Illusory Contours Research Articles

Electrophysiological Modulation in an Effort to Complete Illusory Figures: Configuration, Illusory Contour and Closure Effects.

Figure recognition process: From the coarse configuration standing from the background to the closure of a meaningful shape, was investigated by ERP technique. ERP components at different latencies from stimulus onset allowed to tap into the figure recognition process at discrete time-points when different cognitive operations take place. In this study, we present two experiments where the support-ratio (SR) of illusory figures was manipulated to vary continuously the recognition of geometrical figures. In the first experiment three shapes were used to vary the SR and the P1 component (80-130ms) was modulated by the configuration-effect explained, in part for the first time, with the unbalanced physical stimulation between upper and lower visual field. In the second experiment, we used one shape and varied systematically the SR in a discrimination task. The N1 (130-180ms) and the N2 (230-270ms) were modulated by two effects: The Ic-effect, represented by the N1, and the closure-effect, represented by the N2, being larger when the SR was small and the discrimination more difficult with respect to large SRs and easier discrimination. These results showed that figure recognition proceeded from the coarse parsing of the visual scene (configuration-effect), through the completion of a set of illusory borders (Ic-effect) to the final assembling of a meaningful shape (closure-effect).

Affect from mere perception: Illusory contour perception feels good.

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

Global and local visual processing in autism: An objective assessment approach.

We examined global and local visual processing in autism spectrum disorder (ASD) via a match-to-sample task using Kanizsa illusory contours (KIC). School-aged children with ASD (n = 28) and age-matched typically developing controls (n = 22; 7-13 years) performed a sequential match-to-sample between a solid shape (sample) and two illusory alternatives. We tracked eye gaze and behavioral performance in two task conditions: one with and one without local interference from background noise elements. While analyses revealed lower accuracy and longer reaction time in ASD in the condition with local interference only, eye tracking robustly captured ASD-related global atypicalities across both conditions. Specifically, relative to controls, children with ASD showed decreased fixations to KIC centers, indicating reduced global perception. Notably, they did not differ from controls in regard to fixations to local elements or touch response location. These results indicate impaired global perception in the absence of heightened local processing in ASD. They also underscore the utility of eye-tracking measures as objective indices of global/local visual processing strategies in ASD. Autism Res 2017, 10: 1392-1404. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.

Weak experiences sufficient for creating illusory figures that influence perception of actual lines.

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.

Visual perception in domestic dogs: susceptibility to the Ebbinghaus-Titchener and Delboeuf illusions.

Susceptibility to geometrical visual illusions has been tested in a number of non-human animal species, providing important information about how these species perceive their environment. Considering their active role in human lives, visual illusion susceptibility was tested in domestic dogs (Canis familiaris). Using a two-choice simultaneous discrimination paradigm, eight dogs were trained to indicate which of two presented circles appeared largest. These circles were then embedded in three different illusory displays; a classical display of the Ebbinghaus-Titchener illusion; an illusory contour version of the Ebbinghaus-Titchener illusion; and the classical display of the Delboeuf illusion. Significant results were observed in both the classical and illusory contour versions of the Ebbinghaus-Titchener illusion, but not the Delboeuf illusion. However, this susceptibility was reversed from what is typically seen in humans and most mammals. Dogs consistently indicated that the target circle typically appearing larger in humans appeared smaller to them, and that the target circle typically appearing smaller in humans, appeared larger to them. We speculate that these results are best explained by assimilation theory rather than other visual cognitive theories explaining susceptibility to this illusion in humans. In this context, we argue that our findings appear to reflect higher-order conceptual processing in dogs that cannot be explained by accounts restricted to low-level mechanisms of early visual processing.

Consistency of Border-Ownership Cells across Artificial Stimuli, Natural Stimuli, and Stimuli with Ambiguous Contours.

To distinguish different objects in a natural scene, the brain must segment the image into regions corresponding to objects. The so-called "border-ownership" cells appear to be dedicated to this task, as they signal for a given edge on which side the object is that owns it. Here, we report that individual border-ownership cells are unreliable when tested across a battery of artificial stimuli used previously but can signal border-ownership consistently as a population. We show that these border-ownership population signals are also suited for signaling border-ownership for natural objects and at longer latency, even for stimuli without local edge information. Our results suggest that border-ownership cells integrate both local, low-level and global, high-level cues to segment the scene.

Early information processing contributions to object individuation revealed by perception of illusory figures.

