The Bourdon illusion in haptic space

A strong Bourdon illusion-the apparent bentness of a straight edge-in the active haptic mode was established in two experiments. Blindfolded subjects clasped the opposite surfaces of an object with the same frontal profile as the visual figure between thumb and forefinger and moved the latter together from end to end across the object. When the two triangular components of the objects were rotated through 180 degrees so that their bases instead of their apexes were contiguous, the illusion was reversed in direction and reduced by about half, as with the visual illusion. It was concluded that the active haptic Bourdon illusion is basically the same as the visual illusion and can be accounted for by a compromise in perception between the orientation of the test surface and that of the object of which it is an integral part.     

Determinants of the Bourdon effect

The Bourdon illusion is the apparent inward bending of straight, collinear edges in a solid figure consisting of two elongated triangles meeting at their apexes. This effect was investigated in five experiments. In the first and third experiments, it was shown that the apparent bending is greatest when the apical angles are about 12 deg and the axis of the figure is oriented at about 22 deg from the vertical. The second experiment was a control involving visual acuity for angular departures of two lines from collinearity and served as a basis of selection for subjects in Experiments 3, 4, and 5. Experiments 4 and 5 showed that the illusion occurs strongly in a solid (“filled in”) figure but is notably smaller in outline figures of the same size and shape. It tends to be negative in outline figures with boundaries formed by continuous and broken lines. The relationship between the Bourdon illusion and the “negative” Zöllner illusion is considered.     

The Bourdon illusion occurs with straight-, right-angle-, and parallel-edge figures

The Bourdon illusion is the apparent bentness of the straight edge of a figure consisting of two elongated triangular components arranged apex to apex. In three experiments, the illusion was shown to occur in the opposite direction, with the components arranged base to base. It was also shown to occur with the component edges at right angles and parallel. With the edges at right angles, the illusion also occurred in one direction when the components were apex to apex and occurred in the opposite direction when they were base to base. Supplementary observations indicated that the illusion is stronger when the components are relatively small and widely separated and eliminated when one of the two edges is curved.     

The basis of the Bourdon illusion

Day (1990) and Day, Mitchell, and Stecher (1990) recently reported new data on the Bourdon illusion, showing that the effect occurs in novel variations of the classic figure--for example, with orthogonal and parallel test edges--and also that it occurs in the haptic modality. Day (1990) criticized theories of the Bourdon effect proposed by Wenderoth, Criss, and van der Zwan (1990), Wenderoth and O'Connor (1987a, 1987b), and Wenderoth, O'Connor, and Johnson (1986), and proposed his own "perceptual compromise" hypothesis. It is argued that Day has inaccurately portrayed the Wenderoth et al. theory, and that his own hypothesis lacks predictive power because it lacks any reference to neural mechanisms. Day's theory is thus unable to account for extant data, including his own. It is demonstrated how Day's novel experiments provide insights into the mechanisms of the Bourdon effect, and also how the Wenderoth et al. theory can account for the new data of Day (1990) and Day et al. (1990).     

Determinants of subjective contour: Bourdon illusions and "unbending" effects

Wenderoth and O'Connor (1987b) reported that, although matches to the straight edge of two triangles placed apex to apex revealed an apparent bending in the direction of the chevron formed by the hypotenuse pair (the Bourdon effect), no perceptual unbending of the bent chevron occurred. Using subjective contour figures, Walker and Shank (1988b) found large and approximately equal bending and unbending effects, consistent with two theories that they proposed. In Experiment 1, using adjustable chevron matching and subjective contours, we found that Bourdon effects, equivalent in magnitude to those reported by Walker and Shank, were 4-5 times larger than unbending effects. In Experiment 2, we used a variation of Walker and Shank's measurement technique, in which subjects selected a matching angle from a graded series. We obtained Bourdon effects similar to those in Experiment 1, but much larger unbending effects. Nevertheless, Bourdon effects were significantly larger than unbending effects in one set of data; and in another, Bourdon test means were larger than unbending test means. In both data sets, there was a large and significant pretest bending effect, which enhanced the magnitude of unbending test minus pretest scores. These results were consistent with our theory but not the theories of Walker and Shank. The variance of unbending test matches, 3-4 times that of Bourdon test matches, reflected the task difficulty. We propose that subjective obtuse angle contraction that exceeds real obtuse angle contraction explains the fact that unbending effects are larger in subjective than in real contours.     

