bluuebunny/crossref_metadata_2025 huggingface dataset:
Directionally Selective Channels Mediate the Discrimination of Moving Aperiodic Stimuli
Authors: Shulman Gordon L
Published: January 2007

Adaptation procedures were used to demonstrate that directionally selective channels mediate the discrimination of aperiodic stimuli moving at a common speed. Subjects were adapted to a grating that drifted to either the left or the right. They then determined the orientation of the gap in a ‘C’ that was also moving to either the left or the right. Performance in identifying the gap orientation was most impaired when the C moved in the same direction as the adapting grating.
Illusory Continuous Motion from Oscillating Positive-Negative Patterns: Implications for Motion Perception
Authors: Anstis Stuart M , Rogers Brian J
Published: January 2007

A black and white (positive) grating pattern was superimposed in exact register on its own photographic negative. Four operations were repetitively applied to this positive pattern so that it moved fractionally to the right, grew dimmer, moved back to the left, and grew brighter again. This sequence produced a strong illusion of continuous apparent motion to the right for as long as the cycle was repeated. The small relative motion between the two patterns generated two new illusory effects: enhanced real movement (ERM) and reversed real movement (RRM). The dimming and brightening phases gave rise to reversed apparent movement (RAM). All three effects are attributed to spatial filtering by neural mechanisms, which shifts the effective position of the positive-negative contours.
The Spectral Sensitivities of the Middle- and Long-Wavelength Cones: An Extension of the Two-Colour Threshold Technique of W S Stiles
Authors: Stockman Andrew , Mollon John
Published: January 2007

When a tiny centred test flash is presented on a small concentric background, the threshold rises with background radiance more quickly than Weber's law would predict. It is argued that under such conditions it is possible, by means of a test sensitivity method, to isolate either the M-cone or the L-cone types throughout the visible spectrum. As predicted, double-branched M- and L-cone tvr functions are found when the test flash and the field are of the same wavelength. From the independent vertical displacements of the two branches as test wavelength is varied, it is possible to derive spectral sensitivities that agree well with dichromatic sensitivities and König fundamentals. The test sensitivities deviate from π4 at longer wavelengths and from π5 at shorter wavelengths.
Mapping of Extracorporeal Space by Vibrotactile Reaction Times: A Far-Left-Side Disadvantage
Authors: Pierson-Savage Jane M , Bradshaw John L
Published: January 2007

Vibrotactile reaction times in normal dextrals were measured for the two hands separately when either hand was located at each of seven possible positions: 90°, 45°, and 15° to the left and right of the chest midline, and at the midline itself (0°). Reaction times for the two hands did not differ and there was no Hand by Position interaction. At 90° left, reaction times were significantly slower than at any other position except 45° right. However, none of the other positions, including 45° right, differed from each other. Performance in this task, therefore, was relatively uniform from 90° right to 45° left, but markedly slower at 90° left. This far-left-side disadvantage may reflect a difficulty (for dextrals) in focussing covert attention in the far-left part of space for a block of trials. Since vibrotactile reaction times are sensitive to attentional factors in normal subjects, the paradigm should allow quantification of the clinical symptoms of the hemineglect syndrome; some preliminary observations of this syndrome with another vibrotactile design are reported.
The Way the Ball Bounces: Visual and Auditory Perception of Elasticity and Control of the Bounce Pass
Authors: Warren William H , Kim Elizabeth E , Husney Robin
Published: January 2007

Human observers may perceive not only spatial and temporal dimensions of the environment, but also dynamic physical properties that are useful for the control of behavior. A study is presented in which visual and auditory perception of elasticity in bouncing objects, which was specified by kinematic (spatiotemporal) patterns of object motion, were examined. In experiment 1, observers could perceive the elasticity of a bouncing ball and were able to regulate the impulse applied to the ball in a bounce pass. In experiments 2 and 3, it was demonstrated that visual perception of elasticity was based on relative height information, when it was available, and on the duration of a single period under other conditions. Observers did not make effective use of velocity information. In experiment 4, visual and auditory period information were compared and equivalent performance in both modalities was found. The results are interpreted as support for the view that dynamic properties of environmental events are perceived by means of kinematic information.
The Recovery of Structure from Motion: No Evidence for a Special Link with the Convergent Disparity Mechanism
Authors: Bradshaw Mark F , Frisby John P , Mayhew John E W
Published: January 2007

Convergent and divergent stereo mechanisms were compared in their ability to recover structure from motion. Contrary to a recent result reported by Richards and Lieberman, no difference in their performance was found; both mechanisms appeared equally capable of supporting the perception of good structure from motion. Possible reasons for the disparate results are discussed.
The Adaptive Significance of the Location of the Optic Disk
Authors: Arditi Aries
Published: January 2007