To isolate multiple coherent objects from their surrounds, each object must be represented as a stable perceptual entity across both time and space. Recent theoretical and empirical work has proposed that this process of object individuation is a mid-level operation that emerges around 200-300 ms after stimulus onset. However, this hypothesis is based on paradigms that have potentially obscured earlier effects. Furthermore, no study to date has directly assessed whether object individuation occurs for task-irrelevant objects. In the present study we used electroencephalography (EEG) to measure the time course of individuation, for stimuli both within and outside the focus of attention, to assess the information processing stage at which object individuation arises for both types of objects. We developed a novel paradigm involving items defined by illusory contours, which allowed us to vary the number of to-be-individuated objects while holding the physical elements of the display constant (a design characteristic not present in earlier work). As early as 100 ms after stimulus onset, event-related potentials tracked the number of objects in the attended hemifield, but not those in the unattended hemifield. By contrast, both attended and unattended objects could be individuated at a later stage. Our findings challenge recent conceptualizations of the time course of object individuation and suggest that this process arises earlier for attended than unattended items, implying that voluntary spatial attention influences the time course of this operation.

Stereoscopic surface interpolation from illusory contours

Stereoscopic Kanizsa figures are an example of stereoscopic interpolation of an illusory surface. In such stimuli, luminance-defined disparity signals exist only along the edges of inducing elements, but observers reliably perceive a coherent surface that extends across the central region in depth. The aim of this series of experiments was to understand the nature of the disparity signal that underlies the perception of illusory stereoscopic surfaces. I systematically assessed the accuracy and precision of suprathreshold depth percepts using a collection of Kanizsa figures with a wide range of 2D and 3D properties. For comparison, I assessed similar perceptually equated figures with luminance-defined surfaces, with and without inducing elements. A cue combination analysis revealed that observers rely on ordinal depth cues in conjunction with stereopsis when making depth judgements. Thus, 2D properties (e.g. occlusion features and luminance relationships) contribute rich information about 3D surface structure by influencing perceived depth from binocular disparity.

Illusory and Real Contour Processing: Findings from a Joint Event-related Potential – Functional MRI Analysis

Illusory and Real Contour Processing: Findings from a Joint Event-related Potential – Functional MRI Analysis

Is There a Common Mechanism for Path Integration and Illusory Contour Formation?

Is There a Common Mechanism for Path Integration and Illusory Contour Formation?

Induction of Kanizsa Contours Requires Awareness of the Inducing Context

It remains unknown to what extent the human visual system interprets information about complex scenes without conscious analysis. Here we used visual masking techniques to assess whether illusory contours (Kanizsa shapes) are perceived when the inducing context creating this illusion does not reach awareness. In the first experiment we tested perception directly by having participants discriminate the orientation of an illusory contour. In the second experiment, we exploited the fact that the presence of an illusory contour enhances performance on a spatial localization task. Moreover, in the latter experiment we also used a different masking method to rule out the effect of stimulus duration. Our results suggest that participants do not perceive illusory contours when they are unaware of the inducing context. This is consistent with theories of a multistage, recurrent process of perceptual integration. Our findings thus challenge some reports, including those from neurophysiological experiments in anaesthetized animals. Furthermore, we discuss the importance to test the presence of the phenomenal percept directly with appropriate methods.

Global attention facilitates the planning, but not execution of goal-directed reaches.

In daily life, humans interact with multiple objects in complex environments. A large body of literature demonstrates that target selection is biased toward recently attended features, such that reaches are faster and trajectory curvature is reduced when target features (i.e., color) are repeated (priming of pop-out). In the real world, however, objects are comprised of several features—some of which may be more suitable for action than others. When fetching a mug from the cupboard, for example, attention not only has to be allocated to the object, but also the handle. To date, no study has investigated the impact of hierarchical feature organization on target selection for action. Here, we employed a color-oddity search task in which targets were Pac-men (i.e., a circle with a triangle cut out) oriented to be either consistent or inconsistent with the percept of a global Kanizsa triangle. We found that reaches were initiated faster when a task-irrelevant illusory figure was present independent of color repetition. Additionally, consistent with priming of pop-out, both reach planning and execution were facilitated when local target colors were repeated, regardless of whether a global figure was present. We also demonstrated that figures defined by illusory, but not real contours, afforded an early target selection benefit. In sum, these findings suggest that when local targets are perceptually grouped to form an illusory surface, attention quickly spreads across the global figure and facilitates the early stage of reach planning, but not execution. In contrast, local color priming is evident throughout goal-directed reaching.

