Brain responses strongly correlate with Weibull image statistics when processing natural images.

The visual appearance of natural scenes is governed by a surprisingly simple hidden structure. The distributions of contrast values in natural images generally follow a Weibull distribution, with beta and gamma as free parameters. Beta and gamma seem to structure the space of natural images in an ecologically meaningful way, in particular with respect to the fragmentation and texture similarity within an image. Since it is often assumed that the brain exploits structural regularities in natural image statistics to efficiently encode and analyze visual input, we here ask ourselves whether the brain approximates the beta and gamma values underlying the contrast distributions of natural images. We present a model that shows that beta and gamma can be easily estimated from the outputs of X-cells and Y-cells. In addition, we covaried the EEG responses of subjects viewing natural images with the beta and gamma values of those images. We show that beta and gamma explain up to 71% of the variance of the early ERP signal, substantially outperforming other tested contrast measurements. This suggests that the brain is strongly tuned to the image's beta and gamma values, potentially providing the visual system with an efficient way to rapidly classify incoming images on the basis of omnipresent low-level natural image statistics.

Spatio-temporal context for robust multitarget tracking.

In multitarget tracking, the main challenge is to maintain the correct identity of targets even under occlusions or when differences between the targets are small. The paper proposes a new approach to this problem by incorporating the context information. The context of a target in an image sequence has two components: the spatial context including the local background and nearby targets and the temporal context including all appearances of the targets that have been seen previously. The paper considers both aspects. We propose a new model for multitarget tracking based on the classification of each target against its spatial context. The tracker searches a region similar to the target while avoiding nearby targets. The temporal context is included by integrating the entire history of target appearance based on probabilistic principal component analysis (PPCA). We have developed a new incremental scheme that can learn the full set of PPCA parameters accurately online. The experiments show robust tracking performance under the condition of severe clutter, occlusions, and pose changes.

Point Light Source Position Estimation From RGB-D Images by Learning Surface Attributes.

Light source position (LSP) estimation is a difficult yet an important problem in computer vision. A common approach for estimating the LSP assumes Lambert's law. However, in real-world scenes, Lambert's law does not hold for all different types of surfaces. Instead of assuming all that surfaces follow Lambert's law, our approach classifies image surface segments based on their photometric and geometric surface attributes (i.e. glossy, matte, curved, and so on) and assigns weights to image surface segments based on their suitability for LSP estimation. In addition, we propose the use of the estimated camera pose to globally constrain LSP for RGB-D video sequences. Experiments on Boom and a newly collected RGB-D video data sets show that the state-of-the-art methods are outperformed by the proposed method. The results demonstrate that weighting image surface segments based on their attributes outperform the state-of-the-art methods in which the image surface segments are considered to equally contribute. In particular, by using the proposed surface weighting, the angular error for LSP estimation is reduced from 12.6° to 8.2° and 24.6° to 4.8° for Boom and RGB-D video data sets, respectively. Moreover, using the camera pose to globally constrain LSP provides higher accuracy (4.8°) compared with using single frames (8.5°).

Sparse representation for coarse and fine object recognition.

This paper offers a sparse, multiscale representation of objects. It captures the object appearance by selection from a very large dictionary of Gaussian differential basis functions. The learning procedure results from the matching pursuit algorithm, while the recognition is based on polynomial approximation to the bases, turning image matching into a problem of polynomial evaluation. The method is suited for coarse recognition between objects and, by adding more bases, also for fine recognition of the object pose. The advantages over the common representation using PCA include storing sampled points for recognition is not required, adding new objects to an existing data set is trivial because retraining other object models is not needed, and significantly in the important case where one has to scan an image over multiple locations in search for an object, the new representation is readily available as opposed to PCA projection at each location. The experimental result on the COIL-100 data set demonstrates high recognition accuracy with real-time performance.

The semantic pathfinder: using an authoring metaphor for generic multimedia indexing.

This paper presents the semantic pathfinder architecture for generic indexing of multimedia archives. The semantic pathfinder extracts semantic concepts from video by exploring different paths through three consecutive analysis steps, which we derive from the observation that produced video is the result of an authoring-driven process. We exploit this authoring metaphor for machine-driven understanding. The pathfinder starts with the content analysis step. In this analysis step, we follow a data-driven approach of indexing semantics. The style analysis step is the second analysis step. Here, we tackle the indexing problem by viewing a video from the perspective of production. Finally, in the context analysis step, we view semantics in context. The virtue of the semantic pathfinder is its ability to learn the best path of analysis steps on a per-concept basis. To show the generality of this novel indexing approach, we develop detectors for a lexicon of 32 concepts and we evaluate the semantic pathfinder against the 2004 NIST TRECVID video retrieval benchmark, using a news archive of 64 hours. Top ranking performance in the semantic concept detection task indicates the merit of the semantic pathfinder for generic indexing of multimedia archives.

