CiteULike: Author Koenderink

Spatial properties of the visual detectability of moving spatial white noise

A van Doorn — 2008-07-23T02:43:12-00:00

Experimental Brain Research, Vol. 45, No. 1. (17 January 1982), pp. 189-195.

Summary We determined the spatial parameters that describe the visual detection of spatio-temporal correlation in moving two-dimensional noise patterns. The target field (5.21×5.31 deg of visual angle) was divided into horizontal stripes of equal width D. Adjacent bars alternately contained noise patterns moving with velocity v1 and v2. We varied D, v1 and v2.Roughly three different percepts occurred. If the stripes were very broad the different movements in alternate stripes were perceived together with the division of the field into stripes. If the stripes were very narrow the division into stripes was not seen, but the moving noise patterns with velocities v1 and v2 were perceived as transparent sheets moving through each other. For intermediate stripe widths the target field looked incoherent and the subject was not clear about the percept. In this region the subject found it difficult and sometimes impossible to discriminate these patterns from a completely uncorrelated spatiotemporal white noise pattern (snow).To quantify the detectability, the patterns were masked with snow (spatio-temporal white noise). The r.m.s. contrast of the total stimulus was kept at a constant value, whereas the subject set the signal-to-noise ratio (SNR) to a threshold value. At certain barwidths the thresholds reached a maximum value. These critical barwidths depended on the velocities v1 and v2.These critical barwidths were interpreted in terms of a simple general model for the detection of spatiotemporal correlation. In these terms the span of the elementary correlators rose monotonically with the velocity to which the correlator is most sensitive.

Detectability of velocity gradients in moving random-dot patterns

AJ Van Doorn — 2008-07-23T02:41:44-00:00

Vision Research, Vol. 23, No. 8. (1983), pp. 799-804.

We present a subject with two moving spatial noise patterns at both sides of a common border. The patterns are masked with spatio-temporal white noise and we measure the threshold signal-to-noise ratio at which a subject can discriminate the case of two differently moving patterns from that of a single uniformly moving pattern over the whole field. At some experiments the patterns move in the same direction but the magnitude of the velocity of one of the patterns is varied. In other experiments both patterns move with the same velocity but in different directions. In no case do we find a dependence of the thresholds on the orientation of the common border with respect to the direction of movement of the patterns. In all cases we find a certain region of magnitudes of the variable velocity or of the orientation-difference between the two velocities in which the thresholds are very high, whereas for larger differences either in magnitude or in direction the thresholds are generally at values of less than one for the signal-to-noise ratio. The region of the raised thresholds for both magnitude and orientation differences can be described with a single simple expression: thresholds are high whenever the magnitude of the velocity difference of the two patterns is less than one half of the magnitude of the common component of the two velocities. This is a kind of Weber law for the velocity vectors. It is estimated that you may not pick many more than 150 differently moving patterns (both with respect to magnitude and direction) in such a way that any pair of them leads to easy discrimination (low threshold signal-to-noise ratio).

Representation of local geometry in the visual system

JJ Koenderink — 2007-02-16T14:36:45-00:00

Biol. Cybern., Vol. 55, No. 6. (March 1987), pp. 367-375.

Surface shape and curvature scales

Jan Koenderink — 2006-11-13T10:41:14-00:00

Image and Vision Computing, Vol. 10, No. 8. (October 1992), pp. 557-564.

The classical surface curvature measures, such as the Gaussian and the mean curvature at a point of a surface, are not very indicative of local shape. The two principal curvatures (taken as a pair) are more informative, but one would prefer a single shape indicator rather than a pair of numbers. Moreover, the shape indicator should preferably be independent of the size i.e. the amount of curvature, as distinct from the type of curvature. We propose two novel measures of local shape, the `curvedness' and the `shape index'. The curvedness is a positive number that specifies the amount of curvature, whereas the shape index is a number in the range [-1, +1] and is scale invariant. The shape index captures the intuitive notion of `local shape' particularly well. The shape index can be mapped upon an intuitively natural colour scale. Two complementary shapes (like stamp and mould) map to complementary hues. The symmetrical saddle (which is very special because it is self-complementary) maps to white. When a surface is tinted according to this colour scheme, this induces an immediate perceptual segmentation of convex, concave, and hyperbolic areas. We propose it as a useful tool in graphics representation of 3D shape.