Hypothesis concerning embodied calendars: A case study of number form, color spreading, and taste-color synaesthesia

We propose a hypothesis concerning the neural basis of the mental ‘calendar’ we all carry around in our brains, based on observations we made on a 25year old ‘projector synaesthete’, EA, who displays some novel and instructive features. In addition to her grapheme-color synaesthesia, she has a circular ‘calendar line’, laid out vividly in front of her in the horizontal plane with December 31st passing through the middle of her chest and other months arranged in clockwise sequence ending with December on her right (July was 3 feet in front of her). Her access to episodic memories felt, subjectively, as if it was facilitated by her calendar line. Intriguingly, if she turned her sideways to look to the right, the calendar remained ‘stuck to the chest’ – meaning that it was body centered rather than head centered. Even more surprising is how, when she rotated her head rightward, the left portion of her calendar became “fuzzy”, and memories of February and March became less accessible; a striking example of embodied cognition – memories gated by information from neural networks representing the activity of neck muscles. We postulate that the human calendar is mediated by connections between the angular gyrus and hippocampal place and time cells-via the inferior longitudinal fasciculus.Other aspects of EA’s synaesthesia were also explored. The colors evoked by graphemes spread outside the grapheme itself, but the spread could be blocked by real as well as illusory contours. These interactions might be mediated by cells signaling illusory contours (in V2) and color/texture sensitive cells in (V4). Tastes also evoked colors, but, intriguingly, the colors were experienced inside her mouth rather than out there in the world.We discuss the deeper implications of these findings for understanding the nature of quale, and the manner in which the self anchors itself in space and time.

Perceptual overloading reveals illusory contour perception without awareness of the inducers

Unconscious perception is frequently examined by restricting visual input (e.g., using short stimulus durations followed by masking) to prevent that information from entering visual awareness. Failures to demonstrate perception without awareness may thus be a consequence of this restricted input rather than of limitations in unconscious perception. Here, we demonstrate a novel method that circumvents these significant drawbacks inherent in other methods. Using this new perceptual overloading technique (POT), in which stimuli are repeatedly presented in alternation with a stream of variable masks, we demonstrate illusory contour perception and modal completion even when subjects are completely unaware of the inducing elements. In addition to demonstrating a powerful new method to study consciousness by effectively gating robust visual input from visual awareness, we show that more complex contextual effects, previously considered to be a privilege only of conscious vision, can occur without awareness.

A computational model of topological and geometric recovery for visual curve completion

Visual curve completion is a fundamental problem in understanding the principles of the human visual system. This problem is usually divided into two problems: a grouping problem and a shape problem. On one hand, though perception of the visually completed curve is clearly a global task (for example, a human perceives the Kanizsa triangle only when seeing all three black objects), conventional methods for solving the grouping problem are generally based on local Gestalt laws. On the other hand, the shape of the visually completed curve is usually recovered by minimizing shape energy in existing methods. However, not only do these methods lack mechanisms to adjust the shape of the recovered visual curve using perceptual, psychophysical, and neurophysiological knowledge, but it is also difficult to calculate an explicit representation of the visually completed curve. In this paper, we present a systematic computational model for generating a visually completed curve. Firstly, based on recent studies of perception, psychophysics, and neurophysiology, we formulate a grouping procedure based on the human visual system by seeking a minimum Hamiltonian cycle in a graph, solving the grouping problem in a global manner. Secondly, we employ a Bezier curve-based model to represent the visually completed curve. Not only is an explicit representation deduced, but we also present a means to integrate knowledge from related areas, such as perception, psychophysics, and neurophysiology, and so on. The proposed computational model has been validated using many modal and amodal completion examples, and desirable results were obtained.

Numerosity underestimation in sets with illusory contours

People underestimate the numerosity of collections in which a few dots are connected in pairs by task-irrelevant lines. Such configural processing suggests that visual numerosity depends on the perceived scene segments, rather than on the perceived total area occupied by a collection. However, a methodology that uses irrelevant line connections may also introduce unnecessary distraction and variety, or obscure the perception of task-relevant items, given the saliency of the lines. To avoid such potentially confounding variables, we conducted four experiments where the line-connected dots were replaced with collinear inducers of Kanizsa-type illusory contours. Our participants had to compare two simultaneously presented collections and choose the more numerous one. Displays comprised c-shaped inducers and disks (Experiment 1), c-shaped inducers only (Experiments 2 and 4), or closed inducers (Experiment 3). One display always showed a 12- (Experiments 1–3) or 48-item reference pattern (Experiment 4); the other was a test pattern with numerosity varying between 9 and 15 (Experiments 1–3) or 36–60 items (Experiment 4). By manipulating the number of illusory contours in the test patterns, the level of connectedness increased or decreased respectively. The fitted psychometric functions revealed an underestimation that increased with the number of illusory contours in Experiments 1 and 2, but was absent in Experiments 3 and 4, where illusory contours were more difficult to perceive or larger numerosities were used. Results corroborate claims that visual numerosity estimation depends on segmented inputs, but only within moderate numerical ranges.

