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.     

The tactual horizontal-vertical illusion depends on radial motion of the entire arm

Wesought to clarify the causes of the tactual horizontal-vertical illusion, where vertical lines are overestimated as compared with horizontals in Land inverted-T figures. Experiment 1 did not use L or inverted-T figures, but examined continuous or bisected horizontal and vertical lines. It was expected that bisected lines would be perceived as shorter than continuous lines, as in the inverted-T figure in the horizontal-vertical illusion. Experiment 1 showed that the illusion could not be explained solely by bisection, since illusory effects were similar for continuous and bisected vertical and horizontal lines. Experiments 2 and 3 showed that the illusory effects were dependent upon stimulus size and scanning strategy. Overestimation of the vertical was minimal or absent for the smallest patterns, where it was proposed that stimuli were explored by finger movement, with flexion at the wrist. Larger stimuli induce whole-arm motions, and illusory effects were found in conditions requiring radial arm motion. The illusion was weakened or eliminated in Experiment 4 when subjects were forced to examine stimuli with finger-and-hand motion alone, that is, their elbows were kept down on the table surface, and they were prevented from making radial arm motions. Whole-arm motion damaged performance and induced perceptual error. The experiments support the hypothesis that overestimation of the vertical in the tactual horizontal-vertical illusion derives from radial scanning by the entire arm.     

视错觉产生的神经机制

传统认知心理学对视觉错觉的解释存在局限性。认知神经科学在移动错觉、轮廓错觉、颜色错觉研究中的一批新发现,可以综合并概括为一种视错觉发生的神经关联重叠机制。该重叠性表现为错觉与相应的真实知觉具有本质上相同的神经活动,即错觉与真实知觉享有共同的神经关联,没有只对错觉反应而不对真实知觉反应的特定神经关联物,而错觉神经活动的特别之处是在真实知觉发生的相同脑区有更强烈的激活     

Halt and recovery of illusory motion perception from peripherally viewed static images

We quantitatively investigated the halt and recovery of illusory motion perception in static images. With steady fixation, participants viewed images causing four different motion illusions. The results showed that the time courses of the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion (i.e., "Rotating Snakes") were very similar, while the Ouchi and Enigma illusions showed quite a different trend. When participants viewed images causing the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion, they typically experienced disappearance of the illusory motion within several seconds. After a variable interstimulus interval (ISI), the images were presented again in the same retinal position. The magnitude of the illusory motion from the second image presentation increased as the ISI became longer. This suggests that the same adaptation process either directly causes or attenuates both the Fraser-Wilcox illusion and the modified Fraser-Wilcox illusion.     

Analysis of individual variations in the classical horizontal-vertical illusion

In the horizontal-vertical illusion (HVI), the length of the vertical line is overestimated, whereas in the bisection illusion (BI), the horizontal bisecting line is expected to be overestimated. Here, only half of our 22 observers showed the expected BI, whereas the other half underestimated the bisecting line. Observers also differed in their judgments of the strength of the HVI: The HVI was stronger for observers showing the classical bisection effect, and weaker or absent for those underestimating the bisecting line. To account for these results, we used a linear model to individually estimate the strength of two putative factors underlying both illusions. Whereas the strength of the HVI and BI were highly correlated, the estimated factors were uncorrelated. Therefore, in two control experiments, we then measured the pure horizontal-vertical (pHVI) and bisection (pBI) illusions. A significant correlation between the estimated factors and the measured illusion variants was found. Results were robust against variations of contrast, repetitive presentations, and choice of adjusted line. Thus, the classical HVI as an additive combination of two independent factors was confirmed, but we found considerable interindividual variations in the strength of the illusions. The results stress the importance of analyzing individual data rather than taking sample means for understanding these illusions.     

Encoding specificity principle in motor short-term memory for movement extent

The hypothesis was tested that, when the mode of presentation matches the mode of reproduction in memory for movement extent, there is less error in reproduction than when the modes are not matched. Female undergraduates (n = 24) were tested under active and passive criterion movements presented either under preselected or constrained conditions. All subjects underwent 36 trials involving the combination of three retention conditions (immediate, 20-sec unfilled, and 20-sec filled) and two reproduction conditions (active and passive). Results for absolute error showed that for constrained presentations, when the mode of presentation and the mode of reproduction were the same, accuracy was greater than when the modes of presentation and reproduction were different.     

