An experimental and theoretical appraisal of the perspective and size-constancy theories of illusions

The perspective and size-constancy theories of illusions are described. They both imply that illusory distortions arise as a consequence of mistaken perceptual impressions of depth. Predictions from these theories are tested in six experiments and several ancillary studies. They show that although differences in apparent distance may be sufficient for the appearance of illusions their presence is not a necessary condition. Detailed theoretical implications of these results are considered in discussion.     

Interaction of Perception and Action in Discrete and Continuous Rapid Aiming Tasks

Previously, we have shown that discrete and continuous rapid aiming tasks are governed by distinct visuomotor control mechanisms by assessing the combined visual illusion effects on the perceived and effective index of difficulty (ID). All participants were perceptually biased by the combined visual illusion before they performed the rapid aiming tasks. In the current study, the authors manipulated the order of performing perceptual and motor tasks to examine whether perceptual or motor experience with the illusory visual target would influence the subsequent perceived and effective ID in discrete and continuous tapping tasks. The results supported our hypothesis showing that perceptual experience with the illusory visual target in the discrete condition reduced the effective ID in the subsequent discrete tapping task, and motor experience with the illusory visual target in the continuous condition reduced the illusion effects on the perceived ID in the subsequent perceptual judgment task. The study demonstrates the coinfluence of perception and action, and suggests that perception and action influence one another with different magnitude depending on the spatial frame of reference used to perform the perceptuomotor task.     

Context and the motion aftereffect: occlusion cues in the test pattern alter perceived direction

A horizontally moving vertical grating viewed through a diamond-shaped aperture can be made to appear to move either upwards or downwards by introduction of appropriate depth-ordering cues at the boundaries of the aperture (Duncan et al. 2000 Journal of Neuroscience 20 5885-5897). The grating is perceived to move towards (and sliding under) occluding 'near' surfaces, and parallel to 'far' surfaces. Here we show that these depth-ordering cues affect the perceptual interpretation of the motion aftereffect (MAE) as well. After adaptation to unambiguous horizontal motion, the MAE direction deviates from horizontal towards near surfaces. However, the influence of depth-ordering cues on the illusory motion of the MAE is generally less than that seen for 'real' motion. Implications for theories of depth-motion and depth-MAE interactions are discussed.     

Directional harmonic theory: a computational Gestalt model to account for illusory contour and vertex formation

Visual illusions and perceptual grouping phenomena offer an invaluable tool for probing the computational mechanism of low-level visual processing. Some illusions, like the Kanizsa figure, reveal illusory contours that form edges collinear with the inducing stimulus. This kind of illusory contour has been modeled by neural network models by way of cells equipped with elongated spatial receptive fields designed to detect and complete the collinear alignment. There are, however, other illusory groupings which are not so easy to account for in neural network terms. The Ehrenstein illusion exhibits an illusory contour that forms a contour orthogonal to the stimulus instead of collinear with it. Other perceptual grouping effects reveal illusory contours that exhibit a sharp corner or vertex, and still others take the form of vertices defined by the intersection of three, four, or more illusory contours that meet at a point. A direct extension of the collinear completion models to account for these phenomena tends towards a combinatorial explosion, because it would suggest cells with specialized receptive fields configured to perform each of those completion types, each of which would have to be replicated at every location and every orientation across the visual field. These phenomena therefore challenge the adequacy of the neural network approach to account for these diverse perceptual phenomena. I have proposed elsewhere an alternative paradigm of neurocomputation in the harmonic resonance theory (Lehar 1999, see website), whereby pattern recognition and completion are performed by spatial standing waves across the neural substrate. The standing waves perform a computational function analogous to that of the spatial receptive fields of the neural network approach, except that, unlike that paradigm, a single resonance mechanism performs a function equivalent to a whole array of spatial receptive fields of different spatial configurations and of different orientations, and thereby avoids the combinatorial explosion inherent in the older paradigm. The present paper presents the directional harmonic model, a more specific development of the harmonic resonance theory, designed to account for specific perceptual grouping phenomena. Computer simulations of the directional harmonic model show that it can account for collinear contours as observed in the Kanizsa figure, orthogonal contours as seen in the Ehrenstein illusion, and a number of illusory vertex percepts composed of two, three, or more illusory contours that meet in a variety of configurations.     