Robust photometric invariant features from the color tensor.

Luminance-based features are widely used as low-level input for computer vision applications, even when color data is available. The extension of feature detection to the color domain prevents information loss due to isoluminance and allows us to exploit the photometric information. To fully exploit the extra information in the color data, the vector nature of color data has to be taken into account and a sound framework is needed to combine feature and photometric invariance theory. In this paper, we focus on the structure tensor, or color tensor, which adequately handles the vector nature of color images. Further, we combine the features based on the color tensor with photometric invariant derivatives to arrive at photometric invariant features. We circumvent the drawback of unstable photometric invariants by deriving an uncertainty measure to accompany the photometric invariant derivatives. The uncertainty is incorporated in the color tensor, hereby allowing the computation of robust photometric invariant features. The combination of the photometric invariance theory and tensor-based features allows for detection of a variety of features such as photometric invariant edges, corners, optical flow, and curvature. The proposed features are tested for noise characteristics and robustness to photometric changes. Experiments show that the proposed features are robust to scene incidental events and that the proposed uncertainty measure improves the applicability of full invariants.

Dielectric measurements on electro-manipulation media.

The permittivity and conductivity of solutions of sugars and sugar alcohols, such as are suitable for electric-field work on cells, were measured. The range of concentrations was 0-3 M (subject to solubility), and the frequency range was that commonly used in dielectrophoresis prior to electrofusion (200 kHz-2 MHz). This was widened to 30 kHz-12 MHz, when dispersive behaviour was found. A parallel-plate impedance chamber with variable electrode-spacing, suited to stable measurements in these low-conductivity aqueous media, was used. Errors due to electrode polarisation were effectively removed by linear regression of the series parameters against electrode distance, as well as by subtraction of data obtained on KCl solutions of comparable conductivity. The permittivity of the sugar solutions decreased as only approximately linear functions of concentration, so that a description in terms of both first- and second-order molar dielectric increments (delta 1 and delta 2) provided a better description of the behaviour than a single linear increment (delta). Sugar solutions of concentration of 1.2 M or less showed no change in permittivity or conductivity over the measured frequency range (i.e., were dispersion-free). On the other hand, various high-density media showed dispersion. The stabilised silica sol 'Percoll' showed dispersion over the whole frequency range; solutions of sucrose (at above 1.2 M concentration), as well as of proprietary high-density solutes ('Metrizamide' at above 0.5 M and 'Nycodenz' at above 0.75 M) showed dispersion above 0.6 MHz. Although these media are of interest for electro-manipulation (they can be used to prevent sedimentation), their dispersive properties may make them unsuitable for use with radio-frequency fields.

Alterations in the electrical properties of T and B lymphocyte membranes induced by mitogenic stimulation. Activation monitored by electro-rotation of single cells.

Stimulation of either B or T lymphocytes using specific mitogens results in changes in the passive electrical properties of the cell surface. These effects can be related to growth and secretion. This was possible because the high resolution of the contra-field electro-rotation method, combined with the use of very low conductivity media, allowed accurate and analytically-derived values for the cell surface properties. Increases in effective CM (membrane capacity) and changes in apparent membrane conductivity (reflecting the additive effects of true membrane conductivity GM and surface conductance KS) were measured. After 72 h treatment with concanavalin A, thymocyte CM had increased from 0.76 muF/cm2 to 1.24-1.46 muF/cm2 (7.6 to 12.4-14.6 mF/m2). Allowing for the stimulation-induced size increase (cell radius increased from 2.8 to 4.4 micron) these data imply that the plasma membrane area per cell increases 5-fold during stimulation. Stimulation of B cells (by 3 days incubation with bacterial lipopolysaccharide) increased CM from 0.93 to 1.6-1.7 muF/cm2 (9.3 to 16-17 mF/m2). Incubation without mitogen gave no significant increase in CM or in radius. Control cells of different sizes showed no difference in membrane properties. The increases in effective CM are argued to reflect an increase in membrane ramification (microvilli, folding, etc.). The apparent membrane conductivity of T cells also increased during stimulation, from 5 to 21 mS/cm2 (50 to 210 S/m2). This increase is proportionately much greater than that in CM or in membrane area. It seems to be due to a real increase in GM, but a small increase in KS may also occur. The earliest changes in apparent membrane conductivity were evident between 3 and 5 h after stimulation, before the cells increased in size. This response parallels increases in transmembrane transport reported to follow mitogenic stimulation.

