The influence of visual illusions on grasp position

Visual illusions occur around us all the time, which present in nature and appear when you least expect it. It is well known that visual illusions can have a dramatic effect upon our visual perception of an object 's size and position. However, whether visual illusions have a similar influence on visual guided action or not, it remains the subject of much debate. The purpose of this study is to explore whether the visual information used to identify and categorize an object would be similar to the visual information used to plan and control visually guided movements directed toward that object. The outcome of this study will be helpful for designers to understand our visual information system and apply that knowledge to the design practice. In order to achieve the purpose of this study mentioned above, three visual illusions as the experiment's apparatus are applied to examine the relative effect of visual illusions on both the visual perceived center of an object and the position of a grasp on that object when a balanced lift is required. The results of the experiment show that (1) Illusions influence both the perceived and the grasped estimates of the center position, but the grasp center is more veridical; (2) Judd-illusion, one of the three visual illusions, has a bigger degree of effect than two other illusions on both the perceived and the grasped estimates of the center position; (3) By providing the participants with a mean to guide their visual perception of these illusions, the errors from the estimation of the center position can be significantly reduced.

It has been suggested that goal-directed actions performed under full vision are immune to certain visual illusions, while movements relying on perception-based visual information are deceived by them (Milner & Goodale, 1995). Consequently, pointing movements should be deceived by visual illusions when a delay is introduced (memory demands) or when antipointing (spatial imagery) is required. In 2 experiments, participants performed either propointing or antipointing movements to different versions of the Müller-Lyer illusion in 2 vision conditions (open-loop vs. delay). Apart from open-loop propointing, all conditions should rely on perceptual processing and should therefore yield similarly illusion effects. While we observed illusion effects in all conditions, their magnitude varied in unexpected ways. Most surprisingly, introducing a delay seemed to reduce illusion effects in antipointing. We show that this decrease can be explained by the fact that pointing after delay is less responsive to physical size changes. After correcting for this, illusion effects in antipointing were similar in both vision conditions but still twice as large as in the delayed propointing task. Our findings highlight the necessity of employing a correction procedure when comparing illusion effects across tasks and do not conform well to the predictions derived from the perception-action model.

It has been demonstrated that pictorial illusions have a smaller influence on grasping than they do on perceptual judgments. Yet to date this work has not considered the reduced influence of an illusion as it is measured repeatedly. Here we studied this decrement in the context of a Ponzo illusion to further characterize the dissociation between vision for perception and for action. Participants first manually estimated the lengths of single targets in a Ponzo display with their thumb and index finger, then actually grasped these targets in another series of trials, and then manually estimated the target lengths again in a final set of trials. The results showed that although the perceptual estimates and grasp apertures were equally sensitive to real differences in target length on the initial trials, only the perceptual estimates remained biased by the illusion over repeated measurements. In contrast, the illusion's effect on the grasps decreased rapidly, vanishing entirely after only a few trials. Interestingly, a closer examination of the grasp data revealed that this initial effect was driven largely by undersizing the grip aperture for the display configuration in which the target was positioned between the diverging background lines (i.e., when the targets appeared to be shorter than they really were). This asymmetry between grasping apparently shorter and longer targets suggests that the sensorimotor system may initially treat the edges of the configuration as obstacles to be avoided. This finding highlights the sensorimotor system's ability to rapidly update motor programs through error feedback, manifesting as an immunity to the effects of illusion displays even after only a few trials.

Visual illusions affect planning but not control

This study is about the expression types and characteristics of Trompe l`oeil in modern fashion. The study defines the concept of Trompe l`oeil, examines the Trompe l`oeil in the surrealist works from which the Trompe l`oeil derives from, and analyzes the works that have emerged since 2000. The study used a collection of magazines such as Vogue, Gap, Fashion News, and the Internet. According to the findings, the expression types of Trompe l`oeil in modern fashion were seen in the human body, the wearing style, the details, and accessory effects of clothes. The modern fashion design using the Trompe l`oeil based on such external expressions was characterized by the following: first, the fashion design destroyed the concept of common sense and expressed the purity of the human spirit by revealing the human body hidden in clothes from the combination of clothes and the human body with surrealism. Second, the details or accessories of clothes were printed or painted as if they existed. Pleasure was expressed by applying Trompe l`oeil to the wearing methods and forms of clothes through the optical illusion of materials or colors. Third, the effects of an optical illusion were displayed by transforming and distorting the wearing style and reversing the front, back, exterior and interior of clothes; this optical illusion characterized the deconstruction expressed through the transformation of ideas.

