Search

Illusory contours demonstrate an important function of the visual system-object inference from incomplete boundaries, which can arise from factors such as low luminance, camouflage, or occlusion. Illusory contours can be perceived with varying degrees of clarity depending on the features of their inducers. The present study aimed to evaluate whether illusory contour clarity influences visual search efficiency. Experiment 1 compared visual search performance for Kanizsa illusory stimuli and nonillusory inducer stimuli when manipulating inducer size as a clarity factor. Experiment 2 examined the effects of illusory contour clarity on visual search by manipulating the number of rings with missing arcs (i.e., line ends) comprising the inducers, for both illusory and nonillusory stimuli. To investigate whether surface alterations had an impact on visual search in Experiment 1, Experiment 3 examined search performance for Kanizsa-like stimuli formed from "smoothed" inducers compared with standard Kanizsa figures. The results of Experiments 1 and 2 indicated that while Kanizsa produced inefficient search, this was not contingent on the clarity of the illusory contours. Experiment 3 suggested that surface alterations of Kanizsa figures did impact visual search performance. Together, the results indicated that illusory contour clarity did not have much bearing on search performance. In certain conditions, Kanizsa figures even facilitated search compared with nonillusory stimuli, suggesting that rather than contour inference, surface features might have greater relevance in guiding visual attention., (© 2024. The Psychonomic Society, Inc.)

Object-based warping is a visual illusion in which dots appear farther apart from each other when superimposed on an object. Previous research found that the illusion's strength varies with the perceived objecthood of the display. We tested whether objecthood alone determines the strength of the visual illusion or if low-level factors separable from objecthood also play a role. In Experiments 1-2, we varied low-level features to assess their impact on the warping illusion. We found that the warping illusion is equally strong for a variety of shapes but varies with the elements by which shape is defined. Shapes composed of continuous edges produced larger warping effects than shapes defined by disconnected elements. In Experiment 3, we varied a display's objecthood while holding low-level features constant. Displays with matched low-level features produced warping effects of the same size even when the perceived unity of the elements in the display varied. In Experiments 4-6, we tested whether displays with low-level features predicted to be important in spatial warping produced the visual illusion even when the display weakly configured into a single object. Results showed that the presence of low-level features like contour solidity and convexity determined warping effect sizes over and above what could be accounted for by the display's perceived objecthood. Our findings challenge the view that the spatial warping illusion is solely object-based. Other factors like the solidity of contours and contours' position relative to reference dots appear to play separate and important roles in determining warping effect sizes. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Explanations of the Ponzo size illusion, the simultaneous contrast illusion, and the Craik-O'Brien-Cornsweet brightness illusions involve either stimulus-driven processes (assimilation, enhanced contrast, and anchoring) or prior experiences. Real-world up-down asymmetries for typical direction of illumination and ground planes in our physical environment should influence these illusions if they are experience based, but not if they are stimulus driven. Results presented here demonstrate differences in illusion strengths between upright and inverted versions of all three illusions. A left-right asymmetry of the Cornsweet illusion was produced by manipulating the direction of illumination, providing further support for the involvement of an experience-based explanation. When the inducers were incompatible with the targets being located at the different distances, the Ponzo illusion persisted and so did the influence from orientation, providing evidence for involvement of processes other than size constancy. As defined here, upright for the brightness illusions is consistent with an interpretation of a shaded bulging surface and a 3D object resulting from a light-from-above assumption triggering compensation for varying illumination. Upright for the Ponzo illusion is consistent with the inducers in the form of converging lines being interpreted as railway tracks receding on the ground triggering size constancy effects. The implications of these results, and other results providing evidence against experience-based accounts of the illusions, are discussed.

It is often assumed that results from standard visual search tasks will be replicated in related tasks but his idea is rarely tested. In a conceptual replication of Li, Cave, and Wolfe (2008), we investigated the attentional demands of Kanizsa-style illusory contours using orientation-based search, comparing performance for items defined by real- as compared to illusory contours. After confirming the initial findings in standard search, we tested the same manipulation in multiple-target search, Thornton and Gilden's (2007) hybrid standard/multiple-target search, and simple- and selective enumeration. The RT slope differences between real- and illusory contours did not replicate in Thornton and Gilden's task, though they did in multiple-target search and selective enumeration. In fact, absolute differences between real- and illusory contours in RT costs per distractor were 2 - 6 times larger than in standard search. To determine whether performance differences between real and illusory contours originated from shape-definition (necessary for distinguishing target shapes from distractors) or unit formation (grouping disconnected parts to define an item/unit), simple and selective enumeration were compared. The differences between real- and illusory-contours only emerged in selective enumeration (enumerating targets among distractors), which suggests the discrepancies between conditions originate from shape definition rather than unit formation processes. There was no evidence of subitizing in selective enumeration for illusory contour figures, but contrary to attention-based theories of enumeration, there was no subitizing for the real-contour controls either. This study contributes to research on illusory contours but it is especially important to the study of search and enumeration.

Visual systems have evolved to recognize and extract features from complex scenes using limited sensory information. Contour perception is essential to this process and can occur despite breaks in the continuity of neighboring features. Such robustness of the animal visual system to degraded or occluded shapes may also give rise to an interesting phenomenon of optical illusions. These illusions provide a great opportunity to decipher neural computations underlying contour integration and object detection. Kanizsa illusory contours have been shown to evoke responses in the early visual cortex despite the lack of direct receptive field activation. Recurrent processing between visual areas has been proposed to be involved in this process. However, it is unclear whether higher visual areas directly contribute to the generation of illusory responses in the early visual cortex. Using behavior, in vivo electrophysiology, and optogenetics, we first show that the primary visual cortex (V1) of male mice responds to Kanizsa illusory contours. Responses to Kanizsa illusions emerge later than the responses to the contrast-defined real contours in V1. Second, we demonstrate that illusory responses are orientation-selective. Finally, we show that top-down feedback controls the neural correlates of illusory contour perception in V1. Our results suggest that higher-order visual areas may fill in the missing information in the early visual cortex necessary for illusory contour perception. SIGNIFICANCE STATEMENT Perception of the Kanizsa illusory contours is impaired in neurodevelopmental disorders such as schizophrenia, autism, and Williams syndrome. However, the mechanism of the illusory contour perception is poorly understood. Here we describe the behavioral and neural correlates of Kanizsa illusory contours perception in mice, a genetically tractable model system. We show that top-down feedback controls the neural responses to Kanizsa illusion in V1. To our knowledge, this is the first description of the neural correlates of the Kanizsa illusion in mice and the first causal demonstration of their regulation by top-down feedback., (Copyright © 2020 the authors.)