The influence of tetraphenylborates (hydrophobic anions) on yeast cell electro-rotation.

The action of a series of tetraphenylborate ion (TPB) derivatives on yeast cells was studied by electro-rotation of the pre-treated cells. TPB derivatives in which all four phenyl groups were substituted with fluorine, chlorine or trifluoromethyl were much more toxic than the unsubstituted compound, the effect increasing dramatically with increasing size of substituents. These observations suggest that the toxicity of these hydrophobic ions is determined mainly by their size and possibly also by the chemical inductivity of their substituent groups. The order of the toxicities of these ions was in fair agreement with literature values for their translocation rates across artificial bilayers. Incubation times of 3 h were used as standard, longer incubations (up to 48 h) showed that the number of cells affected by low doses of TPB increased with the logarithm of time after the first hour of incubation. Although measurements of the percentage of cells showing co-field rotation showed that controls were not adversely affected by incubations as long as 9 h, rotation spectra showed that some cells suffer loss of internal conductivity during extended incubations. Decrease of the pH of the incubation medium, or inclusion of high concentrations of NaCl or KCl, potentiated the effects of these hydrophobic ions. The toxicity developed slowly, and the sensitivity of the assay was only very weakly dependent on the cell suspension density.

The effect of mercuric salts on the electro-rotation of yeast cells and comparison with a theoretical model.

The rotational spectrum of yeast cells changed after pre-treatment of the cells with HgCl2 or Hg(NO3)2 and became indistinguishable from that of ultrasonically produced cell walls. The spectrum of the affected cells contained a peak which could only be explained by attributing a conductivity to the cell walls that was higher than that of the medium. Theoretical models of the rotational response are fully in accord with the experimental spectra. It is shown that the rotation method is capable of measuring even the low cell wall conductivity of yeast cells (which was found to be 33 microS/cm at 10 microS/cm medium conductivity). Knowledge of the spectra allowed a field frequency to be selected at which untreated cells showed no rotation, but at which cells affected by treatment with Hg(II) identified themselves by rotating in the same direction as the field. Calculation of the percentage of cells showing this co-field rotation gave an index (termed the co-field rotation value) of the proportion of the cells that were affected. Using this technique, effects of 25 nmol/l Hg(II) could be demonstrated. In media of low conductivity (10 microS/cm) the change in the rotational spectrum was usually 'all-or-none', whereas at 200 microS/cm a graded Hg(II)-mediated change became apparent. The co-field rotation method showed that the action of small quantities of Hg(II) was still increasing after 3 h of incubation and paralleled the Hg(II)-induced K+ release. A rapid reduction of the effects of Hg(II) was seen when 3-30 mM K+ (or Na+) or when 1 mM Ca2+ were present in the incubation medium, or as the pH was increased. At high incubation cell concentrations the toxic effect of Hg(II) was reduced, apparently due to binding by the cells.

Divalent cation-sensitive pores formed by natural and synthetic melittin and by Triton X-100.

Leakage of ions and low-molecular-weight metabolites from Lettre cells is induced by synthetic melittin, as effectively as by melittin isolated from bee venom; in each case leakage is inhibited by Ca2+, Zn2+ or H+. Inhibition of leakage by divalent cations is reversible in that Lettre cells incubated with melittin (or with Triton X-100) in the presence of inhibitory amounts of Zn2+, when freed of Zn2+ by EGTA or by centrifugation, begin to leak (in Zn2(+)-sensitive manner). Electrorotation of Lettre cells is altered by melittin, compatible with membrane permeabilization; melittin plus Zn2+ does not alter electrorotation until Zn2+ (and unbound melittin) are removed. Melittin or Triton X-100 added to calcein-loaded liposomes induces leakage of calcein; divalent cations inhibit. Energy transfer between liposome-associated melittin and 2-, 7- or 12-(9-anthroyloxy)stearate (AS) is maximal with 12-AS; addition of Zn2+ has little effect. Circular dichroism spectra of melittin plus liposomes are unaffected by Zn2+. These results show that the formation of divalent cation-sensitive pores is not dependent on the presence of endogenous membrane proteins and that the action of divalent cations is not by displacement of melittin (or Triton) from the lipid bilayer.