Although the ‘filling in’ of each blind spot by healthy retina in the other eye has long been described as an adaptive property of the spatial arrangement of the optic disks, an explanation of why the disks are specifically located where they are has yet to be proposed. A rationale for their horizontal position in humans is offered that is based on the projections of the blind spots in visual space in relation to fixation distance and to the protrusion of the bony facial occlusion of the nose bridge.
Temporal Separation of Two Part-Letter Arrays and Size Changes in a Nonmasking Word-Superiority Effect
Authors: Solman Robert T
Published: January 2007

In two experiments subjects were asked to report the identity of a position-cued critical letter in an array of four letters. Four types of arrays were used: (i) unpronounceable nonwords; (ii) pronounceable nonwords (‘pseudowords’); (iii) words in which the critical letter was minimally constrained by the context letters; and (iv) words in which the critical letter was maximally constrained by the context letters. All four-letter stimuli were presented in two parts. A leading array in which the information from two quadrants of a vertical by horizontal division of each letter was presented, and, after intervals of 0, 20, 40, 80, 100, 120, 160, 320, and 480 ms and infinity (ie, no trailing array), a trailing array of the complementary letter parts. In experiment 1 a single group of eight subjects responded to the one hundred and sixty combinations of the four types of letter strings, the four serial positions, and the ten stimulus onset asynchrony values. In experiment 2 the stimulus onset asynchrony values were varied among subjects, with twelve subjects responding at each value. The results from these two studies were generally similar. Performance in the word conditions was consistently superior to performance in the nonword conditions, and the magnitude of this difference (ie, the word-superiority effect) increased with increasing stimulus onset asynchrony up to 120 ms, and then gradually declined. The fact that the magnitude of the word-superiority effect initially increased with the separation of leading and trailing arrays was interpreted as support for Johnston's suggestion that letters in words are represented during visual encoding both in the form of individual letter percepts and in a decay-resistant word percept, as opposed to letters in nonwords, which are represented only as decay-susceptible letter percepts. The experimental findings are discussed in relation to the ‘interactive activation’ model of word perception.
Reference Frame Effects on Shape Perception in Two versus Three Dimensions
Authors: Palmer Stephen , Simone Edward , Kube Paul
Published: January 2007

Three experiments are reported in which it is tested whether the Gestalt effect of configural orientation on shape perception operates on two-dimensional (2-D) or three-dimensional (3-D) representations of space. It is known that gravitationally defined squares and diamonds take longer to discriminate in diagonal arrays than in horizontal or vertical arrays. In the first experiment it is shown that this interference effect decreases dramatically in magnitude when pictorial depth information is added so that subjects perceive the target shapes in different depth planes. In the second experiment this difference is shown not to be due to relative size of the target shapes or to occlusion of a background plane. It is also shown, in the final experiment, that this difference is not due to linear perspective information or merely to perception of the target figures in a 3-D scene. The overall pattern of results supports the position that this configural reference frame effect arises primarily when the elements of the configuration are coplanar, and that the principal organization underlying it is the structure of the perceived 3-D environment rather than that of the 2-D image. In all three experiments, however, there is also a small interference effect in the noncoplanar 3-D conditions. This might be due either to some aspect of reference frame selection operating on the 2-D image representation or to the failure of subjects to see depth in the 3-D stimuli on some proportion of the trials.
Psychophysical Studies of Shape with Fourier Descriptor Stimuli
Authors: Alter Isabelle , Schwartz Eric L
Published: January 2007

The Fourier descriptor (FD) method of shape representation provides a convenient description for the outlines of shapes. It can also be used to generate orthogonal patterns (FD stimuli) which are uniquely characterized by their frequency, amplitude, and phase. Psychophysical studies were conducted to assess threshold tuning properties and frequency specificity during adaptation to FD stimuli. The results suggest the operation of filters which are characterized by the parameter ‘curvature frequency’.
The Effect of Feature Uncertainty on Spatial Discriminations
Authors: Vogels Rufin , Eeckhout Hilde , Orban Guy A
Published: January 2007

Just noticeable differences (JNDs) in orientation and spatial frequency were measured under two conditions. In one condition the subject was cued before stimulus presentation as to the feature to be discriminated on that trial, while in the other condition the subject was cued only after stimulus offset. JNDs were larger in the latter, feature uncertainty, condition. This feature uncertainty effect increased with decreasing stimulus processing time. The results suggest that this feature uncertainty effect is of sensorial origin. They also demonstrate that it is possible for humans to address selectively those mechanisms that are most relevant for a given discrimination task.
Alleys on an Extensive Apparent Frontoparallel Plane: A Second Experiment
Authors: Indow Tarow , Watanabe Toshio
Published: January 2007

