bluuebunny/crossref_metadata_2025_split huggingface dataset:
Detection of Relative Motion
Authors: Hogervorst M A , Kappers A M L , Koenderink J J , Bongaerts J 
Published: December 2015

We measured human sensitivity to the relative motion of blobs moving in the peripheral visual field. The stimuli consisted of one or two blobs (Gaussian luminance profiles) oscillating relative to an (invisible) reference frame which rotated with constant angular velocity about a fixation point. We determined thresholds for detecting the oscillation of different configurations of one or two blobs as a function of velocity, eccentricity (viewing distance), and temporal frequency. By determining thresholds as a function of frequency the temporal characteristics of the detection system could be revealed. Thresholds are higher for oscillations in the motion direction of the reference frame than perpendicular to it. No influence has been found of the position of the blobs in the frame of reference. The thresholds are scale-invariant. For low frequencies (<2 Hz) the threshold amplitude of the velocity modulation is constant whereas for high frequencies (>2 Hz) the threshold amplitude of the position modulation is constant. This behaviour can be well described by a model which detects the oscillations whenever, within a critical time (of about 200 ms for two blobs), the relative displacement is larger than a critical distance. The critical distance shows the same dependence on velocity as the span in the bilocal detector model of Koenderink et al (1985 Journal of the Optical Society of America A2 252 – 259).
On the Generation of Antisaccades in Different Conditions
Authors: Fischer B , Weber H 
Published: December 2015

To generate a saccade in the direction opposite to the stimulus site (antisaccade) takes a conscious voluntary effort: first, one needs an intact fixation system to prevent saccades to the stimulus (prosaccades) and second, one has to generate a saccade to a location with no target. While the maintenance of fixation relies on an intact tectum, the correct performance of the antitask relies on an intact frontal system. Valid peripheral precues for attention capture indicating the direction of the antisaccade increase the error rate and the reaction time of the antisaccades. In this study we investigated how different conditions of stimulus presentation influence the performance of the antitask. We varied systematically the duration of the gap (time between fixation point offset and stimulus onset) from zero to 600 ms, and the stimulus eccentricity from 1 to 12 deg. Eye movements were recorded under infrared light. We analysed the reaction time of the correct antisaccades (a-srt) and of the erratic prosaccades (p-srt), and the percentage of errors (%-err) separately for the right and the left stimulus. Increasing the gap to 200 ms increased the %-err by 10% and decreased the a-srt by 30 ms. For longer gap durations these effects disappeared. With larger saccades the p-err rates decreased and the p-srt were reduced by 10 ms mostly staying in the range of express saccades and following the corresponding prosaccades in protasks. The a-srt decreased systematically with increasing eccentricity. The results have implications for voluntary versus involuntary control of saccades and are related to the significance of the antitask in clinical studies.
Visual Attention and Manual Aiming: Evidence for Obligatory and Selective Spatial Coupling
Authors: Deubel H , Schneider W X , Paprotta I 
Published: December 2015

We recently demonstrated that visual attention before saccadic eye movements is focused on the saccade target, allowing for spatially selective object recognition (Deubel and Schneider Vision Research in press). Here we investigate the role of visual selective attention in the preparation of aiming hand movements. The interaction of visual attention and manual aiming was studied in a dual-task paradigm that required manual pointing to a target in combination with a letter discrimination task. Subjects were asked to keep fixation in the centre of a screen. Upon offset of a central cue, they had to aim, with unseen hand, to locations within horizontal letter strings left or right from the central fixation; movements were registered with a Polhemus FastTrack system. The ability to discriminate between the symbol “E” and its mirror image presented tachistoscopically within the surrounding distractors was taken as the measure of visual attention. The results reveal that discrimination performance is far superior when the discrimination stimulus is also the target for manual aiming; when discrimination stimulus and pointing target refer to different objects, performance deteriorates. We conclude that it is not possible to maintain attention on a stimulus while directing a manual movement to a spatially separate object. Rather, our results argue for an obligatory and selective coupling of visual attention and movement programming, just as found for saccadic eye movements. This is consistent with a model of visual attention (proposed by Schneider) in which a unitary attention mechanism selects a goal object for visual processing, and simultaneously provides the information necessary for goal-directed motor action such as saccades, pointing, and grasping.
Misinterpreting Changes of Illuminant on Complex Mondrian Patterns
Authors: Nascimento S M C , Foster D H 
Published: December 2015