Reduction of the Zöllner illusion with motion of inducing figure

The magnitude of the Zöllner illusion was measured when the inducing lines moved rightward or leftward and were tracked by subjects. Motion of the inducing lines significantly reduced the magnitude of illusion, as in the Poggendorff illusion. Increasing velocity markedly increased the reduction, and, again, this effect was not significantly different from that obtained with the Poggendorff illusion. The current evidence seems to support the suggestion outlined earlier in relation to the Poggendorff illusion, that is, moving and stationary figures are processed by separate channels and, therefore, the interaction between them is reduced.     

Determinants of subjective contour: Bourdon illusions and “unbending” effects

Wenderoth and O’Connor (1987b) reported that, although matches to the straight edge of two triangles placed apex to apex revealed an apparent bending in the direction of the chevron formed by the hypotenuse pair (the Bourdon effect), no perceptual unbending of the bent chevron occurred. Using subjective contour figures, Walker and Shank (1988b) found large and approximately equal bending and unbending effects, consistent with two theories that they proposed. In Experiment 1, using adjustable chevron matching and subjective contours, we found that Bourdon effects, equivalent in magnitude to those reported by Walker and Shank, were 4–5 times larger than unbending effects. In Experiment 2, we used a variation of Walker and Shank’s measurement technique, in which subjects selected a matching angle from a graded series. We obtained Bourdon effects similar to those in Experiment 1, but much larger unbending effects. Nevertheless, Bourdon effects were significantly larger than unbending effects in one set of data; and in another, Bourdon test means were larger than unbending test means. In both data sets, there was a large and significant pretest bending effect, which enhanced the magnitude of unbending test minus pretest scores. These results were consistent with our theory but not the theories of Walker and Shank. The variance of unbending test matches, 3–4 times that of Bourdon test matches, reflected the task difficulty. We propose that subjective obtuse angle contraction that exceeds real obtuse angle contraction explains the fact that unbending effects are larger in subjective than in real contours.     

Control of the corridor illusion in baboons (Papio papio) by gradient and linear-perspective depth cues

The corridor illusion was recently demonstrated in baboons with background pictures containing rich depth information (Barbet and Fagot 2002, Behavioural Brain Research 132 111-115). In the current research we determined the contribution of gradient texture and perspective lines to that illusion. In experiment 1, the corridor illusion was tested in two baboons, with pictures of a hallway as backgrounds, or the same image of the hallway represented by perspective lines. Findings confirmed that the baboons experience the corridor illusion with the picture of the hallway, and showed that this illusion remained with the perspective-line backgrounds. The same procedure was adopted in experiment 2, but with a hallway drawn from gradient textures. The two baboons experienced the illusion in this experiment too. Thus both gradient and perspective line cues convey sufficient information to control the corridor illusion in baboons. In baboons, the processing of these two kinds of depth cues interacts with the perception of object size, suggesting homologous processes of pictorial depth perception in humans and non-human primates.     

Correlations among scores on measures of field dependence-independence cognitive style, cognitive ability, and sustained attention

The Children's Embedded Figures Test, the Rod and Frame Test to measure the field dependence-independence cognitive style, Cattell's Culture Fair Intelligence Tests to measure cognitive ability, and two cancellation tasks (Zazzo task and Bourdon task) to assess sustained attention were administered to 179 boys and 110 girls whose average age was 9.0 yr. Correlations between scores on measures of field dependence-independence and cognitive ability were moderated. Average correlations between scores on measures of field dependence-independence, cognitive ability, and measures of sustained attention was .23 for the Zazzo task and quite weak (.06) for the Bourdon task.     

Susceptibility to the flash-beep illusion is increased in children compared to adults

Audio-visual integration was studied in children aged 8-17 (N = 30) and adults (N = 22) using the 'flash-beep illusion' paradigm, where the presentation of two beeps causes a single flash to be perceived as two flashes (fission illusion), and a single beep causes two flashes to be perceived as one flash (fusion illusion). Children reported significantly more fission illusions than adults, indicating that auditory and visual information was integrated more often, and less selectively, than in adults. Within either group, illusion reports did not correlate with either age or motor coordination measures. The current results show that the form of multisensory integration indexed by the illusion is slow to mature in normally developing children.     