Levitation, holding, and rotation of cells within traps made by high-frequency fields.

Biological cells and other particles can be electrically manipulated by means of negative dielectrophoresis within microchambers whose electrode geometry is of the order of the cell size. Very-high-frequency fields (50 MHz and above) and media of increased relative permittivity are especially suitable for the purpose, as shown by experimental data on levitation and rotation. It appears to be possible to move and rotate cells or particles at will using this technology.

The comparative influence of substituted phenols (especially chlorophenols) on yeast cells assayed by electro-rotation and other methods.

The toxicity of 31 phenols was studied by electro-rotation of yeast cells. Control yeast cells show both anti-field and co-field rotation, depending upon the field frequency applied. After treatment with supra-threshold amounts of phenols the anti-field rotation is weakened or abolished and a stronger co-field rotation can be seen. The proportion of cells showing the co-field rotation was found to be a sensitive measure of toxicity. Doses of 2.2 mumol/l of pentachlorophenol, or of 0.3 mumol/l of pentabromophenol were detectable after 3 h incubation at pH 4.0. At a given pH, the toxicity of the chlorophenols correlated extremely well with their octanol:water partition coefficients (Pow). The complete set of phenols showed fair overall correlation with Pow, but less good correlation with their acidity constants (pKa). In particular the toxicity of a given phenol was less than predicted from its pKa if the incubation pH was higher than the pKa. Biochemical assays on 23 of the phenols showed that the rotational sensitivity runs closely parallel to the sensitivities of cell growth rate and of the plasmamembrane ATPase, but less closely to the inhibition of purine incorporation. It appears that the electro-rotation method provides a useful and rapid test for the presence of organic ecotoxins. The test enables us to distinguish differences between single cells, and is comparable in sensitivity to biochemical tests that use vesicles or homogenates derived from a cell population.

Differences in membrane properties between unfertilised and fertilised single rabbit oocytes demonstrated by electro-rotation. Comparison with cells from early embryos.

The apparent membrane capacity of tubular rabbit oocytes increases from 1.7-2.0 microF/cm2 before fertilisation to 3.7-4.0 microF/cm2 after fertilisation. The membrane conductivity measured on single cells was also increased by fertilisation from less than 1 mS/cm2 to 14 mS/cm2. Cells obtained from 2-, 4- or 8-cell embryos exhibited intermediate values of membrane capacity (2.3-2.8 microF/cm2) and conductivity (5-22 mS/cm2). The values quoted are those effective between 1 and 10 kHz, the frequency of the rotating field used. The large apparent capacities are probably due to the presence of structures such as microvilli which cause the actual membrane area to exceed the smooth sphere area. It must be assumed that these structures change in form or number on fertilisation, and that they persist in embryos, at least up to the 8-cell stage. No difference was apparent between cells fertilised in vitro or in vivo. Comparison of the above zona-free data with measurements on zona-complete oocytes indicate how fertilised and unfertilised rabbit eggs may be distinguished from one another, even in the presence of the zona pellucida.

Electro-rotation of mouse oocytes: single-cell measurements of zona-intact and zona-free cells and of the isolated zona pellucida.

Passive electrical properties of oocytes and of zonae pellucidae, and the mechanical coupling between them, can be elucidated by means of rotating-field-induced rotation. In low-conductivity media (25-100 microS/cm) rotation of mouse oocytes (with or without their zonae) requires fields in the 1-100 kHz frequency range. However, an isolated zona shows weak rotation in the opposite direction to that of a cell, and in response to much higher field frequencies (approx. 1 MHz). In zona-intact mouse oocytes, the rotation of cell and zona are not rigidly coupled: thus rotation of the cell can still be induced when the zona is held stationary. However, rotation of freely suspended zona-intact cells is much slower than that of zona-free cells and requires an optimum field frequency that is approximately 1.5 kHz higher. These observations show that the electrical properties of the oocyte that are measured by rotation are altered by the presence of the zona pellucida, even though no such influence has been detected using micro-electrodes. The data are consistent with the zona acting as a porous shell with a conductivity of 40 microS/cm (preliminary estimate made at a single medium conductivity of 26 microS/cm). Measurements on cells from which the zonae had been removed gave values for the membrane capacity and resistivity of 1.2-1.3 microF/cm2 and 400 omega.cm2, respectively. These values may reflect the presence of plasmalemma microvilli. The results strongly suggest that the technique may be useful for studies of cell maturation and for in vitro fertilization, because the cells may be further cultured after measurement.