Early development of object unity: evidence for perceptual completion in newborns

The present study aimed to investigate whether perceptual completion is available at birth, in the absence of any visual experience. An extremely underspecified kinetic visual display composed of four spatially separated fragments arranged to give rise to an illusory rectangle that occluded a vertical rod (illusory condition) or rotated so as not to elicit perceptual grouping (control condition) was constructed. After newborns' ability to detect the particular kind of rod-and-box display used in the present study had been probed (Experiment 1), they were habituated to the illusory rod-and-box display (Experiment 2), to the control display that did not contain illusory contours (Experiment 3), and to a standard real rod-and-box display akin to those used in previous infants' studies (Experiment 4). Newborns perceived the rod as a connected unit either in the illusory condition (Experiment 2) or in the real condition (Experiment 4), as documented by a preference for a broken rod over a complete rod during the test phase, but not when the occluder was absent (Experiment 3). In all experiments newborns showed no preference between the two test stimuli (control condition), avoiding the possibility that newborns have a spontaneous preference for one test display over the other. Overall, the results of the present study provide evidence that the ability to achieve object unity (1) stems from intrinsic properties of the human perceptual system and (2) is operative from birth, given the right conditions.     

Functional theory of illusory conjunctions and neon colors

Illusory conjunctions are the incorrect perceptual combination of briefly presented colors and shapes. In the neon colors illusion, achromatic figures take on the color of an overlaid grid of colored lines. Both illusions are explained by a theory that assumes (a) poor location information or poor spatial resolution for some aspects of visual information and (b) that the spatial location of features is constrained by perceptual organization. Computer simulations demonstrate that the mechanisms suggested by the theory are useful in veridical perception and they are sufficient to produce illusory conjunctions. The theory suggests mechanisms that economically encode visual information in a way that filters noise and fills in missing data. Issues related to neural implementation are discussed. Four experiments illustrate the theory. Illusory conjunctions are shown to be affected by objective stimulus organization, by subjective organization, and by the linguistic structure of ambiguous Hebrew words. Neon colors are constrained by linguistic structure in the same way as illusory conjunctions.     

Mirrors and Misleading Appearances

Although philosophers have often insisted that specular perception is illusory or erroneous in nature, few have stressed the reliability and indispensability of mirrors as optical instruments. The main goal of this paper is to explain how mirrors can contribute to knowledge and at the same time be a source of systematic errors and misleading appearances. To resolve this apparent paradox, I argue that mirrors do not generate perceptual illusions or misperceptions, by defending a view of mirrors as transparent and invisible visual media. I then consider the reasons for which mirrors are said to be misleading. Contrary to the illusory account, I defend a nonperceptual approach to the errors attributable to mirrors, which analyses the kind of errors generated by the use of mirrors in terms of false judgments. I further show that a nonperceptual view of errors extends to all of the cases in which a sensorimotor adaptation is required, such as perception through magnifying or inversing lenses.     

Attention during adaptation weakens negative afterimages of perceptually colour-spread surfaces

The visual system can complete coloured surfaces from stimulus fragments, inducing the subjective perception of a colour-spread figure. Negative afterimages of these induced colours were first reported by S. Shimojo, Y. Kamitani, and S. Nishida (2001). Two experiments were conducted to examine the effect of attention on the duration of these afterimages. The results showed that shifting attention to the colour-spread figure during the adaptation phase weakened the subsequent afterimage. On the basis of previous findings that the duration of these afterimages is correlated with the strength of perceptual filling-in (grouping) among local inducers during the adaptation phase, it is proposed that attention weakens perceptual filling-in during the adaptation phase and thereby prevents the stimulus from being segmented into an illusory figure.     

When does grouping happen?