For most observers, the part of the stimulus that is filled with some visual elements (e.g., distractors) appears larger than the unfilled part of the same size. This illusion of interrupted spatial extent is also known as the \'filled‑space' or 'Oppel‑Kundt' illusion. Although the continuously filled‑space illusion has been systematically studied for over a century, there is still no generally accepted explanation of its origin. The present study aimed to further develop our computational model of the continuously filled‑space illusion and to examine whether the model predictions successfully account for illusory effects caused by distracting line‑segments of various lengths that are\r\nattached to different endpoints (i.e., terminators) of the reference spatial interval of the three‑dot stimulus. Our experiments confirm that the illusion manifests itself along a distracting segment located both inside and outside of the reference interval. In the case of two distractors arranged symmetrically with respect to the lateral terminator, we found that the magnitude of the illusion is approximately equal to the sum of the relevant values obtained with separate distractors. The results of experiments using vertical shifts of distractors supported the model's assumption regarding the two‑dimensional Gaussian profile of hypothetical areas of weighted spatial summation\r\nof neural activity. A good correspondence between the experimental and theoretical results supports the suggestion that perceptual positional biases associated with the context‑evoked increase in neural excitation may be one of the main causes of the continuously filled‑space illusion.

Perception of local properties of the visual field is influenced by aftereffects of adaptation. The tilt aftereffect describes repulsion of the perceived orientation of a line from the orientation of an adapting line. Analogous effects of spatial context are often called illusions. Repulsion of the perceived orientation of a grating from the orientation of a surrounding grating is referred to as the tilt illusion. In the same manner, the size aftereffect and Ebbinghaus illusion form a complementary pair of temporal and spatial context effects of size. Here we report psychophysical evidence for a previously unknown aspect-ratio illusion which causes the perceived aspect-ratio of a rectangle to be repelled from the aspect-ratio of rectangles surrounding it. This illusion provides a spatial analogue to the aspect-ratio aftereffect., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

One way to better understand how animals visually perceive their environment is to assess the way in which visual information is interpreted and adapted based on preconceptions. Domestic dogs represent a unique species in which to evaluate visual perception as recent findings suggest they may differ from humans and other animal species in terms of their susceptibility to geometric visual illusions. Dogs have demonstrated human-like, reversed, and null susceptibility depending on the type of illusion. To further evaluate how dogs perceive their environment, it is necessary to perform additional assessments of visual perception. One such assessment is the perceptual filling-in of figures, which may be invoked when presented with illusory contours. Six dogs were assessed on their perception of the Ehrenstein illusory contour illusion in a two-choice size-discrimination task. Dogs, as a group, demonstrated equivocal perception of illusory contours. Some individual dogs, however, demonstrated human-like perception of the subjective contours, providing preliminary evidence that this species is capable of perceiving illusory contour illusions, thereby improving the current understanding of canine visual perception capabilities. Additional assessments using alternative illusory contour illusions are needed to clarify these results and identify features that underpin the individual differences observed.

The motion-induced contour (MIC) was first described by Victor Klymenko and Naomi Weisstein in a series of papers in the 1980s. The effect is created by rotating the outline of a tilted cube in depth. When one of the vertical edges is removed, an illusory contour can be seen in its place. In four experiments, we explored which stimulus features influence perceived illusory contour strength. Participants provided subjective ratings of illusory contour strength as a function of orientation of the stimulus, separation between inducing edges, and the length of inducing edges. We found that the angle of tilt of the object in depth had the largest impact on perceived illusory contour strength with tilt angles of 20° and 30° producing the strongest percepts. Tilt angle is an unexplored feature of structure-from-motion displays. In addition, we found that once the depth structure of the object was extracted, other features of the display, such as the distance spanned by the illusory contour, could also influence its strength, similar to the notion of support ratio for 2-D illusory contours. Illusory contour strength was better predicted by the length of the contour in 3-D rather than in 2-D, suggesting that MICs are constructed by a 3-D process that takes as input initially recovered contour orientation and position information in depth and only then forms interpolations between them.

Perceptual estimates of spatial dimensions of visual objects depend on their shape and surface attributes. The present psychophysical study emphasizes two main contributors to the Oppel‑Kundt illusion: the outline of the filled space and the mode of filling. In past experiments, both factors have been considered significant. Our experiments were performed by using combined stimuli of the Oppel‑Kundt figures and supplementary objects situated within the empty intervals of the figures. Line segments, empty and filled rectangles, blurred contours, and grey and color images were used for the supplementary stimuli role. The experimental data demonstrated an innate property of the objects to balance the illusion of distance if they were placed within the Oppel‑Kundt figure and to create an illusion of extent when compared with an empty space interval. Both the balance magnitude and the induced illusion strength varied depending on the objects' spatial structure. The supplementary objects showed a tendency to differ from each other by their functional capacity and were ranked from lowest to highest: a line segment, a solid bar with a blurred outline, a contour of a rectangle, a solid fill rectangle, greyscale patterns, and color pictures. The experimental findings provided support for an explanation of the Oppel‑Kundt illusion in terms of the spatial‑temporal summation of excitations representing the object outline and surface attributes at the lower cortical levels of the visual system. Along with the facts already established in current literature, the experimental data gave rise to the assumption that any visual object could appear larger than its occupied area, and that the Oppel‑Kundt illusion could become a separate case in the common sensory phenomenon of object size illusion.

In the present study, we tested the ability of our computational model of the filled‑space illusion to account for data collected in experiments with stimuli comprising single‑dot distractors. In three sets of experiments, we investigated this illusory effect as a function of distance between the distractor and lateral terminator of the reference spatial interval of the three‑dot stimulus. We found that the model calculations properly predicted all of the observed changes in magnitude of the illusion for stimuli with a single distracting dot placed both within and outside the interval, as well as, for stimuli with two distractors arranged symmetrically relative to the lateral terminator. To additionally test the model, in a fourth set of experiments we performed psychophysical examination of the conventional Oppel‑Kundt stimulus with a different number of equally spaced dots subdividing the filled part. Adequate correspondence between the computational and experimental data supports our assumptions concerning the origin of the filled‑space illusion.