Vision for action and vision for perception have been suggested to be mediated by overlapping yet dissociable mechanisms. Recent evidence provided basic psychophysical support for this suggestion. In particular, it has been shown that Weber's law, the decrease in visual resolution with object size, does not affect the maximum grip aperture (MGA) between the fingers prior to grasp. Several studies replicated this result, but have argued that grasping may still obey Weber's law at early movement stages. Here, we show that this apparent adherence to Weber's law was confounded by task demands. Specifically, subjects were asked to keep their fingers closed prior to grasp, which encouraged them to open their fingers faster for larger compared to smaller objects. In two experiments, we tested this proposal by disentangling the effects of velocity from those of Weber's law. In Experiment 1, subjects were asked to keep their fingers open wide-apart prior to grasp, which encouraged them to close their fingers faster, rather than slower, to smaller objects. Now, the apparent adherence to Weber's law was reversed, and higher resolution was found for larger compared to smaller objects, thus indicating a "reversed" Weber's law. In Experiment 2, we manipulated task demands to equate aperture velocities across different objects sizes. When velocity was equated, no effects of Weber's law were found throughout the movement. These findings show that previous studies have confounded visual resolution with fingers' velocity, which led to an erroneous conclusion that Weber's law affected grasping at early stages of the movement.

The visual control of skilled actions, such as reaching and grasping, requires fundamentally different computations from those mediating our perception of the world. These differences in the computational requirements for vision for action and vision for perception are reflected in the organization of the two prominent visual streams of processing that arise from primary visual cortex in the primate brain. Although the ventral stream projecting to inferotemporal cortex mediates the visual processing underlying our visual experience of the world, the dorsal stream projecting to the posterior parietal cortex mediates the visual control of skilled actions. Specialized visual-motor modules have emerged in the posterior parietal cortex for the visual control of eye, hand, and arm movements. Although the identification of goal objects and the selection of an appropriate course of action depend on the perceptual machinery of the ventral stream and associated cognitive modules in the temporal and frontal lobes, the execution of the subsequent goal-directed action is mediated by dedicated online control systems in the dorsal stream and associated motor areas. Ultimately then, both streams work together in the production of goal-directed actions.

Human vision can be divided into vision for perception and vision for action (Milner & Goodale, 1995). In vision for perception the features of an object are processed independently (Kyllingsbaek & Bundesen, 2007). Are features also processed independently in vision for action? 20 different white bars were created using 4 lengths (5-7 cm.) and 5 orientations (30°- 60°). They were presented at arms length for 40 ms. on a black computer screen one at a time on either the left or right side of a central fixation mark. A contrast adjustment procedure lowered the intensity of the bar until barely visible. Vision for perception and vision for action were evaluated in separate blocks by two types of response modes for specifying length and orientation of the bars: Symbolic specification: specifying length and orientation on a keyboard. Motoric specification: moving index and thumb to the endpoints of the bar (using about 500 ms.). Positions of the fingers were measured and specification of raw lengths and orientations calculated. The raw lengths and orientations were then categorized into 4 lengths and 5 orientations. 7 subjects completed 320 trials of both types of specifications in separate blocks. Correct/incorrect symbolic or motoric specifications of length and orientation were entered into a 2x2 chi2. This was used as a measure of independence. In all subjects the chi2 analysis of both types of specifications supported independent processing of the two features. Also, Monte Carlo simulations of specifications of the two features (based on the observed marginal probabilities of correct specification of each feature) showed no significant differences between the observed specifications and the corresponding predictions of the model assuming independent processing of features. The features of an object are processed independently in both "vision for perception" and "vision for action". Meeting abstract presented at VSS 2012

Another Look at the Visual Illusion:The Effect of Muller-Lyer Illusion on Perception and Action

Mirror-induced visual illusion obtained through mirror therapy is widely used to facilitate motor recovery after stroke. Activation of primary motor cortex (M1) ipsilateral to the moving limb has been reported during mirror-induced visual illusion. However, the mechanism through which the mirror illusion elicits motor execution processes without movements observed in the mirrored limb remains unclear. This study aims to review evidence based on brain imaging studies for testing the hypothesis that neural processes associated with kinaesthetic motor imagery are attributed to ipsilateral M1 activation. Four electronic databases were searched. Studies on functional brain imaging, investigating the instant effects of mirror-induced visual illusion among stroke survivors and healthy participants were included. Thirty-five studies engaging 78 stroke survivors and 396 healthy participants were reviewed. Results of functional brain scans (n = 20) indicated that half of the studies (n = 10, 50%) reported significant changes in the activation of ipsilateral M1, which mediates motor preparation and execution. Other common neural substrates included primary somatosensory cortex (45%, kinaesthesia), precuneus (40%, image generation and self-processing operations) and cerebellum (20%, motor control). Similar patterns of ipsilateral M1 activations were observed in the two groups. These neural substrates mediated the generation, maintenance, and manipulation of motor-related images, which were the key processes in kinaesthetic motor imagery. Relationships in terms of shared neural substrates and mental processes between mirror-induced visual illusion and kinaesthetic motor imagery generate new evidence on the role of the latter in mirror therapy. Future studies should investigate the imagery processes in illusion training for post-stroke patients.