Recent research on perceptual grouping is described with particular emphasis on identifying the level(s) at which grouping factors operate. Contrary to the classical view of grouping as an early, two-dimensional, image-based process, recent experimental results show that it is strongly influenced by phenomena related to perceptual constancy, such as binocular depth perception, lightness constancy, amodal completion, and illusory contours. These findings imply that at least some grouping processes operate at the level of phenomenal perception rather than at the level of the retinal image. Preliminary evidence is reported showing that grouping can affect perceptual constancy, suggesting that grouping processes must also operate at an early, preconstancy level. If so, grouping may be a ubiquitous, ongoing aspect of visual organization that occurs for each level of representation rather than as a single stage that can be definitively localized relative to other perceptual processes.     

Sustained perceptual invisibility of solid shapes following contour adaptation to partial outlines

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.     

Perceptual organization: To reverse back the inverted (upside-down) question of feature binding

In addressing the most fundamental question of “Where visual processing begins”, all theories of perception can be segregated into two contrasting lines of thinking: “early feature-analysis” (i.e., from local to global processing) and “early holistic registration” (i.e., from global to local processing). The problem of feature binding is then essentially a consequence of the particular local-to-global assumption. However, from the global-to-local perspective, the problem of feature binding may be a wrong question to ask to begin with, while the Gestalt concept of perceptual organization serves to reverse this inverted position. Inspired by the analysis of invariants over transformations, particularly shape-changing transformations, a topological approach has been proposed to describe precisely the nature and rules of perceptual organization. Evidence supporting topological perception will be illustrated in topics of visual sensitivity, apparent motion, illusory conjunctions, and the relative salience of different geometric invariants.     

Toward a perceptual theory of transparency

Theories of perceptual transparency have typically been developed within the context of a physical model that generates the percept of transparency (F. Metelli's episcotister model, 1974b). Here 2 fundamental questions are investigated: (a) When does the visual system initiate the percept of one surface seen through another? (b) How does it assign surface properties to a transparent layer? Results reveal systematic deviations from the predictions of Metelli's model, both for initiating image decomposition into multiple surfaces and for assigning surface attributes. Specifically, results demonstrate that the visual system uses Michelson contrast as a critical image variable to initiate percepts of transparency and to assign transmittance to transparent surfaces. Findings are discussed in relation to previous theories of transparency, lightness, brightness, and contrast-contrast.     

Privileged directions for subjective contours: horizontal and vertical versus tilted

Subjective contours and brightness enhancement in Ehrenstein-like situations are affected by pattern orientation. If a classic Ehrenstein pattern (with four inducing elements for every gap at intersection points) is observed, a number of anomalous illusory patches usually appear in these gaps, but if the same pattern is observed tilted by 45 degrees the patches disappear and it is possible to see an illusory grid of horizontal and vertical 'streets'. These two perceptual results are mutually exclusive. In a Koffka-cross variant of this pattern, the illusory patches, which are usually square, appear more rounded in the tilted pattern. All these results were confirmed in two experiments by means of a magnitude estimation procedure. It is suggested that the formation of a subjective contour is easier along horizontal and vertical directions and more difficult in an oblique direction, and that this phenomenon, as well as other visual acuity oblique effects, depends in part on the basic functioning of the visual system at the level of sensation.     

Can we see syllables in monosyllabic words? A study with illusory conjunctions

Mathey, Zagar, Doignon, and Seigneuric (2006) reported an inhibitory effect of syllabic neighbourhood in monosyllabic French words suggesting that syllable units mediate the access to lexical representations of monosyllabic stimuli. Two experiments were conducted to investigate the perception of syllable units in monosyllabic stimuli. The illusory conjunction paradigm was used to examine perceptual groupings of letters. Experiment 1 showed that potential syllables in monosyllabic French words (e.g., BI in BICHE) affected the pattern of illusory conjunctions. Experiment 2 indicated that the perceptual parsing in monosyllabic items was due to syllable information and orthographic redundancy. The implications of the data are discussed for visual word recognition processes in an interactive activation model incorporating syllable units and connected adjacent letters (IAS; Mathey et al., 2006).     