To explore mechanisms of object integration, the present experiments examined how completion of illusory contours and surfaces modulates the sensitivity of localizing a target probe. Observers had to judge whether a briefly presented dot probe was located inside or outside the region demarcated by inducer elements that grouped to form variants of an illusory, Kanizsa-type figure. From the resulting psychometric functions, we determined observers' discrimination thresholds as a sensitivity measure. Experiment 1 showed that sensitivity was systematically modulated by the amount of surface and contour completion afforded by a given configuration. Experiments 2 and 3 presented stimulus variants that induced an (occluded) object without clearly defined bounding contours, which gave rise to a relative sensitivity increase for surface variations on their own. Experiments 4 and 5 were performed to rule out that these performance modulations were simply attributable to variable distances between critical local inducers or to costs in processing an interrupted contour. Collectively, the findings provide evidence for a dissociation between surface and contour processing, supporting a model of object integration in which completion is instantiated by feedforward processing that independently renders surface filling-in and contour interpolation and a feedback loop that integrates these outputs into a complete whole. (PsycINFO Database Record, ((c) 2018 APA, all rights reserved).)

The Ebbinghaus illusion, in which a central target surrounded by larger context figures looks smaller than when surrounded by smaller context figures, is usually classified as a size contrast illusion. Thus "size contrast" is the dominant account of this effect. However, according to an alternative "contour interaction" account this phenomenon has little to do with size contrast but is rather caused by distance-dependent attractive and repulsive interactions between neural representation of contours. Here evidence is presented against the size contrast account and consistent with the contour interaction account. Experiment 1 was a control study confirming that the illusion can be obtained using displays consisting only of squares, which are more convenient to manipulate than the standardly used circles. In Experiment 2, the standard configuration involving small context figures surrounding the target was compared to a novel configuration, which involved many "spread" small context figures. The illusory effect of the standard context was stronger than the illusory effect of the spread context, in accord with the prediction of the contour interaction account, and contrary to the prediction of the size contrast account. In Experiment 3 two novel configurations were used, based on standard and spread contexts. The results were in accord with the prediction of the contour interaction account, whereas the size contrast account had no prediction because the stimuli did not involve conventional size contrast. Additional aspects of the stimuli and an account of the illusion based on a perspective interpretation are also discussed., (Copyright © 2018. Published by Elsevier B.V.)

Infants less than 1 year old are known to preferentially look at pictures of motion illusion induced by luminance gradation. However, the mechanisms underlying infant's perception of motion illusion remain unclear. The current study analyzed the eye movement patterns of infants perceiving a motion illusion induced by stationary luminance gradations (a derivative of the Fraser-Wilcox illusion). Infants produced the same movement patterns that increase the magnitude of illusory motion in adults. We conclude that infants and adults similarly perceive motion illusion.

The visual system can represent a partially occluded 3-D surface from images of separated surface segments. The underlying amodal surface integration process accomplishes this by amodally extending each surface segment behind the occluder (amodal surface extension) and integrating the extended surfaces to form a whole surface representation. We conducted five experiments to investigate how depth cues, such as binocular disparity, half-occlusion, and monocular depth cues (T-junctions and L-junctions), contribute to amodal surface extension, and how the geometrical relationship and image similarity among the surface segments affect surface integration. This was achieved by having observers adjust the stereoscopic depth and slant of a comparison stimulus to match those of the tested 3-D stimulus. We found that both binocular disparity and half-occlusion cues are used to determine border-ownership assignment of surface segments and for amodal surface extension. We also found that separated surface segments need to have the same luminance contrast-polarity for them to be integrated as a whole surface. Finally, we found that having the same motion direction, minimum misalignment between boundary contours, and proximity among separated segments facilitate their integration. Overall, our findings reveal a set of "perceptual factors" for amodal surface integration, which arguably reflects our visual system's built-in knowledge of the regularities in natural scenes., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

In the present visual psychophysical study, the Oppel‑Kundt and and Müller‑Lyer illusion magnitudes were measured separately (by single figures) and in combination (by two patterns superposed spatially). Data for 30 subjects revealed extensive variability both for the separate and combined illusion strength. Nevertheless, the effect of addition of the perceived length distortions was established. The combined illusions were significantly stronger than the separate ones. Dynamics of the misperceptions summation was studied by varying length of the Müller‑Lyer wings in the superposed stimuli. According to the experimental data obtained, the two misperceptions in length occurred and combined into sensory response varying in dependence on the spatial parameters of the superposed stimuli and on the individual experimental accomplishment. The data supported an explanation for the origin of the filled/unfilled illusion: overestimate of a filled interval length developed due to the spatiotemporal integration along a continuous excitation path elicited by the real or imaginary contours of the filling.

Previous research on perceptual organization operations still provides contradictory evidence on whether the integration of sparse local elements into coherently unified shapes and the construction of the illusory form are accomplished without the need of awareness. In the present study, three experiments were conducted in which participants were presented with masked (Experiment 1, SOA=27ms; Experiment 2; SOA=53ms) and unmasked (Experiment 3) primes consisting of geometric shapes (a square or a diamond) that could be congruent or incongruent with subsequent probe stimuli (square vs. diamond). Furthermore, the primes were divided into: a grouping condition (where local elements may group together into global shapes), an illusory condition (where the arrangement of local elements produced illusory shapes) and a hybrid condition (where both operations were presented simultaneously). While no priming effects were found for the shortest SOA (27ms), both grouping and illusory primes produced significant priming effects in the longer SOA (53ms). On the other hand, results in Experiment 3 (unmasked) showed strong priming effects for the grouping of the inducers in both the grouping and the hybrid conditions, and also a significant but weaker priming effect for the illusory condition. Overall, our results support the possibility of the integration of local visual features into a global shape in the absence of awareness and, likewise, they suggest an early -subliminal- construction of the illusory shape, implying that feedback projections from higher to lower visual areas are not crucial in the construction of the illusory form., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Can affect be evoked by mere perception? Earlier work on processing fluency, which manipulated the dynamics of a running perceptual process, has shown that efficient processing can indeed trigger positive affect. The present work introduces a novel route by not manipulating the dynamics of an ongoing perceptual process, but by blocking or allowing the whole process in the first place. We used illusory contour perception as one very basic such process. In 5 experiments (total N = 422), participants briefly (≤100 ms) viewed stimuli that either allowed illusory contour perception, so-called Kanizsa shapes, or proximally identical control shapes that did not allow for this process to occur. Self-reported preference ratings (Experiments 1, 2, and 4) and facial muscle activity (Experiment 3) showed that participants consistently preferred Kanizsa over these control shapes. Moreover, even within Kanizsa shapes, those that most likely instigated illusory contour perception (i.e., those with the highest support ratio) were liked the most (Experiment 5). At the same time, Kanizsa stimuli with high support ratios were objectively and subjectively the most complex, rendering a processing fluency explanation of this preference unlikely. These findings inform theorizing in perception about affective properties of early perceptual processes that are independent from perceptual fluency and research on affect about the importance of basic perception as a source of affectivity. (PsycINFO Database Record, ((c) 2017 APA, all rights reserved).)