Small light points were presented, in the dark, around a point in the center which was fixed at a distance of about 3 m from the subject. In experiment 1, the subject adjusted the positions of points so that all were frontoparallel and in three horizontal series, each consisting of five points, with the middle series level with the eyes, to satisfy the following conditions: (i) the three series must appear straight and horizontally parallel; (ii) the points of each of the five triplets must appear equally separated vertically; (iii) the three points of each triplet must appear to move horizontally along straight and parallel paths; (iv) the three points of each triplet must appear to move horizontally with a constant vertical separation. The most distant points were about 0.51 rad to the left and right of center, and about 0.22 rad above and below. In experiment 2, with the configuration of points obtained in experiment 1, the subject assessed ratios of all perceptual distances between points and also from the subject to all points. From experiment 1 (three subjects used), Gaussian curvature K and a constant related to depth perception (σ) were estimated under the assumption that the frontoparallel plane is a Riemannian plane of constant curvature K and that Luneburg's mapping functions between visual space and physical space hold. The analysis was made according to equations different from those used previously. The results of experiment 2 (two subjects used) were analyzed by a new computer program in which no preassumed mapping functions are necessary for the estimation of K. From both analyses it is clear that there is no need to assume any other value of K than 0 (Euclidean) to describe the geometry of the frontoparallel plane. This presents a striking contrast to the results from experiments on parallel and equidistance alleys running toward the subject on the horizontal plane.
Two-Dimensional Tilt Illusions Induced by Orthogonal Plaid Patterns: Effects of Plaid Motion, Orientation, Spatial Separation, and Spatial Frequency
Authors: Wenderoth Peter , Johnstone Syren , Van der Zwan Rick
Published: January 2007

Tilt illusions occur when a drifting vertical test grating is surrounded by a drifting plaid pattern composed of orthogonal moving gratings. The angular function of this illusion was measured as the plaid orientation (and therefore its drift direction) varied over a 180° range, This was done when the test and inducing stimuli abutted and had the same spatial frequency, and when the test and inducing stimuli either differed in frequency by an octave, or were spatially separated by a 2 deg blank annulus, or both differed in frequency and were also separated by the annulus (experiments 1–4). The obtained angular function was virtually identical to that obtained previously with the rod and frame effect and other cases involving orthogonal inducing components, with evidence for illusions induced both by real-line components and by virtual axes of symmetry. Although the magnitude of the illusion was very similar in all four experiments, there was evidence to suggest that largest real-line effects occurred in the abutting same-frequency condition, with a pattern of results similar to that obtained previously with the simple one-dimensional tilt illusion. On the other hand, virtual-axis effects were more prominent with gaps between test and inducing stimuli. A fifth, repeated-measures, experiment confirmed this pattern of results. It is suggested that this pattern-induced tilt effect reflects both striate and extrastriate mechanisms and that the apparent influence of spatially distal virtual axes of symmetry upon perceived orientation implies the existence of AND-gate mechanisms, or conjunction detectors, in the orientation domain.
Pattern Orientation, Not Motion, Determines the Two-Dimensional Tilt Illusion
Authors: Johnstone Syren , Wenderoth Peter
Published: January 2007

In the usual tilt illusion (TI) configuration, an inducing stimulus which has a single orientation is used psychophysically to explore orientation analysis in the human visual system. Recently, this approach has been extended to the use of inducing stimuli which have two orientations. Such a two-dimensional (2-D) stimulus permits investigation of the low-level analysis of visual patterns.

Prior experimentation has left it unclear whether it is the spatial or the motion properties of a moving crossed-grating plaid which determine two-dimensional tilt illusions (2-D TIs) because these two parameters previously were perfectly correlated. In the present experiments pattern orientation and motion were decoupled. It is shown that 2-D TIs are determined by the spatial properties of an inducing annulus and not by its motion properties. The results also support the existence of a mechanism which extracts axes of symmetry, and which is difficult to account for in terms of local cross-orientation domain inhibition.
An Investigation of Trained Neural Networks from a Neurophysiological Perspective
Authors: Moorhead Ian R , Haig Nigel D , Clement Richard A
Published: January 2007

The application of theoretical neural networks to preprocessed images was investigated with the aim of developing a computational recognition system. The neural networks were trained by means of a back-propagation algorithm, to respond selectively to computer-generated bars and edges. The receptive fields of the trained networks were then mapped, in terms of both their synaptic weights and their responses to spot stimuli. There was a direct relationship between the pattern of weights on the inputs to the hidden units (the units in the intermediate layer between the input and the output units), and their receptive field as mapped by spot stimuli. This relationship was not sustained at the level of the output units in that their spot-mapped responses failed to correspond either with the weights of the connections from the hidden units to the output units, or with a qualitative analysis of the networks. Part of this discrepancy may be ascribed to the output function used in the back-propagation algorithm.
Intermodal Transfer of Spatial Arrangement of the Component Parts of an Object in Infants Aged 4–5 Months
Authors: Pineau Arlette , Streri Arlette
Published: January 2007

