Abstract: A neural model is proposed of how the visual system processes natural images under variable illumination conditions to generate surface lightness percepts. Previous models clarify how the brain can compute relative contrast. The anchored Filling-In Lightness Model (aFILM) clarifies how the brain 'anchors' lightness percepts to determine an absolute lightness scale that uses the full dynamic range of neurons. The model quantitatively simulates lightness anchoring properties (Articulation, Insulation, Configuration, Area Effect) and other lightness data (discounting the illuminant, the double brilliant illusion, lightness constancy and contrast, Mondrian contrast constancy, Craik-O'Brien-Cornsweet illusion). The model clarifies how retinal processing stages achieve light adaptation and spatial contrast adaptation, and how cortical processing stages fill-in surface lightness using long-range horizontal connections that are gated by boundary signals. The new filling-in mechanism runs 1000 times faster than diffusion mechanisms of previous filling-in models.

Abstract: Recent evidence indicates that stimulus-driven and goal-directed control of visual selection operate independently and in different time windows (van Zoest et al., 2004). The present study further investigates how eye movements are affected by stimulus-driven and goal-directed control. Observers were presented with search displays consisting of one target, multiple non-targets and one distractor element. The task of observers was to make a fast eye movement to a target immediately following the offset of a central fixation point, an event that either co-occurred with or soon followed the presentation of the search display. Distractor saliency and target-distractor similarity were independently manipulated. The results demonstrated that the effect of distractor saliency was transient and only present for the fastest eye movements, whereas the effect of target-distractor similarity was sustained and present in all but the fastest eye movements. The results support an independent timing account of visual selection.

Abstract: The relationship between distance and size perception is unclear because of conflicting results of tests investigating the size-distance invariance hypothesis (SDIH), according to which perceived size is proportional to perceived distance. We propose that response bias with regard to measures of perceived distance is at the root of the conflict. Rather than employ the usual method of magnitude estimation, the bias-free two-alternative forced choice (2AFC) method was used to determine the precision (1/sigma) of discriminating depth at different distances. The results led us to define perceptual distance as a bias free power function of physical distance, with an exponent of approximately 0.5. Similar measures involving size differences among stimuli of equal angular size yield the same power function of distance. In addition, size discrimination is noisier than depth discrimination, suggesting that distance information is processed prior to angular size. Size constancy implies that the perceived size is proportional to perceptual distance. Moreover, given a constant relative disparity, depth constancy implies that perceived depth is proportional to the square of perceptual distance. However, the function relating the uncertainties of depth and of size discrimination to distance is the same. Hence, depth and size constancy may be accounted for by the same underlying law.

Abstract: A gray region can be made to look colored by a colored surround. This phenomenon, chromatic induction, depends on color differences around the boundary of the region. We performed experiments on chromatic induction with small, initially achromatic, targets on nine different colored surrounds ranging in color from blue to red. Using scaling of saturation as our measure of perceived color strength, we found that chromatic induction is at its maximum when the brightness contrast at the boundary between target and surroundings is minimal. This implies that the neural mechanism in the cerebral cortex that mediates the appearance of brightness at a boundary inhibits the activity of chromatic mechanisms at that same boundary. Observers matched the apparent brightness and luminance of each of the colored surrounds. For surround colors where brightness and luminance matches differ, brightness contrast, not luminance contrast, controls chromatic induction. These new findings, taken together with other evidence, require a new theory of color appearance that includes mutually inhibitory interactions between color and brightness mechanisms that are sensing color and brightness contrast at visual boundaries.

