bluuebunny/crossref_metadata_2025_split huggingface dataset:
Shift-Invariance of Pattern Recognition in the Visual Field?
Authors: Jüttner M , Rentschler I , Unzicker A 
Published: December 2015

The classification behaviour of human observers with respect to compound Gabor signals was tested at foveal and extrafoveal retinal positions. Classification performance was analysed in terms of a probabilistic classification model recently proposed by Rentschler, Jüttner, and Caelli (1994 Vision Research34 669 – 687). The analysis allows inferences about structure and dimensionality of the individual internal representations underlying the classification task and their temporal evolution during the learning process. With this technique it was found that the internal representations of direct and eccentric viewing are intrinsically incommensurable in the sense that extrafoveal pattern representations are characterised by a lower perceptual dimension in feature space relative to the corresponding physical input signals, whereas foveal representations are not (Jüttner and Rentschler, 1996 Vision Research in press). We then addressed the question to what extent observers are capable of generalising class concepts that have been acquired at one particular retinal location to other retinal sites. We found partial generalisation with respect to spatial translation across the visual field. Moreover, there is, in the case of extrafoveal learning, a distinct asymmetry in performance with respect to the visual hemifield in which the signals were originally learned. The latter finding can be related to functional hemispheric specialisation in pattern learning and recognition.
Deficits and Recovery of First-Order and Second-Order Motion Perception in Patients with Unilateral Posterior Parietal Lesions
Authors: Braun D , Fahle M , Schönle P , Zanker J 
Published: December 2015

Our aim was to test whether unilateral posterior parietal lesions degrade first-order and second-order motion differentially, and to investigate the time course of any potential recovery. We tested ten patients with circumscribed parietal lesions. Thresholds were measured for the discrimination of the direction of motion of stimuli presented 5.5° peripherally in the ipsilesional and contralesional visual hemifields. Subjects had to indicate whether a rectangular region (1.6 deg × 3 deg) embedded in dynamic random noise background moved up or down. The region contained moving (signal) and flickering (background) dots and moved for 1 s at 2.36 deg s−1. Signal dots were either (a) coherently moving in the same direction as the region (first-order), (b) stationary (second-order), or (c) coherently moving in the opposite direction (theta). Thresholds were defined as percentage of signal dots within the region yielding 75% correct responses. All patients had higher thresholds for second-order than for first-order motion. When contralesional and ipsilesional thresholds were compared, three patients showed proportional threshold elevations for all three types of motion stimuli in the contralesional hemifield. Two of these three patients were tested again five months later. Both showed considerable recovery: in one patient, the contralesional deficit was no longer present; in the other, it was reduced by about half. None of our patients had lesions affecting first-order or second-order motion differentially; lesions always affected first-order and second-order motion similarly. Owing to recovery, these deficits might be detectable only for a short time.
Converging Evidence for Two Separate Processes in Perceptual Grouping
Authors: Gilchrist I D , Findlay J M , Heywood C A 
Published: December 2015

Using a visual search paradigm, Gilchrist, Humphreys, Riddoch, and Neumann ( Journal of Experimental Psychology: Human Perception and Performance in press) demonstrated that grouping was mediated by a contrast-polarity-dependent surface-based process, and a contrast-independent edge-based process. A vertical pair of circles ‘popped out’ amongst horizontal pairs of circles as distractor only if the items within each pair had the same contrast polarity. Alternatively, if the circles were replaced with squares that had collinear edges, the vertical pair ‘popped out’ regardless of the contrast polarity of the squares. Here we report new data in which the role of these two grouping processes in the control of saccadic eye-movements was investigated. If subjects are presented with two items concurrently in an eye-movement task they will often make a saccade to the midpoint between the two items—this has been called the ‘global effect’. This effect is assumed to reflect some underlying integration of the visual information during the programming of saccade landing position. Distractor items that either shared the same polarity or had collinear edges with the target were found to affect the landing position of saccades. Distractors that had neither collinear edges nor common polarity produced a smaller global effect but the landing position was still affected. This suggests that the integration processes that underlie the global effect are sensitive to both common edge and surface properties. However, some integration may occur as a result of the mere presence of activity in the visual field.
Tracing a Metameric Match to Individual Variations of Colour Vision
Authors: Diaz J A , Chiron A , Viénot F 
Published: December 2015