Geometry of binocular vision and a model for stereopsis

J Koenderink — 2008-06-03T19:00:07-00:00

Biological Cybernetics, Vol. 21, No. 1. (7 January 1976), pp. 29-35.

If a binocular observer looks at surfaces, the disparity is a continuous vector field defined on the manifold of cyclopean visual directions. We derive this field for the general case that the observer is presented with a curved surface and fixates an arbitrary point. We expand the disparity field in the neighbourhood of a visual direction. The first order approximation can be decomposed into congruences, similarities and deformations. The deformation component is described by the traceless part of the symmetric part of the gradient of the disparity. The deformation component carries all information concerning the slant of a surface element that is contained in the disparity field itself; it is invariant for changes of fixation, differential cyclotorsion and uniform aniseikonia. The deformation component can be found from a comparison of the orientation of surface details in the left and right retinal images. The theory provides a geometric explanation of the percepts obtained with uniform and oblique meridional aniseikonia. We utilize the geometric theory to construct a mechanistic model of stereopsis that obviates the need for internal zooming mechanisms, but nevertheless is insensitive to differential cyclotorsion or uniform aniseikonia.

Spatial heterogeneity of inhibitory surrounds in the middle temporal visual area.

DK Xiao — 2008-05-14T14:50:09-00:00

Proceedings of the National Academy of Sciences of the United States of America, Vol. 92, No. 24. (21 November 1995), pp. 11303-11306.

A recurrent theme in the organization of vertebrate visual cortex is that of receptive fields with an associated "silent" opponency component. In the middle temporal area (area MT), a cortical visual area involved in the analysis of retinal motion in primates, this opponency appears in the form of a region outside the classical receptive field (CRF) that in itself gives no response but suppresses responses to motion evoked within the CRF. This antagonistic motion surround has been described as very large and symmetrically arrayed around the CRF. On the basis of this view, the primary function of the surround has long been thought to consist of simple figure-ground segregation based on movement. We have made use of small stimulus patches to map the form and extent of the surround and find evidence that the surround inhibition of many MT cells is in fact confined to restricted regions on one side or on opposite sides of the CRF. Such regions endow MT cells with the ability to make local-to-local motion comparisons, capable of extracting more complex features from the visual environment, and as such, may be better viewed as intrinsic parts of the receptive field, rather than as separate entities responsible for local-to-global comparisons.

Affine structure from motion.

JJ Koenderink — 2008-05-08T14:11:35-00:00

Journal of the Optical Society of America. A, Optics and image science, Vol. 8, No. 2. (February 1991), pp. 377-385.

A mobile observer samples sequences of narrow-field projections of configurations in ambient space. The so-called structure-from-motion problem is to infer the structure of these spatial configurations from the sequence of projections. For rigid transformations, a unique metrical reconstruction is known to be possible from three orthographic views of four points. However, human observers seem able to obtain much shape information from a mere pair of views, as is evident in the case of binocular stereo. Moreover, human observers seem to find little use for the information provided by additional views, even though some improvement certainly occurs. The rigidity requirement in its strict form is also relaxed. We indicate how solutions of the structure-from-motion problem can be stratified in such a way that one explicitly knows at which stages various a priori assumptions enter and specific geometrical expertise is required. An affine stage is identified at which only smooth deformation is assumed (thus no rigidity constraint is involved) and no metrical concepts are required. This stage allows one to find the spatial configuration (modulo an affinity) from two views. The addition of metrical methods allows one to find shape from two views, modulo a relief transformation (depth scaling and shear). The addition of a third view then merely serves to settle the calibration. Results of a numerical experiment are discussed.

Nonequilibrium Microtubule Fluctuations in a Model Cytoskeleton

Clifford Brangwynne — 2008-05-07T21:19:39-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Biological activity gives rise to nonequilibrium fluctuations in the cytoplasm of cells; however, there are few methods to directly measure these fluctuations. Using a reconstituted actin cytoskeleton, we show that the bending dynamics of embedded microtubules can be used to probe local stress fluctuations. We add myosin motors that drive the network out of equilibrium, resulting in an increased amplitude and modified time dependence of microtubule bending fluctuations. We show that this behavior results from steplike forces on the order of 10 pN driven by collective motor dynamics.

What does the occluding contour tell us about solid shape?