Abstract: The perception of natural scenes relies on the integration of pre-existing knowledge with the immediate results of attentional processing, and what can be remembered from a scene depends in turn on how that scene is perceived and understood. However, there are conflicting results in the literature as to whether people are more likely to remember those objects that are consistent with the scene or those that are not. Moreover, whether any discrepancy between the likelihood of remembering schema-consistent or schema-inconsistent objects should be attributed to the schematic effects on attention or on memory remains unclear. To address this issue, the current study attempted to directly manipulate attention allocation by requiring participants to look at (i) schema-consistent objects, (ii) schema-inconsistent objects, or (iii) to share attention equally across both. Regardless of the differential allocation of attention or object fixation, schema-consistent objects were better recalled whereas recognition was independent of schema-consistency, but depended on task instruction. These results suggest that attention is important both for remembering low-level object properties, and information whose retrieval is not supported by the currently active schema. Specific knowledge of the scenes being viewed can result in the recall of non-fixated objects, but without such knowledge attention is required to encode sufficient detail for subsequent recognition. Our results demonstrate therefore that attention is not critical for the retrieval of objects that are consistent with a scene's schematic content.

Abstract: The perception of blur in images can be strongly affected by prior adaptation to blurry images or by spatial induction from blurred surrounds. These contextual effects may play a role in calibrating visual responses for the spatial structure of luminance variations in images. We asked whether similar adjustments might also calibrate the visual system for spatial variations in color. Observers adjusted the amplitude spectra of luminance or chromatic images until they appeared correctly focused, and repeated these measurements either before or after adaptation to blurred or sharpened images or in the presence of blurred or sharpened surrounds. Prior adaptation induced large and distinct changes in perceived focus for both luminance and chromatic patterns, suggesting that luminance and chromatic mechanisms are both able to adjust to changes in the level of blur. However, judgments of focus were more variable for color, and unlike luminance there was little effect of surrounding spatial context on perceived blur. In additional measurements we explored the effects of adaptation on threshold contrast sensitivity for luminance and color. Adaptation to filtered noise with a 1/f spectrum characteristic of natural images strongly and selectively elevated thresholds at low spatial frequencies for both luminance and color, thus transforming the chromatic contrast sensitivity function from lowpass to nearly bandpass. These threshold changes were found to reflect interactions between different spatial scales that bias sensitivity against the lowest spatial grain in the image, and may reflect adaptation to different stimulus attributes than the attributes underlying judgments of image focus. Our results suggest that spatial sensitivity for variations in color can be strongly shaped by adaptation to the spatial structure of the stimulus, but point to dissociations in these visual adjustments both between luminance and color and different measures of spatial sensitivity.

Abstract: Four experiments examined the biases of individual subjects when attempting to bisect vertical lines, and tested various hypotheses concerning the origin of vertical bisection errors (VBEs). In each experiment, individual differences on the vertical line bisection task were compared to individual differences on another task to evaluate whether the tasks were systematically related. In the first experiment, VBEs were found not to correlate with horizontal line bisection errors (HBEs). In the second experiment, VBEs were found not to correlate with the size of the horizontal-vertical illusion (HVI). In the third experiment, VBEs were found not to correlate with differences in perceived length of vertical lines presented in the upper and lower visual fields that were displaced horizontally. In the fourth experiment, VBEs were found to correlate with bisection errors of an open vertical interval. The results of the first three studies are counter to various hypotheses of the origins of vertical bisection errors. The last experiment suggests that lines, per se, are unnecessary for VBEs. Rather, it is important that the upper and lower segments of the stimulus that are judged to be equal on vertical bisection tasks are co-extensive.

Abstract: The amount of depth perceived from a fixed pattern of horizontal disparities varies with viewing distance. We investigated whether thresholds for discriminating stereoscopic corrugations at a range of spatial frequencies were also affected by viewing distance or whether they were determined solely by the angular disparity in the stimulus prior to scaling. Although thresholds were found to be determined primarily by disparity over a broad range of viewing distances, they were on average a factor of two higher at the shortest viewing distance (28.5 cm) than at larger viewing distances (57 to 450 cm). We found the same pattern of results when subjects' accommodation was arranged to be the same at all viewing distances. The change in thresholds at close distances is in the direction expected if subjects' performance is limited by a minimum perceived depth.