The question of whether conscious experience is best viewed as graded or dichotomous has received much scientific attention in recent years as the answer is relevant not only to models of consciousness, but also to the examination of neural markers of consciousness in patients and infants. Although some studies have found evidence of graded perception, it is unclear whether such perception is simply composed of individual stimulus features perceived in an all-or-none manner. Here, we examined whether the Kanizsa triangle (an illusory figure that is supposedly only perceived when all its parts are visible) has an impact on line length discrimination across four degrees of subjective visibility. We found that the presence of the Kanizsa triangle biases line length judgments (a phenomenon called the Ponzo illusion) when participants reported any experience (even a weak glimpse) of the stimulus. The results support the view that consciousness is a graded phenomenon. The strength of this support depends on the assumption that all parts of the illusory figure must be perceived for the illusion to work but this assumption is not resolved in the present literature. Currently, evidence can be found both for and against this notion.

When a bar suddenly appears between two boxes, the bar will appear to shoot away from the box that matches it in colour or in shape-a phenomenon referred to as attribute priming of illusory line motion (ILM; colour ILM and shape ILM, respectively). If the two boxes are identical, ILM will still occur away from a box if it changes luminance shortly before the presentation of the bar ( flash ILM). This flash condition has been suggested to produce the illusory motion due to the formation of an attentional gradient surrounding the flashed location. However, colour ILM and shape ILM cannot be explained by an attentional gradient as there is no way for attention to select the matching box prior to the presentation of the bar. These findings challenge the attentional gradient explanation for ILM, but only if it is assumed that ILM arises for the same underlying reason. Two experiments are presented that address the question of whether or not flash ILM is the same as colour ILM or shape ILM. The results suggest that while colour ILM and shape ILM reflect a common illusion, flash ILM arises for a different reason. Therefore, the attentional gradient explanation for flash ILM is not refuted by the occurrence of colour ILM or shape ILM, which may reflect transformational apparent motion (TAM).

In the filled-space (or Oppel-Kundt) illusion, the filled part of the stimulus for most observers appears longer in comparison with the empty one. In the first two experimental series of the present study, we investigated the illusory effect as a function of continuous filling (by a shaft-line segment) of the reference spatial interval of the three-dot stimulus. It was demonstrated that for the fixed length of the reference interval, the magnitude of the illusion increases non-linearly with the shaft length. For the fixed length of the shaft, the illusion magnitude gradually decreases with the lengthening of the reference interval. In the third series, psychophysical examination of the conventional Oppel-Kundt stimulus with different number of equally spaced elements (dots) subdividing its filled part was performed. Based on the analysis of the functional dependencies established, we have proposed a simple computational model that was successfully applied to fit the experimental data obtained in the present study.

In this study we manipulate the distribution of contrast polarity reversals in inducing configurations to create novel variants of modal and amodal completion. The novel variants, better equated in their geometric and photometric characteristics offer a superior way to probe similarities and differences in the temporal dynamics that underlie different forms of perceptual completion. We use dot localisation to directly compare the spatial characteristics of modally and amodally interpolated contours at presentation durations ranging from 120 to 300ms and find robust differences in the spatiotemporal formation of modally and amodally completed boundaries. Modally completed contours are localised more accurately and with better spatial precision across all presentation durations. Our results challenge the assumption that the boundary interpolation system depends solely on the geometrical relatability of inducing fragments and suggest that boundary interpolation depends on the spatial distribution of local luminance relationships. As an alternative to the strong version of the identity hypothesis, we propose that modal and amodal completion are mediated by different mechanisms, triggered by particular configurations of contrast polarity., (Copyright © 2015. Published by Elsevier Ltd.)