JJ Koenderink — 2008-04-25T15:49:16-00:00

Perception, Vol. 13, No. 3. (1984), pp. 321-330.

A new theorem is discussed that relates the apparent curvature of the occluding contour of a visual shape to the intrinsic curvature of the surface and the radial curvature. This theorem allows the formulation of general laws for the apparent curvature, independent of viewing distance and regardless of the fact that the rim (the boundary between the visible and invisible parts of the object) is a general, thus twisted, space curve. Consequently convexities, concavities, or inflextions of contours in the retinal image allow the observer to draw inferences about local surface geometry with certainty. These results appear to be counterintuitive, witness to the treatment of the problem by recent authors. It is demonstrated how well-known examples, used to show how concavities and convexities of the contour have no obvious relation to solid shape, are actually good illustrations of the fact that convexities are due to local ovoid shapes, concavities to local saddle shapes.

The shape of smooth objects and the way contours end.

JJ Koenderink — 2008-04-25T15:45:35-00:00

Perception, Vol. 11, No. 2. (1982), pp. 129-137.

Depth and shape from differential perspective in the presence of bending deformations.

JJ Koenderink — 2008-04-25T15:42:43-00:00

Journal of the Optical Society of America. A, Optics and image science, Vol. 3, No. 2. (February 1986), pp. 242-249.

It is shown that there exist classes of useful partial solutions to what has become known as the structure from motion problem, that is, the problem of finding the three-dimensional shape as well as the movement in space of objects on the basis of their optical projections in several distinct phases of the movement. The class of solutions treated in this paper is quite different from the usual treatment in that the rigidity condition is relaxed so as to include the important class of bending deformations. For instance, we show a case in which important aspects of shape are obtained for a configuration of seven points, no four of which move in a rigid way, and which can be obtained from only two views. (This violates all requirements of the structure from motion theorem.) We discuss an algorithm that obtains the three-dimensional configuration of a polyhedral vertex up to a relief transformation (a similar transformation to the one that maps object to image space in paraxial optics), in the presence of arbitrary flections of the edges, from the projection and the instantaneous-movement parallax. Such a solution can, for instance, be used to predict side views of the vertex, as is demonstrated for a specific example. The solution can be used in an iterative fashion on triangulations to solve the case of extended curved surfaces suffering arbitrary bendings. The class of bending deformations encompasses all deformations that conserve distances along the surface (but not necessarily through space); thus stretchings are excluded. Practical examples include, e.g., deformations of flexible but nonstretchable shells. Actual numerical examples illustrate the practicability of the method.

The brain a geometry engine.

JJ Koenderink — 2008-04-25T15:31:22-00:00

Psychological research, Vol. 52, No. 2-3. (1990), pp. 122-127.

According to Kant, spacetime is a form of the mind. If so, the brain must be a geometry engine. This idea is taken seriously, and consequently the implementation of space and time in terms of machines is considered. This enables one to conceive of spacetime as really "embodied."

The internal representation of solid shape with respect to vision.

JJ Koenderink — 2008-04-25T15:28:58-00:00

Biological cybernetics, Vol. 32, No. 4. (2 May 1979), pp. 211-216.

It is argued that the internal model of any object must take the form of a function, such that for any intended action the resulting reafference is predictable. This function can be derived explicitly for the case of visual perception of rigid bodies by ambulant observers. The function depends on physical causation, not physiology; consequently, one can make a priori statements about possible internal models. A posteriori it seems likely that the orientation sensitive units described by Hubel and Wiesel constitute a physiological substrate subserving the extraction of the invariants of this function. The function is used to define a measure for the visual complexity of solid shape. Relations with Gestalt theories of perception are discussed.

The singularities of the visual mapping.

JJ Koenderink — 2008-04-25T15:25:09-00:00

Biological cybernetics, Vol. 24, No. 1. (16 September 1976), pp. 51-59.

The structure of images.

JJ Koenderink — 2008-04-25T15:09:59-00:00

Biological cybernetics, Vol. 50, No. 5. (1984), pp. 363-370.