Tactile expectations and the perception of self-touch: An investigation using the rubber hand paradigm

The rubber hand paradigm is used to create the illusion of self-touch, by having the participant administer stimulation to a prosthetic hand while the Examiner, with an identical stimulus (index finger, paintbrush or stick), administers stimulation to the participant’s hand. With synchronous stimulation, participants experience the compelling illusion that they are touching their own hand. In the current study, the robustness of this illusion was assessed using incongruent stimuli. The participant used the index finger of the right hand to administer stimulation to a prosthetic hand while the Examiner used a paintbrush to administer stimulation to the participant’s left hand. The results indicate that this violation of tactile expectations does not diminish the illusion of self-touch. Participants experienced the illusion despite the use of incongruent stimuli, both when vision was precluded and when visual feedback provided clear evidence of the tactile mismatch.     

Visual and haptic perception of the horizontal-vertical illusion

The horizontal-vertical illusion consists of two lines of the same length (one horizontal and the other vertical) at a 90 degree angle from one another forming either an inverted-T or an L-shape. The illusion occurs when the length of a vertical line is perceived as longer than the horizontal line even though they are the same physical length. The illusion has been shown both visually and haptically. The present purpose was to assess differences between the visual or haptic perception of the illusions and also whether differences occur between the inverted-T and the L-shape illusions. The current study showed a greater effect in the haptic perception of the horizontal-vertical illusion than in visual perception. There is also greater illusory susceptibility of the inverted-T than the L-shape.     

Framing effects and the reversed Müller-Lyer illusion.

The enclosure hypothesis of the reversed Müller-Lyer illusion was examined in three experiments. In Experiment 1, the ingoing- and outgoing-wings forms of the illusion were measured separately, as a function of the size of the gap between the ends of the shaft and the apices of the wings. In Experiments 2 and 3, the effects of a square frame and of complete and amputated versions of a rectangle on the perceived length of an enclosed horizontal line were examined. For all non-Müller-Lyer illusion figures, an inverted U-shaped function describes the relationship between illusion magnitude and the length of the test line. The peak overestimation of the test line's length was obtained when the ratio of total figure length to test line length was about 3:2. Taken together, the results of the three experiments suggest that the reversed Müller-Lyer illusion can be explained within current theoretical frameworks, such as assimilation theory, without recourse to a special factor of enclosure.     

Influence of vestibular signals on bodily self-consciousness: Different sensory weighting strategies based on visual dependency

Previous studies showed that the vestibular system is crucial for multisensory integration, however, its contribution to bodily self-consciousness more specifically on full-body illusions is not well understood. Thus, the current study examined the role of visuo-vestibular conflict on a full-body illusion (FBI) experiment that was induced during a supine body position. In a mixed design experiment, 56 participants underwent through a full-body illusion protocol. During the experiment, half of the participants received synchronous visuo-tactile stimulation, and the other half received asynchronous visuo-tactile stimulation, while their physical body was lying in a supine position, but the virtual body was standing. Additionally, the contribution of individual sensory weighting strategies was investigated via the Rod and Frame task (RFT), which was applied both before (pre-FBI standing and pre-FBI supine) and after the full-body illusion (post-FBI supine) protocol. Subjective reports of the participants confirmed previous findings suggesting that there was a significant increase in ownership over a virtual body during synchronous visuo-tactile stimulation. Additionally, further categorization of participants based on their visual dependency (by RFT) showed that those participants who rely more on visual information (visual field dependents) perceived the full-body illusion more strongly than non-visual field dependents during the synchronous visuo-tactile stimulation condition. Further analysis provided not only a quantitative demonstration of full-body illusion but also revealed changes in perceived self-orientation based on their field dependency. Altogether, findings of the current study make further contributions to our understanding of the vestibular system and brought new insight for individual sensory weighting strategies during a full-body illusion.     

Can you believe it? Illusionism and the illusion meta-problem

Illusionism about consciousness is the thesis that phenomenal consciousness does not exist, but merely seems to exist. Embracing illusionism presents the theoretical advantage that one does not need to explain how consciousness arises from purely physical brains anymore, but only to explain why consciousness seems to exist while it does not. As Keith Frankish puts it, illusionism replaces the “hard problem of consciousness” with the “illusion problem.” However, a satisfying version of illusionism has to explain not only why the illusion of consciousness arises, but also why it arises with its particular strength: Notably, why we are so deeply reluctant to recognize the illusory nature of consciousness. Explaining our strong intuitive resistance to illusionism means solving what I call the “illusion meta-problem,” which I think is a part of the illusion problem. In this paper, I argue that current versions of illusionism are unable to solve the illusion meta-problem. I focus on two of the most promising recent illusionist theories of consciousness, and I show why they fail to explain the peculiar reluctance we encounter whenever we try to accept that consciousness is an illusion.     