Introduction: The present study sets out to determine which sensory system mostly influences self-motion perception when visual and vestibular cues are in conflict. We paired caloric vestibular stimulation that signaled motion in either the clockwise or counter-clockwise direction with a visual display that indicated self-rotation in either the same or opposite directions.Methods: In Experiment 1 (E1), caloric vestibular stimulation was used to produce vestibular circular vection. In Experiment 2 (E2), a virtual optokinetic drum was used to produce visual circular vection in a VR headset. Vection speed, direction, and duration were recorded using a potentiometer knob the participant controlled in E1 and E2. In Experiment 3 (E3), visual and vestibular stimuli were matched to be at approximately equal speeds across visual and vestibular modalities for each participant setting up Experiment 4 (E4). In E4, participants observed a moving visual pattern in a virtual reality (VR) headset while receiving caloric vestibular stimulation. Participants rotated the potentiometer knob while attending to visual–vestibular stimuli presentations to indicate their perceived circular vection. E4 had two conditions: 1) A congruent condition where calorics and visual display indicated circular vection in the same direction; 2) an incongruent condition where calorics and visual display indicated circular vection in opposite directions.Results and discussion: There were equal reports of knob rotation in the direction consistent with the visual and vestibular self-rotation direction in the incongruent condition of E4 across trials. There were no significant differences in knob rotation speed and duration in both conditions. These results demonstrate that the brain appears to weigh visual and vestibular cues equally during a visual–vestibular conflict of approximately equal speeds. These results are most consistent with the optimal cue integration hypothesis.

Grasping the Müller-Lyer illusion: The contributions of vision for perception in action

Defining visual illusions in Parkinson's disease: Kinetopsia and object misidentification illusions

Previous studies have shown that both young and older subjects adapt their reaches in response to a visuomotor distortion. It has been suggested that one's continued ability to adapt to a visuomotor distortion with advancing age is due to the preservation of implicit learning mechanisms, where implicit learning mechanisms include processes that realign sensory inputs (i.e. shift one's felt hand position to match the visual representation). The present study examined this proposal by determining if changes in sense of felt hand position (i.e. proprioceptive recalibration) follow visuomotor adaptation in older subjects. As well, we examined the influence of age on proprioceptive recalibration by comparing young and older subjects' estimates of the position at which they felt their hand was aligned with a visual reference marker before and after aiming with a misaligned cursor that was gradually rotated 30 degrees clockwise of the actual hand location. On estimation trials, subjects moved their hand along a robot-generated constrained pathway. At the end of the movement, a reference marker appeared and subjects indicated if their hand was left or right of the marker. Results indicated that all subjects adapted their reaches at a similar rate and to the same extent across the reaching trials. More importantly, we found that both young and older subjects recalibrated proprioception, such that they felt their hand was aligned with a reference marker when it was approximately 6 degrees more left (or counterclockwise) of the marker following reaches with a rotated cursor. The leftward shift in both young and older subjects' estimates was in the same direction and a third of the extent of adapted movement. Given that the changes in the estimate of felt hand position were only a fraction of the changes observed in the reaching movements, it is unlikely that sensory recalibration was the only source driving changes in reaches. Thus, we propose that proprioceptive recalibration combines with adapted sensorimotor mappings to produce changes in reaching movements. From the results of the present study, it is clear that changes in both sensory and motor systems are possible in older adults and could contribute to the preserved visuomotor adaptation.

Visual illusions allow for strong tests of perceptual functioning. Perceptual impairments can produce superior task performance on certain tasks (i.e., more veridical perception), thereby avoiding generalized deficit confounds while tapping mechanisms that are largely outside of conscious control. Using a task based on the Ebbinghaus illusion, a perceptual phenomenon where the perceived size of a central target object is affected by the size of surrounding inducers, we tested hypotheses related to visual integration in deaf (n = 31) and hearing (n = 34) patients with schizophrenia. In past studies, psychiatrically healthy samples displayed increased visual integration relative to schizophrenia samples and thus were less able to correctly judge target sizes. Deafness, and especially the use of sign language, leads to heightened sensitivity to peripheral visual cues and increased sensitivity to visual context. Therefore, relative to hearing subjects, deaf subjects were expected to display increased context sensitivity (ie, a more normal illusion effect as evidenced by a decreased ability to correctly judge central target sizes). Confirming the hypothesis, deaf signers were significantly more sensitive to the illusion than nonsigning hearing patients. Moreover, an earlier age of sign language acquisition, higher levels of linguistic ability, and shorter illness duration were significantly related to increased context sensitivity. As predicted, disorganization was associated with reduced context sensitivity for all subjects. The primary implications of these data are that perceptual organization impairment in schizophrenia is plastic and that it is related to a broader failure in coordinating cognitive activity.