In practice the relevant details of images exist only over a restricted range of scale. Hence it is important to study the dependence of image structure on the level of resolution. It seems clear enough that visual perception treats images on several levels of resolution simultaneously and that this fact must be important for the study of perception. However, no applicable mathematically formulated theory to deal with such problems appears to exist. In this paper it is shown that any image can be embedded in a one-parameter family of derived images (with resolution as the parameter) in essentially only one unique way if the constraint that no spurious detail should be generated when the resolution is diminished, is applied. The structure of this family is governed by the well known diffusion equation (a parabolic, linear, partial differential equation of the second order). As such the structure fits into existing theories that treat the front end of the visual system as a continuous stack of homogeneous layers, characterized by iterated local processing schemes. When resolution is decreased the images becomes less articulated because the extrem ("light and dark blobs") disappear one after the other. This erosion of structure is a simple process that is similar in every case. As a result any image can be described as a juxtaposed and nested set of light and dark blobs, wherein each blob has a limited range of resolution in which it manifests itself. The structure of the family of derived images permits a derivation of the sampling density required to sample the image at multiple scales of resolution.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of element orientation on apparent motion perception.

P Werkhoven — 2008-04-25T12:50:34-00:00

Perception & psychophysics, Vol. 47, No. 6. (June 1990), pp. 509-525.

We present an ambiguous motion paradigm that allows us to quantify the influence of aspects of form relevant to the perception of apparent motion. We report on the role of bar element orientation in motion paths. The effect of orientation differences between bar elements in a motion path is small with respect to the crucial role of the orientation of bar elements relative to motion direction. Motion perception between elements oriented along the motion direction dominates motion perception between elements oriented perpendicularly to motion direction. The perception of apparent motion is affected by bar length and width and is anisotropic.

The cell as a material

Karen Kasza — 2008-04-18T15:21:56-00:00

Current Opinion in Cell Biology, Vol. 19, No. 1. (February 2007), pp. 101-107.

To elucidate the dynamic and functional role of a cell within the tissue it belongs to, it is essential to understand its material properties. The cell is a viscoelastic material with highly unusual properties. Measurements of the mechanical behavior of cells are beginning to probe the contribution of constituent components to cell mechanics. Reconstituted cytoskeletal protein networks have been shown to mimic many aspects of the mechanical properties of cells, providing new insight into the origin of cellular behavior. These networks are highly nonlinear, with an elastic modulus that depends sensitively on applied stress. Theories can account for some of the measured properties, but a complete model remains elusive.

A Quantitative Analysis of Contractility in Active Cytoskeletal Protein Networks

Poul Bendix — 2008-04-15T16:33:51-00:00

Biophys. J., Vol. 94, No. 8. (15 April 2008), pp. 3126-3136.

Cells actively produce contractile forces for a variety of processes including cytokinesis and motility. Contractility is known to rely on myosin II motors which convert chemical energy from ATP hydrolysis into forces on actin filaments. However, the basic physical principles of cell contractility remain poorly understood. We reconstitute contractility in a simplified model system of purified F-actin, muscle myosin II motors, andalpha -actinin cross-linkers. We show that contractility occurs above a threshold motor concentration and within a window of cross-linker concentrations. We also quantify the pore size of the bundled networks and find contractility to occur at a critical distance between the bundles. We propose a simple mechanism of contraction based on myosin filaments pulling neighboring bundles together into an aggregated structure. Observations of this reconstituted system in both bulk and low-dimensional geometries show that the contracting gels pull on and deform their surface with a contractile force of [~]1 microN, or [~]100 pN per F-actin bundle. Cytoplasmic extracts contracting in identical environments show a similar behavior and dependence on myosin as the reconstituted system. Our results suggest that cellular contractility can be sensitively regulated by tuning the (local) activity of molecular motors and the cross-linker density and binding affinity. 10.1529/biophysj.107.117960

Multidrug resistance protein 4 (MRP4/ABCC4): a versatile efflux transporter for drugs and signalling molecules.

Frans G M Russel — 2008-03-23T13:49:50-00:00

Trends Pharmacol Sci (17 March 2008)

Multidrug resistance protein (MRP) 4 is a member of the MRP/ABCC subfamily of ATP-binding cassette transporters, which are capable of pumping a wide variety of endogenous and xenobiotic organic anionic compounds out of the cell. In addition to its role in the body distribution and renal excretion of a wide variety of antiviral, cytostatic, antibiotic and cardiovascular drugs, MRP4/ABCC4 has the remarkable ability to transport molecules involved in cellular signalling. These molecules include cyclic nucleotides, eicosanoids, urate and conjugated steroids. The unique structure, regulation and dual localisation of MRP4 in polarised cells could be connected with a key function in cellular protection and extracellular signalling pathways. This review focuses on recent insights into the versatile transport function of MRP4 and its potential as a new therapeutic target to modulate various pathophysiological signalling processes.