As expressed in the Gestalt law of good continuation, human perception tends to associate stimuli that form smooth continuations. Contextual modulation in primary visual cortex, in the form of association fields, is believed to play an important role in this process. Yet a unified and principled account of the good continuation law on the neural level is lacking. In this study we introduce a population model of primary visual cortex. Its contextual interactions depend on the elastica curvature energy of the smoothest contour connecting oriented bars. As expected, this model leads to association fields consistent with data. However, in addition the model displays tilt-illusions for stimulus configurations with grating and single bars that closely match psychophysics. Furthermore, the model explains not only pop-out of contours amid a variety of backgrounds, but also pop-out of single targets amid a uniform background. We thus propose that elastica is a unifying principle of the visual cortical network., (Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Evidence obtained using the dynamic grouping method has shown that the grouping of an object's connected surfaces has properties characteristic of a nonlinear dynamical system. When a surface's luminance changes, one of its boundaries is perceived moving across the surface. The direction of this dynamic grouping (DG) motion indicates which of two flanking surfaces has been grouped with the changing surface. A quantitative measure of overall grouping strength (affinity) for adjacent surfaces is provided by the frequency of DG motion perception in directions promoted by the grouping variables. It was found that: (1) variables affecting surface grouping for three-surface objects evolve over time, settling at stable levels within a single fixation, (2) how often DG motion is perceived when a surface's luminance is perturbed (changed) depends on the pre-perturbation affinity state of the surface grouping, (3) grouping variables promoting the same surface grouping combine cooperatively and nonlinearly (super-additively) in determining the surface grouping's affinity, (4) different DG motion directions during different trials indicate that surface grouping can be bistable, which implies that inhibitory interactions have stabilized one of two alternative surface groupings, and (5) when alternative surface groupings have identical affinity, stochastic fluctuations can break the symmetry and inhibitory interactions can then stabilize one of the surface groupings, providing affinity levels are not too high (which results in bidirectional DG motion). A surface-grouping network is proposed within which boundaries vary in salience. Low salience or suppressed boundaries instantiate surface grouping, and DG motion results from changes in boundary salience., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Using event-related potentials (ERPs) we investigated the influence of object knowledge on perceiving partly occluded shapes. We created stimuli based on well-known objects of which the middle part was occluded. Object completions that were compatible with structure consisted of the connection of the visible fragments by smoothly extending their contours. In contrast, object completions that were incompatible with structure consisted of separate disconnected completions of the fragments. Furthermore, object completions could be in line with, or conflict with expectations based on knowledge. We measured ERPs when hidden parts were revealed by removing the occluder, and observed an early positive ERP peaking around 115-140ms at occipital sites, presumably triggered by physical differences. Most importantly, we observed a positive ERP peaking around 300-400ms at parieto-occipital sites that could be related to influences of both structure and knowledge. An additional analysis controlling for differential stimulus characteristics revealed similar conclusions. All in all, we demonstrate that the interpretation of partly occluded shapes is not solely driven by stimulus structure, but that it can also be influenced by knowledge of objects., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Spatiotemporal boundary formation (SBF) refers to perception of continuous contours, shape, and global motion from sequential transformations of widely separated surface elements. How such minimal information in SBF can produce whole forms and the nature of the computational processes involved remain mysterious. Formally, it has been shown that orientations and motion directions of local edge fragments can be recovered from small sets of element changes (Shipley & Kellman, (1997). Vision Research, 37, 1281-1293). Little experimental work has examined SBF in simple situations, however, and no model has been able to predict human SBF performance. We measured orientation discrimination thresholds in simple SBF displays for thin, oriented bars as a function of element density, number of element transformations, and frame duration. Thresholds decreased with increasing density and number of transformations, and increased with frame duration. An ideal observer model implemented to give trial-by-trial responses in the same orientation discrimination task exceeded human performance. In a second group of experiments, we measured human precision in detecting inputs to the model (spatial, temporal, and angular inter-element separation). A model that modified the ideal observer by added encoding imprecision for these parameters, directly obtained from Exp. 2, and that included two integration constraints obtained from previous research, closely fit human SBF data with no additional free parameters. These results provide the first empirical support for an early stage in shape formation in SBF based on the recovery of local edge fragments from spatiotemporally sparse element transformation events., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The perception of shape, it has been argued, also often entails the perception of time. A cookie missing a bite, for example, is seen as a whole cookie that was subsequently bitten. It has never been clear, however, whether such observations truly reflect visual processing. To explore this possibility, we tested whether the perception of history in static shapes could actually induce illusory motion perception. Observers watched a square change to a truncated form, with a "piece" of it missing, and they reported whether this change was sudden or gradual. When the contours of the missing piece suggested a type of historical "intrusion" (as when one pokes a finger into a lump of clay), observers actually saw that intrusion occur: The change appeared to be gradual even when it was actually sudden, in a type of transformational apparent motion. This provides striking phenomenological evidence that vision involves reconstructing causal history from static shapes., (© The Author(s) 2016.)

The Poggendorff illusion refers to the phenomenon that the human brain misperceives a diagonal line as being apparently misaligned once the diagonal line is interrupted by two parallel edges, and the size of illusion is negatively correlated with the angle of interception of the oblique, i.e. the sharper the oblique angle, the larger the illusion. This optical illusion can be produced by both real and illusory contour. In this fMRI study, by parametrically varying the oblique angle, we investigated the shared and specific neural mechanisms underlying the Poggendorff illusion induced by real and illusory contour. At the behavioral level, not only the real but also the illusory contours were capable of inducing significant Poggendorff illusion. The size of illusion induced by the real contour, however, was larger than that induced by the illusory contour. At the neural level, real and illusory contours commonly activated more dorsal visual areas, and the real contours specifically activated more ventral visual areas. More importantly, examinations on the parametric modulation effects of the size of illusion revealed the specific neural mechanisms underlying the Poggendorff illusion induced by the real and the illusory contours, respectively. Left precentral gyrus and right middle occipital cortex were specifically involved in the Poggendorff illusion induced by the real contour. On the other hand, bilateral intraparietal sulcus (IPS) and right lateral occipital complex (LOC) were specifically involved in the Poggendorff illusion induced by the illusory contour. Functional implications of the above findings were further discussed., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

In contrast to the previously held notion that mice have a weak visual system, it is now generally accepted that mice can perceive rather complicated figures in various contexts such as in cognitive experiments and in social settings. Here, we show that mice could even be capable of perceiving a visual illusion--subjective contours. This illusion requires the visual system to compensate for a lack of visual information in compressed 2D images on the retina. In this experiment, we trained mice to respond appropriately to a rectangle-shaped rewarded figure of specific orientation in a two-choice visual discrimination task with a touchscreen monitor. In Transfer Test 1, mice could discriminate illusory rectangle-shaped figures significantly as compared with a figure, which did not induce illusory figures. In Transfer Test 2, the choice rate of targets decreased with imperfect illusory figures, which produced weak perception of rotated or deficient inducers. Moreover, in Transfer Test 3, mice could not discriminate the low-resolution illusory figure, which also induced weak perception. These results demonstrated the possibility that mice might be useful for investigating fundamental properties of the neural visual system., (© The Author(s) 2015.)

There is debate as to whether or not the Ebbinghaus illusion is driven by high-level cognitive size contrast mechanisms as opposed to low-level biphasic contour interactions. In this study, we examine the variability in effects that are shared between this illusion and a different illusion that cannot be explained logically by a size contrast account. This comparison revealed that nearly one quarter of the variability for one illusion is shared with the other - demonstrating how a size-contrast account cannot be the sole explanation for the Ebbinghaus illusion., (© The Author(s) 2016.)

The vista paradox is the illusion in which an object seen through a window appears to shrink in apparent size (and appears farther away) as the observer approaches the window. Paradoxically, the distal object appears smaller as its visual angle increases. We investigated the effect in four experiments varying object size, distance, point of fixation, and texture of the frame and of the object. In the first experiment, we tried to confirm the illusion and to test the robustness of the phenomenon. In the second experiment, we manipulated where subjects fixated (on the frame or on the object) as well as the texture of the object and the frame. Fixation was essential for the illusion: fixating the frame led to an apparent shrinking of the object, whereas fixation on the object did not. Texture of the frame intensified the apparent shrinking of the object. In a third experiment, we separated the point of fixation from the frame in a between-subjects design. Finally, in Experiment 4, we showed that the paradox does not require a frame, but it requires a fixation on a location different from the object. That is, the window or frame is dispensable for the vista paradox, but fixation is critical., ((c) 2015 APA, all rights reserved).)