Infants' tactual discriminative abilities and intermodal transfer were investigated in two research studies which endeavoured to answer two questions: can infants aged 4–5 months process and discriminate tactually information about different spatial arrangements of elements of an object, in the absence of visual control, and secondly, can they transfer this information from touch to vision? Two experiments were conducted involving a habituation procedure and a procedure which tests reaction to novelty. Infants were found to be able to discriminate tactually between different spatial arrangements of the three elements of an object (experiment 1), and to transfer this information to the visual mode (experiment 2). These findings constitute new data on intermodal functioning in babies.
Auditory Localization: Effects of Reflecting Surfaces
Authors: Guski Rainer
Published: January 2007

Based on anatomical and evolutionary conceptions of the human ear, an experiment was conducted in which forty-eight human subjects were asked to localize sounds (a human voice) emitted by one of twenty-seven stationary loudspeakers in an anechoic chamber. The position of the active loudspeaker varied with respect to azimuth, distance, and elevation in three steps each. The position of a single sound-reflecting surface (about 6 m2) was varied: on the floor, on the ceiling, to the left, and to the right. The accuracy of identifying the active loudspeaker for each position of the sound-reflecting surface was compared intraindividually with the absence of reflection. The results show an overall increase in correct localizations with a sound-reflecting surface on the floor. Especially the elevation of the sound source can be detected with greater precision. Additionally, the percentage of correct localizations decreased systematically with the presence of a sound-reflecting ceiling, while the presence of sound-reflecting walls did not systematically affect the localization performance. Judgments in the horizontal plane and those of distance were not systematically influenced by the presence of a sound-reflecting surface.
Serial Search for Targets Defined by Divergence or Deformation of Optic Flow
Authors: Braddick Oliver J , Holliday Ian E
Published: January 2007

The optic flow field can be described in terms of the local differential measures, divergence, deformation, and rotation, which are informative about observer motion and the 3-D structure of the environment. Does an explicit representation of these measures exist in human visual processing in the form of a feature map? Triesman's criteria were used to investigate this; ie is there ‘pop-out’ for a target defined as different in local divergence or deformation from surrounding elements, or is a serial search necessary? The stimulus arrays contained 3, 5, or 9 square or rectangular elements, which each underwent repeated cycles of expansion, contraction, or deformation. The time required to detect a target undergoing the opposite transformation increased steeply with the number of elements, implying very slow serial search. (The mean time was 210 ms per element for divergence targets and 542 ms per element for deformation.) The process was clearly still serial when the density and number of elements was increased up to 48 in an array 2.16 deg × 2.16 deg. In contrast, a single line element undergoing the opposite direction of translation motion to the rest of the display did show pop-out.

It is concluded that no parallel processes seem to exist which are sensitive to the spatial uniformity of divergence and of deformation of optic flow. These differential properties may be derived as conjunctions of signals from a primary process which extracts local velocity. This result contrasts with our findings for targets defined by stereo disparity gradient, which show parallel processing in analogous experiments.
Orientation Selectivity in Infancy: Behavioural Evidence for Temporal Sensitivity
Authors: Hood Bruce , Atkinson Janette , Braddick Oliver , Wattam-Bell John
Published: January 2007

One-month-old infants were tested with a habituation—recovery paradigm to determine whether they could discriminate phase-shifting grating patterns that switched between two orientations, three or eight times a second, from grating patterns that only shifted in phase. The infants were found to discriminate patterns switching orientation at the lower temporal rate of 3 reversals s−1, but not 8 reversals s−1. This finding supports the idea that orientation-selective mechanisms improve in their temporal sensitivity during early infancy. Where they can be compared, the results from behavioural and electrophysiological studies agree as to the course of this development.
The Effect of Defocussing the Image on the Perception of the Temporal Order of Flashing Lights
Authors: Luria Saul M , Newacheck James S
Published: January 2007

The temporal interval between flashing lights that is required to perceive nonsimultaneity decreases as the refractive error increases from 0 to +2 diopters. The interval then remains constant with further increases to 3 and 4 diopters. The results are discussed in terms of the relative increase of transient to sustained visual channels and of the increase in apparent movement.
Sighting-Down and the Poggendorff Illusion: A Mystery within a Mystery
Authors: Parks Theodore E
Published: January 2007

As reported before, sighting-down one of the diagonal lines on a typical Poggendorff pattern will reduce the illusion even if that situation is only pictorial. But here it is also argued that sighting-down itself requires further understanding.
Serial pattern complexity: irregularity and hierarchy
Authors: A van der Helm Peter , J van Lier Rob , L J Leeuwenberg Emanuel
Published: January 2007