New motion illusion caused by pictorial motion lines

Motion lines (MLs) are a pictorial technique used to represent object movement in a still picture. This study explored how MLs contribute to motion perception. In Experiment 1, we reported the creation of a motion illusion caused by MLs: random displacements of objects with MLs on each frame were perceived as unidirectional global motion along the pictorial motion direction implied by MLs. In Experiment 2, we showed that the illusory global motion in the peripheral visual field captured the perceived motion direction of random displacement of objects without MLs in the central visual field, and confirmed that the results in Experiment 1 did not stem simply from response bias, but resulted from perceptual processing. In Experiment 3, we showed that the spatial arrangement of orientation information rather than ML length is important for the illusory global motion. Our results indicate that the ML effect is based on perceptual processing rather than response bias, and that comparison of neighboring orientation components may underlie the determination of pictorial motion direction with MLs.     

Illusory volumes from conformation

The purpose of this paper is to offer demonstrations of 'illusory volumes' in the spirit of the illusory flat surfaces described by Kanizsa. These demonstrations of illusory volumes exploit a new cue to the recovery of surface curvature from ambiguous images: conformation. In assuming conformation, the visual system assumes that the surface of a volume conforms to the curvature of its neighboring, underlying, or supporting surface, in the absence of image cues to the contrary. Demonstrations that exploit the assumption of conformation provide several insights into the nature of the inferential processing that underlies contour, surface, and volume formation. In particular, these demonstrations imply that the visual system does not calculate local surface curvature, illusory contours, or occlusion relationships before it analyzes global surface relationships.     

Paranormal believers are more prone to illusory agency detection than skeptics

It has been hypothesized that illusory agency detection is at the basis of belief in supernatural agents and paranormal beliefs. In the present study a biological motion perception task was used to study illusory agency detection in a group of skeptics and a group of paranormal believers. Participants were required to detect the presence or absence of a human agent in a point-light display. It was found that paranormal believers had a lower perceptual sensitivity than skeptics, which was due to a response bias to ‘yes’ for stimuli in which no agent was present. The relation between paranormal beliefs and illusory agency detection held only for stimuli with low to intermediate ambiguity, but for stimuli with a high number of visual distractors responses of believers and skeptics were at the same level. Furthermore, it was found that illusory agency detection was unrelated to traditional religious belief and belief in witchcraft, whereas paranormal beliefs (i.e. Psi, spiritualism, precognition, superstition) were strongly related to illusory agency detection. These findings qualify the relation between illusory pattern perception and supernatural and paranormal beliefs and suggest that paranormal beliefs are strongly related to agency detection biases.     

Wheels: a new illusion in the perception of rolling objects

A new illusory effect is described which consists of a conspicuous perceptual overestimation of the speed at which a wheel rolling across an observer's visual field appears to be rotating. Wheels appear to revolve much faster than is compatible with their linear displacement. Experimental verification of the genuineness and magnitude of the effect is reported, along with a discussion of some of the variables upon which it may depend.     

Rethinking perceptual organization: The role of uniform connectedness

A principle of perceptual organization, calleduniform connectedness (UC), is described, and a theoretical approach to perceptual organization is proposed in which this principle plays a fundamental role. The principle of UC states that closed regions of homogeneous properties—such as lightness, chromatic color, texture, and so forth—tend to be perceived initially as single units. We demonstrate its effects and show that they occur even when opposed by powerful grouping principles such as proximity and similarity. We argue that UC cannot be reduced to such grouping principles, because it is not a form of grouping at all. We then propose a theoretical framework within which UC accounts for the initial (orentry level) organization of the visual field into primitive units. Classical principles of grouping operate after UC, creating superordinate units consisting of two or more basic-level units. Parsing processes also operate after UC, dividing basic-level units into subordinate parts. UC in the retinal image is proposed to be a necessary, but not a sufficient, condition for unit formation, since connected elements on the retina that are perceived to lie in separate depth planes fail to be perceived as units. This fact, together with other evidence that the Gestalt principles of grouping are based onperceived (rather than retinal) relations, suggests that the organization of visual stimulation into UC objects is ultimately achieved within a relatively late, postconstancy representation of environmental surfaces. The implications of this possibility are discussed in light of present theories of visual perception.