Several studies have reported similarities between perceptual processes underlying face and body perception, particularly emphasizing the importance of configural processes. Differences between the perception of faces and the perception of bodies were observed by means of a manipulation targeting a specific subtype of configural processing: the composite illusion. The composite face illusion describes the fact that two identical top halves of a face are perceived as being different if they are presented with different bottom parts. This effect disappears, if both halves are laterally shifted. Crucially, the effect of misalignment is not observed for bodies. This study aimed to further explore differences in the time course of face and body perception by using the composite effect. The present results replicated behavioural effects illustrating that misalignment affects the perception of faces but not bodies. Thus, face but not body perception relies on holistic processing. However, differences in the time course of the processing of both stimulus categories emerged at the N170 and P200. The pattern of the behavioural data seemed to be related to the P200. Thus, the present data indicate that holistic processes associated with the effect of misalignment might occur 200 ms after stimulus onset., (© 2014 The British Psychological Society.)

White's illusion is the perceptual effect that two equiluminant gray patches superimposed on a black-and-white square-wave grating appear different in lightness: A test patch placed on a dark stripe of the grating looks lighter than one placed on a light stripe. Although the effect does not depend on the aspect ratio of the test patches, and thus on the amount of border that is shared with either the dark or the light stripe, the context of each patch must, in a yet to be specified way, influence their lightness. We employed a contour adaptation paradigm (Anstis, 2013) to test the contribution of each of the test patches' edges to the perceived lightness of the test patches. We found that adapting to the edges that are oriented parallel to the grating slightly increased the lightness illusion, whereas adapting to the orthogonal edges abolished, or for some observers even reversed, the lightness illusion. We implemented a temporal adaptation mechanism in three spatial filtering models of lightness perception, and show that the models cannot account for the observed adaptation effects. We conclude that White's illusion is largely determined by edge contrast across the edge orthogonal to the grating, whereas the parallel edge has little or no influence. We suggest mechanisms that could explain this asymmetry.

Unlabelled: In the Barber-Pole Illusion (BPI), a diagonally moving grating is perceived as moving vertically because of the narrow, vertical, rectangular shape of the aperture window through which it is viewed. This strong shape-motion interaction persists through a wide range of parametric variations in the shape of the window, the spatial and temporal frequencies of the moving grating, the contrast of the moving grating, complex variations in the composition of the grating and window shape, and the duration of viewing. It is widely believed that end-stop-feature (third-order) motion computations determine the BPI, and that Fourier motion-energy (first-order) computations determine failures of the BPI. Here we show that the BPI is more complex: (1) In a wide variety of conditions, weak-feature stimuli (extremely fast, low contrast gratings, 21.5 Hz, 4% contrast) that stimulate only the Fourier (first-order) motion system actually produce a slightly better BPI illusion than classical strong-feature gratings (2.75 Hz, 32% contrast). (2) Reverse-phi barber-pole stimuli are seen exclusively in the feature (third-order) BPI direction when presented at 2.75 Hz and exclusively in the opposite (Fourier, first-order) BPI direction at 21.5Hz, indicating that both the first- and the third-order systems can produce the BPI. (3) The BPI in barber poles with scalloped aperture boundaries is much weaker than in normal straight-edge barber poles for 2.75 Hz stimuli but not in 21.5 Hz stimuli., Conclusions: Both first-order and third-order stimuli produce strong BPIs. In some stimuli, local Fourier motion-energy (first-order) produces the BPI via a subsequent motion-path-integration computation (Journal of Vision (2014) 14, 1--27); in other stimuli, the BPI is determined by various feature (third-order) motion inputs; in most stimuli, the BPI involves combinations of both. High temporal frequency, low-contrast stimuli favor the first-order motion-path-integration computation; low temporal frequency, high-contrast stimuli favor third-order motion computations., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The luminance contrast at the borders of a surface strongly influences surface's apparent brightness, as demonstrated by a number of classic visual illusions. Such phenomena are compatible with a propagation mechanism believed to spread contrast information from borders to the interior. This process is disrupted by masking, where the perceived brightness of a target is reduced by the brief presentation of a mask (Paradiso & Nakayama, 1991), but the exact visual stage that this happens remains unclear. In the present study, we examined whether brightness masking occurs at a monocular-, or a binocular-level of the visual hierarchy. We used backward masking, whereby a briefly presented target stimulus is disrupted by a mask coming soon afterwards, to show that brightness masking is affected by binocular stages of the visual processing. We manipulated the 3-D configurations (slant direction) of the target and mask and measured the differential disruption that masking causes on brightness estimation. We found that the masking effect was weaker when stimuli had a different slant. We suggest that brightness masking is partly mediated by mid-level neuronal mechanisms, at a stage where binocular disparity edge structure has been extracted., (Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.)

In the present study, we have tested the applicability of the computational model of centroid extraction to account for the data collected in experiments with stimuli comprising of closed two-dimensional shapes. The outlined or uniformly filled pie-shaped circular sectors (contextual distractors) were arranged according to the Brentano pattern, and three different stimulus parameters (either the radius or the central angle or the tilt angle of the sectors) were used as independent variables in different series of experiments. It was demonstrated that the model calculations adequately predict the variations of illusion magnitude shown by all the subjects for all independent variables and that there is no significant difference between the data obtained for stimuli with the outlined and uniformly filled distractors. A good correspondence between the computational and experimental data provides convincing evidence in support of the "centroid" explanation of illusions of extent of the Müller-Lyer type.