In perception research, various models have been designed for the encoding of, for example. visual patterns, in order to predict the human interpretation of such patterns. Each of these encoding models provides a few coding rules to obtain codes for a pattern, each code expressing regularity and hierarchy in that pattern. Some of these models employ the minimum principle which states that the human interpretation of a pattern is reflected by the simplest code for that pattern. ie the simplest code according to a given complexitv metric. In rhis paper a new complexitv metric is proposed. This metric is based on a formal analysis of the concept of regularitv. Some conclusions of this analysis are sketched. The new metric does not depend on artifacts of the coding rules. [t accounts for the amounts of irregularity and hierarchy as represented in a code of a pattern. such that these two amounts can be added to determine the complexity of a code. An experiment is discussed that shows that the new metric performs significantly better than the metrics used previously. In particular, the new metric predicts more local pattern organizations than the old metrics. This implies that various local pattern organizations do not falsif-v the minimum prirrciple anymore.
Visual Size Invariance Does Not Apply to Geometric Angle and Speed of Rotation
Authors: Werkhoven Peter , Koenderink Jan J
Published: January 2007

The ability of humans to visually estimate geometric angle and speed of rotation was examined as a function of the spatial scale of the stimuli. Both properties are objectively invariant at different spatial scales, but the results of experiments show that the judgement of acute geometric angles as well as that of speed of rotation varies strongly and monotonically with the scale of presentation. If the image is magnified, the perceived geometric angle and the perceived speed of rotation increase. If the image is reduced in size, they decrease. This result imposes strong constraints on perceptual theories.
An Unreported Size Illusion
Authors: Carrasco Marisa , Sekuler Erica B
Published: January 2007

A previously unreported size illusion in some commonly used stimuli—a large (global) letter made up of small (local) letters (eg a large H made up of small Es) is described: the size of the local letters forming the horizontal component of the global letter appears to be overestimated. Global and local letters made up of horizontal and vertical components (E, F, T, and H) were employed as stimulus material. The first experiment showed that the illusion was consistently perceived, and was present for most of the stimuli tested; ie for all global letters and for all local letters except H. A second experiment showed that the size distortion is not specific to a particular dimension: instructing observers to focus on the height or the width of the local letters did not affect the extent of the illusion. The effect of some stimulus manipulations was also investigated.
Colour Pools, Brightness Pools, Assimilation, and the Spatial Resolving Power of the Human Colour-Vision System
Authors: Moulden Bernard , Kingdom Fred , Wink Brian
Published: January 2007

A stimulus is described that demonstrates the spatial pooling of colour information in the visual system. Chequerboards (or gratings) consisting of alternating squares (or stripes) of complementary colours become achromatic at particular spatial scales; such stimuli have been named ‘transchromatic’ stimuli. Colour pools are much larger than the receptive fields that respond to luminance contrast. Some measurements are described which form the basis for estimates of the size of the colour pools. The size of colour pools varies according to the colours involved. For red—cyan and green—magenta complementary pairs colour is pooled at spatial frequencies above about 7–8 cycles deg−1, implying pools whose diameter is around 8 min arc. For yellow—blue complementary pairs the corresponding figures are about 4 cycles deg−1 and 15 min arc. Some phenomena of normal colour vision, colour blindness, and the development of infant vision are discussed in the light of these findings.
Information Processing by Tree Fellers: Signal Detection Analysis
Authors: Henderson Mel , Over Ray
Published: January 2007

Eighteen experienced tree fellers and eighteen forestry students watched video recordings of mature eucalypts being felled by a man using a chain saw, and then rated whether each tree had fallen normally or abnormally. Signal-detection analysis showed that the tree fellers were more accurate than the forestry students in predicting eventual outcome. Further, the tree fellers achieved peak accuracy in discrimination by the time the logger had completed cutting the scarf (typically several minutes before the tree hit the ground), whereas the forestry students predicted outcome most accurately only when a tree was falling (and about 1 s from hitting the ground). Study of the bases for information processing and decision making by tree fellers has implications for personnel selection and training, as well as for formulation of effective work practices.
The Effect of Extraneous Elements Surrounding a Kanizsa-Like Illusory-Figure Pattern
Authors: Parks Theodore E
Published: January 2007

A display in which a Kanizsa-like illusory-figure pattern of three notched circles was accompanied by several other notched circles was found to be relatively ineffective, thus confirming an important, but previously untested, suggestion. This ineffectiveness may have been due to a strengthened tendency of the visual system perceptually to ‘explain’ each edge of each of the three critical notches as belonging to one of the partial circles themselves, thus tending to preclude any other (ie illusory) ‘explanation’ of those edges.
Tests of the Dipole Model of Perceived Movement in Apertures
Authors: Power Roderick P
Published: January 2007

