hr with or without exposure to chemicals. Mean amplitude of lateral head displacement (AALH) increased over the 6 hr period, while curvilinear velocity (Vc) first increased and then decreased. Values for linearity (Lin), or beat cross frequency (BCF), were unchanged. The majority of spermatozoa progressed linearly, with rapid rotation of the sperm head, but subpopulations of spermatozoa with different swimming patterns appeared after 1-3 hr of incubation. Percentage motile sperm and Vc were most sensitive to the action of the compounds (pyrogallol, hydroquinone, ammonium oxalate, triethyl phosphite, and pinocolyl alcohol), while BCF was least affected. The decline in percentage of motile sperm was dependent on duration of exposure and chemical concentration. Mean Vc of the sperm population decreased rapidly upon chemical exposure and remained at a low value until motility ceased. The initial decrease in Vc was dependent on the concentration of the added compound. Motion-based indices--motility concentration (MCI50), motility time (MTI50), and velocity (VI)--were defined and used as toxicological endpoints. The rank order of these indices, the end point of the neutral red in vitro assay for cytotoxicity, and LD50 values for the five compounds were the same, suggesting that chemical inhibition of sperm motility may be useful as a method for the in vitro assessment of chemical cytotoxicity.     

From 1D to 2D via 3D: dynamics of surface motion segmentation for ocular tracking in primates.

In primates, tracking eye movements help vision by stabilising onto the retinas the images of a moving object of interest. This sensorimotor transformation involves several stages of motion processing, from the local measurement of one-dimensional luminance changes up to the integration of first and higher-order local motion cues into a global two-dimensional motion immune to antagonistic motions arising from the surrounding. The dynamics of this surface motion segmentation is reflected into the various components of the tracking responses and its underlying neural mechanisms can be correlated with behaviour at both single-cell and population levels. I review a series of behavioural studies which demonstrate that the neural representation driving eye movements evolves over time from a fast vector average of the outputs of linear and non-linear spatio-temporal filtering to a progressive and slower accurate solution for global motion. Because of the sensitivity of earliest ocular following to binocular disparity, antagonistic visual motion from surfaces located at different depths are filtered out. Thus, global motion integration is restricted within the depth plane of the object to be tracked. Similar dynamics were found at the level of monkey extra-striate areas MT and MST and I suggest that several parallel pathways along the motion stream are involved albeit with different latencies to build-up this accurate surface motion representation. After 200-300 ms, most of the computational problems of early motion processing (aperture problem, motion integration, motion segmentation) are solved and the eye velocity matches the global object velocity to maintain a clear and steady retinal image.     

Gentamicin-based medium for the isolation of group D streptococci and application of the medium to water analysis.

Gentamicin-thallous-carbonate (GTC) medium contained (per liter): 40.0 g of Trypticase soy agar, 5.0 g of KH(2)PO(4), 2.0 g of NaHCO(2), 1.0 g of glucose, 1.0 g of esculin, 0.5 g of thallous acetate (TA), 0.5 g of ferric citrate, 0.75 ml of Tween 80, and 2.5 mg of gentamicin sulfate. The NaHCO(3) (20 ml of a 10% solution that had been heated to boiling) was added after sterilization of the basal medium. The spread plate technique was used to compare GTC agar with Pfizer selective enterococcus, TA, and KF agars by using sewage as well as bovine and swine fecal samples. Significantly greater numbers of group D streptococci were recovered on GTC agar than on Pfizer selective enterococcus or KF agars, within and over all samples. Higher counts also were obtained on GTC than on TA agar, but the differences were not statistically significant. The percentage of false positives was about the same for all four media. Samples of riverwater also were plated on GTC, TA, and KF agars, and significantly higher recoveries were obtained with GTC agar. GTC agar was superior to the other media examined primarily because of increased recoveries of Streptococcus bovis and S. equinus; other advantages of GTC agar were large colony size and short (24-h) incubation period. The percentage of false positives from riverwater was 13% for GTC agar and 0% for TA and KF agars; therefore, confirmation would be necessary when GTC agar is used with some types of environmental samples.     

Molecular analysis of the rfb gene cluster of a group D2 Salmonella enterica strain: evidence for its origin from an insertion sequence-mediated recombination event between group E and D1 strains.

