Evoked activity in the mammalian cortex and the resulting behavioral responses exhibit a large variability to repeated presentations of the same stimulus. This study examined whether the variability can be attributed to ongoing activity. Ongoing and evoked spatiotemporal activity patterns in the cat visual cortex were measured with real-time optical imaging; local field potentials and discharges of single neurons were recorded simultaneously, by electrophysiological techniques. The evoked activity appeared deterministic, and the variability resulted from the dynamics of ongoing activity, presumably reflecting the instantaneous state of cortical networks. In spite of the large variability, evoked responses in single trials could be predicted by linear summation of the deterministic response and the preceding ongoing activity. Ongoing activity must play an important role in cortical function and cannot be ignored in exploration of cognitive processes.

We introduce a new mathematical tool for quantifying the symmetry contents of molecular structures: the Symmetry Operation Measures. In this approach, we measure the minimal distance between a given structure and the structure which is obtained after applying a selected symmetry operation on it. If the given operation is a true symmetry operation for the structure, this distance is zero; otherwise it gives an indication of how different the transformed structure is from the original one. Specifically, we provide analytical solutions for measures of all the improper rotations, S n p, including mirror symmetry and inversion, as well as for all pure rotations, C n p. These measures provide information complementary to the Continuous Symmetry Measures (CSM) that evaluate the distance between a given structure and the nearest structure which belongs to a selected symmetry point-group.

A novel trajectory ensemble model of a stratocumulus cloud is described. In this model, the boundary layer (BL) is fully covered by a great number of Lagrangian air parcels that during their motion can contain either wet aerosols or aerosols and droplets. The diffusion growth of aerosols and droplets, as well as drop collisions, is accurately described in each parcel. Droplet sedimentation is taken into account, which allows simulation of precipitation formation. The Lagrangian parcels are advected by the velocity field generated by the turbulent-like flow model obeying turbulent correlation laws. The output of the numerical model includes droplet and aerosol size distributions and their moments, such as droplet concentration, droplet spectrum width, cloud water content, drizzle content, radar reflectivity, etc., calculated in each parcel.Horizontally averaged values are calculated as well.Stratocumulus clouds observed during two research flights (RF01 and RF07) in the Second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS II) field campaign are simulated. A good agreement between the dynamical and microphysical structures simulated by the model with observations is obtained even in the nonmixing limit. A crucial role of sedimentation for the creation of a realistic cloud microphysical structure is demonstrated. In sensitivity studies, the statistical stability of the model is analyzed.Applications of the model for the investigation of droplet size distribution and drizzle formation are discussed, as is the possible utilization of the model for remote sensing applications.

Detection of low-contrast Gabor patches (GPs) is improved when flanked by collinear GPs, whereas suppression is observed for high-contrast GPs. The facilitation resembles the principles of Gestalt theory of perceptual organization. We propose a model for contour integration in the context of noise that incorporates a temporal element into this spatial architecture. The basic principles are (1) the response increases with increasing contrast, whereas the latency decreases; (2) activity-dependent interactions: facilitation for low and suppression for high activity; (3) the variance increases with contrast for responses, rates, and latency; and (4) inhibition has a shorter time constant than excitation. When a texture of randomly oriented GPs is presented, the response to every element decreases due to fast inhibition between the neighboring elements, shifting the activity toward the range of collinear facilitation. Next, the slower excitation induces selective facilitation along the contour elements. Consequently, the response to the contour increases, whereas the variance of the rate and latency decreases, providing better temporal correlation between the contour elements. Thus, collinear facilitation increases the saliency of contours. Our model may suggest a solution to the binding problem by bridging between the temporal and spatial aspects of lateral interactions that determine the encoding of perceptual grouping.

The computerised bird monitoring system devised on the basis of MRL-5 meteorological radar enables to perform around-the-clock automated bird monitoring. The paper presents an algorithm for identifying bird signals against a background of other objects. The radar signal reflected from a flying bird changes its location in space. In case the flight direction remains unchanged, the coordinates of the corresponding radio echoes obtained as a result of successive azimuth scans form a straight line. This enables to isolate bird radio echoes from those of motionless or chaotically moving objects, such as ground clutter, clouds, precipitation, atmospheric inhomogeneities, etc. In the proposed algorithm, the coordinates thus obtained are used for estimation of vectors of individual birds and bird groups.In order to identify a bird signal, its fluctuation pattern is analysed. A fluctuation pattern is determined by the frequency of wing-flapping that is characteristic of a certain bird species. The analysis of the spectra of signals obtained in the study indicates that the proposed algorithm enables to isolate bird radio echoes with approximately 85% confidence. The technique implying simultaneous measurements of signal fluctuation pattern and of the effective scattering area (ESA) forms a promising basis for further studies aimed at determining the species of flying birds by means of radar.

SUMMARYThis paper uses a 2000-bin spectral microphysics cloud parcel model to investigate the effects of aerosol particles on droplet spectrum evolution and warm rain formation in ascending cloud parcels under maritime, intermediate, and continental unstable thermodynamic conditions. Cloud parcels of different cloud depth are simulated for each thermodynamic condition. Concentration and size distribution of cloud condensation nuclei (CCN) were varied within a wide range for each cloud depth.Cloud parcels were divided into three groups with respect to the relationship between cloud depth H c and the distance of the collision triggering level H col above cloud base. The sensitivity of precipitation to CCN variations is quite different for the different groups. Parcels of the rst group (H c < H col ) do not precipitate. Precipitation from cloud parcels belonging to the second group, in which collisions start at a small distance below the maximum cloud-top height (H c ' H col ), turns out to be highly sensitive to the variation of CCN concentration and size distribution. For these parcels an increase in the concentration of large CCN can result in an increase in precipitation amount by several factors. Many cloud parcels arising under continental and intermediate conditions, as well as not very deep maritime clouds, can be assigned to this group. The precipitation amount from cloud parcels belonging to the third group (H c À H col ), represented by deep tropical clouds, turns out to be insensitive to CCN distribution and droplet concentration within a wide range of their variations. Comparable effects of small and large CCN on precipitation formation are analysed for each group of cloud parcels.These results are closely related to the problem of rain enhancement via hygroscopic seeding. It is shown that there is a certain range of cloud depths within which cloud seeding can potentially lead to a signi cant (several times) increase in rain within ascending parcels. Rain formed in cloud parcels with depths beyond this range is only slightly sensitive to the CCN concentration.

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