Power and Moulden (1993) have proposed a dipole model to account for the apparent movement of gratings in apertures. This includes movement orthogonal to the orientation of the grating, and the barber pole illusion: the illusion that a grating drifting diagonally across a narrow aperture appears to be moving along it. The essence of the model is that movement is signalled by a large number of dipoles, of many orientations and lengths. These dipoles respond if, and only if, one end is stimulated, and then the other. Three experiments intended to test predictions from the model are reported here. In each case a horizontal grating drifted across an aperture and subjects fixated outside the aperture. In experiment 1 subjects fixated just above or below the aperture, and reported the motion aftereffect (MAE) shown by a set of test spots. As predicted by the model, the spots further from the fixation point showed a strong MAE. Experiment 2 combined both viewing conditions of experiment 1, so that test spots above and below the fixation point were viewed simultaneously. The predictions were confirmed, since test spots further from the fixation point exhibited a stronger MAE than test spots closer to the fixation point. In experiment 3 the fixation point in all conditions was below the aperture, and, as predicted, the MAE of a spot near the bottom of the aperture was diagonally upward, although stimulation was horizontal. Again, as predicted, the MAE of a spot in the middle of the aperture appeared to move horizontally. Finally, it was predicted that a test spot at the top of the aperture would appear to move diagonally downwards, but subjects were unable to report unequivocally the direction of motion, since the MAE was occurring too far from the fovea for clear vision. Overall, then, the predictions from the model were confirmed, although there are associated phenomena the model cannot as yet account for.
Shape from Shading. I: Surface Curvature and Orientation
Authors: Johnston Alan , Passmore Peter J
Published: January 2007

The human visual system makes effective use of shading alone in recovering the shape of objects. Pictures of sculptures are readily interpreted—a situation where shading provides virtually the sole cue to shape. However, shading has been considered a poor cue to depth in comparison with retinal disparity and kinetic cues. Curvature discrimination thresholds were measured with the use of a surface-alignment task for a range of surface curvatures from 0.16 cm−1 to 1.06 cm−1. Weber fractions were around 0.1, demonstrating considerable precision in this task. Weber fractions did not vary substantially as a function of surface curvature. Rotation of the light source around the line of sight had no effect on curvature discrimination but rotation towards the viewer increased discrimination thresholds. In contrast, slant discrimination declined with rotation of the light-source vector towards the viewpoint. When a band-limited random grey-level texture was mapped onto the sphere, curvature discrimination thresholds increased gradually as a function of texture contrast, even though texture and shading provided consistent cues to depth. Adding texture also increased slant discrimination thresholds, demonstrating that texture can act as a source of noise in shape-from-shading tasks.

The psychophysical findings have been used to evaluate whether current algorithms for shape from shading in computer vision could serve as models of human three-dimensional shape analysis and to highlight low-level intramodular interactions between depth cues. It is demonstrated that, in the case of surfaces defined by shading, curvature descriptions are primary and do not depend upon the prior encoding of surface orientation, and Koenderink's local-shape index is suggested as an alternative intermediate representation of surface shape in the human visual system.
The Size Illusion in Children from Five Years of Age and Adults
Authors: Laszlo Judith I , Begg Jannine M , Sainsbury Kim M
Published: January 2007

First it was established that the size illusion occurs not only when complex patterns are presented kinaesthetically but even with simple stimuli such as straight lines and circles. Thus it was established that information overload is not the underlying cause of the illusion. The size illusion was investigated in children aged from five to twelve years and in adults. The stimulus circle was presented in passive kinaesthetic, sequential visual, in combined kinaesthetic and sequential visual, in static visual, and in combined kinaesthetic and static visual conditions either at fast or at slow speeds. A between-subjects design was used. In all conditions where the stimulus presentation included the kinaesthetic modality alone or in combination with visual or sequential visual conditions, size overestimation was found, while in the sequential visual and static visual conditions overestimation of the size of the stimulus was absent. Further, the kinaesthetic illusion was stable across the age range tested, indicating that the illusion is a property of the kinaesthetic system and is not influenced by learning, ie is hard wired.
Visual Search for Global and Local Stimulus Features
Authors: Saarinen Jukka
Published: January 2007

The observer looked for a target pattern differing from distractors in orientation at one spatial scale only (either at a global or at a local scale) and ignored the other. The stimulus patterns in the search array were vertical or horizontal bars consisting of four oblique line segments sharing the same orientation (45° or 135°). In the search at the global scale, the target and distractors differed from each other in the bar orientation, but not in the orientation of the line segments, which was random. In the search at the local scale, the observer had to use the line orientation for discriminating the target and distractors (the bar orientation was random). The results showed that even though the search was parallel at both scales (ie the search time did not increase with an increasing number of distractors), target detection at the local scale required considerably more time than at the global scale. This latter finding is in agreement with the phenomenon of ‘global precedence’.
The Natural Moon Illusion: A Multifactor Angular Account
Authors: Plug Cornelis , Ross Helen E
Published: January 2007

It is argued that the failure to explain the celestial illusion results from conceptual confusion about perceived size and from disregard of the observational evidence relating to the natural moon illusion. The evidence shows that the illusion consists of a perceived angular size enlargement of horizon objects, by a factor of about 1.5–2.0 in diameter in comparison with elevated objects. Most measurements of the illusion have been made in terms of angular size, although in some proposed explanations an illusion of linear size is assumed. The magnitude of the illusion varies, particularly with the detail of the horizon scene. The illusion can be explained as the sum of several factors that affect perceived angular size: size contrast, vergence commands and eye or head position, aerial perspective, and colour. The relative contributions of these factors are assessed.
On the Microgenesis of Illusory Figures: A Failure to Replicate
Authors: Parks Theodore E
Published: January 2007

