bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Role of the Vertical Dimension in Stereoscopic Vision
Authors: Longuet-Higgins H Christopher 
Published: January 2007

It is shown how the full three-dimensional structure of a scene may in principle be computed from a correlated pair of retinal images, when all that is known about the orientations of the two eyes is that the planes of their horizontal meridians accurately coincide. The vertical dimension plays a crucial role in the theory; visible points which lie on the horizontal meridian supply no information about the angle of convergence or the direction of gaze. (i) If the scene contains three or more nonmeridional points, not all lying in a vertical plane, then their positions in space are fully determined by the horizontal and vertical coordinates of their images on the two retinas. (ii) If just two nonmeridional points are visible, or more than two, lying in a vertical plane, then their retinal images admit, in general, just two distinct three-dimensional interpretations. One of these is usually unrealistic; but a choice between them may be perceptually difficult if the vertical plane containing the points is nearly perpendicular to the interocular axis. These results suggest that vertical disparities may play an important role in the binocular perception of absolute depth. Elsewhere (Mayhew, this issue) this suggestion is found to provide a quantitative explanation of Ogle's ‘induced effect’.
Amodal Representation of Occluded Surfaces: Role of Invisible Stimuli in Apparent Motion Correspondence
Authors: Shimojo Shinsuke , Nakayama Ken 
Published: January 2007

A series of demonstrations were created where the perceived depth of targets was controlled by stereoscopic disparity. A closer object (a cloud) was made to jump back and forth horizontally, partially occluding a farther object (a full moon). The more distant moon appeared stationary even though the unoccluded portion of it, a crescent, changed position. Reversal of the relative depth of the moon and cloud gave a totally different percept: the crescent appeared to flip back and forth in the front depth plane. Thus, the otherwise-robust apparent motion of the moon crescents was completely abolished in the cloud-closer case alone. This motion-blocking effect is attributed to the ‘amodal presence’ of the occluded surface continuing behind the occluding surface. To measure the effect of this occluded ‘invisible’ surface quantitatively, a bistable apparent motion display was used (Ramachandran and Anstis 1983a): two small rectangular-shaped targets changed their positions back and forth between two frames, and the disparity of a large centrally positioned rectangle was varied. When the perceived depths supported the possibility of amodal completion behind the large rectangle, increased vertical motion of the targets was found, suggesting that the amodal presence of the targets behind the occluder had effectively changed the center position of the moving targets for purposes of motion correspondence. Amodal contours are literally ‘invisible’, yet it is hypothesized that they have a neural representation at sufficiently early stages of visual processing to alter the correspondence solving process for apparent motion.
Evidence for Independent Processing of Subjective Contour Brightness and Sharpness
Authors: Siegel Steven , Petry Susan 
Published: January 2007

Subjective contours have been of considerable interest because of their importance to theories and physiological models of form perception. In particular, they have recently been characterized as the result of magnocellular cortical processing. There is, however, a paucity of parametric data relating to basic psychophysical parameters in this field. Two experiments are reported in which the roles of subjective contour size, retinal eccentricity, and flicker rate in subjective contour salience were investigated. Eleven observers estimated subjective contour magnitude using an Ehrenstein configuration. Configurations ranging in size from 0.25 to 3 deg were presented to three retinal loci (fovea, 2 deg, and 4 deg) at flicker rates ranging from 5 to 15 Hz. Subjective contour brightness and distinctness were measured separately. Brightness was greatest at a subjective contour size of about 1.25 deg, at flicker rates of 5 – 7 Hz, and at 3 deg peripheral for all flicker rates and all but the smallest stimulus sizes. Distinctness decreased with eccentricity and flicker, but remained high at small diameters (thus implicating spatially sensitive mechanisms). Taken together, the results support a magnocellular processing of subjective contours with respect to brightness, but also suggest that there is a parvocellular contribution to subjective contour sharpness.
Higher-Order Factors Influencing the Perception of Sliding and Coherence of a Plaid
Authors: Kooi Frank L , De Valois Karen K , Switkes Eugene , Grosof David H 
Published: January 2007

The effect of several new stimulus parameters on the perception of a moving plaid pattern (the sum of two sine-wave gratings) were tested. It was found that: (i) the degree of perceived sliding is strongly influenced by the aperture configuration through which the plaid is viewed; (ii) the chromaticity of the sinusoidal components affects coherence in that more sliding is observed when the plaid components differ in hue, and there is less sliding when they are of the same hue; (iii) equiluminant plaids made of components equal in color almost never show any sliding; and (iv) sliding increases with viewing time. The coherence—sliding percept must therefore be influenced by color, by global interactions, and by adaptation or learning effects, thus suggesting a higher-level influence. These results are most easily modelled by separating the decision to carry out recombination from the process of recombination.
Trade-Off between Achromatic Colour and Perceived Illumination as Revealed by the Use of Pseudoscopic Inversion of Apparent Depth
Authors: Logvinenko Alexander , Menshikova Galina 
Published: January 2007