A metameric match may be accepted by one observer but rejected by another, indicating that the colour vision of the two observers is different. We analysed the variations of normal colour vision using metameric surfaces. First, we modelled the matches of a theoretical colour-normal observer on the Davidson and Hemmendinger (D&H) colour rule (Kaiser and Hemmendinger, 1980 Color Research and Applications5 65 – 71), using spectroradiometric measurements and a set of fundamentals (Stockman, Macleod, and Johnson, 1993 Journal of the Optical Society of America A10 2491 – 2521). We also derived deviate fundamentals by changing the macular pigment density, the lens density, the photopigment density, and by shifting the long-wave sensitive photopigment. Modelling the deviate normal observer matches yields shifts of no more than 1 sample on the D&H colour rule. The largest shifts are produced by changes in lens density. Second, we asked six observers to perform a match on the D&H colour rule. We also estimated their macular pigment densities and their lens densities by using heterochromatic flicker photometry matches at 466 nm and 413 nm versus a 558 nm reference, and we recorded their Nagel anomaloscope setting. Then, we computed their personalised fundamentals in order to predict their D&H colour matches. As the most frequent match in a sample of young observers is F-15 and the modelling for the theoretical observer yields G-15, a systematic error occurs in the predictions. After correction for this error, the results show that the metameric match of five out of six observers is better predicted by using the personalised correction of the lens and macular pigment optical density only, than by using the theoretical colour observer data.
Texture Segmentation Performance as a Function of Target Size: Psychophysical Data and Simulation Results
Authors: Kehrer L 
Published: December 2015

Texture segmentation performance is little affected by the size of the target patch if targets of 0.76 deg × 0.76 deg and 2.16 deg × 2.16 deg are used (Joffe and Scialfa, 1995 Spatial Vision9 325 – 342). In two experiments we reinspected the results of Joffe and Scialfa, but now also included a rather small target patch (target sizes: 0.3 deg × 0.3 deg, 1.4 deg × 1.4 deg, and 2.3 deg × 2.3 deg). In agreement with Joffe and Scialfa, we found only a small difference in segmentation performance between the two large target sizes (1.4 deg × 1.4 deg and 2.3 deg × 2.3 deg) but a marked difference between 0.3 deg × 0.3 deg and 1.4 deg × 1.4 deg. In line with Joffe and Scialfa, we conclude that segmentation performance is affected by the size of the target in a nonlinear manner, leading to an asymptotic performance level at large target sizes. In a simulation study we asked whether the observed experimental results could be explained within the framework of a standard two-layer ‘back-pocket’ spatial filter model. The results of the computations showed a good fit with the experimental data if spatial filters in the second layer of the model were set to a width of about 1 deg.
Do Difficulties in Stimulus Discrimination Affect Luminance Processing?
Authors: Pins D , Treisman M , Johnston R 
Published: December 2015

Simple reaction time is known to decay as a hyperbolic function of luminance (Piéron's function). An identical relationship has also been demonstrated recently (Pins and Bonnet, 1996 Perception & Psychophysics in press) with different choice-reaction-time tasks. Although mean choice reaction time increased with the complexity of the task, the exponents of the functions relating reaction time (RT) to luminance were found to be equal in each experiment. These results suggest that the task specific time required by the different tasks only adds to the time necessary for luminance processing. In these experiments, the different stimuli presented were easily discriminable. In the present study, we examined the effect of variation in luminance on a more difficult discrimination task involving variation in orientation. Five different luminance levels covering the entire mesopic range were used. In two conditions, tilted lines at nine different angles were used, at a spacing of 2°. In the first condition, the orientations were chosen on both sides of the vertical (the subject responded “left” or “right”); in the second condition, the orientations were on both sides of a line oriented at −40° to the vertical (the subject responded “high” or “low”). The results were compared to those of a second experiment in which only two easily discriminable orientations were used. The results show that RT is greater in the experiments in which nine orientations are used, while the effect of intensity on RT is lower. This effect does not depend on orientation.
A Neuronal Model Detecting Figure—Ground Direction and Contrast Polarity at Illusory Contours
Authors: Heitger F , Peterhans E 
Published: December 2015