Necker cube and autokinetic illusions after awakening from REM and NONREM sleep: lack of effect.

Earlier studies demonstrated differential perceptions of the visual illusions, beta movements and spiral aftereffect, following awakening from REM and NONREM sleep. The present study tested the effects of awakening from the two phases of sleep on two additional visual illusions, the Necker cube and autokinetic illusions. 12 young adults were wakened from REM and NONREM sleep and tested on both illusions for three 1-min. trials, within 1 to 2 min. of the awakening. Comparison of the post-REM and post-NONREM illusions indicated a nonsignificant difference between the two waking conditions in the passive spontaneous reversal rates of the Necker cube. Data for the autokinetic illusion were not reported as they were unreliable.     

Reported serial positions of true and illusory memories in the Deese/Roediger/McDermott paradigm

One of the easiest ways to induce illusory memories in the laboratory is to use the Deese–Roediger–McDermott (DRM) word-list paradigm. Researchers have used this paradigm not only to study people's memories of stimuli that were not actually presented, but also to study the phenomenological qualities of these illusions. In four experiments, the current investigation explored a phenomenological quality of illusory memories that has received almost no attention, specifically, temporality. A serial position task was incorporated into the DRM paradigm to examine temporal attributes of participants' true and false memories. Effects of list strength, presentation order, and types of warnings were examined. Results showed consistent serial position responses for true and false memories. However, only responses for illusory memories were affected by manipulations at study. The current findings thus lend support to encoding-based explanations of false recollections.     

An illusion of movement complementary to the horizontal-vertical illusion

Blindfolded subjects moved a stylus held in the hand over a standard distance of 4.5 ins. in a given direction. They then attempted to move the same distance in a direction at right angles to the first. Eight combinations of movements were investigated. The results reveal an illusion such that the extent of movements to left or right across the body is underestimated, while the extent of movements towards or away from the body in the mid-line is overestimated. The illusion applies to speed as well as extent of movement. Movement up or down in a vertical plane is equivalent to movement towards or away from the body in a horizontal plane.

The interaction of this illusion with the well-known horizontal-vertical illusion of visual perception explains a failure to find any net illusory effect where lines visually displayed in different orientations were matched for length by unseen movements in similar orientations.

Whether the visual and movement illusions simply co-exist or whether they are functionally related is not yet clear.     

Combined visual illusion effects on the perceived index of difficulty and movement outcomes in discrete and continuous fitts’ tapping

The speed-accuracy trade-off is a fundamental movement problem that has been extensively investigated. It has been established that the speed at which one can move to tap targets depends on how large the targets are and how far they are apart. These spatial properties of the targets can be quantified by the index of difficulty (ID). Two visual illusions are known to affect the perception of target size and movement amplitude: the Ebbinghaus illusion and Muller-Lyer illusion. We created visual images that combined these two visual illusions to manipulate the perceived ID, and then examined people’s visual perception of the targets in illusory context as well as their performance in tapping those targets in both discrete and continuous manners. The findings revealed that the combined visual illusions affected the perceived ID similarly in both discrete and continuous judgment conditions. However, the movement outcomes were affected by the combined visual illusions according to the tapping mode. In discrete tapping, the combined visual illusions affected both movement accuracy and movement amplitude such that the effective ID resembled the perceived ID. In continuous tapping, none of the movement outcomes were affected by the combined visual illusions. Participants tapped the targets with higher speed and accuracy in all visual conditions. Based on these findings, we concluded that distinct visual-motor control mechanisms were responsible for execution of discrete and continuous Fitts’ tapping. Although discrete tapping relies on allocentric information (object-centered) to plan for action, continuous tapping relies on egocentric information (self-centered) to control for action. The planning-control model for rapid aiming movements is supported.     

Visual illusions viewed as stabilized retinal images

Some well-known visual illusions have been examined under conditions which remove the effect of eye movements so that the image on the retina is stationary. Under these conditions the simple geometrical illusions are perceived in the normal way. Ambiguous perceptive figures show the reversals at about the usual rate provided that the subject is able to direct his attention to a salient point of the pattern. Certain regular stationary patterns produce illusory shadows which appear to move across the pattern in normal vision. These shadows are not seen when the effect of eye movements is removed.     