The albedo hypothesis was tested under apparent transformations of perceived illumination and achromatic colour induced by pseudoscopic inversion of apparent depth. Looking through a pseudoscope made a cone attached to a vertical white screen look like a conical hole in the screen. This in turn caused the shadow which the now ‘invisible’ cone cast on the screen to change its appearance and to look like a darkly pigmented area. The darkness of the shadow before the pseudoscopic reversal and greyness of colour afterwards were measured by means of psychophysical scales for darkness and greyness set by the bisection method. Contrast of the shaded area was varied from 0.17 to 0.96 in 7 steps, the mean illuminance of the screen having been maintained at 40 lx. Although the albedo hypothesis in its classical form was not confirmed, it was found that darkness of shadow varied linearly in inverse proportion to greyness of colour within the entire contrast range. This is in agreement with the hypothesis that achromatic colour and perceived illumination are inversely proportional to each other while the retinal illumination is constant.
Aftereffects of Apparent Motion: The Existence of an AND-Type Binocular System in Human Vision
Authors: van Kruysbergen Noud A W H , de Weert Charles M M 
Published: January 2007

There has been evidence for the existence of a purely binocular system in human vision that acts as an AND gate on information from the two eyes. There also has been evidence for the nonexistence of such a purely binocular system, indicating only the existence of an OR-type binocular system that responds to input from one or both eyes. As a result there are a number of possible explanations for the differing experimental results: the binocular system is an OR-type system only, it is a facilitating OR system that has AND-type characteristics, or it consists of independent OR and AND subsystems. Monocular adaptation, alternating monocular adaptation, or binocular adaptation were used to demonstrate the existence of the different systems, but in none of the previous experiments was the AND-type binocular system activated directly, and the existence of this AND system was deduced mostly because of differences in aftereffect strengths between monocular and binocular test conditions. Experiments are reported in which stimuli that activate the AND-type binocular system explicitly have been used, and the results show that we need the existence of such an AND-type binocular system to account for the results.
The Relationship between Stereoacuity and Stereomotion Thresholds
Authors: Cumming Bruce G 
Published: January 2007

There are in principle at least two binocular sources of information that could be used to determine the motion of an object towards or away from an observer; such motion produces changes in binocular disparities over time and also generates different image velocities in the two eyes. It has been argued in the past that stereomotion is detected by a mechanism that is independent of that which detects static disparities. More recently this conclusion has been questioned. If stereomotion detection in fact depends upon detecting disparities, there should be a clear correlation between static stereo-detection thresholds and stereomotion thresholds. If the systems are separate, there need be no such correlation. Four types of threshold measurement were performed by means of random-dot stereograms: (1) static stereo detection/discrimination; (2) stereomotion detection in dynamic random-dot stereograms (temporally uncorrelated); (3) stereomotion detection in temporally correlated random-dot stereograms; and (4) binocular detection of frontoparallel motion. Three normal subjects and five subjects with unusually high stereoacuities were studied. In addition, two manipulations were performed that altered stereomotion thresholds: changes in mean disparity, and image defocus produced by positive spectacle lenses. Across subjects and conditions, stereomotion thresholds were well correlated with stereo-discrimination thresholds. Stereomotion was poorly correlated with binocular frontoparallel-motion thresholds. These results suggest that stereomotion is detected by means of registering changes in the output of the same disparity detectors that are used to detect static disparities.
Is the Richness of Our Visual World an Illusion? Transsaccadic Memory for Complex Scenes
Authors: Blackmore Susan J , Brelstaff Gavin , Nelson Kay , Trościanko Tom 
Published: January 2007

Our construction of a stable visual world, despite the presence of saccades, is discussed. A computer-graphics method was used to explore transsaccadic memory for complex images. Images of real-life scenes were presented under four conditions: they stayed still or moved in an unpredictable direction (forcing an eye movement), while simultaneously changing or staying the same. Changes were the appearance, disappearance, or rotation of an object in the scene. Subjects detected the changes easily when the image did not move but when it moved their performance fell to chance. A grey-out period was introduced to mimic that which occurs during a saccade. This also reduced performance but not to chance levels. These results reveal the poverty of transsaccadic memory for real-life complex scenes. They are discussed with respect to Dennett's view that much less information is available in vision than our subjective impression leads us to believe. Our stable visual world may be constructed out of a brief retinal image and a very sketchy, higher-level representation along with a pop-out mechanism to redirect attention. The richness of our visual world is, to this extent, an illusion.
Surface Gradients, Contours and the Perception of Surface Attitude in Images of Complex Scenes
Authors: Christou Chris , Koenderink Jan J , van Doorn Andrea J 
Published: January 2007

