Donald A. Glaser

Large-scale isolation of UV-sensitive clones of CHO cells

We have isolated 54 ultraviolet light (UV) sensitive clones of Chinese hamster ovary (CHO) cells, including two from a parent cell line which is hypersensitive to ethyl methanesulfonate (EMS) and is also sensitive to X rays. A replica plating technique was used for the isolation of two of the clones, and a semiautomated technique was used for the isolation of the other 52 clones. We have observed UV sensitization of up to 5-fold in the mutants relative to the parent in terms of the slopes of the survival curves. Seven of the clones were examined for DNA repair competence using a repair replication assay, and all exhibited a DNA repair defect resembling that seen in human mutant xeroderma pigmentosumcells. We have also demonstrated an approximately 9-fold enhancement in the UV mutagenic response of two of the repair replication-defective clones relative to the parent for resistance to ouabain, 6-thioguanine, and 8-azaadenine.

Depth discrimination of a line is improved by adding other nearby lines

Depth discrimination thresholds are shown to be lowered by up to a factor of 10 when a few reference lines are added to a stimulus containing a single isolated test line. Four reference lines are better than two, which are better than one, and the improvement in performance is greater when the test line lies between the two reference lines in depth. Stereoacuity for the relative depth of a target line, relative to other nearby reference lines, is shown to be insensitive to changes of disparity of the whole pattern of up to about +/- 5 arc min and is only weakly sensitive to larger displacements of up to +/- 10 arc min.

Influence of remote objects on local depth perception

The perceived relative depth of two test dots displayed within the fovea is shown to be influenced by other features in the surrounding area. These features can be as far apart as 51 deg and can have relative disparities as large as 20 deg, much larger than the disparities of the test dots. Since this effect is seen for stimuli presented for 100 msec or less, changes in direction of gaze cannot play a role. The effect varies inversely as the spatial separation between the test dots and the remote features, and is insensitive to the relative disparities of these remote features when they are greater than 2 deg. Observers sometimes differ significantly from each other in their responses to various configurations of the outlying features. This appears to rule out response mechanisms which depend only on the stimulus; some characteristics of the observer must be involved in determining the response. For these briefly presented stimuli, observers are unable to report accurately the relative depth of the central foveated test items if they are also required to report the depths of distant peripheral features.

Metastable motion anisotropy

The phenomenon of apparent motion can arise when two spatially separated visual tokens are presented in temporal sequence. If tokens at opposite corners of a hypothetical square are presented simultaneously followed by simultaneous presentation of tokens at the remaining two corners, an apparent motion percept may occur along either the vertical or horizontal axis. The display is perceptually metastable since most observers will perceive motion along only one axis at a time. The metastable display, however, produces anisotropic results, in that with central fixation, vertical motion is seen more frequently than horizontal motion. The ratio of the vertical to horizontal length of the sides of a rectangle needed to achieve equal frequencies of motion judgments along the respective axes falls in the range of 1.18-1.92 for different observers in our experiments. It appears that signal transmission across the vertical midline is a major determinant of the vertical bias, since the anisotropic effects disappear when the fixation point is sufficiently offset along the horizontal meridian so as to cause a fully homonymous representation of all of the metastable tokens. One of the factors may be signal degradation or delay in callosal transmission which could reduce the strength of the motion signal along the horizontal axis. In addition, there appears to be a strip along the vertical midline with a width of 30-50 min arc within which reduced levels of anisotropy are found. The possibility that this strip is a consequence of a zone of naso-temporal overlap in the projection of the retina to the brain along the vertical meridian will be discussed.

Spontaneous Local Gamma Oscillation Selectively Enhances Neural Network Responsiveness

Synchronized oscillation is very commonly observed in many neuronal systems and might play an important role in the response properties of the system. We have studied how the spontaneous oscillatory activity affects the responsiveness of a neuronal network, using a neural network model of the visual cortex built from Hodgkin-Huxley type excitatory (E-) and inhibitory (I-) neurons. When the isotropic local E-I and I-E synaptic connections were sufficiently strong, the network commonly generated gamma frequency oscillatory firing patterns in response to random feed-forward (FF) input spikes. This spontaneous oscillatory network activity injects a periodic local current that could amplify a weak synaptic input and enhance the networks responsiveness. When E-E connections were added, we found that the strength of oscillation can be modulated by varying the FF input strength without any changes in single neuron properties or interneuron connectivity. The response modulation is proportional to the oscillation strength, which leads to self-regulation such that the cortical network selectively amplifies various FF inputs according to its strength, without requiring any adaptation mechanism. We show that this selective cortical amplification is controlled by E-E cell interactions. We also found that this response amplification is spatially localized, which suggests that the responsiveness modulation may also be spatially selective. This suggests a generalized mechanism by which neural oscillatory activity can enhance the selectivity of a neural network to FF inputs.