Reynolds found in 1981 that with increased viewing time of a pattern which may or may not produce illusory contours there were: first, reports of the pattern without an illusory figure; then, at longer exposures, an increase in the frequency with which illusory figures were reported; and then, with still longer exposures, a decrease in such reports if the pattern contained elements which tended to contradict the possibility of such a figure. Unfortunately, however, three attempts to replicate these potentially very important findings—with the aid of substantially improved methodology—consistently failed to do so. It is suggested that this failure, although it is disappointing to those who subscribe to a ‘problem solving’ explanation of illusory contours, may not constitute a strong refutation of such a theory. Regardless, the purpose in the report is to clarify and rectify the published record concerning this part of the evidential underpinnings of that theory.
Binocular Displacement of Unpaired Region
Authors: Takeichi Hiroshige , Nakazawa Hitoshi
Published: January 2007

Binocular displacement of binocularly unpaired parts of the stimulus was examined by means of the Poggendorff figure. The Poggendorff figure can be used to investigate displacement since lateral displacement of the transversal may cause bias in judgments of its collinearity. In experiment 1, the transversal had a disparity, and thus binocularly unpaired parts, relative to the rectangle. The magnitude of the Poggendorff illusion should not have changed by addition of binocular disparity if displacement occurred. There was no or slight change when the transversal was seen behind the rectangle, but there was significant decrease when the transversal was seen in front of the rectangle, suggesting absence of displacement in this case. There were two possible explanations. One was that displacement depended on the positional relation between the unpaired stimuli and the binocularly presented rectangle, ie the occlusion constraint, which the case with the transversal in front did not satisfy. The alternative was that the decrease was due to the perceived front depth of the transversal, and not related to binocular displacement at all. In order to discriminate between these two possibilities, the transversal was reduced to only the unpaired parts, resulting in dichoptic stimulation in experiment 2. In this stimulus, the positional relation between the unpaired and the paired stimuli was the same as in the previous experiment, yet no front depth could be perceived. The results showed similar asymmetry as in experiment 1. Thus we conclude that binocular displacement depends on the positional relation between the unpaired and the paired stimuli, regardless of their perceived depth. This may imply that binocular displacement is not symmetric about the sign of disparity, hence that it is not just averaging but is a reconstruction of the spatial layout of objects in the outside world to keep the visual direction of the unsuppressed unpaired region veridical by using explicit cues to depth discontinuity.
The Speed Tuning of Medial Superior Temporal (Mst) Cell Responses to Optic-Flow Components
Authors: Orban Guy A , Lagae Lieven , Raiguel Steve , Xiao Dengke , Maes Hugo
Published: January 2007

The responses of macaque medial superior temporal (MST) cells to translation and to the optic-flow components—rotation, expansion/contraction, and deformation—were examined with particular regard to the speed tuning of MST cells for optic-flow stimuli and the effect of removing speed gradients from those stimuli. The use of position invariance as an indispensable criterion for assessing the authenticity of responses to optic flow is reviewed. By extending the scope of testing to include higher speeds it is found, in contrast to in previous reports, that MST cells generally respond to optic-flow components with a speed—response profile which is tuned for a particular range of speeds. Removal of the speed gradient had little effect on this observation. These and other properties of MST cells lead to the conclusion that one of the major functions of MST is the detection and encoding of self-motion.
Factors Influencing the Accuracy of Age Estimates of Unfamiliar Faces
Authors: George Patricia A , Hole Graham J
Published: January 2007

Factors affecting the accuracy with which adults could assess the age of unfamiliar male faces aged between 5 and 70 years were examined. In the first experiment twenty-five ‘young’ adult subjects, aged 16–25, and twenty-five ‘old’ adults, aged 51–60, were used. Each subject saw five versions of three different faces: these consisted of an original version of each face and four manipulated versions of it. The manipulations consisted of mirror reversal, pseudo-cardioidal strain, thresholding, and elimination of all but the internal features of the face. The second experiment was similar except that a between-subjects design was used: each subject saw three faces for each age category of target face, but was exposed to only a single type of manipulation (plus a set of ‘original’ faces which were identical for all groups, so that the comparability of the different groups in age estimation could be checked).