Sophisticated computer graphics were used to generate images of three-dimensional blocks-world scenes to investigate the perception of surface attitude. Three types of surface primitive (planar blocks, cylinders, and ellipsoids) were combined to form structured settings. The experiments were designed to investigate whether surface-based information such as gradients in shading and texture provide any significant advantage in attitude judgments over information derived from object contours. Images of shaded, textured, and line-drawn surfaces formed the stimulus set. The subjects' task consisted of setting an attitude probe on different parts of the scene so that the probe appeared to be locally coplanar with the perceived surfaces. Analysis of settings according to attitude components, slant and tilt, shows remarkable agreement in slant settings for the shaded and line-drawn scenes but poor correlation between shaded and textured scenes. Similarly, tilt was also easily judged in shaded and line-drawn scenes and the experiments indicate that explicit surface boundaries are important for stable tilt perception. In general, the results suggest that, for the simple surfaces employed here, surface cues provide little extra information beyond that which is derived from contours.
Spatial Scale Interactions and Image Statistics
Authors: Brady Nuala 
Published: January 2007

In natural scenes and other broadband images, spatial variations in luminance occur at a range of scales or frequencies. It is generally agreed that the visual image is initially represented by the activity of separate frequency-tuned channels, and this notion is supported by physiological evidence for a stage of multi-resolution filtering in early visual processing. The question whether these channels can be accessed as independent sources of information in the normal course of events is a more contentious one. In the psychophysical study of both motion and spatial vision, there are examples of tasks in which fine-scale structure dominates perception or performance and obscures information at coarser scales. It is argued here that one important factor determining the relative salience of information from different spatial scales in broadband images is the distribution of response activity across spatial channels. The special case of natural scenes that have characteristic ‘scale-invariant’ power spectra in which image contrast is roughly constant in equal octave frequency bands is considered. A review is presented of evidence which suggests that the sensitivity of frequency-tuned filters in the visual system is matched to this image statistic, so that, on average, different channels respond with equal activity to natural scenes. Under these conditions, the visual system does appear to have independent access to information at different spatial scales and spatial scale interactions are not apparent.
Grouping by Proximity or Similarity? Competition between the Gestalt Principles in Vision
Authors: Quinlan Philip T , Wilton Richard N 
Published: January 2007

The nature of the psychological processes that underlie the Gestalt principles of grouping by proximity and grouping by similarity is examined. Similarity was defined relative to the principles of grouping by common colour and grouping by common shape. Subjects were presented with displays comprising a row of seven coloured shapes and were asked to rate the degree to which the central target shape grouped with either the right or the left flanking shapes. Across the displays the proximal and featural relationships between the target and flankers were varied. These ratings reflected persuasive effects of grouping by proximity and common colour; there was only weak evidence for grouping by common shape. Nevertheless, both common colour and common shape were shown to override grouping by proximity, under certain conditions. The data also show that to understand how the Gestalt principles operate it appears necessary to consider processes that operate within and between groups of elements that are initially identified on the basis of proximity. Whether such groups survive further analysis depends critically on the featural content of the constituent elements.
Cues to Viewing Distance for Stereoscopic Depth Constancy
Authors: Glennerster Andrew , Rogers Brian J , Bradshaw Mark F 
Published: January 2007

A veridical estimate of viewing distance is required in order to determine the metric structure of objects from binocular stereopsis. One example of a judgment of metric structure, which we used in our experiment, is the apparently circular cylinder task (E B Johnston, 1991 Vision Research31 1351 – 1360). Most studies report underconstancy in this task when the stimulus is defined purely by binocular disparities. We examined the effect of two factors on performance: (i) the richness of the cues to viewing distance (using either a naturalistic setting with many cues to viewing distance or a condition in which the room and the monitors were obscured from view), and (ii) the range of stimulus disparities (cylinder depths) presented during an experimental run. We tested both experienced subjects (who had performed the task many times before under full-cue conditions) and naive subjects. Depth constancy was reduced for the naïve subjects (from 62% to 46%) when the position of the monitors was obscured. Under similar conditions, the experienced subjects showed no reduction in constancy. In a second experiment, using a forced-choice method of constant stimuli, we found that depth constancy was reduced from 64% to 23% in naive subjects and from 77% to 55% in experienced subjects when the same set of images was presented at all viewing distances rather than using a set of stimulus disparities proportional to the correct setting. One possible explanation of these results is that, under reduced-cue conditions, the range of disparities presented is used by the visual system as a cue to viewing distance.
SatDNA Analyzer: a computing tool for satellite-DNA evolutionary analysis
Authors: Navajas-Pérez Rafael , Rubio-Escudero Cristina , Aznarte José Luis , Rejón Manuel Ruiz , Garrido-Ramos Manuel A. 
Published: January 2007