Clonal variation in colony morphology and growth of CHO cells cultured on agar

Single Chinese hamster ovary (CHO) cells plated on agar form macroscopic colonies with high efficiency. Colonies produced by cells from the uncloned cell line increase in diameter continuously for 10-12 days after plating to form mounds of cells about 1 mm in diameter. With further incubation, some of these colonies do not increase in diameter (arrested dome), some form an expanding annular monolayer of cells around the central mount (fried egg), and some grow by enlarging the central mound into a low multilayered disc (saucer). These colony types on agar appear to be clonal characteristics of the CHO cell line. Cloning the line gives two kinds of isolates: one forms a mixture of arrested dome and fried egg colonies in an inheritable ratio, and the other forms saucer colonies. Cells from saucer colonies form saucer colonies when replated on agar. Cells from all colony types replate with similar efficiency on plastic or agar, and exhibit the same growth rate and cell size in liquid suspension culture. On plastic substrate, all these CHO cells form colonies which increase continuously in diameter for as long as 21 days, and little clonal difference in the morphology of colonies or of single cells is observed. These observations reveal a previously unsuspected heterogeneity in an established line of cultured mammalian cells and provide a method for studying new classes of in vitro growth control phenomena. These control phenomena may help in the building an in vitro model for tumor growth.

Chromosomal sites of DNA-membrane attachment in Escherichia coli.

Abstract Evidence is presented to show that both the chromosomal replication point and the chromosomal origin in Escherichia coli are associated with a structure possessing the sedimentation properties and enzyme sensitivities characteristic of membrane. The data suggest that both newly synthesized DNA strands and template DNA strands are bound at this replication point and that both strands are also bound at the origin.

Synaptic Plasticity Controls Sensory Responses through Frequency-Dependent Gamma Oscillation Resonance

Synchronized gamma frequency oscillations in neural networks are thought to be important to sensory information processing, and their effects have been intensively studied. Here we describe a mechanism by which the nervous system can readily control gamma oscillation effects, depending selectively on visual stimuli. Using a model neural network simulation, we found that sensory response in the primary visual cortex is significantly modulated by the resonance between “spontaneous” and “stimulus-driven” oscillations. This gamma resonance can be precisely controlled by the synaptic plasticity of thalamocortical connections, and cortical response is regulated differentially according to the resonance condition. The mechanism produces a selective synchronization between the afferent and downstream neural population. Our simulation results explain experimental observations such as stimulus-dependent synchronization between the thalamus and the cortex at different oscillation frequencies. The model generally shows how sensory information can be selectively routed depending on its frequency components.

AN AUTOMATED SYSTEM FOR THE GROWTH AND ANALYSIS OF LARGE NUMBERS OF BACTERIAL COLONIES USING AN ENVIRONMENTAL CHAMBER AND A COMPUTER-CONTROLLED FLYING-SPOT SCANNER*

A computer-controlled flying-spot scanner system for biological research is being constructed at the University of California, Berkeley, the purpose of which is to scan and analyze large numbers of photographs taken of colonies of microorganisms growing in a controlled environmental chamber. The chamber now being constructed will be capable of containing I O6 to 1 O8 bacterial colonies which can be photographed automatically on a cyclic basis during their growth on nutrient agar. The immediate objectives of this system are to count and analyze large numbers of colonies of bacteria and other microorganisms grown on nutrient agar and, more importantly, to identify the organisms by observations of colony morphology, growth rate, nutritional requirements, drug resistances, and other characteristics observable during growth on solid media. When properly tabulated and interpreted the results of such large-scale observations can be used, for instance, to select and characterize mutants, to make exhaustive genetic maps of microorganisms, to investigate taxonomic relationships among groups of organisms, to study mutational rates at various sites due to a variety of mutagenic agents, and possibly to identify and assay organisms for medical diagnosis and public health purposes.

Motion detection and characterization by an excitable membrane: The “bow wave” model

Abstract We present a novel method for detecting and characterizing coherent motion in a set of image frames, using a two-dimensional sheet of locally connected neural elements. Externally applied activity leads to traveling activity waves. Activity waves resulting from points or objects in motion have a characteristic “bow wave” shape, which can be used to establish the existence, direction and speed of the objects motion. This mechanism may be used to model motion detection in biological systems, and may help to explain such psychophysical effects as motion pop-out, robustness of motion signals to noise, and the perception of transparent motion.