Abstract: Visibility of an oriented stimulus may be enhanced by nearby stimuli that are co-aligned with the target. However, the underlying mechanism governing this facilitation is controversial. Here we measured the dependence of percent correct detection on the target's contrast (psychometric curve) with and without flankers, where flankers were either collinear with or orthogonal to the target. We find that the effect of collinear flankers can be described as a translation of the psychometric function along the linear contrast axis. This behavior is consistent, within experimental error, with two types of models: (1) non-linear transduction of target contrast with collinear flankers having additive effects on contrast, and (2) uncertainty reduction (Pelli, 1985) by collinear flankers. We discuss properties of collinear facilitation that can help deciding between these two models.

Abstract: In theory, properties of clutter can be defined globally or locally. However, in the literature, the distinction between local and global clutter is arbitrary, where the standard approach of setting the local domain to twice the expected target size, in applying local clutter metrics, is adopted without any justification. This paper addresses this problem and considers the implications for the application of clutter metrics. It was found that the size of the local clutter region around a target has a strong effect on the probability of detection of that target and that this is affected by regions much larger than twice the target size. It was also discovered that this effect was much stronger for targets subtending less than 0.8 degrees of visual angle than for larger targets. In the case of the former, the fall-off in human visual performance with clutter region size was approximately quadratic, compared to a slight linear fall-off for larger targets. A simple model is presented explaining these phenomena, indicating that the auto-covariance function characterising the clutter is the main determinant of the size of the region of local clutter.

Abstract: The 'colour-shading effect' describes the phenomenon whereby a chromatic pattern influences perceived shape-from-shading in a luminance pattern. Specifically, the depth corrugations perceived in sinusoidal luminance gratings can be enhanced by spatially non-aligned, and suppressed by spatially aligned sinusoidal chromatic gratings. Here we examine whether colour contrast can influence perceived shape in patterns that combine shape-from-shading with shape-from-texture. Stimuli consisted of sinusoidal modulations of texture (defined by orientation), luminance and colour. When the texture and luminance modulations were suitably combined, one obtained a vivid impression of a corrugated depth surface. The addition of a colour grating to the texture-luminance combination was found to enhance the impression of depth when out-of-phase with the luminance modulation, and suppress the impression of depth when in-phase with the luminance modulation. The degree of depth enhancement and depth suppression was approximately constant across texture amplitude when measured linearly. In the absence of the luminance grating however, the colour grating had no phase-dependent affect on perceived depth. These results show that colour contrast modulates the contribution of shading to perceived shape in combined shading and texture patterns.

Abstract: The light reflected from an object depends both on the object's surface and on the illuminant. Visual systems attempt to resolve this intrinsic ambiguity by comparing the light reflected from the object with respect to the background by computing the difference between the object-background light sampled by three cones. The cone-contrasts for the sample-background stimulus under the test illumination should correspond to the cone-contrasts for samples matched in appearance under the standard background (C). The validity of this cone-contrast rule (Whittle, 2003) and its possible link with stability of perceived colour was studied here using six test illuminants. A successive asymmetric colour-matching task with 40 simulated Munsell samples (value 7, chroma 4) on a neutral background (N7) was used. The subject adjusted the sample under standard illuminant C to match the colour appearance of the test sample under one of the test illuminants. Brunswik Ratio (BR) was used as an index of stability of colour appearance. When computed with respect to the reference illuminant C, the cone contrast rule was violated (particularly for S-cone-contrast). However, if a new reference point based on the perceived colour of the neutral background under the test illumination was used, the cone contrast rule applied. That is, when cone contrasts of the matching samples are computed with respect to this perceived background, they correspond to cone contrasts of the test stimuli. This represents a form of discounting the illuminant for the purpose of determining an object's cone-contrast against the background, which does not vary with background illumination. These cone contrasts, however, do not provide any information about the colour appearance of objects under particular viewing conditions, unless calibrated against a standard by allowing subjects to learn particular colours.