Population-based incremental interactive concept learning for image retrieval by stochastic string segmentations.

We propose a method for concept-based medical image retrieval that is a superset of existing semantic-based image retrieval methods. We conceive of a concept as an incremental and interactive formalization of the user's conception of an object in an image. The premise is that such a concept is closely related to a user's specific preferences and subjectivity and, thus, allows to deal with the complexity and content-dependency of medical image content. We describe an object in terms of multiple continuous boundary features and represent an object concept by the stochastic characteristics of an object population. A population-based incrementally learning technique, in combination with relevance feedback, is then used for concept customization. The user determines the speed and direction of concept customization using a single parameter that defines the degree of exploration and exploitation of the search space. Images are retrieved from a database in a limited number of steps based upon the customized concept. To demonstrate our method we have performed concept-based image retrieval on a database of 292 digitized X-ray images of cervical vertebrae with a variety of abnormalities. The results show that our method produces precise and accurate results when doing a direct search. In an open-ended search our method efficiently and effectively explores the search space.

Combining strings and necklaces for interactive three-dimensional segmentation of spinal images using an integral deformable spine model.

Segmentation of the spine directly from three-dimensional (3-D) image data is desirable to accurately capture its morphological properties. We describe a method that allows true 3-D spinal image segmentation using a deformable integral spine model. The method learns the appearance of vertebrae from multiple continuous features recorded along vertebra boundaries in a given training set of images. Important summarizing statistics are encoded into a necklace model on which landmarks are differentiated on their free dimensions. The landmarks are used within a priority segmentation scheme to reduce the complexity of the segmentation problem. Necklace models are coupled by string models. The string models describe in detail the biological variability in the appearance of spinal curvatures from multiple continuous features recorded in the training set. In the segmentation phase, the necklace and string models are used to interactively detect vertebral structures in new image data via elastic deformation reminiscent of a marionette with strings allowing for movement between interrelated structures. Strings constrain the deformation of the spine model within feasible solutions. The driving application in this work is analysis of computed tomography scans of the human lumbar spine. An illustration of the segmentation process shows that the method is promising for segmentation of the spine and for assessment of its morphological properties.

Fast occluded object tracking by a robust appearance filter.

We propose a new method for object tracking in image sequences using template matching. To update the template, appearance features are smoothed temporally by robust Kalman filters, one to each pixel. The resistance of the resulting template to partial occlusions enables the accurate detection and handling of more severe occlusions. Abrupt changes of lighting conditions can also be handled, especially when photometric invariant color features are used. The method has only a few parameters and is computationally fast enough to track objects in real time.

Fast anisotropic Gauss filtering.

We derive the decomposition of the anisotropic Gaussian in a one-dimensional (1-D) Gauss filter in the x-direction followed by a 1-D filter in a nonorthogonal direction phi. So also the anisotropic Gaussian can be decomposed by dimension. This appears to be extremely efficient from a computing perspective. An implementation scheme for normal convolution and for recursive filtering is proposed. Also directed derivative filters are demonstrated. For the recursive implementation, filtering an 512 x 512 image is performed within 40 msec on a current state of the art PC, gaining over 3 times in performance for a typical filter, independent of the standard deviations and orientation of the filter. Accuracy of the filters is still reasonable when compared to truncation error or recursive approximation error. The anisotropic Gaussian filtering method allows fast calculation of edge and ridge maps, with high spatial and angular accuracy. For tracking applications, the normal anisotropic convolution scheme is more advantageous, with applications in the detection of dashed lines in engineering drawings. The recursive implementation is more attractive in feature detection applications, for instance in affine invariant edge and ridge detection in computer vision. The proposed computational filtering method enables the practical applicability of orientation scale-space analysis.

Tracking nonparameterized object contours in video.

We propose a new method for contour tracking in video. The inverted distance transform of the edge map is used as an edge indicator function for contour detection. Using the concept of topographical distance, the watershed segmentation can be formulated as a minimization. This new viewpoint gives a way to combine the results of the watershed algorithm on different surfaces. In particular, our algorithm determines the contour as a combination of the current edge map and the contour, predicted from the tracking result in the previous frame. We also show that the problem of background clutter can be relaxed by taking the object motion into account. The compensation with object motion allows to detect and remove spurious edges in background. The experimental results confirm the expected advantages of the proposed method over the existing approaches.