Bending Dynamics of Fluctuating Biopolymers Probed by Automated High-Resolution Filament Tracking

Clifford Brangwynne — 2007-06-11T04:36:02-00:00

Biophys. J., Vol. 93, No. 1. (1 July 2007), pp. 346-359.

Microscope images of fluctuating biopolymers contain a wealth of information about their underlying mechanics and dynamics. However, successful extraction of this information requires precise localization of filament position and shape from thousands of noisy images. Here, we present careful measurements of the bending dynamics of filamentous (F-)actin and microtubules at thermal equilibrium with high spatial and temporal resolution using a new, simple but robust, automated image analysis algorithm with subpixel accuracy. We find that slender actin filaments have a persistence length of [~]17 microm, and display a q-4-dependent relaxation spectrum, as expected from viscous drag. Microtubules have a persistence length of several millimeters; interestingly, there is a small correlation between total microtubule length and rigidity, with shorter filaments appearing softer. However, we show that this correlation can arise, in principle, from intrinsic measurement noise that must be carefully considered. The dynamic behavior of the bending of microtubules also appears more complex than that of F-actin, reflecting their higher-order structure. These results emphasize both the power and limitations of light microscopy techniques for studying the mechanics and dynamics of biopolymers. 10.1529/biophysj.106.096966

Gestalt and phenomenal transparency

Jan Koenderink — 2007-12-28T19:38:54-00:00

J. Opt. Soc. Am. A, Vol. 25, No. 1. (1 January 2008), pp. 190-202.

Phenomenal transparency is commonly studied by using a stimulus configuration introduced by Metelli: a bipartite patch, divided into equal left and right halves is overlaid with a smaller, concentric bipartite patch, divided along the same line. Observers are instructed to report either a transparent patch over an opaque bipartite field or a mosaic of four opaque patches. We show theoretically and empirically that these are only two of five generic perceptual categories, namely, transparent patch, transparent annulus (hole), mosaic, partial transparency, and multiple transparency (ambiguous) cases. Thus Gestalt factors complicate the interpretation "phenomenal transparency." We propose a framework that avoids this complication. There is excellent agreement between predictions and results.

Short-Time Inertial Response of Viscoelastic Fluids: Observation of Vortex Propagation

M Atakhorrami — 2007-11-26T20:36:52-00:00

Physical Review Letters, Vol. 95, No. 20. (2005)

We probe the response of viscous and viscoelastic fluids on micrometer and microsecond length and time scales using two optically trapped beads. In this way we resolve the flow field, which exhibits clear effects of fluid inertia. Specifically, we resolve the short-time vortex flow and the corresponding evolution of this vortex, which propagates diffusively for simple liquids. For viscoelastic fluids, this propagation is shown to be faster than diffusive and the displacement correlations reflect the frequency-dependent shear modulus of the medium.

Psychophysical basis of coincidence mechanisms in the human visual system.

MA Bouman — 2007-11-21T15:59:04-00:00

Ergeb Physiol, Vol. 65 (1972), pp. 126-172.

Experimental evidence for large dynamic effects on the plasmon dispersion of subwavelength metal nanoparticle waveguides

Femius Koenderink — 2007-11-20T20:36:07-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 76, No. 20. (2007)

We present angle- and frequency-resolved optical extinction measurements to determine the dispersion relation of plasmon modes on Ag and Au nanoparticle chains with pitches down to 75 nm. The large splitting between transverse and longitudinal modes and the band curvature are inconsistent with reported electrostatic near-field models and confirm that far-field retarded interactions are important, even for /5-sized structures. The data imply that lower propagation losses, larger signal bandwidth, and larger maximum group velocity than expected can be achieved for wave vectors below the light line. We conclude that for the design of optical nanocircuits coherent far-field couplings across the entire circuit need to be considered, even at subwavelength feature sizes.