Ratios of cone excitations from different surfaces of the same coloured scene are almost invariant under illuminance changes, and might provide the cue by which the visual system discriminates illuminant from non-illuminant changes in coloured scenes. Previous work with pairs of surfaces showed that observers were able to detect small, naturally occurring, violations in these ratios (Nascimento and Foster, 1995 Perception24 Supplement, 60 – 61). In the present study, sensitivity to violations was assessed with more complex, Mondrian patterns. In a two-interval forced-choice experiment, two colour transformations of the same Mondrian pattern were compared by observers. The patterns comprised 7 × 7 coloured patches. Each patch was a simulation of a Munsell surface, and the whole pattern was illuminated by a Planckian illuminant of variable colour temperature. In one of the intervals only the colour temperature of the illuminant changed; in the other, the same colour-temperature change was made, but, in addition, the spectral reflectances of the surfaces were adjusted such that all cone ratios were exactly preserved for the three classes of cone. The task was to identify which of the intervals contained the pure illuminant change. Observers could reliably discriminate the intervals but systematically interpreted colour changes with invariant cone-ratios as being illuminant changes, with a probability that increased as the degree of violation of invariance increased. Performance depended mainly on long-wavelength-sensitive cones, less on medium-wavelength-sensitive cones, and little or not at all on short-wavelength-sensitive cones or luminance signals. Cone-excitation ratios, although sometimes unreliable, appear to be the dominant cue for deciding on the nature of colour changes.
Effects of Element Density on Segmentation by Luminance, Colour, and Motion
Authors: Moeller P , Hurlbert A C 
Published: December 2015

A target composed of random dots can be easily segmented from a background of random dots when each is defined by a different, broad distribution of dot speeds, colours, or brightnesses, even if these distributions overlap extensively. This shows that segmentation can be based on global properties of the target and background. If average feature value is the relevant global property, segmentation threshold should be a monotonically decreasing function of dot density. Observers viewed a 6.5 deg × 6.5 deg square array of between 512 and 36 864 randomly positioned dots (dot size 4.7 or 1.6 min arc) in which the target was a vertical band of dots defined by a different distribution of luminance, colour, or motion from the background. The task was to indicate whether the target appeared on the left or on the right of midline. The stimulus was displayed for 67 ms, followed after 233 ms by a 67 ms mask. We found that motion segmentation thresholds decrease monotonically as a function of density. For luminance and colour, segmentation thresholds initially decrease toward a local minimum, then increase to a local maximum. Thereafter, thresholds decrease monotonically as density increases further. These results suggest that the integrative segmentation process for motion is qualitatively different from those for luminance and colour. The local extrema for luminance and colour suggests that segmentation in these domains is based on forming clusters by linking together dots with similar feature properties. Only when interdot distance is sufficiently small are all dots within an area included in the cluster. Behaviour is then determined by difference in average feature value or target — background contrast.
Isoluminant Colour Contrast Does Not Fill in Surfaces
Authors: Dresp B , Wehrhahn C 
Published: December 2015

It has been suggested (Livingstone and Hubel, 1988 Science240 740 – 749) that the ‘colour-blind’ magnocellular pathways generate the neurophysiological basis of surfaces with illusory contours since the latter do not seem to be perceived in inducing configurations of a given colour which is isoluminant with regard to the colour of the background (equiluminant colour contrast). However, psychophysical data allowing us to assess the relative visibility of illusory surfaces in coloured stimuli with luminance contrast compared to configurations with equiluminant colour contrast are not yet available. We designed a colour-matching experiment where ten naive observers had to adjust the intensity of a red illusory surface so that it appeared to match the intensity of the red background. The configurations used were Kanizsa squares with green inducing elements, isoluminant or not with regard to the background. Isoluminance was assessed individually for each observer by means of a classical flicker test. A brightness-matching procedure was applied to configurations of achromatic inducers on a grey background. In this case, the inducers had either all the same contrast polarity (light), or both polarities (light and dark) within a given configuration. Luminance contrast in the achromatic configuration with only one polarity was the same as in the non-isoluminant colour condition. Luminance contrasts of light and dark inducers in the mixed-polarity condition were physically balanced. The results show that the mean point of subjective equality (PSE) of the test surface corresponds to the physical intensity of the background with equiluminant colour contrast only, indicating the absence of an apparent surface in this condition. This result supports the idea that magnocellular pathways in the human visual system mediate the neurophysiological genesis of illusory surfaces. In all the other stimulus conditions, the PSE does not correspond to the physical intensity of the background. Matching ‘errors’ are significantly stronger in the achromatic conditions, but, paradoxically, strongest in the condition with balanced contrasts of opposite polarity. This finding suggests that luminance contrast is not the only determinant of the perceived strength of illusory surfaces.
A Hierarchical Dynamic Link Network to Solve the Visual Correspondence Problem
Authors: Würtz R P , von der Malsburg C 
Published: December 2015