Vertical and bisection bias in active touch

We investigated the conditions that underlie the vertical and bisection illusion in touch, in order to understand the basis of their similarity to visual illusions, and the means of reducing the biases in length perception by active touch. Movement, speed, and spatial reference cues were tested. Movements in scanning L-shapes in ipsilateral and contralateral (across the body midline) table-top space produced significant underestimation of the vertical line with the right hand, but not with the left hand. Right-handed scanning of L-shapes showed no significant bias when the vertical line in the figure was aligned to the body midline, suggesting that spatial cues were involved. The vertical line was overestimated in inverted T-shapes, but underestimated in rotated T-shapes, implicating line bisection. Holding scanning latencies constant reduced the vertical error for inverted T-shapes, but could not explain the bisection bias. Sectioning biases were predicted by the location of junctions on sectioned lines, showing that junction points act as misleading anchor cues for movement extents. The illusion was significantly reduced when reference information was added by instructing subjects to relate two-handed scanning of the figure to an external frame and to body-centred cues. It is argued that disparities in spatial reference (anchor) cues for movement extents are involved in vertical and bisection biases in active touch. The hypothesis that length illusions depend on disparities in spatial reference information can also account for the similarity of the tactile to the visual horizontal-vertical illusion.     

Response times to Ehrenstein illusions of varying subjective magnitude: complementarity of psychophysical measures

The following experiments investigate the effects of contrast polarity, inducer spacing, and inducer type on three dependent variables measuring the perception of an illusory surface in Ehrenstein figures: subjective magnitude, response time, and frequency of perception. It was found that response time generally decreased when the other two behavioral indicators increased. However, it was also shown that subjective magnitude provided more discriminating measures of relatively strong illusory percepts, whereas frequency of perception and response time provided more discriminating measures of relatively weak illusory percepts. The findings generally confirm earlier work on the effect of inducer spacing and contrast polarity on the perceived strength of brightness illusions, and in particular reveal the complementarity of subjective magnitude, response time, and frequency of perception as critical measures of configurational effects in the perceptual processing of these phenomena. Thanks are due to J. E. Hoffman, T. Parks, and an anonymous referee for their insightful comments and suggestions on an earlier version of this paper.     

‘Eppur si move’: The Association Between Electrophysiological and Psychophysical Signatures of Perceived Movement Illusions

Vibration applied to muscle tendons induces an illusory perception of movement by activating spindle receptors and Ia afferent fibers. While neural structures supporting such illusions are quite defined, functional and psychophysical correlates of their perception have not been deeply investigated. The authors aimed at exploring subjective perception of illusory movements thanks to a repeated-stimulation multimodal approach. The integrated analysis of electroencephalography, source data (standardized low-resolution brain electromagnetic tomography–sLORETA), and psychophysical evaluation pointed at the activation of a frontoparietal sensorimotor network, with prefrontal-medial regions mainly involved in supporting the awareness for illusory movements. Further, illusions showed consistency in terms of presence, vividness, and duration. Finally, superior parietal structures seemed to play an important role in self-attribution of agency and ownership during such experiences.     

Consistent inter-individual differences in susceptibility to bodily illusions

Illusory senses of ownership and agency (that the hand or effector that we see belongs to us and moves at our will, respectively) support the embodiment of prosthetic limbs, tele-operated surgical devices, and human-machine interfaces. We exposed forty-eight individuals to four different procedures known to elicit illusory ownership or agency over a fake visible rubber hand or finger. The illusory ownership or agency arising from the hand correlated with that of the finger. For both body parts, sensory stimulation across different modalities (visual with tactile or visual with kinesthetic) produced illusions of similar strength. However, the strengths of the illusions of ownership and agency were unrelated within individuals, supporting the proposal that distinct neuropsychological processes underlie these two senses. Developing training programs to enhance susceptibility to illusions of agency or ownership for people with lower natural susceptibility could broaden the usefulness of the above technologies.     