Inferring 3D Shapes from 2D Codons

Whitman Richards — 2007-05-07T19:07:58-00:00

No. AIM-840. (1985)

All plane curves can be described at an abstract level by a sequence of five primitive elemental shapes, called "codons", which capture the sequential relations between the singular points of curvature. The codon description provides a basis for enumerating all smooth 2D curves. Let each of these smooth plane curves be considered as the silhouette of an opaque 3D object. Clearly an infinity of 3D objects can generate any one of our "codon" silhouettes. How then can we predict which 3D object...

The structure of images

Jan Koenderink — 2007-03-06T12:55:04-00:00

Biological Cybernetics, Vol. V50, No. 5. (1984), pp. 363-370.

Scale and the differential structure of images

Luc Florack — 2007-03-06T12:15:22-00:00

Image Vision Comput., Vol. 10, No. 6. (1992), pp. 376-388.

High-Frequency Stress Relaxation in Semiflexible Polymer Solutions and Networks

GH Koenderink — 2007-02-08T15:47:03-00:00

Physical Review Letters, Vol. 96, No. 13. (2006)

We measure the linear viscoelasticity of sterically entangled and chemically cross-linked networks of actin filaments over more than five decades of frequency. The high-frequency response reveals rich dynamics unique to semiflexible polymers, including a previously unobserved relaxation due to rapid axial tension propagation. For high molecular weight, and for cross-linked gels, we obtain quantitative agreement with predicted shear moduli in both amplitude and frequency dependence.

Ambiguity and the 'mental eye' in pictorial relief.

JJ Koenderink — 2006-12-11T11:21:00-00:00

Perception, Vol. 30, No. 4. (2001), pp. 431-448.

Photographs of scenes do not determine scenes in the sense that infinitely many different scenes could have given rise to any given photograph. In psychophysical experiments, observers have (at least partially) to resolve these ambiguities. The ambiguities also allow them to vary their response within the space of 'veridical' responses. Such variations may well be called 'the beholder's share' since they do not depend causally on the available depth cues. We determined the pictorial relief for four observers, four stimuli, and four different tasks. In all cases we addressed issues of reliability (scatter on repeated trials) and consistency (how well the data can be explained via a smooth surface, any surface). All data were converted to depth maps which allows us to compare the relief from the different operationalisations. As expected, pictorial relief can differ greatly either between observers (same stimulus, same task) or between operationalisations (same observer, same stimulus). However, when we factor out the essential ambiguity, these differences almost completely vanish and excellent agreement over tasks and observers pertains. Thus, observers often resolve the ambiguity in idiosyncratic ways, but mutually agree--even over tasks--in so far as their responses are causally dependent on the depth cues. A change of task often induces a change in 'mental perspective'. In such cases, the observers switch the 'beholder's share', which resolves the essential ambiguity through a change in viewpoint of their 'mental eye'.

Reflectance and texture of real-world surfaces

Kristin Dana — 2006-09-19T08:32:20-00:00

ACM Trans. Graph., Vol. 18, No. 1. (January 1999), pp. 1-34.

The nonlinear rescaling process

A Salden — 2006-09-07T13:42:38-00:00

No. 3DCV 92-28. (April 1992)

Because the process of observation denes a natural topological nesting of geometric structures of the jet bundle, we propose to model this process by a nonlinear evolution equation that we may obtain upon changing gradually the metric underlying the linear isotropic rescaling process. We a propose to derive a nonlinear rescaling process that is connected to the linear isotropic rescaling process by an one-to-one mapping. In order to study the scale space generated by this nonlinear rescaling...

Supporting knowledge-intensive inspection tasks with application ontologies

Nicole Koenderink — 2006-08-17T12:49:34-00:00

International Journal of Human-Computer Studies, Vol. 64, No. 10. (October 2006), pp. 974-983.

One of the major challenges in computer vision is to create automated systems that perform tasks with at least the same competences as human experts. In particular for automated inspection of natural objects this is not easy to achieve. The task is hampered by large in-class variations and complex 3D-morphology of the objects and subtle argumentations of experts. For example, in our horticultural case we deal with quality assessment of young tomato plants, which requires experienced specialists. We submit that automation of such a task employing an explicit model of the objects and their assessment is preferred over a black-box model obtained from modelling input-output relations only. We propose to employ ontologies for representing the geometrical shapes, object parts and quality classes associated with the explicit models. Our main contribution is the description of a method to develop a white-box computer vision application in which the needed expert knowledge is defined by: (i) decomposing the task of the inspection system into subtasks and (ii) identifying the algorithms that execute the subtasks. This method describes the interaction between the task decomposition and the needed task-specific knowledge, and studies the delicate balance between general domain knowledge and task-specific details. As a proof of principle of this methodology, we work through a horticultural case study and argue that the method leads to a robust, well-performing, and extendable computer vision system.