Abstract: Traditional theories of early visual processing suggest that elementary visual features are handled in parallel by independent neural pathways. We studied the interaction of orientation and spatial frequency in the discrimination of Gabor random fields. Target textures differed from reference textures either in mean feature value, showing an edge-like transition between both textures (edge defined), or in the degree of feature homogeneity with smooth transitions (region defined). Irrespective of the kind of texture definition, we found strong cue summation for targets defined by both cues simultaneously, provided two conditions were fulfilled. First, they were barely discriminable when defined by one cue alone. Second, the target elements formed a closed 2D surface. Only marginal cue summation was observed when target elements were heterogeneously distributed in a predefined area, lacking a clear 2D shape. Our findings indicate that feature synergy enables figure-ground segregation when the information from independent feature-specific pathways is insufficient for solving this task.

Abstract: Collinear facilitation of contrast detection of achromatic stimuli has been studied over the past decade by different groups. We measured collinear facilitation of chromatic contrast detection under equal-luminance (photometric quantity) and under isoluminance (minimum motion technique) conditions, as two different controls. The facilitation was tested for chromatic contrast detection of a foveal Gabor signal flanked by two high chromatic-contrast Gabor signals. The results indicated a significant facilitation in the presence of spatial adjacent collinear chromatic contrast signals, when the flankers were located at a short distance, across all observers for three chromatic channels. The facilitation was compared to a non-collinear flanker configuration. The results indicated no facilitation effect at the opposing phase configuration, at a short flanker distance, whereas a small facilitation was observed with a configuration at a longer flanker distance. The findings suggest that the performance and specificity of chromatic collinear facilitation is not impaired with regard to achromatic mechanisms.

Abstract: We previously showed that interpolation between vertically misaligned luminance edges of same polarity of contrast is preferred to that between co-linear edges of opposite polarity of contrast, although it results in illusory tilt (Roncato and Casco, 2003) We here analyze the spatial conditions that produce this illusory binding of vertically misaligned edges of light and dark tiles, alternated in a row, and in counterphase with those in the rows above and below We find that, independently of scale and number of tiles in a row, the illusion is perceived when the vertical misalignment of more than three tiles is smaller than or equal to 9' and the horizontal separation between co-linear edges is smaller than or equal to 13' These short distances suggest that the underlying mechanism is local Both our phenomenological and psychophysical results support the notion of a local short-range association field, selective to contrast polarity, which produces a binding solution different from that of a phase-independent long-range mechanism We suggest that the occurrence of the illusion at local-level is a result of the activation, within a local short-range association field, of units with orientation different from that stimulated by the physical edges These units are not inhibited when they are close, iso-oriented and co-linear, and the misaligned edges from which they propagate have the same contrast polarity We found that horizontal and vertical spatial limits for the interpolation covary but not such that their ratio is fixed, indicating that the two edges can be connected by projections having a relatively wide range of orientations

Abstract: Recently, Kitagawa and Ichihara (2002) demonstrated that visual adaptation to an expanding or contracting disk produces a cross-modal visually-induced auditory loudness aftereffect (VALAE), which they attributed to cross-correlations of motion in three-dimensional space. Our experiments extend their results by providing evidence that attending selectively to one of two competing visual stimuli of the same saliency produces a cross-modal VALAE that favors the attended stimulus. These cross-modal attentional effects suggest the existence of integrative spatial mechanisms between vision and audition that are affected by attention.

Abstract: Wilson et al.'s (1997) study on Glass patterns suggested that the integration of stimulus features into a linear shape occurs quite locally, whereas curved structures--such as circular--require global summation. Their conclusion was based on experiments in which they varied the size of the signal area containing a spatial structure. In the present study, we tested the integration of constant-sized linear and curved Glass patterns by varying their global irregularity. If the mechanisms underlying the detection of a Glass pattern pool features globally throughout the stimulus, the irregularity should have a strong effect on detection performance. The irregular Glass patterns were composed of a variable number of sub-areas, each of which contained its own linear or curved structure. The structural irregularity impaired the detection of the curved patterns, whereas the thresholds for the linear patterns were not affected. Thus, our results are in line with the notion that the integration of curved Glass patterns occurs more globally than the integration of linear patterns.