Results from both experiments were similar. Age estimates for unmanipulated ‘original’ faces were highly accurate, although subjects were most accurate with target faces that were within their own age range. Results for the manipulated faces implied that the importance of cardioidal strain as a necessary and sufficient cue to age may have been overestimated in previous reports: subjects' age estimates were accurate when cardioidal strain was absent from the stimulus, and poor when cardioidal strain was the only cue available.
Orientation Discrimination for Bars Defined by Orientation Texture
Authors: Regan David
Published: January 2007

A texture pattern consisting of short (0.2 deg) lines contained a 5.0 deg × 1.4 deg texture-defined bar. The bar was rendered visible by the difference in orientation (2θ) between the lines inside the bar and outside the bar. Orientation-discrimination threshold for the texture-defined bar was a U-shaped function of 2θ over the range 2θ = 0° to 2θ = 180°. The lowest threshold was at 2θ = 90°, and was 0.57° for both subjects tested. This threshold was little different from the lowest values of threshold for motion-defined bars, disparity-defined bars, and colour-defined gratings reported elsewhere. A luminance-defined bar was created by switching off all texture lines outside the texture-defined bar. Orientation-discrimination threshold fell to a limiting value as the luminance contrast of this bar was progressively increased. The lowest value of orientation discrimination for the luminance-defined bar (0.42° and 0.35° for the two subjects) was not greatly less than the lowest value for the texture-defined bar.
Spatial-Gradient Limit on Perception of Multiple Motion
Authors: Hermush Yoseph , Yeshurun Yehezkel
Published: January 2007

Motion is perceived whenever a subject is presented with an appropriate spatiotemporal visual pattern. Like many other visual tasks, motion perception involves both local and global processing, and thus might be subject to the well-known paradox that arises from the fact that local features and observations form the basis for global perception, but sometimes this global percept can not be easily derived from any single local observation, as is best exemplified by the aperture problem. Globally, dual (transparent) motion can be readily perceived. Spatial limits on the local ability to perceive multiple motion are sought. By using the framework of apparent motion, it is found that dual, orthogonally oriented motion can be perceived only when the dots that constitute the two motions are separated by some spatial limit. For short-range apparent motion, the limit is found to be comparable to Dmax, and the visual system cannot perceive more than a single coherent motion in a local ‘patch’ of radius Dmax. It was also found that this spatial limit on local-motion perception is not constant, but depends linearly on the spatial organisation of the stimuli, and vanishes for stimuli having reverse contrast. The lower bound on the ability to perceive multiple motion is compared with some well-known bounds in stereopsis, and a cortical columnar architecture that might account for it is proposed.
Presaccadic Processes in the Generation of Pro and anti Saccades in Human Subjects—A Reaction-Time Study
Authors: Weber Heike
Published: January 2007

It has been widely acknowledged that the generation of an anti saccade, ie a saccade towards the direction opposite to that of a visual stimulus, requires the correct function of special brain structures. In the present study attempts were made to measure the time consumption of brain processes preceding the execution of pro and anti saccades. The saccadic eye movements of five adult human subjects were investigated in a series of combined pro/anti saccade tasks with the aid of the gap and the overlap paradigms. The type of trial—pro saccade and anti saccade—was defined by the structure of the stimulus. In some sessions the subjects were, in addition, preinformed about the actual command by a cue at different lead times before stimulus onset. Pro-saccade and anti-saccade trials were randomly intermixed in equal proportions. High error rates (>30% of all trials in some subjects) occurred in the test sessions without preinformative cues. These errors had long reaction times (∼200 ms), whereas the latencies of correct pro or correct anti saccades were even longer (∼350 ms). Analysis of the errors revealed that they were related to the situation in the previous trial: a correct response in the previous trial enhanced the chance of making a saccade of the same type in the actual trial by up to 30%. This pretrial effect occurred whether the actual trial was a pro-saccade or an anti-saccade command. With a cue lead time of 100 ms the numbers of errors decreased, but the latencies of the correct pro or anti saccades were about 70 ms longer than those obtained in the nonrandom control. With a 200 ms cue lead time the reaction times corresponded to those in the control condition. The results suggest that the situation in a given trial creates a kind of default program for the saccade preparation in the next trial. When a cue about the actual command is given early enough, the default program is overridden correspondingly. The perception of the cue and the programming of the corresponding saccade takes an additional 150 to 200 ms.
Adaptation to Spatial Offsets
Authors: Hess Robert F , Doshi Sima
Published: January 2007

After prolonged viewing of a three-element target in which the middle element is spatially offset, subsequent viewing of the same three elements in alignment results in the middle element appearing to be offset in the opposite direction. This adaptational aftereffect to a spatial offset was investigated with elements which were spatial-frequency narrowband and equidetectable to ascertain (a) the properties of the mechanisms involved and (b) the nature of the underlying computation. Evidence is presented in favour of an orientational-grouping, rather than a purely positional computation, underlying this aftereffect. A dual site of adaptation is proposed: one which receives input from the orientation extracted from the output of linear filters, and another which receives input from the orientation derived from grouping processes working on the contrast-energy representation. These may correspond to the mechanisms which are thought to underlie the processing of real and subjective contours.