Applying Transcranial Magnetic Stimulation (TMS) Over the Dorsal Visual Pathway Induces Schizophrenia-like Disruption of Perceptual Closure.

Perceptual closure ability is postulated to depend upon rapid transmission of magnocellular information to prefrontal cortex via the dorsal stream. In contrast, illusory contour processing requires only local interactions within primary and ventral stream visual regions, such as lateral occipital complex. Schizophrenia is associated with deficits in perceptual closure versus illusory contours processing that is hypothesized to reflect impaired magnocellular/dorsal stream. Perceptual closure and illusory contours performance was evaluated in separate groups of 12 healthy volunteers during no TMS, and during repetitive 10Hz rTMS stimulation over dorsal stream or vertex (TMS-vertex). Perceptual closure and illusory contours were performed in 11 schizophrenia patients, no TMS was applied in these patients. TMS effects were evaluated with repeated measures ANOVA across treatments. rTMS significantly increased perceptual closure identification thresholds, with significant difference between TMS-dorsal stream and no TMS. TMS-dorsal stream also significantly reduced perceptual closure but not illusory contours accuracy. Schizophrenia patients showed increased perceptual closure identification thresholds relative to controls in the no TMS condition, but similar to controls in the TMS-dorsal stream condition. Conclusions of this study are that magnocellular/dorsal stream input is critical for perceptual closure but not illusory contours performance, supporting both trickledown theories of normal perceptual closure function, and magnocellular/dorsal stream theories of visual dysfunction in schizophrenia.

The Poggendorff illusion driven by real and illusory contour: Behavioral and neural mechanisms

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.

The Kanizsa triangle illusion in foraging ants

The Kanizsa triangle, wherein three Pac-Man configurations symmetrically face inwards, is a well-known illusion. By exposing foraging ants (Lasius niger) to Kanizsa-shaped honeydew solutions, we studied the origin of this illusion. More specifically, we examined whether foraging ants showed different movement reactions to local honeydew patterns formed by nestmates. This novel phenomenon could serve as an abstract model of the Kanizsa triangle illusion under the assumption that such an illusion could arise through the sum of each agent's limited global cognitions, because each agent could not perceive the entire subjective contours. Even a subjective consciousness consists of some parts which have no identical perception and could be an illusion. We succeeded in inducing foragers to move along the sides of a Kanizsa triangle when Pac-Man-shaped inducers were introduced. Furthermore, foragers appeared to form Y-shaped trajectories when dot-shaped or inverse Kanizsa inducers were used. Based on our findings, we propose an agent-based ant model that compares modelled behaviour with experimental phenomena. Our abstract model could be used to explain such cognitive phenomena for bottom-up processes, because ants cannot perceive the given subjective contours, instead simply move along the edges.

Cue-dependent circuits for illusory contours in humans

Objects' borders are readily perceived despite absent contrast gradients, e.g. due to poor lighting or occlusion. In humans, a visual evoked potential (VEP) correlate of illusory contour (IC) sensitivity, the “IC effect”, has been identified with an onset at ~90ms and generators within bilateral lateral occipital cortices (LOC). The IC effect is observed across a wide range of stimulus parameters, though until now it always involved high-contrast achromatic stimuli. Whether IC perception and its brain mechanisms differ as a function of the type of stimulus cue remains unknown. Resolving such will provide insights on whether there is a unique or multiple solutions to how the brain binds together spatially fractionated information into a cohesive perception. Here, participants discriminated IC from no-contour (NC) control stimuli that were either comprised of low-contrast achromatic stimuli or instead isoluminant chromatic contrast stimuli (presumably biasing processing to the magnocellular and parvocellular pathways, respectively) on separate blocks of trials. Behavioural analyses revealed that ICs were readily perceived independently of the stimulus cue—i.e. when defined by either chromatic or luminance contrast. VEPs were analysed within an electrical neuroimaging framework and revealed a generally similar timing of IC effects across both stimulus contrasts (i.e. at ~90ms). Additionally, an overall phase shift of the VEP on the order of ~30ms was consistently observed in response to chromatic vs. luminance contrast independently of the presence/absence of ICs. Critically, topographic differences in the IC effect were observed over the ~110–160ms period; different configurations of intracranial sources contributed to IC sensitivity as a function of stimulus contrast. Distributed source estimations localized these differences to LOC as well as V1/V2. The present data expand current models by demonstrating the existence of multiple, cue-dependent circuits in the brain for generating perceptions of illusory contours.