Framing effects and the reversed Müller-Lyer illusion

The enclosure hypothesis of the reversed Müller-Lyer illusion was examined in three experiments. In Experiment 1, the ingoing- and outgoing-wings forms of the illusion were measured separately, as a function of the size of the gap between the ends of the shaft and the apices of the wings. In Experiments 2 and 3, the effects of a square frame and of complete and amputated versions of a rectangle on the perceived length of an enclosed horizontal line were examined, For all non-Müller-Lyer illusion figures, an inverted U-shaped function describes the relationship between illusion magnitude and the length of the test line. The peak overestimation of the test line’s length was obtained when the ratio of total figure length to test line length was about 3:2. Taken together, the results of the three experiments suggest that the reversed Müller-Lyer illusion can be explained within current theoretical frameworks, such as assimilation theory, without recourse to a special factor of enclosure.     

Planning to reach for an object changes how the reacher perceives it

Three experiments assessed the influence of the Ebbinghaus illusion on size judgments that preceded verbal, grasp, or touch responses. Prior studies have found reduced effects of the illusion for the grip-scaling component of grasping, and these findings are commonly interpreted as evidence that different visual systems are employed for perceptual judgment and visually guided action. In the current experiments, the magnitude of the illusion was reduced by comparable amounts for grasping and for judgments that preceded grasping (Experiment 1). A similar effect was obtained prior to reaching to touch the targets (Experiment 2). The effect on verbal responses was apparent even when participants were simply instructed that a target touch task would follow the verbal task. After participants had completed a grasping task, the reduction in the magnitude of the illusion remained for a subsequent verbal-response judgment task (Experiment 3). Overall, the studies demonstrate strong connections between action planning and perception.     

The role of distance from the body and distance from the real hand in ownership and disownership during the rubber hand illusion

A key tool for investigating body ownership is the rubber hand illusion, in which synchronous multisensory feedback can induce feelings of ownership over a fake hand. Much research in the field aims to tease apart the mechanisms that underlie this phenomenon. Currently there is conflicting evidence as to whether increasing the distance between the real and fake hands (within reaching space) can reduce the illusion. The current study examines this further by modulating, not only the absolute distance between the real and fake hands but also their relative distance from body midline. It is found that the strength of the illusion is reduced only when the fake hand is both far from the real hand and far from the trunk; illusion scores over a fake hand in the same position can then be increased by moving the real hand nearer. This is related to peripersonal space surrounding the trunk and the hand. Subjective disownership of the real hand, and proprioceptive drift measures were also taken and may be driven by different mechanisms.     

Reduction of the Poggendorff effect by the motion of oblique lines

The magnitude of the Poggendorff illusion was measured when the test lines were moved up or down and tracked by subjects. The difference between test lines and inducing lines caused by motion of the test lines significantly reduced the magnitude of illusion (60%). Supplementary experiments seemed to indicate that location of test lines, perceptual shrinkage of space in the vertical dimension, and effective display time were not the main factors contributing to the reduction in illusion magnitude. Instead, it seems that some reduction in interaction between test and inducing lines was the main cause of the reduction. The rising curve of the reduction was very steep with velocity, and the reduction magnitude was almost constant over most of the range of velocities studied. The current evidence seems to suggest that moving and stationary figures are processed by separate channels and that, therefore, the interaction between them is reduced.     

The Ponzo illusion affects grip-force but not grip-aperture scaling during prehension movements

Contextual cues such as linear perspective and relative size can exert a powerful effect on the perception of objects. This fact is demonstrated by the illusory effects that can be induced by such cues (e.g., the Ponzo railway track and Titchener circles illusions). Several recent studies have reported, however, that visual illusions based on such cues have little or no influence on the visuomotor mechanisms used to guide hand action. Furthermore, evidence of this sort has been cited in support of a distinction between visual perception and the visual control of action. In the current study, the authors investigated the effect of the Ponzo visual illusion on the control of hand action, specifically, the scaling of grip force and grip aperture during prehension movements. The results demonstrate that grip force scaling is significantly influenced by the Ponzo visual illusion, whereas the scaling of grip aperture is unaffected by the illusion.