A previous model of visual cortical contour processing simulated the perception of illusory contours and the direction of figure and ground at such contours (Heitger and von der Heydt, 1993 Proceedings of International Conference on Computer Vision). This model was based on ‘complex type’ operators. Hence, it lacked representations of contrast polarity and could not account for brightness effects of illusory figures. Recently, we found that some neurons of area V2 in monkey visual cortex that signalled illusory contours were also sensitive to the figure — ground direction and contrast polarity at such contours. This allowed us to extend the previous scheme by incorporating the signals of even and odd symmetrical ‘simple type’ operators, particularly at the level of the ‘end-stopped’ operators which served the detection of the inducing elements of illusory figures (line ends, corners). To represent illusory contours, the signals of these operators were grouped with the use of fields of wide spatial support. Line ends and corners were handled differently in this process: at corners, signals parallel to the illusory contour were emphasised (para grouping), whereas at line ends signals orthogonal to the contour (ortho grouping) were emphasised. Further, ortho grouping included only end-stopped operators with even-symmetrical fields, para grouping only operators with odd-symmetrical fields. Thus, both processes contributed to the contrast polarity represented at illusory contours. Results of simulations showed that this model reproduces the shapes of illusory contours, their figure — ground direction, and contrast polarity. Hence, it can explain the selectivity of cortical neurons at such contours and the perceived brightness of illusory figures.
The Changing of Perceived Speed as a Function of Stimulus Contrast: An Attempted Replication with a Variety of Stimuli
Authors: Blakemore M R , Snowden R J 
Published: December 2015

It has been reported that the perceived speed of a target stimulus varies with its physical contrast (Thompson, 1982 Vision Research22 377 – 380). Yet the effect is somewhat ephemeral with intersubject variations, effects of presentation mode, and test speed. So far this effect has only been reported for grating stimuli and it is unclear whether the effect generalises to other stimuli. Subjects reported which of two patterns appeared to move more quickly. Using a forced-choice method we compared performance with three different stimuli: simple gratings in linear motion, radial gratings in rotary motion, and Julesz random-dot patterns in linear motion. We further compared simultaneous and successive presentations. For grating stimuli at slow speeds we obtained an effect of contrast which was greater for simultaneous presentations, thus replicating Stone and Thompson's results (1992 Vision Research32 1535 – 1549). However, at similar temporal frequencies there is no reliable effect of contrast for the radial gratings. With the random-dot patterns we generally found little effect of contrast; however, we did obtain a small effect at the slowest speeds, but only when presenting the two patterns simultaneously. Our results suggest that the contrast effect on perceived speed does not readily generalise to all stimuli. We discuss the implications of these results in terms of attentional tracking mechanisms and models of motion perception.
Is the McCollough Effect Coded in Disparity-Sensitive Units?
Authors: McLoughlin N P 
Published: December 2015

Previous studies have demonstrated that the McCullough effect (ME) is coded by both binocular and monocular units [see for example Vidyasagar, 1976 Nature (London)261 39]. At ECVP95, McLoughlin and Savoy (1995 Perception24 Supplement, 94) presented a contiguity-learning based model capable of accounting for a large dataset of both binocular and monocular ME variants. One issue not directly addressed by this model concerns the site of binocular ME adaptation. Specifically, are the binocular ME ‘units’ disparity selective? The methods used were developed from those of McLoughlin and Savoy. Subjects were exposed to alternating coloured gratings at various crossed and uncrossed disparities (relative to fixation points). A colour cancellation paradigm was used to record the strength of the ME for a series of binocularly disparate composite achromatic test gratings both before and after adaptation. These test gratings had a small random amount of colour added to them so that the subjects could not assume that the patterns were achromatic before making adjustments. Subjects waited 5 min in the dark after adaptation before being tested. This removes any possible confounds associated with retinal afterimages. Preliminary results suggest, in agreement with previous studies, that the main component of the ME is monocular. However, the binocular component appears to be sensitive to disparity. A second series of experiments wherein two opponent MEs were induced, one at crossed and the other at uncrossed disparities, supports this finding. As in the Vidyasagar variant, the magnitude of the ME is extremely weak in this case.
Psychophysical Discrimination of Spatial Structure in Natural Images
Authors: Carlin P , Watt R 
Published: December 2015