Haptic after-effect of successively touched curved surfaces.

IM Vogels — 2006-03-30T11:42:04-00:00

Acta Psychol (Amst), Vol. 106, No. 3. (February 2001), pp. 247-263.

A flat surface is more often judged to be convex after the touching of a concave surface than after the touching of a convex surface. This haptic after-effect increases with the time of contact with the curved surface till it saturates, and it decreases with the time-lapse between the touching of the first surface and the next one. In this paper, the haptic after-effect of two successively touched spherically curved surfaces is investigated. It is found that both surfaces contribute to the after-effect, but the after-effect is not additive. The time course of the after-effect of two successive surfaces can be described by a first-order integrator with a single time constant of about 7 s and an amplitude equal to the difference between the saturation levels of the after-effects of the two surfaces when measured in isolation. The new saturation level is therefore equal to that of the after-effect of the most recently touched surface.

Similar mechanisms underlie curvature comparison by static and dynamic touch.

SC Pont — 2006-03-30T11:27:59-00:00

Percept Psychophys, Vol. 61, No. 5. (July 1999), pp. 874-894.

In four experiments, we tested whether haptic comparison of curvature ranging from -4/m to +4/m is qualitatively the same for static and for dynamic touch. In Experiments 1 and 3, we tested whether static and dynamic curvature discrimination are based on height differences, attitude (slope) differences, curvature differences, or a combination of these geometrical variables. It was found that both static and dynamic hepatic curvature discrimination are based on attitude differences. In Experiments 2 and 4, we tested whether this mechanism leads to errors in the comparison of stimuli with different lengths for static and dynamic touch, respectively. If the judgments are based on attitude differences, subjects will make systematic errors in these comparisons. In both experiments, we found that subjects compared the curvatures of strips of the same length vertically, whereas they made systematic errors if they were required to compare the curvatures of strips of different lengths. Longer stimuli were judged to be more curved than shorter stimuli with the same curvature. We conclude that similar mechanisms underlie static and dynamic haptic curvature comparison. Moreover, additional data comparison showed that static and dynamic curvature comparison is not only qualitatively, but also quantitatively similar.

Haptic detection of sine-wave gratings.

S Louw — 2005-11-24T01:32:06-00:00

Perception, Vol. 34, No. 7. (2005), pp. 869-885.

We studied human haptic perception of sine-wave gratings. In the first experiment we measured the dependence of amplitude detection thresholds on the number of cycles and on the wavelength of the gratings. In haptic perception of sine-wave gratings, the results are in agreement with neural summation. The rate at which detection thresholds decrease with increasing number of cycles is much higher than can be accounted for by probability summation alone. Further, neural summation mechanisms describe the detection thresholds accurately over the whole spatial range probed in the experiment, that is wavelengths from 14 mm up to 225 mm. Earlier, we found a power-law dependence of thresholds on the spatial width of Gaussian profiles (Louw et al, 2000 Experimental Brain Research 132 369-374). The current results extend these findings; the power-law dependence holds not only for Gaussian profiles, but also for a broad range of sine-wave gratings with the number of cycles varying between 1 and 8. Haptic perception involves tactual scanning combined with an active, dynamic exploration of the environment. We measured characteristics of the velocity and force with which stimuli were scanned while performing a psychophysical task. One particularly surprising finding was that, without being instructed, participants maintained an almost constant scanning velocity during each 45-min session. A constant velocity in successive trials of the experiment might facilitate or even be necessary for discrimination. Further, a large systematic dependence of velocity on scanning length was found. An eightfold increase in scanning length resulted in about a fourfold increase in scanning velocity. A second experiment was conducted to study the influence of scanning velocity on psychophysical detection thresholds. This was done by systematically imposing specific scanning velocities to the participants while the thresholds were measured. The main result of the second experiment was that psychophysical detection thresholds are constant over a relatively broad range of scanning velocities.