AbstractSummary: satDNA Analyzer is a program, implemented in C++, for the analysis of the patterns of variation at each nucleotide position considered independently amongst all units of a given satellite-DNA family when comparing it between a pair of species. The program classifies each site accordingly as monomorphic or polymorphic, discriminates shared from non-shared polymorphisms and classifies each non-shared polymorphism according to the model proposed by Strachan et al. in six different stages of transition during the spread of a variant repeat unit toward its fixation. Furthermore, this program implements several other utilities for satellite-DNA analysis evolution such as the design of the average consensus sequences, the average base pair contents, the distribution of variant sites, the transition to transversion ratio and different estimates of intra-specific variation and inter-specific variation. Aprioristic hypotheses on factors influencing the molecular drive process and the rates and biases of concerted evolution can be tested with this program. Additionally, satDNA Analyzer generates an output file containing a sequence alignment without shared polymorphisms to be used for further evolutionary analysis by using different phylogenetic softwares.Availability: satDNA Analyzer is freely available at http://satdna.sourceforge.net/. SatDNA Analyzer has been designed to operate on Windows, Linux and Mac OS X.Contact: rnavajas@uga.edu
The pro‐peptide of proNGF: Structure formation and intramolecular association with NGF
Authors: Kliemannel Marco , Golbik Ralph , Rudolph Rainer , Schwarz Elisabeth , Lilie Hauke 
Published: January 2007

AbstractThe pro‐peptide of human nerve growth factor (NGF) functions as an intramolecular chaperone during oxidative renaturation of proNGF in vitro and interacts intramolecularly with the mature part of native proNGF. Here, we analyzed the structure formation and stability of the pro‐peptide in the context of proNGF and its intramolecular interaction with the native mature part. Folding and unfolding of the NGF‐coupled pro‐peptide, as analyzed by fluorescence, were biphasic reactions with both phases depending on the interaction with the mature part. This interaction was characterized by an overall stability of ΔG = 20.9 kJ/mol that was subdivided into two reactions, native ↔ intermediate state (14.8 kJ/mol) and intermediate ↔ unfolded state (6.1 kJ/mol). An additional very fast unfolding reaction was observed using circular dichroism (CD), indicating the presence of at least two kinetically populated intermediates in the unfolding of proNGF. The part of the pro‐peptide involved in the intramolecular association with mature NGF comprised the peptide Trp−83‐Ala−63as determined by H/D exchange experiments. Spectroscopic analyses revealed that on the NGF side, a surface area around Trp21interacted with the pro‐peptide. Trp21also participates in binding to TrkA and p75 receptors. These overlapping binding sites of the pro‐peptide and the NGF receptors might explain the previously observed lower affinity of proNGF to its receptors as compared to NGF.
Analysis of ligand binding to a ribose biosensor using site‐directed mutagenesis and fluorescence spectroscopy
Authors: Vercillo Natalie C. , Herald Kaitlin J. , Fox John M. , Der Bryan S. , Dattelbaum Jonathan D. 
Published: January 2007

AbstractComputational design of proteins with altered ligand specificity is an emerging method for the creation of new biosensing systems. In this work, we investigated the outcome of site‐directed mutagenesis on the Escherichia coli ribose binding protein (RBP), which is frequently used as a design scaffold for computational searches. A ribose biosensor was first constructed whereby an environmentally sensitive fluorescent probe was covalently attached to RBP at position S265C. This protein conjugate displayed a 54% decrease in emission intensity upon the addition of saturating ribose concentrations and exhibited an apparent dissociation constant (Kd) of 3.4 μM. Site‐directed mutants within the RBP binding pocket were created and examined for ribose binding ability and overall structural stability. Because as many as 12 mutations are needed to alter ligand specificity in RBP, we measured the effect of single and multiple alanine mutations on stability and signal transduction potential of the ribose biosensor. Single alanine mutations had significant impact on both stability and signaling. Mutations of N190A and F214A each produced melting temperatures >8°C below those observed for the wild‐type protein. Residue Q235, located in the hinge region of RBP, appeared to be a hot spot for global protein stability as well. Additional single alanine mutations demonstrated as much as 200‐fold increase in apparent Kd but retained overall protein stability. The data collected from this study may be incorporated into design algorithms to help create more stable biosensors and optimize signal transduction properties for a variety of important analytes.