Conventional neural networks try to solve the problem of object recognition in a single step by building a stimulus — response system that codes its result as cell activities. We take a different approach assuming that recognition is an active process with temporal dynamics and results in an ordered state. We present a structure of neuronal layers, interconnected by dynamic links (von der Malsburg, 1985 Berichte der Bunsengesellschaft für Physikalische Chemie89 703 – 710) that solves the correspondence problem between two images and thus constitutes an important building block for a model of recognition. Images as well as stored models are represented as Gabor pyramids. This allows the dynamics to proceed from coarse to fine scale and reduces the sequential processing time inherent in the concept. Invariance under background changes is also made possible. On the lowest frequency level, a single blob of activity moves across the image and model layer, respectively. Dynamic links between these layers are initialised to the (highly ambiguous) feature similarities. Links grow or decline according to a combination of feature similarity and correlated activation. This enforces correct neighbourhood relationships in addition to feature similarity. On the higher levels the established correspondences are refined by several blobs in parallel. We present an improved version of the dynamical system proposed by Würtz [1995 Multilayer Dynamic Link Networks for Establishing Image Point Correspondences and Visual Object Recognition (Thun, Frankfurt a.M.: Harri Deutsch)] and show, with examples of human faces, that it evolves from an unordered link distribution to any ordered state where only corresponding point pairs are connected by strong links. Correspondences between sample points are population-coded by a set of neighbouring links.
Simulation of Cortex Visual Cells for Texture Segmentation: Foveal and Parafoveal Projections
Authors: Palagi P M , Guérin-Dugué A 
Published: December 2015

The objective of this work is to simulate visual cortical cells, their sensitivities to frequencies and orientations, and their part in texture segmentation. The simulation of these cells is realised through band-pass, oriented filters (Gabor filters), and multiresolution image decomposition. By this means, the filter sensitivities represent cell sensitivities to preferred orientations according to their frequency and orientation bandwidths, and multiresolution represents the different band frequencies. For texture analysis and segmentation, overlaying of band-pass filters is necessary to completely cover the Fourier domain. A continuous sensitivity to frequency and orientation is achieved by the filters overlapping and consequently by their interpolation. We used here four octave frequency bands from 1 to 16 cycles deg−1 and six orientations per band. The results obtained for texture segmentation with these parameters are very promising (up to 97% recognition rate) [Guérin-Dugué and Palagi, 1994 Neural Processing Letters1(1) 25 – 29]. The images analysed cover a multitude of different domains such as psychophysical tests and natural textures of different roughness. In order to create a cortical cell representation closer to neurophysiological data, and to improve texture segmentation results, we represent cell sensitivities by their foveal and parafoveal projections [R L DeValois, K K DeValois, 1988 Spatial Vision (Oxford: Oxford Science Publications)]. Cells receiving projections from the foveal zone are modeled by five octave frequency bands (from 0.5 to 16 cycles deg−1) and six orientations. Cells receiving projections from the parafoveal zone have the same sensitivities but are modeled by four octave frequency bands (from 0.5 to 8 cycles deg−1). By using these two different resolutions, preliminary tests have shown the capability of detecting textured regions by the parafoveal projection and localisation of boundaries by the foveal projection.
Dynamics in Parvocellular and Magnocellular Pathways: Consequences for Luminance and Colour Processing Streams
Authors: Beaudot W H A 
Published: December 2015

An achromatic neuromorphic model of the vertebrate retina has already accounted for X and Y pathways (Beaudot and Hérault, 1994 Perception23 Supplement, 25) and has shown a temporal ‘coarse-to-fine’ processing of spatial information (Beaudot et al, 1995 Perception24 Supplement, 93). This model has been extended to colour vision. By taking into account the chromatic sensitivities of cones, functional properties of the parvocellular pathway are modelled. Approximating the responses of colour-opponent cells, the model provides a spatial multiplexing of luminance and chrominance information: sustained responses show spatial band-pass behaviour to luminance variations and low-pass behaviour to equiluminant colour changes. In addition the spatiotemporal inseparability for luminance in the parvocellular model leads to a temporal multiplexing of spatial luminance information: at higher temporal frequencies the spatial filtering is low-pass, conveying only luminance information. Demultiplexing this mixed information suggests interactions between retinal channels. By locally combining additive and subtractive mechanisms between opposite parvocellular pathways (eg G+/ R−± R+/ G−), and an inhibition from the magnocellular pathway, the existence of at least three functional subchannels is predicted: (i) a transient, spatially low-pass channel, (ii) a sustained, spatially band-pass channel, dedicated to the analysis of luminance information in a spatiotemporally separable way (eg moving shadows and static textures), and (iii) a spatiotemporally low-pass, colour-opponent channel leading to colour induction, which is little affected by the presence of shadows and is more representative of objects. This hypothesis of spatiotemporal demultiplexing of luminance and chrominance information, which should presumably occur at an early cortical level, is in accordance with the multiple-processing-streams organisation of the primate visual system.
The Orthogonal Motion Aftereffect
Authors: Grunewald A , Lankheet M J M 
Published: December 2015