Abstract: In an experiment, 20 participants had to detect a backward masked Gabor luminance-modulation target imposed on a field of uniform luminance at varying eccentricities along the horizontal meridian. Different spatial frequencies were used as target modulations. Results for a 7.0 c/deg target patch showed peak detection performance at the center of the visual field and a steady decrease toward the periphery. For 1.0 c/deg, 0.75 c/deg, and 0.5 c/deg target patches, in contrast, the peak was several degrees off retinal center and decreased steadily toward the center. Findings not only confirmed the familiar sensitivity loss toward peripheral areas for high spatial frequencies, but also indicated a sensitivity loss toward central areas for low spatial frequencies. It is concluded that they further support Gurnsey et al.'s (1996) 'mismatch hypothesis' extending its scope to also include 'first-stage' stimuli.

Abstract: Fechner-Benham subjective color is widely believed to be governed by local interactions in early (probably retinal) mechanisms. Here we report three lines of phenomenological evidence that suggest otherwise: subjective colors seen in spatially extended stimuli (a) are dependent on global aspects of the stimuli; (b) can become multistable in position; and (c) even after being stabilized do not support the creation of McCollough's colored after-effects--a cortically based phenomenon generally thought to be more central than Fechner-Benham color. These phenomena suggest a central locus that controls perception of subjective color, characterized by pattern dependent interactions among cortical mechanisms that draw their inputs from peripheral units.

Abstract: Texture discrimination is sometimes asymmetrical; texture A embedded in texture B is more easily detected than texture B embedded in texture A. Furthermore, texture discrimination often improves as the disparate texture is moved into the periphery; this has been referred to as the central performance drop (CPD). The interaction of these interesting and counter-intuitive aspects of texture discrimination has received very little attention. Using four stimulus pattern pairs that were previously shown to elicit asymmetrical texture discrimination, we examined texture discrimination asymmetries as a function of eccentricity. We found three patterns of results; (i) both texture arrangements (A in B, and B in A) elicit a CPD but do not show an asymmetry, (ii) both texture arrangements elicit a monotonic decrease in performance with eccentricity (i.e. no CPD) but an asymmetry is seen at each eccentricity and (iii) discrimination asymmetries are minimal at fixation and in the far periphery and maximal about 3° from fixation with a CPD generally shown for the 'stronger' member of the pair. These results emphasize that one cannot talk about the 'discriminability' of a particular texture pair without reference to the arrangement of the two textures and the eccentricity of presentation.

Abstract: Three experiments investigated the effects of advance information about orientation on the processing of relevant and irrelevant objects, as indicated by Stroop effects from color words located in either object. Four results were obtained. First, participants showed the expected modulation of the Stroop effect: words in the relevant object produced much larger Stroop effects than words in the irrelevant object or words in the background. Second, blocking of object orientation had no effects. Third, informative orientation cues facilitated processing of the relevant object, but cueing did not affect processing of the irrelevant object. Fourth, effects of informative orientation cues were restricted to the first part of each experiment. Results suggest that observers can use advance information about object orientation for improving attentional selection of a visual object. In addition, the results revealed some constraints for the effective use of orientation cues, and discard possible explanations for the observed modulation of Stroop effects.

Abstract: Dynamic texture spreading is a filling-in phenomenon where a colored pattern perceptually spreads onto an area confined by virtual contours in a multi-aperture motion display. The spreading effect is qualitatively similar to static texture spreading but widely surpasses it in strength, making it particularly suited for quantitative studies of visual interpolation processes. We first carried out two experiments to establish with objective tasks that texture spreading is a genuine representation of surface qualities and thus goes beyond mere contour interpolation. Two subsequent experiments serve to relate the phenomenon to ongoing discussions about potentially responsible mechanisms for spatiotemporal integration. With a phenomenological method, we examined to what extent simple sensory persistence might be causally involved in the effect under consideration. Most of our findings are consistent with the idea of sensory persistence, and indicate that information fragments are integrated over a time window of about 100 to 180 ms to form a complete surface representation.