A generic model of automatic gain control (AGC) is proposed as a general framework for multidimensional automatic contrast sensitivity adjustment in vision, as well as in other sensory modalities. We show that a generic feedback AGC mechanism, incorporating a nonlinear synaptic interaction into the feedback loop of a neural network, can enhance and emphasize important image attributes, such as curvature, size, depth, convexity/concavity and more, similar to its role in the adjustment of photoreceptors and retinal network sensitivity over the extremely high dynamic range of environmental light intensities, while enhancing the contrast. We further propose that visual illusions, well established by psychophysical experiments, are a by-product of the multidimensional AGC. This hypothesis is supported by simulations implementing AGC, which reproduce psychophysical data regarding size contrast effects known as the Ebbinghaus illusion, and depth contrast effects. Processing of curvature by an AGC network illustrates that it is an important mechanism of image structure pre-emphasis, which thereby enhances saliency. It is argued that the generic neural network of AGC constitutes a universal, parsimonious, unified mechanism of neurobiological automatic contrast sensitivity control. This mechanism/model can account for a wide range of physiological and psychophysical phenomena, such as visual illusions and contour completion, in cases of occlusion, by a basic neural network. Likewise, and as important, biologically motivated AGC provides attractive new means for the development of intelligent computer vision systems.

Current accounts of the Ebbinghaus illusion emphasize either size contrast or contour interaction processes. To assess these alternatives, four variants of the Ebbinghaus figure were constructed using 1, 5, 9, or 13 small circles dispersed along the perimeter of larger contextual circles. 30 observers ranked the perceived size of the central circles and a single control circle. The rankings indicated that increasing the number of small circles reduced the perceived size of the central circle. The results parallel the effects of contextual arcs on the Ebbinghaus illusion and suggest that the mis-estimations of central circle size in Ebbinghaus figures result primarily from contour interactions.

In the occlusion illusion, a partly occluded object is perceived as though it were less occluded than it actually is (Palmer, Brooks, & Lai, 2007). We confirm and extend this finding using a stimulus with a moving occluder. In agreement with Palmer et al.'s (2007) findings and their partial-modal-completion hypothesis, we found that the illusion is indeed related to the sensory evidence for occlusion. Our experiments also confirm their speculation that the occlusion illusion involves an intriguing, seemingly paradoxical percept. In our experiments, subjects viewed an opaque disk with an open sector rotating in front of a background and indicated the perceived angular extent (a) of the occluder and (b) of the part of the background experienced as directly visible through the open sector. While the former was judged quite accurately, the latter was clearly overestimated. Thus, the angular extent of the background experienced as occluded and the extent experienced as directly visible sum to more than 360°, which makes the total percept an impossible figure. We argue that the key to resolving this paradox is to question the seemingly self-evident assumption that occluded and unoccluded portions of a visual scene are represented by amodal and modal percepts, respectively. Instead, we propose that visual percepts are experienced as modal whenever they are based on sufficiently conclusive sensory evidence and are otherwise experienced as amodal. Functionally, this perceptual representation of the conclusiveness of the sensory evidence underlying perceptual inferences might be more useful than estimates about optical visibility., (© 2015 ARVO.)

We tested the misperception hypothesis of drawing errors, which states that drawing accuracy is strongly influenced by the perceptual encoding of a to-be-drawn stimulus. We used a highly controlled experimental paradigm in which nonartist participants made perceptual judgements and drawings of angles under identical stimulus exposure conditions. Experiment 1 examined the isosceles/scalene triangle angle illusion; congruent patterns of bias in the perception and drawing tasks were found for 40 and 60° angles, but not for 20 or 80° angles, providing mixed support for the misperception hypothesis. Experiment 2 examined shape constancy effects with respect to reproductions of single acute or obtuse angles; congruent patterns of bias in the perception and drawing tasks were found across a range of angles from 29 to 151°, providing strong support for the misperception hypothesis. In both experiments, perceptual and drawing biases were positively correlated. These results are largely consistent with the misperception hypothesis, suggesting that inaccurate perceptual encoding of angles is an important reason that nonartists err in drawing angles from observation.

Spatially separated object information can be effortlessly completed in the visual system, as demonstrated by the well-known Kanizsa-type illusory contours. The perception of illusory contours is closely associated with the spatial configuration of contour fragments, leading to the long-lasting difficulty in distinguishing the effect of the completion process that interpolates the contour fragments from the effect of the noncompletion process that analyzes the contour fragments. However, a close relationship does not necessarily imply nonindependence, e.g., two people may show similar behaviors in one situation but may not in another situation. Inspired by this simple common sense, we conducted a contour discrimination task (i.e., discriminating between the interpolated contours) and a fragment discrimination task (i.e., discriminating between the physically-specified contour fragments) for Kanizsa squares and Kanizsa circles. The performance difference between the contour and fragment discrimination tasks was much larger for Kanizsa circles than for Kanizsa squares. This independence of the completion effect--as indicated by the performance in the contour task--from the noncompletion effect--as indicated by the performance in the fragment task--provides new insights into the understanding of the mechanism of visual completion.

Despite a relatively constant visual angle, the size of the moon appears very variable, mostly depending on elevation and context factors--the so-called moon illusion. As our perceptual experience of the size of the moon is clearly limited to the perceptual sphere of the sky, however, we do not know whether the typical perception of the moon at its zenith reflects a veridical interpretation of its visual angle of only 0.5 degrees. When testing the moon illusion in a large-scale planetarium, we observed two important things: (a) variation in perceptual size was no longer apparent, and (b) the moon looked very much smaller than in any viewing condition in the real sky--even when comparing it at its zenith. A closer inspection of the control console of the planetarium revealed that classic-analog as well as updated-digital planetariums use projections of the moon with strongly increased sizes to compensate for the loss of a natural view of the moon in the artificial dome of the sky.

In the barber-pole illusion (BPI), a diagonally moving grating is perceived as moving vertically because of the shape of the vertically oriented window through which it is viewed-a strong shape-motion interaction. We introduce a novel stimulus-the moving barber pole-in which a diagonal, drifting sinusoidal carrier is windowed by a raised, vertical, drifting sinusoidal modulator that moves independently of the carrier. In foveal vision, the moving-barber-pole stimulus can be perceived as several active barber poles drifting horizontally but also as other complex dynamic patterns. In peripheral vision, pure vertical motion (the moving-barber-pole illusion [MBPI]) is perceived for a wide range of conditions. In foveal vision, the MBPI is observed, but only when the higher-order modulator motion is masked. Theories to explain the BPI make indiscriminable predictions in a standard barber-pole display. But, in moving-barber-pole stimuli, the motion directions of features (e.g., end stops) of the first-order carrier and of the higher-order modulator are all different from the MBPI. High temporal frequency stimuli viewed peripherally greatly reduce the effectiveness of higher-order motion mechanisms and, ideally, isolate a single mechanism responsible for the MBPI. A three-stage motion-path integration mechanism that (a) computes local motion energies, (b) integrates them for a limited time period along various spatial paths, and (c) selects the path with the greatest motion energy, quantitatively accounts for these high-frequency data. The MBPI model also accounts for the perceived motion-direction in peripherally viewed moving-barber-pole stimuli that do and do not exhibit the MBPI over the entire range of modulator (0-10 Hz) and carrier (2.5-10 Hz) temporal frequencies tested.