A double dissociation between action and perception in the context of visual illusions: opposite effects of real and illusory size

The idea that there are two distinct cortical visual pathways, a dorsal action stream and a ventral perception stream, is supported by neuroimaging and neuropsychological evidence. Yet there is an ongoing debate as to whether or not the action system is resistant to pictorial illusions in healthy participants. In the present study, we disentangled the effects of real and illusory object size on action and perception by pitting real size against illusory size. In our task, two objects that differed slightly in length were placed within a version of the Ponzo illusion. Even though participants erroneously perceived the physically longer object as the shorter one (or vice versa), their grasping was remarkably tuned to the real size difference between the objects. These results provide the first demonstration of a double dissociation between action and perception in the context of visual illusions and together with previous findings converge on the idea that visually guided action and visual perception make use of different metrics and frames of reference.     

Properties of long-range illusory contours produced by offset-arcs

Researchers have used several different types of illusory contours to investigate properties of human perception. One rarely used illusory contour is a combination of the abutting grating and Kanizsa illusions. We call this the offset-arcs illusion and provide an empirical investigation of the illusion. Through a series of four experiments, using different methods of measurement, we show that changes to the phase of the abutting-grating part of the inducing stimulus can dramatically change the perceived strength and clarity of the long-range illusory contour. The easy manipulation of illusion strength should make the offset-arcs illusion applicable to a wide range of studies that utilize long-range illusory contours. The lack of a brightness component to the illusion should allow the offset-arcs illusion to help separate perceptual grouping from surface brightness effects that are often confounded in other illusory contours.     

The Empathy Quotient (EQ) predicts perceived strength of bodily illusions and illusion-related sensations of pain

In this study we test the possible implications of high empathising skills on bodily self–other distinction by measuring the strength of a body ownership illusion and a related experience of illusory pain. One-hundred adult participants completed the Empathy Quotient (EQ) questionnaire. Twenty participants from the top quintile and 20 participants from the bottom quintile of the EQ distribution took part in a laboratory experiment. In the experiment, a classical Rubber Hand Illusion (RHI) was induced followed by the presentation of a series of painful stimuli to the rubber hand. Participants were asked to self-rate the strength of their subjective experience of the RHI and of the illusory pain. A proprioceptive location judgment on the position of the hidden hand was also required before and after RHI induction, to record drifts towards the rubber hand. We found a significant difference between high- and low-empathy participants in RHI and pain score. The EQ was not related with the proprioceptive location judgement drift. It thus appears to be a better predictor of subjective ownership feelings and phenomenological self–other merging than of the behavioural components of bodily illusions.     

Disjunctive illusory inferences and how to eliminate them

The mental model theory of reasoning postulates that individuals construct mental models of the possibilities in which the premises of an inference hold and that these models represent what is true but not what is false. An unexpected consequence of this assumption is that certain premises should yield systematically invalid inferences. This prediction is unique among current theories of reasoning, because no alternative theory, whether based on formal rules of inference or on probabilistic considerations, predicts these illusory inferences. We report three studies of novel illusory inferences that depend on embedded disjunctions—for example, premises of this sort: A or else (B or else C). The theory distinguishes between those embedded disjunctions that should yield illusions and those that should not. In Experiment 1, we corroborated this distinction. In Experiment 2, we extended the illusory inferences to a more stringently controlled set of problems. In Experiment 3, we established a novel method for reducing illusions by calling for participants to make auxiliary inferences.     

Individual differences in susceptibility to experimentally induced phantom sensations

We investigated individual differences in susceptibility to two vibration-induced phantom illusions, i.e. illusory arm extension and nose prolongation (“Pinocchio illusion”). Vibration was applied to the biceps brachii tendon of 32 healthy participants. Susceptibility to the illusions was quantified by vividness ratings and by ratings of the amount of illusory position changes of the arm and illusory shape changes of the nose. Participants also completed the Perceptual Aberration (PA) and the Need for Cognition (NFC) inventories. PA reflects the frequency of spontaneously experienced body schema alterations and NFC a person's tendency to cognitively structure experiences. PA was positively correlated with participants’ susceptibility to illusory arm extension and, exclusively for men, also to nose elongation. A high NFC was weakly associated with a high susceptibility for the Pinocchio illusion. By inference, these findings indicate a physiological basis of PA and a cognitive mediation of experimentally induced phantom sensations.