We report a series of experiments in which subjects were required to make spatial discriminations about naturally obtained images, as follows. Subjects were shown two natural images on a computer screen, side by side and for a period of 500 ms. Subjects were then shown, on a separate part of the computer screen, a small patch of one of the images selected at random. Subjects were required to decide which of the two full images the patch comes from, and whereabouts in that image it is taken from. They indicated their response by clicking the mouse cursor at the appropriate point on the screen. The proportion of trials on which the subjects selected the correct image and the accuracy of the spatial position of their correct responses were calculated. These two measures independently indicate the general discriminability of the two image sets, and the extent to which the spatial layout of the image has been perceived. By comparing response accuracy for images that are from different sets (such as mountain or town scenes) with response accuracy when the two images are from the same set it is possible to establish the image properties that underlie coarse scene apprehension. By using sets of images that are perhaps best described as being natural textures (such as patterns of foliage or rock surface) it is possible to measure the extent to which texture is processed spatially. Our results are interpreted in the light of statistical differences between the image sets that we have used.
Face Detection in Natural Scenes
Authors: Hill H , Watt R 
Published: December 2015

The first task of any face processing system is detection of the face. We studied how the human visual system achieves face detection using a 2AFC task in which subjects were required to detect a face in the image of a natural scene. Luminance noise was added to the stimuli and performance was measured as a function of orientation and orientation bandwidth of the noise. Sensitivity levels and the effects of orientation bandwidth were similar for horizontally and vertically oriented noise. Performance was reduced for the smallest orientation bandwidth (5.6°) noise but sensitivity did not decrease further with increasing bandwidth until a point between 45° and 90°. The results suggest that important information may be oriented close to the vertical and horizontal. To test whether the results were specific to the task of face detection the same noise was added to the images in a man-made natural decision task. Equivalent levels of noise were found to be more disruptive and the effect of orientation bandwidth was different. The results are discussed in terms of models of face processing making use of oriented filters (eg Watt and Dakin, 1993 Perception22 Supplement, 12) and local energy models of feature detection (Morrone and Burr, 1988 Proceedings of the Royal Society of London B235 221 – 245).
One Whole or Two? The Flexible Role of the Observer in Perceptual Grouping
Authors: Freeman E D 
Published: December 2015

The flexibility of perceptual grouping was examined from a functional perspective. If particular wholes are good for particular tasks, we can bend the rules of Gestalt organisation in order to see them? Evidence was gained by observing the effects of Gestalt factors on task performance, in the context of subjects' experience and expectation. Ellipsoids were split into two semi-ellipses by a central gap of variable width, which controlled the relative dominance of a one-whole percept (Narrow-Gap) or a two-parts percept (Wide-Gap). In a sequential comparison task, a reference ellipsoid was paired either with another ellipsoid (Whole/Whole), or with a single semi-ellipse (Whole/Part). Subjects responded “different” to a change in the shape of one semi-ellipse. Stimuli appeared for 50 ms and the reference stimulus was masked with stimulus onset asynchronies (SOAs) of 50 to 250 ms. Subjects were first trained either on Whole/Whole or Whole/Part only; both comparisons types were then mixed randomly. A loss of discriminability was predicted for Whole/Whole with Wide-Gap, and for Whole/Part with Narrow-Gap, but only for untrained and unexpected comparisons. The predicted interaction was significant. The effects were particularly strong with 50 ms SOA, suggesting an early involvement of task-contextual factors in perceptual grouping, and showing no evidence of global priority. Subjects appeared to readily learn flexible grouping strategies, abstracting only the behaviourally relevant relationships, and thus offsetting the effects of stimulus structure on perceptual grouping.
Searching for Objects in the Visual Periphery: Effects of Orientation
Authors: Newell F N 
Published: December 2015

Previous studies have found that the recognition of familiar objects is dependent on the orientation of the object in the picture plane. Here the time taken to locate rotated objects in the periphery was examined. Eye movements were also recorded. In all experiments, familiar objects were arranged in a clock face display. In experiment 1, subjects were instructed to locate a match to a central, upright object from amongst a set of randomly rotated objects. The target object was rotated in the frontoparallel plane. Search performance was dependent on rotation, yielding the classic ‘M’ function found in recognition tasks. When matching a single object in periphery, match times were dependent on the angular deviations between the central and target objects and showed no advantage for the upright (experiment 2). In experiment 3 the central object was shown in either the upright rotation or rotated by 120° from the upright. The target object was similarly rotated given four different match conditions. Distractor objects were aligned with the target object. Search times were faster when the centre and target object were aligned and also when the centre object was rotated and the target was upright. Search times were slower when matching a central upright object to a rotated target object. These results suggest that in simple tasks matching is based on image characteristics. However, in complex search tasks a contribution from the object's representation is made which gives an advantage to the canonical, upright view in peripheral vision.