Abstract: The illumination interpretation approach claims that lightness illusions can be explained as misapplications of lightness constancy mechanisms, processes which usually enable veridical extraction of surface reflectance from luminance distributions by discounting illumination. In particular, luminance gradients are thought to provide cues about the interactions of light and surfaces. Several examples of strong lightness illusions are discussed for which explanations based on illumination interpretation can be proposed. In criticisms of this approach, a variety of demonstrations of similarly structured control displays are presented, which involve equivalent lightness effects that cannot readily be accounted for by illumination interpretation mechanisms. Furthermore, a number of known and novel displays are presented that demonstrate effects of gradients on the qualitative appearance of uniform regions. Finally, some simple simulations of neural effects of luminance distributions are discussed.

Abstract: We report some novel 'lighting' and 'backlighting' effects in plane figures similar to those which induce the 'watercolor illusion', that is, figures made with outlines composed of juxtaposed parallel lines varying in brightness and chromatic color. These new effects show 'illumination' as an emergent percept, and show how arrangements of 'dark and light' along the boundaries of various plane figures model the volume and strengthen the illusion of depth. To account for these various effects we propose several phenomenological 'laws of figurality' to add to the Gestalt laws of organization and figure-ground segregation. We offer a set of meta-laws which are speculative but which serve to integrate and organize the phenomenological laws. These laws indicate how luminance gradient profiles across boundary contours define both the 3D appearance of figures and the properties of the light reflected from their volumetric shapes.

Abstract: In four experiments we address the question whether several visual objects can be selected voluntarily (exogenously) and then tracked in a Multiple Object Tracking paradigm and, if so, whether the selection involves a different process. Experiment 1 showed that items can indeed be selected based on their labels. Experiment 2 showed that to select the complement set to a set that is automatically (exogenously) selected — e.g. to select all objects not flashed — observers require additional time and that given 1080 ms they were able to select and track them as well as those selected automatically. Experiment 3 showed that the additional time needed in the previous experiment cannot be attributed solely to time required to disengage attention from the initially automatic selections. Experiment 4 showed that the added time provides a monotonically greater benefit when there are more targets, suggesting a serial process. These results are discussed in relation to the Visual Index (FINST) theory which assumes that visual indexes are captured by a data-driven process. It is suggested that voluntarily allocated attention can be used to facilitate the automatic attention capture by objects of interest.

Abstract: Two phenomena can be observed in the watercolor illusion: illusory color spreading and figure–ground organization. We performed experiments to determine whether the figure–ground effect is a consequence of the color illusion or due to an independent mechanism. Subjects were tested with displays consisting of six adjacent compartments — three that generated the illusion alternating with three that served for comparison. In a first set of experiments, the illusory color was measured by finding the matching physical color in the alternate compartments. Figureness (probability of 'figure' responses, 2AFC) of the watercolor compartments was then determined with and without the matching color in the alternate compartments. The color match reduced figureness, but did not abolish it. There was a range of colors in which the watercolor compartments dominated as figures over the alternate compartments although the latter appeared more saturated in color. In another experiment, the effect of tinting alternate compartments was measured in displays without watercolor illusion. Figureness increased with color contrast, but its value at the equivalent contrast fell short of the figureness value obtained for the watercolor pattern. Thus, in both experiments, figureness produced by the watercolor pattern was stronger than expected from the color effect, suggesting independent mechanisms. Considering the neurophysiology, we propose that the color illusion follows from the principles of representation of surface color in the visual cortex, while the figure–ground effect results from two mechanisms of border ownership assignment, one that is sensitive to asymmetric shape of edge profile, the other to consistency of color borders.