The Use of Illusory Visual Information in Perception and Action

__Abstract__ Humans constantly interact with objects in the environment (e.g. grasp a pencil for writing or pick up a cup of tea) without making many mistakes in these performed actions. To guide these actions, visual information is used. In order to accurately grasp and pick up the pencil, vision provides information such as the size of the pencil and its position relative to the body and other objects in the environment. But what happens if the interpretation of the visual information differs from physical reality? When this happens, an illusion is experienced (Gillam, 1998; Gregory, 1998). In our everyday experiences, we often perceive the world inaccurately. For example, when a car drives by, at a certain speed it looks like the wheels are turning backwards. Furthermore, distances appear shorter and hills appear steeper than they really are (Proffitt et al., 1995). And estimating a distance from a map is inaccurate when Muller-Lyer elements (figure 1.2b) are present in the map (Gillan et al., 1999). Are these visual illusions also reflected in our actions? Sometimes they are and the performed action does not have the intended goal. When flying an airplane, an illusion of motion direction can make the pilot incorrectly adjust the flying direction, which can cause an accident (Shebilske, 1981). Visual illusions are a tool to fmd out how (illusory) visual information is proces

Misjudging where you felt a light switch in a dark room.

Previous research has shown that subjects systematically misperceive the location of visual and haptic stimuli presented briefly around the time of a movement of the sensory organ (eye or hand movements) due to errors in the combination of visual or tactile information with proprioception. These briefly presented stimuli (a flash or a tap on the finger) are quite different from what one encounters in daily life. In this study, we tested whether subjects also mislocalize real (static) objects that are felt briefly while moving ones hand across them, like when searching for a light switch in the dark. We found that subjects systematically mislocalized a real bar in a similar manner as has been shown with artificial haptic stimuli. This demonstrates that movement-related mislocalization is a real world property of human perception

Fixation locations when grasping partly occluded objects

When grasping an object, subjects tend to look at the contact positions of the digits (A. M. Brouwer, V. H. Franz, D. Kerzel, & K. R. Gegenfurtner, 2005; R. S. Johansson, G. Westling, A. Bäckström, & J. R. Flanagan, 2001). However, these contact positions are not always visible due to occlusion. Subjects might look at occluded parts to determine the location of the contact positions based on extrapolated information. On the other hand, subjects might avoid looking at occluded parts since no object information can be gathered there. To find out where subjects fixate when grasping occluded objects, we let them grasp flat shapes with the index finger and thumb at predefined contact positions. Either the contact position of the thumb or the finger or both was occluded. In a control condition, a part of the object that does not involve the contact positions was occluded. The results showed that subjects did look at occluded object parts, suggesting that they used extrapolated object information for grasping. Additionally, they preferred to look in the direction of the index finger. When the contact position of the index finger was occluded, this tendency was inhibited. Thus, an occluder does not prevent fixations on occluded object parts, but it does affect fixation locations especially in conditions where the preferred fixation location is occluded. © ARVO

Judgments of reachability are independent of visuomotor adaptation

The furthest distance that is judged to be reachable can change after participants have used a tool or if they are led to misjudge the position of their hand. Here we investigated how judged reachability changed when visual feedback about the hand was shifted. We hoped to distinguish between various ways in which visuomotor adaptation could influence judged reachability. Partici-pants had to judge whether they could reach a virtual cube without actually doing so. They indicated whether they could reach this virtual cube by moving their hand. During these hand movements, visual feedback about the position of the hand was shifted in depth, either away from or toward the participant. Participants always adapted to the shifted feedback. In a session in which the hand movements in the presence of visual feedback were mainly in depth, perceived reachability shifted in accordance with the feedback (more distant cubes were judged to be reachable when feedback was shifted further away). In a second session in which the hand movements in the presence of visual feedback were mainly sideways, for some participants perceived reachability shifted in the opposite direction than we expected. The shift in perceived reachability was not correlated with the adaptation to the shift in visual feedback. We conclude that reachability judgments are not directly related to visuomotor adaptation

Grasping the Muller-Lyer illusion: not a change in perceived length

Peak grip aperture has often been used to quantify the influence of illusions on judgments of size for action. However, a larger peak grip aperture need not mean that the object looks larger. It could also mean that it was grasped more carefully. These two possibilities can be distinguished on the basis of the velocity of grip closure just before contact. We let people grasp a bar that was placed on the shaft of a Müller-Lyer figure. The Müller-Lyer figure influenced the peak grip aperture. It did not influence the velocity of grip closure in the way that one would expect if size were misperceived. Thus there is no reason to assume that the perceived size guides the way that we reach and grasp an object. © 2006 Springer-Verlag

Illusions in action : consequences of inconsistent processing of spatial attributes

Many authors have performed experiments in which subjects grasp objects in illusory surroundings. The vast majority of these studies report that illusions affect the maximum grip aperture less than they affect the perceived size. This observation has frequently been regarded as experimental evidence for separate visual systems for perception and action. In order to make this conclusion, one assumes that the grip aperture is based on a visual estimate of the object's size. We believe that it is not, and that this is why size illusions fail to influence grip aperture. Illusions generally do not affect all aspects of space perception in a consistent way, but mainly affect the perception of specific spatial attributes. This applies not only to object size, but also to other spatial attributes such as position, orientation, displacement, speed, and direction of motion. Whether an illusion influences the execution of a task will therefore depend on which spatial attributes are used rather than on whether the task is perceptual or motor. To evaluate whether illusions affect actions when they influence the relevant spatial attributes we review experimental results on various tasks with inconsistent spatial processing in mind. Doing so shows that many actions are susceptible to visual illusions. We argue that the frequently reported differential effect of illusions on perceptual judgements and goal-directed action is caused by failures to ensure that the same spatial attributes are used in the two tasks. Illusions only affect those aspects of a task that are based on the spatial attributes that are affected by the illusion

Planning movements well in advance

It has been suggested that the metrics of grasping movements directed to visible objects are controlled in real time and are therefore unaffected by previous experience. We tested whether the properties of a visually presented distractor object influence the kinematics of a subsequent grasping movement performed under full vision. After viewing an elliptical distractor object in one of two different orientations participants grasped a target object, which was either the same object with the same orientation or a circular object without obvious orientation. When grasping the circular target, grip orientation was influenced by the orientation of the distractor. Moreover, as in classical visuomotor priming, grasping movements were initiated faster when distractor and target were identical. Results provide evidence that planning of visually guided grasping movements is influenced by prior perceptual experience, challenging the notion that metric aspects of grasping are controlled exclusively on the basis of real-time information

Shifted visual feedback of the hand affects reachability judgments in interception

Estimating whether an object is reachable is important if one intends to interact with the object. If an object is moving, it will be reachable only within a certain time-window. In such situations, motion of the object relative to the body has to be taken into account to judge the moment at which the target becomes reachable. We know that judgments of reachability are influenced by displaced visual feedback about the position of the hand when objects are static. Here we examine whether displaced feedback of the hand also influences reachability judgments when reachability is temporally constrained because the object is moving. The task for the subjects was to intercept a virtual cube with their unseen index finger as soon as the cube was considered to be reachable. Subjects received visual feedback about the position of their index finger, but this feedback was shifted in depth by 5. cm, either away from or closer to their body. The region that was judged to be reachable was larger when feedback of the hand was shifted away from the body than when the feedback was shifted closer to the body. This effect was correlated with the spatial error committed at the interception point. We conclude that all judgments about the surrounding space are adjusted in relation to the shifted visual feedback of the hand. © 2013 Elsevier Ltd