Contour adaptation (CA) is a recently described paradigm that renders otherwise salient visual stimuli temporarily perceptually invisible. Here we investigate whether this illusion can be exploited to study visual awareness. We found that CA can induce seconds of sustained invisibility following similarly long periods of uninterrupted adaptation. Furthermore, even fragmented adaptors are capable of producing CA, with the strength of CA increasing monotonically as the adaptors encompass a greater fraction of the stimulus outline. However, different types of adaptor patterns, such as distinctive shapes or illusory contours, produce equivalent levels of CA suggesting that the main determinants of CA are low-level stimulus characteristics, with minimal modulation by higher-order visual processes. Taken together, our results indicate that CA has desirable properties for studying visual awareness, including the production of prolonged periods of perceptual dissociation from stimulation as well as parametric dependencies of that dissociation on a host of stimulus parameters., (Copyright © 2014. Published by Elsevier Inc.)

Background: Schizophrenia is a devastating psychiatric disorder characterized by symptoms including delusions, hallucinations, and disorganized thought. Kanizsa shape perception is a basic visual process that builds illusory contour and shape representations from spatially segregated edges. Recent studies have shown that schizophrenia patients exhibit abnormal electrophysiological signatures during Kanizsa shape perception tasks, but it remains unclear how these abnormalities are manifested behaviorally and whether they arise from early or late levels in visual processing., Method: To address this issue, we had healthy controls and schizophrenia patients discriminate quartets of sectored circles that either formed or did not form illusory shapes (illusory and fragmented conditions, respectively). Half of the trials in each condition incorporated distractor lines, which are known to disrupt illusory contour formation and thereby worsen illusory shape discrimination., Results: Relative to their respective fragmented conditions, patients performed worse than controls in the illusory discrimination. Conceptually disorganized patients-characterized by their incoherent manner of speaking-were primarily driving the effect. Regardless of patient status or disorganization levels, distractor lines worsened discrimination more in the illusory than the fragmented condition, indicating that all groups could form illusory contours., Conclusion: People with schizophrenia form illusory contours but are less able to utilize those contours to discern global shape. The impairment is especially related to the ability to think and speak coherently. These results suggest that Kanizsa shape perception incorporates an early illusory contour formation stage and a later, conceptually-mediated shape integration stage, with the latter being compromised in schizophrenia., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

The snake illusion is an effect in which the lightness of target patches is strongly affected by the luminance of remote patches. One explanation is that such images are decomposed into a pattern of illumination and a pattern of reflectance, involving a classification of luminance edges into illumination and reflectance edges. Based on this decomposition, perceived reflectance is determined by discounting the illumination. A problem for this account is that image decomposition is not unique, and that different decompositions may lead to different lightness predictions. One way to rule out alternative decompositions and ensure correct predictions is to postulate that the visual system tends to classify curved luminance edges as reflectance edges rather than illumination edges. We have constructed several variations of the basic snake display in order to test the proposed curvature constraint and the more general image decomposition hypothesis. Although the results from some displays have confirmed previous findings of the effect of curvature, the general pattern of data questions the relevance of the shape of luminance edges for the determination of lightness in this class of displays. The data also argue against an image decomposition mechanism as an explanation of this effect. As an alternative, a tentative neurally based account is sketched., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Visual illusions can reveal unconscious representations and processes at work in perception. Here we report a robust illusion that involves the misperception of moving, partially occluded objects. When a dynamically occluded object is seen through 2 misaligned apertures, the object appears misaligned in the direction of the apertures, creating the Aperture Capture Illusion. Specifically, when part of a dynamically occluded object disappears behind an occluding surface and then another part of the object comes into view immediately afterward, the 2 parts appear misaligned in the direction of the offset of the apertures through which they were seen. This illusion can be nulled: Separating the 2 object parts to increase the time interval between their appearance produced the percept of alignment. The ability to null the illusion in this manner demonstrates that dynamically occluded regions of moving objects continue to persist in perceptual awareness but, we argue, are perceived to move at a slower velocity than visible regions. We report 7 experiments establishing the existence of the illusion and ruling out several classes of explanation for it. We interpret the illusion and the ability to nullify it within the context of Palmer, Kellman, and Shipley's (2006) theory of spatiotemporal object formation.

Bamboo sharks (Chiloscyllium griseum) were tested for their ability to perceive subjective and illusionary contours as well as line length illusions. Individuals were first trained to differentiate between squares, triangles, and rhomboids in a series of two alternative forced-choice experiments. Transfer tests then elucidated whether Kanizsa squares and triangles, grating gaps and phase shifted abutting gratings were also perceived and distinguished. The visual systems of most vertebrates and even invertebrates perceive illusionary contours despite the absence of physical luminance, color or textural differences. Sharks are no exception to the rule; all tasks were successfully mastered within 3-24 training sessions, with sharks discriminating between various sets of Kanizsa figures and alternative stimuli, as well as between subjective contours in >75% of all tests. However, in contrast to Kanizsa figures and subjective contours, sharks were not deceived by Müller-Lyer (ML) illusions. Here, two center lines of equal length are comparatively set between two arrowheads or -tails, in which case the line featuring the two arrow tails appears to be longer to most humans, primates and birds. In preparation for this experiment, lines of varying length, and lines of unequal length randomly featuring either two arrowheads or -tails on their ends, were presented first. Both sets of lines were successfully distinguished by most sharks. However, during presentation of the ML illusions sharks failed to succeed and succumbed either to side preferences or chose according to chance.

Glass patterns are textural moirés from random dots. Sequential presentation of Glass patterns induces a sense of illusory motion. We evaluated how changes in temporal frequency affected the detection of global form in Glass patterns. We found linear improvement in coherence thresholds with increasing temporal frequency (Experiment 1), particularly in stimuli with large dot-pair separations (Experiment 2). These results support the notion that temporal and orientation information sum to boost sensitivity to visually obscure objects, and are discussed within the framework of "motion streak" detectors., (Copyright © 2013 Elsevier B.V. All rights reserved.)