A recent model of motion perception suggests that the motion aftereffect (MAE) is due to an interaction across all directions, rather than just opposite directions (Grunewald, 1995 Perception24 Supplement, 111). According to the model, the MAE is caused by the interaction of broadly tuned inhibition and narrowly tuned excitation, both in direction space. The model correctly suggests that, after adaptation to opposite directions of motion, no MAE results. Unlike other accounts of the MAE, this model predicts that, after adaptation to opposite but broadly defined directions of motion, a MAE orthogonal to the inducing motions is observed. We tested this counter-intuitive prediction by adapting subjects to two populations of dots, whose average motion vectors were opposite, but which contained motion vectors deviating slightly (up to 30°) from the average direction. During the subsequent test phase, randomly moving dots were displayed. Subjects were asked to indicate whether they perceived any global motion during this phase, and if so, they were asked to indicate the perceived motion axis by aligning a line. Subjects were tested on four pairs of directions: vertical, horizontal, and the two diagonals. In all four conditions subjects reported seeing an MAE, and the axis that they indicated was always orthogonal to the inducing motions (ANOVA: p<0.001, accounted for 95% of variance). This experiment confirms the predictions made by the model, thus further supporting the interaction across all directions of narrowly tuned excitation and broadly tuned inhibition.
On the Psychophysics of Binocular Visual Space
Authors: Heller J 
Published: December 2015

Binocular vision gives rise to a perceptual space that is endowed with a rich geometrical structure, which according to Luneburg [1947 Mathematical Analysis of Binocular Vision (Princeton, NJ: Princeton University Press)] is a non-Euclidean Riemannian geometry of constant curvature. This hypothesis, together with certain psychophysical assumptions, provides a qualitative explanation of classical empirical phenomena. The Luneburg theory has been less successful in quantitatively predicting individual data, a fact that has sometimes been taken as evidence against the presumed geometrical structure. Some of these shortcomings, however, may also be attributed to psychophysical assumptions that do not depend on the geometry of visual space. The locus of perceived egocentric equidistance, for example, has been found to deviate systematically from the Vieth-Müller circle postulated by Luneburg (eg Foley, 1966 Journal of the Optical Society of America56 822 – 827). To account for these findings a generalisation of Luneburg's psychophysical mapping is proposed that actually realises his original intentions. Because of the theoretical basis of its measurement, the adequacy of this general psychophysical frame of reference may be empirically tested. The results of an experimental test of one of the key conditions, which strongly support the presented theory, are reported. It is also shown that psychophysical invariances lead to restrictions on the possible form of the psychophysical mapping, and the approach is discussed from the viewpoint of retinal correspondence.
Is Stereoscopic Capture Possible with Three-Dimensionally Slanted or Curved Illusory Surfaces?
Authors: Häkkinen J , Liinasuo M , Kojo I , Nyman G 
Published: December 2015

The depth of disparate illusory contours affects the perception of background patterns that are enclosed by these contours. If the pattern is repetitive, the depth of the illusory surface is attributed to texture elements even though the texture is in zero disparity. Previous results have suggested that this phenomenon, called stereoscopic capture, is possible only with frontoparallel surfaces (Ramachandran, 1986 Perception & Psychophysics39 361 – 373). We hypothesised that the disruption of stereo capture with three-dimensionally slanted or curved surfaces has been due to the impossibility of consistent rematching of background texture elements. If the texture is designed in such a way that adjacent elements can be rematched to form a surface that is consistent with the three-dimensional structure of the illusory surface, the capture should be possible with complex surfaces. We investigated our hypothesis by showing three-dimensionally slanted and curved surfaces to subjects and changing the pattern of the background texture. According to our results a texture of a constant period can be captured only by a frontoparallel surface; however, if the texture is designed to be consistent with the three-dimensional structure of the complex surface, the texture elements that initially form a frontoparallel surface form a complex surface when they are captured. We conclude that (a) stereoscopic capture is possible with complex three-dimensional surfaces; (b) the possible discrete matches within the background texture determine the possible three-dimensional forms that can be captured and therefore stereoscopic capture should not be characterised only as depth interpolation (Mitchison and McKee, 1987 Vision Research27 285 – 294) between large image elements.