The Salmonella enterica O antigen is a highly variable surface polysaccharide composed of a repeated oligosaccharide (the O unit). The O unit produced by serogroup D2 has structural features in common with those of groups D1 and E1, and hybridization studies had previously suggested that the D2 rfb gene cluster responsible for O-unit biosynthesis is indeed a hybrid of the two. In this study, the rfb gene cluster was cloned from a group D2 strain of S. enterica sv. Strasbourg. Mapping, hybridization, and DNA sequencing showed that the organization of the D2 rfb genes is similar to that of group D1, with the alpha-mannosyl transferase gene rfbU replaced by rfbO, the E1-specific beta-mannosyl transferase gene. The E1-specific polymerase gene (rfc) has also been acquired. Interestingly, the D1-like and E1-like rfb regions are separated by an additional sequence closely related to an element (Hinc repeat [H-rpt]) associated with the Rhs loci of Escherichia coli. The H-rpt resembles an insertion sequence and possibly mediated the intraspecific recombination events which produced the group D2 rfb gene organization.     

Energy filters,motion uncertainty,and motion sensitive cells in the visual cortex: a mathematical analysis

Energy filters are tuned to space-time frequency orientations. In order to compute velocity it is necessary to use a collection of filters, each tuned to a different space-time frequency. Here we analyze, in a probabilistic framework, the properties of the motion uncertainty. Its lower bound, which can be explicitly computed through the Cramér-Rao inequality, will have different values depending on the filter parameters. We show for the Gabor filter that, in order to minimize the motion uncertainty, the spatial and temporal filter sizes cannot be arbitrarily chosen; they are only allowed to vary over a limited range of values such that the temporal filter bandwidth is larger than the spatial bandwidth. This property is shared by motion sensitive cells in the primary visual cortex of the cat, which are known to be direction selective and are tuned to spacetime frequency orientations. We conjecture that these cells have larger temporal bandwidth relative to their spatial bandwidth because they compute velocity with maximum efficiency, that is, with a minimum motion uncertainty.     

Multiplication noise in the human visual system at threshold: 2. Probit estimation of parameters

A mathematical technique is described that relates detection model parameters to stimulus magnitude and experimental probability of detection. The normalizing transform is used to make the response statistics approximately Gaussian. Conventional probit analysis is then applied. From measurements at M stimulus levels, a system of M equations is solved and estimates of M unknown parameters of the detection model are obtained. The technique is applied to a threshold vision model based on additive and multiplicative Poisson noise. Results are obtained for the parameter estimates for individual subjects, and for the standard deviation of the estimates, for various values of the stimulus energy and number of trials. A frequency-of-seeing experiment is performed using a point-source stimulus that randomly assumes 3 energy levels with 200 trials per level. With a central efficiency of 50%, the estimated ocular quantum efficiency for our four subjects lies between 12% and 23%, the average dark count at the retina lies between 8 and 36 counts, and the threshold count for our (low falsereport rate) data lies between 11 and 32. The theoretical results reduce to those obtained by Barlow (J. Physiol. London 160, 155–168, 1962), in the absence of dark light and multiplication noise.This work was supported by the National Science Foundation     

Measurement of visual channels by contrast adaptation.

Inspection of a high-contrast grating pattern affects our ability to detect patterns that are similar. This technique can be used to infer the underlying mechanisms of the visual system. By using this technique, measurements of the bandwidth of orientation channels are taken for different levels of adapting contrast and adapting duration. If the threshold elevation is plotted as the difference between the unadapted and adapted threshold in decibels, then the orientation bandwidth is invariant if taken at some fraction of the maximum elevation. This results from the fact that, as the orientation difference between the adapting and test patterns increases, the function relating threshold elevation to adapting contrast reduces in slope. These data contradict the often-used 'equivalent contrast transformation' (in which the fall off in the adaptation effect with respect to orientation is expressed in terms of an equivalent reduction in adapting contrast) as this would produce quite different bandwidths at different adapting contrasts. The data also address the issue of the neuronal mechanisms of adaptation.     

Encoding of motion in real time by the fly visual system.

Direction-selective cells in the fly visual system that have large receptive fields play a decisive role in encoding the time-dependent optic flow the animal encounters during locomotion. Recent experiments on the computations performed by these cells have highlighted the significance of dendritic integration and have addressed the role of spikes versus graded membrane potential changes in encoding optic flow information. It is becoming increasingly clear that the way optic flow is encoded in real time is constrained both by the computational needs of the animal in visually guided behaviour as well as by the specific properties of the underlying neuronal hardware.